The cytoplasm refers to the entire area of the cell outside of thenucleus. The cytoplasm has two parts, the organelles and the
cytosol,a grayish gel-like liquid that fills the interior of the cell. Thecytosol provides a home for the nucleus and organelles as well as alocation for protein synthesis and other fundamental chemicalreactions.
Cytoskeleton
The cytoskeleton is a protein structure thatmaintains cell shape and helps move organelles around the cell. Thereare two types of cytoskeleton proteins:
microtubules and
microfilaments.Microtubules are thick, hollow rods that provide a strong scaffold forthe cell. The smaller microfilaments are thin rods made of a proteincalled actin; they are strung around the perimeter of the cell to helpit withstand strain. In some organisms, the microtubules power limbscalled cilia and flagella, creating movement. Contraction of themicrofilaments powers muscle movement in animals and facilitates thecreeping motion of creatures like amoebas. The microtubules also formprotein tracks on which organelles can slide around the cell.
The Organelles
Floating in the cytoplasm are the manymembrane-bound organelles, each with a distinct structure and animportant function in the processes of the cell.
Nucleus:
stores the cell’s genetic material in strands of
DNAand choreographs life functions by sending detailed messages to therest of the cell. The interior of the nucleus is separated from thecytosol 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.
Ribosomes:
synthesize proteins for the cell. Someribosomes are mounted on the surface of the endoplasmic reticulum (seebelow), and others float freely in the cytoplasm. All ribosomes havetwo unequally sized subunits made of proteins and a substance calledRNA. All living cells, prokaryotic and eukaryotic alike, haveribosomes. Ribosomes are explained in more detail in the chapter onCell Processes as part of the larger discussion about the way the cellmanufactures proteins.
Mitochondria:
produces energy for the cell through aprocess called cellular respiration (see the chapter on CellProcesses). The mitochondria has two membranes; the inside membrane hasmany folds, called cristae. Many of the key cell-respiration enzymesare embedded in this second membrane. The chemical reactions ofrespiration take place in the compartment formed by the secondmembrane, a region called the
mitochondrial matrix.
Endoplasmic reticulum:
an extensive network of flattened membrane sacsthat manufactures proteins. These proteins are transferred to the Golgiapparatus, from which they will be exported from the cell. There aretwo types of endoplasmic reticulum: rough and smooth. Rough endoplasmicreticulum is studded by ribosomes covering its exterior. Theseribosomes make the rough endoplasmic reticulum a prime location forprotein synthesis. The smooth endoplasmic reticulum moves the proteinsaround the cell and then packages them into small containers calledvesicles that travel to the Golgi apparatus. The smooth endoplasmicreticulum also functions in the synthesis of fats and lipids.
Golgi apparatus:
a complex of membrane-bound sacs that packageproteins for export from the cell. Proteins enter the Golgi complexfrom the endoplasmic reticulum and proceed through the stacks, wherethey are modified and stored before secretion. When proteins are readyfor export, pieces of the Golgi membrane bud off, forming vesicles thatsend them to the cell membrane.
Lysosomes:
small membrane-bound packages of acidic enzymesthat digest compounds and worn-out cellular components that the cell nolonger needs.
| Cell Organelles |
| Organelle | Function | Found in which Type of Cell |
| Cytoplasm | Home for the organelles | Prokaryotes and eukaryotes |
| Cytoskeleton | Maintains cell shape, moves organelles, moves cell | Eukaryotes |
| Nucleus | Contains the genetic material | Eukaryotes |
| Mitochondria | Produce energy for the cell | Eukaryotes |
| Ribosomes | Synthesize proteins | Prokaryotes and eukaryotes |
| Endoplasmic Reticulum | Manufactures and transports proteins, manufactures fats | Eukaryotes |
| Golgi Complex | Packages proteins for secretion | Eukaryotes |
| Lysosomes | Digest wastes | Eukaryotes |
| Chloroplasts | Make food | Plant eukaryotes |
| Vacuoles | Storage | Plant eukaryotes (contractile vacuoles are found in some animal cells) |
| Cell Wall | Stability and protection | Plant eukaryotes; some prokaryotes have a cell wall made of peptidoglycan |
Plant Cell Organelles
The organelles described aboveare found in both animal and plant eukaryotic cells. But plants haveadditional organelles—chloroplasts, vacuoles, and cell walls—thatsupport their unique life cycles.
Chloroplasts:
Animal cells break down the food that theyingest to produce energy. Plants do not need to ingest food; theymanufacture their own from sunlight, using the process ofphotosynthesis (covered in the chapter on Plant Structure andFunction). Chloroplasts are the organelles in which
photosynthesis takes place. They are large oval-shaped structures containing a green pigment called
chlorophyllthat absorbs sunlight. Chloroplasts, like mitochondria, are built fromtwo membranes: an external membrane forming the boundary of theorganelle and a stacked inner membrane within the organelle.
Vacuoles:
large liquid-filled storage containers found inplant cells. Plant cells can put virtually anything in their vacuoles,from nutrients to wastes to water to pigments. Vacuoles can be quitelarge, allowing plant cells to grow to substantial -volumes withoutmaking new cytoplasm. Some animal cells in freshwater microorganismshave specialized contractile vacuoles that pump water out of the cellto prevent bursting.
Cell wall:
Plant cells have a rigid cell wall surroundingtheir cell membrane. This wall is made of a compound called cellulose.The tough wall gives the plant cell added stability and protection fromharm.
