A2 - AQA Biology (Unit 4) - Photosynthesis

These are the notes I made for Photosynthesis during my Unit 4 revision.

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Photosynthesis Notes
1. Overview
The leaf is the main photosynthetic structure.
The Chloroplasts are the cellular organelles within the leaf where photosynthesis takes place.
Leaves are adapted to allow in the materials for photosynthesis, Water, Carbon Dioxide and
Light, and is adapted to remove the products, Oxygen and Glucose.
Photosynthesis Equation: 6CO2 + 6H2O -> C6H12O6 + 6O2
Adaptations of the leaf:
Large Surface Area to collect as much Sunlight as possible.
Leaves arranged to minimise overlapping, allowing as much sunlight into all the leaves as
possible.
Thin because most light is absorbed within the first few millimetres of the leaf, and this
reduces the diffusion pathway.
Transparent Cuticle and epidermis which allow light through to the photosynthetic mesophyll
cells.
Long, narrow upper mesophyll cells have many chloroplasts to maximise photosynthesis.
Lots of Stomata for Gas Exchange. These Stomata can open and close in response to changes
in light intensity.
Air Spaces in the lower mesophyll layer allow diffusion of CO2 and O2.
Xylem vessels water to the leaf cells and Ploem carry away sugars.
Structure of the Chloroplasts
Photosynthesis takes place inside of the
Chloroplast. Chloroplasts are small, flattened
organelles found in plant cells.
They have a double membrane (phospholipid
bi-layer) called the Chloroplast envelope.
Thylakoids are stacked up in the Chloroplasts
into structures called Grana. The Grana are
linked by membranes known as Lamellae.
Chloroplasts contain photosynthetic
pigments (Chlorophyll A, Chlorophyll B and
Carotene). These are coloured substances
that absorb the light energy needed during
Photosynthesis. The pigments are found within the membrane of the Thylakoids and are attached to
proteins. The protein and pigment are called a Photosystem.
The 2 photosystems used in Photosynthesis are:
Photosystem 1 (PS1) ­ This absorbs light best when it has wavelength 700nm.
Photosystem 2 (PS2) ­ This absorbs light best when it has wavelength 680nm.

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These photosystems are both part of the Light-Dependent Reactions in Photosynthesis.
2. The Light-Dependent Reaction (LDR)
As the name suggests, this reaction requires light, and is why light is needed during
Photosynthesis.
The LDR takes place within the Thylakoid membrane, making use of PS1 and PS2.
When light hits PS2 an electron in the pigment gets excited and breaks from the atom. This
electron travels along an electron carrier chain, which is between the double membranes,
until it reaches PS1.…read more

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The Light-Independent Reaction (LIR)
The LIR is also known as the Calvin Cycle.
The Calvin Cycle takes place in the Stroma of the Chloroplast, NOT the Thylakoid like the LDR.
ATP, NADPH and CO2 are used in this reaction.
The purpose of the Calvin Cycle is to reduce CO2 into Glucose using the energy sources made
in the LDR.
The Calvin Cycle
Ribulose Bisphosphate (found in the Stroma) is recycled in the Calvin Cycle
The Calvin Cycle "turns" 6 times to make 1 hexose sugar.…read more

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Factors Limiting Photosynthesis
The ideal conditions for Photosynthesis vary between plant species, but the majority of species living
in the UK are best suited for the following conditions:
High light intensity of a certain wavelength.
o Light is needed to make NADPH and ATP in Photosynthesis. The higher the light
intensity, the more energy the plant is provided.
o Only certain wavelengths of light are used in photosynthesis. The pigments,
Chlorophyll A, Chlorophyll B and Carotene only absorb the red and blue light in
sunlight.…read more

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