This document provides an overview of biological molecules and the cellular basis of life. It discusses the main types of biological molecules including carbohydrates, lipids, proteins, and nucleic acids. It also describes the basic components and functions of cells, and outlines the cell theory which states that cells are the fundamental unit of life and all cells come from pre-existing cells.

1. General Biology
Unit 2
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Biological Molecules

2. 2. Biological Molecules
• Biological molecules is referred to as the molecules of life/bio-
molecules that are basically found in a living cell &
categorized as
– organic & inorganic molecules.
• is vital for every single organism on Earth.
• The organic biomolecules are:-
»proteins
»carbohydrates
»lipids
»nucleic acids
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3. • They are important either
– structurally or functionally for cells &, in most cases,
important in both ways.
• The most commonly known inorganic molecules are
• Water
• Minerals
important for the normal functioning of the cell.
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4. 2.1. Carbohydrates
is made of atoms of C, H & O.
important source of energy
provide structural support for cells and help with
communication between cells (cell-cell recognition).
found in the form of either a sugar or many sugars
linked together, called saccharides.
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5.  Based on the number sugar units they contain, they are
categorized into three:
Monosaccharides: single sugar molecule
Disaccharides: have two sugar molecule &
Polysaccharides: polymers of chains of mono or disaccharide
 Each of the sugar molecules are bonded together through the
glycosidic linkage/s.
 Carbohydrates are polyhydroxy aldehydes or ketones.
e.g; Glucose (aldehyde) while fructose (ketone).
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1-4 glycosidic
bond

6. 1. Monosaccharides
Are simple sugars with multiple OH groups.
• Based on number of carbons (3C, 4C, 5C, 6C), a
monosaccharide is;
Aldose Ketose
– triose (3C): Glyceraldehyde, Dihydroxyacetone
– tetrose (4C): Erythrose, Erythrulose
– pentose (5C): Ribose, Xylose, Arabinose, Ribulose, Xylulose
– hexose (6C): Glucose, Galactose, Mannose, Fructose
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7. • Those with aldehyde group are classified as aldoses.
– Aldoses are reducing sugars.
• Those with a ketone group are classified as ketoses.
– Ketoses are non-reducing sugars.
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8. Glucose
is the most important carbohydrate fuel in human cells.
Its concentration in the blood is about 1 g/dm3.
The small size & solubility in water allows them to pass
through the cell membrane into the cell.
Energy is released when the molecules are metabolized.
glucose + glucose  maltose (dissaccharide).
Starch & cellulose are polysaccharides made of glucose
units.
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9. Galactose
look very similar to glucose molecules.
They can also exist in α and β forms.
Galactose + glucose  lactose (dissaccharide).
However, glucose & galactose cannot be easily converted
into one another.
Galactose cannot play the same part in respiration as
glucose.
This comparison of glucose & galactose shows why the
precise arrangement of atoms in a molecule is so important.
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10. Fructose
 Fructose, glucose & galactose are all hexoses.
 glucose & galactose are aldoses (reducing sugars),
 fructose is a ketose (a non-reducing sugar).
 It also has a five-atom ring rather than a six-atom ring.
 Fructose + glucose  sucrose (dissacharide).
Ribose & deoxyribose
Ribose & deoxyribose are pentoses.
The ribose unit forms part of a nucleotide of RNA.
The deoxyribose unit forms part of the nucleotide of DNA.
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11. 2. Disaccharides
 Monosaccharides are rare in nature.
 Most sugars found in nature are disaccharides.
 formed when two monosaccharides covalently linked.
 are soluble in water, but too big to pass through the cell
membrane by diffusion.
 They are broken down in the small intestine during digestion
to give the smaller monosaccharides that pass into the blood
& through cell membranes into cells.
 This is a hydrolysis reaction & is the reverse of a
condensation reaction & it releases energy.
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12. A condensation reaction takes place by releasing water.
This process requires energy.
A glycosidic bond forms & holds the two monosaccharide.
the 3 most important are sucrose, lactose & maltose.
formed from the appropriate monosaccharides.
Sucrose is a non-reducing sugar.
Lactose & maltose are reducing sugars.
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13. Monosaccharides are used very quickly by cells.
However, a cell may not need all the energy immediately &
it may need to store it.
Monosaccharides are converted into disaccharides in the cell
by condensation reactions.
Further condensation reactions result in the formation of
polysaccharides.
These are giant molecules which are too big to escape from
the cell.
These are broken down by hydrolysis into monosaccharides
when energy is needed by the cell.
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14. 3. Polysaccharides
 Monosaccharides can undergo a series of condensation
reactions, to form very large molecules (polysaccharides).
 This is called condensation polymerisation, & the
building blocks are called monomers.
The properties of a polysaccharide molecule depend on:
Its length (usually very long)
The extent of any branching (addition of units to the side of
the chain rather than one of its ends)
Any folding which results in a more compact molecule
Whether the chain is 'straight' or 'coiled'
» e.g; starch, glycogen & cellulose
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15. Starch
is often produced in plants as a way of storing energy.
exists in two forms: amylose & amylopectin and both are
made from α-glucose.
Amylose is an unbranched polymer of α-glucose.
The molecules coil into a helical structure.
It forms a colloidal suspension in hot water.
Amylopectin is a branched polymer of α-glucose.
It is completely insoluble in water.
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16. Glycogen
 Glycogen is amylopectin with very short distances between
the branching side-chains.
 Starch from plants is hydrolysed in the body to produce
glucose.
 Glucose passes into the cell & is used in metabolism.
 Inside the cell, glucose can be polymerised to make
glycogen which acts as a carbohydrate energy store.
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17. Cellulose
• is a third polymer made from glucose.
• made from β-glucose molecules.
• the polymer molecules are 'straight'.
• Cellulose serves a very different purpose in nature to starch
& glycogen.
• It makes up the cell walls in plant cells.
– These are much tougher than cell membranes.
• This toughness is due to the arrangement of glucose units in
the polymer chain & the hydrogen-bonding b/n
neighboring chains.
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18. • Cellulose is not hydrolysed easily,
– therefore, cannot be digested so it is not a source of energy
for humans.
• The stomachs of Herbivores contain a specific enzyme called
cellulase which enables them to digest cellulose.
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19. 11/13/2023 19

20. 2.2. Lipids
are a highly variable group of molecules that include fats,
oils, waxes & some steroids.
are esters of fatty acids & glycerol (chains of alcohols).
Fatty acids are made mostly from chains of C, H & they
bond to a range of other types of atoms to form d/t lipids.
the primary function of lipids is to store energy.
a lipid called a triglyceride is a fat if it is solid at room temp
& oil if it is liquid at room temp.
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21. triglycerides are stored in the fat cells called adipocytes or
lipocytes
are responsible in storing fats & lipids in animals‟ body.
are categorized in white fat cells & brown fat cells.
The difference is in their ways of storing lipids.
– White fat cells store one large lipid drop
– brown fat cells store smaller and multiple droplets of lipids
Various types of lipids occur in the human body are:-
(1) Triacylglycerol
(2) Cholesterol
(3) Polar lipids, like phospholipids, glycolipids & sphingolipids.
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22.  Plant leaves are coated with lipid waxes to prevent water loss.
 the honeycomb in a beehive is made of beeswax.
 Fatty acid tail is a chain of carbon atoms bonded to H & other C
atoms by single or double bonds.
 Lipids can be;
• Saturated fats:- have tail chains with only single bonds between
the carbon atoms.
- no more hydrogen can bond to the tail.
• Unsaturated fats:- have at least one double bond between
carbon atoms in the tail chain
- can accommodate at least one more H
• Polyunsaturated fats:- fats with more than one double bond in
the tail.
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23. Properties of lipids
• Insoluble in water
• Longer chains
More hydrophobic, less soluble
• Double bonds increase solubility
• Melting points:
Depend on chain length & saturation
Double bonds lead acyl chain disorder & low
melting temperatures
Unsaturated fatty acids are solid at room temp.
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24. Importance of lipids
main component of cell membranes (phospholipids)
Insulation of heat and water
Storing energy, protection and cellular communication.
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25. 2.3. Proteins
is made of small C cpds called amino acids (aa).
Several covalent bonds called peptide bonds join aa together
to form proteins
aa are made of C, N, O, H, & sometimes S.
all aa share the same general structure.
aa have a central carbon atom
C can form four covalent bonds.
– One of those bonds is with H.
– The other three bonds are with
• an amino group (–NH2),
• a carboxyl group (–COOH), &
• a variable group (–R).
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Fig 2.3: Peptide bond

26. The variable group makes each aa different.
There are 20 different variable groups
proteins are made of d/t combinations of all 20 d/t aa.
A peptide forms b/n the amino group of one aa & the carboxyl
group of another.
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Fig: 2.4. Basic structure of amino acid

27. Based on variable groups, proteins have 4 levels of structure.
Primary structure: the number of aa in a chain & the order in
which the aa are joined.
Secondary structure: once aa chain is formed, it folds into a
unique 3-dimensional shape called α-helix or β-pleated sheets.
Tertiary structure: further folding of the secondary structure &
the formation of new bonds to hold it in place.
Quaternary structure: formed when two or more polypeptide
chains (folded into a tertiary structure) become associated in the
final structure of the protein. E.g, hemoglobin, collagen
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28. 11/13/2023 28
Fig 2.6 The levels of structure
of a protein
Fig2.5: A The four polypeptides in haemoglobin’s
quaternary structure; B The three polypeptides in
collagen’s quaternary structure

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 Proteins make up about 15% of your total body mass & are
involved in nearly every function of your body.
e.g, -our muscles,
-skin, &
-hair all are made of proteins.
 Our cells contain about 10,000 different proteins that provide
structural support,
transport substances inside the cell & between cells,
communicate signals within the cell & between cells,
speed up chemical reactions, and
control cell growth.

30. 2.4. Nucleic acids
are complex macromolecules that store & transmit genetic
information.
are made of smaller repeating subunits called nucleotides.
Nucleotides are composed of C, N, O, P, & H atoms.
There are six major nucleotides, all of which have 3 units a
phosphate, a nitrogenous base, & a ribose sugar.
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Fig. 2.7. Basic structure of nucleotide

31. • There are two types of nucleic acids in living organisms:
– deoxyribonucleic acid (DNA)
– ribonucleic acid (RNA)
• Five different bases found in nucleotide subunits that make up
DNA & RNA;
»Adenine (A)
»Thymine (T)
»Guanine (G)
»Cytosine (C)
»Uracil (U)
• Each of these nitrogenous base sticks by hydrogen bonding
with other bases in other nucleic acids.
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32. 11/13/2023 32
Fig 2.8: The structures of DNA and RNA
A-T
G-C
A-U
G-C

33. A nucleotide with three phosphate groups is adenosine
triphosphate (ATP).
ATP is a storehouse of chemical energy that can be used by
cells in a variety of reactions.
It releases energy when the bond between the second & third
phosphate group is broken.
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Fig: 2.9. Nitrogenous bases

34. 2.5. Vitamins
• are organic & needed in small amounts for metabolic
activities.
• Many vitamins help enzymes function well.
• Vitamin D is made by cells in your skin.
• B & K are produced by bacteria living in the large intestine.
• most vitamins cannot be made by the body.
• Some vitamins that are fat-soluble can be stored in small
quantities in the liver & fatty tissues of the body.
• Other vitamins are water-soluble & cannot be stored in the
body.
• Foods providing an adequate level of these vitamins should
be included in a person‟s diet on a regular basis.
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35. 2.6. Water
• formed by covalent bonds that link two H atoms to one O
atom
• most plentiful & essential of compounds,
• tasteless & odorless, existing in gaseous, liquid, & solid states.
• has the ability to dissolve & as a media for transportation of
many other substances.
• Water molecules have an unequal distribution of charges and
are called polar molecules, meaning that they have oppositely
charged regions.
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36. 2.7. Minerals
• are inorganic compounds used by the body as,
– building material, &
– involved with metabolic functions.
e.g, iron is needed to make hemoglobin & it binds to it in RBCs &
is delivered to body cells as blood circulates in the body.
• Calcium, & other minerals, is an important component of
bones & is involved with muscle & nerve functions and they
serve as cofactors for enzymes.
• Magnesium is an important component of the green pigment,
chlorophyll, involved in photosynthesis.
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37. General Biology
Unit 3
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The cellular basis of life

38. 3. The cellular basis of life
Discovery of cell
Robert Hook (1600) was the first to observe plant cells
with a crude microscope.
Then, Mathias Schleiden & Theodore Schwann (1830)
proposed that all living things are composed of cells.
Virchow extended this idea by contending that cells arise
only from other cells.
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39. 3.1 The cell theory
A cell is the basic structural & functional unit of living
organisms.
The activity of an organism depends on both the individual &
the collective activities of its cells.
According to the principle of complementarity of structure &
function, the biochemical activities of cells are dictated by
– their shapes or forms, and
– by the relative number of their specific sub-cellular structures.
All cells arise from pre-existing cells.
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40. A typical eukaryotic cell has 3 major parts:
 The plasma membrane:
» the outer boundary of the cell.
 The cytoplasm:
» the intracellular fluid packed with organelles
 The nucleus:
» an organelle that controls cellular activities.
» Typically the nucleus resides near the cells center.
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41. Fig: 3.1. Generalized animal cell
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42. 3.1.1 Cell organelles
An organelle is a specialized subunit within a cell that has a
specific function.
In eukaryotes an organelle is a membrane bound structure
Prokaryotes do not have membrane bound organelles.
organelles are found in the cytoplasm
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43. two kinds of cell organelles based on membrane covering,
Membranous organelles:
– ER (rough & smooth), Golgi bodies, mitochondria,
chloroplasts, nucleus, lysosomes, peroxisomes & vacuoles.
Non-membranous organelles:
– ribosomes (70s & 80s), centrosomes, cilia & flagella,
microtubules, basal bodies & microfilaments.
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44. 3.1.2 Structure and function of organelles
The nucleus:
• is oval shaped largest central structure
• surrounded by a double-layered membrane.
• In the nucleus, DNA directs protein synthesis
• DNA gives codes, or instruction for directing synthesis of
specific structure and enzymes proteins within the cell.
• the nucleus indirectly governs most cellular activities & serves
as the cell’s master.
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45.  Three types of RNA are involved in protein synthesis.
• mRNA: copies instructions in the DNA & carries these to
the ribosome.
• tRNA: reads mRNA sequence & transfers each amino acid
to ribosome where the protein product is synthesized.
• rRNA: composes the ribosome & binds the corresponding
amino acid to a growing peptide chain.
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46. Generally, the nucleus may be:
roundede.g. in hepatocytes.
indented (segmented)e.g. in neutrophils.
binucleatede.g. in parietal cells, cardiac muscle cells.
multinucleatede.g. in osteoclasts, skeletal muscle cells.
very large (many DNA)e.g. in megakaryocytes.
absente.g. in mature erythrocytes, blood platelets.
 The nucleus is surrounded by a nuclear envelope & contains
chromatin & one or more nucleoli.
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47. The Nuclear envelope
surrounds nuclear material
consists of outer & inner membrane
perforated at intervals by nuclear pores
Through this pores most ions & water soluble molecules to
transfer b/n nucleus & cytoplasm
Chromatin:
term chromatin means "colored material"
Refers it is easily stained for viewing with microscope, &
it is composed mainly of coils DNA bound to basic protein called
histones.
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48. Nucleoli: the nuclei of most cells contain one or more lightly
stained structures called nucleoli
actively engage in synthesizing of ribosomes.
does not have a limiting membrane.
it contains large amounts of RNA & protein.
nucleolus enlarged when a cell is actively synthesizing proteins.
The genes of five separate chromosome pairs synthesize rRNA
& then store it in the nucleolus.
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49. Cytoplasm: cytosol is cell‟s interior not occupied by nucleus
is complex jelly like marrow called cytosol.
All cells contain six main types of organelles-
ER, GC, lysosomes, peroxisomes, mitochondria & vacuoles.
They are similar in all cells with some variations on the cell
specialization.
Each organelle is a separate compartment, different function.
These organelles occupy about half of the total cell volume.
The remaining part of the cytoplasm is cytosol.
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50.  Endoplasmic reticulum (ER)
Rough ER
-Ribosomes attached.
-Works on protein synthesis
-m-RNA carries the genetic
message from the nucleus to
the ribosomes “workshop”
Smooth ER
-Does not have ribosomes
-it looks smooth
-It does not produce proteins.
-smooth ER bud off/pinch off,
giving rise to transport vesicles.
-So, it used to make membranes
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51. Golgi complex:
is associated with the ER &
contains sets of flattened, curved, membrane- enclosed sacs, or
cisternae, stacked in layers.
Number of stacks vary in cells
cells for protein secretion have hundreds of stacks, whereas
some have only one.
 Function
 finishes, sorts, labels & ships proteins
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52. It performs the following important functions.
1. Processing the raw material into finished products.
2. Sorting & directing finished product to their final
destination.
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Fig:3.2. Golgi apparatus

53. Lysosomes:
intracellular “digestive system”.
are membrane-enclosed sacs
contains powerful hydrolytic enzymes capable of digesting &
removing
– Digest unwanted cellular debris & foreign materials like bacteria.
vary in size & shape, & about 300μm in a cell.
Extrinsic material to be attacked by lysosomal enzymes is brought
into the interior by the process of endocytosis.
If the fluid is internalized by endocytosis, the process is called
pinocytosis.
Endocytosis is also accomplished by phagocytosis. This is achieved
by specialized cells- white blood cells.
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54. take up old organelles such as mitochondria & break down into
their component molecules.
– those molecules can be reabsorbed into the cytosol, & the rest
are dumped out of the cell.
The process by which worn-out organelles are digested is called
autophagy a human liver cell recycles about half its content
every week.
– Garbage disposal & recycling.
In the inherited condition known as lysosomal storage disease
(Tay-Sachs disease) lysosomes are not effective because they
lack specific enzymes.
– As a result, harmful waste products accumulate disrupting the
normal function of cells, often with fatal results.
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55. Peroxisome: is membrane-enclosed sacs
containing oxidative enzymes & catalase
– detoxify various wastes such as ethanol (liver & kidneys).
major product generated is a powerful oxidant H2O2.
catalase & antioxidant enzyme decomposing H2O2 into
harmless H2O & O2.
This reaction is an important safety reaction that destroys
deadly H2O2, at the site of production
– thereby preventing possible devastating escape into the cytosol.
Peroximal disorders disrupt the normal processing of lipids &
disrupt the normal function of the nervous system
– by altering the structure of the nerve cell membrane.
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56. Mitochondria:
are the “power houses” of a cell;
they extract energy from nutrients & transform into usable form.
Varies in number based on the energy needs of each cell types.
– A single cell may have few hundreds or thousands.
rod or oval shaped about the size of a bacterium.
Each is enclosed by a double membrane-
– a smooth outer that surrounds the mitochondria, &
– an inner membrane that forms a series of enfolding called
cristae, inner cavity filled with a jelly-like matrix
• cristae contain proteins (the electron transport protein).
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57. • The enfolding increase the surface area for keeping these proteins.
• The matrix contains a mixture of hundreds of different dissolved
enzymes (Citric acid cycle enzymes).
 Function
 make ATP energy from cellular respiration
 sugar + O2  ATP
 fuels the work of life both animal & plant cells
• Found in both animal & plant cells
 Mitochondria are unusual organelles in two ways:
 In the matrix they have their own unique DNA called
mitochondrial DNA.
 Have the ability to replicate themselves even when the cell to
which they belong is not undergoing cell division.
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58. Fig. 3.3 Mitochondrial structure
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59. Chloroplasts
are useful organelles among plastids
participate in the process of photosynthesis
is a process by which plants synthesize their own food.
by converting light energy into chemical energy.
are located in outer surface of the cell to receive enough light.
are green colored due to chlorophyll pigments.
 Plants make their energy in two ways:
 Mitochondria: make energy from sugar + O2
 cellular respiration: sugar + O2  ATP
 Chloroplasts: make energy + sugar from sunlight
 Photosynthesis: sunlight + CO2  ATP + sugar
ATP = active energy
sugar = stored energy
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60. Fig 3.4: Structure of plant cell
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61. Vesicles: are membrane bound sacs
are used to store or transport substances around the cell.
Lysosomes are actually Vesicles.
Vacuoles: are essentially larger vesicles
formed by joining many Vesicles together.
are membrane bound organelles
have no specific shape
contain water with a number of d/t compounds within it.
Their function varies depending on the type cell.
e.g, In plant cells used to maintain Turgor Pressure.
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62. Cytoskeleton:
is a complex protein network act as „bone & muscle‟ of the cell.
This network has at least four distinct elements: Microtubules,
Microfilaments, Intermediate filaments & Microtubular
lattice.
 Generally, cytoskeletons determine/ provide:
distinct shape, size to the cell
structural support
organizing its contents
substances movement through cell (cilia, flagella &
intracytoplasmic vesicles), and
Contribute to movements of the cell as a whole.
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63. Plasma/cell membrane
is extremely thin layer of lipids & proteins forming outermost
boundary of living cell & enclosing the intracellular fluid (ICF).
It serves as a mechanical barrier that traps needed molecules
within the cell;
plays an active role by selective permeability of substances to
pass b/n the cell & its ECF environment.
It is a fluid lipid bilayer embedded with proteins.
It appears as „trilaminar’ layer structure having
– two dark layers separated by a light middle layer
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64. Fig 3.5: Structure of the cell membrane
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65. All plasma membrane are made up of
– lipids & proteins plus small amount of carbohydrates.
Phospholipids are
– most abundant with a lesser amount of cholesterol.
– have a polar hydrophilic (water loving) head having a
negatively charged phosphate group &
– two non-polar (electrically neutral) hydrophobic (water fearing)
fatty acid tails.
Such two-sided molecule self-assemble into a lipid bilayer when
in contact with water.
The hydrophobic tails bury themselves in the center away from
the water, while the hydrophilic heads line up on both sides in
contact with water.
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66. The water surface of the layer is exposed to ECF
the inner layer is in contact with the ICF.
Cholesterol provides to
– the fluidity as well as the stability;
– lies in between the phosphate molecules,
– preventing the fatty acid chain from packing together &
crystallizing that could decrease fluidity of the membrane.
– exerts a regulatory role on some of the membrane proteins.
For fluidity of the membrane,
– it gives flexibility to the cell to change its shape;
– transport processes are also dependent on the fluidity
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67. Lipid bilayer forms the basic structure of the membrane,
– is a passage of water-soluble substances b/n the ICF & ECF;
– is responsible for the fluidity of the membrane.
– forms the primary barrier to diffusion.
Membrane proteins are variety of different proteins within the
plasma membrane; have the following special functions:
 some form water-filled passage ways or channels, across the lipid
bilayer;
 Others serve as carrier molecule that transport specific molecule
that cannot cross on their own.
 Many proteins on the outer surface serve as ‘receptor sites’
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68.  Another group act as membrane-bound enzymes to control
specific chemical reactions
 Some proteins are arranged as filaments network/ meshwork on
the inner side
 Other proteins function as cell adhesion molecules (CAMs).
 Some proteins in conjunction with carbohydrate are used in the
cell’s ability to recognize ‘self’ & in cell-to-cell interactions.
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Fig 3.6: Membrane proteins

69. 11/13/2023 69
 Membrane Carbohydrate:
• Short-chain on the outer membrane surface
• serves as self-identity marker & interact with each other in the
following ways:
 Recognition of „self‟ & cell-to-cell interactions.
 surface markers are important in growth.
 Cells do not overgrow their own territory.
 Some CAMs have carbohydrate, on the outermost tip where they
participate in cell adhesion activity.

70. Functions of biological membranes
Channel protein & carrier protein
Enzymes: membrane proteins sometimes act as enzymes
Receptor molecules
Antigens: these act as cell identity markers or ''name tag''.
Glycolipids- involved in cell-cell recognition.
Energy transfer in photosynthesis & respiration, proteins in the
membranes of chloroplast & mitochondria take part respectively.
Cholesterol: acts like a plug, reducing even further the escape or
entry of polar molecules through the membrane.
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71. 3.1.3. Cellular diversity
Cells are found in different organisms
are very diverse in their size, shape & their internal structure
this also applies to cells found in the same organism.
diversity is influenced by their roles & function within body.
 Cell Shape
Cells have different shapes due to appropriate function.
Body cells have flat, protecting & covering body surface.
Nerve cells have long extensions.
Skin cells have a shape which is flat.
Egg cells have sphere, & some bacteria are rod in shape.
Some plant cells are rectangular.
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72. Cell Size
• Some cell can be seen with naked eye without using
magnification instruments
• Example, egg of birds/reptiles & a neuron cell of
giraffe, which is 2 meters in length.
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73. 3.1.4 Transport across the cell membranes
• plasma membrane is selectively permeable.
• Lipid-soluble & small ions passively diffuse down their electro-
chemical gradients.
• Uncharged/non-polar molecules O, CO2 & fatty acids are highly
lipid-soluble & readily permeate the membrane.
• Charged particle Na/K ions & polar molecules such as glucose &
proteins have low lipid solubility, but are very soluble in water.
• For water-soluble ions of less than 0.8 nm diameters, protein
channels serve as an alternate route for passage
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74. • For the cell to survive some materials need to enter & leave
the cell. There are 4 basic mechanisms:
1. Diffusion & facilitated diffusion
2. Osmosis
3. Active transport
4. Bulk transport
Two forces are involved in facilitating movement across the
plasma membrane:
1. Forces that do not require to expend energy for
movement passive force
2. Forces requiring energy (ATP) to be expended to
transport across the membrane active force
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75. Diffusion
is the net movement of molecules (or ions) from a region of
high concentration to lower concentration.
The molecules move down a concentration gradient.
Molecules have kinetic energy, which makes them move about
randomly.
All molecules in liquid & gases are in continuous random
motion in any direction.
As a result of this random movement, the molecules frequently
collide bouncing off each other in different directions.
The greater the concentration, the greater the likelihood of
collision.
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76.  Additional factors that influence the rate of net diffusion are:
1. permeability of the membrane
2. surface area of the membrane
3. molecular weight of substance (lighter diffuses rapidly)
4. distance through which diffusion must take place
 N.B:- Increasing all the factors increases rate of net diffusion,
except distance - thickness, that if increased, decreases the rate of
diffusion; & molecular weight if increased, decreases rate of
diffusion.
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Fig 3.7: Diffusion of molecules

77.  Movement along electrical gradient
Movement of charged particles is also affected by their electrical
gradient.
If a relative difference in charges exists b/n two adjacent areas,
– the cations tend to move towards more negatively charged area, whereas
the anions tend to move toward the more positively charged areas.
The simultaneous existence of an electrical & concentration
(chemical) gradient for a particular ion is referred to as an electro-
chemical gradient.
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Fig: 3.8: Concentration gradient (A) and diffusion (B)

78. 11/13/2023 78
Fig 3.9: Diffusion of lipid molecules

79. Carrier-mediated Transport
• All carrier proteins span the thickness of the plasma membrane &
are able to undergo reversible changes in shape.
• This transport displays three characteristics:
1. Specificity: each cell possesses protein specified to
transport a specific substance.
2. Saturation: in a given time only a limited amount of a
substance can be transported via a carrier; this limit is
known as transport maximum (Tm). When the Tm is
reached, the carrier is saturated.
3. Competition: closely related compounds may compete for
ride across the plasma membrane on the same carrier.
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80. Facilitated Diffusion
• Facilitated diffusion uses a carrier protein to facilitate the
transfer of a particular substance across the membrane
''downhill'' from higher to lower concentration.
• This process is passive & does not require energy.
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81. Osmosis
• is the net diffusion of water down its own concentration
gradient.
• Water can readily permeate the plasma membrane.
• The driving force for diffusion of water is its concentration
gradient from area of higher water concentration (low solute) to
the area of lower water (high solute) concentration.
• This net diffusion of water is known as osmosis.
• Special mechanisms are used to transport selected molecules
unable to cross the plasma membrane on their own.
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82. 11/13/2023 82

83. Active transport
• Active transport, requires the carrier to expend energy to
transfer its passenger ''uphill'' against a concentration gradient
from an area of lower concentration to an area of higher
concentration.
• ions across a membrane against its natural tendency to diffuse in the
opposite direction. Or transporting against concentration gradient.
• The movement of molecules is in one direction only; unlike diffusion
that is reversible the energy is supplied by the broke down of ATP.
The major ions within the cells & their surrounding are Na+, K+
& Cl-.
the membrane surface of most cell have sodium pump is
coupled with a potassium pump that actively moves K+ from
outside to inside the cell.
The combined pump is called the sodium potassium pump (Na-
K- pump).
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84. Note that for every 2K+ taken into the cell, 3Na+ are removed.
Thus a potential difference is built up across the membrane, with
the inner side of the cell being negative.
This tends to restrict the entry of negatively charged ions (anions)
such as chloride & favoring diffusion of cations into the cell.
This explains why chloride concentration inside red cell is less
than the outside despite the fact that chloride ions can diffuse in
& out by facilitated diffusion
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85. 11/13/2023 85
Fig 3.11: Sodium-potassium pump

86. Na+-K+-pump plays three important roles
• It helps regulate cell volume by controlling the concentration of
solutes inside cell & thus minimizing osmotic effects that would
induce swelling or shrinking of animal cell (osmoregulation).
– If the pump is inhibited, the cell swells and brusts because of the building-up of
Na+, which results in excess water entering in to the cell by osmosis .
• It establishes Na & K concentration gradients across the plasma
membrane of all cells;
– these gradients are important in the nerve & muscle to generate
electrical signals.
– high concentrations of K are needed inside cells for protein
synthesis, glycolysis, photosynthesis & other vital processes.
• The energy used to run the pump also indirectly serves as the
energy source for the co-transport of glucose & amino acids
across the membrane (intestine & kidney cell).
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87. 11/13/2023 87
Table 3.1 How molecules cross cell membrane

88. Exocytocis and Endocytisis
 Vesicular Transport
• cell membrane selectively transport ions & small polar molecules.
• But large polar molecules & multimolecular material may leave or
enter the cell, such as hormone secretion or ingestion of invading
microbe by leukocytes.
• These materials cannot cross the plasma membrane but are to be
transferred between the ICF & ECF not by usual crossing
• This process of transport into or out of the cell in a membrane-
enclosed vesicle is - vesicular transport.
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89. • Transport into the cell is termed endocytosis, whereas transport
out of the cell is called exocytosis.
• Both are active processes involving the bulk transport of
materials through membranes.
• In endocytosis, the transported material is wrapped in a piece of
the plasma membrane, thus gain entrance to the interior cell.
• Endocytosis of fluid is called pinocytosis cell (drinking),
whereas endocytosis of large multimolecular particle is called
phagocytosis (cell eating).
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90. 11/13/2023 90
Fig 3.12: Exocytosis
 Exocytosis is the reverse process of endocytosis.
 Wastes such as solid & undigested remains from phagocytic vacuoles
may be removed from cells or useful materials may be secreted.
 Secretion of enzymes from the pancreas is achieved in this way.