Animal cells

The cells that make up the body of an animal are the most basic units of life. Living cells use nutritional sources for energy to maintain both structure and life processes such as growth and reproduction. Life requires structure, and cells have the minimal architectural design that enables them to retain life and pass it on to future generations of cells.

Typically, cells are joined in the animal body to form larger structures called tissues. A tissue, such as connective or muscle tissue, is an aggregate of similar cells and intercellular materials that combine to perform a common function. An organ, such as the skin or the biceps, is frequently composed of several types of tissues.

Cell Structure

There are essential structural characteristics of animal cells that enable them to maintain life. Most fundamentally, an animal cell must have a border that separates it from its environment or surroundings. In animal cells, that border is called the cell membrane. Within the membrane lies the cytoplasm (literally, the plasm of the cell); outside the membrane is the environment from which the cell must extract its nutritional needs and into which it must pass the waste products that result from the numerous chemical reactions taking place inside the cell.

Structurally, the cell membrane consists of a double layer of lipid molecules. These molecules are bipolar: the head end of each molecule is hydrophilic, meaning it has an attraction to or affinity for water molecules, while the tail end of the molecule is hydrophobic, meaning it tends to repel water molecules. In the formation of the membrane, these molecules are found parallel to one another and arranged in a double row. In the inner row, their heads face the inside of the cell. In the outer row, the heads face their environment. The hydrophobic tails form the interior of the membrane and provide an effective barrier that prevents the free passage of water and water-soluble substances. Interspersed among the lipid molecules are numerous protein molecules.

Some of these proteins, called transmembrane proteins, extend entirely through the lipid bilayer and have ends exposed to both the interior and the exterior of the cell. Others, called integral proteins, are found only on one side of the membrane and extend only partway into the lipid bilayer. Finally, some proteins, called peripheral proteins, are attached to the outside or inside surfaces of the membrane. Transmembrane proteins may function as channels or pores, allowing specific ions or molecules to pass through them. Integral proteins may also function as transmembrane carriers, as they can bind to specific products outside the cell, such as a certain amino acid, and then flip-flop across the membrane to the inner side and release the product into the cell’s interior. Peripheral proteins on the outer surface of the membrane may serve as identification markers for other cells. The membrane itself functions to maintain the integrity of the cell’s cytoplasm by holding essential components inside and preventing the cell from drying out.

Cytoplasm is a general term for the viscous fluid within the cell’s membrane. In the cytoplasm are numerous small, specialized structures called cell organelles. An essential structure for living cells is the nucleus, which contains genetic information in the form of DNA. DNA determines a particular cell’s structure and function. Other organelles in the cytoplasm are designed for specific processes, such as manufacturing energy, building structural proteins, or storing cell products prior to exporting them. Some organelles are enclosed within their own membranes, which are structurally very similar to the membrane forming the cell’s outer boundary.

The Nucleus and Its Contents

The nucleus is an essential control center of the cell. It is enclosed within the nuclear envelope, a unique membrane with relatively large pores. These pores allow large information-carrying molecules called ribonucleic acid (RNA) to pass from the nucleus to the cytoplasm. Resident within the nucleus is a dark material called the chromatin. During cell division, the chromatin material condenses into clearly observable structures called chromosomes. When the cell divides into two, each daughter cell contains an equal number of chromosomes from the original maternal cell. The chromatin material within the cell’s nucleus is made up of two major types of material: DNA and associated proteins.

DNA is organized into discrete packets of information called genes, which form the backbone of the chromosomes. The genes determine the characteristics of the specific cell or organism. The DNA-associated proteins regulate the gene’s expression. At times, the genes may express themselves by replicating their information into chemical messengers called RNA, which then diffuse out through the nuclear pores into the cytoplasm. Within the cytoplasm, RNA binds to an organelle called the ribosome, which produces new protein molecules. Normally, the nucleus also contains one or more dark, round structures called nucleoli, which assist in the production of the cell’s ribosomes. A single nucleolus is made up of protein and RNA.

A particular cell may also contain hundreds of mitochondria, also typical organelles. Mitochondria are complex, double-membraned structures that are found throughout the cytoplasm of the cell. These oval-shaped structures contain numerous enzymes that stimulate energy-producing reactions, the net sum of which results in the formation of high-energy ATP molecules. These molecules diffuse throughout the cell to various other organelles and release their energy, thereby fueling most cell processes.

Lysosomes are cell organelles that are round in shape and are enclosed within a membrane. They contain many different enzymes that break down or digest various substances into simpler substances that the cell can use.

The endoplasmic reticulum (ER) consists of membrane-enclosed spaces in the cytoplasm. These complex membrane arrays are extensions of the outer cell membrane. Some of this membrane system is covered with ribosomes that function to produce protein. These parts of the ER are called rER, for rough ER or ribosomal ER. Other ER portions that lack ribosomes have a smooth appearance and are called smooth ER (sER).

Another cellular organelle, the Golgi apparatus, appears as a bunch of flattened bags. This organelle is usually not far distant from the ER. After the rER produces a protein product, the Golgi apparatus further processes the product and packages it for cellular export.

Cell Size and Function

Animal cells have great variations in size. The smallest may be as little as four micrometers in diameter, where one micrometer equals 0.001 millimeter. The largest known cell is the single-cell ostrich egg, which is about seventy-five millimeters in diameter—more than 750,000 times larger than the smallest bacterium. Somatic cells, the cells that make up the body structures of animals, are typically more intermediate in size. The human red blood cell is about seven micrometers in diameter, while an intestinal epithelial cell is about thirty micrometers in diameter. Most animal cells are approximately the same size and typically have diameters between ten and twenty-five micrometers.

An essential characteristic for cell survival is the ratio of the cell’s surface area to the volume of the cell, or its surface-to-volume ratio. Typically, cells with small diameters have large surface-to-volume ratios, while large cells have small surface-to-volume ratios. Many of the substances that are needed for the cell’s survival, such as oxygen or nutrients, enter the cell through the surface membrane by simple diffusion. The cells that have high rates of metabolism tend to be very small and have larger surface-to-volume ratios. Larger cells either have lower metabolic rates or have specialized shapes, such as numerous membrane enfoldings, to optimize diffusion of essential materials into their interiors.

In multicellular animals, cells are differentiated, meaning they have a specialized modification in their structure to enable them to perform a specific task. Thus, a striated muscle cell contains numerous myofibrils that shorten as the cell contracts, while a glandular cell may contain numerous secretory granules filled with products for export.

Animal cells can be classified on the basis of the number of chromosomes that they contain. Somatic cells, or body cells of an organism, are called diploid, because they contain the total number of chromosome pairs that are characteristic for that organism. For example, somatic cells in mosquitoes contain four pairs of chromosomes, or eight individual chromosomes. Each chromosome in the pair contains genetic information for the same genes that the other paired chromosome contains. Alternatively, sex cells or gamete cells—sperm and egg cells—contain a haploid number of chromosomes, meaning they have a single chromosome from each of the possible pairs. Thus, a human sperm cell contains a total of twenty-three individual chromosomes, while a human skin cell contains forty-six individual chromosomes.

Another way to classify animal cells is to consider the primary way that they use proteins. Some animal cells are primarily protein-secreting and manufacture much of their proteins for body use outside the cell that produces the protein. One example of this is the pancreatic acinar cell, which produces numerous digestive enzymes (proteins) and secretes them into the pancreatic duct for transport into the digestive tract, where the enzymes break down complex foods into simpler forms. Other animal cells are primarily protein-retaining cells, in which much of the manufactured protein is retained for use by the cell itself. An example of this is the keratinocyte, the prominent cell type found in the skin. This cell produces a large amount of the protein keratin, which remains stored in the cell’s cytoplasm. Because of the presence of the keratin, the skin is able to maintain its protective waterproofing function. Without keratin, the skin would lose body water.

Cell Shapes and Types

Animal cells have a wide variety of shapes. Individual cells that are mobile within an aqueous environment tend to be spherical in shape. The neutrophil, a type of white blood cell, is an example of this. Cells that are mobile within tissues and migrate from one area to another often have long cytoplasmic extensions; one such cell is the macrophage, which has long, changeable, armlike processes (projections) that enable it to migrate through tissue spaces and ingest bacteria that may be found there. Other migratory cells have a tail called a flagellum. Sperm cells, for example, use their flagella to propel themselves through the female reproductive tract to reach the egg.

Some cells have branching, stationary cytoplasmic processes through which information molecules move. Often, these processes form a complex network with similar cells, such as the network of neurons in the brain. Body tissues are made of cells that have tight cell-to-cell connections between their membranes. These cells may provide a covering for or form the wall of a particular structure. Protective cells, such as skin cells, are often many layers thick.

A typical animal’s body is composed of about two hundred different recognizable cell types. Most of these fall into four main categories: epithelial cells, connective cells, movement cells, and message cells.

Epithelial cells form a continuous layer over surfaces that are external or internal to the body. Skin is an external protective tissue that is made of many such layers of cells. The inner lining of the digestive tract, for the most part, consists of a single layer of epithelial cells that absorb and secrete materials.

Connective cells provide the structural support of the animal body. Examples of these cells are fibrocytes, found in the dermal layer of the skin, and osteocytes, found in the matrix of bone.

Cells responsible for body movement are typified by the muscle cells. Muscle cells that are attached to bone and cause limb movement are called striated muscle cells. Those found in the walls of body organs such as the stomach are called smooth muscle cells; contraction of these cells causes the contents of the stomach to be mixed and stirred. Cardiac muscle cells are found in the heart, where they contract to force the movement of blood throughout the circulatory system.

Message or conveyance cells are very diverse. Branching nerve cells have long processes that carry information molecules from one part of the body to another, such as from the spinal cord to the finger. Red blood cells carry oxygen from the lungs to the body cells. Gamete cells transfer genetic information from one organism to the next generation of organisms.

Studying Cells

Since the 1600s, when Antoni van Leeuwenhoek first used a simple magnifying lens system to study the structure of single-celled life forms, microscopes have been an important tool in cytology (the study of cells). Several types of microscopes are commonly used to study cells, whether alive or preserved.

The simplest and most common type of microscope used to study animal cells is the light (bright-field) microscope. These microscopes have magnification powers ranging from about forty to two thousand times. In their natural state, most cells are essentially colorless. In order to make microscopic viewing more effective, cells are often stained so that their structures are more readily visible.

A phase-contrast microscope is a modified light microscope that can produce visible images from transparent objects. This type of microscope is frequently used to study unstained or living cells. Its magnification range is similar to the light microscope.

A transmission electron microscope (TEM) uses electrons instead of light rays to visualize objects. A TEM passes its electron beam through very thinly sectioned cells. The density of the stained cellular structures absorbs electrons in a differential fashion so that an image corresponding to the cell’s architecture can be visualized. The TEM can magnify cellular structures from 1,000 to 250,000 times. Consequently, this type of microscopy is often used to study the small subcellular organelles.

Another popular technique used to study animal cells involves growing them in cultures outside the animal’s body. This technique, called in vitro cell culture, involves obtaining a group of living cells from an animal, usually in the form of pieces of tissue. An enzyme solution is commonly used to digest the cell-to-cell connections and produce a suspension of free individual cells. These cells are then placed in petri dishes along with a liquid medium that contains essential nutrients. Some types of animal cells, especially fibrocytes found in the connective tissues, are easily cultured with this technique. These cell cultures, if properly maintained, will grow and reproduce for generations. Experimenters can use such cell cultures to investigate how living cells function and respond to varied environmental influences.

Since cells are the basic units of life, an understanding of their function is essential for comprehending the way living organisms function. Many diseases are caused by a malfunctioning group of cells. For example, a group of cells normally undergoes an orderly sequence of growth and reproduction. At times, however, some cells become disordered and begin to multiply rapidly without stopping. This may be caused by an abnormality that appears in the genetic code or by the presence of a virus that takes over the genetic controls of the cell. This situation is typical of some types of cancer.

As scientists learn more about how cells live and why they die, they will gain valuable insights into the aging process and may thereby increase the span of life. As differentiation is better understood, scientists may be able to change mature cells or even replace them if they become damaged, destroyed, or simply worn out with age.

Principal Terms

Adenosine Triphosphate (ATP): A molecule produced in the cell that provides energy for cell processes

Amino Acid: The subunit that makes up larger molecules called proteins

Cytoplasm: The living portion of the cell that is contained within the cell membrane

Deoxyribonucleic Acid (DNA): The molecular structure within the chromosomes that carries genetic information

Differentiation: The process during development in which specialized cells acquire their characteristic structures and functions

Gamete: The sex cells of an animal, each of which contains only one chromosome from each available pair of chromosomes found in normal body cells

Gene: The part of the chromosome that includes the DNA and is the carrier of heredity

Nucleus (pl. nuclei): A central cell structure that controls the activity of the cell because of the genetic material it contains

Organelle: A subcellular structure found within the cytoplasm that has a specialized function

Protein: A substance made up of amino acids, different types of which are the chief building blocks of cellular structures

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