Biology 12

3.5 The Permeability of the Plasma Membrane

The plasma membrane is selectively permeable, allowing passage of only certain molecules.

Passage of Molecules Across the Membrane• Some substances freely cross the membrane. They move “down” their concentration

gradient (from high concentration to low concentration).• Some substances are unable to freely cross and are transported by proteins or vesicles.

They may go “up,” or against, their concentration gradient.

DiffusionDiffusion is the movement of molecules down their concentration gradient. It does not require energy. The rate of diffusion is affected by factors such as temperature, pressure, and molecule size.

• A solution contains a solute in a solvent. Diffusion occurs until there is an equal distribution of solute and solvent.

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Diffusion of Oxygen

Only a few types of molecules can diffuse across the plasma membrane.• Gases can diffuse across the bilayer

• Oxygen enters cells and carbon dioxide leaves• In lungs, oxygen moves from the alveoli to blood in the capillaries

OsmosisOsmosis is the diffusion of water molecules across a selectively permeable membrane due to a difference in concentration.

• There is a net movement of water and changes in solute concentration on both sides of the membrane

Isotonic, Hypotonic, and Hypertonic Solutions Prefixes:

iso: the same ashypo: less thanhyper: more than

tonicity: refers to osmotic pressure

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Isotonic solutions have the same concentration of solute and solvent as the solution inside the cell, and water will not enter or leave the cell.Hypotonic solutions have a lower concentration of solute than solution inside the cell, and water will enter the cell.Hypertonic solutions have a higher concentration of solute than solution inside the cell, and water will leave the cell.

Transport by Carrier ProteinsThe plasma membrane stops the passage of most molecules into and out of the cell. However, biologically important molecules do pass. They do so because of carrier proteins that exist in the plasma membrane.

• Carrier proteins are specific and each binds to specific molecules• Carrier proteins are required for both facilitated transport and active transport of

substances across the plasma membrane

Facilitated Transport• Assists in transport of molecules across the membrane by binding to those molecules• Occurs down a concentration gradient and does not require ATP

Active Transport• Assists transport of substances across the membrane by binding to them• Occurs against a concentration gradient and requires energy, usually in the form of ATP

Proteins involved in active transport are often called pumps because they use energy to pump substances against their concentration gradient.

• One important carrier protein pump is the sodium-potassium pump. It moves sodium ions to the outside of the cell and potassium ions to the inside of the cell.

Bulk Transport

Prefixes:

iso: the same ashypo: less thanhyper: more than

tonicity: refers to osmotic pressure

Biology 12

Macromolecules are transported into and out of the cell by vesicle formation, called membrane-assisted transport in energy-dependent processes.

• Exocytosis is a way substances can exit a cell• Endocytosis is way substances can enter a cell

ExocytosisDuring exocytosis, a vesicle fuses with the membrane and the substance it is carrying is secreted outside of the cell.

• Neurotransmitters, hormones, and digestive enzymes are examples of substances secreted in this way

EndocytosisDuring endocytosis, cells take in substances by vesicle formation.

• The plasma membrane folds in on itself and then pinches off to form an intracellular vesicle

Endocytosis occurs in one of three ways.• Phagocytosis• Pinocytosis• Receptor-mediated endocytosis

PhagocytosisDuring phagocytosis, the material being taken into the cell is large, such as a food particle or another cell.

• Common in unicellular organisms and occurs in certain types of human white blood cells

PinocytosisDuring pinocytosis, vesicles form around liquid or very small particles.

• Common in blood cells, intestinal cells, and plant root cells

Receptor-Mediated Endocytosis

Biology 12

Receptor-mediated endocytosis is a type of pinocytosis. It involves receptor proteins that only bind to certain molecules.

• The receptors are in coated pits. Once vesicles form, they become uncoated and fuse with lysosomes. Empty vesicles fuse with the plasma membrane and receptors return to their previous locations.