Biology 12

Chapter 3 Cellular Energy • MHR 83

3.3 Photosynthesis

Photosynthesis is one of the most important
chemical processes on Earth. Photosynthesis
involves the use of energy from light to form
carbohydrates. Organisms that manufacture their
own food (autotrophs), such as plants, algae,
cyanobacteria, and photosynthetic bacteria, do so
through photosynthesis. Autotrophs form the base
of the food chain for virtually all communities of
heterotrophs, which must eat to obtain nutrients.
The process of photosynthesis also produces the
oxygen found in the atmosphere. Oxygen is used
by organisms for many processes, such as aerobic
cellular respiration. The overall equation for
photosynthesis is as follows:

The process of photosynthesis is believed to
have originated in bacteria. Some of these bacteria
were able to produce oxygen. Other bacteria were
able to carry out a different form of photosynthesis
but did not produce oxygen. In 2000, biochemists
led by Dr. Carl Bauer at the University of Indiana
found that non-oxygen-producing species (purple
and green bacteria) are the most ancient
photosynthetic bacteria. The oxygen-producing
cyanobacteria that exist today (see Figure 3.19)
evolved from a non-oxygen-producing bacteria
called heliobacteria.
As you may have noticed, the overall equation
for photosynthesis is exactly the opposite of the
equation for aerobic cellular respiration. This does
not mean, however, that the reactions follow the
same course in reverse. Photosynthesis requires
structures and metabolic processes similar to those
used in mitochondria: electron transport chains,
dissolved enzymes, and a membrane-enclosed
space for chemiosmosis.

6CO 2 +6H 2 O+energy→C 6 H 12 O 6 + 6O 2

carbon glucose oxygen

dioxide

water

Figure 3.18The process of photosynthesis is one of the

most important of all life processes. Green plants use

carbon dioxide and water to produce oxygen and food.

Figure 3.19Cyanobacteria eventually gave rise to the

structures that carry out photosynthesis in the algae and

green plants of today.

EXPECTATIONS

Describe the energy transformations that occur in photosynthesis.

Describe the role of enzymes in metabolic reactions in chloroplasts.

Investigate and explain how the structure of molecules can influence

metabolic rate.

Compare the structure and function of chloroplasts and mitochondria.