5. Energy Transfers In & Between Organisms : Key Terms

• Acetyl coenzyme A: A two-carbon molecule formed during the link reaction when acetate combines with coenzyme A. It is then oxidised in the Krebs cycle.
• Adenosine triphosphate (ATP): A universal energy molecule found in all living cells.
• Aerobic respiration: Cellular respiration that occurs in the presence of oxygen, producing carbon dioxide, water, and ATP. It involves four stages: glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation. The overall equation:

$$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$$

• Ammonification: The conversion of organic nitrogen-containing compounds into ammonia by saprobiontic microorganisms, resulting in ammonium ions being added to the soil.
• Anaerobic respiration: Cellular respiration that occurs without oxygen. In animals, lactate is produced, while in plants and microorganisms, ethanol and carbon dioxide are formed. Less ATP is generated compared to aerobic respiration.
• Artificial fertilisers: Man-made substances, often containing nitrogen, phosphorus, and potassium, used to enrich soil nutrients.
• ATP synthase: An enzyme embedded in cellular membranes that synthesises ATP from ADP as protons pass through it.
• Biomass: The total mass of organic material in a specific area over a defined time, measured as dry mass or carbon content.
• Calorimetry: A method for estimating the chemical energy content of dry biomass.
• Carnivores: Animals that consume other animals for energy, functioning as secondary or tertiary consumers.
• Chemiosmotic theory: The production of ATP driven by the movement of protons down their concentration gradient through ATP synthase.

• Chlorophyll: A pigment in chloroplast thylakoids that absorbs light energy and becomes ionised during photosynthesis.
• Coenzymes: Molecules that assist enzyme activity, such as NAD, FAD, and NADP.
• Consumers: Organisms that gain energy by feeding on other organisms.
• Denitrification: The reduction of nitrate ions into nitrogen gas by denitrifying bacteria in anaerobic conditions like waterlogged soils.
• Denitrifying bacteria: Microorganisms that convert nitrate ions to nitrogen gas in oxygen-poor environments.
• Ecosystem: A system comprising living (biotic) organisms and non-living (abiotic) components interacting within a specific area.
• Efficiency of energy transfer: The ratio of energy transferred between trophic levels, calculated as:

$$\text{Efficiency} = \frac{\text{Energy transferred}}{\text{Total energy available}} \times 100$$

• Electron acceptor: Oxygen acts as the final electron acceptor in the electron transport chain:

$$\frac{1}{2}O_2 + 2e^- + 2H^+ \rightarrow H_2O$$

• Electron transfer chain: A series of proteins that transfer electrons through oxidation-reduction reactions, facilitating ATP synthesis.

• Eutrophication: The excessive enrichment of water bodies with nutrients, often from fertilisers, leading to environmental imbalances.

• FAD: A molecule that accepts protons and electrons during the Krebs cycle, forming reduced FAD.

• Food chain: A representation of feeding relationships and energy transfer in a sequence: $\text{Producer} \rightarrow \text{Primary consumer} \rightarrow \text{Secondary consumer} \rightarrow \text{Tertiary consumer}$

• Food web: A network of interconnected food chains within an ecosystem.

• Glycerate 3-phosphate (GP): A three-carbon molecule reduced to triose phosphate (TP) using reduced NADP and ATP during the light-independent stage of photosynthesis.
• Glycolysis: The initial stage of respiration occurring in the cytosol, breaking glucose into two pyruvate molecules, producing ATP and reduced NAD.
• Gross primary production (GPP): The total chemical energy stored in plant biomass in a specific area or volume.
• Herbivores: Primary consumers that feed exclusively on plants.
• Krebs cycle: A series of reactions in the mitochondrial matrix that oxidise acetyl coenzyme A, producing reduced NAD, reduced FAD, ATP, and carbon dioxide.
• Leaching: The loss of nutrients from soil due to rainwater.
• Light-dependent reaction: The stage of photosynthesis where light energy produces ATP, reduced NADP, and oxygen in the thylakoids.
• Light-independent reaction (Calvin cycle): The photosynthesis stage using carbon dioxide, ATP, and reduced NADP to produce sugars in the chloroplast stroma.
• Limiting factor: A variable that restricts the rate of a process, such as light intensity in photosynthesis.
• Link reaction: A step in aerobic respiration that converts pyruvate into acetyl coenzyme A, producing reduced NAD and carbon dioxide.

$$\text{Pyruvate} + \text{NAD} + \text{CoA} \rightarrow \text{Acetyl CoA} + \text{Reduced NAD} + \text{CO}_2$$

• Mycorrhizae: Symbiotic relationships between fungi and plant roots, enhancing water and nutrient absorption.
• NAD: A molecule that accepts protons and electrons in aerobic respiration, forming reduced NAD.
• NADP: A molecule that accepts protons and electrons during the light-dependent reaction, forming reduced NADP.
• Natural fertilisers: Organic materials like compost or manure used to improve soil nutrient content.
• Net primary productivity (NPP): The remaining chemical energy in plants after respiration losses:

$$\text{NPP} = \text{GPP} - \text{Respiratory losses (R)}$$

• Net production of consumers (N): The chemical energy stored in food minus losses due to respiration and excretion:

$$N = I - (F + R)$$

where $I$ is ingested energy, $F$ is energy lost in waste, and is energy used in respiration.

• Nitrification: The oxidation of ammonium ions into nitrite and nitrate ions by nitrifying bacteria.
• Nitrifying bacteria: Aerobic microorganisms responsible for converting ammonium to nitrate ions.
• Nitrogen cycle: The process by which nitrogen moves through living organisms and the environment, involving nitrogen fixation, nitrification, ammonification, and denitrification.
• Nitrogen fixation: The conversion of atmospheric nitrogen into ammonia by nitrogen-fixing bacteria in soil or plant root nodules.
• Nitrogen-fixing bacteria: Microorganisms that fix atmospheric nitrogen into usable nitrogen compounds.
• Oxidation: The loss of electrons or hydrogen or the gain of oxygen.
• Oxidative phosphorylation: ATP synthesis during the electron transfer chain, using reduced coenzymes and oxygen.
• Phosphorus cycle: The movement of phosphate ions between organisms and the environment, involving uptake, digestion, and erosion.
• Photoionisation: The process where chlorophyll absorbs light, exciting electrons to a higher energy level.
• Photolysis: The splitting of water using light energy during photosynthesis, producing oxygen, protons, and electrons:

$$H_2O \rightarrow 2H^+ + 2e^- + \frac{1}{2}O_2$$

• Photosynthesis: A process where plants convert light energy into chemical energy through light-dependent and light-independent reactions:

$$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$$

• Primary productivity: The rate of energy fixation by photosynthesis in a given area over time.
• Producers: Photosynthetic organisms that form the base of the food chain by creating biomass.
• Pyruvate: A three-carbon molecule formed in glycolysis, converted into acetate during aerobic respiration or lactate/ethanol in anaerobic conditions.
• Reduction: The gain of electrons or hydrogen or the loss of oxygen.
• Ribulose bisphosphate (RuBP): A five-carbon molecule that reacts with carbon dioxide in the Calvin cycle to form glycerate 3-phosphate.
• Rubisco: The enzyme catalysing the reaction between RuBP and carbon dioxide.
• Saprobionts: Decomposers that break down dead material into simpler organic substances.
• Secondary productivity: The rate at which consumers convert ingested energy into biomass.
• Substrate-level phosphorylation: ATP formation by transferring a phosphate group from an intermediate compound to ADP.
• Triose phosphate (TP): A three-carbon molecule in the Calvin cycle that can form sugars or regenerate RuBP.
• Trophic level: The position of an organism within a food chain.