Membrane Structure and Function: Passive and Active Transport

Vocabulary and Key Concepts

This section introduces essential terminology for understanding membrane transport processes in cells.

• Tonicity: Refers to the relative concentration of solutes in two solutions separated by a semipermeable membrane. Types include hypotonic, isotonic, and hypertonic.

• Osmosis: The passive movement of water molecules across a selectively permeable membrane down their concentration gradient.

• Endocytosis: The process by which cells internalize substances from their external environment via vesicle formation.

• Exocytosis: The process by which cells expel materials to the external environment through vesicle fusion with the plasma membrane.

• Phagocytosis: A type of endocytosis where cells engulf large particles or microorganisms.

• Pinocytosis: A type of endocytosis involving the uptake of fluids and dissolved solutes.

• Hypercholesterolemia: A condition characterized by high levels of cholesterol in the blood, often linked to defects in receptor-mediated endocytosis.

• Passive Transport: Movement of substances across membranes without energy input.

• Active Transport: Movement of substances against their concentration gradient, requiring energy input (usually ATP).

Learning Outcomes

By studying this topic, students should be able to:

• Explain and compare passive and active transport across selectively permeable membranes.

• Predict animal and plant cell behavior in solutions of different tonicity.

• Identify and describe types of transport: direct passage, mediated passage (passive and active), and vesicular transport.

• Explain the function and mechanism of transport proteins, pumps, and cotransporters.

• Describe specific transport systems such as receptor-mediated endocytosis and LDL receptors.

Traffic Across the Plasma Membrane

Selectively Permeable Membrane

The plasma membrane is selectively permeable, allowing certain substances to cross while restricting others. This property is essential for maintaining cellular homeostasis.

• What can cross? Small nonpolar molecules (e.g., O2, CO2), some small polar molecules, and water can cross directly. Larger or charged molecules require specialized transport mechanisms.

• Mechanisms mediating crossing: Direct passage, transport proteins, and vesicular transport.

Types of Transport Across the Cell Membrane

Substances cross the cell membrane via three main mechanisms:

1. Direct passage through lipid bilayer: Simple diffusion of small, nonpolar molecules.

2. Passage mediated by transport proteins: Includes facilitated diffusion and active transport.

3. Vesicular Transport (Bulk Transport): Movement of large molecules or particles via endocytosis and exocytosis.

Passive vs. Active Transport

Comparison of Transport Types

Passive and active transport differ in energy requirements and direction relative to concentration gradients.

Simple Diffusion

Simple diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, driven by the concentration gradient.

• No energy required

• Example: Oxygen and carbon dioxide gas exchange in lungs

Where is the flux, is the diffusion coefficient, and is the concentration gradient.

Osmosis

Osmosis is the passive diffusion of free water molecules across a selectively permeable membrane down their concentration gradient.

• Occurs when solute concentrations differ across the membrane

• Water moves from regions of low solute concentration to high solute concentration

• Example: Water uptake by plant roots

Tonicity

Tonicity describes how a solution affects cell volume due to water movement.

• Hypotonic: Solution has lower solute concentration than the cell; water enters the cell, causing swelling or lysis.

• Isotonic: Solution has equal solute concentration to the cell; no net water movement.

• Hypertonic: Solution has higher solute concentration than the cell; water leaves the cell, causing shrinkage.

Facilitated Diffusion

Facilitated diffusion is the passive movement of molecules across the membrane via transport proteins.

• Channel proteins: Form pores for specific molecules (e.g., aquaporins for water, ion channels for ions).

• Carrier proteins: Bind and transport molecules by changing shape.

• No energy required; movement is down the concentration gradient

• Example: Glucose transport into cells via GLUT transporters

Active Transport

Active transport moves substances against their concentration gradient, requiring energy input, typically from ATP.

• Pumps: e.g., Sodium-potassium pump (-ATPase)

• Cotransporters: e.g., Symport and antiport systems

• Bulk transport: Movement of large molecules via vesicles (endocytosis, exocytosis)

Vesicular (Bulk) Transport

Bulk transport involves the movement of large particles or volumes of fluid via vesicles.

• Endocytosis: Uptake of materials into the cell. Includes phagocytosis ("cell eating"), pinocytosis ("cell drinking"), and receptor-mediated endocytosis.

• Exocytosis: Expulsion of materials from the cell.

• Example: Uptake of cholesterol via LDL receptors (receptor-mediated endocytosis)

Summary Table: Passive vs. Active Transport

Applications and Examples

• Plant cells in hypotonic solutions: Become turgid, which is ideal for structural support.

• Animal cells in hypertonic solutions: Lose water and shrink (crenate).

• Receptor-mediated endocytosis: Cells take up specific molecules (e.g., cholesterol via LDL receptors).

Additional info: The notes reference "Lecture 7" and required reading from a textbook (Chapter 5: Concepts 8.5-8.7), indicating these topics are foundational for understanding cell membrane transport in General Biology.