AQA A-Level Biology: Topic 5 Energy transfers in and between organisms

Stages of Photosynthesis and Locations

1. Light-dependent reaction (LDR) in the thylakoid membrane of chloroplasts; 2. Light-independent reaction (Calvin cycle) in the stroma of chloroplasts.

Photoionisation in the Light-Dependent Reaction

Chlorophyll absorbs light energy which excites its electrons to a higher energy level, causing electrons to be released from chlorophyll (chlorophyll becomes positively charged).

Chemiosmosis and ATP/Reduced NADP Production in LDR

Excited electrons move along an electron transfer chain, releasing energy used to actively pump protons from the stroma into the thylakoid; protons move by facilitated diffusion down an electrochemical gradient into the stroma via ATP synthase, joining ADP and Pi to form ATP (photophosphorylation); NADP accepts a proton and an electron to become reduced NADP.

Photolysis of Water in the Light-Dependent Reaction

Water splits using light energy to produce protons, electrons, and oxygen (H2O -> 1/2 O2 + 2e- + 2H+); the electrons replace those lost from chlorophyll during photoionisation.

Light-Independent Reaction (Calvin Cycle)

CO2 reacts with ribulose bisphosphate (RuBP), catalysed by rubisco, forming two molecules of glycerate 3-phosphate (GP); GP is reduced to triose phosphate (TP) using reduced NADP and energy from ATP; some TP is converted to useful organic substances (e.g. glucose), while the rest regenerates RuBP using ATP.

Effect of Temperature on Rate of Photosynthesis

As temperature increases, rate increases because enzymes (e.g. rubisco) gain kinetic energy so more enzyme-substrate complexes form; above the optimum temperature, rate decreases as enzymes denature due to hydrogen bonds breaking in their tertiary structure.

Effect of Light Intensity on Rate of Photosynthesis

As light intensity increases, rate increases because more photoionisation of chlorophyll produces more ATP and reduced NADP, allowing more GP to be reduced to TP and more RuBP to be regenerated; above a certain intensity, rate levels off as another factor (temperature or CO2 concentration) becomes limiting.

Effect of CO2 Concentration on Rate of Photosynthesis

As CO2 concentration increases, rate increases because more CO2 combines with RuBP to form GP, so more GP is reduced to TP and converted to organic substances; above a certain concentration, rate levels off as light intensity or temperature becomes limiting.

Evaluating Agricultural Practices for Limiting Factors

Practices (e.g. heating, CO2 enrichment) increase photosynthesis and glucose production for faster respiration and ATP release for growth, increasing yield; however, the profit from extra yield must outweigh the financial and environmental costs.

Required Practical 7: Paper Chromatography Method

Crush leaves with solvent to extract pigments, draw a pencil origin line 1 cm above bottom of chromatography paper, add a spot of extract, stand paper in solvent below origin line with lid on, remove before solvent reaches top, and mark solvent front immediately with pencil.

Required Practical 7: Chromatography Common Questions

Origin drawn in pencil because ink is soluble and would dissolve/mix; origin placed above solvent so pigments do not dissolve into solvent; solvent front marked immediately before it evaporates; Rf value = distance moved by spot / distance moved by solvent front, measured from centre of spot to standardise spread.

Required Practical 8: Dehydrogenase Activity (DCPIP) Method

Extract chloroplasts and place in tubes with DCPIP (redox indicator dye) and water; shine light on tubes and time how long DCPIP takes to turn from blue (oxidised) to colourless (reduced) compared to a colour standard without DCPIP; rate = 1 / time taken.

Required Practical 8: DCPIP Controls and Colorimeter Modification

Control 1 (covered in foil) shows light is required for photoionisation and DCPIP reduction; Control 2 (no chloroplasts) shows chloroplasts are required; measuring light absorbance at intervals with a colorimeter (zeroed with colour standard) overcomes the subjectivity of judging colour change by eye.

Stages and Locations of Aerobic vs Anaerobic Respiration

Aerobic: 1. Glycolysis (cytoplasm), 2. Link reaction (mitochondrial matrix), 3. Krebs cycle (mitochondrial matrix), 4. Oxidative phosphorylation (inner mitochondrial membrane). Anaerobic: 1. Glycolysis (cytoplasm), 2. NAD regeneration (cytoplasm).

Process of Glycolysis

Glucose is phosphorylated to glucose phosphate using inorganic phosphates from 2 ATP, hydrolysed into 2 triose phosphate molecules, and oxidised to 2 pyruvate molecules, reducing 2 NAD and producing 4 ATP (net gain of 2 ATP).

Anaerobic Respiration After Glycolysis

Pyruvate is converted to lactate (in animals and some bacteria) or ethanol and CO2 (in plants and yeast), oxidising reduced NAD back into NAD so glycolysis can continue and produce a small net gain of 2 ATP.

Why Anaerobic Respiration Produces Less ATP Than Aerobic

Anaerobic respiration only involves glycolysis (net gain of 2 ATP), whereas aerobic respiration includes the Krebs cycle and oxidative phosphorylation, which produces the majority of ATP (around 34 molecules).

The Link Reaction

Pyruvate is actively transported into the mitochondrial matrix, oxidised and decarboxylated to acetate (releasing CO2 and reducing NAD), and acetate combines with coenzyme A to form Acetyl Coenzyme A (yields 2 Acetyl CoA, 2 CO2, and 2 reduced NAD per glucose).

The Krebs Cycle

Acetyl CoA (2C) reacts with a 4C molecule, releasing coenzyme A and forming a 6C molecule; through redox reactions, the 4C molecule is regenerated while releasing 2 CO2, reducing NAD and FAD, and generating ATP by substrate-level phosphorylation (yields 6 reduced NAD, 2 reduced FAD, 2 ATP, and 4 CO2 per glucose).

Oxidative Phosphorylation (Chemiosmotic Theory)

Reduced NAD and FAD are oxidised, releasing H atoms that split into protons and electrons; electrons pass down the electron transfer chain via redox reactions, releasing energy to pump protons from the matrix into the intermembrane space; protons diffuse back into the matrix via ATP synthase to make ATP from ADP + Pi; oxygen acts as the final electron acceptor, combining with protons and electrons to form water.

Alternative Respiratory Substrates

Fatty acids from lipid hydrolysis are converted into Acetyl Coenzyme A, and amino acids from protein hydrolysis are converted into intermediates that enter the Krebs cycle.

Required Practical 9: Respirometer for Aerobic Respiration

Place yeast and glucose with a pH buffer and sodium hydroxide (to absorb CO2) in a sealed respirometer in a water bath; leave open for 10 mins to equilibrate, then close and measure distance moved by coloured liquid towards the organism as O2 is consumed and pressure drops; rate = (pi r^2 x distance) / (mass x time).

Required Practical 9: Respirometer for Anaerobic Respiration

Omit the CO2 absorber and create anaerobic conditions (e.g. oil layer over yeast, O2 absorber, or leaving 1 hour for O2 to be used up); yeast releases CO2 during anaerobic respiration, increasing gas volume and pressure so liquid moves away from the organism.

Required Practical 9: Methylene Blue Redox Indicator Method

Add yeast, glucose, and buffer to tubes, equilibrate in a water bath for 5 mins, add methylene blue, shake once, and time how long it takes to turn colourless as methylene blue accepts electrons instead of NAD/FAD; do not shake again because oxygen would re-oxidise methylene blue back to blue; control uses boiled/dead yeast.

Biomass Formation and Measurement

Plants make organic compounds from CO2 during photosynthesis; most sugars are respired and the rest form biomass, measured as mass of carbon or dry mass per unit area; dry mass is found by heating at 100°C and reweighing until mass is constant (all water removed), avoiding variation in water content.

Calorimetry for Estimating Chemical Energy in Biomass

Burn a known mass of dry biomass completely to heat a known volume of water, and use the water's temperature rise to calculate chemical energy; a stirrer distributes heat evenly, insulation reduces heat loss, and water's high specific heat capacity absorbs heat accurately.

Gross Primary Production (GPP) and Net Primary Production (NPP)

GPP is the total chemical energy store in plant biomass in a given area/volume and time; NPP is the chemical energy store left after respiratory losses (NPP = GPP - R), which is available for plant growth, reproduction, and transfer to herbivores and decomposers.

Primary and Secondary Productivity Units

Measured in kJ ha-1 year-1; per unit area standardises for different environment sizes, and per year accounts for seasonal variations in temperature and light, allowing valid comparisons.

Why Most Light Falling on Producers Is Not Used

Light is reflected or is the wrong wavelength, misses chlorophyll/chloroplasts, or another factor such as CO2 concentration or temperature is limiting photosynthesis.

Net Production of Consumers and Energy Transfer Efficiency

Net production of consumers: N = I - (F + R), where I is chemical energy in ingested food, F is energy lost in faeces and urine, and R is respiratory loss; Efficiency (%) = (energy or biomass available after transfer / energy or biomass available before transfer) x 100.

Why Energy Transfer Between Trophic Levels Is Inefficient

Heat energy is lost during respiration, parts of organisms are not eaten (e.g. bones), some ingested food is not digested and is lost as faeces, and energy is lost in excretory products such as urea in urine.

How Crop and Livestock Farming Increase Energy Transfer Efficiency

Crops: herbicides reduce competition, pesticides/fungicides prevent biomass loss to pests/disease, and fertilisers provide nitrates. Livestock: restricting movement and keeping warm reduce respiratory heat loss, slaughtering young maximises growth energy, antibiotics prevent pathogen energy loss, and selective breeding increases growth rates.

Role of Saprobionts and Mycorrhizae

Saprobionts secrete enzymes for extracellular digestion to decompose organic matter (proteins, urea, DNA) into soluble nutrients and mineral ions; mycorrhizae are symbiotic fungi on plant roots whose hyphae increase root surface area for water and mineral ion uptake in exchange for organic carbohydrates.

Nitrogen Cycle: Nitrogen Fixation and Ammonification

Nitrogen fixation: nitrogen-fixing bacteria convert atmospheric nitrogen gas (N2) into ammonia (NH3), forming ammonium ions (NH4+) in soil. Ammonification: saprobionts decompose organic nitrogen compounds (proteins, urea, DNA) in dead matter/waste into ammonia/ammonium ions.

Nitrogen Cycle: Nitrification and Denitrification

Nitrification: nitrifying bacteria oxidise ammonium ions into nitrites and then nitrates in aerobic conditions (ploughing aerates soil to boost nitrification). Denitrification: denitrifying bacteria reduce soil nitrates back into nitrogen gas (N2) in anaerobic conditions (e.g. waterlogged soil).

The Phosphorus Cycle

Weathering/erosion releases phosphate ions from rocks into soil/oceans; plants/algae absorb phosphate ions (aided by mycorrhizae) into biomass; phosphate passes through food chains and returns via excretion or decomposition of dead matter/waste by saprobionts.

Why Fertilisers Are Used and Natural vs Artificial Fertilisers

Fertilisers replace nitrates and phosphates lost when crops and livestock are harvested instead of decomposing on site; natural fertilisers (manure, compost) are organic, less soluble, and release ions slowly via saprobionts, whereas artificial fertilisers contain inorganic nitrogen, phosphorus, and potassium compounds.

Leaching and Eutrophication

Water-soluble nitrates/phosphates leach into rivers/lakes, causing an algal bloom that blocks light; submerged plants die as they cannot photosynthesise; saprobionts decompose dead plants using oxygen in aerobic respiration, depleting oxygen so fish die.