Photosynthesis: Dark cycle reactions,variation in the dark cycle system, protection of the photosynthesis system,control of photosynthetic rate.

Objectives of the lecture:

1. Describe the dark cycle reactions of photosynthesis.

2. Illustrate field measurements of photosynthesis.

3. Discuss how dark cycle reactions can limit the rate of photosynthesis.

4. Describe photorespiration.

5. Describe C4 and CAM photosynthesis.

Text book pages:213-219.

Recap and importance:

The photochemical reactions produce ATP and NADH at sites in the stroma.

The Dark Cycle (Calvin Cycle), or more descriptively, the carbon reactions of photosynthesis

~200 billion tons of CO2 are converted to biomass each year

The enzyme ribulose biphosphate carboxylase/oxygenase, Rubisco, that incorporates CO2 is 40% of the protein in most leaves.

The Calvin cycle proceeds in three stages: carboxylation, reduction, and regeneration

Carboxylation of the CO2 acceptor, ribulose-1, 5-biphosphate, forming two molecules of 3-phosphoglcerate.

Reduction of 3-phosphoglycerate to form glyceraldehyde-3-phosphate which can be used in formation of carbon compounds that are translocated.

Regeneration of the CO2 acceptor ribulose-1, 5-biphosphate from glyceraldehyde-3-phosphate

Rubisco – the enzyme ribulose biphosphate carboxylase/oxygenase

The affinity of Rubisco for CO2 is sufficiently high to

ensure rapid carboxylation at the low concentration of CO2 found in photosynthesizing cells

The negative change in free energy associated with carboxylation of RuBP is large so the forward reaction is favored.

RuBP

Rubisco will also take O2 rather than CO2 and

oxygenate RuBP – called photorespiration.

The rate of operation of the Calvin Cycle can be enhanced by increases in the concentration of its intermediates. That is the cycle is autocatalytic.

Also, if there are insufficient intermediates available, for example when a plant is transferred from dark to light, then there is a lag, or induction period, before photosynthesis reaches the level that the light can sustain. (There can also be enzyme induction.)

Rubisco is notoriously inefficient as a catalyst for the carboxylation of RuBP and is subject to competitive inhibition by O2, inactivation by loss of

carbamylation, and dead-end inhibition by RuBP. These inadequacies make Rubisco rate limiting for photosynthesis and an obvious target for increasing agricultural productivity. Really?

Field measurement of photosynthesis and its control by

environmental conditions

Infra-red Gas Analyzer measures the concentration of CO2 in the air stream before and after it flows across the leaf in the chamber

The chamber is enclosed over the leaf. Light and temperature are measured while photosynthesis is being measured.

Photosynthesis rate calculated from gas flow rate and CO2 concentration difference

LI-COR 6400

Basics of foliage photosynthesis

00

Saturation level. sometimes called photosynthetic capacity.

Compensation pointThe irradiance at which CO uptake is zero2

Photosynthetic efficiency:Increase in photosynthesis per increase in irradiance

Any questions?

Increasing CO2 concentration in the atmosphere can increase the maximum rate of photosynthesis in the short term

Light Reaction Limiting

Dark Reaction Limiting

Measured light response curve of Abies amabilis first year foliage.

Shade foliage with low maximum value and low compensation point.

Observed assimilation rates (µmolCO2/m2s) of Tsuga

heterophylla and Abies amabilis in response to periods of 10 minutes high light (1500µmol/m2s PPFD), with 5 minutes intervals of darkness (shaded parts in the diagram) in between. Values measured using 200 mol/s flow rate.

Species differences in leaf photosynthesisA has the highest photosynthetic rate at light saturation

B has the highest photosynthetic efficiency and the lowest compensation point.

Another important measure is called Water Use Efficiency:the ratio of photosynthesis achieved per unit of water lost.

Units: mmol/mol milli mols of CO per mol of water transpired

2

Units: μmol/m /smicro mols of CO per square meter foliage per second

22

milli [m] 0.001 (a thousandth) micro [µ] 0.000 001 (a millionth)

Any questions?

Wind River Canopy Crane Research Facility

Thuja plicata

Abies grandis

Pseudotsuga menziesii

Tsuga heterophylla

Old-growth species:

Douglas-firPseudotsuga

Western hemlockTsuga

Upper Canopy Lower CanopyPhot. Cap.

13.1

9.0

μmol/m /s2

Water Use Eff.

6.2

4.9

mmol/mol

Phot. Cap.

8.8

3.2

Water Use Eff.

3.5

4.8

Notice the difference in branch structure between the species

The problem of photorespiration and the evolution of photosynthesis

When the enzyme Rubisco uses oxygen to breakdown carbohydrate to CO2 rather than using CO2 to synthesize carbohydrate

How some grasses have evolved a C4 metabolic process and some desert plants have evolved Crassulacean Acid Metabolism

Although Rubisco acts like a carboxylase in photosynthesis, it can also act as an oxygenase when O2 is available.O2 and CO2  compete for the same active site!

This is called Photorespiration

3-phosphoglycerate

2-phosphoglycerate

This becomes a  problem when photosynthesis rates are high, i.e. photosystem II produces lots of  O2 .

P P PPC-C-C-C-C C-C-C + C-C

RuBisCORibulose 1, 5-biphosphate

Enzyme

O2

CO2

It is believed that photorespiration in plants has increased over geologic time due to increasing atmospheric O2 concentration -the

product of photosynthetic organisms themselves.

In the presence of higher O2 levels, photosynthesis rates are lower.

The inhibition of photosynthesis by O2 was first noticed by the German plant physiologist, Otto Warburg, in 1920, and called the "Warburg effect".

275 ppm CO2

73 ppm CO2

Decarboxylation of malate (CO2 release) creates a higher concentration of CO2 in bundle sheath cells than found in photosynthetic cells of C3 plants.

The first product of CO2 fixation is malate (C4) in mesophyll cells, not PGA as it is in C3 plants. This is transported to bundle sheath cells

CO2 is released from malate in bundle sheath cells, where it is fixed again by Rubisco and the Calvin cycle proceeds. PEP is recycled back to mesophyll cells.

This enables C4 plants to sustain higher rates of photosynthesis. And, because the concentration of CO2 relative to O2 in bundle sheath cells is higher, photorespiration rates are lower.

C4 Photosynthesis

Xylem

Bundle sheath cells filled with chloroplasts. CALVIN REACTION SITE

Phloem

Parenchyma filled with chloroplasts

C4 acids synthesized in the parenchyma move to the bundle sheath

Carbon skeleton compounds return to parenchyma

Anatomical separation of the C4 photosynthesis component processes

Crassulacean Acid Metabolism (CAM)

Uses C4 pathways, but segregates CO2 assimilation and Calvin cycle between day and night

CAM plants open their stomates at night. This conserves H2O. CO2 is assimilated into malic acid and stored in high concentrations in cell vacuoles

During the day, stomates close, and the stored malic acid is gradually recycled to release CO2 to the Calvin cycle

First discovered in succulents of the Crassulacea: e.g.,sedums

C3, majority of

speciesC4, e.g., sugar

cane, cornCAM, e.g., cacti

Leaf structure

Typical habitatcharacteristics

Productivity

Optimum Temperature

Efficiency in light

Bundle sheathcells have

chloroplasts

Bundle sheath cells lack

chloroplasts

Mesophyll cells have large vacuoles

Can be sun or shade plants

Ineffective in shade

CO2 capture at night

Requires relatively

moist habitats

Arid or tropicalregions

Arid environments

Moderate High Low

15-25oC 30-40oC 35oC

Things you need to know ...1. The basic reactions of the Calvin Cycle with the names and basic

structures of the principal reactants but not their detailed chemical formulea.

2. The characteristics of Rubisco.

3. The method of field measurement of photosynthesis by gas exchange

4. The light saturation curve of leaf photosynthesis and its important features

5. Water use efficiency. Calculation and value as a physiological measure

6. Why photorespiration is important and the processes of C4 and CAM photosynthesis.