What Is A Cell?

All cells have an outer skin-like layer called the plasma membrane that surrounds the internal cytoplasm. The plasma membrane is made of a double layer of phospholipid molecules, proteins, and cholesterol known as the phospholipid bilayer. Acting like a security guard, the plasma membrane controls what can enter and exit the cell and helps to regulate the internal environment of the cell.

Phospholipids make up most of the membrane and are permeable to very small molecules, such as oxygen and carbon dioxide, and nonpolar substances, such as lipids. Proteins in the plasma membrane perform many roles, including transporting ions and large substances through the membrane and communicating with other cells. Cholesterol helps to plug small gaps in the plasma membrane and makes the membrane more flexible.

The plasma membrane is considered a part of the endomembrane system—more on that below.

The cytoplasm includes all the material of the cell inside the plasma membrane and consists of cytosol and organelles.

• Cytosol is a liquid solution surrounding the organelles and filling the interior of the cell. Many solutes, including oxygen, sugars, ions, and proteins, are dissolved in the watery cytosol and are used by the organelles.
• Organelles are tiny organ-like structures that perform the functions of the cell, including making energy and reproduction.

All cells, whether prokaryotic or eukaryotic, contain a cytoskeleton, DNA, and ribosomes.

• The cytoskeleton is a strong, yet flexible, framework of protein fibers and filaments that span the interior of the cell. It provides shape to the cell and helps to anchor the other organelles.
• DNA, deoxyribonucleic acid, is a complex molecule that stores the genetic code of instructions for the cell’s functions. In prokaryotes, the DNA forms a structure known as a nucleoid, while in eukaryotes it forms chromosomes.
• Ribosomes are small structures that produce proteins. The genetic code in DNA is used to make mRNA molecules that provide the code for the ribosomes to produce proteins.

Some cells, both prokaryotic and eukaryotic, contain structures for movement. Flagella are long, whip-like structures that arise from the cytoskeleton and protrude through the plasma membrane. When flagella rotate, they push the cell like a boat motor. Cilia are shorter hair-like extensions that stick out of the plasma membrane. Cilia move back and forth like the oars of a boat, either moving the cell or moving substances around the cell. The cells of the upper respiratory tract in the human body are ciliated, allowing them to move mucus out of the lungs.

Unlike prokaryotes, eukaryotic cells contain several complex membrane-bound organelles that form the endomembrane system.

The endomembrane system begins with the nucleus, an organelle that houses the DNA and keeps it separate from the rest of the cell. Its exterior consists of a membrane, the nuclear envelope, which contains many small pores to allow small molecules to enter and exit the nucleus.

A long, convoluted network of tube-like membranes, known as the endoplasmic reticulum, extends from the nuclear envelope. Closer to the nucleus, the endoplasmic reticulum (or ER, for short) has many ribosomes embedded into its membrane to facilitate protein production. Viewed under the microscope, the ribosomes make the membrane look rough and irregular, giving it the name rough ER.

Further from the nucleus, there are no ribosomes, resulting in a region of long membranes known as the smooth ER. Small spheres of this membrane break off from the smooth ER to form vesicles, which are used to transport proteins and other large molecules across the plasma membrane.

Another set of flattened membranes, the Golgi apparatus, reside between the smooth ER and the plasma membrane. The Golgi apparatus plays a key role in the final processing and packaging of proteins that are used in the cell or shipped out of the cell.

Another group of membrane-bound organelles are used for transporting substances throughout a cell. Vesicles are tiny spheres made of the same material as the plasma membrane. Vesicles can form during endocytosis, when a section of the plasma membrane surrounds a substance outside of the cell and pulls it into the cell. They can also form from a section of the Golgi apparatus and transport proteins to the plasma membrane. At the membrane, they fuse with the membrane and release their contents out of the cell in a process known as exocytosis.

Some cells (like plant cells) contain vacuoles, which are large vesicles used for long-term storage of substances like water, food, or waste materials.

Finally, lysosomes and peroxisomes are vesicles formed by the Golgi apparatus that contain digestive enzymes. Lysosomes act like the stomach of the cell, chemically digesting larger molecules so that their building blocks can be used by the cell. Peroxisomes contain hydrogen peroxide and play a vital role in the digestion of fatty acids.

Despite their location in membranes, peroxisomes are technically not part of the endomembrane system because they do not receive vesicles from the Golgi apparatus.

A very special set of eukaryotic organelles, the plastids, are membrane-bound organelles that contain their own DNA and reproduce independently from the rest of the cell.

Mitochondria, found in all eukaryotes, are plastids that use aerobic respiration to convert glucose into chemical energy in the form of adenosine triphosphate (ATP). Aerobic respiration greatly increases the amount of ATP produced from glucose. A small portion of our DNA is also found in the mitochondria. This mitochondrial DNA (mtDNA) is of interest in DNA testing.

In autotrophic cells, chloroplasts use sunlight to convert water and carbon dioxide into glucose through the process of photosynthesis. Chloroplasts contain the green pigment chlorophyll, which absorbs sunlight to power the chemical reactions of photosynthesis.

Although cells come in many different varieties and contain different structures, they all perform many of the same functions. The main functions of cells are:

All cells must grow to survive. Sometimes this growth is an increase in size, while at other times it may be the growth of new internal structures.

Cells must be able to move, whether it is movement from one location to another or the movement of substances within the cell. Cells use their plasma membranes to control the transportation of substances into and out of the cell. Chemicals and organelles move through the cytoplasm to reach their destinations within the cell.

All cells are born from other cells through the processes of mitosis and meiosis. To stay alive, cells must replace dead cells.

All cells must perform many chemical reactions to obtain energy and the raw materials to build new structures and to break down old structures. The sum of these reactions is metabolism.

• Catabolic reactions break down large molecules into smaller molecules and release energy
• Anabolic reactions use energy to build large molecules from smaller ones.

All cells must respond to changes in their internal and external environment. They must find food, fight off viruses or other cells, and react to changes in their surroundings.

Cells communicate with one another using several different methods. Chemical messengers, such as hormones and signaling molecules, allow cells to communicate over long distances. Molecules on the surface of cells allow cells to interact with each other and recognize their surrounding cells.

Cells depend on a regulated environment that stays within strict physiological limits. The metabolic pathways of some cells can only function within a certain temperature range. Other cells require a specific pH range to perform their functions. The plasma membrane plays a critical role in controlling the chemical environment of the cell.