Photosynthesis — OCR A-Level Biology

This clause requires you to explain how photosynthesis and respiration depend on each other. In photosynthesis, chloroplasts use light energy to fix carbon dioxide and release oxygen, producing glucose and other carbohydrates. In aerobic respiration, mitochondria oxidise those carbohydrates, using oxygen and releasing carbon dioxide, water and ATP. The two processes are therefore linked by gas exchange: the oxygen released in photosynthesis is used in respiration, and the carbon dioxide released in respiration is used in photosynthesis. They are also linked by organic molecules: the glucose made in photosynthesis is the respiratory substrate, and the ATP and intermediate compounds produced feed biosynthetic reactions. In a plant cell both occur, in different organelles, and their rates vary with light intensity, so net gas exchange depends on which process dominates.

A chloroplast is a lens-shaped organelle bounded by a double membrane, the envelope. Its interior is filled with stroma, a fluid containing enzymes, ribosomes, DNA and starch grains. A third membrane system, the thylakoids, forms flattened sacs; stacked thylakoids are grana, and grana are joined by lamellae. The two main stages occur in different compartments. The light-dependent stage happens on the thylakoid membranes, where photosystems, electron carriers and ATP synthase are embedded. The light-independent stage (the Calvin cycle) happens in the stroma, where the enzymes for carbon fixation are located. So a question may ask which structure houses a named process, or which feature is absent from a given compartment. Match each process to its membrane or fluid, and remember the envelope is distinct from the thylakoid membranes.

Photosynthetic pigments absorb light energy and pass it to the reaction centre, so they are essential for the light-dependent stage. Chlorophyll a is the primary pigment at the reaction centre; accessory pigments such as chlorophyll b and carotenoids absorb wavelengths that chlorophyll a absorbs poorly and transfer the energy to chlorophyll a. This widens the range of wavelengths usable, increasing the efficiency of light absorption. Carotenoids also protect chlorophyll from damage by excess light. Because each pigment absorbs a characteristic range of wavelengths, an absorption spectrum shows which colours are captured, and an action spectrum shows the rate of photosynthesis at each wavelength; the two broadly correspond. A question may ask why several pigments are advantageous rather than one, or which pigment is at the reaction centre.

TLC separates photosynthetic pigments because each pigment partitions differently between a stationary phase (a thin silica layer on a plate) and a moving solvent. A concentrated leaf extract is spotted as a small origin spot, the plate is developed in a shallow solvent so the solvent front rises by capillary action, and pigments travel at different rates according to their solubility in the solvent and their attraction to the stationary phase. More soluble pigments move further; Rf values (distance moved by pigment ÷ distance moved by solvent front) identify pigments such as chlorophyll a, chlorophyll b, carotene and xanthophyll. Good technique uses a pencil origin line, a small spot, a covered tank and a solvent front marked before it dries.

The light-dependent stage occurs on the thylakoid membranes of chloroplasts. Light absorbed by photosystems excites electrons in chlorophyll, which are passed along an electron transport chain, generating ATP by chemiosmosis as protons are pumped into the thylakoid space and flow back through ATP synthase. Water is split by photolysis, releasing electrons to replace those lost, protons and oxygen as a by-product. NADP is reduced to NADPH. The ATP and NADPH produced are used in the light-independent stage. Learners should link each step to its location and to the fate of the products.

The light-independent stage occurs in the stroma and does not need light directly, but depends on ATP and reduced NADP from the light-dependent stage. Carbon dioxide is fixed when it joins the five-carbon acceptor ribulose bisphosphate (RuBP), catalysed by rubisco, forming two molecules of the three-carbon glycerate 3-phosphate (GP). GP is reduced to triose phosphate (TP) using ATP and reduced NADP. Most TP regenerates RuBP, consuming further ATP, so the cycle continues. A useful check: for each CO₂ fixed, one carbon enters the cycle, and the cycle must turn six times to build one hexose. Track the carbon atoms and the ATP and reduced NADP consumed at each step.

Triose phosphate (TP) is the three-carbon product of the light-independent stage, and it has several fates. Most TP is used to regenerate ribulose bisphosphate (RuBP) so the Calvin cycle can continue. Some TP is converted to glycerol and fatty acids, which combine to form lipids. Some TP is built into amino acids, and therefore proteins, when nitrogen is available. Some TP is converted to hexose sugars such as glucose, which can be used in respiration or polymerised into starch for storage or cellulose for cell walls. Sucrose is also formed for transport in the phloem. A useful method is to sort each use into 'regenerate RuBP', 'make carbohydrate', 'make lipid' and 'make amino acid'.

Photosynthesis rate is limited by whichever factor is in shortest supply. Light intensity, carbon dioxide concentration and temperature each act on a different stage, so raising one only increases the rate until another becomes limiting. Light drives the light-dependent stage, producing ATP and reduced NADP; carbon dioxide supply feeds the light-independent stage via rubisco-catalysed carbon fixation; temperature affects enzyme activity, including rubisco, and so changes rate with a distinct optimum before denaturation reduces it. A graph of rate against one factor, with the others held constant, plateaus where a second factor limits. Learners should read such graphs, identify the limiting factor at a given point, and predict the effect of changing conditions.

Practical work measures the rate of photosynthesis by a dependent variable such as oxygen evolved or carbon dioxide taken up. A common method uses pondweed: the number of oxygen bubbles released per minute, or the volume of gas collected, is recorded while one independent variable (for example light intensity, set by distance from a lamp, or carbon dioxide concentration using hydrogencarbonate solution, or temperature using a water bath) is changed and the others are controlled. Rate is calculated from the dependent variable, and results are plotted to show the limiting-factor pattern. Control variables include temperature, carbon dioxide supply and light distance, and repeats improve reliability.