Cell Surface
A thin membrane termed the plasma membrane encloses the contents of all living cells and defines the boundary between the contents of the cell and the surroundings. The plasma membrane is a continuous layer of lipid and protein molecules 8 to 10 nanometres (nm) thick that acts as a selective barrier to regulate the cell's chemical composition. Most water-soluble ions and molecules are unable to cross this barrier spontaneously and require a specific carrier protein or a channel made of protein to enable them to cross. In this way the cell is able to maintain concentrations of ions and small molecules different from those of their surroundings. Another mechanism, involving small membrane vesicles (fluid-filled sacs) that add to or bud from the plasma membrane, allows animal cells in particular to transfer macromolecules and even large particles across their membranes.
Most bacterial and plant cells are also encased in a relatively thick and sturdy cell wall made of polysaccharides (predominantly cellulose in the case of higher plants). The cell wall, which is external to the plasma membrane, maintains the shape of the cell and protects it from mechanical damage, but it also restricts the movements of the cell and limits the entry and exit of materials.
The Nucleus
The most conspicuous organelle (cell organ) in most plant and animal cells is the nucleus, typically a membrane-enclosed, roughly spherical body about 5 µm in diameter. Within the nucleus, molecules of DNA and proteins are organized into chromosomes, which usually occur in identical pairs. Chromosomes are too stringy and intertwined to be separately identified. Just before a cell divides, however, the chromosomes become condensed and thick enough to be seen as separate structures. The DNA inside each chromosome is a single, long, highly coiled molecule containing a linear sequence of genes. Genes contain the coded instructions for the assembly of the proteins and RNA molecules needed to produce a functioning copy of the cell.
The nucleus is surrounded by a two-layered membrane and interaction between the nucleus and the rest of the cell—the cytoplasm—is permitted through holes, called nuclear pores. A specialized region, the nucleolus, assembles particles containing RNA and protein, which migrate through the nuclear pores to the cytoplasm and are then modified to become ribosomes (see below).
The nucleus controls protein synthesis in the cytoplasm by sending molecular messengers. Messenger RNA (mRNA), as it is called, is made according to instructions in the DNA and then leaves the nucleus via the nuclear pores. Once in the cytoplasm, the mRNA attaches to ribosomes and is translated into the primary structure of a specific protein.
Cytoplasm and Cytosol
The entire volume of a cell, excluding the nucleus, is called its cytoplasm. This includes many specialized structures and organelles, as described below.
The concentrated aqueous solution in which organelles are suspended is termed the cytosol. This is a water-based gel containing a host of large and small molecules, and in most cells it is by far the largest single compartment (in bacteria it is the only intracellular compartment). The cytosol is the site of many of the most important housekeeping functions of the cell, including the early stages of breakdown of food molecules and the synthesis of many of the large molecules with which a cell is built. Whereas many molecules in the cytosol exist in true solution, moving rapidly from one location to another by free diffusion, other molecules are more highly ordered. These ordered structures give the cytosol an internal organization that provides a framework for the manufacture and breakdown of large molecules, and they channel many of the chemical reactions of the cell along restricted pathways.
Cytoskeleton
A system of protein filaments in the cytosol termed the cytoskeleton fills the interior of all animal and plant cells. It is especially important in animal cells, which lack a rigid cell wall, as the cytoskeleton maintains the structure and shape of the cell. The cytoskeleton provides a framework for the organization of the cell and an anchorage for organelles and enzymes. It is also responsible for the many movements that the cell can produce. In many cells, the cytoskeleton is not a permanent structure but continually dismantled and reassembled. It is formed from three principal types of protein filaments: microtubules, actin filaments, and intermediate filaments. These are linked to each other and to other structures in the cell by a variety of accessory proteins.
Cell movements in eukaryotic cells almost always depend on actin filaments or microtubules. Many cells possess flexible “hairs” on their surface, called cilia or flagella, which contain a core bundle of microtubules capable of regular energy-driven bending movements. Sperm cells swim by means of flagella, for example, and cells lining the intestine or other ducts in the vertebrate body carry fields of cilia on their surfaces that sweep fluids and particles in a specific direction. Large bundles of actin filaments are found in muscle cells where, together with the protein called myosin, they produce forceful contractions. The movements associated with cell division in animals or plants depend on actin filaments and microtubules, which carry chromosomes and other components of the cell into the two segregating daughter cells. Many other internal movements are needed by plant and animal cells in order to develop a particular shape or to maintain their complicated internal structure.
Mitochondria and Chloroplasts
Mitochondria are among the most conspicuous organelles in the cytoplasm, and are present in nearly all eukaryotic cells. They have a distinctive structure when seen in the electron microscope: each mitochondrion is usually sausage-shaped, several micrometres long, and enclosed in two separate membranes, the inner one being highly folded. Mitochondria are the energy-producing organelles. Cells need energy to grow and replicate and mitochondria supply most of this energy by performing the last stages of the breakdown of food molecules in the process known as the Krebs cycle. These stages involve the consumption of oxygen and the production of carbon dioxide—a process called respiration because of its similarity to breathing. Without mitochondria, animals and fungi would be unable to use oxygen to extract the full amount of energy from the food they consume to fuel their growth and replication. Various organisms that live in environments that lack oxygen are said to be anaerobic and they all lack mitochondria.
Chloroplasts are large green organelles that are found only in cells of plants and algae, not in cells of animals or fungi. They have an even more complex structure than mitochondria: in addition to the two surrounding membranes, they have multiple sacs in their interior formed from a membrane that contains the green pigment chlorophyll. Chloroplasts carry out an even more essential task than mitochondria: photosynthesis—that is, they use the energy of sunlight to drive the manufacture of small, energy-rich, carbon-containing molecules. In the process they release oxygen. Thus chloroplasts generate both the food molecules and oxygen that mitochondria use.
Internal Membranes
Nuclei, mitochondria, and chloroplasts are not the only membrane-bounded organelles inside eukaryotic cells. The cytoplasm also contains a complex profusion of other organelles, each enclosed by a single membrane, that perform many distinct functions. Most of these functions are concerned with the cell's need to import raw materials and export manufactured substances and waste products. Thus organelles of one class are enormously enlarged in cells that are specialized for secretion of proteins; organelles of another class are especially plentiful in cells in higher vertebrates that capture and digest viruses and bacteria that have invaded the body.
An irregular three-dimensional network of spaces enclosed by a membrane, called the endoplasmic reticulum, is where most cell membrane components are made, as well as materials destined for export from the cell. Attached to the endoplasmic reticulum or floating in the cytoplasm are ribosomes—particles composed of protein and RNA that help manufacture new protein molecules. Stacks of membrane-bounded flattened sacs constitute the Golgi apparatus. This receives the molecules made in the endoplasmic reticulum, processes them, and then directs them to various locations in the cell. Lysosomes are small, irregularly shaped organelles that contain stores of enzymes responsible for the digestion of many unwanted molecules in cells. Peroxisomes are small, membrane-bounded vesicles that provide a contained environment for reactions where dangerously reactive hydrogen peroxide is generated and degraded. Membranes form numerous other small vesicles involved in the transport of materials between one organelle and another. In a typical animal cell the membrane-bounded organelles may occupy up to a half the total cell volume.
Secretion and Endocytosis
One of the most important functions of vesicles is to carry materials to and from the plasma membrane, thereby providing a means of communication between the interior of the cell and its surroundings. There is a continual exchange of materials between the endoplasmic reticulum, the Golgi apparatus, the lysosomes, and the outside of the cell. The exchange is mediated by small membrane-bounded vesicles that pinch off from one membrane and fuse with another. At the surface of the cell, for example, portions of the plasma membrane continually bud inward to form vesicles that carry material captured from the external medium to the cell interior (a process termed endocytosis). Very large particles or even entire foreign cells can be engulfed in this way. The reverse process, called secretion, when vesicles from inside the cell fuse with the plasma membrane and release their contents into the external medium, is also common in many cells.
Cell Division
Plants and animals are built from many billions of individual cells joined together into tissues and organs with specific functions. All the cells in any individual plant or animal are produced from a single cell—the fertilized egg—by a process of division. The fertilized egg divides, to produce two identical daughter cells, each containing a set of chromosomes identical to those of the parent cell. Each daughter cells divides again, and so on. Except for the earliest division of the egg, a cell generally grows to roughly twice its original size before it divides. In doing so it duplicates its DNA so that each chromosome is doubled, and duplicate sets are transported into each of the newly formed daughter cells—this process is called mitosis (see Genetics: Mitosis). During mitosis an array of microtubules (spindle-fibres), which forms at the beginning of mitosis, pulls one complete set of chromosomes to each of the two forming daughter cells.
Differentiation
The cells found in the various tissues of a multicellular organism often differ dramatically in both structure and function. Differences between a mammalian nerve cell, a liver cell, and a white blood cell, for example, are so extreme that it is difficult to imagine that the two cells contain the same genetic information. Since all cells in an animal or plant are produced by successive divisions from the same fertilized egg, most of the cells contain the same genetic information. They become different from one another because they synthesize and accumulate different sets of RNA and protein molecules without altering the sequence of their DNA. This process, termed differentiation, involves switching genes on and off selectively in a programmed succession. These orchestrated changes in cell characteristics are often irreversible, so that a human nerve cell cannot easily change to a white blood cell or revert to the rapidly dividing state of an immature cell in the embryo from which it came.
Intercellular Junctions
To form a multicellular organism, cells must not only differentiate into specialized types, but also be bound together into tissues and organs. Eukaryotes have evolved a number of different ways to satisfy this need. In higher plants, the cells not only remain connected by cytoplasmic bridges (called plasmodesmata), they are also imprisoned in a rigid honeycomb of chambers walled with cellulose that the cells themselves have secreted (cell walls). The cells of most animals are bound together by a relatively loose network of large extracellular organic molecules (called the extracellular matrix) and by adhesion between plasma membranes. Often attachments between the cells hold them together to form a multicellular sheet, or epithelium. Epithelial sheets frequently form the outer boundary of a tissue or organ, providing a surface barrier that regulates the entry and exit of material.
Cell Signalling
During the development of an embryo, each different type of cell becomes programmed to respond in a particular way, and there is consequently a need for messages or signals to pass between different cells. The cell also has to work in harmony with its surroundings, which in a multicellular organism means co-operating with its neighbours. The importance of such “social controls” on cell division becomes apparent when the controls fail, resulting in cancer, which usually kills the organism. Cells co-ordinate their many activities through a system of signalling reactions that serves a role similar to the electrical system of a motor car or the nervous system of a small animal. Molecules, often produced by other cells, act on cell-surface receptors which initiate cascades of biochemical reactions in the cytoplasm of the target cell. Changes in the concentration of the chemical signals (specific ions and molecules) regulates the activity of proteins and the expression of genes in the target cell.
