Cell Transport

Plasma Membrane Structure and Function

The plasma membrane is composed of a fluid-mosaic model, which consists of a phospholipid bilayer with hydrophobic tails facing inward and hydrophilic heads facing the surfaces.

The fluid-mosaic model is a model that describes the structure of the plasma membrane as a fluid phospholipid bilayer with embedded proteins.

Embedded within the phospholipid bilayer are integral proteins, which are embedded within the bilayer, and peripheral proteins, which are attached to the inner surface of the bilayer. Other components of the plasma membrane include glycoproteins, glycolipids, and cholesterol.

Functions of Membrane Proteins

Membrane proteins perform a variety of functions, including:

  • Channel proteins: form channels for substances to move across the membrane

  • Receptor proteins: bind to substances in the environment and trigger cell responses

  • Carrier proteins: transport specific substances across the cell membrane

  • Enzymes: catalyze chemical reactions for cell metabolism

Membrane Protein Diversity

The diversity of membrane proteins is due to the variety of functions they perform.

Membrane protein diversity refers to the variety of different proteins that are embedded within the plasma membrane, each with unique functions and properties.

Carbohydrate Chains

Carbohydrate chains are bound to the outer surface of cell recognition proteins and form a glycocalyx, or sugar coat. The diversity of carbohydrate chains produces an individual fingerprint, allowing for recognition of self vs. nonself and triggering immune responses.

Permeability of the Plasma Membrane

The plasma membrane is selectively permeable, allowing some substances to pass through freely while others do not. There are two types of transport mechanisms: passive transport, which requires no cellular energy, and active transport, which requires ATP.

Passive Transport

Passive transport mechanisms include:

  • Diffusion: the movement of molecules from an area of high concentration to an area of low concentration

  • Facilitated diffusion: the movement of molecules across the membrane with the help of carrier proteins

Active Transport

Active transport mechanisms include:

  • Active transport: the movement of molecules against a concentration gradient using ATP

  • Exocytosis: the transport of cell products and wastes out of the cell by vesicle formation

  • Endocytosis: the transport of substances into the cell by vesicle formation

Diffusion and Osmosis

Diffusion is the movement of molecules from an area of high concentration to an area of low concentration.

Osmosis is the diffusion of water across a semipermeable membrane, resulting in the movement of water from an area of high concentration to an area of low concentration.

The osmotic pressure is the force that causes water to move in a direction, and is due to the number of nondiffusable particles in solution. There are three types of solutions:


SolutionEffect on Cells

Hypotonic

causes cells to swell and burst

Hypertonic

causes cells to shrink or crenate

Isotonic

no change

Transport by Carrier Proteins

Carrier proteins are specific for the molecules they transport and can facilitate both passive and active transport. The sodium-potassium pump is an example of a carrier protein that uses ATP to transport sodium and potassium ions against their concentration gradients.

Exocytosis and Endocytosis

Exocytosis is the process by which vesicles containing cell products fuse with the plasma membrane, releasing the products and incorporating the vesicle membrane into the plasma membrane. Endocytosis is the process by which an area of the cell membrane invaginates and surrounds a substance, pinching off to form a vesicle. There are three types of endocytosis:

  • Phagocytosis: the uptake of large particles, such as bacteria or cell debris

  • Pinocytosis: the uptake of liquid or small particles

  • Receptor-mediated endocytosis: a specific type of pinocytosis that occurs in response to receptor stimulation