Cell membrane

The smallest unit of living matter that can exist by itself is the cell. Some organisms, such as bacteria, consist of only a single cell. Others, such as humans and oak trees, are composed of many billions of cells.

Cells exist in a variety of shapes and sizes. Red blood cells are disk-shaped, while some skin cells resemble cubes. A single cell could be as large as a tennis ball or so small that thousands would fit on the period at the end of this sentence. Regardless of size, however, every cell contains the components needed to maintain life. Cells normally function with great efficiency, though they are vulnerable to disease.
Cell size is usually measured in microns. A micron is equal to about one millionth of a meter, and about 25,000 microns equal 1 inch. The smallest bacteria are about 0.2 micron in diameter. The diameter of the average human cell is roughly 10 microns, making it barely visible without a microscope.
The study of cells is the branch of biology called cytology, and the scientists who specialize in this field are called cytologists. A related field is molecular biology, which examines large molecules such as nucleic acids and proteins and their roles in cell structure and function.

Prokaryotes and Eukaryotes

Most scientists today agree that all living organisms can be divided into two major groups—prokaryotes and eukaryotes—based on fundamental differences in cell structure. Within these groups, the organisms are further classified into kingdoms, based on a variety of characteristics. Prokaryotes consist of a single kingdom—Monera—that is made up entirely of bacteria. The eukaryotes include the animal, plant, fungi, and protist kingdoms.
Prokaryotic and eukaryotic cells are distinguished by several key characteristics. Both cell types contain DNA as their genetic material However, prokaryotic DNA is single-stranded and circular, and it floats freely inside the cell; eukaryotic DNA is double-stranded and linear and is enclosed inside a body called the nucleus. Eukaryotes also have membrane-bound organelles—specialized structures that do much of the cell's work. Prokaryotes lack organelles, though they must accomplish many similar vital tasks. This inability to “delegate” tasks makes prokaryotes less metabolically efficient than eukaryotes.

Cell Structure and Function

All cells consist of protoplasm, a living jellylike substance made up of water, proteins, and other molecules surrounded by a membrane. The protoplasm within the main body of the cell is called cytoplasm. This is the site of much of the cell's work. Structures inside eukaryotic cells, such as the organelles, contain their own protoplasm.

Cell Membrane

Cells can survive only in a liquid medium that brings in food and carries away waste. For unicellular (single-celled) organisms, such as bacteria, algae, and protists, this fluid is an external body of water, such as a lake or stream. For multicellular (many-celled) organisms, the medium is part of the organism. In plants, for example, it is the sap; in animals, the blood.
The cell membrane is semipermeable—that is, some substances can pass through it but others cannot. This characteristic enables the cell to admit and reject substances from the surrounding fluid and enables the cell to excrete waste products into its environment.
The cell membrane is composed of two thin layers of phospholipid molecules studded with large proteins. Phospholipids are chemicals similar to stored fat that give the membrane its fluid quality. Some of the membrane proteins are structural; others form pores that function as gateways to allow or prevent the transport of substances across the membrane.
Substances pass through the cell membrane in several ways. Small uncharged molecules, such as water, pass freely down their concentration gradient (from the side of the membrane where they are in higher concentration to the side of lower concentration). This movement is called diffusion. Other materials, such as ions (charged molecules), must be transported through channels—membrane pores that are regulated by chemical signals from the cell. This facilitated transport requires energy for substances moving against a concentration gradient.

Passive transport

Substances such as glucose or ions enter the cell through specific channels, traveling down their concentration gradient. Since the process does not require energy, it is called passive transport.

Active transport

Molecules moving against their concentration gradient must be “escorted” across the cell membrane. This is called active transport, and it requires the cell to spend energy. Chemical signals in the cell tell the membrane channels when to start and when to stop the transport process.

Endocytosis and exocytosis

Endocytosis is a process used by cells to take in certain materials. The cell membrane forms a pocket around a substance in its environment. The filled pocket breaks loose from the membrane, forming a bubblelike vacuole that drifts into the cytoplasm, where its contents are “digested”: the vacuole wall is broken down and the contents are released into the cytoplasm. The process is called pinocytosis (pino- is from the Greek pinein, meaning “to drink”) when the material is dissolved in fluid and phagocytosis (phago- is from the Greek phagein, meaning “to eat”) when the cell ingests larger, particulate matter, such as another cell. The reverse process, exocytosis, is used to remove material from the cell.

Cell Wall

Almost all prokaryotes, as well as the cells of plants, fungi, and some algae, have a cell wall—a rigid structure that surrounds the cell membrane. Most cell walls are composed of polysaccharides—long chains of sugar molecules linked by strong bonds . The cell wall helps maintain the cell's shape and, in larger organisms such as plants, enables it to grow upright. The cell wall also protects the cell against bursting under certain osmotic conditions
Plant cell walls, as well as those of green algae and some other protists, are made mostly of the polysaccharide cellulose . In some plants, the cellulose is mixed with varying amounts of other polysaccharides, such as lignin, an important component of tree bark and wood. In some fungi the cell wall is composed of chitin, a polysaccharide that also forms the exoskeleton of many invertebrates such as insects and crabs. The bacterial cell wall is composed mostly of peptidoglycan, which is made up of polysaccharides and amino acids. Diatom cell walls have a high concentration of silica, which gives them a glasslike appearance.

Cytoplasm

Water is the largest component of cytoplasm. Depending on the cell and its needs and conditions, water concentration varies from about 65 percent to roughly 95 percent. Suspended in the cytoplasm are various solids such as proteins, carbohydrates, fat droplets, and pigments. As such, cytoplasm is a colloid rather than simply a solid or a liquid.
Changes in the concentration of solids produce an apparent streaming of the cytoplasm from place to place within the cell. When viewed through a microscope, membranes and fibrous structures are more readily visible in the cytoplasm when the concentration of solids increases. This visibility decreases as the solid content decreases.

Organelles and Their Functions

Cells are constantly working to stay alive. Food molecules are changed into material needed for energy, and substances needed for growth and repair are synthesized, or manufactured. Some of these tasks occur in the cytoplasm; in eukaryotes, however, most specialized tasks take place inside membrane-bound bodies in the cytoplasm called organelles.

Plastids

Plastids are found in plant cells and in protists such as algae that use photosynthesis to manufacture and store food. Chloroplasts, chromoplasts, and leucoplasts are the most common plastids. Photosynthesis takes place inside chloroplasts, which contain chlorophyll, a green pigment that captures energy from the sun and converts it into sugar  Chromoplasts, most commonly found in fruits and flower petals, contain other pigments, such as the orange carotenes, yellow xanthophylls, and red and blue anthocyanins. These pigments give fruits and flowers their colors and produce the brilliant fall hues seen in many tree species. Leucoplasts are colorless and usually contain starch granules or other materials.
All plastids have an inner and an outer membrane; the inner membrane is highly impermeable, while the outer is semipermeable. Plastids have their own DNA; it is distinct from the DNA found in the cell's nucleus and is replicated and inherited independently. Plastids manufacture some of their own proteins but rely on the cell's DNA and ribosomes to synthesize others.

Mitochondria

Often called the powerhouses of the cell, the sausage-shaped mitochondria produce the energy needed by the cell to function. Food molecules that pass into the cytoplasm are taken into the mitochondria and oxidized, or burned, for energy. Like plastids, mitochondria have an inner and an outer membrane. Also like plastids, they depend upon the cell's DNA for certain proteins though they have their own DNA.

Endoplasmic reticulum and ribosomes

The endoplasmic reticulum (ER), a network of membranous tubes and sacs, twists through the cytoplasm from the cell membrane to the membrane surrounding the nucleus. Located along portions of the endoplasmic reticulum are ribosomes, tiny bodies made of ribonucleic acid (RNA) that play a vital role in the manufacture of proteins. Ribosomes are also found scattered throughout the cytoplasm; distinct sets of ribosomes are found in plastids and mitochondria.The portions of the endoplasmic reticulum that contain ribosomes are called rough endoplasmic reticulum (RER). Areas of the network that do not contain ribosomes are called smooth endoplasmic reticulum (SER). The latter is predominant in cells involved in the synthesis and metabolism of lipids and the detoxification of some drugs.

Golgi complex

The Golgi complex, or Golgi apparatus, is a membranous structure composed of stacks of thin sacs. Newly made proteins and lipids move from the RER and SER, respectively, to the Golgi complex. The materials are transported inside vesicles formed from the ER membrane. At the Golgi complex, the vesicles fuse with the Golgi membrane and the contents move inside the Golgi's lumen, or center, where they are further modified and stored. When the cell signals that certain proteins are needed, the latter are “packaged” by the Golgi for export—part of the Golgi membrane forms a vesicle that then buds off, or breaks away, from the larger apparatus. The vesicle may migrate to the cell membrane and export its contents via exocytosis or it may travel to an intracellular location if its contents are needed by the cell. Lipids are processed by the same methods.

Vacuoles

Vacuoles drift through the cytoplasm and usually carry food molecules in solution. Vacuoles also regulate the water content of some unicellular organisms. For example, when an amoeba absorbs too much water, it forms a contractile vacuole against the membrane. The vacuole fills with water and then contracts to squeeze the excess liquid out of the cell.
Vacuoles in cambium cells in plants develop large central vacuoles that play a role in building stalks and stems. If a cambium cell is to become bark or wood, its membrane grows into the vacuole and deposits layers of cell wall to increase stiffness. In cells that become part of a vascular bundle that transmits sap, the vacuole becomes cylindrical and develops openings at each end that pass sap from cell to cell.

Lysosomes and peroxisomes

Lysosomes are similar in appearance to vacuoles. Each lysosome is filled with enzymes that help the cell to digest certain materials, such as cell parts that are no longer functional, and foreign particles, such as bacteria. Similar to lysosomes are peroxisomes, which contain enzymes that destroy toxic materials such as peroxide. Lysosomes are produced in the Golgi complex, while peroxisomes are self-replicating.

Centrosomes

Near the nucleus of animal, fungus, and algal cells is a spherical structure called the centrosome. During cell division, the centrosome divides into two centrosomes. Each of these then travels to opposite ends of the cell. The centrosomes contain a pair of structures called centrioles, which produce microtubules. These protein tubes form “spindles” that extend toward the nucleus and help the cell's chromosomes separate during cell division . Plant cells lack centrioles, but they do have centrosomes, which serve a function similar to that in animal cells.

Cytoskeleton

The cytoskeleton helps the cell maintain its shape, aids in cellular movement, and helps with internal movement. Found only in eukaryotic cells, the cytoskeleton is a network of protein filaments and tubules that extends throughout the cytoplasm. Microtubules help form structures such as cilia and flagella, which help in cell movement, and the spindle fibers that help chromosomes move during cell division. Microfilaments give the cell its shape and help it contract; intermediate filaments give it strength.

Nucleus

Near the center of the cell is the nucleus. The nucleus is the control center of the cell. It also contains the structures that transmit hereditary traits . A nucleus not undergoing division has at least one nucleolus, which is the site of RNA synthesis and storage.
The nucleus is enclosed by a two-layered membrane and contains a syrupy nucleoplasm and strands of DNA wrapped around proteins in a manner that resembles a string of beads. Each strand contains a long series of genes—segments of DNA inherited from the previous generation. Each gene determines a heritable characteristic of the organism . Genes also regulate the production of RNA, which in turn controls the manufacture of specific proteins.
The DNA strands, which are called chromatin because they readily stain with dyes, are usually too thin to be seen with an optical microscope. When a cell begins to divide, the chromatin–protein strands coil repeatedly around themselves, condensing into thicker structures called chromosomes.

How Cells Divide

Prokaryotes reproduce by several means, including simple fission (in which the cell divides after replicating its DNA) and conjugation (a form of simple sexual reproduction). Eukaryote cells undergo a more complex process.

Mitosis

The division of eukaryote somatic cells—any cell type except germ cells, such as sperm and eggs—is called mitosis. When the cell needs to divide, the nucleus signals the chromatin to condense into chromosomes. Each chromosome then makes an exact duplicate of itself. A body called the centromere holds the original and duplicate chromosomes together. As mitosis begins, the paired chromosomes gather in a line at mid-cell. The two centromeres (and centrioles, in cells that have them) divide to form a second pair and produce asters—lengths of microtubules. The centrosomes move to opposite ends of the cell, producing more tubules—called spindle fibers at this stage—as they move. As the centromeres split, the chromosome pairs are separated; each moves along the spindle toward its respective centrosome. Eventually the cell divides, producing two daughter cells—each with an identical complement of chromosomes.
Each cell of a given species has a characteristic number of chromosomes. Human somatic cells normally contain 46 chromosomes—23 pairs. Mitosis ensures that both daughter cells have the full set of chromosomes characteristic of their species.

Meiosis

Germ, or sex, cells produce gametes—sperm and eggs in humans—by meiosis. This involves two divisions. During the first division, the chromosomes pair up and duplicate themselves, sometimes exchanging genes through a process called crossing over. The first division produces two cells, each with a full set of chromosomes. During the second division, the chromosomes in the two cells do not duplicate themselves. The second division produces four gametes, each containing only one chromosome from each chromosome pair, or only half the number of chromosomes characteristic of the species. The full complement of chromosomes is restored when a male gamete combines with a female gamete. For example, a human sperm and a human egg each contain 23 chromosomes. When the sperm fertilizes the egg, the two gametes fuse, forming a cell with a complete set of 46 chromosomes. This new cell is called a zygote, and it is the beginning of a new organism.

History of Cell Theory

Cells were first described by the English scientist Robert Hooke, who in 1665 published a book about his findings. Hooke had sliced thin sections of cork to view under a microscope of his own design. He was able to see the minute, boxlike units of which the cork was made up. Hooke called these structures cells because he thought the boxes looked like monastery cells. The first description of living cells was provided in 1674 by Dutch scientist Anthony van Leeuwenhoek, who observed bacteria and protozoa under his microscope. Ten years later Leeuwenhoek gave the first accurate description of red blood cells.
Improvements in microscopes by the 19th century allowed more detailed investigations. In the 1830s Scottish botanist Robert Brown discovered the cell nucleus, and two German scientists, Matthias J. Schleiden and Theodor Schwann, concluded independently that cells were the basis of all life, a view called the cell theory. Rudolf Virchow, another German scientist, stated in 1858 that all cells develop from previously existing cells. During the late 19th century, techniques of fixing and staining tissues to preserve cells opened the way for intensive research.
Scientists use a variety of microscopes to study cells. In the light microscope, the background is brightly lit, the objects studied are dark, and the power of magnification is about 1,000. In some microscopes, however, the background is dark, and the objects examined are bright. Electron microscopes, which were first developed in the early 20th century, use magnetic fields and waves of electrons (negatively charged particles) to get an image. Electron microscopes can magnify images up to one million times, allowing biologists to examine the structure and contents of cells at an extremely fine scale.