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