Photosynthesis

Chapter 6

Carbon and Energy Sources

Photoautotrophs

Carbon source is carbon dioxide

Energy source is sunlight

Heterotrophs

Get carbon and energy by eating

autotrophs or one another

Linked Processes

Photosynthesis

Energy-storing pathway

Releases oxygen

Requires carbon dioxide

Aerobic Respiration

Energy-releasing pathway

Requires oxygen

Releases carbon dioxide

Autotrophs are the producers of the biosphere Photoautotrophs

capture sunlight energy and use it to carry out photosynthesis.

Plants, algae, some protists, and bacteria are photosynthetic autotrophs They are the ultimate

producers of food consumed by virtually all organisms

On land, plants such as oak trees and cacti are the predominant producers

In aquatic environments, algae and photosynthetic bacteria are the main food producers

Photosynthesis occurs in chloroplasts

In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts

A chloroplast contains: Stroma (fluid) grana (stacks of

thylakoids)

The thylakoids contain chlorophyll Chlorophyll is the green

pigment that captures light for photosynthesis

Location and Structure of Chloroplasts

Photosynthesis Equation

12H2O + 6CO2 6O2 + C2H12O6 + 6H2OLIGHT ENERGY

Photosynthesis is a redox process, as is cellular respiration

Water molecules are split apart and electrons and H+ ions are removed, leaving O2 gas These electrons and H+ ions are transferred

to CO2, producing sugar

Reduction

Oxidation

Two Stages of Photosynthesis

Visible Light

Humans perceive different wavelengths as different colors

Violet (380 nm) to red (750 nm) Longer wavelengths, lower energy Shorter wavelengths, higher energy

Pigments

Light-absorbing molecules

Absorb some wavelengths and transmit others

Color you see are the wavelengths NOT absorbed Wavelength (nanometers)

chlorophyll a

chlorophyll b

Excitation of Electrons

Excitation occurs only when the quantity of energy in an incoming photon matches the amount of energy necessary to boost the electrons of that specific pigment

Amount of energy needed varies among pigment molecules

Pigments in Photosynthesis

Bacteria Pigments found in plasma membranes

Plants Pigments embedded in thylakoid membrane

system Pigments and proteins organized into

photosystems Photosystems located next to electron

transport systems Pigments include chlorophyll a, chlorophyll b and carotenoids

Light-Dependent Reactions

Pigments absorb light energy, give up e- which enter electron transport systems

Water molecules are split, ATP and NADPH are formed, and oxygen is released

Pigments that give up electrons get replacements from photosystem

Photosystem Function: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 reduced to level that can be

captured by molecule of chlorophyll a

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

Reaction center accepts energy and donates electron to acceptor molecule

Light Dependent Reactions

Making Sugar from CO2: The Calvin Cycle The Calvin–Benson cycle makes sugar from CO2.

Overall reactants

Carbon dioxide

ATP

NADPH

Overall products

Glucose

ADP

NADP+

Making Sugar from CO2: The Calvin Cycle The Calvin cycle has four phases:

fixation of CO2 Energy consumption and reduction carbohydrate production (release of

G3P)regeneration of RuBP.

Calvin Cycle

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 in plants

Photosynthesis Summary

Photorespiration in C3 Plants

On hot, dry days stomata close Inside leaf

O2 levels rise

CO2 levels drop

Rubisco attaches RuBP to oxygen instead of carbon dioxide

Only one PGA forms instead of two

Photorespiration in C4 Plants

Carbon dioxide is fixed twice

In mesophyll cells, carbon dioxide is

fixed to form four-carbon compound

(oxaloacetate)

Carbon dioxide is released and fixed

again in Calvin cycle

Photorespiration in CAM Plants

Carbon is fixed twice (in same cells) Night

Carbon dioxide is fixed to form organic acids

DayCarbon dioxide is released and fixed

in Calvin cycle