Photosynthesis

• What does magnesium deficiency in plants cause?

  Chlorosis (yellowing of the leaves due to insufficient chlorophyll production)
• What is the role of magnesium in plant metabolism?

  It is a central component of chlorophyll.
• What does nitrogen deficiency in plants cause?

  Stunted growth and chlorosis.
• What is the role of nitrogen in plant metabolism?

  Nitrogen is essential for the synthesis of amino acids, nucleotides, and chlorophyll.
• What factors limit the rate of photosynthesis?

  • Light intensity (light-dependent stage)
  • Light wavelength (absorption by chlorophyll)
  • CO2 concentration (light-independent stage)
  • Temperature (enzyme-controlled reactions)
  • pH (enzyme-controlled reactions)
• What is a limiting factor in photosynthesis?

  A variable that limits the rate of a particular reaction.
• How are nutrients produced as a result of photosynthesis?

  • Formation of amino acids from GP (requires nitrates and sulfates)
  • TP molecules used to produce sugars (e.g., glucose, fructose, sucrose)
• What happens during regeneration of the Calvin cycle?

  • 5C compound ribulose monophosphate (RuP) forms
  • Ribulose biphosphate (RuBP) is regenerated from RuP using 1× ATP
  • Forms 1× ADP
• What happens during reduction of the Calvin cycle?

  • 2× G3P are reduced to 2× triose phosphate (TP)
  • Requires 2× reduced NADP and 2× ATP from the light-dependent reaction
  • Forms 2× NADP and 2× ADP that enter the light-dependent reaction
• What happens during carbon fixation of the Calvin cycle?

  • Reaction between CO2 and ribulose bisphosphate (RuBP) catalysed by RuBisCo
  • Forms unstable 6C intermediate that breaks down into two molecules of glycerate 3-phosphate (G3P)
• What are the three main stages of the Calvin cycle?

  1. Carbon fixation
  2. Reduction
  3. Regeneration
• What is the light-independent stage of photosynthesis also known as?

  The Calvin cycle.
• Describe the light-independent stage of photosynthesis.

  • Second stage of photosynthesis
  • Calvin Cycle produces glucose
  • Does not require light energy and takes place in the stroma
  • Uses carbon dioxide and products of the light-dependent stage to build organic molecules
• How does the electron transfer chain produce reduced NADP?

  NADP acts as a final electron acceptor and is subsequently reduced.
• What happens to the products of photolysis?

  • H+ is used in proton pumping and to reduce NADP
  • e- replaces electrons lost from chlorophyll a in PSII
  • O2 is a by-product, used for respiration or diffuses out of the leaf
• What is photolysis?

  The splitting of a molecule of water in the presence of light during the light-dependent stage of photosynthesis.
• How does chemiosmosis produce ATP in the light-dependent stage?

  • Protons flow down their concentration gradient from the thylakoid space into the stroma via ATP synthase
  • ATP synthase phosphorylates ADP to form ATP as protons flow through it
• What is the purpose of non-cyclic photophosphorylation?

  It produces ATP and reduced NADP for the Calvin cycle.
• Outline non-cyclic photophosphorylation.

  • Involves Photosystem I and II
  • Excited electrons enter the electron transport chain to produce ATP
  • NADP acts as a final electron acceptor and is reduced
  • Water is photolysed to compensate for electrons lost from Photosystem II
• Define non-cyclic photophosphorylation.

  The formation of ATP and reduced NADP involving both Photosystems I and II.
• What is the purpose of cyclic photophosphorylation?

  It produces additional ATP to meet surplus energy demands of the cell.
• Outline cyclic photophosphorylation.

  • Involves Photosystem I only (no electrons supplied from PSII)
  • Excited electrons enter the electron transport chain to produce ATP and then return to Photosystem I
  • No reduction of NADP and no water required to replace lost electrons
• Define cyclic photophosphorylation.

  The formation of ATP involving Photosystem I only, where NADP+ is not reduced.
• What are the two processes of ATP generation by the light-dependent reactions?

  • Cyclic photophosphorylation
  • Non-cyclic photophosphorylation
• Describe the light-dependent stage of photosynthesis.

  • First stage of photosynthesis
  • Takes place in the thylakoids of the chloroplast
  • Uses light energy to produce ATP, reduced NADP, and oxygen
• What is an action spectrum?

  A graph of the rate of photosynthesis against each wavelength of light absorbed by a pigment.
• What is an absorption spectrum?

  A pattern of bands that occurs when a substance absorbs different wavelengths of light.
• How do Photosystems I and II differ?

  They absorb different wavelengths of light.
• Describe the process of light harvesting in photosynthesis.

  • Antenna complex absorbs light energy of varying wavelengths and transfers it to the reaction centre
  • Energy absorbed by two chlorophyll a molecules which emit ‘excited’ electrons
• What is a Photosystem?

  • Protein complex consisting of an antenna complex and reaction centre
  • Involved in the absorption of light and transfer of electrons in photosynthesis
  • Two types: Photosystems I (PSI) and II (PSII)
• Name the two stages of photosynthesis.

  • Light-dependent stage
  • Light-independent stage
• What is photosynthesis?

  A complex metabolic pathway that synthesises organic molecules from carbon dioxide and water in the presence of light.
• What is the overall equation for photosynthesis?

  6CO2 + 6H2O ⟶ C6H12O6 + 6O2
• Give some examples of photosynthetic pigments.

  • Chlorophylls a and b
  • Beta carotene
  • Xanthophylls
• What is the purpose of chromatography?

  To separate different products from a mixture.
• What is a photosynthetic pigment?

  A molecule present in chloroplasts that absorbs light energy for photosynthesis.
• What is a transducer, and why are chloroplasts described as transducers?

  A transducer converts one type of energy into another; chloroplasts transduce light energy into chemical energy of ATP.
• How are chloroplasts adapted for photosynthesis?

  • Thylakoids provide a large surface area for light-independent reactions
  • Photosynthetic pigments arranged into photosystems to maximize light absorption
  • Stroma surrounds grana for direct diffusion of products
  • Contain their own DNA (cpDNA) and ribosomes
  • Inner membrane less permeable than outer, controlling substance movement
• How are angiosperm leaves adapted for photosynthesis?

  • Large surface area for light absorption
  • Thin to reduce diffusion distance for CO2
  • Upper epidermis transparent for light penetration
  • Palisade cells densely packed with chloroplasts
  • Air spaces reduce diffusion distance for CO2
  • Vein network transports water and minerals
  • Stomata allow CO2 diffusion into the leaf
• Where are chloroplasts located in a leaf?

  Mainly found in the palisade layer.