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Summary AQA A level Biology (A2) - Energy Systems
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Full lesson and revision notes on AQA A2 Biology: Energy transfers in and between organisms
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PhotosynthesisGreen tissues on plants are able to photosynthesise as they have chloroplasts in them (which gives
them the green colour). Palisade cells are the main site of photosynthesis but mesophyll cells also
have photosynthesis capabilities. Photosynthesis is a two stage process:
1. Light dependant stages – Light is an absolute requirement. It occurs within the thylakoid
membrane.
2. Light independent stages – Does not require light. It occurs within the stroma.
Light Dependant Reactions
Light of the correct wavelength – red or blue – strikes the Photosystem II (chlorophyll molecule) (1).
The electron in the Photosystem II gets excited and leaves to travel to a higher energy level (2). This
is called Photoionisation. This is then accepted by an electron carrier molecule in a reduction
reaction (3). In a series of Redox reactions (4), the electron is passed down a series of electron
carriers (5). Energy from the transfer is used to move H+ across the thylakoid membrane into the
thylakoid space (6). This generates a high H+ concentration in the thylakoid space (7). Then light from
the correct wavelength strikes the Photosystem I (8). Photoionisation then occurs (electron gets
excited and leaves) and is then accepted by an electron carrier molecule (9). Then, in a series of
redox reactions (11), an electron is passed down a series of electron carriers (12) before being finally
accepted by NADP (together with an H+) and is reduced to NADPH (13).
Photolysis of water
H2O is split in the presence of light to produce:
• An electron (e-) to replace those lost by chlorophyll molecules.
, • H+ used to move across thylakoid membrane and to reduce NADP.
• ½ O2 is waste produce and diffuses out of chloroplast.
Chemiosmosis
H+ molecules flood back through the ATP synthase down the massive concentration gradient from
the thylakoid space back into the stroma. The ATP synthase re-generates ATP from ADP +Pi using the
kinetic energy from the H+ concentration. The process in which sunlight (indirectly) forms ATP is
called photophosphorylation.
Light Independent Reactions – The Calvin Cycle
, RuBP combines with CO2 (1) to produce two molecules of GP (2). The GP is then reduced to TP (3)
using energy from ATP (4) and the reducing power/electron from NADPH (5). The TP is then
converted back to RuBP (6) using energy from ATP (7) which is then converted to a glucose molecule
and other organic molecules (8).
Photosynthesis Summary
Photosynthesis Major Players
1. Adenosine Triphosphate (ATP)
• The universal cellular form of energy.
• Immediate form of energy as energy is released in single reactions in manageable
amounts. OILRIG
• It’s recyclable. Oxidation
2. Nicotinamide Adenine Dinucleotide Phosphate (NADP) Is
• An electron carrier. Lost
• Gains/loses an electron in a redox reaction Reduction
(oxidation/reduction). Is
3. Chlorophyll Gain
• A light absorbing pigment.
• There are different types of chlorophyll.
• They are arranged in photosystems
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