Cell Transport

The eukaryotic cell can be divided into two major parts: The nucleus is a separate compartment that contains the DNA of the cell. The cytoplasm is the fluid portion of the cell outside the nucleus and any of the cells internal sub-structures, called organelles. The cytoplasm is separated from the external environment by a phospholipid bilayer called the plasma membrane.

Plasma Membrane The plasma membrane of the cell provides these functions: Physical isolation between the internal cellular and external environments. Regulation of nutrient/waste exchange. Sensing changes in the environment or receiving communication signals from other parts of the body. Connecting to other cells, giving body tissues a stable structure.

The main component of plasma membranes is phospholipids. A class of lipid where one of the fatty acids is replaced by a phosphate (PO4). The two fatty acid tails are hydrophobic; insoluble in water. The phosphate head is hydrophilic; water-soluble.

The basic design of the cell membrane is a bilayer, a two- molecule thick film of phospholipids. The phosphate heads form the outer part that is in contact with the water. The fatty acids form the inner part that is away from water.

Membrane Proteins Proteins embedded in the phospholipid bilayer determine most of the membrane’s specific functions.

Receptor proteins will respond to specific chemical signals. Carrier proteins transport substances across the cell membrane.

Enzymes aid in chemical reactions. Anchoring proteins attach the plasma membrane to other structures. Recognition proteins identify the cell to the immune system as self or nonself; normal or abnormal.

Membrane Carbohydrates In addition to proteins, chains of carbohydrates are also found throughout the surface of the plasma membrane. These carbohydrates vary among species, individuals, and even cell types in an individual. Blood type (A, B, AB, O) is determined by carbohydrate markers on your red blood cell membranes.

Permeability of the Lipid Bilayer The cell membrane has selective permeability, meaning some molecules pass through, others cannot.

Permeability of the Lipid Bilayer Nonpolar (hydrophobic) molecules can dissolve in the lipid bilayer and pass through the membrane rapidly Examples: Oxygen, carbon dioxide, hormones made of lipids Large polar (hydrophilic) molecules cannot cross the membrane as easily. Examples: Glucose, sucrose, proteins

Permeability of the Lipid Bilayer The cell membrane has selective permeability, meaning some molecules pass through, others cannot.

Passive Transport Diffusion is the movement of molecules areas of greater concentration to areas of lower concentration. This is considered passive transport because no energy is required. The difference between the high and low concentrations represents a concentration gradient.

Osmosis is the diffusion of water through channel proteins embedded in the cell membrane. The direction of osmosis is determined by the concentration of solvent and solutes in a solution. Solutes are solid substances dissolved in a solution, like sugar or salts. The solvent is the liquid component of the solution.

Water follows the same rules of diffusion, moving from high to low concentration. In other words, solutions high in solutes (salt, sugar, etc) will draw in more water molecules.

Isotonic solutions, where the concentration of water molecules is the same inside and outside the cell, do not have a net movement of water. Cells placed in isotonic solutions are unaffected.

Hypotonic solutions, where there is a greater concentration of water outside the cell, will cause a net movement of water into the cell. This causes the cell to swell and burst.

Hypertonic solutions have a lower concentration of water than the interior of the cell, causing a net movement of water out of the cell. The cell shrivels.

Reverse Osmosis Filtration Reverse osmosis filters work by moving water through a selective membrane against the normal concentration gradient. This requires the application of force (pressure). The resulting water has fewer dissolved solutes.

In some forms of membrane transport, proteins aid the movement of molecules across the plasma membrane. If this process works with the concentration gradient, it is called facilitated diffusion. This process is often used when the molecule is too pass through the cell membrane on its own.

Active Transport When carrier proteins move substances against their concentration gradient, ATP energy is required. This is active transport.

Vesicular Transport Cells can move large amounts of materials at the same time through membranous sacs called vesicles. If the material is imported into the cell, the process is called endocytosis. If material is exported out of the cell, the process is called exocytosis.

Pinocytosis is a form of endocytosis also called “cell drinking,” because it occurs when the cell takes in large amounts of liquid. Phagocytosis, also called “cell eating,” occurs when the cell takes in large solid objects.

The Cytoskeleton The cytoskeleton is a framework of protein filaments that provides structure and support to the cell. The cytoskeleton of some cells forms finger-shaped projections called microvilli that increase surface area. Needed by cells that absorb a lot of material (intestines, kidneys, etc)

A centriole is a cylindrical structure made of cytoskeleton fibers. Direct the movement of chromosomes during cell division. Cilia are long, slender extensions of the plasma membrane. Able to independently move using energy from ATP.

Flagella are similar to cilia, but much longer. Sperm cells are the only human cells with flagella.