Biology Chapter: Plasma Membrane

Study Notes

Plasma Membrane

The plasma membrane is differentially or selectively permeable, allowing certain substances to move across while restricting others.

Permeability Types

Impermeable: No substances can pass through the plasma membrane.
Permeable: All substances can pass through the plasma membrane.

Key Characteristics of Permeability

The permeability of the plasma membrane permits some substances, like small, uncharged molecules (e.g., CO2, O2, H2O, glycerol, alcohol), to move freely, whereas larger molecules, ions, and charged species require assistance.

Transport Mechanisms

Passive Movement: Some substances can move across the plasma membrane freely, following their concentration gradient from high to low.
Active Transport: May require carrier proteins and ATP (energy) for movement across the membrane.

Molecule Types

Small/Uncharged Molecules: Move freely across membranes and follow concentration gradients.
Large/Charged Molecules: Cannot cross without assistance, using protein channels and carrier proteins.

Vesicle Formation

Molecules can exit (exocytosis) or enter (endocytosis) cells via vesicle formation, typically for macromolecules.

Diffusion

Defined as the movement of molecules from higher to lower concentration until equilibrium is reached, e.g., gases like oxygen and carbon dioxide can diffuse across the plasma membrane.

Factors Affecting Diffusion Rate

Influenced by temperature, pressure gradient, concentration gradient, molecular size, charge, and electrical currents.

Osmosis

Refers to the diffusion of water from areas of high water concentration to low water concentration.

Osmotic Pressure

The pressure that builds due to osmosis; higher osmotic pressure indicates a greater tendency for water to diffuse in that direction.

Solutions and Concentration

Isotonic Solutions: Solute concentration is equal inside and outside the cell; no net water movement.
Hypotonic Solutions: Lower solute concentration outside; cells can gain water, swell, or burst (cytolysis).
Hypertonic Solutions: Higher solute concentration outside; cells lose water, shrink or shrivel up (crenation).

Turgor Pressure

The pressure exerted by plant cell contents against the cell wall due to water influx, preventing cell bursting.

Freshwater Organisms

Use contractile vacuoles to regulate internal environments and prevent excess water uptake.

Plasmolysis

Occurs when cells lose water in hypertonic solutions, causing contraction of cell contents; marine animals employ adaptations to retain water.

Description

This quiz covers the key concepts of the plasma membrane, including its selective permeability and different transport mechanisms. Understand how substances move across the membrane, whether passively or actively, and the distinctions between different types of molecules involved in these processes.