Photosynthesis

📚The biology

Light energy to chemical energy

In photosynthesis, light energy is transformed into chemical energy stored in carbon compounds. Plants, algae and cyanobacteria use it to make their own food, and every consumer and decomposer that feeds on them, directly or indirectly, depends on this chemical energy. It is the main entry point of energy into ecosystems (C4.2).

The overall reaction

Carbon dioxide is converted to glucose using hydrogen obtained by splitting water. The oxygen released is a by-product, and it comes from the water, not from the carbon dioxide. Oxygen is produced as a by-product by plants, algae and cyanobacteria.

Separating pigments by chromatography

Chloroplasts contain several photosynthetic pigments, which can be separated by paper chromatography or thin-layer chromatography:

1. Grind leaves with a little sand and a solvent (such as propanone) to extract the pigments.
2. Place a small, concentrated spot of extract on a pencil line near the bottom of the paper or TLC strip.
3. Stand the strip in a small volume of solvent, with the spot above the solvent level, in a closed container.
4. The solvent moves up, carrying the pigments. Each pigment travels a different distance depending on its solubility in the solvent and its attraction to the stationary phase.
5. Remove the strip before the solvent reaches the top, and mark the solvent front.

Pigments are identified by their colour and their R_f value, which is always between 0 and 1. The typical order, from top to bottom, is: carotene (orange-yellow, near the solvent front), xanthophylls (yellow), chlorophyll a (blue-green) and chlorophyll b (yellow-green). Exact R_f values depend on the solvent and the medium used.

Absorbing light

Pigments absorb light because light energy can excite electrons in the pigment molecule, raising them to a higher energy level. Only photons of particular wavelengths carry exactly the amount of energy that matches the jumps available to electrons in a given pigment, so each pigment absorbs only certain wavelengths and reflects or transmits the rest. The energy of the excited electrons is what is transformed into chemical energy.

Chlorophyll absorbs mainly blue and red light, and reflects green, which is why leaves look green. Accessory pigments such as carotenoids absorb other wavelengths, mainly blue-green, widening the range of light the plant can use.

An absorption spectrum is a graph of how much light a pigment absorbs at each wavelength. Its horizontal axis should show both wavelength (roughly 400–700 nm) and the corresponding colours of light, from violet and blue at the short end to red at the long end.

Absorption and action spectra

Similarities: both show peaks in the blue and red regions and a trough in the green, because wavelengths that are absorbed are the ones that drive photosynthesis. Differences: the action spectrum does not drop as low in the green as chlorophyll’s absorption spectrum does, because accessory pigments absorb some of those wavelengths and pass the energy on; and not all absorbed light is used with equal efficiency.

To make an action spectrum, measure the rate of photosynthesis under light of different wavelengths — for example, oxygen production by pondweed or the uptake of CO₂ (shown by a hydrogencarbonate indicator) — keeping light intensity the same, and plot rate against wavelength.

Limiting factors

The rate of photosynthesis depends on light intensity, carbon dioxide concentration and temperature. At any moment, the factor in shortest supply limits the rate: increasing any other factor makes no difference until the limiting factor is increased. On a graph of rate against light intensity, the rate rises and then levels off; at the plateau, something other than light is now limiting.

A typical experiment uses an aquatic plant such as Elodea or Cabomba, measuring the rate by counting oxygen bubbles or collecting the gas, or leaf discs that float as they photosynthesize.

In each case, identify the independent variable (the factor you change), the dependent variable (the rate, e.g. bubbles per minute) and the controlled variables (the other two factors, the plant, the time allowed to adjust). A hypothesis may be suggested before the experiment, from theory, or after it, from the pattern in the results; either way it is provisional and needs further testing.

Carbon dioxide enrichment experiments

Atmospheric CO₂ is rising (D4.3). Because CO₂ is often a limiting factor, biologists want to predict how future concentrations will affect photosynthesis and plant growth.

This illustrates a general point: control of variables is easier in the laboratory, but some questions can only be answered in the field.

Photosystems AHL

A photosystem is a large array of chlorophyll and accessory pigment molecules, held in place by proteins, that absorbs light and emits excited electrons. Photosystems are always located in membranes: the thylakoid membranes of chloroplasts in eukaryotes, and internal membranes in cyanobacteria.

Light energy absorbed anywhere in the array is passed from pigment to pigment until it reaches a special pair of chlorophyll a molecules at the reaction centre. There, an electron is excited enough to be emitted to an electron acceptor. There are two types, photosystem II and photosystem I.

Why an array? AHL

A single molecule of chlorophyll, on its own, could not perform any part of photosynthesis: it would absorb occasional photons, and the energy would simply be lost as heat or light. The structured array works because:

• many pigment molecules together capture far more light, and funnel the energy to one reaction centre;
• different types of pigment absorb a wider range of wavelengths;
• the proteins hold the reaction centre next to an electron acceptor, so an emitted electron is captured instead of falling back.

Photolysis of water AHL

When photosystem II emits an electron, it must be replaced. Photosystem II takes electrons from water, splitting it: photolysis.

The electrons replace those lost by photosystem II; the protons contribute to the proton gradient and are used to reduce NADP; the oxygen is a waste product. The evolution of oxygen-producing photosynthesis had immense consequences for life and for geology: oxygen accumulated in the oceans and atmosphere, oxidizing iron into the banded iron formations, forming the ozone layer, and making aerobic respiration possible.

ATP production by chemiosmosis AHL

Excited electrons from photosystem II pass along a chain of electron carriers in the thylakoid membrane. As they move, energy is released and used to pump protons from the stroma into the thylakoid space. Together with the protons from photolysis, this builds a proton gradient. Protons flow back to the stroma through ATP synthase, which uses the energy to make ATP — chemiosmosis, as in mitochondria. This is photophosphorylation.

Reduction of NADP AHL

Photosystem I absorbs light and emits excited electrons. These pass to the enzyme that reduces NADP. NADP accepts two electrons from photosystem I and a hydrogen ion from the stroma, becoming reduced NADP. Electrons lost from photosystem I are replaced by electrons arriving from photosystem II along the carrier chain. (Use the terms consistently: either “NADP and reduced NADP” or “NADP⁺ and NADPH”, not a mixture.)

The thylakoid as a system AHL

Carbon fixation by Rubisco AHL

The light-independent reactions (the Calvin cycle) take place in the stroma. The first step is carbon fixation: the enzyme Rubisco catalyses the reaction of carbon dioxide with a 5C sugar, ribulose bisphosphate (RuBP). The unstable 6C product immediately splits into two molecules of glycerate 3-phosphate (GP), a 3C compound.

Rubisco is the most abundant enzyme on Earth. High concentrations of it are needed in the stroma because it works relatively slowly, catalysing only a few reactions per second, and is not effective at low carbon dioxide concentrations.

Synthesis of triose phosphate AHL

Glycerate 3-phosphate is converted into triose phosphate (TP), a 3C sugar, using ATP (for energy) and reduced NADP (which supplies hydrogen, reducing GP). Both come from the light-dependent reactions.

Regeneration of RuBP AHL

For the cycle to continue, RuBP must be regenerated. Five molecules of triose phosphate (15 carbons) are converted into three molecules of RuBP (15 carbons), using more ATP. Only the remaining TP is available to make other products.

If glucose is the product: fixing 6 CO₂ produces 12 TP. Of these, 10 are used to regenerate 6 RuBP, and 2 (6 carbons) make one glucose. So five-sixths of all the triose phosphate made must be recycled into RuBP.

Making other carbon compounds AHL

All the carbon in every compound in a photosynthesizing organism is fixed in the Calvin cycle. Glucose, sucrose, starch and cellulose are made from triose phosphate; other compounds are made by pathways that can be traced back to an intermediate of the cycle, often with mineral nutrients absorbed from the soil: amino acids need nitrogen (and some need sulfur); nucleotides need nitrogen and phosphorus; fatty acids and glycerol for lipids are also built from Calvin cycle intermediates.

Interdependence of the two stages AHL

• No light: the light-dependent reactions stop, so no ATP or reduced NADP is made. GP cannot be converted to TP, and RuBP cannot be regenerated, so the Calvin cycle also stops within seconds.
• No CO₂: the Calvin cycle stops using ATP and reduced NADP, so NADP is not regenerated. Without NADP to accept electrons, electrons back up along the chain, and photosystem II cannot function even in bright light.

The light-dependent reactions supply ATP and reduced NADP to the Calvin cycle; the Calvin cycle returns ADP, phosphate and NADP. Neither can continue for long without the other.