Biology unit 4 module 4.5 revision

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Module 4.5
In photosynthesis light energy is transferred to ATP in the light dependent reactions and used to
make carbohydrate in the light independent reactions.
Know photosynthesis has both light dependent and light independent reactions.
Know that in the light dependent reactions light energy excites electrons, the energy they contain
is used to generate ATP and reduced NADP as they pass along electron transport chains in the
chloroplast membranes, photolysis of water produces protons, electrons and oxygen.
Know that in the light independent reactions ribulose bisphosphate (RuBP) and CO2 join and form
two molecules of glycerate phosphate (GP); ATP and reduced NADP are required (obtained from
LDR); RuBP is regenerated in the Calvin cycle. Triose phosphate (TP) can be converted to
carbohydrates, fats and proteins as needed by the plant.
Appreciate how temperature, light intensity, CO2 concentration affect the rate of photosynthesis
and can be limiting factors.
Thylakoid: series of flattened membranous sacs in a chloroplast that contain chlorophyll and the
associated molecules needed for the light-dependent reaction of photosynthesis.
Grana: a stack of thylakoids in chloroplasts that resembles a pile of coins. This is the site of the
light-dependent reaction of photosynthesis.
Stroma: matrix of a chloroplast where the light-independent reaction of photosynthesis takes place.
Light Dependent reactions: stage of photosynthesis in which light energy is required to produce ATP
and reduced NADP.
Light independent reactions: stage of photosynthesis which does not require light energy directly
but does need the products of the light dependent reaction to reduce carbon dioxide and so form
NADP: (nicotinamide adenine dinucleotide phosophate) ­ a molecule that carries electrons produced
in the light-dependent reaction of photosynthesis.
Photolysis: splitting of water molecule by light such as occurs in the light dependent reaction of
RuBP: Ribulose Bisphosophate
GP: Glycerate phosophate
TP: Triose phosphate
Calvin Cycle: a biochemical pathway that forms part of the light-independent reaction of
photosynthesis, during which carbon dioxide is reduced to form carbohydrate.

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Limiting factor: a variable that limits the rate of a chemical reaction.
Light dependent reaction:
o To add an inorganic phosphate molecule to ADP to make ATP
o To split water into H+ ions (protons) and OH- ions. As the splitting is caused by light ­
The making of ATP
o Chlorophyll absorbs light energy.
o The electrons gain energy and rise to a higher energy state.
o Electrons are said to be excited ­ so they leave the chlorophyll.…read more

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The loss of electrons when light strikes a chlorophyll molecule leaves it short of
o If the chlorophyll molecule is to continue absorbing light energy it needs a
replacement of electrons.
o They are provided from water molecules.
o Photolysis also yields H+ ions.
o 2H2O 4H+ + 4e- + O2
o The hydrogen ions are taken up by an electron carrier called NADP.…read more

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The light-independent reaction
o Products from the light dependent
reaction ­ ATP + reduced NADP are used
to reduce carbon dioxide.
o Takes place in the stroma of
o The Calvin cycle.
1. Carbon dioxide from the
atmosphere diffuses into the leaf through
the stomata and dissolves in water around
the walls of the mesophyll cells. It then
diffuses through the plasma membrane into the stroma of the chloroplasts.
2.…read more

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Most TP molecules are used to regenerate ribulose bisphosphate using ATP from the light
dependent reaction.
Site of the light-dependent reaction
Takes place in the stroma of chloroplasts.
The fluid of the stroma contains all the enzymes needed to carry out the light-independent
reaction (reduction of carbon dioxide)
The stroma fluid surrounds the grana and so the products of the light-dependent reaction in
the grana can readily diffuse into the stroma.…read more

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In the light-dependent reaction of photosynthesis, light energy generates ATP. Describe how:
Light energy excites/raises energy level of electrons in chlorophyll.
Electrons pass down electron transfer chain;
Electrons reduce carriers/passage involves redox reactions
Electron transfer chain/role of chain associated with chloroplast membranes/in
Energy released/ carriers at decreasing energy levels;
ATP generated from ADP and Pi/ phosphorylation of ATP
Example such as chemiosmosis
In the light-independent reaction of photosynthesis, the carbon in carbon dioxide becomes carbon in
triose phosphate.…read more


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