Plant Power
Photosynthesis
• Photosynthesis makes sugar and oxygen
gas from carbon dioxide and water
– Carried out by plants, algae, and some bacteria
– The ultimate source of all food eaten by
animals
– A major source of heat, light, and fuel
Summary Equation for
Photosynthesis
Overview of Photosynthesis
• Autotrophs are the producers of the
biosphere
– Autotrophs produce their own food
– Autotrophs produce the food supply for the
global ecosystem
– Photoautotrophs produce organic molecules
using light energy
• Plants, algae, and photosynthetic bacteria
Photosynthesis occurs in
Chloroplasts
• In plants, photosynthesis occurs primarily
in leaves
– CO2 enters and O2 exits through stomata
• Chloroplasts are the site of photosynthesis
– Concentrated in mesophyll cells of leaves
Chloroplast Structure
• Stroma
– Fluid enclosed by inner membrane
– Location of sugar synthesis
• Thylakoids
– Suspended in stroma
– Interconnected sacs
– Stacked in grana
– Contain chlorophyll in membranes
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Location & Structure of Chloroplasts
O2 Gas Produced by Splitting Water
• Experiments have made it possible to
follow all the atoms in photosynthesis
• O2 is both a product and a reactant in
photosynthesis
• The O2 liberated by photosynthesis is made
from the oxygen in water
Oxygen Bubbles on Aquatic Plant
Experiments Tracking Oxygen
Fate of All Atoms in Photosynthesis
Photosynthesis & Cellular
Respiration are Redox Processes
• Photosynthesis
– H20 is oxidized
– C02 is reduced
– Electrons gain energy
• Cellular respiration
– Glucose is oxidized
– 02 is reduced
– Electrons lose potential energy
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Photosynthesis
Cellular Respiration
Overview of Photosynthesis
Overview of Photosynthesis
• Light reactions
– Occur in thylakoid membranes
– Convert light energy to chemical energy as ATP and
NADPH
– Produce O2 as a waste product
• Calvin cycle
– Occurs in stroma
– Assembles sugar molecules from CO2 using ATP and
NADPH from light reactions
Electromagnetic Spectrum
Pigments
Absorb some wavelengths of light; reflect or
transmit others
• Chlorophyll a
– Absorbs blue-violet and red light, reflects green light
– Participates directly in the light reactions
• Accessory pigments
– Chlorophyll b
– Carotenoids
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Interaction of Light and Chloroplast
Photons
• Light also behaves as photons
– A fixed quantity of energy
– Specific amounts of energy in photons
absorbed by different pigments
Photosystems
Light-Excited Chlorophyll
• When a pigment molecule absorbs a
photon
– An electron is raised in energy from a ground
state to an unstable excited state
– The electron rapidly drops back to the ground
state
– The electron releases excess energy as heat
Photosystems
• The thylakoid membrane contains two types of
Chorophyll Embedded in
Photosystem
photosystems, which consist of
– Light-harvesting complexes of chlorophyll and other
pigments and proteins
– A reaction center that contains
• A chlorophyll a molecule
• A primary electron acceptor molecule
– Receives excited electrons from the chlorophyll and
transfers them to an electron transport chain
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Photosystem II
• A pigment molecule absorbs a photon
• The energy is ultimately transferred to
chlorophyll P680
• The excited electrons from P680 are passed to
the primary electron acceptor, then to an electron
transport chain
• Electrons shuttle down the chain from
photosystem II to photosystem I, providing
energy to make ATP
• Water is split, replacing electrons lost by P680
and releasing O2
Photosystem I
• A photon is absorbed and excites an
electron of chlorophyll P700
• The excited electron passes through a
short electron transport chain, reducing
NADP+ to NADPH
Electron Flow in Photosystems
Mechanical Analogy of Light Reactions
Chemiosmosis
Photophosphorylation
Powers ATP synthesis in the light reactions
• Energy of the concentration gradient drives
H+ back across the membrane through ATP
synthase
• The electron transport chain
– Pumps H+ from the stroma into the thylakoid
space
– Generates a concentration gradient across the
thylakoid membrane
• ATP synthase couples flow of H+ to the
phosphorylation of ADP to produce ATP
• Light provides the initial energy for the
phosphorylation
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Chemiosmosis
Calvin Cycle
• The Calvin cycle makes sugar in the
chloroplast
– Inputs
• Carbon from CO2
• Energy from ATP
• High-energy electrons from NADPH
– Outputs
• Energy-rich G3P
• Can be used to make organic molecules
Calvin Cycle
Steps of Calvin Cycle
1. Carbon fixation
2. Reduction
3. Release of one molecule of G3P
4. Regeneration of RuBP
Photosynthesis Reviewed
Photosynthesis uses light energy to make food
molecules
• The light reactions
Photosynthesis Summary
– Capture light energy
– Generate NADPH and ATP
– Release O2 and water
• The Calvin cycle
– Manufactures sugar
• Cells use many of the same mechanisms in
photosynthesis and cellular respiration
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