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Chapter 6
Photosynthesis
Section 6.1
Energy Processes for Life

Autotrophs manufacture their own food from
inorganic substances

Use photosynthesis to convert light energy
from the sun into chemical energy

Heterotrophs cannot manufacture their own
food

Must obtain food by eating autotrophs or other
heterotrophs
Photosynthesis

Involves a complex series of chemical
reactions in which the product of one reaction
is consumed in the next reaction

Biochemical pathway
6CO2 + 6H2O + solar energy  C6H12O6 + 6O2
Solar energy drives a series of chemical
reactions that require carbon dioxide and water
Light Absorption in Chloroplasts

Light reactions- the initial reactions in
photosynthesis

Begin with the absorption of light in
chloroplasts
Chloroplasts

Each chloroplast is surrounded by a pair of
membranes

Inside the membranes are stacks of thylakoids

The stacks are called grana

The fluid surrounding the grana is called stroma
Light and Pigments

Visible spectrum- white light can be separated
into an array of colors when passed through a
prism

ROY G. BIV
Light

Light travels in waves

The distance from the crest of one wave to the
crest of another wave is called a wavelength
Chloroplast Pigments


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Pigments- compounds that absorb light
Chlorophyll a – absorbs more red light
Chlorophyll b – absorbs more blue light
Carotenoids – absorbs more green light

Only chlorophyll a is directly involved in the
light reactions of photosynthesis

Chlorophyll b assists chlorophyll a in
capturing light energy (accessory pigment)

In the leaves of a plant, chlorophylls are
abundant

In Fall, plants lose their chlorophylls and their
leaves take on the color of the carotenoids
Electron Transport

Each cluster of pigment molecules is a
photosystem

Two types of photosystems: photosystem I &
photosystem II
Light Reactions

1. light energy excites electrons in chlorophyll
a molecules of photosystem II

2. These electrons move to a primary electron
acceptor

3. The electrons are then transferred along a
series of molecules called an electron
transport chain

4. light excites electrons in chlorophyll a molecules
of photosystem I. As these electrons move to another
primary electron acceptor, they are replaced by
electrons from photosystem II

5. The electrons from photosystem I are transferred
along a second electron transport chain. At the end of
this chain, they combine with NADP+ and H+ to make
NADPH
Restoring Photosystem II

Replacement electrons come from water
molecules

An enzyme inside the thylakoid splits water
molecules into protons, electrons, and oxygen

2H2O  4H+ + 4e- + O2
Chemiosmosis

Synthesis of ATP

Depends on a concentration gradient of
protons across a thylakoid membrane

The concentration of protons is higher inside
the thylakoid

ATP synthase makes ATP by adding a
phosphate group to ADP

ATP synthase functions as a carrier protein