UWCSEA - IB Biology - Topic 8.3 - Photosynthesis

Explain photophosphorylation in terms of chemiosmosis.

production of ATP by build up of proton gradient using light energy is called photophosphorylation; electrons cause pumping of protons into the thylakoid compartment; proton gradient used by ATP synthase to drive ATP production; by adding phosphate to ADP; protons flow back into stroma; from the thylakoid compartment (space);

Explain the light-dependent reactions (the entire sequence) including chemiosmosis and photophosphorylation

*Chlorophyll* is a pigment that absorbs light energy; this causes electrons in chlorophyll to be raised to a *higher energy level*; this means they are *photoactivated*; *photolysis* of water *replaces* electrons which are excited and leave the chlorophyll molecules; the electrons are passed from photosystem II to a chain of *electron carrier proteins* (in thylakoid membrane); the *energy from electrons* is used to *pump protons* into the *thylakoid space* from the stroma; small thylakoid space enhances this build up; the protons *move* through *ATPsynthase* to drive ATP production; by adding *inorganic phosphate/Pi* to *ADP*;ATP synthase is a complex in the *thykakoid membrane*; production of ATP using light energy called *photophosphorylation*; ATP synthesis using a *gradient* of protons built up using *electron transport* is called *chemiosmosis*; electrons are passed to photosystem I by electron carriers; electrons are re-excited and leave *photosystem I*; electrons are then passed by electron carriers and used to *reduce NADP+*; (NADP reducatase is the enzyme responsible); *reduced NADP* is produced; also known as NADPH;

What is the role of the ATP and reduced NADP in the light-independent reactions?

ATP provides energy; for the reduction of RUBP; by reduced NADP (NADPH + H+); which provides hydrogen atoms; carbon can be added to RUBP;

How were the metabolic pathways for the fixation of carbon first shown? By who? What techniques were used?

Discovered by Calvin; algae placed in thin glass container, the "lollipop" apparatus; given plenty of light and bicarbonate/CO2 ; at start of experiment algae supplied radioactive carbon, 14C; then samples taken at intervals and alcohol killed samples the Carbon compounds were separated by chromatography; the radioactive-compounds identified by autoradiography; results showed that RuBP was phosphorylated; and after five seconds there was more glycerate-3-phosphate than any other compound; this showed that glycerate-3-phosphate was the first first stable product; next compound to be detected containing radioactive carbon was triose phosphate; he also showed that a wide range of carbon compounds was quickly made in sequence; showed that a cycle of reactions was used to regenerate RuBP;

What is photolysis and what does it create?

It is the splitting of water using an enzyme which is associated with photosystem II; leading to electrons which replace those lost from photosystem II; produced oxygen which is a waste product; 2H2O → O2 + 4H+ + 4e-

Where do the light-dependent reactions take place? (the ones requiring sunlight to take place)

Light-dependent reactions take place in the intermembrane space of the thylakoids, which are located in chloroplasts; the *thylakoid space*; And within the thylakoid membrane; thylakoids are membrane-bound compartments in the chloroplasts;

What does the thylakoid membrane contain? What are the components?

Photosystem II; ATP synthase; a chain of electron carriers; Photosystem I;

What is produced by the light-dependent reactions?

Reduced NADP (NADPH+ H+); and ATP are produced in the light-dependent reactions;

What is the first carbohydrate produced by the light-independent reactions? What is done with it?

Triose phosphate; It is used to regenerate RUBP; Some is used to form glucose, starch, hexose (other sugars/polysaccharides);

What is chemiosmosis?

The production of ATP using a gradient of protons; and energy from electron transport; Protons flow down their concentration gradient; from the thylakoid compartment; thought ATP synthase; producing ATP

Draw and label a diagram showing the structure of a chloroplast as seen in electron micrographs.

a) Cell wall (b) Double membrane (c) Starch grain (d) Grana (e) Thylakoid (f) Stroma

Explain chemiosmosis as it occurs in photophosphorylation

a. photophosphorylation is the production of ATP; b. (some of the) light absorbed by chlorophyll or photosystem II; c. photolysis splits water into hydrogen ions and electrons; d. the electron transport system moves the electrons through a series of carriers; e. (electron transport system occurs) in the thylakoid membrane; f. electron transport linked to movement of protons into thylakoid space; g. a proton gradient builds up (in the thylakoid space); h. small thylakoid space enhances the gradient; i. hydrogen ions move by diffusion through the ATP synthase; j. ADP + inorganic phosphate (Pi) forms ATP; k. (the kinetic energy from) movement of hydrogen ions (through ATP synthase) generates ATP; l. ATP synthase is a protein complex in the thylakoid membrane; m. formation of ATP synthesis linked to electron transport is chemiosmosis;

What does the absorption of light lead to? What is the consequence?

absorption of light by photosystems generates excited electrons; excited electrons carry energy; this energy is used to pump protons (H+/hydrogen ions) into the thylakoid space; creating a proton gradient; electrons ultimately reduce NADP+ to NADPH + H+ (reduced NADP)

How and why are electrons transferred in the thylakoid membrane?

by electron carriers; such as plastoquinone; so that protons can be pumped; so that they can eventually reduce NADP+ to reduced NADP (NADPH + H+)

Explain the light-independent reactions

happen in the stroma; light-independent reaction fixes CO2 into carbohydrates; using RUBP and RUBP carboxylase; to make glycerate 3-phosphate (GP); which becomes reduced; to triose phosphate (TP); using NADPH (reduced NADP+); and ATP; triose phosphate then used to form glucose; RUBP is regenerated from triose phosphate; using ATP;

Explain the relationship between the structure of the chloroplast and its function.

light dependent reaction occurs in the thylakoid membrane; thylakoids provide a large surface area; chlorophyll is located in the membrane; in groups of (hundreds of) molecules called photosystems; folds in thylakoid allow photosystems to be close to each other; electron carriers embedded in membrane; NADP+ accepts two high energy electrons and an H+ from stroma to form NADPH + H+ (reduced NADP); high proton gradient formed in space between thylakoids due to it being small; ATP synthase embedded in thylakoid; photolysis of water occurs in thylakoid space;

What reactions are considered to be light-independent?

light-independent reactions - carbon fixation - carboxylation of RuBP - production of triose phosphate - ATP and NADPH being used as energy sources - ATP being used to regenerate RuBP - ATP being used to produce carbohydrates

What reactions are considered to be light-dependent?

light-independent reactions are - photolysis - photoactivation - electron transport - chemiosmosis - ATP synthesis - reduction of NADP+

How are the light-independent reactions linked to the light-dependent (why would they stop eventually without light)?

reduced NADP (NADPH) produced in the light-dependent reactions; through reduction of NADP+ using electrons which are photoactivated; ATP produced; using the proton gradient created when protons are pumped into thylakoid space by electron carriers; this is called photosphorylation; as light-dependent reactions produce ATP and NADPH, and they are used up and would run out without light; if there is no ATP and reduced NADP, Glycerate-3-phosphate cannot be reduced to triose phosphate; RuBP is therefore not regenerated, (as triose phosphate is used to regenerate RuBP); and as ATP required for RuBP regeneration from triose phosphate; carbon dioxide fixation therefore stops; Glycerate-3-phosphate accumulates; and there would be no RuBP to add to carbon dioxide; stomata also close in the dark; carbon dioxide is therefore not absorbed;

What is carbon fixation? Where does it occur?

taking atmospheric carbon dioxide and fixing it (turning it into) into glycerate-3-phosphate (GP); by reacting with RUBP; using RuBP carboxylase; in the stroma of chloroplasts;