Photosynthesis

Photosynthesis is essentially the reverse of respiration. It is usually simplified to:

carbon dioxide + water (+ light energy) →glucose + oxygen

• The light-dependent reactions use light energy to split water and make ATP, oxygen and energetic hydrogen atoms. This stage takes place within the thylakoid membranes of chloroplasts, and is very much like the respiratory chain, only in reverse.
• The light-independent reactions don’t need light, but do need the products of the light-dependent stage (ATP and H), so they stop in the absence of light.  This stage takes place in the stroma of the chloroplasts and involves the fixation of carbon dioxide and the synthesis of glucose.
• Like respiration, oxygen and carbon dioxide are quite separate. Plants do not turn carbon dioxide into oxygen; they turn carbon dioxide into glucose, and water into oxygen.

Chloroplasts

• Photosynthesis takes place entirely within chloroplasts.
•  Like mitochondria, chloroplasts have a double membrane, but in addition chloroplasts have a third membrane called  the thylakoid membrane.
•  This is folded into thin vesicles (the thylakoids), enclosing small spaces called the thylakoid lumen.
• The thylakoid vesicles are often layered in stacks called grana.
• The thylakoid membrane contains the same ATP synthase particles found in mitochondria.
• Chloroplasts also contain DNA, tRNA and ribososomes, and they often store the products of photosynthesis as starch grains and lipid droplets.

Chlorophyll

• Chloroplasts contain two different kinds of chlorophyll, called chlorophyll a and b, together with a number of other light-absorbing accessory pigments, such as the carotenoids and luteins(or xanthophylls).

• These different pigments absorb light at different wavelengths, so having several
  different pigments allows more of the visible spectrum to be used.
•  A low absorption means that those wavelengths are not absorbed and so cannot be used, but instead are reflected or transmitted.
• Different species of plant have different combinations of photosynthetic pigments, giving rise to different coloured leaves.
•  In addition, plants adapted to shady conditions tend to have a higher concentration of chlorophyll and so have dark greenleaves, while those adapted to bright conditions need
  less chlorophyll and have pale green leaves.
• By measuring the rate of photosynthesis using different wavelengths of light, an action spectrumis obtained.
• Chlorophyll is a fairly small molecule (not a protein) with a structure similar to haem, but with a magnesium atom instead of iron.

• Chlorophyll and the other pigments are arranged in complexes with proteins, called photosystems.
•  Each photosystem contains some 200 chlorophyll molecules and 50
  molecules of accessory pigments, together with several protein molecules (including enzymes) and lipids.
• These photosystems are located in the thylakoid membranes and they hold the light-absorbing pigments in the best position to maximise the absorbance of photons of light.
• The chloroplasts of green plants have two kinds of photosystem called photosystem I(PSI) and photosystem II(PSII).

The Light-Dependent Reactions

• The light-dependent reactions (or photophosphorylation) take place on the thylakoid membranes using four membrane-bound proteins: photosystem I (PSI), photosystem II (PSII), cytochrome (C) and ferredoxin (FD).
•  In these reactions light energy is used to split water, oxygen is given off, hydrogen is produced and ADP is phosphorylated to make ATP.

I

• Chlorophyll molecules in PSII absorb photons of  light, exciting chlorophyll electrons to a higher energy level and causing a charge separation within PSII.
• This charge separation drives the splitting (or photolysis) of water molecules to make oxygen (O2), protons (H+) and electrons (e-):
  2H2O  → O2+ 4H+ + 4e-
• Water is a very stable molecule and it requires the energy from 4 photons of light to split one water molecule.
• The oxygen produced diffuses out of the chloroplast and eventually into the air; the proton sbuild up in the thylakoid lumen causing a proton gradient; and the electrons from water replace the excited electrons that have been ejected from chlorophyll.

II

• The excited, high-energy electrons are passed along the chain of proteins in the thylakoid membrane, in a similar way to the respiratory chain.

• In PSI more light energy is absorbed and passed to the electron, which gains energy as it goes.
• The energy of the electrons is used to pump protons from stroma to lumen, creating a proton gradient across the thylakoid membrane.

III

• Finally, the electron is recombined with a proton to form a hydrogen atom, which is taken up by the coenzyme NADP, reducing it to NADPH.
  NADP + H+ + e- →NADPH

IV

• The combination of the water splitting and proton pumping causes a proton gradient across the thylakoid membrane.
•  This gradient is used to make ATP using the ATP synthase enzyme in exactly thesame way as respiration.
• This synthesis of ATP is called photophosphorylation because it uses light energy to phosphorylate ADP (ADP + Pi →ATP).

1. Light energy is absorbed by chlorophyll and used to photolyse water (H2O → O2+ H+ + e-).

2. The high-energy electron is passed along the chain of proteins in the thylakoid membrane, gaining energy from light as it goes.

3. The electron is taken up by NADP, which is reduced to NADPH (NADP + H++ e →NADPH).

 4. The energy from the light is used to make ATP in the ATP synthase enzyme (ADP + Pi →ATP).

The Light-Independent Reactions

• The light-independent, or carbon-fixing reactions, of photosynthesis take place in the stroma of the chloroplasts and comprise another cyclic pathway, called the Calvin Cycle, after the American scientist who discovered it.

I

• Carbon dioxide binds to the 5-carbon sugar ribulose bisphosphate (RuBP)to form 2 molecules of the 3-carbon compound glycerate phosphate.
• This carbon-fixing reaction is catalysed by the enzyme ribulose bisphosphate carboxylase, always known as rubisco.
• It is a very slow and inefficient enzyme, so large amounts of it are needed (recall that increasing enzyme concentration increases reaction rate), and itcomprises about 50% of the mass of chloroplasts, making it the most abundant protein in nature.

II

• Glycerate phosphate (C3H4O4.PO3) is an acid, not a carbohydrate, so it is reduced and activated to form triose phosphate(C3H6O3.PO3), the same 3-carbon sugar as that found in glycolysis.
• Two ATP and two NADPH molecules from the light-dependent reactions  provide the energy for this step.
• The ADP and NADP return to the thylakoid membrane for recycling.

III

• Triose phosphate is a branching point.
•  Most of the triose phosphate continues through a complex series of reactions to regenerate the RuBP and complete the cycle.
•  5 triose phosphate molecules (5 x 3C = 15 carbon atoms) combine to form 3 RuBP molecules (3 x5C = 15 carbon atoms).  

IV

• Every 3 turns on average of the Calvin Cycle 3 CO2 molecules are fixed to make 1 new triose phosphate molecule (3CO2+ 6H →C3H6O3).
• This triose phosphate leaves the cycle, and two  of these triose phosphate molecules combine to form one glucose molecule using the glycolysis enzymes in
  reverse.

V

• The light-independent reactions are now finished, and the glucose can now be transported out of the chloroplast and used to make all the other organic  compounds that the plant needs (cellulose, lipids, proteins, nucleic acids, etc).
•  Some of these need the addition of mineral elements like N, P or S.
•  Plants are very self-sufficient!

• The rate of photosynthesis by a plant or algae can be measured by recording the amount of oxygen produced, or carbon dioxide used, in a given period of time.
•  But these measurements are also affected  by respiration, which plants do all the time, so the respiration rate must be measured separately.
• A plant’s growth (or productivity) depends on the difference between the rates of photosynthesis and respiration.
•  The conditions at which the rates of photosynthesis and respiration are equal, so there is no net change in oxygen or carbon dioxide concentration, is called the compensation
  point.
• Many of the environmental factors that affect photosynthesis also affect respiration.

Temperature.

• Temperature affects the rates of all enzyme reactions, so the rates of photosynthesis and respiration are both affected.
• Photosynthesis is more sensitive to temperature with an optimum of about 30-35°C, whereas respiration often has an optimum nearer to 45°C.

• So there is a temperature compensation point around 40°C (A), and above this temperature plants lose mass as the rate of respiration is greater than the rate of photosynthesis.

Carbon dioxide concentration.

• Carbon dioxide is the substrate for the enzyme rubisco in the light-independent stages of photosynthesis, so the higher the carbon  dioxide concentration the faster the rate of the Calvin cycle.
• The rate of respiration is not affected by carbon dioxide concentration, and the carbon dioxide compensation  pointis usually very low, at about 50ppm (A).
•  Normal carbondioxide concentration in the air is about 400ppm (B), whereas the optimum concentration for most plants is nearer to 1000ppm, so carbon dioxide is often the limiting factor.

Light intensity.

• Light is the source of energy for the production of ATP and NADPH in the light-dependent stages of photosynthesis, so the higher the light intensity the faster the rate of photosynthesis.
• The rate of respiration is not affected by light intensity, and the light compensation point is
  usually low.
•  Shade plants are adapted to growing in low light conditions (such as a forest floor), so have a very low light compensation point and a low optimum intensity.
• Shade plants make good house plants, since they are adapted to the low light intensities indoors.
• Sun plants have a higher compensation point, and have a very high optimum near the light intensity of a bright summer’s day .

Time of Day. 

• Both photosynthesis and respiration are affected by time of day: photosynthesis by changes in light and respiration by changes in temperature.
•  At night respiration exceeds photosynthesis, while during the day photosynthesis
  exceeds respiration, so there are two compensation  points each day.
• Over a 24-hour period the amount of photosynthesis is greater than the amount of respiration, so plants gain mass and have a net uptake of carbon dioxide.

Understanding how factors affect photosynthesis andrespiration is very important for farmers and commercial growers.

For example in a closed greenhouse with lots of plants the carbon dioxide
concentration can fall very low, so it can be worth increasing the CO2 concentration in the greenhouse to increase the rate of photosynthesis.

This is most efficiently done by burning a fuel, since this releases CO2 and raises the temperature. It’s hard to beat the intensity of daylight, but day length can be increased with
artificial lighting.

Limiting Factors

• Although all these factors (and many others) could affect the rate of photosynthesis, at any given time there can only be onefactor that is actually controlling the rate – the limiting factor.
• This is the factor that is in shortest supply, or furthest from its optimum.
•  It’s a bit like a chain that is only as strong as its weakest link.
• We can observe this in an experiment to investigatethe rate of photosynthesis.

I

• At low light intensities the rate of photosynthesis increases as the light intensity increases.
• This must mean that light is the limiting factor, since the rate depends on it.

II

• At higher light intensities the rate of photosynthesis stays the same even if the light intensity increases.

• This means that light is not the limiting factor, since the rate doesn’t depend on it.
• This isn’t surprising since there is now plenty of light. The rate of photosynthesis must be limited by some other factor.

III

• If we repeat the experiment at a higher concentration of carbon dioxide we get a higher rate, showing that a higher rate is possible, and that carbon dioxide concentration was rate limiting at point.