Topic 8.2 Photosynthesis

Question 1

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

Answer 1

Question 2

8.2.2 Photosynthesis consists of what type of reactions reactions?

Answer 2

Photosynthesis consists of light-dependent and light independent reactions.

Question 3

8.2.3 Explain the light-dependent reactions.

Answer 3

• Photosynthesis in chloroplasts (contain chlorophyll - pigment in thylakoid membranes)
• Chlorophyll molecules arranged groups called photosystems (2 types of photosystems: 1 & 2)
• Photosystem 2:
  
  • chlorophyll molecule absorbs light (photon); energy from light excites and raises electron within chlorophyll molecule to higher energy state
  • e^- received by electron acceptor (chlorophyll molecule is photoactivated)
  • e^- passed from one chlorophyll mol. to the next until it reaches a special chlorophyll mol. in reaction centre of photosystem
  • special chloro. mol. passes e^- to chain of electron carriers (found in thylakoid membrane)
  • e^- releases energy as it passes from one electron carrier to the next
  • energy used to pump H⁺ across thylakoid membrane into space within thylakoids
    
    • forms proton gradient
  • protons travel back across membrane through ATP synthase (located in thylakoid membrane)
  • ATP synthase uses energy released from movement of protons down concentration gradient to synthesize ATP from ADP
    
    • synthesis of ATP in this manner is called non-cyclic photophosphorylation
  • ​e^- from e^- transport chain accepted by P1
  • P1 absorbs light and becomes photoactivated; e^- becomes excited again and raises to higher energy state
  • e^- passed along short chain of e^- carriers and are eventually used to reduce NADP+ in the stroma
  • NADP⁺ accepts 2 excited e^- from chain of carriers and 1 H⁺ ion from stroma to form NADPH + H⁺

Question 4

8.2.3 (Cont.) Explain the light-dependent reactions in terms of cyclic photophosphorylation.

Answer 4

• NADP⁺ is needed for normal flow of e^- in thylakoid membranes as it’s the final e^- acceptor
• if NADP⁺ isn’t available, then normal flow of e^- is inhibited
• if light intensity is not a limiting factor, there will usually be a shortage of NADP⁺ as NADPH accumulates within stroma
• Therefore, alternate pathway for ATP production called cyclic photophosphorylation is available
  
  • Photosystem 1 absorbs light, becomes photoactivated
  • excited e^- passed along to chain of e^- carriers between P1 and P2; e^- travels back to P2
  • electrons travel along the chain of carriers back to Photosystem I, creating proton gradient & producing ATP

Question 5

8.2.3 (Cont.) Explain the light-dependent reactions in relation to photolysis

Answer 5

• when special chlorophyll molecule at reaction centre passes on electrons to the chain of electron carriers, it becomes positively charged
• enzyme at reaction centre splits water molecules within the thylakoid space
• 2e^-, 2H⁺ ions & O₂ are formed
• electrons given to chlorophyll
• O₂ = waste product
• this process, called photolysis, only occurs in presence of light

Question 6

8.2.4 Explain photophosphorylation in terms of chemiosmosis.

Answer 6

Photophosphorylation: production of ATP using energy of sunlight

Chemiosmosis: movement of ions across a selectively permeable membrane, down their concentration gradient

• on the electron transport chain between P1 & P2, electrons travel trhough chain of electron carriers and release energy
• energy is used to pump hydrogen ions across thylakoid membrane into space within thylakoid
  
  • concentration gradient of protons form
• protons move back across thylakoid membrane (down concentration gradient) through ATP synthase
• ATP synthase uses energy released from movement of protons down concentration gradient to synthesize ATP from ADP

Question 7

8.2.5 Explain the light-independent reactions.

Answer 7

Calvin Cycle; occurs in stroma of chloroplast; involve conversion of CO₂ and other compounds into glucose

• Carbon fixation: CO₂ reacts with 5-carbon sugar (RuBP) to form 6-carbon compound
  
  • reaction catalyzed by RuBP carboxylase
  • 6-carbon compound splits to form 2 molecules of glycerate 3 phosphate (GP)
• Reduction reactions: GP reduced to triose phospate (TP)
  
  • energy and H⁺ supplied by 12ATP & 12NADPH + H⁺
  • 2 TP molecules react to form glucose phosphate
  • of 12 TP produced during reactions, 2 used to synthesize glucose phosphate & 10 remaining TP used to regenerate 6RuBP (with 6ATP & 6NADPH + H⁺)

Question 8

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

Answer 8

• Stroma: similar to cytosol of cell; allows an area for reactions to take place; contains many enzymes, including rubisco, which are important for the reactions of the Calvin cycle.
• Thylakoids: have a large surface area for greater light absorption
• Lumen: space within thylakoids allows faster accumulation of protons to produce concentration gradient.
• Double membrane: on outside, isolates working parts and enzymes of chloroplast from surrounding cytosol

Question 9

8.2.7 Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants.

Answer 9

Action spectrum: shows rate of photosynthesis for each wavelength of light

• least = green-yellow light (525 - 625 nm)
• good = red-orange light (625 - 700 nm)
• best = violet-blue light (400 - 525 nm)

Absorption spectrum: shows percentage of light absorbed by pigments within chloroplast, for each wavelength of light

• least absorption = green-yellow light
• good absorption = red-orange light
• best absorption = violet-blue light

Relationship: usually abundance of chlorophyll in plants. Therefore, rate of photosynthesis will be greatest at wavelengths of light best absorbed by chlorophyll. Note, other pigments may be present in plants (e.g. carotene can absorb wavelengths of light chlorophyll cannot) and will contribute to photosynthetic rate (but not seen on absorption spectrum - chlorophyll absorption only)

Question 10

8.2.8 Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide.

Answer 10

usually only 1 limiting factor at a time; rate-limiting step is step occuring most slowly in photosynthetic process, caused by limiting factor

1. Light: when light intensity is poor, shortage of ATP & NADPH - reduction of GP is slowed - rate of photosynthesis limited
2. CO₂: low concentration, production of GP is limited, RuBP & NADPH accumulate - rate of photosynthesis limited
3. Temperature: low temperatures, enzymes work slowly, NADPH accumulates; high temperatures, enzymes (e.g. rubisco) denatured, NADPH accumulates - photosynthesis affected