Review of Act 1of photosynthesis: the light reactions

• ____ energy is converted to ____ energy– Light… chemical

• What are the products? – NADPH, ATP (both used in the Calvin cycle) & oxygen

• Why not stop after light reactions? – NADPH & ATP aren’t stable; energy can not be stored or

transported in these forms

– carbon compounds are needed for organisms to grow

Act 2 of photosynthesis:Calvin Cycle

General equation for photosynthesis:

6CO2 + 6H2O + Sunlight C6H12O6 + 6O2

Note: This is for the light reactions and the Calvin cycle combined

The mission:

Store chemical energy in the form of 3-carbon sugars made from CO2

http://www.khanacademy.org/video/photosynthesis---calvin-cycle?playlist=Biology

Calvin Cycle, a.ka. “dark reactions”

• “light independent reactions”

• The Calvin cycle does not operate at night• requires ATP and NADPH, which are produced in the light reactions

• several of the Calvin cycle enzymes are activated by light

• little CO2 is available when the stomates are closed at night.

http://simple.wikipedia.org/wiki/Stomates

The Players

RuBP (Ribulose bisphosphate) - a 5-carbon sugar CO2 - Carbon Dioxide Rubisco (Ribulose bisphosphate carboxylase-oxygenase) - an

enzyme that allows CO2 to attach to RuBP PGA- (3-Phosphoglyceric acid)- 3-carbon carboxylic acid ATP – (Adenosine triphosphate) - the main energy source DPG (1,3-Disphosphoglycerate)- 3-carbon molecule that has a

phosphate group on both ends NADPH- (Nicotinamide adenine dinucleotide phosphate) product

of light reaction PGAL OR G3P- (Glyceraldehyde-3-phosphate)- the simplest

sugar known, food for plants

The SceneThese reactions occur in the stroma of chloroplasts

Scene 1: Carbon Fixation

• CO2 comes into the stroma of the chloroplast via the stomata of the leaves. 

• Rubisco catalyzes the bonding of CO2 to RuBP to create an unstable 6-carbon molecule that instantly splits into two 3-carbon molecules of PGA

• CO2 + RuBP -------> unstable molecule-------> 2 PGA

http://simple.wikipedia.org/wiki/Stomates

Scene 2: Reduction

• ATP phosphorylates (adds a phosphate group to) each PGA molecule and creates DPG.

– This in turn results in the loss of the terminal phosphate group from ATP thus making ADP

• NADPH reduces DPG which causes the phosphate group to break off once again.  The molecule then picks up a proton (H+) from the medium to become PGAL. 

• NADPH is oxidized (loses an electron) by this process and becomes NADP+.

Scene 3: Regeneration

• For every six molecules of PGAL created, five molecules continue on to phase 3 which leave one to be used to make glucose. 

• ATP is once again needed.  However, this time it phosphorylates PGAL to regenerate RuBP after some rearrangement.

• http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

Accounting

• For every 6 CO2s IN, there is 1 molecule of glucose OUT.

– 2 PGAL (3C) molecules combine and leave the Calvin Cycle, where they are linked to form glucose;

– 10 PGAL (3C) molecules are rearranged to form RuBP

• For every glucose made, 18 ATP and 12 NADPH are used.

Why bother with all this work??!

• Photosynthesis is very "costly" to the cell, requiring a lot of energy from the sun as well as a cast of molecules that make the needed energy and rearrange the chemical bonds needed to make sugar.

• The payoff - when sugar is broken down, it yields even more chemical energy needed to do all other cellular reactions in cells - growth, reproduction, metabolism.... This is the subject of the next chapter, Cellular Respiration.

What affects photosynthesis rates?

• Light intensity

• Temperature

• CO2 concentration

www.ksphotography.com.au/

How does light intensity affect the rate of photosynthesis?

• The rate of photosynthesis levels off. The light reactions are saturated.

• Photoinhibition can occur when plants are exposed to full light where some of the extra energy gets passed to oxygen molecules and hydrogen peroxide forms which damages chloroplasts.

How does temperature affect the rate of photosynthesis?

• The rate of the photosynthesis reaction speeds up as higher temperatures provide more energy, however if the temperature gets too high, the proteins become denatured.

How does carbon dioxide concentration affect the rate of photosynthesis?

• An increase in CO2 increases the rate of photosynthesis to a maximum point, after which the rate levels off.

• What effect will climate change have???

What’s photorespiration?

• O2 can have an inhibitory effect upon photosynthesis

• In the presence of elevated O2 levels, photosynthesis rates are lower due to competition between O2 and CO2 on the Rubisco enzyme

Photorespiration cont.• Recall, the "normal" reaction to start the Calvin cycle has CO2 joined with

RUBP to form 2 molecules of 3PGA.

• In the process called photorespiration, O2 replaces CO2 in a non-productive, wasteful reaction

• It is believed that photorespiration in plants has increased over geologic time and is the result of increasing levels of O2 in the atmosphere--the byproduct of photosynthetic organisms themselves.

• The appearance of C4-type plants appears to be an evolutionary mechanism by which photorespiration is suppressed.

C3vs. C4 vs. CAM

C3 Photosynthesis : C3 plants

•Called C3 because the CO2 is first incorporated into a 3-carbon compound.

•Stomata are open during the day.

•Uses RUBISCO to fix CO2.

Photosynthesis takes place throughout the interior of the leaf.

Adaptive Value: more efficient than C4 and CAM plants under cool and moist conditions and under normal light because it requires less machinery (fewer enzymes and no specialized anatomy).95% of all plants are C3.

C4 Photosynthesis : C4 plants

Called C4 because the CO2 is first incorporated into a 4-carbon compound.

• Stomata open & close during the day.

• Uses PEP Carboxylase in the uptake of CO2, then it "delivers" the CO2 directly to RUBISCO

• Photosynthesis takes place in inner cells of leaf (called the bundle sheath)

• Adaptive Value:

– Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration.

– Is more efficient because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much for the same amount of CO2 gain for photosynthesis.

• Examples: sugarcane, corn, and many of our summer annual plants.

CAM Photosynthesis : CAM Plants(Crassulacean Acid Metabolism)

The CO2 is stored in the form of an acid before use in the Calvin Cycle.

Stomata open only at night (when evaporation rates are usually lower).

CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis

Adaptive Value:

• More efficient with water than C3 plants due to opening stomata at night (no sunlight, lower temperatures, lower wind speeds, etc.).

• When conditions are extremely arid, CAM plants can just leave their stomata closed night and day.

• Examples: succulents such as cactuses and agaves and also some orchids and bromeliads

www.livefortheoutdoors.com/