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Photosynthesis - A-Level Biology notes
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Photosynthesis - A-Level Biology notes
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Chapter 11: Photosynthesis11.1 Overview of photosynthesis
- The leaf is the main photosynthetic structure in eukaryotic plants
- Chloroplasts are the cellular organelles in the leaf where photosynthesis occurs
Adaptation of the leaf:
- Large surface area for maximum absorption of light
- And arrangement of leaves that minimises overlapping
- Thin for a short diffusion path
- Transparent cuticle allowing epidermis to let light through to mesophyll cells
- Long and narrow upper mesophyll cells are packed with chloroplasts
- Numerous stomata for gas exchange
- Stomata open and closes which helps control water loss
- Many airspaces in the lower mesophyll to allow rapid diffusion of CO2 and O2
- A network of xylem and phloem
Main stages of photosynthesis:
1. Capturing of light energy
2. Light dependent reaction
3. Light independent reaction
Structure and role of chloroplasts in photosynthesis:
- Disk shaped, 2-10 um, long and 1um wide.
- Double membrane
- Grana are stocks of up to 100 thylakoids where the light dependent reaction occurs
- Some thylakoids have tubular extensions that join up with thylakoids in adjacent grana
- Stroma is a fluid filled matrix where the light independent reaction occurs. Within this stroma there are a
number of other structures like starch grains
11.2 The light dependent reaction
Light energy is used to:
- Add inorganic phosphate to ADP to make ATP Oxidation: when a substance loses electrons and energy is given out
- Split water to H and OH by photolysis Reduction: when a substance gains electrons and energy is taken in
Making of ATP:
1. A chlorophyll molecule absorbs light energy and excites a pair of electrons within the molecule
2. The pair of electrons reach a high energy level and leave the molecule
3. The chlorophyll molecule becomes ionised by photoionization
4. The pair of electrons that left, are picked up by an electron carrier
5. Chlorophyll molecule becomes oxidised and the electron carrier is reduced
6. The pair of electrons pass along a number of electron carriers in a series of oxidation-reduction reactions. These
electron carriers form a transfer chain located in the thylakoid membrane
7. As the electrons move along they lose energy which is then used to phosphorylate ADP.
Chemiosmotic theory:
1. Each thylakoid is an enclosed chamber into what H protons are pumped from this stroma using proton pumps in
the membrane
2. Energy for this process comes from photolysis of water
3. Photolysis of water also produces protons which further increase their concentration inside the thylakoid space
4. Overall this creates and maintains a concentration gradient of protons with a high concentration in the thylakoid
space and a low concentration in the stroma.
5. Protons can only cross the thylakoids membrane through ATP synthase channel proteins. These channels form
small granules on the surface membrane
6. As the protons pass through ATP synthase channels they cause changes to the structure of the enzyme which
then catalyses the phosphorylation of ADP.
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