5.2 Energy for biological processes

Acetyl coenzyme A

​A two-carbon molecule formed in oxidative decarboxylation when an acetyl group is bound by coenzyme A. It is oxidised in the Krebs cycle.

Adenosine triphosphate (ATP)

​The universal energy carrier found in all living cells. Aerobic respiration

Alcoholic fermentation

​A type of fermentation that takes place in plant root cells and yeast cells, and produces ethanol and carbon dioxide.

Anaerobic respiration

​A form of cellular respiration that takes place in the absence of oxygen. It produces less ATP than in aerobic respiration.

ATP synthase

​An enzyme found embedded in cellular membranes that phosphorylates ADP to form ATP as protons flow through it.

Calvin cycle

​See ‘Light-independent reaction’.

Chemiosmotic theory

​The synthesis of ATP through the movement of protons down their concentration gradient across a semipermeable membrane, catalysed by ATP synthase.

Chlorophyll

​ A photosynthetic pigment located in the thylakoids of chloroplasts that absorbs light energy. There are two main types, chlorophyll an and chlorophyll b.

Chloroplast

​ An organelle found in plants and algae that is the site of photosynthesis.

Citrate

A six-carbon molecule formed in the first stage of the Krebs cycle from the reaction

of acetyl coenzyme A and oxaloacetate.

Coenzyme A

​A coenzyme that binds an acetyl group to form acetyl coenzyme A during oxidative decarboxylation of aerobic respiration.

Coenzymes

​ Molecules that help enzymes carry out their function e.g. NAD, FAD, coenzyme A and NADP.

Cristae

​Folds of the inner mitochondrial membrane that provide a large surface area for oxidative phosphorylation.

Cyclic photophosphorylation

​The formation of ATP involving photosystem I only.

Decarboxylation

​The removal of a carbon dioxide molecule.

Dehydrogenation

​The removal of a hydrogen atom.

Electron acceptor

​Oxygen acts as the final electron acceptor in the electron transfer chain

½ O₂ + 2e^- + 2H⁺ --> H₂O

Electron carriers

​Protein molecules that accept and release electrons e.g. NAD, NADP.

Electron transport chain

​A series of electron carrier proteins that transfer electrons in a chain of oxidation-reduction reactions.

FAD

​A coenzyme that becomes reduced when it takes up hydrogen atoms during the Krebs cycle, forming reduced FAD.

Fermentation

​A type of anaerobic respiration that does not involve an electron transport chain.

Glycerate 3-phosphate (GP)

​A three-carbon molecule that is reduced by reduced NADP in the light-independent stage of photosynthesis to form two molecules of TP. This requires ATP.

Glycolysis

​An anaerobic process that takes place in the cytosol of the cell and breaks down glucose into two molecules of pyruvate. Two molecules of ATP and two molecules of reduced NAD are also formed.

Grana

​Stacks of thylakoids connected by intergranal lamellae.

Hexose bisphosphate

​The compound formed from the phosphorylation of glucose in glycolysis. It splits into two molecules of triose phosphate.

Inner mitochondrial membrane

​The mitochondrial membrane that segregates the matrix from the intermembrane space. It is the site of the electron transport chain.

Intermembrane space

​The small space between the inner and outer mitochondrial membranes. The electron transport chain results in a high proton concentration here.

Krebs cycle

​A series of oxidation-reduction reactions in the matrix of the mitochondria in which acetyl coenzyme A is oxidised, generating reduced NAD, reduced FAD, ATP and carbon dioxide.

Lactate dehydrogenase

​An enzyme that catalyses the conversion of pyruvate to lactate. Lactate fermentation

Lamellae

​Membranous channels that connect adjacent grana in a chloroplast.

Light-dependent reaction

​The first stage of photosynthesis that uses light energy to produce ATP, reduced NADP and oxygen. It takes place in the thylakoids of the chloroplast.

Light-harvesting system

​A collection of protein and chlorophyll molecules found in the thylakoid membranes of chloroplasts that absorbs light energy of varying wavelengths and transfers it to the reaction centre. It is also known as an antennae complex.

Light-independent reaction

​The second stage of photosynthesis, also known as the Calvin cycle, in which the products of the light-dependent stage and carbon dioxide are used to build organic molecules. It does not require light energy and takes place in the stroma.

Limiting factor

​A variable that limits the rate of a particular reaction. Link reaction

Matrix

​The fluid-filled space within the inner membrane of the mitochondria which contains mitochondrial DNA and enzymes required for aerobic respiration.

Mitochondrion

​An organelle found in eukaryotic cells that is the site of aerobic respiration. NAD respiration, forming reduced NAD.

NADP

​A coenzyme that becomes reduced when it takes up hydrogen atoms during the light-dependent stage of photosynthesis, forming reduced NADP.

Non-cyclic photophosphorylation

​The formation of ATP and reduced NADP involving both photosystems I and II.

Outer mitochondrial membrane

​The membrane segregating the contents of the mitochondrion from the rest of the cell. It creates optimal conditions for aerobic respiration.

Oxaloacetate

​A four-carbon molecule that combines with acetyl coenzyme A to produce six-carbon citrate in the first stage of the Krebs cycle. It is eventually regenerated, allowing the cycle to continue.

Oxidative decarboxylation

​The first stage of aerobic respiration (also known as the ‘link reaction’) that takes place in the mitochondrial matrix and converts pyruvate into acetyl coenzyme A and carbon dioxide. Reduced NAD is also formed. Overall:

Pyruvate + NAD + CoA --> acetyl CoA + reduced NAD + CO₂

Oxidative phosphorylation

​The synthesis of ATP from reduced coenzymes and oxygen in the electron transport chain of aerobic respiration.

Photolysis

​The splitting of a molecule of water in the presence of light that occurs during the light-dependent stage of photosynthesis. This produces protons, electrons and oxygen:

H₂O --> 2H⁺ + 2e^- + ½ H₂O

Photophosphorylation

​The harnessing of light energy in photosynthesis to phosphorylate ADP, forming ATP.

Photosynthesis

​A complex metabolic pathway that synthesises organic molecules in the presence of light. It consists of three main stages:

1. capturing of light energy

2. light-dependent reaction

3. light-independent reaction.

Overall:

6CO₂ + 6H₂O —> C₆H₁₂O₆ + 6O₂

Photosynthetic pigments

​Molecules present in chloroplasts that absorb certain wavelengths of light e.g. chlorophyll a, chlorophyll b, xanthophylls and carotenoids.

Photosystem

​A protein complex consisting of a light-harvesting system and reaction centre, that is involved in the absorption of light and transfer of electrons in photosynthesis.

Pyruvate

​A three-carbon molecule produced in glycolysis. In the link reaction of aerobic respiration, it is oxidised to acetate. During fermentation, it is converted to lactate (animals) or ethanol and carbon dioxide (plants and microorganisms).

Reaction centre

​The region of a photosystem where energy is funneled and photosynthetic reactions take place. It contains two chlorophyll a molecules.

Respiration

​A set of metabolic reactions that take place in organisms and break down respiratory substances, such as glucose, into smaller inorganic molecules, like water and carbon dioxide. This is linked to the synthesis of ATP.

Respiratory quotient (RQ)

​The ratio of carbon dioxide produced to oxygen consumed during respiration. Calculated using:

RQ = CO₂ produced / O₂ consumed

Respiratory substrate

​An organic molecule that can be broken down via the respiratory pathways to produce ATP.

Respirometer

​A device used to determine respiration rate in living organisms by measuring the change in volume of oxygen or carbon dioxide.

Retention value (R​f)​ ​

Calculated using the equation:

Rf = distance travelled by solute / distance travelled by solvent

Ribulose bisphosphate (RuBP)

​A five-carbon compound that reacts with carbon dioxide in the light-independent stage of photosynthesis, forming two molecules of GP.

Ribulose bisphosphate carboxylase (RuBisCO)

​An enzyme that catalyses the reaction of RuBP and carbon dioxide in the light-independent stage of photosynthesis.

Stroma

​The fluid interior of chloroplasts that contains the enzymes required for the light-independent reaction.

Substrate-level phosphorylation

​The synthesis of ATP by the transfer of a phosphate group from a phosphorylated intermediate to ADP.

Thin-layer chromatography (TLC)

​A technique used to separate photosynthetic pigments by their rate of movement when carried by a solvent, across an inert surface. Differences in rate arise due to varying solubilities in the mobile phase and interactions with the stationary phase.

Thylakoids

​A series of flattened membrane-bound compartments in chloroplasts. They are stacked to form grana and contain the chlorophyll and other molecules needed for the light-dependent reaction.

Triose phosphate (TP)​

A three-carbon compound formed in glycolysis and the light-independent stage of photosynthesis. It may serve as a starting material for the formation of organic molecules or be used to regenerate RuBP.