bio

• Photosynthesis as an example of biochemical pathways
• Inputs, outputs and locations of the light dependent and light independent stages of photosynthesis in C3 plants

  • Inputs, outputs and locations of the light dependent and light independent stages of photosynthesis in C3 plants (details of biochemical pathway mechanisms are not required)
• Chloroplasts


  • Site of photosynthesis, an overview of their structure and evidence of their bacterial origins
• Rubisco


  Role in photosynthesis, including adaptations of C3, C4 and CAM plants to maximise the efficiency of photosynthesis
• Factors that affect the rate of photosynthesis

  • Light
  • Temperature
  • Carbon dioxide concentration
• Cellular respiration as an example of biochemical pathways
• Main inputs, outputs and locations of glycolysis, Krebs Cycle and electron transport chain including ATP yield

  • Main inputs, outputs and locations of glycolysis, Krebs Cycle and electron transport chain including ATP yield (details of biochemical pathway mechanisms are not required)
• Location, inputs and the difference in outputs of anaerobic fermentation in animals and yeasts

  • Location, inputs and the difference in outputs of anaerobic fermentation in animals and yeasts
• Factors that affect the rate of cellular respiration

  • Temperature
  • Glucose availability
  • Oxygen concentration
• Potential uses and applications of CRISPR-Cas9 technologies to improve photosynthetic efficiencies and crop yields
• Uses and applications of anaerobic fermentation of biomass for biofuel production
• Purpose of Photosynthesis


  A plant can not walk to the fridge to find food, so it must make its own where it is
• Photosynthesis


  The process of using light energy and converting it into chemical energy in the form of glucose
• Photosynthesis is a series of biochemical reactions that are driven and controlled by enzymes
• Chloroplasts


  • Predominantly occurs in the chloroplasts within the green parts of a plant (can be in other coloured regions), and in some protists (eg. Algae)
• Characteristics of Chloroplasts


  • Outer membrane
  • Inner, highly folded membrane that forms stacks called grana
  • Enzymes found embedded on the grana, whilst others are found in the surrounding solution called the stroma (some DNA here too)
• Photosynthesis


  Occurs in two stages: Light Dependent reaction and Light Independent reaction
• Each stage of photosynthesis is confined to an area of the chloroplast
• Inputs and Outputs of Light Dependent Reaction

  • Inputs: Light energy, Water

  Outputs: Oxygen, Energy (ATP, NADPH)
• Light Dependent Reaction


  • Light energy is absorbed by different pigments with the thylakoid membranes of chloroplasts, including chlorophylls, carotenoids and xanthophylls
• Light Dependent phase


  1. NADP+ reductase

  2. Electron transport chain

  3. Photolysis of water
• Light Independent stage


  Occurs in the stroma of the chloroplast (liquid part), can occur with or without light, but requires the products from the light dependent stage
• Light Independent Stage


  H+ ions (carried by NADPH) and energy (ATP) from the light dependent reaction are used in conjunction with carbon dioxide to produce glucose in a process called the Calvin-Benson Cycle (C3 pathway – because the first stable compound has 3 carbon atoms)
• Rubisco


  Enzyme that drives carbon fixation, most abundant enzyme in the world
• It takes 2 Calvin-Benson cycles to produce one C6H12O6
• Summary of inputs and outputs (Light Dependent)

  • Inputs: Light energy, Water

  Outputs: Oxygen, Energy (ATP, NADPH)
• Summary of inputs and outputs (Light independent)

  • Inputs: CO2, H from light dependent stage

  Outputs: Glucose
• C3, C4 and CAM plants


  The C differentiates between the size of the first stable carbon compound formed in the light independent stage (Calvin cycle)
• C3 plants


  • On hot days, they'll close their stomata to reduce H2O evaporation. Downside is CO2 can't get in. When CO2 levels are low the enzyme involved in carbon fixing reacts with O2 instead of CO2 (called photorespiration) – the oxygen reacts with the glucose to produce CO2 but without the production of ATP. The CO2 can then be used for photosynthesis. The first stable compound is a 3-carbon compound.
• C3 Plants


  CO2 + 5C = RuBP (carbon fixation) (Rubisco catalyses)

  RuBP breaks into 2 x 3C (therefore a C3 plant)

  Lots of steps + ATP + NADPH = C6H12O6

  BUT! On hot/dry days: Stomata close, Low CO2 / high O2 PHOTORESPIRATION (O2 competitive inhibitor), Therefore slow carbon fixation, Therefore low growth
• C4 plants


  • In tropical areas a number of plants, such as sugarcane, corn and tropical grasses are able to photosynthesis (fix carbon) when the levels of CO2 within the leaf are low. The first stable carbon compound formed by these C4 plants contains 4 carbon atoms. A different enzyme which does not react with O2 is present and allows photosynthesis to continue in low concentrations of CO2 – therefore stomata can remain closed conserving water loss. C4 plants are more productive than C3 plants at high temperatures, but at lower temperatures C4 plants lose out to C3 species, presumably because the activity of their (C4 plants) enzymes is depressed by photorespiration.
• C4 Plants


  CO2 comes in through stomata and diffuses

  CO2 + C3 = C4

  C4 goes to the bundle sheath cells

  C4 -> CO2 + C3

  CO2 used by Rubisco in Calvin Cycle

  Partitioning means Rubisco is not exposed to lots O2 but CO2 instead

  Therefore high carbon fixation at higher temps and limited photorespiration
• CAM plants


  • CAM plants (such as cacti and other desert plants) don't open their stomata during the day (too hot and risk too much H2O evaporating). Instead, they open their stomata during the night (cooler) to let in the CO2. Calvin cycle occurs. In prolonged droughts they don't open stomata at all, instead use the CO2 produced in aerobic respiration. CAM plants grow slowly but are able to compete with C3 and C4 plants in arid areas because they can conserve water.
• CAM Plants


  In hot climates so stomata open at night

  CO2 is then fixed to form MALATE

  During the day Calvin cycle happens

  Malate is processed to free up CO2

  This uses more ATP therefore slower growth

  When there is lots of rain it reverts to a C3 pathway and grows faster (the desert blooms after rain!)

  In times of extended draught, stomata stay shut so have to use CO2 from cellular respiration for photosynthesis so grow much slower
• Three main factors that affect the rate of photosynthesis

  • Light intensity
  • Carbon dioxide levels
  • Temperature
• Light intensity and photosynthesis

  As the first stage depends on light, it makes sense that as light levels increase, so does the making of products from the independent stage. This occurs until the chloroplasts are saturated with light. About 20% of light that hits the leaf is reflected away. Only about 1% of light absorbed is converted into chemical energy
• Carbon dioxide levels and photosynthesis

  For most plants, carbon dioxide from air dissolves in extracellular fluid before entering photosynthetic cells. There are local variations in carbon dioxide levels in air, in different habitats and at different times of the day. Aquatic plants can also use hydrogen carbonate (carbonic acid), which forms when carbon dioxide dissolves in water. CO2 released as a product of cellular respiration can also be used for photosynthesis, but usually only provides a small amount of the total carbon dioxide requirements. As the concentration of carbon dioxide increases, the amount of photosynthesis also increases provided there are sufficient enzymes to catalyse the reaction and a sufficient supply of NADPH and ATP from the light dependent reaction.
• Effect of Temperature on Photosynthesis

  Photosynthesis increases with increasing temperature until around 20-40°C, depending on plant species, then it declines again. Plants that live in hotter climates are at higher end of the range. As the temperature increases the rate of reaction will increase as particles will have greater kinetic energy and therefore a higher number of random collisions between substrate and enzyme. However temperatures too high will cause the enzymes to denature and therefore decrease the rate of photosynthesis.
• Indirect Factors affecting Photosynthesis

  Water: Required in photosynthesis, only 1% of water passing up the xylem is used in photosynthesis. The rest is used in other chemical reactions, to hydrate cells or is lost in transpiration. If there is not enough water to hydrate the cells and keep them turgid, the stomata close. This prevents carbon dioxide entering the leaves, therefore photosynthesis decreases.

  Level of chlorophyll: Limits photosynthesis
• Aquatic plants can also use hydrogen carbonate (carbonic acid), which forms when carbon dioxide dissolves in water