8.3 (Photosynthesis HL)

Question 1

Where do light dependent and independent reactions occur and what are their products?

Answer 1

Light dependent:

• Thylakoid membrane
• Photolysis and phosphorylation
• H₂O and NADP⁺ (ADP) in
• O₂ and NADPH (ATP) out

Light independent:

• Stroma of chloroplast
• Carbon fixation, calvin cycle and synthesis of carbohydrate
• CO₂ and NADPH (ATP) in
• Glucose phosphate and NADP⁺ (ADP) out

Question 2

Outline light dependent reactions.

Answer 2

• Chlorophyll in PSII absorbs light
• Raises energy level of electrons in chlorophyll (photoactivation)
• Photoactivated electrons are passed along the membrane by electron carriers
• Energy from photoactivated electron is used to pump protons across the thylakoid membrane
• Hydrogen ions accumulate in the thylakoid space
• Oxygen is a waste product
• Electrons are replaced through photolysis of water
• Non-cyclic phosphorylation produces ATP
• Hydrogen ions accumulate in the thylakoid space
• Generates a high H⁺ concentration gradient
• Protons move through ATP synthase through chemiosmosis
• Flow of electrons through ATP synthase couples ADP and Pi ot make ATP
• NADP⁺ is reduced to NADPH
• Electrons in photosystem I are activated by light
• These are received by ferredoxin and used to reduce NADP⁺ with a H⁺ ion
• NADPH is carried to the light independent reactions
• The concentration gradient of H⁺ protons is maintained

Question 3

What do the action and absorption spectrums show (respectively)?

Answer 3

• Action: The rate of photosynthesis for all the wavelengths of light as a % of the maximum possible rate
• Absorption: shows the absorbance of light by photosynthetic pigments (here chlorophyll) for all the wavelengths of light

Question 4

Outline the calvin cycle (light independent reactions).

Answer 4

• Glucose phosphate (6C) is produced, which is either stored as starch, used for growth (as cellulose) or used in respiration
• CO₂, ATP and NADPH are used (produced by light dependent reactions)

Question 5

Outline carbon fixation (light independent reactions).

Answer 5

• RuBP is carboxylated with CO₂
• Catalysed by enzyme rubisco
• The 6C product immediately splits into 2x glycerate 3-phosphate
• Most of the triose phosphate produced is used to regenerate RuBP
• Some of the triose phosphate molecules are linked to form glucose phosphate

Question 6

Explain the relationship between chloroplast structre and function.

Answer 6

• Thylakoid membrane and stacked discs (grana):
• Thylakoids provide a large surface area for light absorption and light dependent reactions
• Chlorophyll (and other pigments) molecules are grouped together to form the photosystems which are embedded in the membrane along with the electron carriers.
• Folds in thylakoid allow photosystems and electron carriers to be close together
• Thylakoid spaces:
• The spaces collect H+ for chemiosmosis, the low volume enables a the H+ gradient to generated rapidly.
• H+ flows back to the stroma, down the H+ gradient, through ATP synthase channels (embedded in thylakoids membrane) to produce ATP
• The Stroma:
• Contains rubisco for carboxylation of RuBP along with all the other enzymes required for the Calvin cycle

Question 7

Compare chloroplasts and mitochondria.

Answer 7

• Chloroplast envelope and outer mitochondrial membrane: membranes which compartmentalise the organelles in the cell’s cytoplasm
• Thylakoid membrane and inner mitochondrial membrane: membranes which carry out electron transport train, have ATP synthase, generate ATP and make use of chemiosmosis of H⁺ ions
• Stacked membranes (grana/thylakoid stacks) and invaginated membranes (cristae): maximise surface area for reactions
• Low-volume inter membrane spaces: rapid generation of H⁺ concentration gradient
• Stroma (calvin cycle) and matrix (Krebs cycle): fluid medium for diffusion of molecules and containing enzymes for cyclic reactions

Question 8

Outline Calvin’s experiment.

Answer 8

• Used Chlorella algae which was placed in a thin glass vessel (called the lollipop vessel)
• The algae was given plenty of light, carbon dioxide and hydrogen carbonate containing normal carbon
• At the start of the experiment the carbon compounds were replaced with compounds containing radioactive carbon
• Samples of algae were taken at different time intervals
• The carbon compounds were separated by chromatography and the compounds containing 14C identified by autoradiography

Key technological developments making experiment possible:

• The discovery of 14C in 1945 by Kamen and Ruben
• The use of autoradiography to produce patterns of radioactive decay emissions (autoradiograms)