Photosynthesis and the environment Flashcards
Total net photosynthesis
Total net photosynthesis depends on interaction of:
- light - CO2 - temperature - water availability
all of these factors change during the day
Light
Leaf architecture: chl absorbs red and blue light
-> transmitted/reflected is green light: why leaves appear green
palisade cells – several layers under epidermis
Palisade cells
plant cells located within the mesophyll in leaves, right under the upper epidermis. They are vertically elongated (2-3 layers). Their chloroplasts absorb a major portion of the light energy used by the leaf. Most photosynthesis takes place here.
(spongy mesophyll with spread out cells is beneath them - more air pockets)
blue dots are chloroplast
Spongy mesophyll cells
- consisting of loosely arranged, irregularly shaped cells that have chloroplasts
- has many spaces between cells to facilitate the circulation of air and the exchange of gases
- it lies just below the palisade layer
- less likely to go through photosynthesis than those in the palisade mesophyll
sieve effect
(chl is in the chloroplasts)
> some light passes through cells without being absorbed
- to reduce sieve effect, plants develop multiple layers of photosynthetic cells
-> reflection, refraction, and scattering of light may also reduce the sieve effect
light guide effect (light channeling)
> some light is channeled through intercellular spaces (fiber optics)
it is efficient for leaves to have several layers of palisade cells (to avoid sieve and light guide effect)
More cells = greater probability that the photon will be absorbed
Why don’t plants have many layers of palisade cells?
- the cells all have to be supported
- trade off
- lower efficiency
- expensive metabolic energy (burden) because plants
will end up wanting to absorb many photons
light guide effect (light channeling)
- some light is channeled through intercellular spaces (fiber optics)
- efficient for leaves to have several layers of palisade cells
spongy mesophyll
- under palisade cells
- irregularly shaped with large air spaces to facilitate the circulation of air and the exchange of gases.
- ## air / water interfaces (light is bent) –> much light refraction –> increase probability of light absorption
Chloroplast and Leaf Movement
Plants can adapt to optimize light absorption
- light is dilute
- plants compete for light
Chloroplasts can move
- LOW light: chlpts PERPENDICULAR to light for MAXIMUM absorption (less air spaces)
- HIGH light: chlpts PARALLEL for MINIMUM light absorption (minimize absorption of photons) (more air spaces)
- chlpts move along actin filaments in cytoplasm
solar tracking
- leaf moves to optimize light absorption (leaf perpendicular to light)
- pulvinus controls leaf orientation and movement
- change in osmotic potential changes leaf orientation
- Plant movement is affected by photoreceptors and pulvinus
plant movement (solar tracking)
Light comes in at a certain angle. leaf changes their orientation and face towards the light
Photosynthesis as Function of Light
graph:
- CO2 uptake increases with photon flux (absorbed light)
- At light compensation point:
net photosynthesis = 0 because CO2 uptake equals CO2 evolution (release)
- Lesser light = negative photosynthesis -> respiration may proceed (dark respiration rate, light is limited)
There is a positive x intercept. Why? absorbed light is either 0 or positive
Shade Plants
- Shade plants have LOWER maximum photosynthetic rates than sun plants
- Shade plants have LOWER light compensation points than sun plants
C3 and C4 plants
- C3 plants saturate (20-40% of full sunlight): photoinhibition when light is too high
- C4 plants DON’T light saturate: Why? C4 plants don’t have much photorespiration (PR) because they pump CO2 in
C3 and C4 plants
C4 plants are MORE productive than C3 plants – Why isn’t the world filled of C4 plants? C4 plants outperform C3 plants when there is a lot of light, but require better ATP and better light conditions.
- Regions are diverse, different climates.
C3 vs C4 plants
- C3 plants are called temperate or cool-season plants. They reduce (fix) CO2 directly in the chloroplast. (less efficient in photosynthesis, perform photosynthesis only when stomata is open)
- C4 plants are often called tropical or warm season plants. They reduce carbon dioxide captured during photosynthesis
to useable components. (more efficient in photosynthesis, perform photosynthesis even when stomata is closed)
Compensation point
- when net photosynthesis is equal 0 (CO2 uptake and CO2 release cancel each other out)
–> CO2 fixation = CO2 evolution - varies with species:
sun plants 10-20 uE /m2 / sec
shade plants 1-5 uE /m2 /sec
–> for plants to grow, light must exceed
compensation point
shade plants vs. sun plants
- shade plants: lower respiratory rates, more chl/reaction center, higher chl b / chl a content, thinner leaves (less metabolic effort)
- sun plants (can capture a lot of photons), more RuBP carboxylase / oxygenase
Leaf grown in sun – more layers of cells (more palisade and spongy mesophyll cells)
Leaf grown in shade – less layers of cells
CO2
[CO2] = 350 ppm
350 ppm = 0.035%
increases 1 ppm / year
greenhouse effect
CO2 plays same role as glass
- radiation from the sun warms both the atmosphere and the earth. the earth then radiates infrared back into the atmosphere. it is either reflected back to earth or absorbed by atmospheric gases (CO2, H2O vapor), thus preventing its escape. some of the trapped infrared is reradiated back to earth giving rise to increased temperatures.
greenhouse
Made of glass
Sunlight comes in through glass
Sunlight (photon) is absorbed
Electrons in an excited stage, returns to ground state/heat
Infrared radiation (heat) is produced in greenhouse and stays trapped within glass ->
Greenhouse gets warm (even in the winter)
CO2 compensation point
net photosynthesis = 0
CO2 fixation = CO2 evolution
- C3 plant: as CO2 increases, so does photosynthesis, positive x-intercept
- C4: at high CO2 levels, they under perform C3 plants (don’t perform as well - reach a constant maximum point in the graph)
- > C3 plants exceed C4 plants because C4 plants are burning extra ATP (linear increase in the graph)
- At normal conditions, C4 plants behave better.
- CO2 compensation point -> C4 plants at zero because they have low photorespiration (they don’t need much CO2 at all)
- C3 and C4 plants have different compensation points because C3 plants have Photo respiration
- There is no difference in compensation point under low PR conditions
C3 plants vs. C4 plants
C3 plants have higher photosynthesis than C4 plants at high CO2 concentrations.
Respiration rates are about the same.
photosynthesis and CO2 concentration
> At low CO2 concentration, photosynthesis is limited by CO2
-> At high CO2 concentration, photosynthesis (in C3 plants is limited by the Calvin Cycle
Temperature
Plants exist in a wide variety of temperatures: Antarctica –> Death Valley
- Biological reactions (processes) have a temperature optimum (T opt)
- Temperature affects respiration and photorespiration
- Net photosynthesis = gross photosynthesis - respiration
- Temperature stimulates photosynthesis up to an optimum
Net photosynthesis
Net photosynthesis = gross photosynthesis - respiration
NP = GP - R
C4 and C3 plants (quantum yield)
- C4: quantum yield constant due to low PR
- C3: quantum yield drops due to PR;
- > therefore, higher energy demand per CO2 fixed.
- C4 plants have no PR (constant line in graph)
- C3 plants have PR (as temp goes up, quantum yield drops) (decreasing line in graph)
C3 and C4 plants performance
- C3 do better at higher CO2 concentrations and lower temperatures (cold)
- C4 plants do better at low CO2 levels and higher temperatures.- C4 plants perform better at warmer temperatures (hot)
Table: Comparison of significant features of C3 and C4 plants
C3 plants:
has PR, 20-100 CO2 compensation, T opt: photosynthesis (20-25) and Rubisco (20-25), quantum yield decreases, transpiration ratio (500-1000), light saturation (400-500)
C4 plants:
No PR, 0-5 CO2 compensation, T opt: photosynthesis 30-45) and PEPcase (30-35), quantum yield is steady/constant, transpiration ratio (200-350), light saturation (none - does not saturate)
Water
- Water is crucial and plants conserve it
- CO2 enters and O2 and water leave through stomata (abundant at bottom of leaves)
- -> stomata: little holes where water can get out
Stomata
- CO2 enters and O2 and water leave through stomata
- Stomata is open under conditions of:
1) adequate water
2) low CO2 (wants more CO2)
3) light (let CO2 in)
changes in osmotic potential leads to ions taken up by guard cells causing them to expand and the stomata to open
- less water: somata is closed to conserve water
Stomata and water stress
During water stress, stomata close
Hormonal control
- What happens to a plant when the stomata close?
oxygen can’t get out, Carbon dioxide can’t get in
Water stress: Reduces turgor pressure Reduces leaf expansion Reduces surface area Reduces total photosynthesis
productivity
Productivity = P - PR – R (P) Photosythesis: takes in carbon (PR) Photorespiration: releases carbon
plant productivity
- Overall plant productivity depends on photosynthesis, photorespiration and respiration
Productivity = P - PR – R - Photosynthesis depends on light, CO2, temperature and water availability.
–> These also affect
photorespiration and
respiration. - Respiration can be growth related or maintenance related.
–> Reduce respiration
through breeding programs
Summary
- Total net photosynthesis depends on the interaction of
light, CO2, temp, water availability - All of these factors change during the day
- Plants can optimize light absorption
- Shade plants have adaptations for their environment
