Unit 5 - Energy Transfers

AQA GCE A-Level Biology - Unit 5 - Energy Transfers

[3.5.1] What is photosynthesis?

The process in which light energy is absorbed and converted into organic molecules (e.g. glucose).

[3.5.1] What is the equation for photosynthesis?

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

[3.5.1] What is the compensation point?

The rate at which the rate of photosynthesis and the rate of respiration are equal. The rate of photosynthesis depends on the light intensity, so light intensity at different times can alter the rate of photosynthesis to a point where it is equal to rate of respiration.

The net uptake of CO₂ or O₂ at the compensation point is zero.

[3.5.1] What is the process of the light dependent reaction?

• When light energy is absorbed by the chlorophyll of photosystem II, two electrons are excited. They leave the chlorophyll and are taken up by an electron acceptor. This is known as photoionisation.

• The electrons pass along a series of electron carriers and the energy released is used to generate ATP from ADP and Pi in a process called chemiosmosis.

• The electrons are accepted by the chlorophyll of photosystem I which again absorbs photons and loses two electrons to a second electron acceptor at a higher energy level.

• The electrons pass down another series of electron carriers and the final electron acceptor is NADP which is reduced into NADPH.

• The energy associated with excitation of electrons causes water to dissociate in photolysis. The hydrogen ions released are used to reduce NADP and the electrons replace the electrons lost by the first chlorophyll molecule. (O₂ is released as a biproduct).

[3.5.1] What is photoionisation?

Light energy excites electrons in the chlorophyll, causing the electrons to leave the chlorophyll.

[3.5.1] What is chemiosmosis?

Chemiosmosis is the process of electrons releasing energy as they pass down electron carriers, creating a proton gradient to drive ATP synthesis.

• Light energy is absorbed by the chlorophyll of PSII, exciting a pair of electrons to a higher energy level.

• Electrons are then passed down a series of electron carriers and an electron transport chain.

• Excited electrons lose energy as they move from carrier to carrier.

• Energy lost is used to actively transport protons into the thylakoid; these accumulate so there is a greater concentration of ions in the thylakoid than in the stroma.

• Protons diffuse back down into the stroma through ATP synthase which releases energy, used to generate ATP from ADP and Pi.

[3.5.1] What is photolysis?

Splitting of water to release electrons to replace those lost from the chlorophyll.

[3.5.1] What is photophosphorylation?

The process of forming ATP in photosynthesis.

[3.5.1] What is the process of the light independent reaction?

The Calvin Cycle takes place in the stroma.

• Carbon Dioxide diffuses into the leaf through the stomata and into the stroma of the chloroplast.

• Ribulose Bisphosphate (RuBP) combines with a molecule of CO₂, catalysed by RuBP Carboxylase.

• An unstable 6-carbon-compound is formed which immediately breaks down into two 3-carbon-molecules of glycerate-3-phosphate (GP).

• The glycerate-3-phosphate is then reduced to triose phosphate using electrons and H⁺ from NADPH and energy from ATP (produced in the light dependent reaction).

• 1/6 of the triose phosphate produced is used to make hexose sugars, and the other 5/6 is used to resynthesise RuBP.

• NADP, ADP and Pi return to the thylakoid membrane for use in the light dependent reaction.

[3.5.1] Which factors might limit the rate of photosynthesis?

Carbon Dioxide Concentration, Light Intensity, Temperature, Wavelength of Light, Water Levels, etc.

[3.5.2] What is respiration?

• Aerobic Respiration is where O₂ is used to convert organic molecules into ATP.

• Anaerobic Respiration is where organic molecules are converted into ATP, without O₂, and less ATP is produced.

[3.5.2] What is the process of glycolysis?

Glycolysis takes place in the cytoplasm.

• Glucose is activated using phosphate from ATP.

• This produces glucose phosphate which is then further phosphorylated into an unstable 6-carbon-compound which immediately breaks down into 2 molecules of triose phosphate.

• The triose phosphate is then oxidised into pyruvate by removal of a proton which reduces NAD to NADH.

• This releases energy which forms 4 ATP from ADP and Pi.

The ATP is produced via SUBSTRATE-LEVEL PHOSPHORYLATION.

[3.5.2] What is the process of the link reaction?

The link reaction takes place in the matrix.

• Pyruvate (produced in glycolysis) is oxidised into acetate, and a molecule of CO₂ is released (NAD forms NADH).

• Acetate binds to coenzyme A to form acetyl coenzyme A.

[3.5.2] What is the process of the Krebs cycle?

The Krebs cycle takes place in the matrix.

• Acetyl coenzyme A enters the cycle and combines with a 4 carbon acid (oxaloacetate) to form a 6 carbon acid (citrate).

• The 6 carbon acid then releases a molecule of CO₂ to form a 5 carbon acid (NAD forms NADH).

• The 5 carbon acid then releases another molecule of CO₂ to form a 4 carbon acid; ATP is formed from ADP and Pi. (NAD forms NADH and FAD forms FADH).

• The reduced NAD and FAD then carries the hydrogen ions to the electron transport chain for oxidative phosphorylation.

[3.5.2] What is the process of the electron transport chain?

• NADH and FADH attach to the cristae on the mitochondrial membrane.

• NADH and FADH donate electrons from their hydrogen atom to the first molecule in the electron transport chain.

• The electrons are passed down a series of carriers which decrease in energy. The energy lost is used to actively transport H⁺ into the intermembrane space.

• This creates a higher concentration of H⁺ in the intermembrane space compared to the matrix, so they diffuse back into the matrix through ATP synthase.

• The energy released is used to convert ADP + Pi into ATP.

• At the end of the chain of electron carriers, oxygen accepts the electrons and combines with hydrogen ions to form water. OXYGEN IS THE FINAL ELECTRON ACCEPTOR.

[3.5.2] What is the process of glycolysis in anaerobic respiration?

The pyruvate produced in glycolysis is converted into ethanol (alcoholic fermentation) in plants or lactate (lactate fermentation) in animals using reduced NAD.

The production of lactate or ethanol regenerates NAD. This means glycolysis can continue even when there isn’t much oxygen so a small amount of ATP can still be produced.

[3.5.3] What is a trophic level?

A trophic level describes the position of an organism in a food chain or food web and its feeding relationship with other organisms.

[3.5.3] What happens to the amount of energy at each trophic level?

• Decreases down a food chain.

• Decreases as trophic levels increase because energy is lost to the environment as heat.

[3.5.3] What is a food chain?

Food chains are the sequence of feeding interactions between organisms in a given habitat.

[3.5.3] What is a food web?

Food webs are where organisms consume each other, so are interdependent. They show how food chains are interlinked with one another.

[3.5.3] What are saprobiotic microorganisms?

Decomposers such as bacteria or fungi which carry out extracellular digestion by secretion of enzymes onto dead or decaying organic matter in order to hydrolyse molecules which are then absorbed.

[3.5.3] What are detritivores?

Organisms such as earthworms or maggots which feed on dead or decaying organic matter and digest the food molecules internally.

[3.5.3] What is biomass?

The mass of living material in a plant.

[3.5.3] What do we measure biomass in terms of?

Mass of carbon or dry mass of tissue in a given area, in a given time.

[3.5.3] What units are typically used to measure biomass?

Kgm⁻²yr⁻¹

[3.5.3] Why is dry mass a better indication of biomass than wet mass?

The water content of tissue varies.

[3.5.3] How would you measure the dry mass of a sample?

• Heat the sample at a low temperature in an oven (high enough to evaporate water but not high enough to burn the sample).

• Weigh the sample at regular intervals and continue heating between weighing.

• Stop once a constant mass is reached.

[3.5.3] How can the chemical energy store in a sample of plant dry biomass be estimated?

Calorimetry.

[3.5.3] What is GPP?

Gross Primary Productivity is the chemical energy store in plant biomass in a given area or volume, in a given time.

[3.5.3] What is the link between GPP and the rate of photosynthesis?

The higher the rate of photosynthesis, the higher the gross primary productivity.

[3.5.3] What is NPP?

Net Primary Productivity is the chemical energy store in plant biomass after respiratory losses in the environment have been taken into account.