Plasma Membrane Structure and Transport Mechanisms

Structure of the Plasma Membrane

Membrane Amphipathic Properties

The plasma membrane is amphipathic:

• Fatty acid tails are hydrophobic (repel water).
• Phosphate head groups are hydrophilic (attract water).

Transport Through the Membrane

Factors Affecting Transport

• The concentration of substances.
• The type of molecules.

Transport Mechanisms

Based on these factors, transport can occur by three phenomena:

• Passive transport
• Active transport
• Vesicle-mediated transport

Passive Transport

In passive transport, molecules move down their concentration gradient: from regions of higher concentration to regions of lower concentration. The cell does not expend energy; the process occurs spontaneously.

Types of Passive Transport

• Simple diffusion
• Osmosis
• Facilitated diffusion

Simple Diffusion

Simple diffusion is transport in which substances freely cross the bilayer. Examples: oxygen, carbon dioxide, nitrogen, urea.

Osmosis

Osmosis is the movement of water through the bilayer toward regions of lower water concentration (or higher solute concentration).

Facilitated Diffusion

Facilitated diffusion is mediated by proteins. Molecules that do not freely cross the membrane are transported by membrane proteins.

Permeases (Protein Carriers)

Permeases, or protein carriers, are transmembrane proteins that carry substances such as glucose and amino acids. When the transported substance binds to the carrier protein, it causes a change in protein conformation and the substance is released on the other side of the membrane.

Channel-Forming Proteins

Channel-forming proteins are transmembrane proteins that form a pore which can open and close to allow the passage of ions.

Active Transport

Active transport moves substances against their concentration gradient and requires cellular energy.

For example, in the liver large amounts of glucose are stored as the more complex sugar glycogen. Glucose concentration is higher in liver cells than in the blood; the liver also takes up blood glucose against its concentration gradient.

Vesicle-Mediated Transport

Endocytosis

Endocytosis is the entry of large particles into the cell:

• The particles adhere to the membrane.
• The membrane then engulfs them.
• The particles are incorporated into the cell.

Phagocytosis

Phagocytosis: the entering particle is solid. Example: white blood cells ingest bacteria.

Pinocytosis

Pinocytosis: entry of liquid or dissolved substances. For example: materials requiring nutrient uptake.

Exocytosis

Exocytosis: substances produced within the cell are released to the outside. Example: hormones.

Common Cellular Response Pattern

Pattern common to animals, plants, bacteria, and other cells:

1. Reception of a stimulus.
2. Processing.
3. Response.

Responses at the cellular level:

• Movement.
• Defense.
• Cell differentiation.
• Degradation.
• Death.