Cell Structure

Within humans and other multicellular organisms, many different cell types can be found. These include blood cells, nerve cells, and muscle cells. Many of these cells bind together to form more complicated tissues that perform specific functions for the body. Each of these cells starts its life as a generic cell, with no primary function. During development, these generic cells are exposed to different chemical signals that guide them down specific pathways. These signals cause the cells to become more specialized through the activation of certain genes found within their nuclei. These genes turn on or off, causing the cell to develop particular traits and functions.

Cell differentiation is the process by which cells change from one type to another. As cells differentiate, they form different structures within them. For example, cells found within the heart would have high concentrations of mitochondria in order to provide a constant supply of energy for the contracting tissues.

An example of cellular differentiation is the process that occurs in the very early stages of human development when the human embryo forms from a structure called a gastrula. The gastrula is formed from three distinct layers of cells called germ layers. Each germ layer gives rise to specialized cells that will go on to form specific organs of the body. The endoderm is the innermost layer and gives rise to the lining of the digestive tract, respiratory tract, pancreas, and liver. The mesoderm is the middle layer and gives rise to organs such as the heart, blood vessels, muscles, and bones. The ectoderm, the outer cell layer, gives rise to the nervous system, skin, and structures derived from the skin (hair and nails, for example).

Human cells are classified as eukaryotic cells. A eukaryote is a cell type with DNA enclosed in a distinct nucleus and membrane-bound organelles. This means the DNA is separated from the rest of the cell in the nucleus by the nuclear membrane. This separation is important in the regulation of protein synthesis, as the cellular structures that produce proteins are found outside the nucleus. The nuclear membrane limits the type of molecules that can reach the DNA and also limits the type of molecules that can leave the nucleus and reach the protein-synthesis machinery of the cell. Eukaryotic cells have very precise regulations that guide the production of proteins and their ribonucleic acid (RNA) intermediates. RNA is a nucleic acid that carries instructions from DNA for protein synthesis (however, some viruses carry genetic information in their RNA).

One feature of human cells is the variety of their shapes and sizes. The shape of the cell is related to the function it performs. Nerve cells, for example, have finger-like projections that allow them to communicate over a distance. In addition, cells aggregate to form larger structures, such as tissues and organs, that perform specific functions.

There is a limit to how large cells can get. Most cells remain small in order to maintain continuity of the internal parts and to ensure that materials can be easily transported back and forth across the membrane. Having a small size also increases the surface area to volume ratio of the cell. As cells grow larger, their internal volume increases. At the same time, the amount of surface area, per unit volume, across which materials can pass decreases. This means there is less area for nutrients and gases to move back and forth. When cells get too large, they divide, thereby decreasing the internal volume and increasing their surface area.

Human cells contain a variety of internal structures, each called an organelle. An organelle is a structure in a cell that has a specific task, such as a mitochondrion, Golgi body, or centriole.

The nucleus directs a cell's growth, division for reproduction, activities, and death. The nucleus houses most of the DNA. Within the nucleus is the nucleolus, a structure in which clusters or subunits of ribonucleic acid (RNA) form. RNA carries the message encoded on DNA from the nucleus into the cytoplasm, the watery interior of the cell.

An endomembrane system is a system of membranes suspended within eukaryotic cells to partition the cells into functional compartments, such as the Golgi apparatus and endoplasmic reticulum. The endoplasmic reticulum (ER) is a network of membranes that helps process molecules in a cell and transports cell materials. The endoplasmic reticulum may be rough, if studded with ribosomes (organelles that bind messenger RNA and transfer it to produce proteins), or smooth, without ribosomes. The Golgi apparatus is an organelle that attaches chemical markers to molecules produced in the endoplasmic reticulum in order to transport the molecules to their places inside or outside a cell. It is a group of stacked and flattened membrane sacs. Molecules move from the endoplasmic reticulum through the successive sac of the Golgi apparatus, where chemical markers are added. These markers direct the molecules to their correct locations inside or outside a cell.

A lysosome is an organelle that digests bacteria that enter a cell, eliminates toxins, and recycles unneeded cell materials. A peroxisome is a structure in eukaryotes that transforms fatty acids into sugars. Peroxisomes also contain enzymes involved in the degradation of lipids and alcohols.

Called the power plant of eukaryotic cells, a mitochondrion is an organelle that changes energy from food into energy a cell can use. It has a smooth outer membrane and an inner membrane that is highly folded. Human cells have numerous mitochondria, which are the sites of cellular respiration, an activity that harnesses chemical energy from food. Mitochondria also have their own DNA and can replicate independently of the cell's replication.

There are two other common structures in cells. The cytoskeleton is a network of filaments that gives the cell its shape and forms the support network for cell functions, such as cell division. The centrosome is a structure in the cytoplasm of a cell that coordinates the formation of microtubules, which allows cell division to proceed during reproduction. The centrosome directs the formation of the spindle involved in cell division. Together, the centrosome and the spindle ensure that each new cell has an equal number of chromosomes (structures of nucleic acids and proteins that carry genetic material) by pulling the chromatids (two strands that result from the duplication of DNA) apart during reproduction.

Cells of the human body are surrounded by a structure called the cell membrane. The cell membrane is a structure that encloses the cell, made of the phospholipid bilayer. The cell membrane has many functions. First, it defines a cell by enclosing cellular contents within the cell.

Secondly, it creates a barrier that selectively controls what enters the cell from the outside environment and what substances leave the cell. Because of selective permeability, only specific molecules are able to cross the cell membrane. Essential molecules such as nutrients are able to enter the cell, while waste is able to exit the cell. Molecule size also plays a role in this permeability. Small molecules such as water and oxygen are able to freely cross the cell membrane, but larger molecules such as sugars are regulated.

Lastly, the cell membrane aids in cell communication. Receptors are proteins that are found on the surface of cell membranes or inside the cell. Receptors can serve as either receivers of extracellular signals or activators of intracellular processes. For example, some cells have receptors for hormones that play a role in the regulation of biological processes in which the cell takes part. The binding of the hormone to the receptor changes the activity of the cell. Cells also have receptors that can bind to circulating immune cells, helping the immune system detect the presence of disease-causing organisms or viruses.

Cell membranes all have the same basic structure, composed of two facing layers of lipids. A phospholipid is a lipid molecule composed of two fatty acid tails. Both fatty acid tails are attached to a phosphate group head by a glycerol molecule. The glycerol molecule also attaches to a phosphate group. Phospholipids create a specific structural design of the cell membrane, called the phospholipid bilayer. A phospholipid bilayer is a cell membrane structure made of a double layer, or bilayer, of hydrophilic phospholipid heads that connect the extracellular fluid and the cytoplasm with a middle region of hydrophobic tails. Their behavior in aqueous solutions assembles them into bilayers. The phospholipid heads of the bilayer are hydrophilic, which means having a strong affinity toward water, so the heads face outside the cell in contact with the extracellular fluid (watery solution outside cells) and inside the cell in contact with the cytoplasm (contents of the cell enclosed by the cell membrane, not including the nucleus). The tails are hydrophobic, which means having a weak or no affinity to water. Therefore, the tails of each layer face each other because the tails are resistant to water. Within the cell membrane, protein and lipid molecules can have short carbohydrate chains coming off the cell surface. These are called glycoproteins and glycolipids, respectively. The cholesterol component of cell membranes stabilizes the membrane and plays a role in cell signaling.