Lecture 7: photosynthesis

Question 1

formula for photo

Answer 1

• 6CO2 + 12H20 -> C6H12O6 + 6O2 + 6H2O

Question 2

differences between C3 and C4

Answer 2

C3 -> temperate plants, chloroplasts exist in mesophyll cells, photorespiration, low water use efficiency, grow in low temps (optimum = 20-25°C), high CO2 concentrations, low photosynthetic rate, rubisco

C4 -> tropical plants, chloros. exist in mesophyll + bundle sheath cells, grow at high temps (optimum = 25-30°C), high water use efficiency, low CO2 concentration, high photosynthetic rate, no rubisco

Question 3

where does photosythesis occur in all plants

Answer 3

• chloroplasts
• surface of thykaloids (unit of stacks) and in stroma (fluid)

Question 4

what is the process of light dependent

Answer 4

absorb energy of sunlight by chlorophyll to oxidise water to O2 and energy

Question 5

what is the process of light independent

Answer 5

carbon fixation using energy from light reactions to produce carbs/sugars. Uses CO2 as a source

Question 6

what is the process of the C3 photo

Answer 6

• CO2 fixed into RuBP (5-C sugar)
• reaction of fixing catalysed by Rubisco enzyme
• photorespiration may occur (Rubisco oxygenates RuBP back to CO2 when temps are high and higher light intensity (This ONLY occurs in C3 plants) – occurs if CO2 is low)
• grow in low temps (optimum = 20-25°C)

Question 7

what is the process of the C4 photo

Answer 7

• two different cells photosynthesise – different cells do carbon fixation and carbohydrate synthesis
• mesophyll chloros. produce energy but DO NOT have Rubisco
• CO2 fixed as 4-carbon compound in mesophyll and transferred to bundle sheath where CO2 is concentrated

Question 8

what is gross photo

Answer 8

total energy produced via photo -> determined by efficiency and light intercepted by leaves

Question 9

what is net photo

Answer 9

• gross – respiration and death

Question 10

what is leaf area index?

Answer 10

Leaf Area Index (LAI) = surface area of leaves (m^2)/ground area (m^2)
- maximum light that can be intercepted even if LAI keeps increasing

Question 11

why does LAI keep increasing but photo plateaus?

Answer 11

• larger plants with more leaves require more energy, will use more carbohydrates in respiration, so eventually LAI will be too high and will reduce net photosynthesis and growth

Question 12

What are the 3 principles of how to use pastures for grazing

Answer 12

1. Optimum Growth Phase
2. need to rest grasses after grazing
3. grazing systems

Question 13

What does optimum growth phase principle mean?

Answer 13

• understanding plant growth cycles and using this to determine grazing times
• Phase 1 plants = slow growing, using root energy storage, grow leaves and photosynthesis begins
• Phase 2 plants will have rapid growth after photosynthesis can occur (leaves grown)
• Phase 3 plants will have so many leaves that too much respiration and shading of leaves causes decrease in growth.
• should graze during phase 2

Question 14

what does the need to rest grasses after grazing principle tell us

Answer 14

• plants only sustain max number of leaves
• need time for regrowth after grazing
• if plants only grow some leaves back and animals allowed to graze again, loss of productivity from plant and nutirition
• Need to wait until all leaves have grown for constant turnover to be higher
• If grazing before max leaf number, major loss of growth -> roots damaged as can’t grow deeper = less moisture + less energy reserves = worse growth
  -short grazing times = roots grow, leaf size increase, increased plant chance of survival

Question 15

What does grazing systems principle tell us

Answer 15

• Need to balance growth and utilisation to optimise feed conversion
• to optimise utilisation, balance leaf area to provide enough photosynthesis for growth, with amount of leaf consumed
• want utilisation to be around 50% so material can grow back (only eat half of what is there)