Unit 10: Biological Molecules and Metabolic Pathways Learning Aim C: Explore the factors that can affect the pathways and the rate of photosynthesis in plants. This assignment and unit was graded an overall DISTINCTION. It is an extremely detailed assignment that meets all criteria in the best of d...
Light dependent stage of photosynthesis
Importance
● The light dependent stage of photosynthesis is an important stage as it is the first
step in the process of converting light energy into chemical energy in the form of ATP
and NADH which is then used in the second stage of photosynthesis, the light
independent stage.
The light dependent stage is the first phase of photosynthesis, and it takes place in the
thylakoid membranes of the chloroplasts.
1. Sunlight light is absorbed by the plant cells. This excites, and hence releases, the
electrons in the chlorophyll, causing them to pass to an electron acceptor at the start
of an electron transport chain, a process known as photo-ionisation.
2. Electrons are transferred from one electron carrier to the next via redox reactions.
This helps in the cyclic or non-cyclic process of photophosphorylation, which
produces ATP from ADP and inorganic phosphate. Reduced NADP is also produced
from NADP.
3. Water molecules are broken down by the light energy into protons (H+), electrons,
and oxygen (waste). In the first stage, the electrons are used to replenish those lost
from the chlorophyll. Photolysis is the process by which water bonds break.
Chemiosmosis is the process by which protons are pushed across the membrane
utilising the ATP produced in the following phase, resulting in a chemical potential
gradient.
4. There is less NADP created. This results from the transfer of a proton (NADPH) and
electrons from the electron transport chain to NADP.
5. The synthesis of ATP occurs. An ATP synthase enzyme helps the proton cross the
membrane again and create an ATP molecule. About four protons are needed to
create one ATP molecule.
Reduced NADP is a source of H+ for the light-independent stage, whilst ATP is a source of
energy.
, Light independent stage of photosynthesis
Importance
● The light independent also known as the calvin cycle is the second stage of
photosynthesis, it is important for the synthesis of molecules such as carbohydrates
from CO2 and other molecules
The light independent reactions are the second and final phase of photosynthesis taking
place in the stroma of the chloroplasts. The light-independent reactions are collectively
known as the Calvin cycle and use ATP and reduced NADP from the light-dependent stage
to produce glucose.
1. Carbon dioxide and 5-carbon ribulose bisphosphate (5C RuBP) combine. The
RuBisCo enzyme is responsible for catalysing the reaction, which results in the
carbon fixation process.
2. Two molecules of glycerol 3-phosphate (GP) are produced from the RuBP.
3. The GP molecules are reduced into triose phosphate (TP), often referred to as
glyceraldehyde phosphate (GALP), using reduced NADP and ATP molecules. The
oxidation of the reduced NADP occurs during this reduction.
4. Some molecules of TP/GALP are used as a raw material to create glucose, which
can then be converted into essential organic compounds. It takes 2 cycles to create 1
glucose molecule. A few examples of organic substances are lipids, nucleic acids,
amino acids, and mono- and polysaccharides.
5. RuBP is regenerated. ATP helps to turn the remaining TP/GALP molecules back into
RuBP.
The light independent stage cannot function without the products of the light dependent
stage (reduced NADP, ATP).
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