The cell membrane is the outer living boundary of the cell. It helps give a cell mechanical strength and shape and regulates the passage of molecules into and out of the cell. Because of this second function it is largely responsible for maintaining cellular homeostasis. The structure of the cell membrane is largely related to its function.

The membrane is composed of a phospholipid bilayer (which has a fluid consistency) in which proteins are wholly or partially imbedded (in a mosaic pattern).

The membrane forms a solubility barrier which separates the contents of the enclosed volume from its surroundings.

Phospholipids - Contain a phosphate group in place of the third fatty acid. The phosphate group can ionize forming a polar head while the two fatty acids form a nonpolar tail. The cell membrane is a phospholipid bilayer in which the heads face outward because they are hydrophilic (water loving) and the tails face inward because they are hydrophobic (water repelling).

Integral membrane proteins are imbedded in the membrane (non-polar R groups of the protein participate in hydrophobic interaction with non-polar interior of lipid bilayer). Peripheral proteins associate with membrane surface by ionic interactions and hydrogen bonds. Supported by evidence from freeze-fracture electron microscopy.

Carbohydrates if present are components of glycolipids or glycoproteins and are oriented to the outside of the plasma membrane. Glycolipids are constructed similarly to phospholipids except that the polar head consists of a chain of carbohydrate molecules. Glycoproteins are protein molecules that span the membrane with attached carbohydrate chain. Animal cell membranes also contain cholesterol molecules. [These molecules lend stability to the lipid bilayer and prevent a drastic decrease in fluidity at low temperatures.]

Enzymatic proteins catalyze a specific reaction

Membrane transport properties: The structure of the lipid bilayer dictates that most polar compounds are unable to pass through it unless assisted by some transport mechanism. Large molecules (macromolecules) cannot freely cross the membrane. Small noncharged molecules, particularly if they are lipid soluble, have no difficulty crossing the membrane. Gases can also diffuse through the lipid bilayer (ex. Lungs). Water passes into and out of cells with relative ease.

Because passage is restricted, the cell membrane is said to be selectively permeable (or differentially permeable)

Diffusion (shown left) is the random movement of molecules in a solution which leads to a generally uniform dispersion of molecules. Diffusion occurs spontaneously and minimizes concentration differences. Diffusion is the net movement of a substance (liquid or gas) from an area of higher concentration to one of lower concentration. Since the molecules of any substance (solid, liquid, or gas) are in motion when that substance is above absolute zero (0 degrees Kelvin or -273 degrees C), there is available energy for movement of the molecules from a higher potential state to a lower potential state. The majority of the molecules move from higher to lower concentration, although there will be some that move from low to high. The overall (or net) movement is thus from high to low concentration. Eventually, if no energy is input into the system the molecules will reach a state of equilibrium where they will be distributed equally throughout the system. Water, carbon dioxide, and oxygen are among the few simple molecules that can cross the cell membrane by diffusion (or a type of diffusion known as osmosis ). Diffusion is one principle method of movement of substances within cells, as well as method for essential small molecules to cross the cell membrane. Gas exchange in gills and lungs operates by this process. Carbon dioxide is produced by all cells as a result of cellular metabolic processes. Since the source is inside the cell, the concentration gradient is constantly being replenished/re-elevated, thus the net flow of CO2 is out of the cell. Metabolic processes usually require oxygen, which is in greater concentration outside the cell, thus the net flow of oxygen is into the cell.

Osmosis (Study fig. 4.5 pg. 66, a thistle tube) is the diffusion of water across a selectively permeable membrane to an area of higher solute concentration. More on that later in the notes.

Membrane transport properties: Most polar compounds are unable to pass through the lipid bilayer unless assisted by some transport mechanism. Transport may be passive, facilitated or active.

Passive transport (left) involves movement through actual penetration of the compound into and through the lipid bilayer or it may involve passage through a pore. Some integral membrane proteins serve as channel proteins which form a pore or small opening through which compounds can pass. Passive transport requires no energy from the cell. Examples include the diffusion of oxygen and carbon dioxide, osmosis of water, and facilitated diffusion Facilitated transport (facilitated diffusion) [fig 4.7] involves the participation of an integral membrane protein which may change conformation to assist passage of the solute. The shape change facilitates release of the solute on the other side of the membrane. Passive and facilitated transport always occur down a concentration gradient and therefore do not require energy.

Active transport [fig. 4.8] may involve movement of solute up a concentration gradient but can do so only with expenditure of energy from some energy source. An example is the sodium, potassium pump which utilizes ATP to pump sodium ions out and potassium ions in to cells [fig. 4.9]. The sodium-potassium pump is associated with nerve and muscle cells. A change in carrier shape after the attachment and again after the detachment of a phosphate group allows it to alternately combine with sodium ions and potassium ions. The phosphate group is donated by ATP. Up to one-third of the ATP used by a resting animal is used to reset the Na-K pump.

[Diffusion is the random movement of molecules in a solution which leads to a generally uniform dispersion of molecules. Diffusion occurs spontaneously and minimizes concentration differences.]

During diffusion, molecules move from higher to lower concentration.

What factors affect the rate of diffusion?

The higher the temperature the faster the rate of diffusion (Why? Collisions)

Higher the solute concentrations will also increase the collisions and therefore rate.

Agitation will also increase the rate of diffusion (More collisions)

The greater the amount of exposed cell membrane surface area will also play a large role in the rate of diffusion across a membrane.

Endocytosis (left): process of cellular uptake of macromolecules and particulate matter whereby a membrane invaginates to enclose the material and then pinches off to form an intracellular vesicle. Requires energy. Phagocytosis is the endocytosis of very large molecules [bacteria, red blood cells. This process is visible with light microscope] Pinocytosis is the endocytosis of large molecules [proteins. This process is visible only with the electron microscope.]

Osmosis is the diffusion of water across a selectively permeable membrane. Net movement of water is from a hypotonic solution (low solute concentration or high water concentration) to hypertonic solution (higher solute concentration or lower water concentration). If solutions are isotonic there is no net movement of water across the membrane.

hypotonic solution - lower solute concentration or higher water concentration outside of cell

hypertonic solution- higher solute concentration or lower water concentration outside of cell

isotonic - solute concentration or water concentration outside of cell same as inside of cell

hypotonic solution - there is a net movement of water into the cell (may cause lysis)

hypertonic solution - there is no net movement of water out of the cell

isotonic - there is no net movement of water across the membrane

hypotonic solution - may cause lysis in animals, turgor pressure develops in plants

hypertonic solution - causes crenation in animals, causes plasmolysis in plants

See the diagram below to better understand the effects of different solutions

Complete potato disc assignment.

What limits cell size? Since volume of a sphere increases as the cube of the radius, while surface area only increases as the square of the radius, the surface area will relatively decrease in size as a cell grows larger. Smaller cells will have a relatively larger ratio of surface area to volume than larger cells. Conversely, as a cell grows larger, its surface area to volume ratio will decrease. Since the surface area (cell membrane) is the only entry or exit for substances needed by or excreted from cells, the relative size of the cell membrane is of prime importance to the efficiency with which cells can metabolize or synthesize.