INS AND OUTS OF PHOTOSYNTHESIS

Autotrophic nutrition involves the use of an external energy source to make the complex organic molecules that we call food. Plants use light energy from the sun as their external energy source, this type of autotrophic nutrition is photosynthesis. During photosynthesis carbon dioxide and water are used to manufacture carbohydrates. Light energy is trapped by chlorophyll and used to generate ATP (cyclic photophosphorylation) and also to split water (the light reaction); the hydroxyl (-OH group) is recombined to make oxygen (which is excreted); the hydrogen is later used to fix carbon dioxide and reduce it to form carbohydrates. Plants manufacture all the other organic molecules (DNA, vitamins, lipids etc. etc.) that they need from these carbohydrates and very small quantities of mineral ions obtained from the soil. The biochemistry of photosynthesis. N.B. The biochemistry of photosynthesis is very complex. Any account you read in a textbook will be simplified, different authors will simplify in different ways.

There are two basic stages to photosynthesis.

1. The Light Dependent Reaction

This takes place in the thylakoids of a chloroplast.

During this stage light energy is trapped by chlorophyll molecules in the light harvesting stage. This raises the energy level of electrons in the chlorophyll molecules. In the process of non-cyclic photophosphorylation this energy is used to convert NADP to NADPH + H+ and ADP to ATP. Non-cyclic photophosphorylation involves photosytems II and I. (“the Z plan”) During this process water is split into oxygen - a waste gas (most of which will be released from the plant) - and into hydrogen ions and electrons. These are used to generate both NADPH + H+ and ATP. In cyclic photophosphorylation only photosystem I is involved and only ATP is produced.

2. The Light Independent Reaction

This takes place in the stroma of a chloroplast. The process is known as The Calvin Cycle.

Carbon dioxide fixation takes place. Initially the carbon dioxide combines with ribulose bisphosphate (RuBP), a compound containing five carbon atoms. The enzyme RuBisCO catalyses this reaction. Thus life on Earth largely depends on this one chemical! This reaction can also be called a carboxylation, since carbon dioxide is being added: it gives a six carbon intermediate compound, which instantly splits into 2 molecules of glycerate-3- phosphate (GP) a 3-carbon compound. The GP is then phosphorylated (phosphate is added), using the ATP generated in the Light- dependent reaction and then reduced to glyceraldehyde-3-phosphate (GALP) These reactions use up the NADPH + H+ and ATP which were produced in the Light Dependent reaction. NADP and ADP are regenerated. Most of the GALP is used to regenerate RuBP, but some is used to produce 6 carbon sugars (monosaccharides), which can then be used to produce a range of organic molecules.

These include: - 1. Starch, a polysaccharide, stored temporarily in the leaf, or in organs such as potato tubers for more long-term storage. 2. Sucrose, a disaccharide transported to other parts of the plant in the phloem. 3. Lipids, for energy storage, in seeds, for example. 4. Amino acids and proteins - the manufacture of these requires nitrogen, the source of this is normally nitrate ions from the soil.

NB You should also know

1. The factors that affect the rate of photosynthesis and their interactions. a. Temperature – higher temperatures raise the speed of the light-independent reactions, up to about 35-40oC (= gross photosynthetic rate); it has little effect on the speed of the light reactions. However, they also raise the speed of respiration in the plant and so NET photosynthesis slows down at higher temperatures. Peak crop yields are at about 25oC b. Carbon Dioxide – normally 360 ppm, growers raise this to about 1000 ppm in greenhouses to raise yields. Peak photosynthesis is at even higher concentrations, but it is not economical to produce this. N.B. Higher world CO2 levels = better plant growth. c. Light - the more the better (brightness); 16-20 hours a day (duration); blue and red must be in balance (i.e. filament lamps are no good!) d. Water – often a limiting factor – plants shut their stomata long before they wilt. Shut stomata = no gas exchange and no photosynthesis!

2. The structure of a chloroplast (be able to draw and label one!)

3. The action and absorption spectra of chlorophyll (virtually the same, with clear peaks in the blue/violet and red light regions).

4. How leaves are adapted to carry out photosynthesis effectively: stomata for gas exchange (largely on the underside to maximise palisade cells) thin to allow gas to reach mesophyll cells quickly moist to allow gasses to dissolve waterproof cuticle to reduce transpiration and so water stress good transport system xylem - to bring water and minerals phloem – to carry away the products of photosynthesis large surface area – to trap light palisade mesophyll – near top to trap maximum light spongy mesophyll to aid in light absorption leaves arranged in space so as not to shade each other. Transparent cuticle/upper epidermis to allow maximum light penetration