Cell Membrane Dynamics: Models, Roles, and Molecular Transport

Plasma Membrane: Structure and Function

Key Functions of the Plasma Membrane

• Protects cell shape and provides rigidity.
• Regulates the entry and exit of substances.

Models of the Plasma Membrane

The Daniel-Davson Model

In this model, proteins are found in blocks or internal/external layers, separated by a lipid bilayer. This bilayer is predominantly composed of phospholipids, which are bipolar.

The Singer-Nicholson Fluid Mosaic Model

In this model, proteins are internalized within the phospholipid bilayer. Some smaller proteins are located on the outer and inner sides. The membrane also contains a small proportion of carbohydrates that tend to form complexes with proteins and lipids. Among the proteins that span the membrane, defined pores allow the entry of molecules, including smaller ones. The membrane does not function like a rigid sieve; instead, it exhibits movement. Therefore, this membrane is known as a fluid mosaic.

Types of Membrane Transport

Glossary of Related Terms

ATP (Adenosine Triphosphate)

The main chemical used by living systems for energy storage, consisting of a base (adenine) attached to a sugar (ribose) and three phosphate groups.

Sodium-Potassium Pump

A mechanism that uses energy from ATP to restore the original distribution of sodium and potassium concentrations across the membrane after nerve impulse transmission.

Diffusion

The spontaneous movement of particles from an area of high concentration to an area of low concentration.

Endocytosis

(Greek Endon = inside; kytos = cell): The incorporation of material from outside the cell inwards by the formation of a vesicle in the plasma membrane that surrounds the material, allowing it to be incorporated into the cell. Includes:

1. Phagocytosis
2. Pinocytosis
3. Receptor-mediated endocytosis

Exocytosis

The process in which a vesicle first fuses with the plasma membrane and then opens, releasing its contents to the outside.

Phagocytes

(Greek phages = glutton; kytos = cell): Literally "cell binge," derived from Phagocytosis, a form of endocytosis in which the cell engulfs solid particles, bacteria, or viruses for destruction.

Phagocytosis

A form of endocytosis in which blood cells (phagocytes) engulf particles (including bacteria).

Glycolysis

The universal cellular metabolic process in which glucose (6 carbons) is broken down into two molecules of pyruvate (three carbons), producing ATP and NADH in the process.

Homeostasis

(From Greek homoios = similar; stasis = standing still): The ability to maintain a relatively constant internal environment.

Osmosis

(Greek osmos = impulse/push): The movement of water molecules through a membrane in response to differences in solute concentration. Water moves from areas of high water concentration/low solute concentration to areas with low water concentration/high solute concentration. This is the movement of water through a semipermeable barrier, such as the cell membrane, from a high water potential to a low water potential.

Active Transport

The transport of molecules against a concentration gradient (from regions of low concentration to regions of high concentration) using cell membrane proteins and energy from ATP.

Passive Transport

Diffusion through the cell membrane without energy expenditure by the cell.

Vacuoles

Membrane-bound spaces found in most animal and plant cells that store foodstuffs and remove waste products.

Vesicles

Small, membrane-bound spaces in most animal and plant cells that transport macromolecules to the outside and inside of the cell, and between cellular organelles.