Transport Across Cell Membranes - Diffusion, Osmosis, and Membrane Transport Mechanisms

Flashcards cover diffusion principles (Fick's Law), osmosis, tonicity (isotonic, hypotonic, hypertonic), and transporter-mediated membrane transport (facilitated diffusion, primary and secondary active transport, and symport/antiport mechanisms).

What factors affect the rate of diffusion according to Fick's Law?

Concentration gradient, distance/thickness of the diffusion barrier, and molecular weight of the diffusing substance (heavier molecules diffuse more slowly; thicker barriers slow diffusion).

What does Fick's Law of Diffusion state?

The rate of diffusion is proportional to the concentration gradient and inversely proportional to the diffusion distance (thickness); substances move down their concentration gradient.

Define osmosis.

Net diffusion of water across a selectively permeable membrane down its own concentration gradient.

What is tonicity?

The effect of the concentration of non-penetrating solutes on cell volume.

Isotonic solution

Same concentration of non-penetrating solutes as inside normal body cells; no net water movement and no change in cell volume.

Hypotonic solution

Lower concentration of non-penetrating solutes than inside the cell; water moves into the cell and the cell may swell.

Hypertonic solution

Higher concentration of non-penetrating solutes than inside the cell; water moves out of the cell and the cell may shrink.

What is facilitated diffusion?

Passive transport that uses membrane transport proteins to move substances down their concentration gradient; no ATP used; characterized by specificity, saturation, and competition.

What is active transport?

Transport that requires energy to move substances against their concentration gradient.

Primary active transport

Direct use of ATP by a transporter protein to move a substance across the plasma membrane.

Secondary active transport

Energy from ATP creates a ion concentration gradient, which is then used to move another substance across the membrane.

What are the two types of secondary transport?

Symport (cotransport) and Antiport (countertransport).

Describe symport (cotransport).

Two substances move in the same direction; the transported solute moves against its gradient, driven by the ion moving down its gradient.

Describe antiport (countertransport).

Two substances move in opposite directions; one is driven by the gradient of the other across the membrane.

What is the role of the driving ion in secondary active transport?

The driving ion moves down its concentration gradient (high to low) to provide the energy for transporting the other substance.

What distinguishes primary from secondary active transport?

Primary uses direct ATP to move solutes; secondary uses energy stored in an ion gradient created by primary transport to drive another transport process.