The relationship between light and photosynthesis is very important as plants cannot begin to synthesise without the energy produced by light. The more light received the increased rate of photosynthesis due to the presence of light giving energy to the reactions to photosynthesis. The intensity of...
UNIT 10: BIOLOGICAL MOLECULES AND METABOLIC PATHWAYS
EXPLORE THE FACTORS THAT CAN AFFECT THE PATHWAYS AND RATE OF PHOTOSYNTHESIS IN PLANTS
PHOTOSYNTHESIS IN PLANTS.
C.P5 EXPLAIN THE STAGES INVOLVED IN PHOTOSYNTHESIS IN PLANTS.
The relationship between light and photosynthesis is very important as plants cannot begin to synthesise without the
energy produced by light. The more light received the increased rate of photosynthesis due to the presence of light
giving energy to the reactions to photosynthesis. The intensity of light in relation to the inverse square law is the energy
of intensity changes in inverse proportion to the square of the distance from the source. The further you are from the
energy source, the decrease in energy. So, the decrease in intensity is proportional to the distance squared. A limiting
factor is something that is limiting the rate of a reaction. When light intensity is changed the outcome will change, as
the process of photosynthesis will change. When the intensity of light is high the rate of photosynthesis will drop as light
begins to damage the plant and the heat produced by the plant starts to denature the photosynthetic enzyme.
PHOTOSYNTHESIS: Takes place in chloroplasts.
WATER + CARBON DIOXIDE GLUCOSE + OXYGEN
6H20 + 6CO2 C6H1206 + 6O2
Plants obtain reactants for photosynthesis by the absorption of water from soil by root hair cells in the roots of the
plants which is then transported throughout the plant via osmosis. Carbon dioxide from the atmosphere is used for
photosynthesis as it enters through the stomata and diffuses through the cell membrane into the chloroplast. Theses
reactants are used during photosynthesis by the breakdown of water during photolysis step of LDS, breaking it down
into protons electrons and oxygen. Carbon dioxide is used to carboxylate RuBP during the Calvin cycle. The main uses of
these products are the breakdown of glucose in the plant cell during respiration. The ATP released by respiration is used
for the synthesis of protein or cell division allowing the plant to survive and mature. Plants use oxygen for aerobic
respiration, any excess leaves the leaf through the stomata into the atmosphere.
The grana and the stroma are two regions of the chloroplast the grana are the thylakoids stacked up; one on top of each
other. Thylakoids are individual membrane- bound sacs, which contain chlorophyll and their job role is to absorb light
and synthesises ATP. There are folds in the inner membrane of the chloroplast called lamella. Interregnal lamella are
between the grana. Thylakoid membranes give a large surface area for the photosynthetic pigment, electron carriers
and ATP synthase enzymes, all of which are used in the light-dependent stage. The stroma is a fluid filled matrix where
the enzymes needed for the light-independent stage are found.
PHOTOSYNTHETIC PIGMENT:
The pigments are arranged in special structure called photosystem. The funnel shape allows the structure to give the
maximum absorption of light energy, this is in the thylakoid membrane which held the photosystems in place by
proteins. In plants there is a green pigment called chlorophyll, it contains a mixture of 2 pigments known as P 680 and P700
and carotenoids. The pigments are found in the primary pigment reaction centre, which is the centre of the
photosystem.
P680 is found in photosystem II and absorbs red light strongly at a wavelength of 680nm.
P700 is found in photosystem I and absorbs red light strongly at a wavelength of 700nm.
CAROTENOIDS:
Carotenoids are pigments that reflect yellow and orange light and absorb blue light. Carotene and xanthophyll are the
main carotenoid pigments. Carotene is orange and xanthophyll is yellow. Carotenoids absorb wavelength which
chlorophyll cannot and pass light energy to chlorophyll, known as accessory pigments.
LIGHT- DEPENDANT STAGE (LDS): LDS is a stage of photosynthesis which takes place on the thylakoid membrane of the
chloroplast.
PHOTOLYSIS
, UNIT 10: BIOLOGICAL MOLECULES AND METABOLIC PATHWAYS
EXPLORE THE FACTORS THAT CAN AFFECT THE PATHWAYS AND RATE OF PHOTOSYNTHESIS IN PLANTS
PHOTOSYNTHESIS IN PLANTS.
(The splitting of water in the presence of light.) due to energy breaking the bonds.
Photosystem II(PSII) has an enzyme which splits the water in the presence of light into protons (H + ions), electrons, and
oxygen. The symbol equation for this is:
2H2O 4H+ + 4e- + O2
The equation shows how each product is used in photolysis:
H+ ions are moved to produce a proton gradient across the
thylakoid membrane during photophosphorylation through
ATP- synthase to produce ATP and to reduce the co-
enzyme NADP during photophosphorylation, which is used
in the LIS.
The electrons are transported along the thylakoid
membrane by electron carrier and are used to replace
electrons lost by oxidised chlorophyll.
Oxygen is used during aerobic respiration of plants and is
released via the stomata into the atmosphere.
PHOTOPHOSPHORYLATION
In photophosphorylation light hits the chlorophyll pigment in a photosystem, the energy is absorbed by the electrons
inside which become excited. As they pass along the electron carriers within the thylakoid membrane, they release
energy; used for chemiosmosis and therefore the production of ATP.
During photophosphorylation a photon hits PSII, the electrons inside absorb the energy and become excited, the
electrons are picked up by the electron acceptors and are passed along a series of electron carriers in the thylakoid
membrane energy is released as they pass, the energy released due to the movement of electrons pumps protons
across the thylakoid membrane into the thylakoid spaces, which creates a proton gradient. The protons travel down the
gradient through channels within ATP synthase enzyme. ATP synthase joins ADP and a phosphate group to make ATP.
CYLCIC AND NON CYLCIC ARE TWO TYPES OF PHOTOPHOSPHORYLATION
CYCLIC NON-CYCLIC
Only a small amount of ATP is produced. Both reduced NADP and ATP are produced.
Excited electrons pass to an electron acceptor and The electrons from the oxidised photosystem II
then back to the chlorophyll molecule that they replace the electrons lost from photosystem I.
were lost from. electrons lost by oxidised chlorophyll in PSII are
replaced with some from photolyzed water.
LIGHT- INDEPENDANT STAGE (LIDS): LIDS uses no light and takes place in the stroma and is also called Calvin cycle.
During this stage products of light dependant stage are used. If light becomes a limiting factor light independent stage
will stop.
Large organic molecules are produces from the carbon dioxide, this comes from the atmosphere and diffuses into the
open stomata in the leaf; through the spongy mesophyll layer across the palisade mesophyll cell wall and membrane
into the stroma.
THE CALVIN CYCLE
1. Carbon dioxide combines with a carbon dioxide acceptor called ribulose bisphosphate (RuBP). In the reaction
the enzyme Ribulose Bisphosphate carboxylase oxygenase (RuBisco) is needed.
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