Photosynthesis- F214

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  • Created by: Laura
  • Created on: 02-04-13 14:42


Photosynthesis: the process whereby light energy from the sun is transformed into chemical energy and used to synthesise large organic molecules from inorganic substances.

Photosynthetic pigments: have a long phyto1 (hydrocarbon) and a porphyrin (magnesium).

  • Chlorophyll a: P680 found in PSII, P700 found in PSI. Chlorophyll b: absorbs 500-680nm.
  • Carotenoids: absorb blue pigments, reflect red and orange pigments.

Types of Organisms:

  • Autotrophs- use light/chemical energy to synthesis complex organic molecules.
  • Heterotrophs- organisms that ingest and digest complex organic molecules, releasing the chemical potential energy stored. 
  • Chemoautotrophs- prokaryotes that synthesus complex organic molecules using energy derived from exergonic chemical reactions (reactions that release energy).
  • Photoautotrophs- plants, some bacteria and some protoctists that gather their energy from sunlight.
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Light dependent stage

Photophosphorylation- the making of ATP from ADP and Pi in the presence of light.

Non-cyclic phosphorylation: Occurs in the GRANA.

1. Light hits photosystem II, exciting a pair of electrons. They leave PSII. These electrons travel along the elctron carrier chain (made up of Ferredoxin, an Fe-S complex that carries electrons). Energy is released to produce ATP. 

2. Light strikes photosystem I, exciting a pair of electrons. The electrons leave PSI, and join with NADP and the protons produced at PSII (from photolysis of water) to make reduced NADP. This is catalysed by NADP reductase.

3. The electrons produced during photolysis replace those lot from photosystem II.

4. Some protons and electrons from photophosphorylation are used in chemiosmosis in respiration to make ATP.

In cyclic photophosphorylation there is NO PHOTOLYSIS OF WATER. This means there is no reduced NADP formed. This is used in guard cells to bring in K+, lowers the water potential to increase intake of water and carbon dioxide as guard cells swell.

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Light independent stage

This is used either after the light dependent stage, or when there is a shortage of light. 

1. Carbon dioxide diffuses into the leaf and then through the chloroplast envelope into the STROMA.

2. Carbon dioxide joins with ribulose bisphosphate to make an unstable 6 carbon intermediate, this is catalysed by rubisco.

3. This intermediate is immediately broken down to form two 3- carbon molecules of glycerate 3-phosphate

4. Glycerate-3-phosphate is reduced and phosphorylated to form triose phosphate (TP). In the process, reduced NADP from the light dependent stage is oxidised to form NADP and ATP loses a phosphate group, ATP--> ADP.

5. 5/6 molecules of triose phosphate are recycled to form 3 molecules ribulose bisphosphate. The rest are converted into hexose sugars or fatty acids.

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Chloroplast Adaptations

- The inner membrane controls the entry of substances between the cytoplasm and the stroma. This means that it can limit how much water and carbon dioxide enters and can help get rid of waste products like oxygen.

- The grana provide a large surface area for the reaction to take place. The larger the surface area, the larger the space for successful collisions to take place.

- Proteins are embedded in the grana. These help to hold the photosystems in place.

- Chloroplasts can make needed proteins for the reactions using their DNA. This means that they can create rubisco for the light independent reaction. NADP is also a protein as it is a coenzyme (molecules that help enzymes carry out oxidation or reduction reactions. They carry atoms/molecules from one enzyme controlled reaction to another).

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Limiting Factors

Limiting factor- the factor that is present at the least favourable value. 

Effects of light:

  • Stomata open so carbon dioxide can enter.
  • Light is trapped by chlorophyll which excites electrons in the photosystems.
  • It splits water to make protons.

Light intensity is a limiting factor until it reaches a maximum, at which point the amount of light has no effect on the rate of reaction as it is working at its optimum.

Effects of temperatuure:

  • If the temperature is too high, the rate of photosynthesis decreases. This is because oxygen competes for the active site in rubisco. 
  • If too high, there will be a large water loss, triggering a stress response in the stomata which closes them, so less carbon dioxide can enter and the rate continues to decrease.
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