AQA A2 BIOLOGY UNIT 4: Photosynthesis

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  • Created on: 18-04-14 14:18
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6CO2 + 6H2O C6H12O6 + 6O2
In plants, light energy is transformed into the chemical energy of the molecules formed during
These molecules are used by the plant to produce ATP during respiration.
The leaf is the main photosynthetic structure.
The chloroplasts are the cellular organelles within the leaf where photosynthesis takes place.
Leaves are adapted to bring together the three raw materials of photosynthesis ­ water, carbon
dioxide and light.
These adaptations include:
Adaptation Benefits
Large surface area To collect as much sunlight as possible
Leaf arrangement on plant To minimise overlapping and so avoids the shadowing of one
leaf by another
Thin Keeps diffusion path short
Transparent cuticle and epidermis Allows light through to the photosynthetic mesophyll cells
Long, narrow upper mesophyll cells Allows light to be collected
packed with chloroplasts
Numerous stomata Gas exchange
Stomata that open and close In response to light intensity
Many air spaces in mesophyll layer Allows diffusion of carbon dioxide and oxygen
Network of xylem Brings water to leaf cells
Phloem Carries away the sugars produced in photosynthesis

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Photosynthesis is a complex metabolic pathway involving many intermediate reactions.
It is a process of energy transformation in which light energy is firstly changed into electrical energy
and then into chemical energy.
There are three main stages to photosynthesis:
Stage Process
1. Capturing light energy By chloroplast pigments such as chlorophyll.
1. The light-dependent In which light energy is converted into chemical energy.…read more

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The light dependent reaction of photosynthesis involves the capture of light whose energy is used for
two purposes:
- To add an inorganic phosphate molecule to ADP thereby making ATP
- To split water by the process of photolysis, into H+ ions and OH- ions
The reaction needs light energy.
It takes place in the thylakoid of the chloroplasts.
The light dependent reaction is useful for making three things:
1. Making ATP from ADP and inorganic phosphate
2. Making reduced NADP from NADP
3.…read more

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Light energy hits photosystem 2
This light energy excites the electrons and causes the chlorophyll to lose electrons
These electrons must be replaced
Light energy also causes water to split into protons, electrons and oxygen
The electrons from photolysis are used to replace the electrons lost from the chlorophyll
2. The excited electron that has come from the chlorophyll has a raised energy level
The electron is captured by an electron acceptor and enters an electron chain
3.…read more

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Chemiosmosis is the name of the process where the movement of H+ ions across a membrane
generates ATP ­ this also occurs in respiration.
The excited electrons lose energy as they move along the electron transport chain.
This energy is used to transport protons into the thylakoid so that the thylakoid has a higher
concentration of protons than the stroma.
This forms a proton gradient across the membrane.
Protons move down their concentration gradient, into the stroma via an enzyme ATP synthase.…read more

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The products of the light-dependent reaction of photosynthesis, ATP and reduced NADP are used to
reduce carbon dioxide in the second stage of photosynthesis.
The light-independent reaction takes place in the stroma of the chloroplasts.
The reaction is also called the Calvin cycle.
1.…read more

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Site of the Light-Dependent Reaction
The light dependent reaction of photosynthesis takes place in the thylakoids of chloroplasts.
The thylakoids are disc like structures that are stacked together in groups called grana.…read more

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Factors Affecting Photosynthesis
A limiting factor limits the rate at which a process can take place.
Processes such as photosynthesis are made up of a series of small reactions.
It is the slowest of these reactions that determines the overall rate of photosynthesis.
The law of limiting factors is expressed as:
At any given moment, the rate of a physiological process is limited by the factor that is at its least
favourable value.…read more

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The rate of photosynthesis is measured in two
The volume of oxygen released by a plant
The volume of carbon dioxide taken up by a plant
Effect of Light Intensity on the Rate of
When light is the limiting factor, the rate of
photosynthesis is directly proportional to light
intensity.…read more

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Growers use information about limiting factors to increase plant growth
Commercial growers know the factors that limit plant growth.
This means that they can create an environment where plants get the right amount of everything that
they need, which increases growth and so increases yield.
Growers create optimum conditions in the following ways:
Limiting Factor Management in Glasshouse
Carbon Dioxide Concentration CO2 is added to the air
E.g.…read more


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