A2 AQA Unit 4 Photosynthesis

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Photosynthesis
3.1 Overview of Photosynthesis
Humans, along with almost every other living organism, owe their continued
existence to photosynthesis
The energy we use, whether it comes from food when we respire or from the
wood, coal, oil or gas we burn in or homes, has been captured by
photosynthesis from sunlight
Photosynthesis likewise produces the oxygen we breathe by releasing it from
water molecules
Energy flows through all organisms and how it enters an organism depends on
its type of nutrition
In plants, light energy is transformed into the chemical energy of the
molecules formed during photosynthesis
These molecules are used by the plant to produce ATP during respiration
Nonphotosynthetic organisms feed on the molecules produced by plants and
then also use them to make ATP during respiration
Site of Photosynthesis
The leaf is the main photosynthetic structure
The chloroplasts are the cellular organelles in the leaf where photosynthesis
occurs
Structure of the Leaf
Photosynthesis takes place largely in the leaves which are adapted to bring
together the 3 raw materials of photosynthesis (H2O, CO2 and light) and
remove its product (O2 and glucose)
These adaptations include:
o A large surface area that collects as much sunlight as possible

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An arrangement of leaves on the plant that minimises overlapping and
so avoids the shadowing of one leaf by another
o Thin, as most light is absorbed in the first few millimetres of the leaf
and so the diffusion distance is thus kept short
o A transparent cuticle and epidermis that let light through to the
photosynthetic mesophyll cells beneath
o Long, narrow upper mesophyll cells packed with chloroplasts that collect
sunlight
o Numerous stomata for gaseous exchange
o Stomata that open and close in…read more

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They are surrounded by a double membrane and inside the membrane there
are 2 distinct regions
o The grana are stacks of up to 100 disclike structures called
thylakoids where the lightdependent 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 intergranal lamellae
o The stroma is a fluidfilled matrix where the lightindependent stage of
photosynthesis takes place.…read more

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The LightDependent Reaction
The lightdependent reaction of photosynthesis involves the capture of light
whose energy is used for two purposes:
1) To add an inorganic phosphate molecule to ADP, thereby making ATP
2) To split water into H+ ions (protons) and OH ions.…read more

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The loss of electrons when light hits a chlorophyll molecule leaves it short of
electrons
If the chlorophyll molecule is to continue absorbing light energy, those
electrons must be replaced
The replacement electrons are provided from water molecules that are split
using light energy
This photolysis of water also yields H+ ions
The equation for this process is:
These H+ ions are taken up by an electron carrier called NADP
On taking up the H+ the NADP becomes reduced
The reduced NADP then enters the…read more

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The LightIndependent Reaction
The products of the lightdependent reaction, namely ATP and reduced NADP,
are used to reduce CO2 in the 2nd stage
Unlike the 1st stage, this stage doesn't require light directly and, in theory,
occurs whether or not light is available
It is therefore called the lightindependent reaction
In practice, it requires the products of the lightdependent stage and so
rapidly ceases when light is absent
The lightindependent stage takes place in the stroma of the chloroplasts
The details of this stage were…read more

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Most triose phosphate molecules are used to regenerate ribulose
bisphosphate using ATP from the lightdependent reaction
Site of the LightIndependent Reaction
The lightindependent reaction takes place in the stroma
The chloroplast is adapted to carrying out the lightindependent reaction of
photosynthesis in the following ways:
o The fluid of the stroma contains all the enzymes needed to carry out
the lightindependent reaction (reduction of CO2)
o The stroma fluid surrounds the grana and so the products of the
lightdependent reaction in the grana can readily…read more

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At this point a different factor, such as temperature, is the limiting
factor and only an alteration in its level will affect the rate of
photosynthesis
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
In turn, it is the level of factors such as temperature and the supply of raw
materials that determines the speed of each step
The law of limiting factors can then be…read more

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It's still one of the rarest gases present and is often the factor that limits
the rate of photosynthesis under normal conditions
The optimum concentration of CO2 for a consistently high rate of
photosynthesis is 0.…read more

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