Transcript Photosynthesis - The Pingry School

Photosynthesis
Chapter 6
Carbon and Energy Sources
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
Photoautotrophs

Carbon source is carbon dioxide

Energy source is sunlight
Heterotrophs

Get carbon and energy by eating
autotrophs or one another
Linked Processes
Photosynthesis
Aerobic Respiration

Energy-storing
pathway

Energy-releasing
pathway

Releases oxygen

Requires oxygen

Requires carbon
dioxide

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
LIGHT ENERGY
12H2O + 6CO2
6O2 + C2H12O6 + 6H2O
Photosynthesis is a redox
process, as is cellular
respiration
Reduction
Oxidation

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
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
chlorophyll a
chlorophyll b
Wavelength (nanometers)
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


Plants
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
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Pigments found in plasma membranes
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 ewhich 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
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The Calvin–Benson cycle makes sugar from CO2.
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Overall reactants


Carbon dioxide

ATP

NADPH
Overall products

Glucose

ADP
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NADP+
Making Sugar from CO2:
The Calvin Cycle
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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
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O2 levels rise
 CO2 levels drop

Rubisco attaches RuBP to oxygen
instead of carbon dioxide
 Only one PGA forms instead of two
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Photorespiration in C4 Plants
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Carbon dioxide is fixed twice

In mesophyll cells, carbon dioxide is
fixed to form four-carbon compound
(oxaloacetate)
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Carbon dioxide is released and fixed
again in Calvin cycle
Photorespiration in CAM
Plants
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Carbon is fixed twice (in same cells)
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Night

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Carbon dioxide is fixed to form organic
acids
Day

Carbon dioxide is released and fixed
in Calvin cycle