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DISTINCTION - Unit 10 Biological Molecules and Metabolic Pathways - Assignment C
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factors that can affect the pathways and the rate of photosynthesis in plants
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UNIT 10 ASSIGNMENT CC.P5 - explain the stages involved in photosynthesis in plants
C.P6 - carry out investigation into a factor that affects the rate of photosynthesis
C.M4 - analyse primary and secondary data to explain the outcomes of an investigation into
a factor that affects the rate of photosynthesis
C.D3 - evaluate the effect of factors of photosynthetic efficiency
There are two stages that make up photosynthesis
1. The light dependant reaction - As the name implies, this process requires light
energy. It occurs in the chloroplast's thylakoid membranes. Light energy is absorbed
and transformed to chemical energy by photosynthetic pigments in photosystems.
The energy is used to form ATP by adding a phosphate group to ADP and to decrease
NADP to form reduced NADP. (Because it can transmit hydrogen and hence electrons
to other molecules, reduced NADP is an energy-rich molecule.) ATP transfers energy
to the light dependant reaction and reduced NADP transfers hydrogen to the light
dependant reaction. during this process water is oxidised to oxygen
2. The light independent reaction - This is also known as the Calvin cycle, and as the
name implies, it does not directly use light energy, but rather relies on the products
of the light dependant reaction. It takes place in the chloroplast stoma. ATP and
reduced NADP from the light independent reaction provide the energy and hydrogen
required to produce glucose from CO2.
(Fielding and Anderson, n.d.)
Photophosphorylation produces ATP in the light-dependent reaction. The light energy
absorbed by photosystems is used for three purposes in the light-dependent reaction:
1. making ATP from ADP and inorganic phosphate. Photophosphorylation is the term
for this reaction.
2. creating reduced NADP from NADP
3. Separating protons (H+ ions), electrons, and oxygen from water this is referred to as
photolysis.
(Fielding and Anderson, n.d.)
The Calvin cycle is another name for the light independent reaction that occurs in the stoma
of chloroplasts. Carbon dioxide fixation refers to the process by which carbon from CO2 is
fixed into an organic molecule. From CO2 and ribulose bisphosphate, it creates a molecule
known as triose phosphate. Triose can be broken down into glucose and other
useful organic compounds. Because the process is a cycle the starting compound ribulose
bisphosphate is regenerated.
1. CARBON FIXATION: Carbon dioxide is combined with ribulose bisphosphate to form
two molecules of glycerate 3 phosphate
CO2 enters the leaf via the stomata and diffuses into the chloroplast stroma. It is combined
with ribulose bisphosphate (RuBP), a 5-carbon compound. This produces an unstable 6
carbon compound, which quickly breaks down into two molecules of a 3-carbon
compound called glycerate 3 phosphate (GP). The reaction between CO2 and ribulose
bisphosphate is catalysed by ribulose bisphosphate carboxylase (RuBisCO).
2. REDUCTION: ATP and reduced NADP are required for the reduction Of GP triose
phosphate
, Now, the energy provided by ATP from the light-dependent reaction is used to convert the
3-carbon compound GP into a different three carbon compound known as triose phosphate
(TP). This reaction also requires H+ ions, which are obtained from reduced NADP [another
light-dependent reaction]. Reduced NADP is recycled to NAPD [for use in the light-
dependent reaction again]. Triose phosphate is then transformed into a variety of useful
organic compounds
3. REGENERATION: Ribulose bisphosphate is regenerated
Five out of every six molecules of TP produced in the cycle are used to regenerate RuBP
rather than to produce hexose sugars. The remaining ATP produced by the light-dependent
reaction is used to regenerate RuBP.
TP and GP can be converted into useful organic compounds such as glucose. TP and GP are
used in the synthesis of carbohydrates, lipids, and amino acids.
• Carbohydrates – Hexose sugars are formed by joining two triose phosphate molecules,
and larger carbohydrates are formed by joining hexose sugars in various ways.
• lipids- glycerol is synthesised from triose phosphate, and fatty acids are synthesised from
glycerate 3 phosphate.
• amino acids- some amino acids are synthesised from glycerate 3 phosphate.
(Fielding and Anderson, n.d.)
Figure 1 show a diagram of how the Calvin cycle works
Photosynthesis must take place under ideal conditions in terms of light intensity,
temperature, and carbon dioxide, or it will be limited. Even if the other two factors are at
their optimal levels, the plant's rate of photosynthesis will be reduced if one of these factors
is too high (above the optimum level) or too low (below optimum level)
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