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. 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. 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. 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. 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. 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. 8.4 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. 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
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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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
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
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. 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. 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. 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. 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. 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. 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
