Transport Across the Cell Membrane

B. Transport proteins—these will expend ATP (cellular energy) in order to transport molecules into or out of the cell. This is referred to as “active transport” because it requires energy in order to allow for the transport.

C. Recognition proteins—these are cells that allow others to recognize the cell as being a

self-cell and distinguish the cell as being of a certain type. They have short polysaccharide (or sugar) chains extending from the surface of the cell.

D. Adhesion proteins—these are proteins that attach to neighboring cells. They provide anchors for the internal filaments and tubules within the cells to provide stability and structure for the cell.

E. Receptor proteins—these are located in and on the membrane and allow trigger

molecules such as hormones to act on the interior of the cells. These are basically binding proteins.

F. Electron transfer proteins—these are involved from one molecule to another during various chemical reactions in the cell membrane.

TRANSPORT ACROSS THE CELL MEMBRANE

There is passive and active transport across the cell membrane. Passive transport does not require the use of energy. Simple diffusion is the net movement of substances from an area of higher concentration to an area of lower concentration. There is random and constant movement of molecules that pass through the cell membrane to an area of lower concentration (also referred to as going down a gradient).

Facilitated diffusion is the diffusion of solutes through the aid of proteins in the membrane. The biggest example of this is the facilitated diffusion of water with the help of aquaporins. This does not require ATP energy input but requires the aquaporin proteins. Osmosis refers to the diffusion of water molecules across a semipermeable membrane. When water moves into a

cell by osmosis, hydrostatic pressure, also called osmotic pressure, can build up in the cell. On

the other hand, dialysis is the term used to describe the diffusion of solutes (usually of a small size) across a selectively permeable membrane.

Active transport involves the movement of solutes against a gradient. It requires the expenditure of ATP energy. There are two possible mechanisms by which this can occur. The first is through protein pumps. These are also known as ion pumps, which are membranebound transport pumps that transfer ions (potassium, sodium, hydrogen, and chloride, for example), amino acids, or monosaccharides across a membrane.

In active transport, ATP changes the protein’s shape so that it will release the bound molecule on the opposite side of the membrane. The protein pumps are specific to the molecule being transported. The protein pumps are ATPases (ATP enzymes) that catalyze the reaction that takes ATP and turns it into ADP, releasing energy used to transport the molecule. The most well-known pump is the sodium-potassium pump or Na+/K+-ATPase enzyme, which actively transports sodium out of the cell and potassium into the cell. These are found on the membranes of every cell in the body.

Vesicular transport involves the transport of large particles and large macromolecules across the plasma membrane. There are two major types of vesicular transport: endocytosis and exocytosis. Exocytosis involves the fusion of vesicles made inside the cell to the cell membrane. The vesicle opens up and extrudes the contents out of the cell. In endocytosis, vesicles are outside of the cell membrane and fuse with the cell membrane; the contents of the vesicle are discharged into the cell.

There are three kinds of endocytosis, known as the following:

A. Phagocytosis—this is referred to as “cell eating.” The dissolved materials enter the cell

with the solid material forming a phagocytic vesicle in the cell. They fuse with lysosomes inside the cell, allowing for the solid material to be broken down.

B. Pinocytosis—this is referred to as “cell drinking.” The plasma membrane folds inward to form a channel in which the dissolved substances enter the cell and are enclosed in a pinocytic vesicle.

C. Receptor-mediated endocytosis—this occurs when specific molecules in the extracellular space bind to a receptor site. The plasma membrane folds inward and forms a vesicle inside the cell that contains the molecules.

CELL CYTOPLASM

The cytoplasm is the gel-like material that makes up the bulk of the inside of the cell. It consists of the cytosol (the liquid component), which is 89-90 percent water. It also contains salts, organic molecules, and numerous enzymes. Also dissolved in the cytosol are proteins and cell nutrients. In addition, the cytoplasm contains the organelles that serve functions inside the cell. There is a cell matrix that keeps the organelles and other compartments separated within the cell interior.

The cytoskeleton consists of threadlike proteins that continually adapt to the cell’s needs, which are continually changing. The cytoskeleton helps to maintain their shape and allows the contents of the cell to move. The cytoskeletal network is made of three components:

A. Microtubules—these function as the framework for organelles and vesicles to move within the interior of the cell. These are the thickest of the structures of the cytoskeleton. They are made from the tubulin protein subunit, forming the mitotic spindles that allow the cell to divide during mitosis. Along with the intermediate filaments and microfilaments, the microtubules will help give the cell its shape.

B. Microfilaments—these provide mechanical support for the cell, enable cell movements, and determine cell shape. They have an arrow shape, with a barbed end and a pointed end. The protein subunit of microfilaments is actin. They predominate in muscle cells and in cells that need to change their shape quickly. Phagocytes also contain a lot of microfilaments.

C. Intermediate filaments—these are intermediate in diameter compared to microtubules and microfilaments. In contrast to microfilaments and microtubules, the intermediate filaments are not directly involved in cell movements. Instead, they appear to play a structural role by providing mechanical strength to cells and tissues.

ORGANELLES

Organelles are the structures inside the cytoplasm that physically separate the different

metabolic activities that occur within the cells. The organelles are each responsible for doing a particular function within the cell. There are numerous types of organelles within the cell structure—from the nucleus, which is prominent within the cell, to the small lysosomes, which contain enzymes that break down particles and unwanted waste within the cell.

NUCLEUS

The nucleus ultimately controls the cell and contains DNA, which is the genetic material inside

the cell. It is the largest organelle in most cells, although some cells can have more than one nucleus or no nucleus at all. Skeletal muscle is an example of a cell type that has multiple nuclei, while RBCs have lost their nucleus by the time they are mature. The nucleus has a phospholipid bilayer, which is similar to the plasma membrane.

Most of the genetic information of the cell is contained within the nucleus. DNA is made into 22 matching pairs of chromosomes and a nonmatching “pair,” the X and Y chromosomes. Chromosomes are not actually visible until the time of cell division when the chromatin condenses and forms thicker chromosomes that can be pulled apart into two separate cells. Histone proteins bind and organize the DNA into bundles, known as nucleosomes. Nucleoli can be within the nucleus, which are made from DNA that is in the process of making ribosomes. These ribosomes are sent to the cytoplasm to manufacture proteins from amino acids.