Photosynthesis 

Important definitions • Compensation point – point where there is no net exchange of gases into or out of a plant • Energy – the ability to do work • Limiting factor – at any given moment, the rate of a physiological process (e.g. photosynthesis) is limited by the factor that is at the least favourable value • Metabolism – a series of metabolic reactions • Oxidation – the loss of electrons • Photolysis – splitting water molecules into H+ and O2 using light energy • Reduction – the gain of electrons 

Chloroplasts Membranes within the chloroplast is arranged into stacks called grana A granum is made of stacks of thylakoids (inside their membrane is where the chlorophyll pigment is found) Granal membranes have stalked particles (used during ATP synthesis) Stroma is the fluid interior (containing enzymes needed to complete photosynthesis and produce glucose) 

Absorption spectrum Indicates how much light a particular pigment absorbs at each wavelength Chlorophyll absorbs wavelengths in blue and red parts of the spectrum The greater the range of pigments in a leaf, the wider the range of wavelengths of light that can be absorbed 

Action spectrum Shows the rate of photosynthesis at different wavelengths of light There is a close correlation between the absorption and action spectra for chlorophyll suggesting that these pigments are responsible for absorbing light that will be used in photosynthesis 

The light dependent stage 1. Occurs in the thylakoid membrane, Chlorophyll in photosystem II absorbs light energy 2. 2 electrons become excited and rise to a higher energy level where they are taken up by an electron carrier 3. The excited electrons a passed along a number of electron carriers in a series of oxidation-reduction reactions. Each carrier is at a lower energy level than the previous and so electrons lose energy at each stage 4. The energy lost during the electron carrier chain is used to make ATP (photophosphorylation) 5. Chlorophyll in photosystem I absorbs light energy 6. 2 electrons become excited and rise to a higher energy level where they are taken up by an electron carrier 7. Electrons can either join the electron carrier chain (cyclic photophosphorylation) or join a shorter chain (non-cyclic photophosphorylation) 8. Electrons join with NADP and H+ (from photolysis of water) to produce NADPH (reduced NADP) 9. Photolysis of water provides replacement electrons to those lost in PS II and PS I 10. O2 is used in respiration or diffused out of the leaf as a waste product of photosynthesis 

1 2 3 4 5 6 7 8 9 10 Cyclic photophosphorylation 

The light independent stage (Calvin cycle) 1. Occurs in the stroma, CO2 combines with ribulose bisphosphate (5 carbon compound) with the aid of the enzyme RUBISCO. This forms 2 molecules of glycerate – 3 – phosphate (3 carbon compound) 2. ATP and NADPH (from light dependent stage) are used to reduce glycerate – 3 – phosphate to triose phosphate (3 carbon compound) 3. 1/6 of triose phosphate forms useful organic substances like glucose 4. 5/6 of triose phosphate is used to regenerate ribulose bisphosphate using ATP (from light dependent stage) 

CO2 RUBISCO Ribulose bisphosphate (5C) Glycerate – 3 – phosphate (3C) Triose phosphate (3C) Useful organic substances like glucose 1/6 of TP 5/6 of TP ATP ADP + Pi ATP ADP + Pi NADPH NADP 1 2 3 4 

Estimating the rate of photosynthesis • Measure volume of O2 released per unit time • Measure volume of CO2 taken in per unit time • Measure amount of carbohydrate produced per unit time • Use a potometer • Assumes all water going in = all water coming out although some will be used for photosynthesis • Factors limiting photosynthesis: the rate of photosynthesis is limited by the factor at its least favourable value 

Light intensity effecting photosynthesis • When light is the limiting factor, rate of photosynthesis is directly proportional to light intensity • As light increases, the volume of O2 produced and CO2 absorbed will increase, so increases rate of photosynthesis • Further increases cause a proportional increase in O2 given off and CO2 absorbed • A point will be reached where any further light intensity increase will not affect the rate of photosynthesis • A compensation point may be reached (no net exchange of gases) 

Carbon dioxide effecting photosynthesis • Optimum concentration of CO2 for a constant high rate of photosynthesis is 0.1% • Normal atmospheric concentration is 0.04% • Greenhous users enrich the air inside with CO2 to provide higher yields • CO2 concentration effects enzyme activity especially RUBISCO 

Temperature effecting photosynthesis • If no other limiting factors, rate of photosynthesis is directly proportional to temperature • Between 0°C and 25°C (the rate doubles for each 10°C rise approx.) • Many plants have an optimum of 25°C anything above will denature enzymes • Light dependent stage is not usually affected