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 cell.

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 proteins.

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 needs.

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 harm.