Photosynthesis Flashcards
Give the symbol equation for the overall reaction of photosynthesis
6CO2 + 6H2O –> C6H12O6 + 6O2
Explain how a leaf is adapted for photosynthesis
Leaves arranged so that they don’t overlap to prevent shadows forming on the leaves below
Large surface area to absorb as much light as possible
Transparent cuticle and epidermis to let light through to the chloroplasts in the mesophyll
Network of xylem and phloem to transport water and sugars
Many stomata that can open and close so that all mesophyll cells are only a short distance away from a stoma, increasing the rate of diffusion. They open and close to balance gas exchange with water loss, responding to changes in light intensity
Air spaces within the mesophyll to allow rapid diffusion of gases throughout the leaf
Upper mesophyll cells contain many chloroplasts to absorb sunlight
Explain how a chloroplast is adapted for its function
Thylakoid membranes have a large surface area so more space for attachment of chlorophyll, electron carriers and enzymes
Network of proteins in the grana to hold chlorophyll in the right position to absorb as much light as possible
Granal membranes contain selectively permeable ATP synthase channels to synthesis ATP and establish a proton concentration gradient
Contain DNA and ribosomes to manufacture proteins needed for photosynthesis reactions
Stroma fluid contains enzymes and substrates and is enclosed by a membrane so the enzymes and substrates are kept distinct from the cytoplasm of the cell
Stroma fluid surrounds the grana so there is a short diffusion pathway for products of the light dependent reaction to diffuse from the grana into the stroma
Describe the light dependent reaction of photosynthesis
Light is absorbed by photosystem II, exciting the electrons in chlorophyll.
The electrons leave the chlorophyll molecule (photoionisation).
The electrons are transferred between electron carriers, as they are transferred they lose energy.
Meanwhile, the electrons in chlorophyll are replaced through photolysis, where water is split into protons, electrons and oxygen by light energy.
The energy lost from the electrons is used to actively transport protons from the stroma into the thylakoids, ensuring that there is a high concentration of protons in the thylakoids.
There is now a high concentration of protons in the stroma and so they diffuse through ATP synthase channels (chemiosmosis). The movement of protons through the ATP synthase channel causes the shape of ATP synthase to change, catalysing the formation of ATP from ADP and Pi. Electrons continue to be transferred through the electron carrier chain before reaching photosystem I. Here they are excited once again by light energy. These electrons and protons in the stroma are taken up by NADP which becomes reduced.
Describe the light independent reaction of photosynthesis
CO2 diffuses in through the stomata and dissolves in the moisture of the palisade cell walls. It then diffuses into the stroma.
CO2 combines with a five carbon molecule called ribulose bisphosphate forming an unstable six carbon molecule. This reaction is catalysed by rubisco.
This unstable molecule doesn’t last long, and breaks into two molecules of the three carbon compound glycerate-3-phosphate
Energy from ATP and H+ from reduced NADP are used to reduce glycerate-3-phosphate. This forms triose phosphate
Pairs of triose phosphate can be combined to make a hexose sugar. These hexose sugars can be polymerised to form longer chained molecules such as starch
Triose phosphate can also be used to regenerate ribulose bisphosphate. Energy from ATP is also required for this. Five out of every six molecules of triose phosphate are used in this way. As each cycle produces two triose phosphate, this means the light independent reaction has to happen six times per molecule of hexose sugar.
What is photoionisation?
Where chlorophyll is ionised - light excites electrons in the chlorophyll causing them to leave the molecule.
What is photolysis? Write a balanced equation
Where light energy is used to split water molecules into electrons, protons and oxygen
H2O -> 2H+ + 2e- + 1/2 O2
What is chemiosmosis?
The movement of protons (H+) down a concentration gradient, through a channel protein (usually ATP synthase)
What is oxidation?
Gain of oxygen, loss of electrons or hydrogen
What is reduction?
Loss of oxygen, gain of electrons of hydrogen
Where does the light dependent reaction take place?
Thylakoid membranes
Where does the light independent reaction take place?
The stroma
What is the light compensation point? Sketch what this looks like on a graph of light intensity vs rate of photosynthesis
The light compensation point is the point at which there is no net exchange of gases into or out of the plant as respiration and photosynthesis are balanced.
What is the law of limiting factors?
At any given moment, the rate of a physiological process is limited by the factor that is at its least favourable value
Suggest environmental factors which limit the rate of photosynthesis
Carbon dioxide
