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Aqa unit 2 biology


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AS biology unit 2 revision aid

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Aqa unit 2 biology

  1. 1. AQA Biology AS Unit 2
  2. 2. 7.1 Investigating Variation Interspecific Variation: One species differs from another Intraspecific Variation: Members of the same species differ Why Sampling Might not be Representative: • Sampling Bias: Investigators might deliberately or unwittingly choose their area of work • Chance: Pure chance can ruin representative results • The best way to remove sampling bias is reduce human involvement as much as possible Random Sampling • 1. Divide study area into a grid of numbered lines • 2. Use random numbers from a number generator to obtain coordinates • 3. Take samples at the intersection of each pair of coordinates How to Minimise Chance • Large sample size: Reduces the probability of chance influencing results increasing reliability • Analyse data: Use statistical tests to see how much chance influenced the data Genetic Differences Causes Environmental Influences • Mutations: Cause sudden changes to genes/chromosomes or genes may not have passed on to the next generation • Meiosis: Genetic information is passed to the gametes by meiosis which gives varied genetic information • Fusion of Gametes: Characteristics inherited from both parents, it is a random process increasing variety • Variety in asexual reproducing organisms occurs due to mutation • The environment can influence how genes are expressed • Genes set limits but the environment decides where in those limits an organism is • Temperature, nutrients, chemicals can all affect the environment • Variation is usually a combination of genetic differences and environmental influences
  3. 3. 7.2 Types of Variation Due to Genetic Factors • No intermediate values: they are categorical • Characters like blood type are usually only controlled by a single gene • Data can be represented in a bar chart or pie chart • Environmental factors have little influence to this type of variation Due to Environmental Influences • Characters are not controlled by single gene but many (polygenes) • This data usually forms a normal distribution curve (bell shaped curve) • The values can be continuous Mean: measurement at the maximum height of the curve, it provides an average value but doesn't give use information about the range of values Standard Deviation: measure of the width of the curve, gives indication of the range either side of the mean. 68% lies within +1 standard deviation, 95% is +2 and 99% is +3 Calculate Standard Deviation • 1. Calculate mean value • 2. Subtract the mean from each measured value • 3. Square all numbers • 4. Add the squared numbers together • 5. Divide this number by the original • 6. Square root the results
  4. 4. 8.1 Structure of DNA DNA (Deoxyribonucleic acid) is the chemical that determines inherited characteristics and is made up of 3 components that form a nucleotide A nucleotide is made up of deoxyribose (sugar), a phosphate group and organic base The two types of base are: • Single ring bases: Cytosine (C) and Thymine (T) • Double ring bases: Adenine (A) and Guanine (G) The deoxyribose, phosphate group and organic base are combined due to condensation reactions A dinucleotide is formed from two mononucleotides and if the mononucleotides continue linking they form a polynucleotide DNA Structure • Made up of two strands of nucleotides that are joined together by hydrogen bonds between the bases • In a ladder shape the deoxyribose and phosphates for the poles and the bases for the steps Pairing of Bases • The bases contain nitrogen and are complementary of each other • The double ring structured bases (A and G) have longer molecules than the single ring structured bases (C and T) • A and T are paired by 2 hydrogen bonds, C and G are paired by 3 hydrogen bonds • A and T will always have the same number as each other just like C and G to each other • The DNA forms a double helix in which every turn has 10 bases Function of DNA • It passes genetic information from cell to cell and generation to generation • There is almost an infinite amount of sequences for bases along the DNA • DNA is stable and difficult to be changed as well as being large carrying much genetic information • The separate strands are able to split apart during protein synthesis and DNA replication due to being held by hydrogen bonds • Bases protected by the deoxyribose-phosphate backbone from chemical/physical forces
  5. 5. 8.2 The Triplet Code What is a Gene? The Triplet Code • Genes are sections of DNA that contains information for polypeptide production • The information has a specific sequence of bases along the DNA molecule • Polypeptides form proteins such as enzymes • Enzymes control chemical reactions so are responsible for organism development and activities • A Polypeptide is basically a sequence of amino acids • 20 amino acids occur regularly in proteins • Each amino acid has its own code of bases, some have more than one code • The code is known as the degenerate code due to some amino acids having more than one code • The code is also non overlapping
  6. 6. 8.3 DNA and Chromosomes In eukaryotic cells the DNA molecules is large, linear and occurs in association with proteins to form chromosomes In prokaryotic cells the DNA molecules are smaller, form a circle and are not associated with protein molecules so they have no chromosomes Chromosome Structure • Only visible when a cell is dividing and appear as two threads (each known as a chromatid) joined at a single point (centromere) • DNA in chromosomes is held in place by proteins • In DNA the helix is wound around proteins, this DNA-protein complex is then coiled • The coil is looped and further coiled to pack into the chromosome • The single molecule of DNA in a chromosome contains many genes that occupy specific positions on the DNA molecule • The number of chromosomes in species varies however they are always in pairs called homologous pairs
  7. 7. DNA and Chromosomes Cont. Homologous Chromosomes • In sexually produced organisms half the DNA comes from each parent • One of each pair of chromosomes comes from each parent, they have the same gene loci and are known as homologous pairs • Cells that contain two sets of chromosomes in the nucleus are known as diploids • A homologous pair possess information for the same thing e.g. eye colour but the chromosomes may carry different alleles e.g. brown colour or blue colour • During meiosis the halving of chromosomes ensures that each daughter cell receives one chromosome from each homologous pair • When the haploid cells combine they form a diploid What is an Allele? • Allele: One of the forms of a gene • You receive one allele from each parent, different alleles code for different polypeptides • Any differences in base sequence of an allele can result in different sequence of amino acids being coded causing different polypeptide production
  8. 8. 8.4 Meiosis and Genetic Variation Why is Meiosis Necessary? • Meiosis produces four daughter nuclei each with half the number of chromosomes as the parent cell • In sexual reproduction two gametes fuse to create the offspring with the full amount of chromosomes needed • Meiosis is needed as by halving the number of chromosomes it makes sure the offspring with have the right amount of chromosome and it won't have doubled • During meiosis the chromosome pairs separate so only once chromosome enters the gamete (haploid) • When two haploids gametes fuse the diploid number is restored
  9. 9. The Process of Meiosis It involves two nuclear divisions that occur straight after each other: Meiosis brings Genetic Variation by: Independent Segregation Variety from New Genetic Combinations Genetic Recombination by Crossing Over • Meiosis 1: Homologous chromosomes pair up and their chromatids wrap around each other. Equivalent portions of the chromatids exchanged in crossing over. By the end the homologous pairs have separated with one chromosome from each pair going into one of two daughter cells • Meiosis 2: Chromatids move apart, 4 cells are formed each with a single chromatid • Meiosis produces genetic variation allowing offspring that can adapt and survive • Independent segregation of homologous pairs • Recombination of homologous pairs by crossing over • Gene: section of DNA that codes for a polypeptide • Locus: the position of a gene on a chromosome/ DNA molecule • Allele: one of the different forms of a particular gene • When homologous pairs arrange themselves it is done randomly • The combination of chromosomes into the daughter cell is completely random • The independent segregation of the chromosomes produces new genetic combinations • Gametes produced from meiosis will be genetically different due to the different combination of maternal and paternal chromosomes • Each gamete had a different make up and due to random fusion variety is produced • The chromatids of each pair become twisted around each other • During this tensions are created and portions of chromatids break off • These broken portions then re-join with the chromatid of its homologous pair • New genetic combinations are produced • Recombination: broken off portions of chromatid recombine with another chromatid • Crossing over increases genetic variety as it produces 4 cells with different genetic composition
  10. 10. Meiosis Process
  11. 11. 9 Diversity Similarities and differences between organisms can be defined in terms of variation of DNA DNA difference leads to genetic diversity Selective Breeding The Founder Effect Genetic Bottleneck • Also known as artificial selection • It uses the desired characteristics from one animal • Offspring are produced with the desired characteristic • If the offspring doesn't have the characteristic it can be killed or stopped from breeding • Alleles for unwanted characteristics are bred out • Selective breeding reduces genetic diversity • It is carried out to produce high yield plants or animals • When few individuals from a population colonise a new region • The individuals have few alleles • When the colony repopulate they have less genetic diversity • In time the population may develop into a separate species • They are less adapt to changing conditions since they have fewer alleles • Natural disasters can cause this • The survivors will possess smaller variety of alleles then original population • When repopulating the genetic diversity will remain restricted • The fewer alleles mean they are less adapt to change in conditions
  12. 12. 10.1 Haemoglobin Primary structure of a protein is the sequence of amino acids determined by DNA that makes up a polypeptide chain It is this sequence that determines how the polypeptide chain is shaped into its tertiary structure Haemoglobins: group of protein molecules that have a quaternary structure Haemoglobin Molecules • Haemoglobins are a group of chemically similar molecules found in organisms • The structure is made up of: • Primary Structure: consisting of four polypeptide chains • Secondary Structure: Each polypeptide chain is coiled into a helix • Tertiary Structure: Each polypeptide chain is folded into a precise shape allowing ability to carry oxygen • Quaternary Structure: All four polypeptides are linked together. Each polypeptide is associated with a haem group which contains a ferrous (Fe2+) ion. Each Fe2+ ion can combine with an O2 molecule
  13. 13. The Role of Haemoglobin Transport oxygen efficiently • It can readily associate with oxygen at the surface where gas exchange occurs • It can readily dissociate from oxygen at tissues requiring it These two requirements are achieved by haemoglobin being able to change its affinity for oxygen under different conditions • Haemoglobin can change shape in the presence of certain substances such as CO2 • With CO2 present haemoglobin molecule binds more loosely to oxygen Why have Different Haemoglobin • Haemoglobin with high affinity for oxygen: take up O2 easily but release it less easily • Haemoglobin with low affinity for oxygen: take up O2 less easily but release it easily • Organism living in low O2 area requires high affinity haemoglobin • Organism with high metabolic rate needs low affinity haemoglobin Why different Haemoglobins have different Affinities • Due to slightly different amino acid sequences Loading and Unloading Oxygen • Loading/Associating: process haemoglobin combines with oxygen • Unloading/dissociating: process haemoglobin releases oxygen
  14. 14. 10.2 Oxygen Dissociation Curves When haemoglobin is exposed to different partial pressures of oxygen it does not absorb O2 evenly Low Concentrations: Four polypeptides of haemoglobin molecule are closely united so difficult to absorb first O2 molecules Once loaded the O2 causes the polypeptides to load remaining O2 easily Graph Reading •The further to the left the curve the greater the affinity of haemoglobin for oxygen (it takes up oxygen readily but releases it less easily) •The further to the right the curve the lower the affinity of haemoglobin for oxygen
  15. 15. Effects of CO2 Concentration •Haemoglobin has a reduced affinity for O2 in the presence of CO2 •The Bohr Effect: the greater the concentration of CO2 the more readily O2 is released •At gas exchange surfaces CO2 level is low due to it being expelled. The affinity of haemoglobin for oxygen is increased meaning O2 is readily loaded by haemoglobin. The reduced CO2 level shifts the graph to the left •In rapidly respiring tissues the CO2 level is high, the affinity for oxygen is reduced causing the oxygen to be readily unloaded from the haemoglobin. The increased CO2 level shifts the graph to the right •Carbon Dioxide is acidic so lowers pH causing haemoglobin to change shape Loading, Transport and Unloading Oxygen •The higher the rate of respiration->the more carbon dioxide the tissue produces->the lower the pH->the greater the haemoglobin shape change->the more readily oxygen is unloaded-> the more oxygen available for respiration •In humans haemoglobin becomes saturated in the lungs so they carry the 4 oxygen molecules •When the haemoglobin reaches a low respiratory rate tissue one of the oxygen molecules is released •At a very active tissue 3 oxygen molecules will be unloaded from each haemoglobin
  16. 16. 10.3 Starch, Glycogen and Cellulose Starch Suited as a energy store: • Found in plants: seeds and storage organs • Forms important part of diets as an energy source • Made up of alpha-glucose monosaccharides linked by glycosidic bonds • Glycosidic bonds formed by condensation reactions • Unbranched chain wound into a coil so compact • • • • Insoluble so doesn't draw in water by osmosis Doesn't diffuse out of cells easily as insoluble Compact: lots can store in small space Hydrolysed into alpha glucose used in respiration
  17. 17. Glycogen • Similar structure to starch • Short chain and highly branched • Major carbohydrate storage in animals • Stored as small granules in muscles and liver • Readily hydrolysed due to small chains • Not found in plant cells Cellulose • Made of Beta-Glucose monomers: position of -H group and OH group on single carbon atom are reversed • -OH group is above the ring so to form glycosidic bonds the monomers are rotated 180 degrees • The -CH2OH group on each Beta-Glucose molecule alternates • Forms straight unbranched chain that runs parallel allowing hydrogen bonds to form cross links between adjacent chains • Cellulose is strengthened by the multiply hydrogen bonds • The cellulose molecules group to form microfibrils which form fibres • Major component of plant cell walls as provides rigidity • Prevents cell bursting when water enters as it exerts an inward pressure • Herbaceous parts of plants are semi-rigid as the cells push against each other • Important in stems and leaves • Provide maximum surface area for photosynthesis
  18. 18. 10.4 Plant Cell Structure Plant cells are eukaryotic cells: have distinct nucleus and membrane bound organelles like mitochondria and chloroplasts Leaf Palisade Cell • Function: Photosynthesis • Features: • Long, thin cells form continuous layer (Absorb Sunlight) • Chloroplast arrange themselves for maximum light • Chloroplast carries out the photosynthesis • Large vacuole pushes cytoplasm and chloroplast to edge of cell Chloroplasts • Typically disc shaped • Features: • Chloroplast Envelope: double plasma membrane surrounds organelle, selects what enters and leaves the cell • Grana: Stacks of discs called thylakoids, thylakoids contain chlorophyll. The first stage of photosynthesis occurs here • Stroma: fluid filled matrix that contains other structures such as starch grains. The second stage of photosynthesis occurs here • Adaptations for Photosynthesis: • Granal membranes: large surface area for chlorophyll attachment, electron carriers and enzymes that carry out 1st stage of photosynthesis • Fluid of stroma possess enzymes needed for 2nd stage of photosynthesis • Chloroplast contains DNA and ribosomes that allow manufacture of proteins for photosynthesis
  19. 19. Cell Wall • Consists of microfibrils of cellulose embedded in a matrix • Features: • Consists of many polysaccharides • Middle Lamella marks boundary between adjacent cell walls that sticks them together • Function: • Provide mechanical strength to stop cell bursting • Mechanical strength to whole plant • Allow water to pass along it Root Hair Cell • Absorb water and mineral ions • Water absorbed by osmosis • Roots have high concentration of ions and sugar compared to soil • Uptake of mineral ions is against the concentration gradient so used active transport • Special carrier proteins use ATP to absorb mineral ions Xylem Vessels • Transport water • Thick cell walls • Formed from dead cells • Lignin often forms rings around the vessel
  20. 20. 11 Replication of DNA Cell division occurs in two main stages: • Nuclear Division: nucleus divides either in mitosis or meiosis • Cell Division: follows nuclear division, it is where the whole cell divides • Before a nucleus divides its DNA must be replicated • This makes sure daughter cells have genetic information to produce enzymes and other needed proteins Semi-Conservative Replication • Has four requirements: • Four types of nucleotide must be present with their bases adenine, guanine, cytosine and thymine • Both DNA molecule strands must act as a template for the attachment of nucleotides • The enzyme DNA polymerase is needed to catalyze the reaction • A source of energy is required Semi-Conservative Replication Process • DNA helicase breaks hydrogen bonds linking base pairs • The double helix separates into two strands • Each exposed polynucleotide strand acts as a template to which complementary nucleotides are attracted • Energy used to activate the nucleotides • Activated nucleotides are joined together by DNA polymerase to form missing polynucleotide strand • Each new DNA molecule has one original DNA strand and one new DNA strand
  21. 21. Mitosis Division of the nucleus of a cell that results in each daughter cell having exact copy of the DNA of the parent cell Genetic make up of the two daughter nuclei is identical to the parent unless mutation The period when the cell is not dividing is called interphase Interphase: cell is actively synthesising proteins, chromosomes are visible and DNA replicates Mitosis is divided into four stages: • Prophase: Chromosomes are visible, nuclear envelope disappears • Metaphase: Chromosomes arrange themselves at the equator of the cell, spindles form • Anaphase: The chromatids are pulled towards poles as the spindles contract • Telophase: Nuclear envelope reforms, spindles disappear and cell division commences The Importance of Mitosis • It makes daughter cells identical to the parent cells • Growth: When two haploid cells fuse to form a diploid cell the diploid has all genetic information needed to from the new organism. All the cells must have same set of genetic information so it resembles its parents • Differentiation: Cells change to give specialised cells, the cells divide by mitosis to give tissues made of identical cells which perform particular functions. Essential for efficient functioning of cells • Repair: If cell is damaged or dies it needs to be replaced with a cell identical in structure and function. Without mitosis an identical cell would not be formed
  22. 22. The Cell Cycle Cells do not divide continuously, they have a cycle of division separated by periods of cell growth. The Cell Cycle which has 3 stages: Cancer • Interphase: most of cell cycle, no division occurs, divided into 3 parts: • G1 (first growth): Proteins from which cell organelles are synthesized are produced • S (Synthesis): DNA is replicated • G2 (second growth): Organelles grow and divide, energy stores are increased • Nuclear Division: Nucleus divides by mitosis or meiosis • Cell Division: Whole cell divides into two (Mitosis) or four (meiosis) • Caused by growth disorder of cells • Result of damage to the genes that regulate mitosis and the cell cycle • Leads to uncontrolled growth of cells, forming abnormal cells known as a tumour • The tumour can expand
  23. 23. 12 Cellular Differentiation The process by which cells in Multicellular organisms become specialised for different functions. Cells organised into tissues and tissues organise organs which organise organ systems All cells in an organism are initially identical. As a cell matures, it takes on its own individual characteristics that suit it to the function that it will perform, when it has matured. Each cell is specialised in structure to suit the role it will carry out. Every cell contains the same genes for its development, the cell differentiates due to the number of genes switched on (Expressed) or off. Tissues: • Collection of similar cells aggregated together to perform a specific function • For them to work efficiently cells are Aggregated together. • E.g epithelial cells and xylem. Organs: • Aggregation of tissues performing physiological functions. • Combination of tissues that are co-ordinated to perform a variety of functions, although they do have one predominant function • Lungs, heart, stomach, leaf • Artery and vein : made up of many tissues- epithelial, muscle and connective. Have one predominant function- to carry blood (transportation of blood) • These systems may be grouped together to perform particular functions more efficiently. • Digestive system, respiratory system, circulatory system .
  24. 24. 13 Exchange between Organisms and Environment Substances need to be interchanged: • Respiratory gases (Oxygen and Carbon Dioxide) • Nutrients (fatty acids, glucose, amino acids, vitamins and minerals) • Excretory products (urea and Carbon Dioxide) • Heat The exchange takes place: • Passively (no energy required): diffusion and osmosis • Actively (energy required): active transport Surface Area: Volume Ratio • Small organisms have surface area large enough for efficient exchange across their body • Larger organisms cannot do this so have adaptations: • Flattened shape so cells close to surface • Specialised exchange surface with large area to increase surface area: volume ratio Features of Specialised Exchange Surfaces • Large surface area to volume ratio: increases rate of exchange • Thin: diffusion distance is short so exchange is rapid • Partially permeable: allow selected materials to cross without obstruction • Movement of environmental medium e.g. air • Movement of internal medium e.g. blood Fick's Law: • Diffusion = Surface Area X Difference in Concentration iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiLength of Diffusion Path
  25. 25. Gas Exchange in Single Celled Organisms/Insects Single Celled Organisms • Are small so have large surface area: volume ratio • Oxygen is absorbed by diffusion across the body which is covered by a cell surface membrane • Carbon dioxide diffuses out the body surface • Living cells with cell walls are completely permeable so no barrier for diffusion Gas Exchange in Insects • Insects are terrestrial so suffer from water evaporation easily making them dehydrated • To reduce water lose terrestrial organisms have: • Waterproof covering • Small surface area to volume ratio: minimize area for water loss • Insects have tracheae (internal network of tubes supported by strengthened rings) • Tracheoles (smaller tubes tracheae divide into): Extend through the body so air is brought directly to respiring tissues • Respiratory gases move in and out: • Along a diffusion gradient: Cell respiration uses oxygen so concentration towards Tracheoles end falls creating a diffusion gradient. Gradient causes oxygen to diffuse from the atmosphere along tracheae. Cells respiring produce CO2 is removed into the atmosphere. It is quick as diffusion in air is more rapid than in water • Ventilation: Movement of muscles creates mass movements of air in and out of tracheae speeding up exchange of gases • Gas enters and leaves through spiracles (tiny pore) on the body surface, the spiracles open and close by a valve • Spiracles are usually kept closed to reduce water lose • The tracheal system relies on diffusion, which needs a short pathway, this limits the size of an insect
  26. 26. Gas Exchange in Fish • Fish have waterproof, air tight outer covering • Have relatively small surface area to volume ratio • They have specialised internal gas exchange surface: Gills Structure of the Gills • • • • Made up of gill filaments stacked up in a pile Right angles to the filaments are gill lamellae which increase surface area Water taken in through the mouth and forced over the gills, goes out opening sides of body The flow of blood and the flow of water are in opposite directions: countercurrent flow The Countercurrent Exchange Principle • Blood well loaded with oxygen meets water, water has maximum concentration of oxygen so diffusion to the blood occurs • Blood with little oxygen meets water that has most oxygen removed so diffusion occurs • If the flow was parallel only 50% of oxygen from the water would diffuse into the blood
  27. 27. Gas Exchange in Leaves • When photosynthesis occurs, some CO2 comes from respiring cells, most CO2 comes from external air. Oxygen from photosynthesis is used in respiration but most is diffused out • When photosynthesis isn't occurring oxygen diffuses into the leaf as needed by the cells during respiration. Carbon dioxide produced during respiration diffuses out Structure of a Leaf • Plant leaves have large surface area and short diffusion path allowing quick diffusion • Leaves have many stomata in lower epidermis and air spaces throughout mesophyll Stomata • Surrounded by guard cells to prevent water loss but also allow gases in and out
  28. 28. Circulatory System of a Mammal How Large Organisms move Substances around their Bodies • Specialist exchange surface absorbs nutrients, respiratory gases and removes waste • Transport system required to take materials from cells to exchange surfaces and from exchange surfaces to cells • Tissues and organs have been specialised for their job • The lower the surface area: volume ratio and the more active an organism the greater the need for specialised transport system with a pump Features of Transport Systems • Suitable medium to carry materials e.g. blood (normally liquid based in water as it readily dissolves substances and moves easily) • Form of mass transport • Closed system of vessels that form a branching network • A mechanism to move the medium which requires pressure (Animals have the heart) • Mechanism to maintain the mass flow movement in one direction e.g. valves • Means of controlling the flow of medium to suit changes How Blood is Circulated in Mammals • There is a closed blood system • A muscular pump (heart) circulates the blood around the body • Mammals have a double circulatory system • Blood passes through the heart twice since: • After passing through the lungs its pressure is reduced, this would make circulation slow if passed around rest of the body • Blood is returned to the heart to boost pressure and then passed through the body • This is needed to keep the body temperature and metabolism in safe conditions • The blood is passed into the cells be diffusion over a large surface area with a short distance and steep diffusion gradient
  29. 29. Blood Vessels Structure Arteries: Carry blood away from the heart into arterioles Arterioles: Smaller arteries that control brood flow from arteries to capillaries Capillaries: Tiny vessels that link arterioles to veins Veins: Carry blood from capillaries back to the heart Arteries, arterioles and veins have same basic structure: • • • • • Tough outer layer: Resist pressure changes from both within and outside Muscle layer: Can contract and control blood flow Elastic layer: Help maintain blood pressure by stretching and springing back Thin inner lining (Endothelium): Smooth to prevent friction and thin to allow diffusion Lumen: Central cavity of the blood vessel The vein's lumen is larger than the arteries
  30. 30. Structure and Function: Artery Function: Transport blood rapidly under high pressure from heart to tissues Thick muscle layer: Smaller arteries can constrict and dilate to control volume of blood passing through them Thick elastic layer: To keep blood pressure high to reach body extremities. It can then stretch and recoil according to when the heart beats allowing it to maintain a high pressure Thickness of wall is large: Resists the vessel bursting under pressure No Valves: Blood under constant high pressure so doesn't flow backwards Arteriole Structure and Function • Function: Carry blood under lower pressure than arteries to the capillaries • Thicker muscle layer: Contractions of the muscle layer allow constriction of the lumen, this restricts the flow of blood and controls movement • Thinner elastic layer: Blood is under lower pressure
  31. 31. Structure and Function: Veins Function: Transport blood under low pressure to the heart Thin muscle layer: Veins carry blood away from tissues so don't need to control flow to them Thin elastic layer: Low pressure stops the bursting Small thickness of wall: No need for thick wall as the pressure of the blood is too low to burst the vein, it also allows them to flatten easily Valves: Ensure blood flows in the right way, when body muscles contract they compress the veins and pressurize them, the valves stop backwards flow Structure and Function: Capillary Function: Exchange metabolic materials like oxygen, carbon dioxide and glucose Walls with lining layer: Allow short diffusion distance for rapid diffusion of materials between blood and cells Branched: Many of them providing large surface area Narrow diameter: Permeate tissues Narrow Lumen: Red blood cells flattened so closer to cells Spaces between endothelial cells: Allows white blood cells to escape and help infection
  32. 32. Tissue Fluid Tissue Fluid • Capillaries cannot reach every cell directly so tissue fluid bathes the tissues • It contains glucose, amino acids, fatty acids, salts and oxygen • It supplies these substances to the tissues in return for CO2 and waste materials • It is formed from blood plasma and is controlled by homeostatic systems Formation • Blood pumped by the heart passes along arteries, then the arterioles and then the capillaries which causes hydrostatic pressure at the arterial end of the capillaries • This pressure forces tissue fluid out of the blood plasma • The outward pressure is opposed by two forces: • Hydrostatic pressure of tissue fluid outside the capillaries prevents outward movement of liquid • The lower water potential of the blood due to the plasma proteins pulls water back into the blood within the capillaries • Ultrafiltration occurs as the pressure is only enough to force small molecules out of the capillaries leaving cells and proteins in the blood
  33. 33. Tissue Fluid Return to the Circulatory System Once it exchanges metabolic materials it must return to the circulatory system Most tissue fluid returns to the blood plasma directly via capillaries: • The loss of tissue fluid from the capillaries reduces hydrostatic pressure inside them • By the time blood reaches the venous end of the capillary network its hydrostatic pressure is less than tissue fluid outside of it • Tissue fluid is then forced back into the capillaries by high hydrostatic pressure outside them • Osmotic forces resulting from the proteins in the blood plasma pulls water back into the capillaries The remainder tissue fluid is carried back via the lymphatic system; a system of vessels than begin in the tissues that resemble capillaries but then merge into larger vessels The larger vessels drain their contents back to the bloodstream via two ducts that join veins close to the heart The contents of the lymphatic system are moved by; • Hydrostatic pressure of the tissue fluid that left the capillaries • Contraction of body muscles squeeze the lymph vessels, valves in the lymph vessels ensure fluid inside them moves away from the tissues in the direction of the heart
  34. 34. Movement of Water through Roots Uptake of Water by Root Hairs • Plants lose water by transpiration so the water is replaced through the root hairs • Root hairs are long, thin extensions of a root epidermal cell Efficient surface for exchange of water and minerals: • Provide large surface area • Thin surface layer The soil the root hairs go into is mostly water so has a high water potential The root hair cells have sugars and amino acids dissolved in them This causes water to move by osmosis into the root hair cells The water travels via: • The Apoplastic pathway • The Symplastic pathway
  35. 35. The Apoplastic Pathway The Symplastic Pathway: Through the cells • Water drawn into the endodermal cells • It pulls more water in due to its cohesive property • This creates tension that draws water along the cell walls of the cells of the root cortex • The mesh like structure of the cellulose cell walls contains many water filled spaces • These water filled spaces provide little resistance to the pull of water along the cell walls • Takes place across the cytoplasm of the cells of the cortex due to osmosis • Water passes through the cell walls along tiny openings called plasmodesmata • Each plasmodesmata is filled with thin strand of cytoplasm • There is a continuous column of cytoplasm extending through the root hair cell to the xylem at the center of the root • Water moves along the column: • Water enters by osmosis increasing the water potential of the root hair cell • The root hair cell has a higher water potential than the first cell in the cortex • Water then movers from the root hair cell to the first cell in the cortex • This is then repeated through the cortex • Water is then pulled in as the water potential of the first cortical cell is lowered again making the water travel by osmosis from the root hair cell to the first cell of the cortex • A water potential gradient is set up along the cells
  36. 36. Passage of Water into the Xylem The waterproof band that makes up the casparian strip prevents water from passing further through the cell wall due to the apoplastic pathway Water is forced into the living protoplast of the cell It joins the water from the symplastic pathway Active transport of mineral salts can take the water into the xylem Endodermal cells actively transport salts into the xylem As the process requires energy it can only occur in living cells It takes place along carrier proteins in the cell surface membrane The active transport of mineral ions into the xylem by the endodermal cells creates a lower water potential so water can move into the xylem by osmosis along a water potential gradient The movement of the mineral ions creates a force that helps to move water up the plant: The is Root Pressure Evidence root pressure is due to the pumping of salts into the xylem: Pressure increases with a rise in temperature Metabolic inhibitors e.g. cyanide prevent most energy release by respiration and stop root pressure Decrease in availability of oxygen causes a reduction in root pressure
  37. 37. Movement of Water up Stems • The main force that pulls water up the stem is transpiration • Transpiration is the evaporation of water from the leaves How Water moves through the Leaf • When stomata are open water vapour molecules diffuse out of the air spaces • The water is replaced by water evaporating from the cell walls of the mesophyll cells • Water from the mesophyll cells is then replaced by water in the xylem by the apoplastic or symplastic pathways • In symplastic pathways it occurs: • Mesophyll cells lose water to air spaces • Cells now have lower water potential so water enters by osmosis • The neighboring cells lose water lowering their water potential • The neighbouring cells then take water from the cells next to them • This establishes a water potential gradient to pull the water from the xylem How Water moves up the Xylem • The two main factors that cause water movement up the xylem are cohesion tension and root pressure • Cohesion Tension operates: • Water evaporates from leaves due to transpiration • Water molecules form hydrogen bonds between them this is cohesion • Water form continuous pathway across the mesophyll cells and down the xylem • As water evaporates from the mesophyll cells in the cells in leaves into the air spaces beneath the stomata molecules of water are brought up • Water is pulled up the xylem due to transpiration pull • Transpiration pull put the xylem under tension
  38. 38. Evidence of the Cohesion Tension Theory Change in diameter of tree trunks • During the day transpiration is at its greatest so more tension in the xylem • This causes the trunk to shrink • At night transpiration is at its lowest so little tension in the xylem • Diameter then increases at night If Xylem vessel is broken and air enters it • The tree can no longer draw up water as no continuous column • If broken air is drawn in Transpiration pull is a passive process so doesn't require metabolic energy As the xylem is dead it can form series of continuous unbroken tubes from root to leaves These tubes are essential to the cohesion tension theory Energy is needed for transpiration and it comes from the heat of the sun
  39. 39. 13.9 Transpiration Why Transpiration Occurs •Leaves have a large surface area to absorb light and stomata to allow diffusion of CO2 into the plant •Both these features cause a huge lose in water •Mineral ions, sugars and hormones are moved around the plant in dissolved water by the transpiration pull •Without transpiration water wouldn't be plentiful and transport of materials would be slow Factors Factor How Factor Affects Increase in Transpiration caused by Decrease in transpiration caused by Light Stomata open in the light and close in the dark Higher light intensity Lower light intensity Temperature Alters the kinetic energy of the water molecules and the relative humidity of the air Higher temperature Lower temperature Humidity Affects the water potential gradient between the air spaces in the leaf and the atmosphere Lower humidity Higher humidity Air Movement Changes the water potential gradient by altering the rate at which moist air is removed from around the leaf More air movement Less air movement
  40. 40. 13.10 Limiting water loss
  41. 41. 14 Classification The Organisation of living organisms into groups The Binomial System: Grouping Species Together: Phylogeny • Species: Similar to one another but different to members of other species that are capable of breeding to produce living, fertile offspring • Based on Greek or Latin • First name (generic name) denotes the genus • The second name (specific name) denotes the species • Taxonomy: Theory and practice of biological classification • Artificial classification: divides organisms due to differences useful at the time. Features such as number of legs are analogous characteristics that have the same function but not evolutionary origins • Natural Classification: based on evolutionary relationships, classifies species in groups using shared features and arranges the group in hierarchy • Natural classification is based upon homologous characteristic so have similar evolutionary origins • Evolutionary relationship between organisms • It reflects the evolutionary branch that led up to an organism
  42. 42. Rank Kingdom Phylum Class Order Family Genus Species
  43. 43. 15.1 Genetic Comparison DNA determines proteins including enzymes and proteins determine features of an organism When a species arises from another due to evolution the DNA will initially be similar Due to mutations the sequences of nucleotide bases in the DNA will change DNA Hybridisation • DNA from two species is extracted, purified and shortened • DNA from one species is labelled with radioactive/fluorescent marker so it can be identified • The mixture of DNA is heated to separate the strands • It is then cooled so the strands recombine that have complementary base sequence • Hybrid strands are formed when one strand of each species combines and can be separated by certain temperatures • If the species are closely related they will share many complementary nucleotide bases so there will be more hydrogen bonds linking the strands • The greater the number of hydrogen bonds the higher the temperature needed. This means the higher the temperature the more closely related Comparison of Amino Acid Sequences in Proteins • DNA determines amino acids in proteins • Similarity in the amino acid sequence of the same protein in two species will reflect how closely related they are • Amino acid sequences for a certain protein can be compared Immunological Comparison of Proteins • Proteins of different species can be compared as antibodies of one species will respond to specific antigens on proteins e.g. albumin in the blood serum of another • Serum albumin from species A is injected into species B • B produces antibodies specific to all antigen sites on albumin from A • Serum is extracted from B containing antibodies specific to the antigens on the albumin of A • Serum from B is mixed with serum from species C • Antibodies respond to their corresponding antigens on the albumin in serum C • The response forms a precipitate, the more precipitate the more similar the antigens and the more closely related
  44. 44. 15.2 Courtship Behaviour Physical and Chemical make-up of organisms help distinguish members of own species Behaviour of members of the same species is more alike than other species Behaviour is genetically determined and has evolved it influence chance of survival Courtship is Necessary • Reproduction is important so a species can survive • Courtship behaviour helps achieve chance of offspring by: • Allowing members for same species to recognise each other so fertile offspring can be produced • To identify mate capable of breeding • Form a pair bond to lead to successful mating and raising of offspring • Synchronise mating so it takes place when maximum chance of fertilisation • Males carry out a specific action which stimulates the female to respond and her action then causes him to react • It is a stimulus-response chain where if the species is the same the chain of actions will be the same
  45. 45. 16.1 Genetic Variation in Bacteria Adaptation can occur in natural selection Long term reproductive success of a species is increased by adaptation Bacteria is adaptable and able to develop resistance to antibiotics Changes in DNA can occur by mutation or recombing DNA of two individuals Bacteria can increase genetic diversity by mutations and conjugation Mutations • Changes in DNA that cause different characteristics • Bases can be added, deleted or replaced during replication • Difference in bases can cause a change in amino acid which will then cause a difference in polypeptide causing a different protein to be formed Conjugation • When one bacterial cell transfers DNA to another bacterial cell • One cell produces thin projection that meets the other cell and forms a thin conjugation tube between them • The donor cell replicates one of its circular DNA pieces (plasmid) • The DNA is broken to make it linear and is then passed along the tube to the recipient cell • The contact is brief so only portion of the donor DNA is transferred • The recipient cell acquires new characteristics Horizontal gene transmission: From one species to another Vertical gene transmission: From one generation of a species to another
  46. 46. 16.2 Antibiotics How Antibiotics Work • Prevent bacteria forming a normal cell wall • Osmotic lysis causes the cell to burst when water enters the cell • Due to the cell wall surrounding the bacteria the content will expand and push against the wall • The wall resists expansion stopping further water entry • Certain antibiotics kill bacteria by preventing cell wall formation • They inhibit synthesis and assembly of the peptide cross linkages in the bacteria cell walls weakening the wall • The walls are then unable to withstand pressure and water enters the bacteria causing it to burst Antibiotic Resistance • Due to the chance mutation within bacteria • The mutation can result in certain bacteria being able to make new proteins • E.g. resistance to penicillin as the bacteria mutate and can produce penicillinase which breaks down penicillin • The resistance can be passed on by vertical gene transmission • When an antibiotic is used it kills the bacteria without the mutation leaving the mutated bacteria able to multiply • Due to this the allele pool is reduced and the resistance bacteria can increase in population • The allele is carried in the plasmid which can be transferred in conjugation so other bacterial species can gain the resistance
  47. 47. 16.3 Antibiotic Use and Resistance Tuberculosis • Treatment involves antibiotics for 6-9 months • When people believe they have recovered they stop using the antibiotics so only the least resistant strains of mycobacterium are killed • The most resistant remain, survive and multiply • A cocktail of antibiotics is used MRSA • Staphylococcus can be carried in the throat • Staphylococcus aureus can cause major health issues if forms methicillin resistant staphylococcus aureus which is resistant to many antibiotics • It is easily spread in hospitals due to weak immune systems and people living close together causing transmission • It is very difficult to treat
  48. 48. 17.1 Species Diversity Biodiversity: The variety in the living world • Species diversity: Number of species and individuals of each species within a community • Genetic diversity: Variety of genes possessed by individuals in the species • Ecosystem diversity: Range of different habitats within particular area • Biodiversity can be measured by; the number of different species in a given area and the proportion of the community that is made up of an individual species Measuring Species Diversity • Using the Species Diversity Index • The higher the value of D the greater the species diversity
  49. 49. 17.2 Species Diversity and Human Activity Impact of Agriculture Impact of Deforestation • Natural ecosystems develop to form complex communities with many different species • Agricultural ecosystems are controlled by humans • Farmers select species for particular qualities that are productive • As a result genetic variety of alleles is reduced to ‘desired features’ • Any area can only support a certain amount of biomass so the more of one species the less room for another and so they have to compete • Overall reduction in species diversity causes a decrease in species diversity index so there are low agricultural ecosystems • Forests form many different habitats and species diversity is high • Deforestation e.g. by forest fires, acid rain or human intervention permanently clear forests • The land is converted for grazing, housing or reservoirs • Biodiversity is decreased