Biology 12

Chapter 3 Cellular Energy • MHR 93

These three ways will now be described.
Autotrophs, such as green plants, produce a
molecule used for energy storage, called starch.
The starch is a large, branched polysaccharide
composed of hundreds of glucose molecules linked
by condensation reactions. Plants convert glucose
to starch in the stroma. During peak hours of bright
daylight, plants may produce more starch than they
can use. This starch is stored in cells and is ready
to be broken down into glucose for use in cellular
processes. In the Thinking Lab on page 92, you will
consider how the structure of starch can influence
metabolic processes.
In another series of reactions, plants may form
another kind of polysaccharide that is the building
block of cell walls — cellulose. PGAL is first
exported from the chloroplast into the cytoplasm
where condensation reactions take place to link
glucose molecules.
The formation of sucrose (the transport sugar in
plants) also occurs in the cytoplasm. In order for
glycolysis and cellular respiration to take place in
the cytosol and mitochondria of plants, glucose is
required. Because plants cannot move glucose
molecules through the phloem (vascular tissue that
transports organic material), they convert PGAL to
glucose in the cytoplasm of leaf mesophyll cells.

Glucose and fructose are then converted to sucrose.

Sucrose is a molecule of fructose covalently

bonded to a molecule of glucose. After sucrose

is formed it is actively transported to the phloem,

and then moved to locations in the plant that

metabolize glucose.

Photosynthesis Versus

Aerobic Cellular Respiration

Both plant and animal cells have mitochondria and

carry out aerobic cellular respiration. However,

only plants use photosynthesis. The cellular

organelle for photosynthesis is the chloroplast,

while the cellular organelle for aerobic cellular

respiration is the mitochondrion. Figure 3.33

compares the processes of photosynthesis and

respiration. Both processes have an electron

transport chain located on membranes in the

chloroplast and mitochondrion. ATP is produced

on these membranes through the process of

chemiosmosis. In photosynthesis, water is oxidized

and oxygen is produced. In aerobic cellular

respiration, oxygen is reduced to form water.

Reactions in the chloroplast and mitochondrion

are catalyzed by enzymes. These enzymes help

to reduce CO 2 to glucose in the chloroplast and

oxidize glucose to CO 2 in the mitochondrion.

Figure 3.33Photosynthesis versus cellular respiration. How are products
produced by one organelle used by the other organelle? The organelles shown are
not drawn to scale, a chloroplast is about four times larger than a mitochondrion.

photosynthesis

chloroplast mitochondrion

aerobic cellular respiration

membranes

ADP ATP

NADPH enzymes NAD NADH

NADP+ +

H 2 O O 2 O 2 H 2 O

CO 2 C 16 H 12 O 6 C 16 H 12 O 6 CO 2