5.1 Photosynthesis Flashcards
What does light depend on?
Continuous transfers of energy
Where does the light dependent reaction take place?
Thylakoid membrane in chloroplast
Where does the light independent reaction take place?
Stroma in chloroplast
What is the relationship between chloroplast structure and function:
Thylakoid membranes and stacks (grana)
- Large surface area for light absorption and reactions
- Membrane-bound ATP Synthase and photosystems
- High density of chlorophyll for light absorption
Low-volume thylakoid spaces
- Rapid generation of H+ gradient for chemiosomosis
Enzyme-rich stroma
- Contains rubisco for carboxylation of RuBP fluid, for easy diffusion
- High density of chloroplasts in the palisade layer of the leaf, for efficient absorption of light
What are the adaptations of a leaf that help with efficient photosynthesis?
- Large surface area that absorbs as much sunlight as possible
- An arrangement of leaves on the plant that minimises overlapping and so avoids the shadowing of one leaf by another
- Thin, as most light is absorbed in the first few micrometres of the leaf and the diffusion distance for gases is kept short
- A transparent cuticle and epidermis that let light through to the photosynthetic mesophyll cells beneath
- Long, narrow upper mesophyll cells packed with chloroplasts that collect sunlight
- Numerous stomata for gaseous exchange so that all mesophyll cells are only a short diffusion pathway from one
- Stomata that open and close in response to changes in light intensity
- Many air spaces in the lower mesophyll layer to allow rapid diffusion in the gas phase of carbon dioxide and oxygen
- A network of xylem that brings water to the leaf cells, and phloem that carries away the sugars produced during photosynthesis
What is the overall equation for photosynthesis:
6CO2 + 6H2O -> C6H12O6 + 6O2
How do the two photosystems differ?
Photosystem One - PS I: its primary pigment if a molecule of chlorophyll a with an absorption peak at 700nm. P700
Photosystem Two - PS II: its primary pigment is a molecule of chlorophyll b with an absorption peak at 680nm. P680
What happens in the first stage of the LDR?
- Chlorophyll molecules in PSII absorb photons of light, exciting 2 chlorophyll electrons to a higher energy level
- This causes the electrons to move to an electron acceptor protein molecule
- This causes the splitting (photolysis) of water: 2H2O -> O2 + 4H+ + 4e-
- The oxygen produced diffuses out of the chloroplast and eventually into the air
- The protons are relased into the thylakoid lumen (space)
- The electrons replace the excited electrons that have been ejected from chlorophyll (in PSII)
The photoionized chlorophylls electrons in photosystem II are replaced by the electrons from photolysis of water
What happens in the second stage of the LDR?
- The excited electrons pass along a chain of electron transport proteins in the thylakoid membrane
- The energy in the electrons is used to pump protons from stroma to thylakoid lumen, creating a proton gradient across the thylakoid membrane
- In PSI more light energy is absorbed and passed to the electrons (causing further excitation)
What happens in the third stage of the LDR?
- The electrons are eventually recombined with some of the protons to form hydrogen atoms
- These are taken up by molecules of NADP so reducing it to reduced NADP: NADP + H+ + e- -> REDUCED NADP
- The reduced NADP moves to the stroma for the light-independent reaction
What happens in the fourth stage of the LDR?
- The remaining protons diffuse down their gradient through the ATP synthase enzyme, producing ATP: ADP + Pi -> ATP
- This is called photophosphorylation (uses light energy to phosphorylate ADP)
What is chemiosmosis:
- H+ (protons) move from an area of high concentration (thylakoid space) to low concentration (stroma)
- Via ATP synthase
- This spins the enzyme and allows production of ATP
- ADP + Pi -> ATP
What are the products of the light dependent reaction:
ATP -> light dependent reaction
Reduced NADP -> light independent reaction
Oxygen -> leaves cell as a by-product or used in respiration
What is the first stage of the light-independent reaction?
- Carbon dioxide binds to the 5-carbon sugar Ribulose biphosphate (RuBP) to form 2 molecules of the 3-carbon compound glycerate-3-phosphate (GP)
- This reaction is catalysed by the enzyme ribulose biphosphate carboxylase (Rubisco)
What is the second stage of the light-independent reaction
- GP is reduced and activated to form triose phosphate (TP)
- ATP and reduced NADP from the LDR provide the energy for this step-one molecule of each per GP
- The ADP, Pi and NADP return to the thylakoid membrane for recycling
