Plant Cells
Figure 1.1: Generalized Plant Cell
Structurally, plant and animal cells are very similar because they are both
eukaryotic cells.
They
both contain membrane-bound
organelles
such as the
nucleus,
mitochondria,
endoplasmic
reticulum, golgi
apparatus,
lysosomes, and
peroxisomes. Both
also contain
similar membranes,
cytosol, and
cytoskeletal
elements. The functions
of these organelles are extremely similar between the two classes of cells
(peroxisomes perform additional complex functions in plant cells having to do
with cellular respiration). However, the few differences that exist between
plant and animals are very significant and reflect a difference in the functions
of each cell.
Plant cells can be larger than animal cells. The normal range for an animal
cell varies from 10 to 30 micrometers while that for a plant cell stretches from
10 to 100 micrometers. Beyond size, the main structural differences between
plant and animal cells lie in a few additional structures found in animal cells.
These structures include: chloroplasts, the cell wall, and vacuoles.
Figure 1.2: Plant Cell v. Animal Cell
Chloroplasts
In animal cells, the mitochondria produces the majority of the cells energy from food. It does not have
the same function in plant cells. Plant cells use sunlight as their energy source; the sunlight must be
converted into energy inside the cell in a process called photosynthesis. Chloroplasts are the
structures that perform this function. They are rather large, double membrane-bound structures (about 5
micrometers across) that contain the substance chlorophyll, which absorbs sunlight. Additional
membranes within the chloroplast contain the structures that actually carry out photosynthesis.
Chloroplasts carry out energy conversion through a complex set of reactions
similar to those performed by mitochondria in animals. The double membrane
structure of chloroplasts is also reminiscent of mitochondria. The inner
membrane encloses an area called the stoma, which is analogous to the
matrix in mitochondria and houses DNA, RNA, ribosomes, and different
enzymes. Chloroplasts, however, contain a third membrane and are generally
larger than mitochondria.
The Cell Wall
Another structural difference between in plant cells is the presence of a rigid
cell wall surrounding the cell membrane. This wall can range from 0.1 to 10
micrometers thick and is composed of fats and sugars. The tough wall gives
added stability and protection to the plant cell.
Vacuoles
Vacuoles are large, liquid-filled organelles found only in plant cells. Vacuoles can occupy up to 90% of
a cell's volume and have a single membrane. Their main function is as a space-filler in the cell, but
they can also fill digestive functions similar to lysosomes (which are also present in plant cells).
Vacuoles contain a number of enzymes that perform diverse functions, and their interiors can be used as
storage for nutrients or, as mentioned, provide a place to degrade unwanted substances.
