A2 Biology Term 2

Question 1

Define the term limiting factor in terms of photosynthesis

Answer 1

The factor that is in the shortest supply and so will limit the rate of photosynthesis.

Question 2

Describe the effect of temperature on the rate of photosynthesis

Answer 2

At low light intensities a rise in temp has little effect as the light dependent reaction cannot occur. This means light intensity is the limiting factor.

At high light intensities a rise in temp has a large effect as the light dependent reaction occurs and the rate is affected by the temperature sensitive light independent reaction. Temperature is then the limiting factor.

Question 3

Describe how limiting factors are exploited in intensive agriculture

Answer 3

Light Intensity: lamps that provide photosynthetically active radiation

Heat: Moderate increases in ambient temperatures

CO2 concentration: Increase in CO2 levels from 340 ppm to 1000 ppm

Question 4

Describe how the rate of photosynthesis can be investigated using an isolated chloroplast suspension and state the Hill reaction.

Answer 4

1. The chloroplasts can be isolated from the leaves and suspended in a buffer solution of the same concentration of the cytosol. This can be done using a centrifuge.
2. A redox indicator is used to detect the oxygen given off to measure the rate of photosynthesis. Such indicators include:
   - An oxygen electrode is used in research laboratories.
   - A hydrogen-acceptor dye can be used. An example is DCPIP which goes from blue to colourless.

Hill Reaction: 2DCPIP + 2H2O -> 2DCPIPH2 + O2

Question 5

Describe how different structures in the chloroplast relates to its function

Answer 5

Double Membrane: Contains grana and stroma and is permeable to CO2, O2, ATP, sugars etc.

Photosystems in thylakoid membranes: Provide large surface area for max light absorption

Thylakoid spaces within grana: Regions for accumulation of protons and establishment of gradient.

Stroma: Site of enzymes for fixation, reduction and regeneration of acceptors.

Question 6

Describe photorespiration

Answer 6

• Occurs in plants in climates with high light intensity and high temperatures meaning carbon dioxide concentration is a limiting factor.
• In such conditions oxygen acts as a competitive inhibitor of rubisco.
• Photorespiration occurs instead of carbon fixation in which RuBP forms a two carbon compound which has no effect in the plant.

Question 7

Describe the mechanism in C4 plants

Answer 7

• photosynthetic cells are surrounded by special mesophyll cells in which carbon dioxide is fixed into a 4-carbon organic acid called malate.
• as malate is produced it is moved into the photosynthetic cells called bundle sheath cells.
• The organic acid is broken down here to form CO2, ensuring its concentration is high. This prevents photorespiration from occuring.
• Photosynthesis occurs at a high rate due to the high temperatures and high light intensities found in tropical conditions.

Question 8

Describe the opening of the stomata

Answer 8

• Potassium ions are pumped into the guard cell vacuole from surrounding cells by proteins in the cell membrane triggered by light.
• Starch is converted into organic acids such as malate and move into the vacuole.
• The accumulation of these substances causes the water potential in the vacuole to become more negative meaning there is a net uptake of water and the guard cells become extremely turgid.

Question 9

Describe the closing of the stomata

Answer 9

• Reversal of opening steps that occurs in the dark
• ABA, a plant growth inhibitor substance, is produced in chloroplasts during drought.
• ABA triggers release of calcium ions from the cell sap in the vacuoles into the cytosol.
• The increase in calcium ion concentration triggers:
  1. an efflux of anions, such as chloride ions, from the cytosol through activated anion channels.
  2. an efflux of K+ ions by activated K+ ion pumps
  3. inhibition of efflux of K+ ions into guard cells.
• This raises the water potential in the vacuole and so water moves out causing the cells to become flaccid and the pore to close.

Question 10

Describe the rapid closure of the Venus Flytrap and explain why it is needed

Answer 10

• Found in conditions where ions do not enter the soil due to being washed away by high rainfall
• The leaves have short, stiff hairs that when bent, cause the two halves of the plant to snap shut
• Hydrolytic enzymes are secreted to digest soft tissue
• It occurs mainly due to a sudden change in the turgidity of cells in the hinge region of the leaf blade.

Question 11

Describe the role of auxin in plant growth

Answer 11

• auxin is a plant growth regulator that promotes the elongation of cells
• it stimulates proton pumps in the cell membrane causing the cell wall to become more acidic
• low pH triggers breakage of cross links between cellulose microfibril and binding polysaccharides
• hydrolytic enzymes attack exposed binding polysaccharides
• wall resistance to stretching decreases and turgor causes elongation of cell wall

Question 12

Describe the role of gibberellin in the germination of barley

Answer 12

• gibberellic acid is formed in the embryo as germination begins. The gene for its synthesis is activated by the uptake of water.
• It then diffuses to the protein store where it triggers synthesis of hydrolytic enzymes.
• The hydrolytic enzymes catalyse the mobilisation of the food reserves to the embryos where they are respired and used to build new cells.

Question 13

Describe the role of gibberellin in stem elongation in terms of pea plants

Answer 13

• Gibberellins switch on the genes that promote germination and go on to promote additional mitosis
• Tall pea plants contain the dominant Le allele which codes for a protein that functions normally in the gibberellin-synthesis pathway
• Dwarf pea plants carry two le alleles that code for a non-functional protein, preventing the synthesis of gibberellin.

Question 14

Define homologous chromosomes

Answer 14

Chromosome pairs containing DNA which contain the same genes and join to form homologous pairs.

Question 15

Define haploid and diploid

Answer 15

Haploid: A eukaryotic cell or organism containing only one complete set of chromosomes.

Diploid: A eukaryotic cell or organism containing two complete sets of chromosomes.

Question 16

Explain the need for a reduction division before fertilisation can occur

Answer 16

The gametes fuse in sexual reproduction resulting in a zygote cell in which the chromosome number is the sum of the two gametes. Nuclear division in which the chromosome number is halved prevents the doubling of chromosome number in every generation.

Question 17

Describe the process of meiosis

Answer 17

Prophase I:

• chromosomes already consist of two chromatids and they begin to shorten and thicken.
• Homologous chromosomes then pair up as they continue to shorten and thicken.
• Centrioles duplicate
• Crossing over occurs and homologous chromosomes repel each other

Metaphase I: Nuclear envelope breaks down and spindle forms. Homologous pairs line up at the equator attached by centromeres

Anaphase I: Homologous chromosomes are pulled apart towards opposite poles of the spindle

Telophase I: Nuclear envelope reforms around daughter nuclei and chromosomes decondense to some extent.

Prophase II: Chromosomes condense and centrioles replicate

Metaphase II: Nuclear envelope breaks down and spindle forms. Chromosomes attach to spindle.

Anaphase II: Chromatids separate at their centromeres and move to opposite poles of spindle.

Telophase II: Chromosomes consisting of one chromatid decondense. Nuclear envelope reforms and cytoplasm divides.

Question 18

Describe the process of crossing over in meiosis

Answer 18

1. Homologous chromosomes pair up
2. Breakages occur in parallel non-sister chromatids at identical points
3. Rejoining of non-sister chromatids forms chiasmata.

Question 19

Describe the ways in which genetic variation occurs during meiosis

Answer 19

• Independent assortment: The way the chromosomes of each homologous pair line up is random. This means which chromosome goes to which pole is random.
• Crossing over: Formation of chiasmata generates great variation as there are many combinations of genes on chromosomes as a result.
• Random fusion of gametes

Question 20

Define gene and locus

Answer 20

Gene: A length of DNA that codes for a protein

Locus: The position of a gene on a chromosome

Question 21

Define the terms allele, dominant, recessive and codominant

Answer 21

Allele: One of two or more alternative nucleotide sequences at a gene locus. They are variant forms of a gene.

Dominant: An allele which has a phenotype that is always expressed when the allele is present.

Recessive: An allele which has a phenotype that is not expressed when the dominant allele is present.

Codominant: Alleles that are both expressed if present together in a heterozygous organism.

Question 22

Define genotype and phenotype

Answer 22

Genotype: the particular alleles of a gene on the specific chromosomes.

Phenotype: The physical expression of the alleles of a gene.

Question 23

Define homozygous and heterozygous

Answer 23

Homozygous: a diploid organism that has the same allele of a gene on both copies of the homologous chromosomes.

Heterozygous: A diploid organism that has different alleles of a gene on each homologous chromosome.

Question 24

Define a test cross

Answer 24

Testing a suspected heterozygote by crossing it with a known homozygous recessive. If any homozygous recessive offspring is produced then the organism is heterozygous.

Question 25

Define autosomal and sex linkage

Answer 25

Autosomal linkage: When multiple genes are close together on the same chromosome resulting in them acting like one gene.

Sex linkage: Genes that are more likely to be expressed in males due to the short length of the Y chromosome meaning it does not mask the X chromosome if a recessive allele is present on the chromosome.

Question 26

Describe how different mutations affect the phenotype

Answer 26

Base addition: Brings about frame shifts in the code which can change the tertiary structure or to cause no protein to be coded for.

Base deletion: Brings about frame shifts in the code which can change the tertiary structure or to cause no protein to be coded for.

Base substitution: May cause protein to be changed but more often has no effect due to amino acids having more than one triplet code.

Question 27

Describe the mutation that leads to sickle cell anaemia

Answer 27

• base substitution occurs
• A replaced by T causing glutamic acid to be replaced by valine
• the molecules of the mutated haemoglobin tend to clump together and form long fibres that cause the red blood cells to become sickle shaped.
• The alleles are codominant meaning heterozygous individuals are mildly anaemic and have the sickle cell trait.

Question 28

Describe the mutation that leads to albinism

Answer 28

• glutamine is replaced by arginine resulting in a gene that fails to code for the tyrosinase protein.
• active tyrosinase is absent and so melanin is absent from pigment forming cells in the body.
• the mutated allele is recessive so albinism only occurs in homozygous recessive individuals.

Question 29

Describe the mutation that leads to Huntington’s disease

Answer 29

• due to an autosomal dominant allele
• CAG is repeated between 36 and 120 times for the mutation
• causes progressive mental deterioration

Question 30

Describe continuous and discontinuous variation

Answer 30

Continuous: Results in a continuous distribution of values. These are usually controlled by groups of genes.

Discontinuous: arises when the characteristic concerned is one of two or more discrete types with no intermediate forms. Usually controlled by one or few genes.

Question 31

Describe how the environment may affect the phenotype with examples

Answer 31

Phenotype = genotype + influences of the environment.

• If plants of a tall variety are deprived of nutrients when growing, full size may not be reached and a tall plant may appear dwarf.
• honey bees have three phenotypes but only two genotypes. The queen differs from workers as she is fed protein rich food in the larval stage.

Question 32

Explain why variation is important in selection

Answer 32

• as the environment changes, some individuals are better placed to survive and prosper.
• This is because they have abilities and features that better suit their survival.
• If there is no variation, environmental changes may mean that individuals lack the features required to survive and so the species may go extinct.

Question 33

Describe the term struggle for existence

Answer 33

• Natural populations have the potential to increase rapidly in numbers provided they have access to the essential resources they require.
• The size of the population are limited by environmental factors which means the environment can only support a certain number of organisms.
• These factors may be abiotic, the chemical and physical aspects of the environment, and the biotic, the interactions between organisms.

Question 34

Describe the different types of natural selection

Answer 34

Stabilising selection: Occurs when environmental conditions are largely unchanging. Maintains favourable characteristics and eliminates variants and abnormalities that are useless or harmful.

Directional selection: May result from changing environmental conditions. The majority of an existing form of organism may no longer be best suited for the environment. Some other phenotypes may have a selective advantage.

Disruptive selection: Environment conditions favour the extremes of a phenotypic range over intermediate phenotypes meaning the gene pool becomes split into two distinct gene pools.

Question 35

Describe the factors that cause allele frequencies to change in populations

Answer 35

• Selection
• Genetic drift: Chance disappearance of particular alleles as individuals die and do not reproduce
• Founder effect: A small group of a large breeding population may become isolated creating a population in which some alleles may be lost
• Emigration/Immigration: Introduces new alleles into population
• Mutation: Random, spontaneous changes in the genes that occur in gonads leading to new characteristics in offspring.

Question 36

Define evolution

Answer 36

The gradual development of life over time in terms of the accumulation of genetic differences.

Question 37

State sources of genetic variation

Answer 37

• Mutations: chromosome mutations, gene mutations
• Random assortment of chromosomes during meiosis
• Recombination of segments of homologous chromosomes (crossing over)
• Random fusion of male and female gametes in sexual reproduction

Question 38

Describe the molecular evidence for evolutionary relationships

Answer 38

Evidence from protein sequence data:

• biochemical commonality is a way of measuring how close organisms are related
• Relatedness can be measured by amount of difference between specific molecules such as DNA, proteins and enzyme systems

Mitochondrial DNA:

• short, circular and changes occur at a quicker rate than chromosomal DNA
• at fertilisation, all mtDNA comes from the egg and so differences are easier to interpret

Question 39

Define speciation and the types of speciation

Answer 39

Speciation: When there is no gene flow between populations resulting in them evolving to a point where they cannot reproduce and have fertile offspring and so are not of the same species.

Allopatric speciation: Due to the physical separation of the gene pool by geographical isolation.

Sympatric speciation: Due to an isolating mechanism within a gene pool preventing production of viable organism

Question 40

Describe the different ways reproductive isolation occurs

Answer 40

Pre-zygotic:

• Habitat differences prevent meeting
• Behavioural differences e.g. mating rituals
• Temporal differences
• Mechanical differences, sex organs incompatible
• Gametic differences, gametes do not recognise each other

Post zygotic:

• Hybrids are not viable and die prematurely
• Hybrids are infertile
• Hybrids formed have low fertility

Question 41

Define structural and regulatory genes

Answer 41

Structural genes: Code for a structural protein, enable functioning of cell or required to create or maintain the structure.

Regulatory genes: are involved in controlling the expression of one or more genes

Question 42

Define inducible and repressible enzymes

Answer 42

Inducible enzymes: Produced under specific conditions such as presence of particular substrate. In absence of substrate, gene action is switched off.

Repressible enzymes: Produced continuously and are formed unless a signal, such as an excess of product, turns their production off.

Question 43

Describe gene regulation in the lac operon

Answer 43

If lactose is present:

1. Lactose combines with repressor and inhibits its action
2. RNA polymerase binds to promoter gene and transcribes mRNA using gene so that lactase will be translated.
3. Lactase metabolises all lactose and repressor will bind to operator again.

Without lactose:

1. Repressor binds to operator
2. RNA polymerase is obstructed from binding to promoter.

Question 44

Describe gene regulation in eukaryotes

Answer 44

• Transcription factors must first bind to promoter to allow transcription to occur.
• Rate of transcription can be increased by binding specific transcription factors on enhancer site of gene.
• When an activator protein binds to the enhancer site it makes contact with the polymerase-transcription factor complex. The rate of gene expression is increased.

Question 45

Describe how gibberellin activates genes

Answer 45

• transcription factors are bound to DELLA proteins in nucleus.
• As GA increases it labels the DELLA proteins with ubiquitin and the protein is degraded by another complex
• This allows GA to trigger the expression of genes coding for hydrolytic enzymes.