Lecture Seven - Energy transformations in cells (photosynthesis) Flashcards
Describe the role of pigments in photosynthesis.
Leave gather light energy with pigments, molecules which momentarily promote an electron to a higher energy state upon absorbing a photon of visible light.
Light harvesting pigments pass their energy between adjacent pigment molecules, until the energy arrives at a ‘reaction centre.’
Ultimately, photosynthetic organisms convert the light energy to ATP and NADPH, to high energy compounds that in turn, are harnessed to proser the assembly of carbon dioxide and water into organic compounds such as glucose.
Primary pigment in leaves: Chlorophyll
Absorbes - red and blue light
Reflects - green light
Accessory pigment in leaves: Carotenoids
Absorbes - purple and blue light
Reflects - green and yellow light
What is the structure of chloroplasts?
Chloroplasts have an outer and an inner membrane, which encloses the fluid filled stroma.
Thylakoids are compartments distributed in the stroma that form stake of disks called grans.
The photosynthetic pigments are embedded in the thylakoid membranes.
Describe the sequence of events that occur during the light dependant reactions.
During photosynthesis, some of the absorbed light energy is stored as chemical energy in ATP as enzymes use the excited electrons to cleave water molecules in to oxygen, protons and electrons.
Electrons are transported through a chain of complexes in the thylakoid membrane by electron carriers, including plastoquinone (PQ) and plastocyanin (PC).
This chain of events stores energy as a proton gradient, which is then transformed to mechanical energy by ATP synthase and finally converted to chemical energy in ATP.
NADP+ accepts high energy electrons from the electron transport chain.
NADPH and ATP provide energy and electrons for the carbon fixation stage.
Other enzymes extract additional energy from the excited electrons by delivering them to the coenzyme NADP+, reducing it to NADPH.
Describe the sequence of events that occur during the light independent reactions.
In this process, carbon dioxide is captured and incorporated or ‘fixed’ into sugars such as glucose.
Begins in the stroma and continues in the cytosol.
Carbon is fixed in three stages:
1) Added to a 5C accepter molecule, ribulose-1, 5-bisphosphate, by an enzyme commonly known as RuBisCO.
The resulting 6C molecule spontaneously breaks into two copies of the 3C molecule glycerate-3-phosphate.
2) Glycerate-3-phosphate is converted to glyceraldehyde-3-phosphate in a two step reaction, which is the revese of the corresponding steps in glycolysis.
In this stage, ATP is consumed and NADPH is oxidised.
Note that biosynthetic pathways such as carbon fixation generally use NADPH, where as catabolic pathways such as glycolysis use NADH.
3) The original 5C substrate is regenerated, making it a cyclic pathway.
Carbohydrates are synthesised from glyceraldehyde-3-phosphate that exits the cycle.
This three step process is called the Calvin Cycle. In this cycle, the carbon in carbon dioxide is reduced while NADPH is oxidised. This process requires an input of energy and is driven by ATP hydrolysis.
What are the main structures in the plant that take in nutrients.
Water is absorbed through the tissues and taken up from the soil by the roots and transported to the chloroplast’s stroma, where carbon fixation occurs.
Carbon dioxide is taken in through the stomata, which regulates the flow of gases in and out of the leaf.
Once inside the leave, carbon dioxide diffuses into the stroma.
What is a chemoautotroph?
An organism that uses energy from oxidation of reduced formed of inorganic compounds such as H2S and NH4+.
What is a phototroph?
An organism that uses energy from sunlight.
Explain the electromagnetic spectrum with relation to photosynthesis.
Ozone filters off much of the radiation smaller than 300nm.
Water vapour in the atmosphere reduces infra-red components.
Wave lengths approximately 300-1400nm reach Earths surface.
Photosynthetically active radiation (PAR) = 400-700nm (used by plants).
How many photons are formed per NADPH molecule and oxygen?
4 photons per NADPH.
4 photons per 0.4 O2 evolved.
8 photons per O2 - this is the theoretical quantum requirement of photosynthesis. Most plants have a higher requirement.
What are C4 plants?
Plants that assimilate carbon dioxide by mechanisms in addition to the calvin cycle.
One such group is termed the C4 plants because the initial product of carbon dioxide assimilation is a 4C, di-carboxylic acid.
In C4 plants, carbon dioxide is first fixed by PEP carboxylase in the mesophyll cells.
C4 acids are transported to bundle sheath cells which contain the calvin cycle.
here, C4 acids are decarboxylated and the carbon dioxide released is fixed but he calvin cycle.
What are CAM plants?
Some plants take up carbon dioxide in the dark.
Maltate is formed in the same way as in C4 plants, i.e. via PEPCase, but it is stored in vacuoles during the night and decarboxylated and fixed by the calvin cycle during the following day.
This is termed Crassulacean Acid Metabolism (CAM).
What are the benefits of being a CAM or a C4 plant?
C4 and CAM metabolism can confer an ecological advantage over C3 plants under some circumstances.
Because of the higher affinity of PEPCase for carbon dioxide, C4 plants need fewer stomata to take up carbon dioxide. This also means they lose less water.
CAM plants lose less water because they only open their stomata at night.
This CAM and C4 plants have a higher water use efficiencies and perform better than C3 under warm, dry conditions.
