Bio 360 September 20, 2001

Photosynthesis II - Environmental Effects

I. . Dark reactions

CO₂ + 2 NADPH + 2 H⁺ + 3 ATP -> (CH₂O) + 2 NADP + 3 ADP + H₂O

1. Process by which gaseous CO₂ is fixed into an organic compound and then

2. The organic compounds go through a series of reactions to make carbohydrates

3. These reactions occur in the stroma of the cholorplasts

4. Three different means of fixing CO₂

A_. C₃

In the initial reaction, CO₂ combined with ribulose bis phosphate to form 2, 3- carbon compounds.

However, Rubisco also reacts with O₂

This second reaction results in photorespiration and a net loss of carbon dioxide.

The other means of carbon dioxide fixation avoid the problem with Rubisco.

B. C₄ - separation of two parts of CO₂ fixation in space

1. A different enzyme is responsible for the first steps of CO₂fixation

PEP carboxylase - has a much higher affinity for

CO_2, photorespiration is not a problem

Initially CO₂ is bound to a C ₄ compound

RuBP-carboxylase is still present

2. Leaf anatomy is different (Kranz anatomy)

3. The two parts of CO₂ fixation are separated in space.

CO₂ fixation occurs in the mesophyll cells where C₄ compounds are made by PEP carboxylase, an enzyme with high affinity for CO₂

C₄ compounds are transported to the bundle sheath cells where CO₂ is released and used by RuBP to make carbohydrates.

4. Benefits

a. There is little oxygen competition for RuBP, so more CO₂can be fixed.

b. Since, CO₂is used efficiently, the stomata do not have to be open as much and so water is conserved.

5. Costs - energy must be expended, 2 additional ATP's per CO₂

6. Occurrence - Found in many grasses, such as corn and sugarcane.

C. CAM - separation of two parts of CO₂ fixation in time

a. Night, stomata open

CO₂is fixed by PEP carboxylase to form malic acid. Starch is broken down to form PEP

b. Day, CO₂ is released from malic acid and is used by RuBP to form carbohydrates. Stomata are closed.

3. Costs - - energetically expensive, 3.5 ATP/CO₂

carbon is fixed very slowly, so plants grow very slowly

4. Occurrence - Cacti, bromeliads, succulents, plants of very hot, dry climates

II. Efficiency of photosynthesis - Efficiency = energy stored/energy input

A. Light reactions

1. Using pure red light at 680nm, 38% light energy -> chemical energy

2. For white light, maximu is 20%

3. In the field, the efficiency is typically less than 10%

B. Dark reactions

1. Maximum possible efficiency in light -> hexose = 35%

2. Little energy is lost

3. But, in the field, measured efficiencies are closer to 5-10%

C. In the field, typically about 1% of the available light energy is converted to stored energy.

What limits photosynthesis in the field

1. Light

2. CO₂

3. Water - to absorb CO₂, the stomata must open when the stomata are open, H₂O is lost

4. Temperature

5. Nutrients

III. What limits photosynthesis in the environment?

A. Photosynthesis - respiration balance

1. Respiration is the opposite of photosynthesis

C₆H₁₂O₆ + 6 O₂ -> 6 CO₂ + 6 H₂O + energy

2. Plants undergo both respiration (CO₂ loss) and photosynthesis (CO₂ gain)

Net rate of photosynthesis = Carbon gained - carbon lost

3. Can be -, 0, or +

4. Under which environmental conditions is the net rate of photosynthesis positive?

B. Light

2. Rate of photosynthesis vs. light availability for single leaves

a. C₃ plants

1. How much light to make the net rate of photosynthesis 0? This is the light compensation point.

2. Light is not limiting in full sunlight.

3. Light is limiting for plants in the shade.

b. C₄ plant

1. Light compensation point is much higher

2. Maximum photosynthesis is not reached at full sun light

3. Light limited

4. Why?

c. CAM - will also need high light

3. Rate of photosynthesis for entire plants

a. Takes more light to saturate entire plants

C. Carbon dioxide

1. C₃ plants

a. The CO₂compensation point is at about 50ppm

b. Atmospheric concentration is 360ppm

c. The plant is not CO₂ saturated at 360ppm, so it is CO₂ limited

d. CO₂concentrations

atmosphere - .036%

canopy at full sun - .026%

canopy in dark - .04%

2. C₄ plants

a. Compensation pt = 0-5ppm

b. Saturated at atmospheric concentration

c. CO₂ is not limiting

d. Why?

3. CAM - compensation point is 0-5ppm

4. The difference between C₃ and C₄ is based on photorespiration

D. Water

1. Low water availability limits photosynthesis in two ways

a. stomata close

b. reduced turgor pressure limits leaf expansion

2. There is a basic tradeoff between water loss and carbon gain

b. For CO₂ to be absorbed, the plant must be vulnerable to water loss

3. C₃ vs. C₄ vs. CAM

a. C₃ plants have the largest water loss due to inefficient CO₂ absorption

b. C₄ plants do not have to have the stomata open often

c. CAM plants open stomata at night

E. Temperature - Effects of increasing temperature

1. May increase the rate of action of enzymes till they denature

2. May change membrane fluidity

3. Increase water loss

4. C₃ vs. C₄ vs. CAM

a. Light reactions - rate stays the same

b. CO₂ diffusion - rate stays the same

c. RUBP-carboxylase - rate of action increases

d. ratio of dissolved O₂:CO₂ increases so,

e. Photorespiration becomes more important in C₃ plants

f. C₄ plants are unaffected by photorespiration and, in consequence, have maximum photosynthesis at high temperatures

g. CAM plants also have high temperature optima

5. The actual optimum temperature of photosynthesis for a plant can vary based on growing conditions

F. Nutrients

1. The rate of photosynthesis increases at the concentration of nitrogen increases

2. Many proteins are involved in photosynthesis

3. About 75% of leaf protein is involved directly in photosynthesis

G. Summary of differences between C3, C4 and CAM plants

Under what conditions would you expect each type of plant to do well or poorly in competition?

IV. Photosynthesis and leaf characters

A. Leaf age

1. Each leaf costs something to build and maintain and

2. Young leaves - construction costs

3. Newly expanded leaves - profitable

4. Old leaves - inefficient photosynthesis, dropped

B. Leaf orientation

1. Tracking the sun

2. Leaf angle

C. Sun vs. shade leaves - within or between species differences

Sun Shade

narrow, dissected entire. broad

thick thin

more RuBP more chlorophyll

thick cuticle thin cuticle

Example: Strawberry

D. Canopy structure

1. Number of leaf layer

2. Leaf Area Index = LAI = area of leaf surface/area of ground

E. Selectable features in crops 1. Selection on the photosynthetic machinery has not worked well. 2. Can breed for leaf shape, area, orientation