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.
BTEC Level 3 Extended Diploma in Applied Science
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The Light dependent stage
This is a reaction of stages is built up of many different reactions that rely on light in order to take
place. The light dependant reactions starts off in the thylakoid membranes and spaces, a section in
the chloroplast which has the most surface area. A larger surface area is created by builds up to form
grana, making it an ideal location as the thylakoids are more capable of absorbing more sunlight to
promote the number of light dependent reactions. The thylakoids are a system of membranes which
are made up of flat fluid rich sacs located in the stroma of the chloroplast. Within the thylakoid
membrane contain the necessities for light dependent reactions, these include pigments, enzymes
and electron carriers make up an electron transport chain which transport electrons through the
membrane. This allows electron energy to be utilised to migrant H+ ions across the membrane and
donate electrons to water molecules which are accepted by NADP molecules to ultimately form
NADPH. (1)
In short, light dependent stage involves photolysis, a process where light energy used and ATP made
and therefore can also be called photophosphorylation. Photosystem one and two are systems of
networks that play a role in this stage. They are compromised of pigments that are vital in
production of photons and utilisation of light energy to catalyse the main photosynthetic endergonic
stages. (2)
Photosystems are photosynthetic pigment molecules that arrange chlorophyll molecules and is
located in the thylakoid membrane which are arranged in light harvesting clusters which work
together to take in light energy. Their role is to absorb photos to energise electrons. The structure is
shaped like a funnel where the Photosystems moves energy down to the next Photosystem in the
cluster to reach its final, the primary pigment reaction centre (2). There are two types of
Photosystems, I and II:
• Photosystem II takes place at the beginning of the electron transport chain and absorbs light
through a primary pigment at a wavelength of 680nm which gives it the name P680
In the chlorophyll molecule/ primary pigment molecule, two electrons are moved to a higher
energy level to be released from the chlorophyll molecule through photoionisation. Before
reaching Photosystem I, electron is transported down the electron transport chain, this process
triggers chemiosmosis.
Chemiosmosis is where protons are moved across a low concentration to an area of high
concentration In the thylakoid lumen due to the energy given by the electrons passing through
the electron transport chain. This results in a proton gradient across the membrane which
stimulates the ATP creation during photophosphorylation later used for Photosystem 1.
Within the Photosystem II is a H2O emitting exposure known as, oxygen evolving complex which
catalyses the breakdown of photolysis. The equation to demonstrate this is:
H2O —> 2H+ + 2e- + 1/2 O2
Finally, the electrons are replaced by electrons from photolysis of and and the old electrons
fuelled with energy exit to move on to Photosystem I.
• Photosystem I also produces light energy and occurs in the middle of the electron transport
chain and absorbs light through its primary pigment at a wavelength of 700nm (P700).
During photo activation in Photosystem II, electrons on photosystem I go through
photoionisation. As the electrons go through the electron transport chain, they fuse with the
hydrogen ions to create photolysis of H2O to be taken out of the thylakoid lumen via ATP
synthase. They also fuse with the NADP carrier molecule which leaves us with reduced
NADP, also known as 2H+ + 2e- + NADP —> reduced NADP.
The product (NADP) later travels through the light independent stage vital for the synthesis
of carbohydrates and the lost electrons in Photosystem I are restored by de energised
electrons from Photosystem II. (3)
Photophosphorylation and Chemiosmosis
During photophosphorylation, as the energised electrons (from the Photosystem complex
mentioned above) are transported along the electron transport chain which gains electrons to
, Fatima Abdallah
10C
reduce the carriers and later become oxidised as a result of losing electrons/ passing them down to
the upcoming carrier. The electrons don’t suddenly lose their electrons, the energy is slowly emitted
via the transport chain to be used to actively transport positive H ions using a proton pump to take
the protons the other side of the thylakoid membrane, from the stroma to the thylakoid lumen.
The proton gradient created here by moving protons from an area of high concentration (thylakoid
lumen) to an area of low concentration (stroma) is done through facilitated diffusion via
chemiosmosis which is where transmembrane ATP synthase enzymes. Chemiosmosis is vital for
synthesising ATP as it supplies the plant with energy to add inorganic phosphate groups to ADP. The
equation to show this process is : ADP + Pi -> ATP.
Role of Photosynthetic Pigments
Within chloroplasts are multiple types of photosynthetic pigments that absorb different
wavelengths of light inside the thylakoids. The two groups of pigments are:
1) Chlorophylls which is a primary pigment also known as chlorophyll a and chlorophyll b which
absorb wavelengths in the blue- purple and red areas of the light spectrum.
2) Carotenoids which is an accessory pigment also known as B carotene and xanthophyll that
absorb wavelengths of light in the blue – purple region of the spectrum.
In Photosystem 2, the primary pigment is chlorophyll b whereas Photosystem 1 has chlorophyll a as
its primary pigment. Surrounding the primary pigment are the accessory pigments which work to
absorb a range of wavelengths of light to the chlorophyll.( 4)
The Light-Independent Reaction
The light independent reaction is also known as the stages of the Calvin cycle and it is the next step
after the light dependent reaction. The Calvin cycle does not use light energy however, it uses the
ATP energy and hydrogen from reduced NADP that was made from the previous cycle to create
carbohydrates (e.g starch, sucrose and cellulose).
The light independent reaction consists of 3 stages to create a carbohydrate:
1) Carbon fixation- this is where rubisco catalyses the reaction where carbon dioxide bonds
with ribulose bias phosphate ( a p5 carbon sugar) (5). The product of this reaction is a 6
carbon compound which splits into two because it is unstable, as a result, we are left with a
3 carbon compound known as glycerine-3-phosphate. Now the CO2 has been made part of
the plant cell (fixed).
2) Reduction of glycerate 3-phosphate: in this stage, glycerate 3-phosphate (GP) into a
phosphorylated 3C sugar called TP- triode phosphate. This is done using the products from
the light dependent stage; the energy from ATP and hydrogen from the reduced NADP. 1/6
of the TP is used to produce organic molecules for example, triode phosphate is condensed
to turn into hexose phosphate a 6 carbon sugar which is used for starch or sucrose or
cellulose. Another organic molecule made is trikes phosphate which is used to produced
amino acids for protein synthesis in the plant. Triode phosphate is also an organic molecule
used by the plant, this is converted to glycerol and glycerate to fatty acids to bond with
libidos for cell membranes.
3) Regeneration of ribulose bisphosphate is the final stage in the Calvin cycle and it’s uses ATP
and 5/6 of the TP molecules to recreate RuBP (ribulose bisphosphate).
The image below demonstrates the Calvin cycle.
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