Exemplar assignment for Unit 10C, the final assignment in BTEC Applied Science Unit 10, which is about the stages of photosynthesis and how efficient photosynthesis is maintained in greenhouses . This assignment contains all the criteria needed to achieve a distinction. If you take anything from ...
Unit 10: Biological Molecules and Metabolic Pathways
C: Explore the factors that can affect the pathways and the rate of photosynthesis in plants
Photosynthesis in plants
This report will outline the processes for light independent and light dependent reactions and investigate how
carbon dioxide concentration affects the rate of photosynthesis. Analysis of my investigation was conducted and this
data was compared to data gathered from other sources.
Light dependent reactions:
Light dependent reactions take place in the thylakoid membrane of the chloroplasts. It is an important stage due to
the products produced (ATP and reduced NADP). These products are essential components for the light independent
reaction of photosynthesis. Light dependent reactions are made up of four stages:
1. Light harvesting
Light harvesting involves absorbing light using pigments. Light harvesting takes place in photosystems -
photosystems are funnel-like structures located in the thylakoid membrane. Photosystems consist of several
pigments, each of which absorbs a specific wavelength. Light is funnelled to the primary pigment.
2. Photolysis of water
Light hits photosystem 2. This causes water to split into protons, electrons and oxygen. This can be
represented by the half equation: 2H2O -> 4H+ + 4e- + O2.
3. Phosphorylation
A photon of light hits photosystem 2. Energy is then transferred to the primary pigment reaction centre.
Some energy excites a pair of electrons; these electrons escape from the chlorophyll molecule and are
passed along electron carriers (proteins containing iron ions). Electrons from photosystem 1 are replaced by
electrons from photosystem 2. These electrons lose energy at each stage and are used to pump protons
across the thylakoid membrane into the thylakoid space. Electrons are captured by one molecule of
chlorophyll A in photosystem 1. The protein iron-sulphur-complex (ferrodoxin) accepts electrons from
photosystem 1 and passes them to NADP in the stroma. Protons accumulate in the thylakoid membrane,
causing a proton gradient to form across the membrane. The protons diffuse down the concentration
gradient through channels linked to ATP synthase. This causes ADP and an inorganic phosphate to bind
together to form ATP.
4. Formation of reduced NADP
Protons are accepted by NADP, which becomes reduced to NADP (NADPH). This reduction is managed by
NADP reductase and occurs through ferredoxin. Reduced NADP and ATP and now in the stroma ready for
the light independent stage.
The light-dependent reaction mechanism of photosynthesis is important because it turns water and carbon dioxide
into oxygen and high-energy molecules. These two products are essential for the production of glucose and other
carbohydrates, which helps plants and other organisms to grow and survive. The production of oxygen is also
necessary for aerobic respiration in living organisms.
Light independent reactions:
Light independent reactions take place in the stroma of the chloroplasts. It makes use of the Calvin cycle, which
involves a series of smaller reactions. The Calvin cycle can only run during the day because it is dependent on the
products of the light dependent reaction (ATP and reduced NADP).
, Unit 10: Biological Molecules and Metabolic Pathways
C: Explore the factors that can affect the pathways and the rate of photosynthesis in plants
1. Carbon fixation
Carbon dioxide binds to RuBP and is catalysed by the enzyme RuBISCO (ribulose bisphosphate carboxylase).
This causes RuBP to become carboxylated. The carbon dioxide now becomes fixed by forming an unstable
hexose molecule and then breaking this molecule into two molecules of glycerate 3-phosphate.
2. Reduction
Hydrogens from the reduced NADP then reduce glycerate 3-phosphate to form triose phosphate. One
molecule of carbon dioxide is made for every two molecules of ATP - this provides energy for the reaction.
3. Regeneration of RuBP
One in every six triose phosphate is used to form glucose. The remaining triose phosphate is used to
regenerate RuBP to restart the Calvin cycle, which requires ATP.
The light independent reaction mechanism of photosynthesis is important because the conversion of carbon dioxide
to glucose can be used as energy sources for plants. This would allow plants to grow, survive and reproduce
properly.
Limiting factors
The law of limiting factors states that if a process is influenced by more than one factor, the rate of photosynthesis
will be limited by the factor nearest to its lowest rate.
Limiting factors that affect the rate of photosynthesis:
● Light intensity
● Carbon dioxide concentration
● Temperature
Light intensity:
Light is needed for light dependent reactions, opening the stomata for gas exchange and transpiration and light
independent reactions since it requires products from the light dependent reaction. In the light dependent reaction,
the levels of GP accumulate since it cannot be reduced to TP. Therefore, TP levels fall and RuBP cannot be
regenerated.
Carbon dioxide concentration:
In low CO2 concentrations (less than 0.01%.), there is not enough COz for RuBP to become carboxylated. There is no
formation of GP and RuBP accumulates, causing TP concentration to fall.
In high CO2 concentrations (more than 0.08% ), RuBP levels decrease as it becomes carboxylated. There is an
increase in GP and TP levels but a decrease in these levels happens when RUBP is regenerated and some TP is
converted to glucose.
Temperature:
At 25-30°C, rate increases providing that all other factors are sufficient.
At temperatures greater than 30°C, photorespiration happens where oxygen competes for the Rubisco's active site,
reducing CO2's ability to bind.
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