Cell

A typical animal cell, sliced open to reveal cross-sections of organelles. Source: National Institute of General Medical Sciences.

The cell is the most basic unit of life. All life, including humans, is composed of cells. Some microscopic organisms are single-celled. Some organisms, like algae, have many cells, but they are very similar to each other. In plants and animals, there may be hundreds of different types of cells, with very specialized functions.

Types

There are two basic kinds of cells: prokaryotic and eukaryotic.

• Prokaryotic cells, also called bacteria, do not have a nucleus.
• Eukaryotic cells, which includes the cells of animals, plants, and fungi, and protozoa and other protists, have a nucleus.

Types of Human Cells

Human cells are usually described by their shape, nature (somatic cell, germ cell, early embryonic stem cell or extra-embryonic cell), and degree of differentiation. Differentiated cells can also be classified into several hundred specific cell types.

Types of human cells by shape

• Names that describe cells shaped like various polyhedrons.
  • Cuboidal cells
  • Columnar (prismatic) cells
  • Squamous (flat) cells
  • Polyhedral cells
  • Pyramidal cells
• Names that describe rounded cells
  • Spherical cells
  • Ovoid cells
  • Fusiform (spindle-shaped) cells
• Names that describe cells with processes
  • Ameboidal cells
  • Dendritic cells
  • Stellate cells

Types of human cells by nature and degree of differentiation

• Embryonic stem cells - totipotential stem cells
• Somatic cells
  • Pluripotential stem cells
  • Multipotential progenitor cells
  • Committed progenitor cells
  • Maturing precusor cells ("blasts")
  • Mature differentiated cells
• Germ cells
  • Primordial germ cells
  • Gonocytes (pro-oogonia and pro-spermatogonia)
  • Gametogonia (oogonia and spermatogonia)
  • Gametocytes (oocytes and spermatocytes)
  • Gametids (spermatids)
  • Mature gametes (spermatozoa)
• Extra-embryonic cells (cells of amnion, chorion, yolk sac and allantois)

Description

The human body is composed of trillions of cells. They provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions. Cells also contain the body’s hereditary material and can make copies of themselves. It is estimated that the human body contains 100,000,000,000,000 cells.^[1]

The cytoplasm surrounds the cell’s nucleus and organelles. Source: U.S. National Library of Medicine.

All cells are surrounded by a plasma membrane, which is composed of a lipid bilayer. This cell membrane defines the boundaries of the cell. Eukaryotic cells, such as those found in all multicellular organisms, are made up of cytoplasm and nuclei. Most eukaryotic cells have one nucleus.

Cytoplasm

The cytoplasm consists of the plasma membrane, cytosol, membrane-bound organelles, cytoskeleton and inclusions.

• The Plasma membrane (cytoplasmic membrane or plasmalemma) is the boundary of the cell, its protective coat. It is composed of a semi-permeable lipid bilayer membrane with integrated and peripheral membrane proteins and a glycocalyx.
• The Cytosol, the cell's inner fluid or space, comprises about half the cell volume, and contains RNAs, ribosomes and proteins. Its functions include imtermediary metabolism, and protein synthesis and degradation.
• Membrane-bound Organelles are structures within the cell that are often compared to organs in the human body. Cell membranes in the cytoplasm define these cellular compartments, which have special components and functions. Some organelles of human cells are listed here:
  • Rough Endoplasmic Reticulum (RER or Nissl substance) contains ribosomes, which make proteins for lysosomes and exocytosis.
  • The Golgi Apparatus is the site of post-translational modification of proteins and sorting of these proteins for differential vesicular compartmentalization and transport.
  • Exocytotic Vesicles are involved in constitutive and regulated secretory pathways.
  • Lysosomes contain acid hydrolases for degradation of macromolecules.
  • Endocytotic Vesicles contain ingested substances. In most cells they are small (pinocytotic vesicles), but in some specialized cells, they may be large (phagocytotic vesicles).
  • Smooth Endoplasmic Reticulum is most abundant in liver. steroid hormone-secreting cells (adrenal gland and gonads) and striated muscle (where it is called sacroplasmic reticulum).
  • Peroxisomes contain catalase and flavin oxidases, which degrade some macromolecules. Peroxisomes are largest and most abundant in the liver and kidney.
  • Mitochondria are double-membrane-bound organelles in which oxidative phosphorylation is energy generating (producing most of the cell's ATP).
• The Cytoskeleton is the cell's internal scaffolding of fibrous proteins, which influence cell shape and movement. The cytoskeleton also plays a role in cell division, and intracellular transport of vesicles and other organelles.
  • Microtubules (composed of tubulin polymers)
  • Thin filaments (composed of actin polymers)
  • Intermediate filaments (e.g., keratin, vimentin, desmin, glial fibrillary acid protein, and neurofilaments).
• Cytoplasmic Inclusions include glycogen granules, lipid droplets, and melanin and lipofuscin pigments.

The nucleus contains most of the cell’s genetic material. Source: U.S. National Library of Medicine.

Nucleus

The nucleus is surrounded by a double membrane, the nuclear envelope. With a light microscope, three features can be distinguished: euchromatin, heterochromatin, and the nucleolus. Chromatin is composed mainly of DNA and basic proteins (histones). Euchromatin is electron-lucent and represents active DNA. Heterochromatin is electron-dense and represents inactive DNA. The nucleolus is the site of assembly of ribosomal subunits, and thus is rich in RNA. The basic functions of the nucleus are DNA replication, transcription into RNA, and assembly of ribosomal subunits.

Role of Cell in the Body

The role of cells in the body is well summarized by the Cell Theory, which was developed in the mid-19th century:

• All living things are composed of one or more cells.
• The cell is the most basic unit of life.
• All cells come from pre-existing cells.

How Cells Work

The cell is the basic unit of life, so it is not surprising that many of the basic functions of the cell are similar to those of the body as a whole.

Irritability

Cells can respond to various stimuli.

• Specialized cells (e.g., neurons and muscle cells) have specialized responses:
  • Nervous irritability (conductivity)
  • Muscular irritability (contractility)

Reproduction

Cells reproduce by cell division, which usually consists of division of the nucleus (mitosis or meiosis) and cytoplasm (cytokinesis).

Absorption

The plasma membrane is a semi-permeable limiting membrane.

• Lipid-soluble molecules, water, oxygen and carbon dioxide can pass through the plasma membrane by osmosis.
• Ions and small hydrophilic molecules may enter the cell by facilitated diffusion (passive transport by specific shuttles or pores) or active transport (requiring energy, usually ATP hydrolysis).
• Macromolecules are ingested by endocytosis.

Digestion

After fusion of endocytic vacuoles and lysomes, ingested macromolecules are digested by lysosomal acid hydrolases. Lysosomes and peroxisomes also degrade macromolecules from internal sources for recycling of simple biochemical building blocks.

Assimilation and Growth

Simple biochemicals derived from absorption or from digestion of macromolecules are used to build macromolecular components of the cell.

Respiration

Cells exchange gases with the extracellular fluid. Oxygen, which is required for oxidative phosphorylation to produce energy-rich molecules, enters cells, and the waste product carbon dioxide leaves.

Secretion and Excretion

Cells produce secretions (useful substances) and excretions (waste products). For example, many cells constituitively secrete components of their extracellular matrix.

• Specialized cells (e.g., glandular epithelial cells and neurons) secrete cell products, usually by regulated exocytosis (via secretory vesicles).
  • Exocrine glandular epithelial cells secrete their products out of the body onto cutaneous surfaces, or into mucosa-lined hollow viscera.
  • Endocrine glandular epithelial cells secrete their products into extracellular fluids, including the blood stream.
  • Neurons synthesis and release neurotransmitters.

Related Professions

A cell biologist is a scientist that studies the structure and function of cells.
A cytologist is an anatomist that specializes in the structure of cells.
A cell physiologist is a scientist that studies cell functions and processes.
A cytopathologist (or diagnostic cytologist) is a pathologist trained to examine isolated cells for signs of disease.

History

"Of the Schematisme or Texture of Cork, and of the Cells and Pores of some other such frothy Bodies." Scheme XI from Hooke's Micrographia. Source: Wikimedia Commons

How the cell was named

Robert Hooke (1635-1703) described the texture of cork (1665), as observed with a compound microscope (30X magnification), as an array of 'little boxes or cells". ^[2] In choosing the word cell, which comes from the Latin cellula (a small room), Hooke was comparing the cork cells he saw through his microscope to the small rooms in which monks lived.
We now know that the pattern in plant sections described and illustrated by Hooke (1665), Nehemiah Grew (1671, 1682) and Marcello Malphigi (1675, 1679) is generated by the plant cell walls. During the early 19th century, botanists gradually recognized that cells were entities rather than cavities.^[3]

How cells were discovered

Cells were first described by Antonie van Leeuwenhoek. Using simple microscopes that magnified up to 200- or 500-fold, he was the first to describe red blood cells (1674), protozoa (1675), spermatozoa (1677), and bacteria (1683). But it wasn't until the early 19th century that these corpuscles and "animalcules" were seen as cells, and that cells were found to be important components of plant and animal tissues.^[4]

Cell Theory

The cell theory,^[5] as first developed in 1839 by Matthias Jakob Schleiden (1804-1881) and Theodor Schwann (1810-1882), states that all living things are composed of one or more cells, and that cells are the basic unit of life.
Rudolf Virchow (1821-1902) corrected Schleiden and Schwann's ideas about the origin of cells (1855, 1858), and added the third basic tenet of the cell theory: All cells come from pre-existing cells ("Omnis cellula e cellula").

Light microscopy

Scientists first saw cells by using traditional light microscopes that were plagued by optical aberrations. Scientists gradually got better at grinding glass into lenses and at whipping up chemicals to selectively stain cellular parts so they could see them better. Robert Brown (1773-1858) is generally credited with being the first to appreciate that the nucleus was an important component of the cells of plants and animals (1833). By the late 1800s, biologists already had identified some of the largest organelles (e.g., mitochondria, and Golgi).

Researchers using high-tech light microscopes and glowing molecular labels can now watch biological processes in real time. The scientists start by chemically attaching a fluorescent dye or protein to a molecule that interests them. The colored glow then allows the scientists to locate the molecules in living cells and to track processes—such as cell movement, division, or infection—that involve the molecules.

Fluorescent labels come in many colors, including brilliant red, magenta, yellow, green, and blue. By using a collection of them at the same time, researchers can label multiple structures inside a cell and can track several processes at once. The technicolor result provides great insight into living cells—and is stunning cellular art.^[6]

Electron microscopy

Scanning electron microscopes allow scientists to see the three-dimensional surface of their samples. Source: National Institute of General Medical Sciences.

In the 1930s, scientists developed a new type of microscope, an electron microscope that allowed them to see beyond what some ever dreamed possible. The revolutionary concept behind the machine grew out of physicists' insights into the nature of electrons.

As its name implies, the electron microscope depends not on light, but on electrons. The microscopes accelerate electrons in a vacuum, shoot them out of an electron gun, and focus them with doughnut-shaped magnets. As the electrons bombard the sample, they are absorbed or scattered by different cell parts, forming an image on a detection plate.

Although electron microscopes enable scientists to see things hundreds of times smaller than anything visible through light microscopes, they have a serious drawback: they can't be used to study living cells. Biological tissues don't survive the technique's harsh chemicals, deadly vacuum, and powerful blast of electrons.

Electron microscopes come in two main flavors: transmission and scanning. Some transmission electron microscopes can magnify objects up to 1 million times, enabling scientists to see viruses and even some large molecules. To obtain this level of detail, however, the samples usually must be sliced so thin that they yield only flat, two-dimensional images. Photos from transmission electron microscopes are typically viewed in black and white.

Scanning electron microscopes cannot magnify samples as powerfully as transmission scopes, but they allow scientists to study the often intricate surface features of larger samples. This provides a window to see up close the three-dimensional terrain of intact cells, material surfaces, microscopic organisms, and insects. Scientists sometimes use computer drawing programs to highlight parts of these images with color.

References

1. ↑ National Center for Biotechnology Information. What is a cell?
2. ↑ Hooke R. Micrographia: Or Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses with Observations and Inquiries thereupon. London: Jo. Martyn and Ja. Allestry, 1665 Full text
3. ↑ Hall TS. History of General Physiology. Chicago: The University of Chicago Press, 1969
4. ↑ Hughes A. A History of Cytology. London: Abelard-Schuman, 1959
5. ↑ Mazzarello P. A unifying concept: the history of cell theory. Nature Cell Biology. 1999; 1:E13 - E15. Full Text
6. ↑ National Institute of General Medical Sciences. Inside the Cell. Chapter 1: An Owner's Guide to the Cell.

External Links

Genetics Home Reference

National Center for Biotechnology Information

Alberts B et al. Molecular Biology of the Cell. New York and London: Garland Science, 2002. NCBI Bookshelf

Cooper GM. The Cell - A Molecular Approach. Sunderland (MA): Sinauer Associates, Inc., 2000. NCBI Bookshelf

Lodish H et al. Molecular Cell Biology. New York: W. H. Freeman & Co., 1999. NCBI Bookshelf