Chapter 8:
Photosynthesis
Big Idea: How do plants
and other organisms
capture energy from the
Sun?
8.1 Energy and Life
Why is ATP useful to cells?
 ATP (Adenosine triphosphate) is cellular
energy. It is used by all cell types.
 Without the ability to obtain and use energy
life would not exist.
 Energy is needed to grow, respond, move
materials, and build new molecules
 ADP (has 1 less phosphate group) and is
how energy is stored. Once the extra
phosphate group is added the cell has
enough energy to be used.
 ATP can easily release and store energy
by breaking and re-forming the bonds
between its phosphate groups. This
characteristic makes ATP very useful as
a basic energy source for all cells.
 One way cells use energy is active
transport.
 Many membranes have sodiumpotassium pumps (Na-K), which powers
movement, and keeps ions balanced on
both sides of the membrane.
 ATP molecules are large so it is more
efficient for cells to only store a small
amount at a time.
What happens during the process of
photosynthesis?
Heterotrophs vs. Autotrophs
Heterotrophs are organisms that obtain food by
consuming other living things (Ex: mammals, insects,
amphibians, reptiles, birds, fish, decomposers).
Autotrophs are organisms that make their own food
(Ex: plants)
In the process of photosynthesis, plants convert the
energy of sunlight into chemical energy stored in
the bonds of carbohydrates
History of Photosynthesis
Many scientists have contributed to
understanding how plants carry out
photosynthesis. Early research focused
on the overall process. Later,
researchers investigated the detailed
chemical pathways.
8.2 Overview of Photosynthesis
What role do pigments play in
photosynthesis?
 Pigments are light absorbing molecules
that capture energy from sunlight.
 A plants main pigments are chlorophyll
a and chlorophyll b. They do not absorb
green light on the light spectrum, but
reflect green very well (which makes
plants green).
Thylakoids are sac-like photosynthetic
membranes that are arranged in stacks,
known as grana. This is where chlorophyll is
located
Stroma is the fluidportion of a chloroplast
A fraction of the light energy absorbed
by plants goes directly to the electrons
in the chlorophyll molecule. This raises
the energy, giving the plant a steady
supply of electrons to help
photosynthesis occur
What is an electron carrier molecule?
A molecule that can accept a pair of
high-energy electrons, and transfer
them, along w/ most of their energy, to
another molecule
 Ex of a carrier molecule: NADP+ accepts and
holds 2 high-energy electrons and a hydrogen
ion, which helps to trap light energy and convert
it into a chemical form.
What are the reactants and products of
photosynthesis?
 Photosynthesis used the energy of sunlight to
convert water and carbon dioxide (reactants)
into high-energy sugars and oxygen (products)
Light-dependent vs. Light-independent
reactions
Photosynthesis involves 2 sets of reactions
The first are light-dependent reactions, which
use energy from sunlight to produce energy
rich compounds such as ATP
- Occur in the thylakoid membranes, oxygen is
produced as a byproduct
The second reactions are light-independent
use ATP and NADPH to produce high-energy
sugars.
-occur in the stroma
8.3 The Process of
Photosynthesis
 The light-dependent reactions use energy
from sunlight to produce oxygen and
convert ADP to NADP+ into the energy
carriers ATP and NADPH
 Photosystems are proteins located in
thylakoids that absorb light and generate
high energy electrons, that generate the
light-independent reactions
The electron-transport chain is a series of
electron-carrier proteins shuttling highenergy electrons from photosystem II
photosystem I.
In photosystem I electrons are reenergized
and carried into another electron transport
chain
The pumping of electrons builds a gradient
on both sides of the thylakoids, giving the
chloroplast energy to make ATP
The light-independent reactions do NOT
require light to occur and are commonly
called the Calvin cycle
During the light-independent reactions, ATP
and NADPH from the light-dependent
reactions are used to produce high-energy
sugars.
The Calvin cycle used 6 molecules of CO2 to
produce a single 6-carbon sugar. It uses
energy produced from the light-dependent
reactions and CO2
The most important factors that affect the rate of
photosynthesis are temperature, light intensity, and
the availability of water.
Extreme conditions:
C4 Photosynthesis: allows photosynthesis to occur
in plants located in hot, dry conditions w/ low levels
of CO2
CAM Plants: Present in dry climates. Allow plant to
obtain CO2
While minimizing water loss by sealing the leaves
pores during the day and opening them during
night.