Chapter 4 - Photosynthesis

6CO2 + 6H2O + light C6H12O6 + 6O2

Photosynthesis occurs in• land plants,• algae,• cyanobacteria

The capture and conversion of solar energy to chemical bond energy

How is photosynthesis important?

Universal energy source 100-200 billion metric tons/yr.

• Humans use 40% entire output

Photosynthesis produces oxygen

Photosynthesis & the carbon cycle

Summary of photosynthesis

Light-dependent reactions

Light-independent reactions

See Figure 4-12

Chlorophyll absorbs some wavelengths of light

Accessory pigments

What happens when light is absorbed by a molecule?

“Free” electrons are a source of kinetic energy

Summary of photosynthesis

Light-dependent reactions

See Figure 4-12

Light-dependent reactions Photosystems embedded in the

thylakoid membranes consist of chlorophyll molecules (100s) and electron transport systems

Light energy captured and stored in energy carriers (ATP & NADPH) via chemiosmosis

Light absorption by chlorophyll

Photolysis replaces electrons removed from chlorophyll

Fig. 4-10

Moving electrons provide the energy to move H+ ions inside the thylakoid membrane

NADP+ NADPH

H+ ions

e-

H20 →2H+ + 2e- + O

Summary of photosynthesis

Light-dependent reactions

Light-independent reactions

Light-independent reactions (Calvin Cycle)

Occurs in the stroma Several enzyme-

catalyzed steps Requires energy Produces sugars

1940s-50s – Melvin Calvin

Nobel prize in chemistry - 1961

Calvin’s experiment (see Fig. 4-6)

Calvin’s results

Photosynthesis animation

Rate of photosynthesis varies with environmental conditions

What is meant by the “rate of photosynthesis”?

What environmental conditions affect the rate of photosynthesis?

Factors influencing the rate of photosynthesis

Carbon dioxide concentration Water availability Light intensity Temperature

Photorespiration and rubisco

Rubisco – Ribulose 1,5-bisphosphate carboxylase/oxygenase Most abundant protein on earth (25-

40% of total plant protein) Activity depends on carbon dioxide

concentration in the leaf

Rubisco as a carboxylase

Rubisco as a carboxylase

Rubisco activity and CO2 concentration

If [CO2] > 50ppm, carboxylase activity If [CO2] < 50ppm, oxygenase activity

When would photorespiration occur and why?

When are CO2 concentrations low?

–When stomata close•Hot days•Moisture stress

Variations of photosynthesis

85% of plants use “C3” photosynthesis C3 cycle = Calvin cycle

C4 photosynthesis C4 plants include many tropical monocots,

other grasses (0.4% of plants) Crassulacean Acid Metabolism (CAM)

CAM plants – cacti, orchids, bromeliads, succulents (~10% of plants)

C4 photosynthesis is more efficient than C3 under certain conditions

C4 photosynthesis

C4 plants reduce photorespiration by concentrating CO2 in the bundle sheath cells

Crassulacean Acid Metabolism Evolved in areas of high light intensity,

little precipitation Stomata closed during the day, open at

night

Photosynthesis research

Why study photosynthesis?

Photosynthesis Research

Photosynthesis Center at Arizona State

Oak Ridge National Laboratory Photosynthesis Research Unit

Photosynthesis Research

Photosynthesis and temperature tolerance

Photosynthesis at high CO2 levels CO2 Science Magazine

Engineering C4 genes in C3 plants Production of pharmaceuticals in

chloroplasts

Cellular respiration Plants utilize aerobic respiration as

do animals