AQA A Level Biology Topic 5: Energy Transfers in and between Organisms

Comprehensive question-and-answer flashcard set covering AQA A Level Biology Topic 5: Energy Transfers in and between Organisms, including photosynthesis, respiration, required practicals 7-9, ecosystems, and nutrient cycles.

What are the two main stages of photosynthesis and where in the chloroplast does each stage occur?

1. Light-dependent reaction: occurs in the thylakoid membrane of the chloroplast.

2. Light-independent reaction: occurs in the stroma of the chloroplast.

What structural features of the chloroplast are depicted in this diagram?

The double membrane, the stroma (containing the thylakoid membrane, small / 70S ribosomes, circular DNA, and starch granules / lipid droplets), lamellae (thylakoid linking grana), and grana (stacks of thylakoids).

Describe the process of photoionisation in the light-dependent reaction of photosynthesis.

Chlorophyll absorbs light energy which excites its electrons to a higher energy level, causing electrons to be released from chlorophyll so that chlorophyll becomes positively charged.

How is energy from released electrons conserved to produce ATP and reduced NADP during the light-dependent reaction as shown in this thylakoid membrane diagram?

1. Electrons move along the electron transfer chain releasing energy.
2. This energy is used to actively pump protons from the stroma into the thylakoid.
3. Protons move by facilitated diffusion down an electrochemical gradient into the stroma via ATP synthase.
4. Energy is used to join ADP and Pi to form ATP (photophosphorylation).
5. NADP accepts a proton and an electron to become reduced NADP.

What is the chemical equation for the photolysis of water in the light-dependent reaction, and what is the role of the released electrons?

The equation is $H_2O \rightarrow \frac{1}{2} O_2 + 2e^- + 2H^+$. The released electrons replace those lost from chlorophyll.

Describe the steps of the light-independent reaction (Calvin cycle) as illustrated in this diagram.

1. $CO_2$ reacts with ribulose bisphosphate (RuBP), catalysed by rubisco, forming 2 glycerate 3-phosphate (GP) molecules.
2. GP is reduced to triose phosphate (TP) using reduced NADP and energy from ATP.
3. Some TP is converted to useful organic substances (e.g. glucose).
4. Some TP is used to regenerate RuBP in the Calvin cycle using energy from ATP.

Describe and explain how temperature affects the rate of photosynthesis at stages 1 and 2 on this curve.

1. As temperature increases, rate increases because enzymes like rubisco gain kinetic energy, forming more enzyme-substrate (E-S) complexes.
2. Above an optimum temperature, rate decreases because enzymes denature as hydrogen bonds in their tertiary structure break, resulting in fewer E-S complexes forming.

Describe and explain how light intensity affects the rate of photosynthesis at regions 1 and 2 on this graph.

1. As light intensity increases, rate increases because the light-dependent reaction increases (e.g., more photoionisation of chlorophyll), producing more ATP and reduced NADP for the light-independent reaction.
2. Above a certain light intensity, rate stops increasing because another factor becomes limiting (e.g. temperature or $CO_2$ concentration).

Describe and explain how $CO_2$ concentration affects the rate of photosynthesis as shown on this curve.

1. As $CO_2$ concentration increases, rate increases because the light-independent reaction increases as more $CO_2$ combines with RuBP to form GP, which is reduced to TP.
2. Above a certain $CO_2$ concentration, rate stops increasing because another factor is limiting (e.g. temperature or light intensity).

What key financial and biological considerations must be evaluated regarding agricultural practices used to overcome limiting factors?

Practices should increase photosynthesis rate and yield (more glucose for faster respiration, yielding more ATP for growth), but the profit gained from the extra yield must be greater than the financial and environmental costs.

Describe the steps for isolating plant leaf pigments using paper chromatography as shown in this apparatus setup.

1. Crush leaves with solvent to extract pigments.
2. Draw a pencil line on filter/chromatography paper, $1\,cm$ above the bottom.
3. Add a drop of extract to the line (point of origin).
4. Stand paper in a boiling tube of organic solvent below the point of origin.
5. Add a lid and leave to run as solvent moves up carrying dissolved pigments.
6. Remove before solvent reaches the top and mark the solvent front with pencil.

In paper chromatography of photosynthetic pigments, why must the origin line be drawn in pencil rather than ink?

Ink is soluble in the solvent, so it would mix with the pigments and move up the paper with the line.

Why must the point of origin be placed above the level of the solvent in paper chromatography?

Pigments are soluble in the solvent, so if immersed, they would run off the paper or dissolve directly into the solvent.

How is the Rf value calculated, and why are Rf values used instead of absolute distances moved by pigment spots?

$\text{Rf value} = \frac{\text{distance moved by spot}}{\text{distance moved by solvent front}}$. Rf values are used because solvent and pigment move different absolute distances across runs, but the Rf value is constant for a given pigment and can be compared.

What is the biological role of the enzyme dehydrogenase in photosynthesis?

Dehydrogenase catalyses the reduction of NADP in the light-dependent reaction as NADP accepts electrons from the photoionisation of chlorophyll or photolysis of water.

In Required Practical 8, what are the roles of Tube A (Control 1), Tube B (Control 2), and Tube C (Standard) shown in this setup?

Tube A (covered in foil) shows light is required for DCPIP to decolourise; Tube B (without chloroplasts) shows chloroplasts are required; Tube C (without DCPIP) acts as a colour standard to identify the endpoint.

Why does DCPIP turn from blue to colourless during the dehydrogenase activity experiment?

DCPIP is a redox indicator that acts as an electron acceptor; it becomes reduced by electrons released during the photoionisation of chlorophyll.

What is a key limitation of determining the endpoint in the DCPIP dehydrogenase experiment, and how can it be overcome quantitatively?

The endpoint colour change is subjective. It can be overcome by using a colorimeter to measure light absorbance of the sample at set time intervals after zeroing the colorimeter using the colour standard.

Identify the structural features of a mitochondrion shown in this diagram and state what the matrix contains.

Outer membrane, cristae (inner membrane folds), and matrix. The matrix contains small (70S) ribosomes and circular DNA.

What are the four stages of aerobic respiration and where does each stage occur?

1. Glycolysis: cytoplasm (anaerobic).
2. Link reaction: mitochondrial matrix.
3. Krebs cycle: mitochondrial matrix.
4. Oxidative phosphorylation: inner mitochondrial membrane.

Describe the main steps and net yields of glycolysis as shown in this pathway diagram.

1. Glucose is phosphorylated to glucose phosphate using inorganic phosphates from 2 ATP.
2. Hydrolysed to 2 triose phosphate molecules.
3. Triose phosphate is oxidised to 2 pyruvate, reducing 2 NAD and regenerating 4 ATP (giving a net gain of 2 ATP).

What happens to pyruvate in anaerobic respiration in animals versus plants/yeast, and why is this step necessary?

Pyruvate is converted to lactate in animals (and some bacteria) or ethanol and $CO_2$ in plants and yeast. This oxidises reduced NAD back to NAD so glycolysis can continue producing ATP.

Why does anaerobic respiration produce significantly less ATP per molecule of glucose than aerobic respiration?

Anaerobic respiration involves only glycolysis, which produces a net gain of only 2 ATP molecules, whereas aerobic respiration includes oxidative phosphorylation which yields the majority of ATP (around 34 molecules).

Describe the link reaction and state its products per molecule of glucose.

Pyruvate is oxidised and decarboxylated to acetate (producing $CO_2$ and reduced NAD), then acetate combines with coenzyme A to form Acetyl Coenzyme A. Products per glucose: 2 Acetyl Coenzyme A, 2 $CO_2$, and 2 reduced NAD.

Describe the Krebs cycle as shown in this diagram, and list its products per molecule of glucose.

Acetyl Coenzyme A (2C) combines with a 4C molecule to form a 6C molecule, releasing CoA. A series of redox reactions regenerates the 4C molecule, losing 2 $CO_2$, reducing NAD and FAD, and producing ATP via substrate-level phosphorylation. Products per glucose: 6 reduced NAD, 2 reduced FAD, 2 ATP, and 4 $CO_2$.

Describe the process of oxidative phosphorylation on the inner mitochondrial membrane as illustrated in this diagram.

1. Reduced NAD/FAD oxidised to release H atoms, split into $H^+$ and $e^-$.
2. Electrons move along the ETC via redox reactions.
3. Energy released by electrons actively pumps $H^+$ from matrix into intermembrane space.
4. $H^+$ diffuses back into matrix down an electrochemical gradient via ATP synthase, synthesising ATP from ADP + Pi.
5. Oxygen acts as the final electron acceptor, combining with protons and electrons to form water.

How are lipids and proteins utilised as alternative respiratory substrates in respiration?

Fatty acids from lipid hydrolysis are converted into Acetyl Coenzyme A. Amino acids from protein hydrolysis are converted into intermediate compounds in the Krebs cycle.

Explain why the liquid droplet moves towards the organism in a respirometer measuring aerobic respiration, as shown in this diagram.

Organisms aerobically respire, taking in $O_2$ and releasing $CO_2$, which is absorbed by sodium hydroxide solution. The volume of gas and pressure inside decrease, causing fluid in the capillary tube to move down a pressure gradient towards the organism.

Why is a respirometer apparatus left open for 10 minutes before starting measurements, and why must it be airtight during the experiment?

Left open to allow the apparatus to equilibrate, accommodate overall pressure changes, and stabilize the respiration rate. It must be airtight to prevent outside air from entering or leaving, which would alter volume/pressure and affect fluid movement.

How is the volume of oxygen consumed calculated in a capillary respirometer, and what are standard units for rate of respiration per mass?

Volume is calculated by finding the cross-sectional area of the capillary tube ($\pi r^2$) and multiplying by the distance moved by the liquid. Units for respiration rate are volume per unit time per unit mass, e.g., $\text{cm}^3\,\text{min}^{-1}\,\text{g}^{-1}$.

How is a respirometer adapted to measure anaerobic respiration, and why does the liquid move away from the yeast?

The chemical absorbing $CO_2$ is removed and an oil/paraffin layer is added above yeast to create anaerobic conditions. The liquid moves away because yeast anaerobically respire releasing $CO_2$, increasing gas volume and pressure.

How do redox indicator dyes like methylene blue measure respiration rate as depicted in this procedure, and why must the tube NOT be shaken after adding the indicator?

Methylene blue accepts electrons released during respiration, becoming reduced and turning colourless (rate = $1 / \text{time taken}$). The tube must not be shaken because oxygen from air would re-oxidise methylene blue, returning it to its original blue colour.

How can biomass be measured, and how is dry mass of tissue determined experimentally?

Biomass is measured as mass of carbon or dry mass of tissue per given area. Dry mass is determined by drying a sample in an oven at $100^\circ\text{C}$ and weighing it at regular intervals until the mass remains constant (all water evaporated).

Explain how the structural features of a bomb calorimeter (shown here) enable accurate measurement of chemical energy stored in dry biomass.

Combusting dry biomass heats a known volume of water. The stirrer evenly distributes heat energy in the water, air/insulation minimizes heat loss to surroundings, and water's high specific heat capacity allows accurate calculation of released energy.

Define Gross Primary Production (GPP) and Net Primary Production (NPP), and state the mathematical relationship between them.

GPP is the total chemical energy store in plant biomass in a given area/volume in a given time. NPP is the chemical energy store remaining after accounting for respiratory losses ($R$). Formula: $\text{NPP} = \text{GPP} - R$.

What is the formula for net production of consumers ($N$), and what do its variables represent?

Formula: $N = I - (F + R)$, where $I$ is the chemical energy store in ingested food, $F$ is chemical energy lost in faeces and urine, and $R$ is respiratory losses to the environment.

What units are used for primary or secondary productivity, and why are area and time included in these units?

Units: $\text{kJ}\,\text{ha}^{-1}\,\text{year}^{-1}$. Per unit area standardises results to compare environments of different sizes; per year accounts for seasonal variations in temperature and biomass.

How do crop farming practices increase the efficiency of energy transfer to human food chains?

They simplify food webs to reduce energy loss to non-human chains (herbicides kill weeds to remove light competition; pesticides kill pests; fungicides prevent fungal infections) and use fertilisers (e.g. nitrates) to prevent poor growth.

How do livestock farming practices increase the efficiency of energy transfer?

By reducing respiratory losses (restricting movement and keeping animals warm), slaughtering animals while young when growth rate is highest, giving antibiotics to prevent pathogen energy loss, and selective breeding for rapid growth.

What are the distinct roles of saprobionts and mycorrhizae in nutrient cycling?

Saprobionts decompose organic compounds in dead matter/waste via extracellular digestion (secreting enzymes) and release mineral ions. Mycorrhizae are symbiotic fungal associations with plant roots that increase surface area for water and inorganic ion absorption.

Describe the four main biological processes and bacterial roles in the nitrogen cycle as shown in this diagram.

1. Nitrogen fixation: Nitrogen-fixing bacteria convert $N_2$ gas to ammonia/ammonium ions.
2. Ammonification: Saprobionts break down organic nitrogen compounds to ammonia/ammonium ions.
3. Nitrification: Nitrifying bacteria oxidise ammonium ions to nitrites then nitrates.
4. Denitrification: Denitrifying bacteria convert nitrates back to $N_2$ gas in anaerobic conditions.

Why does ploughing (aerating) soil increase soil fertility?

Aeration creates aerobic conditions, which increases nitrification by nitrifying bacteria (converting more ammonium to nitrites/nitrates) and decreases denitrification by denitrifying bacteria (preventing conversion of nitrate to nitrogen gas).

Describe the main steps of the phosphorus cycle.

1. Weathering/erosion of rocks releases phosphate ions into soil/water.
2. Producers absorb phosphate ions (aided by mycorrhizae) into biomass.
3. Phosphate ions pass through food chains as consumers eat producers.
4. Phosphate ions are excreted in waste.
5. Saprobionts decompose dead organic matter, releasing phosphate ions back into soil/water.

Explain the process of eutrophication resulting from fertiliser leaching.

1. Leached nitrates/phosphates cause rapid algal growth (algal bloom), blocking light.
2. Submerged plants die due to lack of light for photosynthesis.
3. Aerobically respiring saprobionts decompose dead plants, consuming oxygen.
4. Dissolved oxygen levels drop, causing fish to die from lack of aerobic respiration.

What are two key advantages of natural organic fertilisers over artificial inorganic fertilisers?

Natural fertilisers are less water-soluble, reducing leaching and eutrophication risk. They also require slow breakdown by saprobionts, resulting in a controlled, gradual release of nitrates and phosphates.