Chapter 3 (photosynthesis)

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  • Created by: AbzzStarr
  • Created on: 23-05-16 12:20

3.1 Structure of leaves + p/s equasion


  • Large surface area to collect sunlight
  • Arranged so one leaf isn't shadowed by another
  • Thin = short diffusion pathway
  • Transparent cuticle & epidermis - let light through to photosynthetic mesophyll cells beneath
  • Long, narrow upper mesophyll cells packed with chloroplasts to collect light
  • Lots of stomata for gaseous exchange
  • Stomata that open & close in response to light intensity
  • Lots of air spaces in lower mesophyll layer to allow diffusion of Carbon dioxide and oxygen
  • A network of xylem that brings water to cells in the leaf
  • Phloem to carry away sugars produced by photosynthesis

Overall photosynthesis equasion:

6CO2 + 6H2O ----> C6H1206 (glucose) + 6O2

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3.2 light - dependent reaction

Light used in 2 ways: To join P onto ADP making ATP and photolysis of water

  • Chlorophyll absorbs light - excites electrons
  • Electrons leave chlorophyll for electron carrier
  • Electrons move along ETC in O-R reactions in thylakoid membranes - energy combines P with ADP
  • Electrons replaced using photolysis, which also yields H ions (protons)
  • Protons taken up by NADP (becomes reduced NADP)
  • Reduced NADP then enters LIR along with the electrons 
  • The oxygen by-product of photolysis is either used in respiration or diffuses out of the leaf 

This happens in thylakoids of chloroplasts, chloroplasts are adapted for p/s in the following ways:

  • Thylakoid membranes have big SA for chlorophyll, electron carriers & enzymes to attach to
  • Proteins in grana hold chlorophyll in a way to allow maximum light absorption
  • Granal membranes have enzymes that help make ATP
  • Chloroplasts have DNA & ribosomes - they can make proteins needed for LDR 
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3.3 LIR (Calvin Cycle)

1. CO2 from the atmosphere diffuses into leaf through stomata & ends up in the stroma

2. Once in stroma, CO2 combines with RuBP (5 carbons) using an enzyme

3. This produces 2 molecules of G3P

4. ATP and reduced NADP from the LDR reduce G3P to TP

5. NADP is reformed and returns to LDR to be reduced again

6. Some TP converted to useful organic substances, such as glucose

7. Most TP used to regenerate RuBP using ATP from the LDR

Stroma adaptations for LIR: contains enzymes for reduction of CO2, surrounds grana (products of LDR can get there quickly), has DNA & ribosomes so it can quickly make proteins needed for LIR

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3.4 factors affecting photosynthesis

"At any given moment, the rate of a physiological process is limited by the factor at its least favourable value" e.g. light intensity:

  • In darkness, lack of light stops p/s - temperature & CO2 changes have no effect
  • Add light = increases rate of p/s
  • Add more light = rate of p/s increases further but not indefinitely
  • Now another factor (e.g CO2) is at its least favourable value & limits p/s
  • Providing more CO2 increases p/s
  • Further increases in CO2 have no effect (temperature now limits)

P/S measured by volume of O2 released or volume of CO2 taken up .

Light compensation point  = Point at which there is no net exchange of gases in/out of the plant (O2 produced & CO2 absorbed due to p/s is exactly balanced by O2 absorbed and CO2 produced by respiration)

Optimum CO2 concentration = 0.1% - gardeners/farmers often add more 

Optimum temperature = 25C - any higher & enzymes will denature & slow p/s

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