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C.P. Biology Chapter 4 Photosynthesis
The chemical energy used by most cell processes is carried by ATP
Chemical energy is used by all organisms and is needed for all life processes. The
chemical energy that all cells use in the form of ATP, which stands for adenosine
triphosphate. ATP is a molecule that carries energy that cells can use. This energy
comes from the breakdown of food molecules. The energy from the food is
transferred by ATP. When ATP gets broken down, the energy is released.
ATP has three phosphate groups. The bond that holds the third phosphate group is
easily broken. When the third phosphate is removed, energy is released. Then, the
molecule becomes ADP, which stands for adenosine diphosphate. ADP does not
carry energy that the cells can use. The difference between ATP and ADP is:
 ATP has three phosphate groups and is high energy
 ADP has two phosphate groups and is lower energy
ADP is a lower-energy molecule. If another phosphate is added to ADP
(phosphorylation), it becomes ATP once again, and is high energy. The energy that
comes from breaking down food is used to convert ADP into ATP.
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Organisms break down carbon-based molecules to produce ATP
Foods that you eat do not contain ATP that your cells can use. First, the food must
be digested. Digestion breaks down the food into smaller molecules – such as
carbohydrates, lipids, and proteins – that your cells can use to make ATP.
The amount of ATP produced depends on the type of molecule that is broken down.
A molecule of the simple sugar glucose produces about 36 molecules of ATP. A
typical fat molecule produces about 146 molecules of ATP. A protein molecule
produces about 36 molecules of ATP, similar to sugar. However, protein is usually
not used for energy. Instead, proteins are broken down into amino acids that are
used to build more proteins.
Plant cells also need ATP. But plant cells do not eat food the way animal must. In
stead, plants make their own food. Through he process of photosynthesis, plants
use energy from sunlight to make sugars. Plant cells then break down these sugars
to produce ATP, just like animal cells do.
A few types of organisms do not need sunlight and photosynthesis as a source
of energy
Some organisms, such as plants, use sunlight and photosynthesis to make their own
source of energy. Other organisms, like us, need sunlight and photosynthesis for our
source of energy, too, because we eat plants. We also eat other animals – but those
animals ate plants that got their energy from sunlight and photosynthesis. Most
organisms need sunlight and photosynthesis either directly or indirectly for energy.
But some organisms do not need sunlight. In the deep ocean, for example, there are
areas too deep for sunlight to reach, but some organisms are able to live there.
These organisms live near cracks in the ocean floor that release chemical
compounds, such as sulfides. The organisms that live there use these chemicals as
their energy source.
Chemosynthesis is a process by which some organisms use chemical energy –
instead of light energy – as an energy source to make their own food. These
organisms still need ATP for energy. The way their make their ATP is very similar to
how other organisms make ATP.
Section 4.2 Overview of Photosynthesis
Photosynthetic organisms are producers
Some organisms are called producers because they produce their own sources of
chemical energy. Other organisms also use the chemical energy that producers
make. Plants are producers. Plants capture energy from sunlight, and store it as
chemical energy in the form of sugars. These sugars are made through
photosynthesis. Chloroplasts are organelles in plant cells where photosynthesis
takes place.
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Chlorophyll is a molecule in chloroplasts that absorbs some of the energy in visible
light. Plants use energy in visible light for photosynthesis. Visible light looks white,
but it is made up of different wavelengths, or colors, of light.
Plant have two kinds of chlorophyll, called chlorophyll a and chlorophyll b. These
two types of chlorophyll absorb red and blue wavelengths of light. Neither type of
chlorophyll absorbs much green light. Plants are green because green wavelengths
of light are reflected by chlorophyll.
Photosynthesis in plants occurs in chloroplasts
Chloroplasts are plant cell organelles. Two main parts of chloroplasts are needed
for photosynthesis.
 The grana (singlular, granum) are stacks of compartments called thylakoids.
Thylakoids are shaped like coins, flat and circular. The thylakoid
compartments are enclosed by membranes. These membranes contain
chlorophyll.
 The stroma is fluid that is all around the grana inside the chloroplast.
There are two stages of photosynthesis, which are called the light-dependent
reactions and the light-independent reactions.
The light-dependent reactions capture energy from sunlight. These reactions
happen in the thylakoids and their membranes. Chlorophyll absorbs energy from
sunlight. The energy moves along the thylakoid membrane and is transferred to
molecules that carry energy, such as ATP. During this process, water (H2O)
molecules are broken down, and oxygen (O2) molecules are released.
The light-independent reactions use the energy from the light-dependent
reactions to make sugars. These reactions happen in the stroma. During this
process, carbon dioxide (CO2) and energy from the light-dependent reactions are
used to build sugars, usually glucose (C6H12O6).
The equation for the whole process of photosynthesis is shown below.
6CO2
+ 6H2O

C6H12O6
+ 6O2
carbon dioxide + water sunlight and enzymes
a sugar
+ oxygen
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Simple sugars, such as glucose, are not the only carbohydrates that come from
photosynthesis. Plants use simple sugars to build starch and cellulose molecules.
Another important produce of photosynthesis is the release of oxygen.
Section 4.3 Photosynthesis in Detail
The first stage of photosynthesis captures and transfers energy
During the light-dependent reactions, energy is captured from sunlight and moved
along the thylakoid membrane. This process involves two groups of molecules
called photosystems. The two photosystems are called photosystem I and
photosystem II. Both photosystems absorb energy from sunlight.
Photosystem II and electron transport
In photosystem II, several things happen
 Chlorophyll and other light-absorbing molecules absorb energy from
sunlight. The energy is transferred to electrons (e-) that leave the
chlorophyll. These high-energy electrons enter the electron transport
chain, which is made up of proteins in the thylakoid membrane.
 Water molecules are split apart into oxygen hydrogen ions, and electrons.
The oxygen is released – the same oxygen you breathe. The electrons from
water take the place of the electrons that left the chlorophyll.
 Electrons in the electron transport chain move from protein to protein.
Their energy pumps hydrogen ions (H+) across the thylakoid membrane,
from outside the thylakoid to inside. The transport of H+ ions across the
membrane makes the inside of the thylakoid have a higher concentration of
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H+ ions than on the outside of the thylakoid membrane. When there is a
difference in the concentration of a substance – like there is here with H+ ions
– it is a called a concentration gradient.
Photosystem I and Energy-Carrying Molecules
In photosystem I, several things happen.
 The electrons from photosystem II move on to photosystem I. In addition,
chlorophyll absorbs energy from sunlight, which results in every more highenergy electrons.
 The electrons are added to NADP+, a molecule that is similar to ADP. This
makes NADPH, a molecule that acts a lot like ATP.
 The concentration gradient in the thylakoid provides the energy to make
ATP. The H+ ions will diffuse, or flow, back out of the thylakoid through a
channel in the membrane. This channel is part of an enzyme called ATP
synthase. As the H+ flow through ATP synthase, phosphate groups are
added to ADP to make ATP. The energy from both ATP and NADPH is used
later to make sugars.
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