Chapter 6

PHOTOSYNTHESIS

A. LightVisible light makes up only a small portion

of the electromagnetic spectrum.

Sunlight consists of:

4% Ultraviolet (UV) radiation

44% Visible light 52% Infrared (IR)

radiation

Characteristics of Visible Light: is a spectrum of colors ranging from

violet to red consists of packets of energy called

photons photons travel in waves, having a

measurable wavelength () = distance a photon travels during a complete vibration [measured in nanometers (nm)]

A photon’s energy is inversely related to its wavelength......the shorter the , the greater the

energy it possesses.Which of the following photons possess

the greatest amount of energy?

Green photons = 530nmRed photons = 660nmBlue photons = 450nm

What happens to light when it strikes an object?

reflected (bounces off)

Only absorbed wavelengths of light function in photosynthesis.

transmitted (passes through)

absorbed

B. Photosynthetic PigmentsMolecules that capture photon

energy by absorbing certain wavelengths of light.

1. Primary pigmentsBacteriochlorophyll - green

pigment found in certain bacteria.Chlorophylls a & b - bluish green

pigments found in plants, green algae & cyanobacteria.

Chlorophyll a is the dominant pigment in plant cells.

2. Accessory PigmentsCarotenoids - red, orange, yellow

pigments found in plants, algae, bacteria & archaea.

Phycoerythrin - red pigment found in red algae.

Phycocyanin - blue pigment found in red algae & cyanobacteria.

Each pigment absorbs a particular range of wavelengths.

C. ChloroplastsSites of photosynthesis in plants &

algae.Concentrated in mesophyll cells of

most plants.

Chloroplast structure:

Stroma - gelatinous matrix; contains ribosomes, DNA & various enzymes.

Thylakoid - flattened membranous sac; embedded with photosynthetic pigments.

D. Photosynthesis

Occurs in two stages: Light reactions - harvest photon

energy to synthesize ATP & NADPH. Carbon reactions (Calvin cycle) -

use energy from light reactions to reduce CO2 to carbohydrate.

6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O

Overview of Photosynthesis

1. Light Reactionsrequire lightoccur in thylakoids of chloroplasts involve photosystems I & II (light

harvesting systems).

Photosystems contain antenna complex that captures photon energy & passes it to a reaction center.

Light Reactions of Photosynthesis

ATP Production by Chemiosmotic Phosphorylation

2. Carbon Reactions (Calvin cycle; C3 cycle) do NOT require light (occur in both

darkness & light as long as ATP & NADPH are available)

occur in stroma of chloroplastsrequire ATP & NADPH (from light

reactions), and CO2

Calvin Cycle

Plants that use only the Calvin cycle to fix carbon are called C3 plants.

Ex. cereals, peanuts, tobacco, spinach, sugar beets, soybeans, most trees & lawn grasses.

E. PhotorespirationProcess that counters

photosynthesis.Occurs when stomata close under hot,

dry conditions:O2 levels in plant increase

CO2 levels in plant decrease

Under these conditions, rubisco fixes O2 (rather than CO2).

Thus, PGAL is NOT produced.

F. C4 and CAM Photosynthesis

Adaptations that allow certain plants to conserve water and reduce photorespiration at higher temperatures.

1. C4 Photosynthesis

C4 plants reduce photorespiration by physically separating the light reactions and Calvin cycle.

Leaf anatomy of a C4 plant

C4 Photosynthesis: Light reactions occur

in chloroplasts of mesophyll cells.

Calvin cycle occurs in chloroplasts of bundle sheath cells.

2. CAM PhotosynthesisCAM plants reduce photorespiration

by acquiring CO2 at night.

Night: mesophyll cells fix

CO2 as malic acid malic acid is stored

in vacuoles.

Day: malic acid releases

CO2 which enters Calvin cycle.

Malic acid