Mrs. Schertz' Science Stuff

Chapter 8 Photosynthesis
Plants and some other living things can use light energy from
the sun to make food. These organisms are called autotrophs.
Many organisms cannot use the sun’s energy directly. These
organisms, called heterotrophs, get energy from their food.
Adenosine triphosphate, or ATP, is a compound cells use to
store and release energy. ATP is the basic energy source of all
cells. Cells use energy from ATP to carry out many activities.
These include active transport, synthesis of proteins and nucleic
acids, and responses to chemical signals at the cell surface. ATP is
made up of adenine, a 5-carbon sugar called ribose, and three
phosphate groups.

Adenosine diphosphate (ADP) is a compound similar to ATP.
Unlike ATP, ADP has only two phosphate groups. When energy
is available, a cell can store small amounts of energy by adding a
phosphate group to ADP to form ATP (ADP P➝ATP). Energy
stored in ATP is released by breaking the bond between the second
and third phosphate groups (ATP ➝ADP P).

8–2 Photosynthesis: An Overview
Research into photosynthesis began centuries ago. The experiments
of van Helmont, Priestly, and Ingenhousz led to work by other
scientists. These scientists found that in the presence of light,
plants change carbon dioxide and water into carbohydrates and
give off oxygen. This process is called photosynthesis.
The overall equation for photosynthesis is:
6CO2 6H2O
light
C6H12O6 6O2
carbon dioxide water light
light
sugars oxygen

Photosynthesis uses the energy of sunlight to convert water
and carbon dioxide into high-energy sugars and oxygen. Plants
get the carbon dioxide needed for photosynthesis from the air
or from the water in which they grow. Plants use the sugars
produced during photosynthesis to make complex carbohydrates
such as starches.

Photosynthesis also requires light and chlorophyll. Plants
gather the sun’s energy with light-absorbing molecules called
pigments. The main pigment in plants is chlorophyll. A compound
that absorbs light also absorbs the light’s energy. When
chlorophyll absorbs sunlight, much of the light energy is sent
directly to electrons in the chlorophyll molecules. This raises the
energy levels of the electrons.

The visible spectrum is made up of wavelengths of light you
can see. This spectrum contains all the colors. Chlorophyll absorbs
light in the blue-violet and red regions of the visible spectrum
well. Chlorophyll does not absorb light in the green region well.
Plants look green because their leaves reflect this green light.

8–3 The Reactions of Photosynthesis
In plants and other photosynthetic prokaryotes, photosynthesis
takes place inside the chloroplasts. Chloroplasts have saclike
photosynthetic membranes called thylakoids. Proteins in the
thylakoid membrane organize chlorophyll and other pigments
into clusters known as photosystems. The photosystems are the
light-collecting units of chlorophyll.

When sunlight excites electrons in chlorophyll, the electrons
gain energy. The electron transfers its energy to another molecule.
The energy continues to move from molecule to molecule until it
gets to the end of the chain.
The reactions of photosynthesis occur in two parts: lightdependent
reactions and light-independent reactions.

1. The light-dependent reactions produce oxygen gas and
convert ADP and NADP into ATP and NADPH. These
reactions need light and they occur in the thylakoid
membranes. The light-dependent reactions can be divided
into four processes: light absorption, oxygen production,
electron transport, and ATP formation. The light-dependent
reactions use water, ADP, and NADP. They produce oxygen,
ATP, and NADPH.

2. The light-independent reactions are also called the Calvin
cycle. These reactions do not need light. The Calvin cycle uses
ATP and NADPH from the light-dependent reactions to produce
high-energy sugars. The Calvin cycle takes place in the
stroma of chloroplasts. The Calvin cycle uses carbon dioxide in
its reactions. As photosynthesis proceeds, the Calvin cycle
works steadily to remove carbon dioxide from the atmosphere
and turn out energy-rich sugars. Six carbon dioxide molecules
are needed to make a single 6-carbon sugar.
Many factors affect the rate of photosynthesis. Such factors
include water availability, temperature, and the intensity of light.