5. Energy Transfers In & Between Organisms: Definitions (A Level only) (AQA A-Level Biology): Revision Notes

5. Energy Transfers In & Between Organisms: Definitions (A Level only)

Acetyl coenzyme A: A two-carbon compound formed during the link reaction when acetate combines with coenzyme A. This molecule is subsequently oxidised in the Krebs cycle to release energy.

Adenosine triphosphate (ATP): The universal energy currency found in all living cells, providing readily available energy for cellular processes through the hydrolysis of its phosphate bonds.

Aerobic respiration: A form of cellular respiration occurring in the presence of oxygen, producing carbon dioxide, water and ATP. The process involves four main stages: glycolysis, link reaction, Krebs cycle, and oxidative phosphorylation. The overall equation is:

$\text{C}*6\text{H}*{12}\text{O}\_6 + 6\text{O}\_2 \rightarrow 6\text{CO}\_2 + 6\text{H}\_2\text{O}$

Ammonification: The process by which saprobiontic microorganisms decompose organic nitrogen-containing compounds, releasing ammonium ions which are subsequently added to soil.

Anaerobic respiration: A form of cellular respiration occurring without oxygen. In animals, this produces lactate, while in plants and microorganisms, ethanol and carbon dioxide are produced. This process yields less ATP than aerobic respiration.

Artificial fertilisers: Manufactured compounds typically containing nitrogen, phosphorus and potassium that are applied to soils to increase their mineral content and enhance plant growth.

ATP synthase: An enzyme embedded in cellular membranes that catalyses the formation of ATP from ADP as protons flow through it down their concentration gradient.

Biomass: The total mass of organic material in a specified area measured over a defined time period. This can be expressed as dry mass or mass of carbon per given area.

Calorimetry: An experimental technique used to determine the chemical energy stored in dry biomass by measuring the heat released during combustion.

Carnivores: Animals that feed on other animals, occupying secondary or tertiary consumer positions in food webs.

Chemiosmotic theory: The mechanism of ATP synthesis involving the movement of protons down their concentration gradient across a semipermeable membrane, catalysed by ATP synthase.

Chlorophyll: A photosynthetic pigment found in the thylakoids of chloroplasts that absorbs light energy and becomes ionised during photosynthesis.

Coenzymes: Helper molecules that assist enzymes in carrying out their functions, examples include NAD, FAD, and NADP.

Consumers: Organisms that obtain energy by feeding on other organisms, unable to produce their own food through photosynthesis.

Denitrification: The conversion of nitrate ions to nitrogen gas by denitrifying bacteria, typically occurring in waterlogged soils with limited oxygen.

Denitrifying bacteria: Anaerobic microorganisms found in waterlogged soils that reduce nitrate ions to nitrogen gas, completing the nitrogen cycle.

Ecosystem: The community of living organisms (biotic factors) and non-living components (abiotic factors) in an area, including all their interactions.

Efficiency of energy transfer: The effectiveness of energy transfer between trophic levels, calculated using:

$\text{percentage efficiency} = \frac{\text{energy available after transfer}}{\text{energy available before transfer}} \times 100$

Electron acceptor: Oxygen serves as the final electron acceptor in the electron transfer chain:

$\frac{1}{2}\text{O}\_2 + 2\text{e}^- + 2\text{H}^+ \rightarrow \text{H}\_2\text{O}$

Electron transfer chain: A series of electron carrier proteins that transfer electrons through a sequence of oxidation-reduction reactions during respiration.

Eutrophication: The process whereby a body of water becomes excessively enriched with nutrients, often from fertiliser runoff, leading to algal blooms and oxygen depletion.

FAD: A carrier molecule that becomes reduced when it accepts protons and electrons during the Krebs cycle, forming reduced FAD.

Food chain: A linear sequence describing feeding relationships between organisms and the resulting stages of biomass transfer: producer → primary consumer → secondary consumer → tertiary consumer

Food web: The complex interconnection of multiple food chains within a habitat, showing the feeding relationships between different organisms.

Glycerate 3-phosphate (GP): A three-carbon molecule formed when ribulose bisphosphate is reduced by reduced NADP in the light-independent stage of photosynthesis, leading to the formation of two molecules of triose phosphate. This process requires ATP.

Glycolysis: The initial stage of both aerobic and anaerobic respiration that occurs in the cytosol, breaking down glucose into two molecules of pyruvate. This process also produces ATP and reduced NAD.

Gross primary production (GPP): The total amount of chemical energy stored in plant biomass within a specified area or volume over a given time period.

Herbivores: Animals that consume plants, also referred to as primary consumers in food webs.

Krebs cycle: A series of oxidation-reduction reactions occurring in the mitochondrial matrix where acetyl coenzyme A is oxidised, generating reduced NAD, reduced FAD, ATP and carbon dioxide.

Leaching: The loss of nutrients from soil due to their dissolution and removal by rainwater percolation.

Light-dependent reaction: The second stage of photosynthesis that utilises light energy to synthesise ATP, reduced NADP and oxygen (as a by-product). This stage occurs in the thylakoids of chloroplasts.

Light-independent reaction: The third stage of photosynthesis, also known as the Calvin cycle, where products from the light-dependent stage combine with carbon dioxide to form simple sugars. This stage does not require light energy and occurs in the stroma of chloroplasts.

Limiting factor: A variable that restricts the rate of a particular biological reaction or process.

Link reaction: The second stage of aerobic respiration occurring in the mitochondrial matrix, converting pyruvate into acetyl coenzyme A and carbon dioxide. Reduced NAD is also produced. The overall reaction is:

$\text{Pyruvate} + \text{NAD} + \text{CoA} \rightarrow \text{acetyl CoA} + \text{reduced NAD} + \text{CO}\_2$

Mycorrhizae: Mutualistic associations between certain fungal species and plant roots that help plants retain water and minerals around their root systems.

NAD: A carrier molecule that becomes reduced when it accepts protons and electrons during aerobic respiration, forming reduced NAD.

NADP: A carrier molecule that becomes reduced when it accepts protons and electrons during the light-dependent stage of photosynthesis, forming reduced NADP.

Natural fertilisers: Dead and decaying organic matter used to increase the mineral content of soils, providing nutrients for plant growth.

Net primary productivity (NPP): The chemical energy store remaining when energy losses due to respiration are subtracted from the total energy store. This energy is available for plant growth, reproduction, and other trophic levels:

$\text{NPP} = \text{GPP} - \text{R}$

Net production of consumers (N): Calculated by subtracting the chemical energy lost through respiration (R) and as a result of excretion and egestion (F) from the chemical energy stored in ingested food (I):

$\text{N} = \text{I} - (\text{F} + \text{R})$

Nitrification: The conversion of ammonium ions to nitrate ions by nitrifying bacteria, occurring in two stages: ammonium ions are oxidised to nitrite ions, then nitrite ions are oxidised to nitrate ions.

Nitrifying bacteria: Aerobic microorganisms found in soil that are responsible for the oxidation of ammonium ions to nitrate ions.

Nitrogen cycle: The biogeochemical cycle through which nitrogen moves between living organisms and the environment, involving ammonification, nitrification, nitrogen fixation and denitrification.

Nitrogen fixation: The conversion of atmospheric nitrogen gas into nitrogen-containing compounds by nitrogen-fixing bacteria in soil or root nodules of leguminous plants.

Nitrogen-fixing bacteria: Microorganisms that convert atmospheric nitrogen gas into nitrogen-containing compounds. These can be free-living or form mutualistic relationships with plants.

Oxidation: The loss of electrons, gain of oxygen, or loss of hydrogen in a chemical substance during a reaction.

Oxidative phosphorylation: The synthesis of ATP from reduced coenzymes and oxygen through the electron transfer chain during aerobic respiration.

Phosphorus cycle: The biogeochemical cycle through which phosphorus (in the form of phosphate ions) moves between living organisms and the environment through processes including absorption by plants, feeding, digestion, excretion, sedimentation, erosion, and decay.

Photoionisation: The process by which a chlorophyll molecule is ionised when it absorbs light energy, causing electrons to become excited and raised to higher energy levels before leaving the molecule.

Photolysis: The splitting of water molecules in the presence of light during the light-dependent stage of photosynthesis, producing protons, electrons and oxygen:

$\text{H}\_2\text{O} \rightarrow 2\text{H}^+ + 2\text{e}^- + \frac{1}{2}\text{O}\_2$

Photosynthesis: A complex metabolic pathway consisting of three main stages: light energy capture, light-dependent reaction, and light-independent reaction. The overall process in the presence of light is:

$6\text{CO}\_2 + 6\text{H}\_2\text{O} \rightarrow \text{C}*6\text{H}*{12}\text{O}\_6 + 6\text{O}\_2$

Primary productivity: The rate of primary production, representing the energy fixed by photosynthesis in a given area over a specified time period (measured in kJ ha⁻¹ year⁻¹).

Producers: Photosynthetic organisms at the beginning of food chains that manufacture biomass using light energy, carbon dioxide, water and mineral ions for all living organisms.

Pyruvate: A three-carbon molecule produced during glycolysis. In aerobic respiration, pyruvate is oxidised to acetate in the link reaction, while in anaerobic respiration it is converted to lactate (in animals) or ethanol and carbon dioxide (in plants and microorganisms).

Reduction: The gain of electrons, loss of oxygen, or gain of hydrogen in a chemical substance during a reaction.

Ribulose bisphosphate (RuBP): A five-carbon compound that reacts with carbon dioxide in the light-independent stage of photosynthesis to form two molecules of glycerate 3-phosphate.

Rubisco: An enzyme that catalyses the reaction between ribulose bisphosphate and carbon dioxide during the light-independent stage of photosynthesis.

Saprobionts: Microorganisms that decompose dead plant and animal material into simpler organic compounds to obtain nutrients. Also known as saprophytes.

Secondary productivity: The rate of secondary production, representing the rate at which animals convert chemical energy from plants they consume into their own biomass within a given area over a specified time period (measured in kJ ha⁻¹ year⁻¹).

Substrate-level phosphorylation: The synthesis of ATP through the direct transfer of a phosphate group from a phosphorylated intermediate compound to ADP.

Triose phosphate (TP): A three-carbon compound formed during the light-independent stage of photosynthesis that may be converted into useful organic substances or used to regenerate ribulose bisphosphate.

Trophic level: The position of an organism within a food chain, indicating its feeding relationship with other organisms in the ecosystem.