Chapter 7

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

Overview of PhotosynthesisPhotosynthesis- process by which plants, algae and some microorganisms harness solar energy to make biochemicals.

Occur in organelles – chloroplasts

Two stages – light reaction and carbon reaction

The products of photosynthesis, glucose and other carbohydrates – photosynthate.

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 (λ) λ = 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 λ, the greater the

energy it possesses.Which of the following photons

possess the greatest amount of energy?

Green photons λ = 530nm

Red photons λ = 660nm

Blue photons λ = 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 pigments• Bacteriochlorophyll - 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 Pigments• Carotenoids - red, orange, yellow pigments

found in plants, algae, bacteria & archaea.• Xanthophylls – red and yellow pigments

found in plants, algae & bacteria.• Fucoxanthin –brown pigment found in brown

algae, diatoms, & dinoflagellates• Phycoerythrin - red pigment found in red

algae.• Phycocyanin - blue pigment found in red

algae & cyanobacteria.• Bacteriorhodopsin – purple pigment found in

halophilic archaeaEach pigment absorbs a particular range

of wavelengths.

Light- form of energy- exists as photons. Photons possess different wavelengths that represent different energy levels.

Different wavelengths seen as different colors. Pigment molecules possess different abilities to absorb wavelengths and appear as different colors.

Chlorophyll is the major pigment molecule and appears as green.

Plants and other photosynthetic species use different pigments to absorb different wavelengths and use light more efficiently.

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.

Chloroplasts – type of plastid- unique organelles with multiple layers which increase surface area to improve efficiency.

Chlorophyll is imbedded within the membrane layers in complexes that maximize the absorption and transduction of energy.

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

Molecules in thylakoid membrane capture sunlight energy and transfer energy to molecules of ATP and NADPH. Enzymes of caebon reactions use this energy to capture CO2 abd build glucose.

1. Light Reactions• require light• occur 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

•Light reactions of photosynthesis boost electrons into higher energy levels.

•Transfer them to the carriers NADPH and ATP for use in the cell.

•Additional energy is used to pump hydrogen ions into lumen of thylakoids- establishing gradient called proton motive force.

•Protons escape through a membrane-bound ATP synthase – uses the energy release to phosphorylate ATP (chemiosmotic phosphorylation)

•Electrons – ultimately replaced by converting water to protons and oxygen.

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 chloroplasts• require 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.

Carbon fixation uses energy from ATP and NADPH to convert gaseous carbon dioxide to organic molecules such as glucose.

The enzyme system constantly recycles its components, forming Calvin cycle.

Key enzyme – rubisco attaches carbon to the carrier ribulose bisphosphate.

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

Inefficiency of rubisco causes photorespiration.

To live in hot climates, plants adept at manipulations that reduce photorespiration.

C4 plants use a intermediate to separate the light and carbon reactions from each other within different cell types.

Resulting in higher carbon dioxide concentration within bundle-sheath cells – reduce photorespiration.

CAM plants fix carbon at night when temperatures are lower and water loss is less of a problem.