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Edexcel Biology Unit 4 - Photosynthesis Notes
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This document contains detailed notes and diagrams for the Photosynthesis section of Unit 4 Edexcel biology for A Level Students
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BIOLOGY A2 NOTESUNIT 4
Topic 5A: Photosynthesis
KEY DEFINITIONS FOR THIS TOPIC
Phosphorylation Adding phosphate to a molecule (e.g., ADP is phosphorylated to form ATP)
Photophosphorylation Adding phosphate to a molecule using light.
Photolysis The splitting of a molecule using light energy
Hydrolysis The splitting of a molecule using water
Redox Reaction Reactions that involve oxidation AND reduction.
Autotrophic Those that make their own food, usually do this by photosynthesis.
Organisms They capture energy from the sun, transfer it into chemical energy in the
bonds of organic molecules such as glucose or starch.
Heterotrophic Those who generally eat plants or other animals. They indirectly use the
Organisms products of photosynthesis for making necessary molecules and to supply
energy for activities.
IMPORTANCE OF ATP
Adenosine Triphosphate
- When energy is needed, the third phosphate bond is broken by a hydrolysis reaction
- This reaction is catalysed by ATPase
- This results in the formation of ADP (Adenosine Diphosphate)
- In order to form ATP again, from ADP: The enzyme ATP Synthase is used and an inorganic
phosphate.
- This conversion is reversible.
- The energy released in catabolic reactions is used to drive the production of ATP
PHOTOSYTHESIS
This process involves:
- Reduction of carbon dioxide to a carbohydrate in the Calvin Cycle (the light independent
stage). These carbohydrates can be used to provide energy in respiration.
- The hydrogen for this process comes from splitting of water by light. And the waste oxygen
produced from the photolysis of water, is released into the atmosphere.
- This process also involves co-enzymes. (NADP is a co-enzyme used here – transfers hydrogen
from one molecule to another).
Products of Photosynthesis:
Sugars which can then be converted to:
- Polysaccharides (multiple sugars)
- Lipids (glycerol + may need a phosphate group + fatty acid chains which contain carbohydrates)
- Nucleic Acids
- Amino Acids (from sugars + mineral ions) which can then be made into proteins
, ROLE OF CHLOROPLAST IN PHOTOSYNTHESIS
Thylakoids Membrane discs in which chlorophyll pigment are embedded in. Here is where the light-
dependent stage of photosynthesis occurs.
Grana layers of thylakoid membranes within a chloroplast and grana are connected by stromal
thylakoids.
Lamellae Extension of thylakoid membranes which connect 2 or more grana and act as a
supporting skeleton in the chloroplast. They maintain a working distance so that they
receive the maximum light and function as efficiently as possible.
Stroma The matrix which surrounds the grana and contains all the enzymes needed to complete
the process of photosynthesis and produce glucose. Here is where the light independent
stage of photosynthesis occurs.
CHLOROPHYLL
This light capturing photosynthetic pigment is a mixture of closely related pigments
It contains the following:
- Carotene (ORANGE) (absorb blue and violet light)
- Phaeophytin (GREY) (absorb light in the middle of visible range, useful in dim light)
- Xanthophyll (YELLOW) (absorb blue and violet light)
- Chlorophyll A (BLUE-GREEN) (absorb red and blue light)
- Chlorophyll B (YELLOW – GREEN) (absorb red and blue light)
Different pigments react differently to varying amount of light intensity.
Hence, having more than pigment allows for more efficient absorption of light.
ABSORPTION SPECTRA
This described the different amounts of light of different wavelengths that a photosynthetic pigment
absorbs.
- It is possible to find absorption spectra of different pigments by measuring absorption of light of
different wavelengths
- It is also possible to produce an absorption spectrum for whole chloroplasts.
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