Biology 010
Audersirk, Life on Earth
Chapter 7: Capturing Solar Energy: Photosynthesis
I.
What is Photosynthesis – Organisms need energy to live. The first law of
thermodynamics tells us that energy cannot be created, so living things cannot
manufacture their own energy. Some organisms can capture energy from sunlight and
convert it to chemical energy. This process is called photosynthesis.
a. Life on Earth Depends on Photosynthesis – The solar energy captured by
photosynthesis is stored in sugar and other organic molecules. Also, during the
process of photosynthesis carbon dioxide (CO2) and water (H2O) are consumed
and oxygen (O2) is released.
b. Photosynthesis Converts Carbon Dioxide and Water into Glucose – The energy
captures by photosynthesis is stored in bonds of glucose (C6H12O6). The overall
reaction is:
6CO2 + 6H2O + light energy  C6H12O6 + 6O2
II.
c. Plant Photosynthesis Takes Place in Leaves– Plant photosynthesis takes place
mainly in the leaves. They are flat so that a large surface area is exposed to the
sun and thin so that the light can penetrate them. The leaf obtains CO2 from the
air through tiny hole on the underside called Stomata. Inside the leaf there are a
few layers of chloroplast containing cells called mesophyll. Photosynthesis occurs
within these cells.
d. Leaf Cells Contain Chloroplasts – Chloroplasts are a cell organelle with a double
outer membrane, which encloses a semi-fluid substance called the stroma. There
are disk shaped stacks of membrane sacs called thylakoids in the stroma where
the light-dependent reactions of photosynthesis occur.
e. Photosynthesis Consists of Light-Dependent and Light-Independent Reactions –
There are 2 sets of reactions during the process of photosynthesis, the lightdependent and the light-independent reactions. They occur in 2 different parts of
the chloroplasts but are linked by the energy-carrier molecules adenosine
triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH).
1. In the light-dependent reactions, molecules in the membranes of the
thylakoids capture sunlight energy and convert some of it into chemical
energy stored, for the short term, in ATP and NADPH. The light-dependent
reactions consume water and release oxygen as a byproduct.
2. In the light-independent reactions, enzymes in the stroma use the chemical
energy of ATP and NADPH molecules produced by the light-dependent
reactions to power the manufacture of glucose or other organic molecules
consuming carbon dioxide in the process. As energy is extracted from ATP
and NADPH, these energy carriers are converted into ADP and NADP+.
How is Light Energy Converted to Chemical Energy – Sunlight consists of
electromagnetic waves. The waves come in different sizes (wavelengths). We can see
wavelengths between 400 and 750 nm. Most leaves appear green because the
chlorophyll in them absorbs the violet, blue and red wavelengths of light but reflects
green.
a. Light Energy is First Captured by Pigments in Chloroplasts – The job of
absorbing light energy is carried out by pigments. Chlorophyll is the key lightcapturing (pigment) molecule. Thylakoids also contain other molecules called
accessory pigments like carotenoids which are yellow/orange.
1. In thylakoid contains thousands of photosystems (proteins, chlorophyll and
accessory pigments embedded in the thylakoid phospholipid bilayer).
2. Photosystem II generates ATP.
3. Photosystem I generates NADPH
b. Splitting Water Maintains the Flow of Electrons Through the Photosystems –
Overall electrons flow from the reaction center of photosystem II, through an
electron transport chain (generating ATP), then to photosystem I, through an
electron transport chain (generating NADPH). Therefore this is a one-way flow of
electrons. Water is split in order to replenish this system.
H2O  ½ O2 + 2H+ + 2eIII.
IV.
V.
How is Cellular Energy Stored in Glucose Molecules – In the stroma of the
chloroplasts, both ATP and NADPH provide the energy that drives the lightindependent reactions (The Calvin cycle) that synthesize glucose from CO2 and H2O.
What is the Relationship Between Light-Dependent and Light-Independent
Reactions. Overall the light-dependent reactions produce the energy carrier ATP and
the electron carrier NADPH. Energy from these carriers is used in the synthesis of
organic molecules during the light-independent reactions. The depleted carriers, ADP
and NADP+, return to the light-dependent reactions for recharging.
How Does the Need to Conserve Water Affect Photosynthesis – Photosynthesis
requires carbon dioxide. Therefore, you might think that the ideal leaf would be very
porous to allow lots of carbon dioxide to enter the plant from the atmosphere. But for
land plants this would be a problem because water would evaporate through those
same pores and the plant would dry out. So plants have stomata which open and
close.
a. When Stomata Are Closed to Conserve Water, oxygen tends to build up in the
plant. This causes a wasteful process known as photorespiration. In order to
compensate for this and other problems, like living in a very sunny, hot and dry
desert, some plants have alternate routes (forms) of photosynthesis. Examples
include C4 and CAM photosynthesis.