Photosynthesis, the key to plant productivity

Question 1

Terrestrial net primary production (NPP) is monitored by NASA

Answer 1

Terrestrial photoautotrophic production accounts for approx. 50% of the total production

The other 50% is mostly oceanic

Question 2

Why is photosynthesis so important?

Answer 2

Carbon capture and oxygen release
See: https://mlg.eng.cam.ac.uk/carl/words/carbon.html

Question 3

Photosynthesis: recap/ overview of the process

Answer 3

6 CO2 + 12 H2O + light → C6H12O6 + 6 O2 + 6 H2O
see diagrams in notes

Question 4

Leaves as light harvesting organs

Answer 4

Direct sunlight or diffuse skylight hits the upper surface of the upper leaves

Light is transmitted through the upper leaves to those below

Light is then reflected from the ground, or leaves below,

or (in some cases) reflective pigment on the underside of the leaves e.g. in tradescancia or pepperomia

Some leaves are iridescent such as those of the Begonia pavonina (native to Malaysian rainforest floor).Thylakoids in the epidermal iridoplasts are arranged in a regular structure which improves leaves’ ability to absorb red light. Jacobs etal. (2016)

Absorption spectra varies between different photosynthetic pigments.

Phenotypic plasticity in leaves has been observed in Polygonum lapathifolium which express different pigments in diff conditions to absorb different light frequencies (Sultan 2000)

Question 5

PAR: Photosynthetically active radiation

Answer 5

See diagram

• The solar radiation that reaches Earth is: 1300 W m-2
• PAR in direct sunlight at top of canopy is :
  2000 mmol m-2 s-1 (900 W m-2)
• PAR beneath dense canopy is :
  10 mmol m-2 s-1 (4.5 W m-2)

Question 6

Leaves utilise epidermal focusing

Answer 6

Taking: Ambient light of 2000 μmol m-2 s-1

And increasing it to: Internal light of up to 6000 μmol m-2 s-1

Question 7

Competition for light

Answer 7

see graph

Adaptations to competition for light

Bluebell flowers and reproduces in early spring before deciduous trees come into leaf and shade them

Anemone nemorosas flowers and leaves exhibit solar tracking making it a diaheliotropic plant

Wild garlic has dark green foliage due to high chlorophyll content – traps shade light under the woodland canopy effectively – hence a common feature in woodland plants

Question 8

The light reaction of photosynthesis

Answer 8

see: https://www.youtube.com/watch?v=SnnmmKApT-c

Photosystems 1 and 2 are numbered according to when they were discovered – infact the light reactions begin in photosystem 2

Photosystems are located in the thylakoid membranes – being linked to the membrane keeps them in the right place for the most efficient transport of energy

Question 9

Light harvesting antennae

Answer 9

Light harvesting antennae:
- Both photosystems have a reaction centre for electron removal
- The antenna complex made up of multiple chlorophyll molecules, channels light energy to the centre

Evidence for interaction between antenna pigments and the reaction centre:
- 2500 chlorophyll molecules required for maximum yield
- Above max. Yield chlorophylls are fully saturated and there is another limiting factor – no longer limited by light availablilty.
- Quantum yield is between 0 and 1 at one every photon is being used
- Maximum yield in plants tends to be 0.95
- In excess it results in fluorescence

Antenna systems have:
Chlorophyll and accessory pigments that can absorb other wavelengths arranged in a way to absorb wavelengths in order of energy from short high energy wavelengths to longer low energy wavelengths channeling them towards the reaction centre.
As the energy passes between pigments small amounts of energy are lost
Energy moves by fluorescence resonance energy transfer ( think of the transfer of sound resonance between two tuning forks)

Question 10

Z scheme

Answer 10

• Photosystem 1 absorbs far red light to 700nm (longer wavelengths)
• Photosystem 2 absorbs red light (slightly shorter wavelengths)

In natural light there are equal amounts of red and far red light
Reducing adds electrons & oxidising releases electrons
H ions and electrons come from water
This progression of electrons is referred to as the Z scheme

(see z scheme diagram)

Question 11

Light and shade leaves

Answer 11

Biochemically different

Sun exposed :more rubisco and also xanthophyll accessory pigments which clear up resulting products especially reactive oxygen compounds

Shade grown: larger and thinner leaves with more chlorophyll on PSll to balance with PSl which will operate at a higher level in shade due to more availability of long wavelength light compared to shorter ones used in PSl – both photosystems must work at the same rate for optimal photosynthesis

Question 12

Protein complexes

Answer 12

Electrons must be carried from PSll to PSl via free moving electron carriers
Phosphorylation reaction of residues integrates the two photosystems
Kinase occurs when there’s reduced plastoquinones (PQ) when PSll is being activated more than PSl

Four major protein complexes in the thylakoids:
-PSII (stacked regions),
- PSI and ATP synthase (unstacked regions)
- and cytochrome-b6f (both)

• Kinase cascade reaction shares energy between PS
• Avoids rate of electron flow being limited by part receiving
  least energy
• Phosphorylation of specific Thr on LHCII protein (antenna
  protein) makes it migrate out of stacked region and deliver
  more energy to PSI
  *Kinase is activated when reduced PQ accumulates (i.e.
  when PSII is activated more than PSI)

Question 13

Synthesis of ATP and NADPH

Answer 13

Ultimate goal of photosynthesis

When electrons are stripped away in the lumen H+ ions accumulate

H+ ions also move from inside to outside the membrane via plastoquinones

(see diagram in notes)

Question 14

In summary this lecture covered:

Answer 14

• Principles of photosynthesis
• Leaves as light harvesting organs
• The role of pigments
• The light-dependent reactions of photosynthesis:
• Photosystems: light-absorbing antennae, electron transport and reaction centres
• Synthesis of ATP and NADPH