5 Energy transfers- Photosynthesis Flashcards
What are the two stages of photosynthesis and where do they occur?
1= Light-dependent reaction; in the thylakoid membranes
2= Light-independent reaction; in the stroma
What are chloroplasts?
Flattened organelles with a double membrane within plant cells.
Photosynthesis occurs here
What is the stroma?
-The space within chloroplasts
-Starch grains here are where carbohydrates produced during photosynthesis are stared
What are thylakoids and grana?
Fluid-filled sacs stacked up inside chloroplasts
Stacks of thylakoids- grana
What are lamellae?
-Thylakoid membranes which link grana
-Contain photosynthetic pigments of different kinds; each absorb specific wavelengths of visible light, e.g chlorophyll a/b and carotene
What are photosystems?
Within the thylakoid membranes, photosynthetic pigments are attached to proteins. Together they are called a photosystem. Two are used in photosynthesis, mostly in the LDR:
- photosystem l (PSI)
- photosystem ll (PSII)
What is the first stage of the light-dependent reaction and what occurs?
Absorption of light energy by chlorophyll/ photoionisation of chlorophyll
- light energy is absorbed by chlorophyll in the photosystems.
- this excites the electrons within the photosystem.
- the electrons are hence released from the chlorophyll
- the chlorophyll is now photoionised
- this also results in a release of energy, used in photophosphorylation, reduction and photolysis.
What occurs in the electron transport chain and chemiosmosis in the LDR?
- The electrons released from the chlorophyll are transferred to an electron carrier (proteins in the thylakoid membranes), then along a chain of electron carriers, ETC (electron transport chain)
- as they move down the etc, they lose energy
- the energy pumps protons from the stroma → thylakoids. As they’re transported against their concentration gradient, needing energy.
- as protons build up inside the thylakoids, a proton gradient forms
- Chemiosmosis; the protons diffuse down the concentration gradient across the thylakoid membrane via ATP synthase & energy is released as protons diffuse through the enzyme.
- the energy converts ADP and inorganic phosphate to ATP
How is reduced NADP made in the LDR?
Some of the electrons released from the chlorophyll aren’t passed along the ETC, instead they’re transferred directly to NADP.
The electrons react with a proton in the stroma, making reduced NADP
What is photolysis in the LDR?
-Electrons released from chlorophyll can be replaced by photolysis
-Here, light energy splits water into protons, electrons and oxygen.
-The electrons can then replace those released when light is absorbed
What is non-cyclic photophosphorylation in the LDR?
-This produces NADP & ATP.
-NADP is made when light is absorbed by PSI
-Electrons aren’t recycled in non-cyclic photophosphorylation
-Both types make ATP
What does cyclic-photosphorylation involve?
-Only involves PSI
-Initiated when light energy is absorbed by PSI + subsequent photoionsiation of chlorophyll
-Electrons cycle continuously though the electron carriers in the ETC to PSI
-The same process occurs in the LDR (proton gradient, chemiosmosis)
How do cyclic and non-cyclic photophosphorylation?
Cyclic:
-ATP made
-No reduced NADP made
-Electrons are continuously recycled
-No photolysis
Non-cyclic:
ATP & reduced NADP made
-Electrons in PSII replaced by photolysis
What happens during carbon fixation in the LIR?
-CO² diffuses into leaves through stomata, combines with ribulose biphosphate (RuBP), gives unstable 6-carbon compound. This is catalysed by rubisco
-The unstable compound splits into 2 molecules of glycerate 3-phosphate (GP)
How is GP reduced in the LIR?
-The 2 GPs are reduced to triose phosphate (TP), which is driven by energy from 2 ATP and protons from 2 reduced NADP. These are provided by the LDR and are recycled after their use
-1/6 TP molecules is converted into hexose sugars, 5/6 continue in LIR
