Capturing Solar Energy: Photosynthesis

Chapter 6

Photosynthesis

– Converts water and carbon dioxide into carbohydrates (glucose)

– Releases oxygen as a by product

– Energy source is sunlight

Photosynthesis Equation

Overview

Chloroplasts

Organelles of photosynthesis

Two Stages of Photosynthesis

Sunlight Energy

• Continual input of solar energy into Earth’s atmosphere

• Almost 1/3 is reflected back into space

• Of the energy that reaches Earth’s surface, about 1% is intercepted by photoautotrophs

Visible Light

• Wavelengths humans perceive as different colors

• Violet (380 nm) to red (750 nm)

• Longer wavelengths, lower energy

T.E. Englemann’s Experiment

Pigments

• Light-absorbing molecules

• Absorb some wavelengths and transmit others

• Color you see are the wavelengths NOT absorbed

Pigment Structure

• Light-catching part of molecule often has alternating single and double bonds

• These bonds contain electrons that are capable of being moved to higher energy levels by absorbing light

Excited Electrons

• When energy in an incoming photon matches the amount of energy necessary to boost the electrons of that specific pigment, “excitation” occurs

• Amount of energy needed varies among pigment molecules

Variety of Pigments

Chlorophylls a and b

Carotenoids

Others

Chlorophylls

Main pigments in most photoautotrophs

Carotenoids

• Found in all photoautotrophs

• Absorb blue-violet and blue-green that chlorophylls miss

• Reflect red, yellow, orange wavelengths

• Two types

– Carotenes - pure hydrocarbons

– Xanthophylls - contain oxygen

Others: Anthocyanins & Phycobilins

Red to purple pigments

• Anthocyanins

– Give many flowers their colors

• Phycobilins

– Found in red algae and cyanobacteria

Pigments in Photosynthesis

• Plants

– Pigments embedded in thylakoid membrane system

– Pigments and proteins organized into photosystems

– Photosystems located next to electron transport systems

• Bacteria

– Pigments in plasma membranes

Light-Dependent Reactions

• Pigments absorb light energy, eject e^- which enter electron transport systems

• Water molecules are split, ATP and NADH are formed, and oxygen is released

• Pigments that gave up electrons get replacements

Photosystem Function: Harvester Pigments

• Most pigments in photosystem are harvester pigments

• When excited by light energy, these pigments transfer energy to adjacent pigment molecules

• Each transfer involves energy loss

Photosystem Function: Reaction Center

• Energy is transferred to a molecule of chlorophyll a

• This molecule (P700 or P680) is the reaction center of a photosystem

• Reaction center accepts energy and donates electrons to electron transport system

Pigments in a Photosystem

Electron Transport System

• Adjacent to photosystem

• Acceptor molecule accepts donated electrons from reaction center

• As electrons flow through system, energy they release is used to produce ATP or NADPH

Energy Changes

Electron Flow

• Two-step pathway for light absorption and electron excitation

• Uses two photosystems: type I and
type II

• Produces ATP and NADPH

• Involves photolysis - splitting of water

Electron Flow

Chemiosmotic Model
of ATP Formation

• When water is split during photolysis, hydrogen ions are released into thylakoid compartment

• More hydrogen ions are pumped into the thylakoid compartment when the electron transport system operates

Chemiosmotic Model
of ATP Formation

• Electrical and H⁺ concentration gradient exists between thylakoid compartment and stroma

• H⁺ flows down gradients into stroma through ATP synthesis

• Flow of ions drives formation of ATP

Light-Independent Reactions

• Synthesis part of photosynthesis

• Can proceed in the dark

• Take place in the stroma

• Calvin-Benson cycle

Calvin-Benson Cycle

• Overall reactants

– Carbon dioxide

– ATP

– NADPH

Calvin- Benson Cycle

Building Glucose

• PGA accepts

– phosphate from ATP

– hydrogen and electrons from NADPH

• PGAL (phosphoglyceraldehyde) forms

• When 12 PGAL have formed

– 10 are used to regenerate RuBP

– 2 combine to form phosphorylated glucose

Using the Products of Photosynthesis

• Phosphorylated glucose is the building block for:

– sucrose

• The most easily transported plant carbohydrate

– starch

• The most common storage form

The C3 Pathway

• In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA

• Because the first intermediate has three carbons, the pathway is called the C3 pathway

Photorespiration in C3 Plants

• On hot, dry days stomata close

• Inside leaf

– Oxygen levels rise

– Carbon dioxide levels drop

• Rubisco attaches RuBP to oxygen instead of carbon dioxide

• This wastes energy

C4 Plants

• Carbon dioxide is fixed twice

– In mesophyll cells, carbon dioxide is fixed to form four-carbon oxaloacetate

– Oxaloacetate is transferred to bundle-sheath cells

– Carbon dioxide is released and fixed again in Calvin-Benson cycle

C4 Plants and Photosynthesis

CAM Plants

• Carbon is fixed twice (in same cells)

• Night

– Carbon dioxide is fixed to form organic acids

• Day

– Carbon dioxide is released and fixed in Calvin-Benson cycle

Summary of Photosynthesis

Linked Processes

Photosynthesis

• Energy-storing pathway

• Releases oxygen

• Requires carbon dioxide

Aerobic Respiration

• Energy-releasing pathway

• Requires oxygen

• Releases carbon dioxide

Back to Biology

Dr. Latham’s Home Page

Merced College Home Page