Chapter 2: Cell Structure
All organisms, including humans, are composed of cells. From the single-celled bacteria to plants and complex animals such as ourselves, the cell is the fundamental unit of life.
The Cell Theory
A cell is the basic unit of life. According to the cell theory, nothing smaller than a cell is considered to be alive. A single-celled organism exhibits the basic characteristics of life. No smaller unit of life is able to reproduce and grow, respond to stimuli, remain homeostatic, take in and use materials from the environment, and become adapted to the environment. In short, life has a cellular nature.
All living organisms are made up of cells. While many organisms, such as the bacteria, are single-celled, other organisms, including humans and plants, are multicellular. In multicellular organisms, cells are often organized as tissues, such as nervous tissue and connective tissue. Even bone consists of cells (called osteocytes) surrounded by the material that they have deposited. In general, it is important to recognize that the structure of a cell is directly related to its function.
How Cells Are Organized
Biologists classify cells into two broad categories—prokaryotes and eukaryotes. The primary difference between a prokaryotic cell and a eukaryotic cell is the presence or absence of a nucleus, a membrane-bound structure that houses the DNA. Prokaryotic cells lack a nucleus, whereas eukaryotic cells possess a nucleus. The prokaryotic group includes two groups of bacteria: eubacteria and archaebacteria.
Despite their differences, both types of cells have a plasma membrane, an outer membrane that regulates what enters and exits a cell. The plasma membrane is a phospholipid bilayer—a
"sandwich" made of two layers of phospholipids. Their polar phosphate molecules form the top and bottom surfaces of the bilayer, and the nonpolar lipid lies in between. The phospholipid bilayer is selectively permeable, which means it allows certain molecules—but not others—to enter the cell. Proteins scattered throughout the plasma membrane play important roles in allowing substances to enter the cell. All types of cells also contain cytoplasm, which is a semifluid medium that contains water and various types of molecules suspended or dissolved in the medium. The presence of proteins accounts for the semifluid nature of the cytoplasm.
The cytoplasm of a eukaryotic cell contains organelles, internal compartments that have specialized functions. Originally the term organelle referred to only membranous structures, but we
will use it to include any well-defined subcellular structure. Eukaryotic cells have many types of organelles. Organelles allow for the compartmentalization of the cell. This keeps the various cellular activities separated from one another.
The Plasma Membrane and How Substances Cross It
The plasma membrane is a phospholipid bilayer with attached or embedded proteins. A phospholipid molecule has a polar head and nonpolar tails. When phospholipids are placed in water, they naturally form a spherical bilayer. The polar heads, being charged, are hydrophilic (attracted to water). They position themselves to face toward the watery environment outside and inside the cell. The nonpolar tails are hydrophobic (not attracted to water). They turn inward toward one another, where there is no water.
At body temperature, the phospholipid bilayer is a liquid. It has the consistency of olive oil. The proteins can change their position by moving laterally. The fluid-mosaic model is a
working description of membrane structure. It states that the protein molecules form a shifting pattern within the fluid phospholipid bilayer. Cholesterol lends support to the membrane.
Short chains of sugars are attached to the outer surface of some protein and lipid molecules. These are called glycoproteins and glycolipids, respectively. These carbohydrate chains, specific
to each cell, help mark the cell as belonging to a particular individual. Other glycoproteins have a special configuration that allows them to act as a receptor for a chemical messenger, such as
a hormone. Some plasma membrane proteins form channels through which certain substances can enter cells. Others are either enzymes that catalyze reactions or carriers involved in the passage
of molecules through the membrane.
Plasma Membrane Functions
The plasma membrane isolates the interior of the cell from the external environment. In doing so, it allows only certain molecules and ions to enter and exit the cytoplasm freely. Therefore, the
plasma membrane is said to be selectively permeable. Small, lipid-soluble molecules, such as oxygen and carbon dioxide, can pass through the membrane easily. The small size of water
molecules allows them to freely cross the membrane by using protein channels called aquaporins. Ions and large molecules cannot cross the membrane without more direct assistance.
Diffusion
Diffusion is the random movement of molecules from an area of higher concentration to an area of lower concentration until they are equally distributed. Diffusion is a passive way for molecules to enter or exit a cell. No cellular energy is needed to bring it about.
Osmosis
Osmosis is the net movement of water across a selectively permeable membrane. The direction by which water will diffuse is determined by the tonicity of the solutions inside and outside the cell.
Tonicity is based on dissolved particles, called solutes, within a solution. The higher the concentration of solutes in a solution, the lower the concentration of water, and vice versa. Typically water will diffuse from the area that has less solute (low tonicity, and therefore more water) to the area with more solute (high tonicity, and therefore less water).
Facilitated Transport
Many solutes do not simply diffuse across a plasma membrane. They are transported by means of protein carriers within the membrane. During facilitated transport, a molecule is transported across the plasma membrane from the side of higher concentration to the side of lower concentration. This is a passive means of transport because the cell does not need to expend energy to move a substance down its concentration gradient. Each protein carrier, sometimes called a transporter, binds only to a particular molecule, such as glucose. Type 2 diabetes results when cells lack a sufficient number of glucose transporters.
Active Transport
This is the most complicated and the most common type of transport in the cell. So basically during active transport, a molecule is moving from an area of lower to an area of higher concentration. One example is the concentration of iodine ions in the cells of the thyroid gland. In the digestive tract, sugar is completely absorbed from the gut by cells that line the intestines. In another example, water homeostasis is maintained by the kidneys by the active transport of sodium ions (Na+) by cells lining kidney tubules.
Active transport requires a protein carrier and the use of cellular energy obtained from the breakdown of ATP. When ATP is broken down, energy is released. In this case, the energy is used to
carry out active transport. Proteins involved in active transport often are called pumps. Just as a water pump uses energy to move water against the force of gravity, energy is used to move substances against their concentration gradients. One type of pump active in all cells moves sodium ions (Na+) to the outside and potassium ions (K+) to the inside of the cell. This type of
the pump is associated especially with nerve and muscle cells.
The passage of salt (NaCl) across a plasma membrane is of primary importance in cells. First sodium ions are pumped across a membrane. Then chloride ions diffuse through channels that allow their passage. In cystic fibrosis, a mutation in these chloride ion channels causes them to malfunction. This leads to the symptoms of this inherited (genetic) disorder.
Bulk Transport
Cells use bulk transport to move large molecules, such as polysaccharides or polypeptides, across the membrane. These processes use vesicles rather than channel or transport proteins. During endocytosis, a portion of the plasma membrane invaginates or forms a pouch, to envelop a substance and fluid. Then the membrane pinches off to form an endocytic vesicle inside the cell. Some white blood cells are able to take up pathogens (disease-causing agents) by endocytosis. This process is given a special name: phagocytosis. Usually, cells take up small molecules and
fluid, and then the process is called pinocytosis.
During exocytosis, a vesicle fuses with the plasma membrane as secretion occurs. Later in this chapter, we will see that a steady stream of vesicles moves between certain organelles, before finally fusing with the plasma membrane. This is the way that signaling molecules, called neurotransmitters, leave one nerve cell to excite the next nerve cell or a muscle cell.
One form of endocytosis uses a receptor, a form of membrane protein, on the surface of the cell to concentrate specific molecules of interest for endocytosis. This process is called receptor-mediated endocytosis. An inherited form of cardiovascular disease occurs when cells fail to take up a combined lipoprotein and cholesterol molecule from the blood by receptor-mediated endocytosis.
CREATORS' THOUGHTS 
THEARACHNID
THEARACHNID I am preparing for my first med school final exam. Wish me luck.
— New chapter is coming soon — Write a review