The cytoplasm refers to the entire area of the cell outside
of the nucleus. The cytoplasm has two parts, the organelles and
the cytosol, a grayish gel-like liquid that fills the
interior of the cell. The cytosol provides a home for the nucleus
and organelles as well as a location for protein synthesis and other
fundamental chemical reactions.
The cytoskeleton is a protein structure that maintains
cell shape and helps move organelles around the cell. There are
two types of cytoskeleton proteins: microtubules and microfilaments.
Microtubules are thick, hollow rods that provide a strong scaffold
for the cell. The smaller microfilaments are thin rods made of a
protein called actin; they are strung around the perimeter of the
cell to help it withstand strain. In some organisms, the microtubules power
limbs called cilia and flagella, creating movement. Contraction
of the microfilaments powers muscle movement in animals and facilitates
the creeping motion of creatures like amoebas. The microtubules
also form protein tracks on which organelles can slide around the
Floating in the cytoplasm are the many membrane-bound
organelles, each with a distinct structure and an important function
in the processes of the cell.
stores the cell’s genetic material in strands of DNA and
choreographs life functions by sending detailed messages to the
rest of the cell. The interior of the nucleus is separated from
the cytosol by a membrane called the nuclear envelope,
which lets only select molecules in and out. The DNA itself is wrapped
around proteins known as histones in an entangled fibrous
network called chromatin. When the nucleus is about
to split in two, this amorphous mass coils more tightly, forming
distinct structures called chromosomes. The nucleus
also houses a small, dark structure called the nucleolus,
which helps manufacture ribosomes.
synthesize proteins for the cell. Some ribosomes are mounted
on the surface of the endoplasmic reticulum (see below), and others
float freely in the cytoplasm. All ribosomes have two unequally
sized subunits made of proteins and a substance called RNA. All
living cells, prokaryotic and eukaryotic alike, have ribosomes.
Ribosomes are explained in more detail in the chapter on Cell Processes
as part of the larger discussion about the way the cell manufactures
produces energy for the cell through a process
called cellular respiration (see the chapter on Cell Processes).
The mitochondria has two membranes; the inside membrane has many
folds, called cristae. Many of the key cell-respiration enzymes
are embedded in this second membrane. The chemical reactions of
respiration take place in the compartment formed by the second membrane,
a region called the mitochondrial matrix.
an extensive network of flattened membrane sacs that manufactures
proteins. These proteins are transferred to the Golgi apparatus,
from which they will be exported from the cell. There are two types
of endoplasmic reticulum: rough and smooth. Rough endoplasmic reticulum
is studded by ribosomes covering its exterior. These ribosomes make
the rough endoplasmic reticulum a prime location for protein synthesis.
The smooth endoplasmic reticulum moves the proteins around the cell
and then packages them into small containers called vesicles
that travel to the Golgi apparatus. The smooth endoplasmic reticulum
also functions in the synthesis of fats and lipids.
a complex of membrane-bound sacs that package proteins
for export from the cell. Proteins enter the Golgi complex from
the endoplasmic reticulum and proceed through the stacks, where
they are modified and stored before secretion. When proteins are
ready for export, pieces of the Golgi membrane bud off, forming
vesicles that send them to the cell membrane.
small membrane-bound packages of acidic enzymes that digest
compounds and worn-out cellular components that the cell no longer
||Found in which Type of Cell
||Home for the organelles
||Prokaryotes and eukaryotes
||Maintains cell shape, moves organelles, moves
||Contains the genetic material
||Produce energy for the cell
||Prokaryotes and eukaryotes
||Manufactures and transports proteins, manufactures
||Packages proteins for secretion
||Plant eukaryotes (contractile vacuoles are
found in some animal cells)
||Stability and protection
||Plant eukaryotes; some prokaryotes have a cell
wall made of peptidoglycan
Plant Cell Organelles
The organelles described above are found in both animal
and plant eukaryotic cells. But plants have additional organelles—chloroplasts,
vacuoles, and cell walls—that support their unique life cycles.
Animal cells break down the food that they ingest to produce
energy. Plants do not need to ingest food; they manufacture their
own from sunlight, using the process of photosynthesis (covered
in the chapter on Plant Structure and Function). Chloroplasts are the
organelles in which photosynthesis takes place. They
are large oval-shaped structures containing a green pigment called chlorophyll that
absorbs sunlight. Chloroplasts, like mitochondria, are built from
two membranes: an external membrane forming the boundary of the
organelle and a stacked inner membrane within the organelle.
large liquid-filled storage containers found in plant
cells. Plant cells can put virtually anything in their vacuoles,
from nutrients to wastes to water to pigments. Vacuoles can be quite
large, allowing plant cells to grow to substantial -volumes without making
new cytoplasm. Some animal cells in freshwater microorganisms have
specialized contractile vacuoles that pump water out of the cell
to prevent bursting.
Plant cells have a rigid cell wall surrounding their cell
membrane. This wall is made of a compound called cellulose. The
tough wall gives the plant cell added stability and protection from