Photosynthesis

Photosynthesis and Respiration

Practical Investigations

The Light-Dependent Stage

The Light-Independent Stage

Factors affecting Photosynthesis

Chloroplast and Photosynthetic Pigments

Photosynthesis

Autotrophic Nutrition - Organic molecules are synthesised from non-organic material (water and carbon dioxide)

Plants are photoautotrophs, light energy is the source for autotrophic nutrition

6CO2 + 6H20 + energy from photons --(chlorophyll)--> C6H12O6 + H2O

Carbon Fixation - the process by which CO2 is converted into sugars, it is a reduction reaction and endothermic, requiring energy, it helps regulate CO2 levels

Respiration

Heterotrophs - obtain energy by digesting organic molecules

All living organisms respire, oxidising organic material, releasing chemical energy (exothermic)

C6H12O6 + 6O2 --> 6H2O + 6CO2 + energy

How they Interrelate

Photosynthesis and Respiration are opposites, allowing a balance of substances

Compensation Point

When photosynthesis and Respiration proceed at the same rate, with no net gain or loss of carbon dioxide

The time of day at which the plant is at compensation point is called the compensation period, this is different for each species

Chloroplasts

Grana

Stroma

Photosynthetic Pigments

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Photosystems

Water

Photophosphorylation

Non-Cyclic

Cyclic

Photosystem I (PSI) has a peak absorption of 700nm (P700)

Photosystem II (PSII) has a peak absorption of 680nm (P680)

An enzyme in PSII splits water in the presence of light, this is photolysis

2H20 --> 4H+ + 4e- + O2

Photolysis creates a source of protons for photophosphorylation and donates electrons to chlorophyll to replace the lost electrons

Electrons in the photosystem II are hit by light and become excited which therefore leave the chlorophyll(magnesium) which becomes oxidised

The electrons move through a series of electron carriers (redox reactions of iron) in the thykaloid membrane, losing energy each stage. The energy goes to help pump protons into the lumen

The electrons are replaced by electrons from photolysis

The electrons are captured by photosystem I, replacing those lost from PSI due to excitation by light energy

Ferredocxin accepts electrons from PSI and passes them to NADP in the stroma

Protons build up in the thykaloid, creating a proton gradient for chemiosmosis

Protons diffuse out of ATP Synthase, the flow of protons causing ADP and Pi to for ATP

The protons are accepted by NADP (along with the electrons) becoming reduced NADP, This is catalysed by NADP reductase

The Calvin Cycle

Light strikes PSI, releasing a pair of electrons into the electron carriers

ATP is generated and eventually the electrons are cycled back into PSI

Ribulose biphosphate (RuBP) accepts CO2, forming a 6C molecule before breaking down into 2 glycerate-3-phosphae (GP) (3C) molecules, catalysed by RuBisCO

2GP is reduced by 2 reduced NADP using the energy from 2ATP into 2 triose phosphate (TP)

It takes 6 turns of the cycle to create 12TP molecules, of which 10 are recycled and 2 are the product

ATP is used to convert 2TP into RuBP

It takes place in the stroma

TP Uses

TP can synthesis glucose

TP can synthesis amino acids, fatty acids and glycerol (lipds)

5 molecules of TP can create 3 RuBP molecules

RuBisCO has a optimum pH of 8, and is activated by large amounts of ATP and the cofactor magnesium

Light Intensity

Carbon Dioxide

Temperature

Water Stress

the conditions a plant will experience when water supply is limited

Cells become plasmolyed

Plant roots produce abscisic acid that, when transloacted to the leaves, cause stomata to close, reducing gas exchange

The rate of photosynthesis is reduced

Tissues become flaccid and leaves wilt

Roots are unable to replace water lost by transpiration

Increases in temperature increase rate of photosynthesis up to 30*C

Above 30*C, rates may reduce due to photorespiration: oxygen competing with carbon dioxide for RuBisCO

Above 45*C, enzymes may denature, reducing photosynthetic rates