These revision notes provide an in depth summary of this specific chapter of AQA Biology in the A Level Specification -
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Biology revision – Photosynthesis (Unit 11)
Overview of photosynthesis
- In plants, energy is absorbed by chlorophyll and then transferred into the chemical
energy of the molecules formed during photosynthesis
- These molecules are used by the plant to produce ATP during respiration
- Non-photosynthetic organisms feed on the molecules produced by plants and then
also use them to make ATP during respiration
Site of photosynthesis
- The leaf is the main photosynthetic structure in eukaryotic plants
- Chloroplasts are the cellular organelles within the leaf where photosynthesis takes
place
Structure of the leaf
- Takes place largely in the leaf, the structure is show in the image
- Leaves are adapted to bring together the three raw materials of photosynthesis –
water, CO2 and light – and remove its products – oxygen and glucose
- These adaptations include:
o Large SA that absorbs as much sunlight as possible
o Arrangement of leaves on plant minimising overlapping and thus
avoids shadowing of one leaf by another
o Thin, as most light is absorbed in the first few micrometres of the
leaf and the diffusion distance for gases is short
o Transparent cuticle and epidermis – lets light through to
photosynthetic mesophyll cells beneath
o Long, narrow upper mesophyll cells packed with chloroplasts that
collect sunlight
o Many stomata for gas exchange so all mesophyll cells are only a
short diffusion pathway from one
o Stomata that open and close in response to changes in light
intensity
o Many air spaces in lower mesophyll to allow rapid diffusion in gas
phase of CO2 and O2
o Network of xylem bringing water to leaf cells and phloem that
carries away sugars produced during photosynthesis
Outline of photosynthesis
- Overall equation is as follows:
6CO2 + 6H2O C2H12O6 + 6O2
This is in the presence of both light and chlorophyll
- This equation is very simplified
o Photosynthesis is a complex metabolic pathway involving many intermediate
reactions
- It’s a process of energy transferral in which some of the energy in light is conserved in
the form of chemical bonds
FOR DIAGRAM SEE PAGE 269 – IMPORTANT!!!!
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