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

VOCABULARY flashcards covering photosynthesis, respiration, ecosystems, and nutrient cycles from AQA A Level Biology Topic 5.

Photoionisation

The process where chlorophyll absorbs light energy which excites its electrons to a higher energy level, causing them to be released so the chlorophyll becomes positively charged.

Photolysis of water

The light-driven splitting of water into protons, electrons, and oxygen, described by the equation: $H_2O \rightarrow \frac{1}{2} O_2 + 2e^- + 2H^+$.

Chemiosmotic theory (in LDR)

The mechanism where energy from electrons moving along the electron transfer chain is used to actively pump protons into the thylakoid, creating a gradient that drives ATP synthesis via ATP synthase as protons move back into the stroma.

Photophosphorylation

The process of joining ADP and inorganic phosphate ($P_i$) to form ATP using energy derived from light during the light-dependent reaction.

Calvin cycle

The light-independent reaction of photosynthesis occurring in the stroma, where $CO_2$ reacts with ribulose bisphosphate (RuBP) to form glycerate 3-phosphate, which is then reduced to triose phosphate.

Rubisco

The enzyme that catalyses the reaction between carbon dioxide ($CO_2$) and ribulose bisphosphate (RuBP) in the light-independent reaction.

Triose phosphate (TP)

A three-carbon intermediate in the Calvin cycle formed by the reduction of glycerate 3-phosphate (GP) using reduced NADP and energy from ATP.

Rf value

A ratio used in chromatography to identify pigments, calculated as: $\text{Rf value} = \frac{\text{distance moved by spot}}{\text{distance moved by solvent front}}$. outdoor

Dehydrogenase

An enzyme in photosynthesis that catalyses the reduction of NADP by allowing it to accept electrons from the photoionisation of chlorophyll or the photolysis of water.

DCPIP

A redox indicator dye used to measure dehydrogenase activity; it turns from blue when oxidised to colourless when reduced by accepting electrons.

Glycolysis

The first stage of respiration occurring in the cytoplasm, where glucose is phosphorylated to glucose phosphate and then split and oxidised into two molecules of pyruvate.

Substrate-level phosphorylation

The direct transfer of a phosphate group from an intermediate compound to ADP to form ATP, occurring in both glycolysis and the Krebs cycle.

Link reaction

The process in the mitochondrial matrix where pyruvate is oxidised and decarboxylated to acetate, which then combines with coenzyme A to form Acetyl Coenzyme A.

Krebs cycle

A series of oxidation-reduction reactions in the mitochondrial matrix that regenerates a 4C molecule and produces reduced NAD, reduced FAD, ATP, and $CO_2$.

Oxidative phosphorylation

The final stage of aerobic respiration where energy from electrons released by reduced NAD and FAD is used to synthesise ATP via the electron transfer chain and chemiosmosis, with oxygen as the final electron acceptor.

Biomass

The mass of living or organic material in an organism, measured in terms of mass of carbon or dry mass of tissue per given area.

Calorimetry

A method used to estimate the chemical energy store in dry biomass by burning it and measuring the temperature increase of a known volume of water.

Gross Primary Production (GPP)

The total chemical energy store in plant biomass in a given area or volume, in a given time, representing the total energy transferred from light during photosynthesis.

Net Primary Production (NPP)

The chemical energy store in plant biomass after respiratory losses ($R$) to the environment have been taken into account, calculated as: $NPP = GPP - R$.

Saprobionts

Microorganisms that decompose dead organic matter by secreting enzymes for extracellular digestion and absorbing the released nutrients.

Mycorrhizae

Symbiotic associations between fungi and plant roots that act as an extension of the root system to increase the surface area for the absorption of water and inorganic ions.

Nitrogen fixation

The conversion of atmospheric nitrogen gas ($N_2$) into ammonia ($NH_3$) and then ammonium ions ($NH_4^+$) by nitrogen-fixing bacteria.

Ammonification

The process where saprobionts break down nitrogen-containing compounds from dead organisms or waste into ammonia, which then forms ammonium ions in the soil.

Nitrification

The two-step oxidation reaction where nitrifying bacteria convert ammonium ions in the soil into nitrites ($NO_2^-$) and then into nitrates ($NO_3^-$).

Denitrification

The conversion of nitrates in the soil into nitrogen gas by denitrifying bacteria, typically occurring in anaerobic conditions such as waterlogged soil.

Leaching

The process where water-soluble nutrients like nitrates and phosphates dissolve in water and are washed out of the soil into lakes or rivers.

Eutrophication

Nutrient enrichment of water bodies leading to algal blooms, which block light and cause submerged plants to die, followed by oxygen depletion by aerobic saprobionts decomposing the dead matter.