Cells Defined

One of the basic tenets of biology is that all living things are composed of cells. Some organisms consist of a single cell, while others have multiple cells organized into tissues and tissues organized into organs. In many living things, organs function together as an organ system. However, even in these complex organisms, the basic biology revolves around the activities of the cell.

One of the first scientists to observe cells was the Englishman Robert Hooke. In the mid 1600s, Hooke examined a thin slice of cork through the newly invented microscope. The microscopic compartments in the cork impressed him and reminded him of rooms in a monastery, known as cells. He therefore referred to the units as cells. Later in that century, Anton Van Leeuwenhoek, a Dutch merchant, made further observations of plant, animal, and microorganism cells. In 1838, the German botanist Matthias Schleiden proposed that all plants are composed of cells. A year later, his colleague, the anatomist Theodore Schwann, concluded that all animals are also composed of cells. In 1858, the biologist Rudolf Virchow proposed that all living things are made of cells and that all cells arise from preexisting cells. These premises have come down to us as the cell theory.

Prokaryote and Eukaryote Cell Structure

During the 1950s, scientists developed the concept that all organisms may be classified as prokaryotes or eukaryotes. The cells of all prokaryotes and eukaryotes possess two basic features: a plasma membrane and cytoplasm. However, the cells of prokaryotes are simpler than those of eukaryotes. For example, prokaryotic cells lack a nucleus, while eukaryotic cells have a nucleus. Prokaryotic cells lack internal cellular bodies (organelles), while eukaryotic cells possess them. Examples of prokaryotes are bacteria and cyanobacteria (formerly known as blue-green algae). Examples of eukaryotes are protozoa, fungi, plants, and animals.

Plasma membrane
All prokaryote and eukaryote cells have plasma membranes. The plasma membrane (also known as the cell membrane) is the outermost cell surface, which separates the cell from the external environment. The plasma membrane is composed primarily of proteins and lipids, especially phospholipids. The lipids occur in two layers (a bilayer). Proteins embedded in the bilayer appear to float within the lipid, so the membrane is constantly in flux. The membrane is therefore referred to as a fluid mosaic structure. Within the fluid mosaic structure, proteins carry out most of the membrane's functions.

Cytoplasm and organelles
All prokaryote and eukaryote cells also have cytoplasm (or cytosol), a semiliquid substance that composes the foundation of a cell. Essentially, cytoplasm is the gel-like material enclosed by the plasma membrane.

Within the cytoplasm of eukaryote cells are a number of membrane-bound bodies called organelles (“little organs”) that provide a specialized function within the cell.

One example of an organelle is the endoplasmic reticulum (ER). The endoplasmic reticulum is a series of membranes extending throughout the cytoplasm of eukaryotic cells. In some places, the ER is studded with submicroscopic bodies called ribosomes. This type of ER is referred to as rough ER. In other places, there are no ribosomes. This type of ER is called smooth ER. The ER is the site of protein synthesis in a cell. Within the ribosomes, amino acids are actually bound together to form proteins. Cisternae are spaces within the folds of the ER membranes.

Another organelle is the Golgi body (also called the Golgi apparatus). The Golgi body is a series of flattened sacs, usually curled at the edges. In the Golgi body, the cell's proteins and lipids are processed and packaged before being sent to their final destination. To accomplish this function, the outermost sac of the Golgi body often bulges and breaks away to form droplike vesicles known as secretory vesicles.

An organelle called the lysosome (see Figure 1 ) is derived from the Golgi body. It is a droplike sac of enzymes in the cytoplasm. These enzymes are used for digestion within the cell. They break down particles of food taken into the cell and make the products available for use. Enzymes are also contained in a cytoplasmic body called the peroxisome. 

The organelle that releases quantities of energy to form ATP (adenosine triphosphate) is the mitochondrion (the plural form is mitochondria). Because mitochondria are involved in energy release and storage, they are called the “powerhouses of the cells.”

Green plant cells contain organelles known as chloroplasts, which function in the process of photosynthesis. Within chloroplasts, energy from the sun is absorbed and transformed into the energy of carbohydrate molecules. Plant cells specialized for photosynthesis contain large numbers of chloroplasts, which are green because the chlorophyll pigments within the chloroplasts are green. Leaves of a plant contain numerous chloroplasts. Plant cells not specializing in photosynthesis (for example, root cells) have few chloroplasts and are not green.

An organelle found in mature plant cells is a large, fluid-filled central vacuole. The vacuole may occupy more than 75 percent of the plant cell. In the vacuole, the plant stores nutrients as well as toxic wastes. Pressure within the growing vacuole may cause the cell to swell.

An organelle called the cytoskeleton is an interconnected system of fibers, threads, and interwoven molecules that give structure to the cell. The main components of the cytoskeleton are microtubules, microfilaments, and intermediate filaments. All are assembled from subunits of protein.

The centriole organelle is a cylinder-like structure that occurs in pairs. Centrioles function in cell division.

Many cells have specialized cytoskeletal structures called flagella and cilia. Flagella are long, hairlike organelles that extend from the cell, permitting it to move. In prokaryotic cells, such as bacteria, the flagella rotate like the propeller of a motorboat. In eukaryotic cells, such as certain protozoa and sperm cells, the flagella whip about and propel the cell. Cilia are shorter and more numerous than flagella. In moving cells, the cilia wave in unison and move the cell forward. Paramecium is a well-known ciliated protozoan. Cilia are also found on the surface of several types of cells, such as those that line the human respiratory tract.

Prokaryotic cells lack a nucleus; the word prokaryotic means “primitive nucleus.” Eukaryotic cells, on the other hand, have a distinct nucleus.

The nucleus of eukaryotic cells is composed primarily of protein and deoxyribonucleic acid, or DNA. The DNA is organized into linear units called chromosomes, also known as chromatin when the linear units are not obvious. Functional segments of the chromosomes are referred to as genes. Approximately 100,000 genes are located in the nucleus of all human cells. Nuclear proteins belong to a class of proteins called histones. The chromosome is coiled around the histones.

The nuclear envelope, an outer membrane, surrounds the nucleus of a eukaryotic cell. The nuclear envelope is a double membrane, consisting of two lipid layers (similar to the plasma membrane). Pores in the nuclear envelope allow the internal nuclear environment to communicate with the cell cytoplasm.

Within the nucleus are two or more dense organelles referred to as nucleoli (the singular form is nucleolus). In nucleoli, submicroscopic particles known as ribosomes are assembled before their passage out of the nucleus into the cytoplasm.

Although prokaryotic cells have no nucleus, they do have DNA. The DNA exists freely in the cytoplasm as a closed loop. It has no protein to support it and no membrane covering it. A bacterium typically has a single looped chromosome with about 4,000 genes.

Cell wall
Many kinds of prokaryotes and eukaryotes contain a structure outside the cell membrane called the cell wall. With only a few exceptions, all bacteria have thick, rigid cell walls that give them their shape. Among the eukaryotes, fungi and plants have cell walls. Cell walls are not identical in these organisms, however. In fungi, the cell wall contains a polysaccharide called chitin. Plant cells, in contrast, have no chitin; their cell walls are composed exclusively of the polysaccharide cellulose.

Cell walls provide support and help cells resist mechanical pressures, but they are not solid, so materials are able to pass through rather easily. Cell walls are not selective devices, as plasma membranes are.

Movement Through the Plasma Membrane

In order for the cell cytoplasm to communicate with the external environment, materials must be able to move through the plasma membrane. This movement occurs through several mechanisms.

One method of movement through the membrane is diffusion. Diffusion is the movement of molecules from a region of higher concentration to one of lower concentration. This movement occurs because the molecules are constantly colliding with one another. The net movement of the molecules is away from the region of high concentration to the region of low concentration.

Diffusion is a random movement of molecules down the pathway called the concentration gradient. Molecules are said to move down the concentration gradient because they move from a region of higher concentration to a region of lower concentration. A drop of dye placed in a beaker of water illustrates diffusion as the dye molecules spread out and color the water.

Another method of movement across the membrane is osmosis. Osmosis is the movement of water from a region of higher concentration to one of lower concentration. Osmosis often occurs across a membrane that is semipermeable. A semipermeable membrane lets only certain molecules pass through while keeping other molecules out. Osmosis is really a type of diffusion involving only water molecules.

Facilitated diffusion
A third mechanism for movement across the plasma membrane is facilitated diffusion. Certain proteins in the membrane assist facilitated diffusion by permitting only certain molecules to pass across the membrane. The proteins encourage movement in the direction that diffusion would normally take place, from a region with a higher concentration of molecules to a region of lower concentration.

Active transport
A fourth method for movement across the membrane is active transport. When active transport is taking place, a protein moves a certain material across the membrane from a region of lower concentration to a region of higher concentration. Because this movement is happening against the concentration gradient, the cell must expend energy that is usually derived from a substance called adenosine triphosphate or ATP. An example of active transport occurs in human nerve cells. Here, sodium ions are constantly transported out of the cell into the external fluid bathing the cell, a region of high concentration of sodium. (This transport of sodium sets up the nerve cell for the impulse that will occur within it later.)

The final mechanism for movement across the plasma membrane is endocytosis, a process in which a small patch of plasma membrane encloses particles or tiny volumes of fluid that are at or near the cell surface. The membrane enclosure then sinks into the cytoplasm and pinches off from the membrane, forming a vesicle that moves into the cytoplasm. When the vesicle contains particulate matter, the process is called phagocytosis. When the vesicle contains droplets of fluid, the process is called pinocytosis. Along with the other mechanisms for transport across the plasma membrane, endocytosis ensures that the internal cellular environment will be able to exchange materials with the external environment and that the cell will continue to thrive and function.