Types of nutrition
• All living organisms-nutrients for growth,
maintenance and repair of damaged tissues.
• Nutrition is the entire process by which
organisms obtain energy and nutrients from
food, for growth, maintenance and repair of
• The process by which autotrophs (autos: self;
trophos: feed) synthesise complex organic
compounds from raw, simple inorganic
substances using light or chemical energy.
• Photosynthesis (photos: light) - the process
through which green plants (called
photoautotrophs) produce organic molecules
from CO2 and H2O using light as a source of
• Chemosynthesis (chemo: chemical) - the process
by which chemoautotrophs synthesise organic
compounds from CO2 and H2O without the help
of light, but obtain energy by oxiding inorganic
substances such as hydrogen sulphide and
• A type of nutrition in which an organism
(cannot synthesise their own nutrients)
• Holozoic nutrition (holo: like; zoon: animal) -
ingesting solid organic matter
• Saprophytism - dead and decaying organic
• Parasitism - absorbs readily digested food
from its host.
• Balanced diet = a diet consisting of all the
nutrients in the correct proportiond to meet
the requirements of the body.
The necessity for a balanced diet
• Nutrients provide the body with its basic
– source of energy,
– chemical building blocks for growth and repair of
damaged body tissues
– metabolic reactions.
• Every individual requires an adequate daily
– Energy-providing foods (carbohydrates and lipids),
– Growth-providing foods (proteins), and
– Sufficient amounts of minerals, vitamins, water
and roughage to maintain health.
Daily energy requirement
• A balanced diet -daily energy requirement
• The sum of all chemical reactions in the body
is called metabolism.
• The energy consumed by the body in a day
while carrying out all the basic processes
known as the basic metabolic rate (BMR).
• The minimum daily energy requirement varies
for different individuals and is dependent
upon various factors
• Burning a known mass of the food completely
in the presence of oxygen
• To calculate the energy value of various types
of food samples.
• Energy value = the amount if heat generated
from the combustion of one gram of food.
The unit used to describe energy values in
food is joule per gram (Jg-1).
• The amount of energy in food can also be
expressed in terms of calories.
• 1calorie (cal) = 4.2 joules (J). 4.2 joules of
energy are needed to raise the temperature of
water by 10C.
Food class Energy value (kJg-1)
• The energy value of food can be calculated as
• Essential for the maintenance of good health
and efficient metabolism.
• Fat-soluble vitamins - are vitamins A, D, E and
K- can be stored in body fat.
• Water-soluble vitamins - include vitamins B
and C- cannot be stored in the body, and have
to be constantly supplied in the daily diet
Sources, functions and effects of vitamin
Vitamin Dietary sources Functions Symptoms of deficiency
Egg yolk, butter, fish
liver oil, dairy
Needed for the formation of light-sensitive pigment in the
Acts as an antioxidant by combining with free radicals to
minimize its damaging effects. Free radicals are
produced when body cells generate and use energy.
Free radicals react with DNA, leading to serious damage;
and in some cases, causing cancer.
Maintenance of epithelial tissues.
Delays the ageing process.
Vision problems (night
Cornea becomes dry and
D Dairy products, egg
yolk, cod liver oil,
milk, (also made in
human skin in the
presence of sunlight)
Aids in the absorption of calcium and phosphorus ions
in the small intestine and the use of these ions in the
formation of bones and teeth.
Promotes bone growth.
Rickets – a disease
characterized by poor teeth
and bone formation in
Deformities such as bowed
legs and knock knees
Stunted growth in children
Osteomalacia – softening of
bones in adults
Wheat germ, nuts,
grains, olive oil, milk
Produces red blood cells.
Acts as an antioxidant, as it combines easily with
unstable metabolic products (free radicals). Thus, vitamin
E protects unsaturated fatty acids with vitamins A and C.
Prevents damage to phospholipids in cell membranes, so
maintaining their structure.
A range of disorders in
different species, including
muscular dystrophy, liver
damage and infertility
May also cause anaemia
Vitamin Dietary sources Functions Symptoms of deficiency
spinach, a form of
the vitamin is
Important in blood
Defective blood clotting which leads
to extensive bleeding
nuts, milk, liver,
Precursor of a
Beri-beri (muscle weakness, nerve
disorder, heart disorder, swollen
feet and loss of skin sensitivity)
milk, liver, eggs
Sore eyes and swollen tongues
Skin lesions at the corner of mouth,
nose and ears
Liver, lean meat,
fish, yeast extract
Pellagra (skin and gastrointestinal
lesions, nervous, mental disorders
and loss of appetite)
Functions Symptoms of deficiency
liver, fish, meat
Component of coenzyme A,
with a role in energy
Muscle cramps, fatigue,
impaired motor coordination
Coenzymes in amino acid
A coenzyme in nucleic acid
Synthesis of red blood cells.
Folic acid Green
Acts as a coenzyme in
nucleic acid and amino acid
Functions Symptoms of deficiency
A coenzyme in the synthesis
of fat, glycogen and amino
Required in the synthesis of
Maintenance of cartilage,
bone and dentin.
A strong antioxidant.
Aids in detoxification.
Improves ferum absorption.
Scurvy – symptoms
bleeding gums and
Degeneration of blood
vessels, muscles and
• Minerals are simple inorganic nutrients which
must be obtained through the diet, either
from food or dissolved in drinking water, cause
the body cannot manufacture them.
• Minerals do not provide energy,
• Major minerals, called macrominerals, are
required in relatively large quantities.
Examples of macrominerals are calcium,
magnesium, phosphorus and sodium
• Some of the macrominerals are recommended
in amounts more than 100mg per day.
• Microminerals are required in trace amounts
of less than 20mg per day.
• Examples of microminerals are cobalt,
fluorine, iodine, manganese, zinc and
molybdenumj. These minerals have very
• If a particular mineral is deficient in a person’s
diet, the normal health and metabolism of
that person can be affected.
• This will result in a deficiency disease with
• A deficiency disease can be avoided by
supplementing the diet with the necessary
Sources, functions and effects of mineral
Mineral Sources Functions Symptoms of deficiency
Calcium Milk, cheese,
Bone and tooth formation.
Aids in blood clotting.
Needed in muscle and nerve
Transmission of nerve
Rickets in children
Delayed blood clotting
the elderly due to loss
of bone mass
Magnesium Green leafy
Activates most types of
enzymes in protein synthesis.
Maintains normal function of
muscles and nerves.
Bone and tooth formation.
Retarded function if
muscles and nerves
egg yolk, meat,
Component of haemoglobin
needed for oxygen transport in
Component of enzymes
involved in cellular respiration.
Reduced reistance to
Mineral Sources Functions Symptoms of
Sodium Table salt An important component in
Maintains the acid-base
balance and water balance.
Normal muscle and nerve
Involved in nerve impulse
Loss of appetite
Chlorine Table salt Acid-base balance
Loss of appetite
Potassium Meat, dairy
Needed for the correct
functioning of the heart.
Maintenance of acid-base
balance and water balance.
Muscle and nerve function.
Mineral Sources Functions Symptoms of
Component of the thyroxine
Sulphur Meat, fish, nuts,
Component of certain
Needed for muscle growth.
Bone and tooth formation.
(component of DNA and
Involved in the transfer of
energy in ATP.
Rickets in children
Loss of calcium and
fluorine Drinking water,
Maintenance of strong teeth
and probably bone
Helps resist tooth decay.
High frequency of
• Dietary fibre -indigestible part of plant food
which consists mainly of cellulose.
• Not a nutrient, dieticians -25-50g of fibre
should be eaten day to ensure good health.
• Foods high in fibre content include fruits,
vegetables, nuts and whole meal grains
• Foods that contain dietary fibre satisfy the
appetite and delay hunger.
• Dietary fibre has no nutritional value.
• Water is very essential to the survival of
humans as all metabolic reactions in the
human body tale place in solutions.
• Water males up about 70% of the total body
Functions of Dietary fibre
• High water holding capacity
• Dietary fibre aids in peristalsis.
• Aids in bowel movement.
• Deficiency constipation
• Dietary fibre lowers the cholesterol level in
• It also reduces the risk of heart disease and
• Is the medium for all cellular biochemical
• Is the medium of transportation for
respiratory gases and nutrients.
• Regulates body temperature.
• Excretory waste such as lactic acid, urea and
excess mineral salts through perspiration and
• Maintains osmotic pressure in the tissue fluid
and blood plasma. Osmotic pressure is
determined by the amount of water and
mineral salts in the blood plasma or tissue
• Aids peristalsis movement.
• Dissolves most chemical substances.
• Enables hydrolysis of food substances during
• For our bodies to function efficiently, we need to
maintain a proper water balance in the body.
• A normal and healthy adult requires about 2 to
2.5 litres of water daily to replace water lost
– through the skin during perspiration,
– through evaporation from the lungs during breathing,
– in the form of urine from the kidneys and also in the
• Water loss can have an adverse effect on the
physiological processes and physical
performance of the body.
• Failure to replace the water lost will result in
dehydration. A severe loss of water can be
Choosing an appropriate diet menu
for different target groups
• The balanced diets of different people vary
according to their age, lifestyle, health
conditions and specific nutritional needs.
• Each target group needs to choose a diet
appropriate to its needs.
• The various target groups are as follows:
• Calcium and phosphorus for the formation of
strong bones in the growing fetuses
• Folic acid and ferum for the formation of red
• Proteins for the formation of new tissues.
• Green vegetables and whole grains as a source of
fibre to prevent constipation.
• Fats and sugar, and caffeinated drinks should be
reduced or avoided.
Infants and children
• Proteins, carbohydrates, lipids, vitamins and
minerals to help them grow.
• Calcium and phosphorous-formation of strong
bones and teeth.
• Proteins and vitamins
• Fruits and vegetables to supply the important
nutrients needed, including vitamins and
• Ferum to prevent iron-deficiency anaemia.
• Plant proteins. Plant protein sources are
grains, soya beans, tofu and nuts.
• Weight lifting -high protein foods to build new
tissues and strong muscles.
• A runner -proteins, vitamins, minerals and
carbohydrates to provide constant energy
during training and competition.
• Athletes -calcium, sodium and potassium to
prevent muscle cramps.
• Proteins, vitamins such as D, B6, folic acid and
minerals such s calcium and phosphorus to
• Vitamin d help prevent osteomalacia and folic
acid helps synthesise red blood cells.
• Reduce their intake of salt, carbohydrates and
fats to -high blood pressure, diabetes and
coronary heart disease.
People with specific diseases
• Diabetes should avoid sugary and high
carbohydrate food to maintain a normal level
of sugar in his blood.
• High risk of coronary heart disease should
reduce the consumption of saturated fats and
• Malnutrition results from an unbalanced diet,
in which certain nutrients are deficient, in
excess, or are in the wrong proportions.
• If this condition persists over a long period of
time, the person’s health will be adversely
– Frequently occur in children aged between 9 and
– Marasmus is the general wasting of the body due
to protein deficiency combined with a lack of
– The child becomes very thin with wrinkled skin.
– A child suffering from kwashiorkor does not
receive sufficient proteins in his diet.
– The child has flaky skin, thin muscles, thin hair and
a swelling of the body due to retention of fluid in
tissues. The child normally experiences stunted
– In both cases, the mental and physical
development of the child is severely impaired
Effects of Calcium, Phosphorus and Vitamin D
• Bones to become brittle, porous and crack easily.
• Not treated, bones will fracture easily, especially
at the hips, backbone and wrists. The backbone
may shorten until a person becomes severely
• Calcium and phosphorus supplements and having
regular exercise can counteract the development
• Osteomalacia (soft bones)- pregnant women.
• Teenagers and adolescents whose diets lack
calcium, phosphorus and vitamin D are at a
higher risk of getting this disease at old age.
This is because bone growth reaches its
maximum density before the age of 35.
• women need to consume food rich in calcium,
phosphorus and vitamin D, such as milk, eggs
• Vitamin D is also required in calcium absorption.
• Vitamin D deficiency results in reduced calcium
absorption from digested food. This will
encourage calcium intake from bones.
• Exercise is also important for children and
teenagers as this will enable them to achieve
maximum bone density. Examples of exercises
that help strengthen bones are walking and
Excessive intake of lipids
• Body fat- diet rich in saturated fats can result
• cardiovascular diseases
• Arteries become clogged by plaques, the threat
of heart attacks and strokes becomes even
• If the coronary artery is partially blocked, the
person may feel chest pains (angina pectoris).
• heart is not receiving sufficient oxygen.
• strenuous activity or when feeling intense
• A fully blocked coronary artery will result in a
heart attack or myocardial infarction.
• Hypertension or high blood pressure.
• Arteriosclerosis raises the blood pressure by
narrowing the lumen of blood vessels and
reducing their elasticity.
– The normal blood pressure for adults is 120/80.
– Blood pressure can cause the small arteries to burst
and can lead to strokes if these occur in the brain.
– Strokes occur as a result of the death of nerve tissue
in the brain, usually caused by a blockage of the
arteries in the brain. In some cases, strokes can be
Excessive intake of minerals
• An excessive intake of vitamins and minerals can also bring about adverse
effects to our health.
• An excess of minerals will cause an imbalance in the osmotic pressure of
• An excess of sodium is associated with high blood pressure, a major factor
that contributes to heart disease and strokes in some people.
– Excessive salt and a lack of water can lead to the formation of kidney stones.
Kidney stones can develop when crystalds form in the urine and build up in
the inner surfaces of the kidney.
– Most stones are made of calcium and oxalate. Oxalate is a substance found in
nuts, leafy greens, chocolates and vitamin C.
– The stones may either remain in the kidney or travel down the ureter. This
will complicate kidney function and urinating will be painful.
• The kidneys of people who lack exercise and do not perspire work hard to
remove the excess salt. Eventually, this will lead to kidney failure.
• Excess calcium can also increase the risk of
kidney stone formation.
• Excess ferum can lead to liver and kidney
damage, kidney toxicity, and death, especially
Excessive intake of vitamins
• Generally, an excessive intake of water-soluble
vitamins is not harmful to the body, as excess
vitamins are excreted in the urine.
• However, excess fat-soluble vitamins are not
excreted from the body but are deposited in body
fat over time.
• Therefore, overdoses of fat-soluble vitamins will
result in an accumulation of these organic
compounds to toxic levelos in the body. Effects of
an overdose of these vitamins are given in Table
Effects of overdoses of vitamins
C Gastrointestinal upset
A Hair loss, vomiting, bone ache, joint pain, liver and bone damage
E Kidney damage
D Too much calcium in the blood and widespread calcification of soft
tissues which interferes with the functions of muscles and heart
K Liver damage and anaemia
B6 Numb feet and poor coordination
~ Flushed face and hands
~ Liver damage
Excessive intake of proteins
• increase the uric content in the blood.
• Uric acid forms crystals in the soft tissues of
the joints. This condition leads to gout.
• Uric acid can also crystallize and form stones
in the kidney and cause kidney damage.
• To prevent gout and formation of kidney
stones, avoid foods high in purine-contaning
nucleic acids such as liver, kidneys and
• complex organic molecules which are too
large to pass through plasma membranes and
enter body cells.
• form that can be readily absorbed by the body
• The process that breaks down complex food
substances into simpler, soluble molecules
that are small enough for the body to absorb
is called digestion.
• Digestion breaks down
– starch into glucose molecules,
– proteins into amino acids, and
– lipids into glycerol and fatty acids.
• These essential substances are required by the body
cells to carry out metabolic processes. For example,
• glucose is oxidized to generate energy,
• amino acids are used to synthesise new proteins, such
as enzymes and hormones,
• lipids form a major component of plasma membranes
Digestion in the mouth
• The chewing
• Secretion of saliva by three pairs of salivary
• The tongue
• Amylase which begins the hydrolysis of starch
• Bolus in preparation for swallowing.
• Swallowing, the bolus enters the throat.
• When swallowing, a cartilage flap called the
epiglottis temporarily closes the airway to
prevent food from entering the trachea.
• Oesophagus, a muscular tube lined with
epithelium and mucous glands.
• The mucus lubricates the movement of the
bolus along the oesophagus by peristalsis, a
series of wave-like muscular contractions
along the oesophagus wall
• When the cardium sphincter relaxes, the bolus
enters the stomach.
Digestion in the stomach
– The stomach is a thick-walled, sausage-shaped organ situated below the diaphragm.
– It is a muscular sac with a highly folded inner wall.
– The epithelial lining of the stomach contains gastric glands.
– Food stays in the stomach for a number of hours.
– During this period, the food is thoroughly churned and mixed the gastric juice by the peristaltic contraction
of the stomach wall.
– Eventually, the contents of the stomach become a semi-fluid called chyme.
– Relaxation of the pyloric sphincter allows the chime to gradually enter the duodenum.
– Health Watch: Excessive secretions of hydrochloric acid and pepsin are thought to cause peptic ulcers.
Recently, this condition has been linked to the presence of Helicobacter pylori (a type of bacteria) in the
stomach. There is also evidence which links H. pylori to certain types of stomach cancer.
– Hydrochloric acid
• Creates an acidic condition (pH 1.5-2.0) which is optimal for the action of the enzymes in the stomach.
• Stops the activity of salivary amylase.
• Helps to kill bacteria in food.
• - pepsin and rennin
• Pepsin starts the hydrolysis of large protein molecules into smaller chains of polypeptides by breaking specific peptide
• Rennin coagulates milk by converting the soluble milk protein, caseinogen, into insoluble casein.
– Digestion in the small intestine
– The small intestine consists of the duodenum, jejunum and the highly coiled ileum.
– The duodenum, the first part of the small intestine, receive chime from the stomach and
secretions from the gall bladder and pancreas.
» Liver secretes bile, an alkaline greenish-yellow liquid produced by the liver and stored in the gall bladder.
» Bile does not contain any digestive enzymes.
» Bile creates an alkaline environment (pH 4.6-8.6) for the enzyme action in the duodenum.
» It helps to reduce the acidity of chime and optimizes the pH for enzyme action.
» Bile salts emulsify lipids, transforming large lumps of lipids into tiny droplets, thus providing a greater
surface area for digestion by enzymes. This allows lipids digestion to procedd more rapidly.
• Pancreas secretes pancreas juice which contains the enzymes pancreatic amylase,
trypsin and lipase.
• The optimum pH required for the action of enzymes in the pancreatic juice is
between 7.14 and 8.2.
• ** Bile enters the duodenum via the bile duct.
• ** Pancreatic juice is secreted into the duodenum by the pancreas via the
• The digestion of the starch, proteins and lipids takes place in the duodenum.• Pancreatic amylase
• Pancreatic amylase completes the digestion of starch and maltose. Starch +
water maltose• trypsin
• Trypsin digests polypeptides into shorter chains of peptides. Polypeptides +
water peptides• lipase
• Lipase completes the digestion of lipids into fatty acids and glycerol, which are
small enough to be absorbed by the epithelial lining of the small intestine. Lipids
droplets + water glycerol +
• Hydrolysis of lipids is especially difficult because lipids are insoluble in water. Bile
acts as an emulsifier which lowers the surface tension of the lipids and coats tiny
fat droplets to form a stable emulsion.
• Self-digestion occurs when digestive juices secreted into the alimentary canal
destroy the epithelial lining.
• Glands in the wall of the ileum secrete intestinal juice which contains digestive
enzymes needed to complete the digestion of peptides and disaccharides.
• The intestinal enzymes require an alkaline medium to act at an optimal rate.
• At the end of the digestive process, all carbohydratesare digested into
monosaccharides such as glucose, fructose and galactose.
• Proteins are digested into amino acids and lipids into fatty acids and glycerol.
• Vitamins and the minerals are extremely small and soluble and need not be
• Dietary fibre cannot be digested in the human body as the enzyme cellulose is not
produced in the alimentary canal.
• The appendix is a small tubular appendage that extends outwards from the
caecum of large intestine. It does not play a vital role in the human body. When
this organ becomes inflamed, a condition called appendicitis develops.
• Protein digestion• erepsin
• Peptides are digested by erepsin (a peptidase) into amino acids.
Digestive system in ruminants and
• The digestive system of ruminants
• > Herbivores like ruminants and rodents feed on plants which contain a high percentage of
cellulose, a polysaccharide which is extremely insoluble.
• > Therefore, much of the energy in their diets is stored in this complex carbohydrate. The
breakdown of cellulose requires the enzyme cellulase.
• > Ruminants obtain most of their energy from the breakdown of cellulose of plant cell walls by
• > Although ruminants do not produce cellulose, their digestive system are specially adapted to
carry out cellulose digestion.
• > Ruminants like cows and goats have stomachs which are divided into four chambers, namely
rumen, reticulum, omasum and abomasums.
• > This adaptation enables ruminants to carry out rumination, the process of regurgitating food and
• > The first two chambers, the rumen and reticulum, are specialized compartments which have large
communities of bacteria and protozoa.
• > These microorganisms are able to secrete cellulose to digest cellulose.
• > In many cases, the microorganisms also use the sugars and other products of cellulose digestion
along with minerals to synthesise certain nutrients, such as vitamins and amino acids, which are
essential to the ruminants.
• > Figure 6.13 shows the processes involved in cellulose digestion in the digestive system of a cow.
• The digestive system of rodents
• > In rodents like rabbits and rats, the caecum and appendix are
enlarged to store the cellulose-producing bacteria (Figure 6.14).
• > Unlike ruminants, the breakdown products pass through the
alimentary canal of rodents twice.
• > The faeces in the first batch are usually produced at night, and are
soft and watery.
• > These are eaten again to enable the animals to absorb the
products of bacterial breakdown as they pass through the
alimentary canal for the second time.
• > The second batch of faeces become drier and harder.
• > This adaptation allows rodents to recover the nutrients initially
lost with the faeces.
The differences between the
processes of cellulose digestion in
Humans Ruminants Rodents
-humans do not produce enzymes that can digest cellulose.
-cellulose does not provide any nutrients for humans.
-the cellulose fibrils pass through the digestive tract and
stimulate the intestinal lining to secrete mucus, which aids in the
movement of food through the intestinal tract.
-cellulose digestion by symbiotic microorganisms (for example,
cellulose-producing bacteria) occurs in the rumen and reticulum
of the stomach.
-these microorganisms not only digest the cellulose into simple
sugars but also convert the sugars into a variety of nutrients
essential to the ruminants.
-cellulose digestion by symbiotic bacteria occurs in the large
intestine as well as in the caecum.
-nourishing by-products of fermentation by bacteria are obtained
when rodents eat some of their faeces and the partially digested
food passes through the alimentary canal the second time.
Problems Associated with Food
– The function of the digestive system depends
largely on proper nutrition.
– Proper nutrition can help the digestive system
function at its best.
– There are many probles associated with the
digestion of food.
– Incomplete digestion of food
• Incomplete digestion of food may cause severe pain in
the abdomen followed by nausea, vomiting and a
• Incomplete digestion of food is caused by excessive
intake of food, eating too much oily food or eating too
• If the food is not chewed properly before swallowing
the stomach cannot properly digest the food and this
decreases the effectiveness of the digestive enzymes.
• Eating moderately and chewing food properly help
prevent incomplete digestion of food.
– Reduced production of specific digestive enzymes
• Reduced production of specific digestive enzymes
can cause digestive problems. Adults usually find
it difficult to digest lactose (milk sugar) compared
to a baby or a child because of the lack of lactase.
• Damage to organs such as the pancreas causes
reduced production of digestive enzymes for the
digestion of starch, proteins and lipids. As a
result, digestion of these foods will be disrupted
and the body will not be able to obtain sufficient
– Gallstone preventing the flow of bile
• A person who often eats fatty food encourages the formation of gallstones
in the bile duct and gall bladder.
• Gallstones are caused by the hardening of cholesterol. It is also caused by
the excessive secretion of bilirubin and bile salts.
• The size of the gallstones may be as a grain of sand or as big as a golf ball.
• When the gallstones block the bile duct, bile cannot be channeled out. As
a result, lipids cannot be emulsified and are difficult to digest.
• Symptoms associated with the presence of gallstones include fever,
vomiting, jaundice and continuous pain in the upper abdomen.
• If the gallstones are found in the pancreas and this results in severe pain
and inflammation of the pancreas.
• Formation of gallstones usually occurs in obese people.
Absorption and Assimilation of Digested Food
• To enter the body cells, nutrients in the lumen
of the small intestine must be transported
across the intestinal lining into the
• Iluem is the major site of nutrient absorption.
– Adaptive Characteristics of the Digestive System
• The wall of the small intestine is covered with epithelial cells that are
specialized to complete the digestive process and absorb the resulting nutrient
• The small intestine, with a length of about 6m, is the longest section of the
• The intestinal lining is highly folded and covered entirely by tiny, finger-like
projections called villi.
• The epithelial cells of a villus has a fringe of microscopic projections called
• The total area exposed to the lumen of the small intestine has been estimated
to be 300m2 . Each villus is only about 1mm long and less than 0.25mm thick.
There are about 40 villi on 1mm3 of the intestinal lining. Estimate the total
number of villi present in the human intestine.
• If the small intestine is simply a smooth tube, without villi on its inner lining, it
would have to be 500 to 600m long to achieve a surface area of 300m2.
• HOW ARE THE VILLI ADAPTED FOR THE PROCESS OF
• The villi
– are numerous, thus increasing the internal surface area of the
ileum for absorption.
– are very thin-walled (only one cell thick); thus, undigested food
can be absorbed rapidly.
– contain a network of blood capillaries for the efficient
transport of digested food.
– contain special structure (lacteals) for absorbing fatty acids
• It is important for the body to retain as much water and
minerals as possible. This is to prevent the body from
becoming severely dehydrated and to maintain a balanced
• Health Watch : Excessive consumption of alcohol can lead
to cirrhosis, a condition where the liver is severely
damaged. The liver cells are forced to break down
excessive amounts of alcohol which is the converted to
fatty acids. This in part contributes to severe liver tissue
damage due to the formation of fibrous scar tissue.
• The liver also acts as a storage place for minerals like
copper, potassium and iron. Iron is needed for making
haemoglobin. It also stores fat-soluble vitamins such as
vitamins A and K.
– Assimilation of Digested Food
» Some of the products of digestion are brought directly to
the liver for processing and prepared for metabolic
processes or assimilation.
» Assimilation takes place in the cells where the nutrients
are used to form complex compounds or structural
components. For example, the transformation of amino
acids to proteins in the protoplasm.
» The liver acts as a checkpoint which controls the amount
of nutrients released into the blood circulatory system.
» From the liver, glucose and amino acids are transported
by the blood circulatory system to the heart to be
pumped to all body cells.
Amino acids Glucose Lipids
The process of
assimilation in the liver
~Amino acids have to pass through the liver before they
reach the blood circulatory system.
~The liver synthesizes plasma proteins from amino acids.
~Plasma proteins have various functions, for example,
blood clotting and osmoregulation.
~When there is a short supply of glucose and glycogen,
the liver converts amino acids into glucose.
~Excess amino acids cannot be stored in the body and
are broken down in the liver by a process called
~Urea, the common nitrogenous waste produced and
transported to the kidneys to be excreted.
~Glucose in the liver is used for respiration. According
to the needs of the body, excess glucose is converted
into glycogen and stored in the liver.
~When the blood sugar level falls and the body needs
energy, the stored glycogen is converted back into
~Once the glycogen store in the liver is full, excess
glucose is converted into lipids by the liver.
~Lipids which enter the heart through the subclavian
veins are transported in the bloodstream to body
The process of
assimilation in the cells
~Amino acids which enter the cells are used for the
synthesis of new protoplasm and the repair of damaged
~They are also important building blocks in the synthesis
of enzymes and hormones.
~Amino acids are also used in the synthesis of proteins
of plasma membranes.
~When the glucose molecules reach the body cells,
they are oxidized to release energy during cellular
~Energy is required for the various chemical processes
which take place in the cell, for example, in muscle
contraction and synthesis of proteins.
~Excess glucose is also stored as glycogen in the
~Glycogen is a long-chained molecule that is insoluble.
~Lipids like phospholipids and cholesterol are major
components of plasma membranes.
~Fats that are stored around organs act as cushions
that protect organs from injuries.
~Excess fats are stored in the adipose tissue
underneath the skin as reserve energy.
~When the body lacks glucose, fats are oxidized to
WHAT ARE THE FUNCTIONS OF THE
– Secretes bile
• >Bile is a mixture of bile pigments and bile salts which are delivered
to the duodenum. Bile is greenish in colour due to the presence of
the pigment bilirubin.
• >Bile pigments do not participate in digestion. They are waste
products from the liver’s destruction of old red blood cells.
• >Bile pigments are eliminated together with faeces. An
accumulation of bile pigments in the body results in a condition
• >Bile salts disperse the lipid droplets present in chime into as
emulsion of tiny droplets. This increases the surface area for the
action of lipase. Bile is stored and concentrated in the gall bladder.
• 2. Site for the synthesis of blood plasma proteins
• >Fibrinogen, prothrombin and many other proteins, which
are vital clotting agents, are synthesized in the liver. The
liver also maintains blood protein concentration within a
• 3. Regulation of blood glucose concentration
• >The liver removes glucose from the blood converting it into
glycogen. The process is stimulated by the pancreatic
• >If the blood glucose level is low, the liver releases glucose
into the blood.
• >The liver also converts amino acids into glucose during
• 4. Detoxification
• >The liver detoxifies blood by removing and metabolizing poisonous substances.
• >The liver absorbs or chemically modifies substances before these substances reach
the rest of the body.
• >Ingested alcohol and drugs are metabolized by liver cells.
• >Toxins, pesticides, carcinogens and poisons are detoxified and eliminated from the
• 5. Storage of nutrients
• >Excess glucose is transformed into fats in the liver and later stored in other parts
of the body. The liver also stores fat-soluble vitamins, A, D, E and K, vitamin B12
and ferum from the haemoglobin of disintegrated red blood cells.
• >Storage of ferum
• The red blood cells of the body become worn out after some time. These cells are
destroyed in the spleen. The haemoglobin is broken down in the liver. Ferum is
released during the breakdown of haemoglobin and stored in the liver.
• 6. Deamination of amino acids
• >Excess amino acids cannot be stored in the
body and are broken down in the liver into
erea, through a process called deamination.
In this process, the amino group (-NH2) is
removed and the remaining part of the
molecule is respired or converted into
6.6 Formation of Faeces and
• The Process of Defaecation
• -After the absorption of nutrients has taken place
in the small intestine, the intestinal contents
enter the colon.
• -The intestinal contents consist of a mixture of
water, undigested food substances and
indigestible fibre, most of which is cellulose from
plant cell walls.
• -The movement of this undigested materials
along the colon is slow and helped by peristalsis.
• Colon – Reabsorption of water and minerals
• # The colon reabsorbs almost 90% of water and
minerals into the bloodstream.
• # Absorption of water from the undigested remains in
the colon results in the formation of faeces, which are s
• # Faeces also contain dead cells shed by the intestinal
lining as well as waste products like bile pigments and
toxic substances which to be eliminated from the body.
• # The wall of the colon secretes mucus which helps to
bind the faeces and lubricates the movement of faeces
along the colon.
• # After 12-24 hours in the colon, the faeces
pass to the rectum for temporary storage.
• # After water is absorbed, the undigested
residue hardens to form faeces.
• # As the faeces accumulate, pressure in the
rectum increase, causing a desire to expel the
faeces from the body.
• The process of defacation
• # The elimination of faeces is known as
• # This process is controlled by muscles around
the anus, the opening of the rectum.
• # When the rectum is full, the muscles of the
wall of the rectum contract to eject the faeces
via the anus.
• Microorganisms in the colon
• # The most common microorganisms found in the colon are
Escherichia coli. E. coli is not harmful to us but lives symbiotically in
the intestines by digesting organic substances in the colon. E. coli
synthesizes vitamins B and K as by-products of their metabolism.
• # The presence of useful microorganisms is important because
– they secrete antibiotics that inhibit the life cycle of harmful
microorganisms, for example, Lactobacillus acidophilus secretes
– the microbial population maintains a stable environment in the
alimentary canal. However, the overuse of antibiotics can reduce the
microbial population. As a result, food digestion and absorption of
nutrients in the intestine will be affected.
Problems Related to Defaecation
• -One of the most common intestinal bacteria is Escherichia coli which feeds on
unabsorbed nutrients. Some of these bacteria synthesise several types of vitamins
B and K as by-products of their metabolism. The synthesis of vitamins by the
colonic bacteria provides a valuable supplement to our dietary intake, especially
vitamin K, which is often deficient in a normal diet.
• -Constipation is caused by faeces moving too slowly through the colon.
• -As a result, a greater amount of water is reabsorbed in the colon, making the
• -This will lead to painful defaecation.
• -Constipation can be avoided by drinking a lot of water.
• -The daily diet should contain sufficient amounts of fibre.
• -Intake of food rich in fibre can help in the movement of undigested food
substances along the colon and during defaecation.
• -The water absorbed softens the fibre and increases its bulk. This stimulates the
muscles to push out the residue, thereby preventing constipation.
• -A person needs at least 30g of fibre every day. Dietary fibre is found only in plant
foods. The best sources include whole grain and bran cereals, fruits and
• -Chronic constipation is associated with haemorrhoids and some cases can
lead to colon cancer.
• Haemorrhoids are abnormally swollen veins in the rectum and anus.
• When bulging haemorrhoidal veins are irritated, they cause the
surrounding membranes to swell, burn, itch, become very painful and
• Haemorrhoids are caused by too much pressure in the rectum, forcing
blood veins to stretch, bulge and sometimes rupturing them.
– Tumours of the colon and rectum are growths arising from the inner wall of
the large intestine. Malignant tumours of the large intestine are called colon
cancer or colorectal cancer.
– Diets high in fats are believed to cause colon cancer. It is believed that the
breakdown of products from fat metabolism leads to the formation of cancer-
causing chemicals (carcinogens).
– Diets high in vegetables and high fibre foods such as wholemeal bread and
cereals may rid the bowel of these carcinogens and help reduce the risk of
• -Some hints to help increase fibre intake
• Eat wholemeal or high fibre white bread.
• Eat brown or unpolished rice which contains more fibre than polished rice.
• Eat more fruits and vegetables (at least 2 servings each).
• Use wholemeal flour whenever possible in baking or cooking.
• Eat breakfast cereals like oats and bran.
• Eat wholemeal biscuits, for example, digestives, wholemeal crackers, oatmeal and
• OTHER IMPORTANT BENEFITS OF A HIGH FIBRE DIET INCLUDE:
• Better blood sugar control in diabetics. Fibre such as guar gum reduces the rate of
glucose absorption, thus, preventing high swing in blood sugar level.
• Helps control overeating or snacking. Foods rich in fibre are usually more filling.
This discourages over-consumption of other foods rich in calories which can be
– Evaluating Eating Habits
– It is important to practice good eating habits.
– Good eating habits mean taking food in the correct quantity at the
– Good eating habits include : ~ taking meals at the appropriate time.
We have to eat breakfast, lunch and dinner at the correct times.
• ~ refraining from overeating or eating too little during a meal.
• ~ eating a variety of foods to satisfy the body’s nutrient
• ~ eating a balanced diet.
• ~ eating sufficient amounts of fibre from fruits and vegetables.
• ~ drinking at least 2 to 3 litres of water daily. This will help to flush
out toxins from the body.
• ~ avoiding excessive fatty food and food rich in sugar.
– Poor eating habits are associated with various health problems
which affect many people.
– In most cases, the symptoms of gastritis are relatively mild and
short-lived. The most common symptoms include abdominal
pain, nausea, belching, vomiting, a burning sensation in the
upper region of the abdomen and loss of appetite. Blood in
vomit or faeces may be a sign of bleeding in the stomach.
– The Body Mass Index (BMI) can be used to determine whether a
person is underweight, overweight or obese. BMI is calculated
• BMI = body mass (kg) / height2 (m2)
• Compared your BMI with the values of the standard chart
– Choosing a Healthy Menu
• Choose plain rice and eat it with dishes that contain less fats but more vegetables.
• Choose wholemeal bread.
• Remove all visible fat from meat.
• Grill, steam or poach vegetables, meat and fish.
• Buy vegetable oils and polyunsaturated spreads instead of animal fats and butter.
• Replace full fat cheese with low fat alternatives.
• Take cereals for breakfast instead of fried food.
• Replace full cream milk with semi-skimmed milk.
• Cut down on sugar foods and sweets.
• Use only a little bit of margarine, butter and jam with toasted bread and biscuits.
• Eat fruits every day. Make it a habit to eat fruits as a snack and as a desert.
• Drink fruit juices instead of cordial and carbonated drinks.
• Eat lots of fresh vegetables and anything that can be eaten raw, for example, cucumbers, tomatoes
• Eat proteins from various animal and plant sources.
• Eat more fish and grains.
• Reduce intake of fried food.
• Eat low fat, low sugar snacks without monosodium glutamate (MSG).
– Health Problems Related to Eating Habits
– Gastritis is a condition where the epithelial lining of the stomach becomes inflamed.
– The stomach lining is usually covered with a layer of mucus which protects it from hydrochloric acid and digestive
– Gastric juice is secreted into the lumen of the stomach whenever one feels hungry.
– If food is not taken at regular times, the absence of food in the stomach will result in the acidic gastric juice acting
on the epithelial lining of the stomach wall.
– Gastritis occurs when the protective mechanism in the stomach is disrupted and the stomach lining is damaged.
– When the breach in the stomach lining develops into a hole, a gastric ulcer results.
– Inflammation and damage to the stomach lining can also be caused by
• Excessive alcohol consumption and stress.
• Taking aspirins and other pain relievers regularly.
• Susceptibility to ulcer is also increased by the presence of Helicobacter pylori.
Infection by these bacteria weakens the mucosal barriers and damages the mucus
layer, causing the stomach to be exposed to the action of acid and digestive
enzymes. Treatment with antibiotics reduces the symptoms and may also cure the
• For most types of gastritis, treatment involves taking medication such as antacids
(sodium bicarbonate and magnesium hydroxide). Antacids can neutralize the
hydrochloric acid in gastric juice.
• Obesity is defined as the excessive storage of energy in
the form of fats which results from as imbalance
between food intake and energy expenditure.
• An obese person is predisposed to a number of
diseases, including cardiovascular diseases,
hypertension and diabetes mellitus.
• Factors which lead to obesity include eating excessively,
eating too much fats and oily foods as well as a lack of
• Obesity can be overcome by practicing a balanced diet
and eating not more than what is required by the body.
– Anorexia nervosa
» People with anorexia nervosa experience an intense fear of gaining weight.
» They are persistently concerned about their body shape and weight.
» Individuals with this disorder have a distorted body image which convinces them
that they are fat.
» This eating disorder occurs frequently among female adults and teenagers.
» People with anorexia nervosa lose their appetite to eat or do not want to eat at all.
» They intentionally deprive themselves of food to achieve severe loss of body
weight, often 15% or more below their normal body weight.
» This condition is potentially fatal and is recognized as a psychological disorder.
» In as attempt to boost their self-esteem, they refrain from eating to the extent that
they become extremely emaciated.
» They lose both fat and muscle and this eventually leads to a disruption of the
functions of the heart, endocrine system and reproductive system.
» Early treatment through nutrition and gradual restoration of body mass can
correct some of the physical symptoms.
» Counseling is also needed to help the patients to correct any distorted belief the
patients may have about their body shape and weight, as well as to help the
patients overcome their emotional distress.
• Unlike anorexics, victims of bulimia may have a normal body mass.
• Bulimia is characterized by sequences of excessive food intake and
purging to counteract the effects of the binge.
• The victims consume huge amounts of food in a short period of time.
• The victims feel out of control and are unable to stop eating during a
binge. They also feel guilty, ashamed, disgusted or depressed
following a binge.
• This is followed immediately by purging through self-induced
vomiting or misuse of laxatives or diuretics.
• Repeated purging results in serious injury to the digestive tract and
can cause an imbalance of mineral salts in the blood. The victims will
experience dehydration, irregular periods or the periods may stop
completely. Symptoms associated with bulimia include malnutrition,
hormonal imbalance, increased risk of diseases such as influenza,
kidney and cardiovascular problems and liver disease.
• Bulimia nervosa can be overcome by observing the patient to ensure
that the person eats correctly, obtains counseling and medication.
• Warning Signs
• Mood swings, depression
• Frequent trips to the bathroom after eating
• Hiding food in unusual places
• Highly self-critical
• Difficulties in breathing or swallowing
• Fainting spells
• Irregular menstrual periods
• Frequent weight fluctuations
• Pronounced jaw swelling from frequent vomiting
• Nutrient Content in Food
– Good eating habits also involve wisely choosing the type of food we eat.
– The nutrient content of commercially packaged foods is provided on the
– The labels on food packets and containers provide complete information
about the total calories per serving and the contents of various nutrients.
– By studying the food labels, we can make informed choices amount the type
of food we consume.
– When choosing food, we must consider the
• Nutritional contents of food
• Freshness of food (check the expiry date)
• Presence of food additives such as food flavouring, coluring and preservaties.
– The nutritional information obtained from these sources will help reduce health problems
related to poor eating habits. Consumers should practice a critical attitude when choosing
commercial food. Take note of the way the food is cooked and the ingredients used in
The Importance of a Healthy Digestive System
• Effects of A Defective Digestive System
• If the alimentary canal is not healthy, the rest of
the body will be affected because it is the point of
• For example, a defective pancreas or stomach
may disrupt food digestion because the secretion
of gastric and pancreatic juices may be halted.
• If the gut lining becomes inflamed or damaged,
food absorption will be disrupted.
• It is important to take care of our digestive
system by practicing good eating habits.
• Taking Care of the Digestive System
• Our general health and well-being depend to a large extent
on a healthy digestive system.
• We need to be concerned about what we eat so that we
can lead healthy and active lives.
• One way to take good care of the digestive system is to take
proper meals at regular times of the day.
• We should avoid taking junk food which includes foods that
are high in salt, sugar, fats and low in nutritional value.
• Junk food offers little in terms of proteins, vitamins or
minerals and contains lots of calories from sugar or fat.
• The term ‘empty calories’ refers to the lack of nutrients in
these types of food.
– The Importance of Macronutrients and Micronutrients in Plants
• Elements Required by Plants
• Plants need water, carbon dioxide and sunlight to synthesise carbohydrates during photosynthesis.
• To synthesise nutrients and other organic substances, plants need additional elements.
• Mineral elements are essential chemical elements requires by plants to achieve optimal growth and development.
• Minerals that are needed by plants can be divided into macronutrients and micronutrients.
• Macronutrient are elements required by plants in relatively large amounts. These macronutrients are carbon (C ), hydrogen (H), oxygen (O), nitrogen
(N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S).
• In the water culture method, macronutrients are needed in a few hundred parts per million.
• Carbon (C ), hydrogen (H) and oxygen (O) are macronutrients that can be easily obtained from carbon dioxide in the atmosphere and water from the
soil. Therefore, deficiency in these nutrients rarely occurs.
• They are the most abundant elements found in a plant and form the major ingredients of organic compounds, most of which are carbohydrates.
• The remaining mineral elements are obtained in the form of inorganic ions from the soil.
• Micronutrients are elements that are required by plants in small quantities. These micronutrients are boron (B), copper (Cu), ferum (Fe), manganese
(Mn), molybdenum (Mo) and zinc (Zn).
• In the water culture method, the quantities of micronutrients that are needed are as little as one part per million.
• To determine which elements are required for normal growth, plant seedings are growth in complete culture solutions, also known as Knop’s
solution. The solution contains the following ingredients:
• Calcium (Ca(NO3)2) 0.8g
• Potassium nitrate (KNO3) 0.2g
• Potassium dihydrogen phosphate (KH2PO4) 0.2g
• Magnesium sulphate (MgSO4) 0.2g
• Ferum (III) phosphate (FePO4) trace
• Distilled water 1000cm3
• Experiments can be carried out to examine the importance and effects of the lack
of a certain element to the plant by eliminating it from Knop’s solution. For
example, to examine the effects of the lack of calcium, the calcium nitrate in
Knop’s solution is replaced with sodium nitrate. The growth of this plant can be
compared to the growth of another plant that is grown in a complete Knop’s
• Macronutrients and micronutrients are involved in the synthesis of chemical
substances essential for the healthy growth of plants.
• They are also required for the various metabolic processes which take place in
• The absence of one or more of these nutrients can lead to mineral deficiencies in
• The symptoms of a mineral deficiency depend on the function or functions of the
mineral in the plants.
• The functions and effects of deficiencies in macronutrients and micronutrients are
given in Table 6.11 and Table 6.12 respectively.
The functions and effects of
Macronutrient Functions Effects of deficiency
Nitrogen A major component of proteins, nucleic acids, chlorophyll and
enzymes for photosynthesis and respiration.
Important for rapid stem and leaf growth.
Increases seed and fruit yields.
Chlorosis, in which the synthesis of chlorophyll is inhibited, results in pale yellow
Phosphorus Synthesis of nucleic acids, adenosine triphosphates (ATP), and
phospholipids of plasma membranes.
Acts as a coenzyme in photosynthesis and respiration.
Poor root growth.
Formation of dull, dark green leaves.
Red purple spots on old leaves.
Potassium Protein synthesis.
A cofactor for many enzymes.
Maintains turgidity in plants.
Reduced proteins synthesis.
Premature death of plants.
Calcium A major constituent of the middle lamella of cell walls.
Formation of spindle fibres during cell division.
Leaves become distorted and cupped.
Areas between leaf veins become yellow.
Magnesium The main structural component of the pigment chlorophyll.
Activates many plant enzymes.
Involved in carbohydrate metabolism.
Yellowing of the regions between the veins of mature leaves.
Red spots on leaf surfaces.
Leaves become cupped.
Sulphur A component of certain amino acids.
A constituent of vitamin B and some coenzymes.
General yellowing of the effected leaves or the entire plant.
Micronutrient Functions Effects of deficiency
Boron Aids in calcium ion uptake by roots and translocation of sugars.
Involved in carbohydrate metabolism.
Aids in the germination of pollen grains.
Required for normal mitotic cell division in the meristems.
Acts as a cofactor for chlorophyll synthesis.
Death of terminal buds.
Abnormal plant growth.
Leaves become thick, curled and brittle.
Copper An important component of enzymes.
Involved in nitrogen metabolism and photosynthesis.
Important for reproductive growth and flower formation in plants.
Death of tips of young shoots.
Brown spots appear on terminal leaves.
Plants are stunted.
Ferum A cofactor in the synthesis of chlorophyll.
Essential for young growing plants.
Yellowing of young leaves.
Manganese An activator of enzymes in photosynthesis, respiration and also
A network of green veins on a light green background.
Brown or grey spots between the veins.
Molybdenum Involved in nitrogen fixation.
Reduction of nitrates during protein synthesis.
Chlorosis in the areas between the veins of mature leaves.
Pale green leaves.
Reduction in crop yields.
Zinc Formation of leaves.
Synthesis of auxin (a type of growth hormone in plants).
Acts as a cofactor in carbohydrate metabolism.
Mottled leaves with irregular areas of chlorosis.
• ~ Two German botanists, Julius Sachs and Wilhelm
Knop, grew plants in culture solutions to determine the
roles of macronutrients in plant growth.
• ~ Sachs showed that a plant would grow well in a
culture solution containing potassium nitrate, sodium
chloride, calcium sulphate, magnesium sulphate and
ferum (II) chloride.
• ~ Knop discovered that sodium chloride could be
omitted without affecting the plant’s growth. He also
altered the solution’s components by supplying the
same elements in different compounds.
Aristotle Soil had the ability to convert dead organic matter into useful nutrients which can be absorbed by plants.
Jean Baptiste van Helmont
(1640; a Belgian physician)
Conducted an experiment to investigate how plants grow.
He planted a seeding weighing 2.3kg in a pot that contained 90.7kg of soil.
The rim of the pot was kept covered with an iron plate pierced with tiny holes.
This prevented dust from mixing with the soil but allowed air and water to enter.
After five years, the seeding had grown into a tree weighing 76.9kg, but only 57g of the soil was lost from the pot.
He concluded that plant growth was mainly due to the water which was regularly added and not the result of the soil.
Joseph Priestly (1772) Demonstrated that green plants can restore air and make it capable of supporting combustion and respiration.
He placed a burning candle in an upturned glass jar and lat the candle burn until the flame went out.
He placed a sprig of mint plant in the jar for ten days.
Another burning candle was then placed inside the jar.
The candle burned perfectly well in it. Not only that, the air in the jar which contained the plant was able to support a mouse placed in it.
Although Priestly did not know about oxygen, his work showed that plants release oxygen into the atmosphere.
Jan Ingenhousz Discovered that plants only release oxygen in the presence of sunlight and that only the green parts of plants could release oxygen.
Recognized the importance of sunlight and chlorophyll in photosynthesis and that carbon dioxide is the source of carbon for green plants.
Jean Senebier (1780s) Discovered that carbon dioxide is used by plants during photosynthesis.
de Saussure (1804) Showed that water is required for photosynthesis.
Robert Mayer (1845) Recognized that plants convert solar energy into chemical energy during photosynthesis.
Blackman (1905) Discovered that photosynthesis involves two reactions.
He showed that photosynthesis involves a photochemical reaction which is light dependent and a biochemical reaction which is light independent.
The photochemical reaction is now referred to as the light reaction whilst the biochemical reaction is referred to as the dark reaction.
Robert Hill (1937) Showed that isolated chloroplasts placed in water in the presence of a suitable oxidizing agent were able to release oxygen.
He proved that chloroplasts are able to produce oxygen by splitting water molecules in the absence of carbon dioxide.
• A Brief History of Discovery of Photosynthesis
• In conclusion, scientists have shown that plants:
– require carbon dioxide (from the air) and water (from
the soil) to synthesise food in the presence of light
– synthesise carbohydrates (glucose) and release oxygen
• carry out photosynthesis in the green parts of
plants which contain chloroplasts
• Leaves are the main photosynthetic organs in
• They are adapted to carry out photosynthesis
• How are the physical structures of leaves adapted for photosynthesis?
– A leaf consists of a flat, thin lamina which is joined to the stem by a petiole. From the petiole,
a main vein leads down the leaf and branches out into side veins which support the lamina.
This petiole holds the leaves in the best position to receive the maximum amount of sunlight.
– The flattened shape of the lamina has a large surface to trap sunlight and it is thin so that light
– The thinness of the lamina also allows diffusion of gases involved in photosynthesis to take
place efficiently in the leaf.
– The veins that connect the leaf to the rest of the plant contain xylem and phloem tissues.
Xylem transports water absorbed by the roots to the leaf while phloem transports products of
photosynthesis away from the leaf.
– The leaves of a whole plant are arranged so that they shade each other as little as possible.
This arrangement, called a leaf mosaic (Photograph 6.1), allows hardly any light to pass
through. In this arrangement, the leaves of a plant are in an optimal position for
– Many plants can also detect the direction of light, so that their leaves are always in the best
position to absorb light.
Adaptation of the structure of a leaf
Structure Features Functional adaptations
Cuticle >A waxy covering which protects the leaf. >The cuticle is waterproof to help prevent excessive water loss.
>It is transparent to allow light to enter the leaf.
Figure 6.21 Epidermis cells
>Coated with cuticle
>Consists of a single layer
>Does not contain chloroplasts
>The upper epidermis of a leaf is thin and transparent. This allows light to
penetrate the leaf and reach the light-trapping chloroplasts inside.
>Sunlight can penetrate easily because the epidermal cells do not contain
Figure 6.22 Palisade mesophyll and spongy
>Palisade cells are packed tightly together in an upright
arrangement near the upper surface of the leaf.
>These cells have a high density of chloroplasts.
>The cell walls of palisades are coated with a film of water.
>Receive the maximum amount of light.
>These cells are the most active cells in photosynthesis. The chloroplasts
within these cells are able to move about and arrange themselves to carry out
maximum light absorption.
>Respiratory gases can dissolve in the film of water before diffusing into the
Spongy mesophyll (Figure 6.22) >The spongy mesophyll consists of cells which have an irregular
>The cells have fewer chloroplasts than palisade cells.
>The cells are loosely arranged and between each of them are air
spaces that connect the mesophyll to the stomata.
>The cell walls of spongy mesophyll are coated with a film of
>The irregular shape of these cells increase the internal surface area for
>Less important for photosynthesis than the palisade mesophyll layer.
>The large air spaces allow for easy diffusion of water and carbon dioxide
through the interior of the leaf to the palisade cells.
>The moist surfaces allow gaseous exchange to take place efficiently in the
Figure 6.23 Vascular bundle
>Consists of xylem and phloem >Xylem transports mineral ions and water to the leaf.
>Phloem transports products of photosynthesis away from the leaf.
Figure 6.24 Guard cells and a stoma
>A layer of epidermal cells that forms the lower protective
boundary to the leaf.
>Does not contain chloroplasts except for guard cells which are
specialized epidermal cells.
>Each stoma is flanked by two guard cells which regulate the size
of the pore.
>Stomata are normally more abundant on the lower epidermis of
the leaf than the upper epidermis.
>Guard cells enable the opening and closing of stomata.
>Stomata support photosynthesis by allowing the exchange of gases between
the inside of the leaf and its surroundings.
>Carbon dioxide from the atmosphere diffuses into the leaf through the
stomata and oxygen (a by-product of photosynthesis), diffuses out of the leaf
via the same route.
• Adaptation of Plants from Different Habitats
to Carry Out Photosynthesis
» Plants from different habitats show a variety of
adaptations to carry out photosynthesis optimally.
» The distribution of stomata and chloroplasts within plants
varies according to their environmental conditions.
Habitat Distribution of stomata Distribution of chloroplasts
Land plants that live in a tropical area, for example, hibiscus. >Large numbers of stomata on the lower epidermis of the leaf which allow maximum carbon
>The upper epidermis does not have many stomata since direct exposure to sunlight would
lead to excessive evaporation and water loss.
>Most of the chloroplasts are found in the palisade mesophyll cells and the spongy mesophyll
cells. This facilitates maximum absorption of sunlight for photosynthesis.
Habitat Distribution of stomata Distribution of chloroplasts
Plants that float on the surface of the water. Floating plants are of two
types: those that are rooted with floating leaves (for example, water
lilies) and those that are not rooted in the sediment, but just float on
the surface (for example, duckweeds)
>The stomata are mostly distributed on the upper epidermis of the leaves.
>This upper epidermis is often covered by a thick, waxy cuticle to repel water and to keep the
>Air-filled internal cavities are also present.
>Chloroplasts are found mainly in the upper epidermis. This maximizes the absorption of
>Weak stems produce a massive floating canopy of leaves which allow maximum absorption of
sunlight for photosynthesis.
Desert plants, for example, cacti. >The leaves contain very few stomata and this helps to prevent excessive loss of water through
>The stomata are normally located in the grooves along the stem.
>Some cacti such as the Dessert Trumpet (Eriognum inflatum) open their stomata at night
when it is cooler, rather than during the day when it is hotter. They absorb and store carbon
dioxide during the night. the carbon dioxide is used during the day when the stomata are
forced to close to reduce the loss of water through transpiration.
>Some shrubs in the dessert, such as Hakea sp., have embedded or sunken stomata in the
leaves to reduce water loss through transpiration.
>In a cactus plant, the stem performs virtually all of the photosynthesis since cacti have
reduced leaves and most of the leaves are modified to become thorns.
>Hence, chloroplasts are found all over the plant, that is, in the thorns and stems that are green
Aquatic plants that are submerged, for example, Hydrilla sp. >The epidermal layer of aquatic plants does not have cuticles.
>The cells on the surface are able to absorb water, nutrients and dissolved gases directly from
>Hence, stomata are not found on the leaves.
>Air-filled cavities often extend through-out the leaves and stems of aquatic plants, providing
an internal atmosphere where gaseous exchange can take place.
>Aquatic plants have thin-textured, feathery and highly dissected or divided leaves.
>This adaptation has the advantage of creating a very large surface area for absorption and
>The leaves and stems are green in colour.
>Hence, chloroplasts are found all over the surface of the plant to miximise the absorption of
>This is an important adaptation because of the low intensity of sunlight in water.
The Mechanism of Photosynthesis
• The parts of Chloroplasts in Relation to Photosynthesis
– There are two main stages in photosynthesis the light reaction and dark reaction.
– The light reaction occurs only in the presence of light while the dark reaction happens during
the day and night.
– The two reactions of photosynthesis occur at different sites in the chloroplasts.
• Chloroplasts contain membranous structure which is piled in stacks called grana
• Grana contain the light-trapping pigment chlorophyll. Chlorophyll is the most
abundant photosynthetic pigment in plants.
• Grana are embedded in a gel-like matrix called stroma.
• The dark reaction of photosynthesis takes place in the stroma.
• The enzymes responsible for the dark reaction of photosynthesis are also found
• Starch grains which act as a temporary storage place for the products of
photosynthesis are also found in the stroma.
Light reaction Differences Dark reaction
Day time Time of reaction Day & night
Grana Site of reaction Stroma
Water Substances required for reaction Carbon dioxide
Oxygen & water Products of reaction Glucose & water
Occur Photolysis of water Does not occur
Needed Light energy Not needed
Factors Affecting Photosynthesis
• - The rate of photosynthesis which takes place
in plants is affected by factors such as
• (a) light intensity
• (b) concentration of carbon dioxide
• (c) temperature
• (d) water supply
• Light is essential during the light reaction of photosynthesis
• When the concentration of carbon dioxide & temperature are
controlled at constant levels, the rate of photosynthesis is directly
proportional to light intensity up to a certain point
– As the light intensity increases, the rate of photosynthesis is increases
up to saturation point at P
– A further increase in light intensity does not increase the rate of
photosynthesis because of limiting factors such as the concentration
of carbon dioxide & temperature which stop the rate of reaction from
increasing further along PQ
– When the carbon dioxide concentration of the environment is raised
up to 0.13%, the rate of photosynthesis at each light intensity is higher
than the rate of photosynthesis of the corresponding light intensity
– At every high light intensity, the rate of photosynthesis slow down
because the pigment chlorophyll is damaged by ultraviolet rays
Concentration of carbon dioxide
• Carbon dioxide is needed in the dark reaction as a raw
material used in the synthesis of glucose
• The concentration of carbon dioxide in the atmosphere
varies between 0.03% & 0.4%
• If there is no other factor limiting photosynthesis, an
increase in the concentration of carbon dioxide results in an
increase in the rate of photosynthesis
• As in the case of light intensity, the rate of photosynthesis
level off at a certain point (saturation point)
• Although the concentration of carbon dioxide increases,
the rate of carbon dioxide will not increase further because
light intensity acts as a limiting factor
• The dark reaction of photosynthesis catalysed by the
photosynthetic enzymes & therefore changes in
temperature will affect the rate of photosynthesis
• Generally, an increase of 10˚C in the surrounding
temperature will double the rate of photosynthesis
• The optimum temperature varies for the different
species of plants, but most plants have an optimum
temperature of between 25˚C & 30˚C
• However, when the temperature is too high, the
photosynthetic enzymes are destroyed (denaturation)
& photosynthesis stops altogether
• Water is needed for photosynthesis but water
also is rarely the limiting factor in photosynthesis
because the amount of water required is small
• If water is not supplied, wilting occurs & the
stomata will close. This prevent the diffusion of
carbon dioxide into the leaves
• As a result, the rate pf photosynthesis decreases
as the lower concentration of carbon dioxide
becomes the limiting factor