Photosynthesis

Contents:

  • Structure of the leaf
  • Structure and role of chloroplasts
  • The light-dependent reaction
  • The light-independent reaction
  • Factors affecting photosynthesis
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  • Created by: Emilie
  • Created on: 17-06-15 15:10
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Photosynthesis
The leaf is the main photosynthetic structure. The chloroplasts are the cellular organelles within
the leaf where photosynthesis takes place.
Structure of the leaf
Leaves are adapted to bring together the three raw materials of
photosynthesis (water, carbon dioxide and light) and remove its
products (oxygen and glucose). These adaptations include:
A large surface area that collects as much sunlight as
possible
An arrangement of leaves on the plant that minimises overlapping and so avoids the
shadowing of one leaf by another
Thin, as most light is absorbed in the first few millimetres of the leaf and the diffusion
distance is thus kept short
A transparent cuticle and epidermis that let light through to the photosynthetic mesophyll
cells beneath
Long, narrow upper mesophyll cells packed with chloroplasts that collect sunlight
Numerous stomata for gaseous exchange
Stomata that open and close in response to changes in light intensity
Many air spaces in the lower mesophyll layer to allow diffusion of carbon dioxide and oxygen
A network of xylem that brings water to the leaf cells, and phloem that carries away the
sugars produced in photosynthesis
Structure and role of chloroplasts
Chloroplasts vary in shape and size but are typically disc shaped, 2-10µm long and 1µm in
diameter. They are surrounded by a double membrane and have two distinct regions within the
membrane:
The grana are stack of up to 100 disc-like structures called thylakoids where the
light-dependent stage of photosynthesis takes place. Within the
thylakoids is the photosynthetic pigment called chlorophyll. Some
thylakoids have tubular extensions that join up with thylakoids in
adjacent grana; these are called inter-granal lamellae.
The stroma is a fluid-filled matrix where the light-independent
stage of photosynthesis takes place. Within the stroma are a
number of other structures such as starch grains.
The overall (simplified) equation for photosynthesis is:
6CO2+ 6H C
2O
6H
12O 6+ 6O2
There are three main stages to photosynthesis:
1. Capturing of light energyby chloroplast pigments such as chlorophyll

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The light-dependent reaction , in which light energy is converted into chemical energy
3. The light-independent reaction , in which protons (hydrogen ions) from the LDR are used to
reduce carbon dioxide to produce sugars and other organic molecules
The light-dependent reaction
The LDR 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 into H+ ions and OH-ions.…read more

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The light-independent reaction
The products of the LDR (namely ATP and NADPH) are used to reduce carbon dioxide in the
second stage of photosynthesis. Unlike the first stage, this stage does not require light directly
and, in theory, occurs whether or not light is available. In practice, however, it requires the
products of the LDR and so rapidly ceases when light is absent. The LIR takes place in the stroma of
the chloroplasts and is referred to as the Calvin cycle.
1.…read more

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Factors affecting photosynthesis
In a complex process such as photosynthesis, the factors that affect it all operate together.
However, the rate of photosynthesis at any given moment is not affected by all the factors, but
rather by the one whose level is at the least favourable value. This is the limiting factor because it
limits the rate at which the process can take place.
Effect of light intensity
When light is the limiting factor, the rate of photosynthesis is directly proportional to light
intensity.…read more

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