5.1 Photosynthesis

• What is the equation for photosynthesis?

  6CO2 + 6H2O --> C6H12O6 + 6O2
• What is the equation for respiration in plants?

  C6H12O6 + 6O2 --> 6CO2 + 6H2O
• What is the photolysis of water?

  The photolysis of water is when light energy causes water to dissociate into protons and electrons and oxygen is produced.

  H2O ⇌ 2H+ + 2e- + ½O2

  These electrons from water replace those lost by chlorophyll in photoionisation.
• What is photoionisation?

  Photoionisation is when light energy excites chlorophyll electrons and raises them to higher energy levels, electrons are lost from chlorophyll which becomes positively charged.
• What is the electron transfer chain?

  Electrons pass down an electron transfer chain using REDOX reactions where electron carriers are reduced, energy is lost as they pass down the chain. These electrons reduce NADP to form NADPH.
• What is the chemiosmosis Theory?

  The energy from the electron transfer chain is used to transport protons across the membrane from the stroma to inside the thylakoid, creating a proton concentration gradient. Protons will then diffuse through ATP synthase into the stroma, ATP synthase spins releasing the energy to phosphorylate ADP with Pi forming ATP.
• NADP is the electron carrier used in photosynthesis.
• The light dependent stage takes place in the thylakoid membranes of chloroplasts.
• Label the Calvin cycle:

  A) Rubisco

  B) Ribulose bisphosphate

  C) Glycerate 3-phospahte

  D) Triose phosphate

  E) Glucose

  F) Ribulose phosphate

  
• What is the role of ATP from the light-dependent reaction in the light-independent reaction?

  ATP provides energy for glycerate 3- phosphate → triose phosphate and provides the inorganic phosphate for triose phosphate → ribulose bisphosphate.
• What is the role of NADPH from the light-dependent reaction in the light-independent reaction?

  NADPH provides electrons and protons for glycerate 3- phosphate → triose phosphate.
• What happens at stage 1 of the Calvin cycle?

  Ribulose bisphosphate and carbon dioxide react to from two mole of glycerate 3-phosphate in the presence of the rubisco enzyme.

  
• What happens at stage 2 of the Calvin cycle?

  ATP provides the energy required for glycerate 3-phosphate to turn into triose phosphate while NADPH provides the electrons and protons required for this reaction.

  
• What happens at stage 3 of the Calvin cycle?

  Triose phosphate will then either turn into glucose or ribulose phosphate, it takes 6 rotations of the Calvin cycle to form one molecule of glucose. Additionally, ATP provides the inorganic phosphate required for the triose phosphate to turn into ribulose phosphate.

  
• What happens at stage 4 of the Calvin cycle?

  If the triose phosphate turns into ribulose phosphate then ATP will provide the energy required for it to react and turn into ribulose phosphate.

  
• What are the limiting factors of photosynthesis and what stage of photosynthesis will they effect?

  1. Low carbon dioxide concentration - affects the light independent reaction.
  2. Low light intensity - affects both stages.
  3. Low/High temperature - affects the enzymes of the light independent reaction.
• With limited light how is the light dependent stage of photosynthesis impacted?

  Fewer electrons are released and lost from the chlorophyll, resulting in less photoionisation. Less photolysis will occur so less dissociation occurs and fewer electrons are released by water.
• With limited light how is the light independent stage of photosynthesis impacted?

  No ATP or NADPH is produced in the light dependent stage so glycerate 3-phosphate cannot be reduced to triose phosphate, leading to a build up of glycerate 3-phosphate.
• With low temperatures how is photosynthesis impacted?

  There is less kinetic energy and so fewer enzyme-substrate complexes form through the enzyme rubisco.
• With high temperatures how is photosynthesis impacted?

  Enzymes are denatured (rubisco) and so enzyme-substrate complexes cannot form. This leads to a decrease in the rate of the light independent stage of photosynthesis.
• With limited carbon dioxide how is photosynthesis impacted?

  The rate of the Calvin cycle decreases as less ribulose bisphosphate is converted into glycerate 3-phosphate, this leads to the accumulation of ribulose bisphosphate and a decrease in glycerate 3-phosphate.
• What agricultural practices can be used to optimise the levels of limiting factors? How would this lead to increased productivity and yields?

  We can use artificial light to provide more light energy.

  We can use heaters to provide the optimum temperature.

  We can burn fossil fuels to provide more carbon dioxide.

  
  

  As a result of this there will be more glucose for respiration, providing the energy required for growth.
• During photosynthesis, plants produce organic compounds which contain carbon, such as carbohydrates, lipids and proteins. Most of the sugars synthesised by plants are used by the plant in respiration . The rest are used to make other groups of biological molecules. These biological molecules form the biomass of the plants. Biomass can be measured in terms of mass of carbon per given area per given time. The chemical energy store in dry biomass can be estimated using calorimetry .