Cell Structure 51
cell membrane. The basal body or plate has a cylindrical
structure like the centriole, that is, it is also composed- of
nine sets of triplet fibers.
Plastids of Plant Cells
In our laboratory exercise on cells, we will examine and
compare plant cells with animal cells. Therefore, it is nec-
essary to discuss these organelles found only in plant cells.
There are three plastids found in plant cells. The most
common and most numerous of these are the chloroplasts-
( KLOR-oh-plastz) that cause plants to look green.
Chloroplasts are large organelles found mainly in plant
cells (Figure 3-14). They contain the green pigment-
chlorophyll. These organelles are the site of photosyn-
thesis. It is here that the light energy of the sun is con-
verted into chemical energy and food for use by both plants
and animals. Without plants and their chloro-plasts,
animals could not survive on this planet. The process of
photosynthesis occurs inside the chloroplast. This chemical
equation is:
6CO 2 1 12H 2 O S C 6 H 12 O 6 or glucose (sugar, food) 1
6O 2 (the air we breathe) 1 6H 2 O
Chloroplasts are large enough to be easily seen with a
light microscope. They are enclosed by an outer mem-brane
but the second internal membranous structure is quite
complex. The inside contains many stacks of mem-branes
called a granum. A granum is made of a stack of individual
double membranes called a thylakoid (THIGH-lah-koid).
The grana (plural) are connected to one an-other by a different
system of membranes called -lamellae. The grana are made
of proteins, enzymes, chlorophyll, and other pigments
arranged in a layered structure.
Plant cells also have two other types of plastids.
Chromoplasts- (KROH-moh-plastz) are similar in struc-
ture to chloroplasts but they contain other pigments
®^
LearningDouble (^) Granum Cengage^
membrane Thylakoid Lamella ©^
Figure 3- 14 The internal anatomy of a chloroplast.
like the carotenoid pigments. The carotenoid pigments are
xanthophyll (ZAN-tho-fill), which produces a yel-low
color (the skin on a banana), and carotene (KAR-oh-
teen), which produces a red-orange color (tomatoes and
carrots). These pigments also produce the colors of flower
petals and fruits. Another type of plastid is the leucoplast
(LOO-koh-plast). Leucoplasts do not have any pigments—
they are storage plastids. An onion bulb is full of
leucoplasts where sugar is stored and a potato contains
leucoplasts where starch is stored.
The Cell Wall of Plant Cells
The cell membrane of plant cells is surrounded by a
semirigid covering called the cell wall made of a complex
carbohydrate called cellulose (SELL-you-lohs). Cellu-
lose is synthesized by Golgi bodies by linking up glucose
units. Animal cells do not have cell walls. This material is
what we call fiber in our diet. It cannot be digested; thus, it
keeps our stools soft. We eat fruits and vegetables to
maintain a balance of fiber in our diet. This fiber may help
prevent the development of colon cancer.
As the Body Ages
Apoptosis (a-po-TOE-sis) is a natural process by which cells in the body die and is
controlled by specific genes. This process eliminates damaged cells, cells that
have outlived their natural lifetime and need to be replaced, cells that are in-fected
with pathogens, and certain cancer cells. As we age, cellular repair and cell
division to replace old cells become more difficult to achieve. Some biolo-gists
believe that there are actually “death genes” that become activated later in life,
resulting in cellular deterioration and death.