Cell Membrane Structure, Transport, and Osmosis

Cell Membrane Structure and Function

Question: What is the role of organic molecules in the cell membrane?

Answer: Organic molecules contribute to cell–cell adhesion and protect the cell surface from chemical damage.

How Are Substances Transported Across the Membrane?

There are three main types of membrane transport:

• Passive transport — a type of membrane transport that does not require energy to move substances across cell membranes. Passive transport can use diffusion or osmosis and depends on the concentration gradient.
• Facilitated transport — molecules are transported across the plasma membrane with the help of membrane proteins (examples in common descriptions include oxygen, CO2, and glucose).
• Active transport — uses different protein carriers or protein pumps to move substances against the concentration gradient (requires ATP energy). Examples of transported macromolecules include sugars, amino acids, and ions.

Diffusion

Diffusion: movement of molecules from a region of higher concentration to a region of lower concentration until equilibrium is reached.

Example: In the body, gases diffuse across the cell membrane; digested nutrients diffuse from the small intestine into the blood.

Functions of the Cell Membrane

• Separates the cell from the external environment.
• Controls entry and exit of substances in and out of the cell.
• Helps maintain a constant internal environment of the cell (homeostasis).

Membrane Composition

What proteins make up most of the cell membrane? The membrane is primarily composed of proteins and phospholipids.

Selective Permeability

Why is the cell membrane described as selectively permeable? It allows some things to pass through freely (small and uncharged molecules) and regulates or denies other molecules from entering.

Solution, Solute, and Solvent

Solution: made up of a solute (a substance dissolved) and a solvent (a fluid).

Osmosis

Osmosis: diffusion of water — the movement of water from low solute concentration to high solute concentration.

Tonicity Terms

Isotonic — equal concentration of solute inside and outside the cell (water moves in and out at equal rates).

Hypotonic — lower solute concentration outside the cell (water moves in and the cell swells).

Hypertonic — higher solute concentration outside the cell; this draws water out of the cell toward the dissolved solutes.

Endocytosis and Vesicle Transport

Endocytosis — the taking in of matter by a living cell through invagination of its membrane to form a vacuole.

Pinocytosis vs Phagocytosis

Pinocytosis — liquids or small molecules in solution are taken into the cell by vesicle formation.

Phagocytosis — large materials, such as food particles or another cell, are taken into the cell by vesicle formation.