Membrane Structure and Function: Study Guide

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Membrane Structure and Function

Membrane Fluidity

The plasma membrane is a dynamic structure that exhibits fluidity, allowing its components to move laterally within the layer. This property is essential for membrane function and cell viability.

Fluidity of Membranes: Biological membranes are not rigid; their lipid and protein molecules can move sideways within the layer, similar to icebergs floating in a sea of lipids.
Factors Affecting Fluidity: The degree of unsaturation of fatty acid tails, cholesterol content, and temperature all influence membrane fluidity. Unsaturated fatty acids increase fluidity, while cholesterol acts as a fluidity buffer.
Example: At lower temperatures, cholesterol prevents membranes from becoming too rigid; at higher temperatures, it prevents them from becoming too fluid.

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a mosaic of various proteins embedded in or attached to a fluid lipid bilayer.

Mosaic: The membrane is composed of a variety of proteins that float in or on the fluid lipid bilayer like boats on a pond.
Components: Includes phospholipids, cholesterol, proteins, and carbohydrates.
Example: Integral proteins span the membrane, while peripheral proteins are attached to the surface.

Selective Permeability

Cell membranes are selectively permeable, meaning they allow some substances to cross more easily than others.

Definition: Selective permeability is the property of a membrane that allows it to regulate the passage of substances.
Example: Small nonpolar molecules (e.g., O2, CO2) pass easily, while ions and large polar molecules require transport proteins.

Phospholipid Bilayer

The fundamental structure of the membrane is the phospholipid bilayer, which forms spontaneously due to the amphipathic nature of phospholipids.

Bilayer: Two layers of phospholipids with hydrophobic tails facing inward and hydrophilic heads facing outward.
Self-Assembly: Occurs spontaneously in water due to hydrophobic interactions.
Example: Liposomes are artificial bilayer vesicles formed in the lab.

Passive Transport

Passive transport is the movement of substances across a membrane without the input of cellular energy.

Definition: Diffusion of a substance across a membrane with no energy investment.
Types: Includes simple diffusion, facilitated diffusion, and osmosis.

Diffusion

Diffusion is the tendency of molecules to spread out evenly into the available space, moving from areas of high concentration to low concentration.

Definition: The net movement of molecules down their concentration gradient.
Spontaneity: Diffusion is a passive and spontaneous process driven by the kinetic energy of molecules.
Equation:

Where J is the flux, D is the diffusion coefficient, and \frac{dC}{dx} is the concentration gradient.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane.

Definition: Movement of water from an area of lower solute concentration to an area of higher solute concentration.
Prediction: Water moves to balance solute concentrations on both sides of the membrane.
Equation:

Where \Psi is water potential, \Psi_s is solute potential, and \Psi_p is pressure potential.

Tonicity

Tonicity describes the ability of a surrounding solution to cause a cell to gain or lose water.

Hypertonic: Solution has a higher solute concentration than the cell; water leaves the cell.
Hypotonic: Solution has a lower solute concentration than the cell; water enters the cell.
Isotonic: Solution has equal solute concentration; no net water movement.

Effects of Tonicity on Cells

Cells respond differently to tonicity depending on the presence or absence of a cell wall.

Animal Cells: In a hypertonic solution, cells shrink (crenate); in a hypotonic solution, they swell and may burst (lyse); in an isotonic solution, they remain normal.
Plant Cells: In a hypertonic solution, cells undergo plasmolysis; in a hypotonic solution, they become turgid (normal); in an isotonic solution, they are flaccid.

Water Balance Regulation

Cells regulate water balance through osmoregulation, which is crucial for survival in varying environments.

With Cell Wall: Plant, fungal, and bacterial cells use their rigid cell wall to prevent excessive water uptake.
Without Cell Wall: Animal cells rely on mechanisms such as contractile vacuoles (in protists) or ion pumps to maintain water balance.

Facilitated Diffusion

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

Transport Proteins: Channel proteins (e.g., aquaporins) and carrier proteins assist in the movement of specific molecules.
Example: Glucose transporters facilitate the entry of glucose into cells.

Comparison of Transport Mechanisms

Different mechanisms exist for the movement of substances across membranes.

Process	Energy Required?	Direction	Protein Involved?
Simple Diffusion	No	Down gradient	No
Facilitated Diffusion	No	Down gradient	Yes
Osmosis	No	Down water potential gradient	Sometimes (aquaporins)
Active Transport	Yes (ATP)	Against gradient	Yes

Active Transport

Active transport moves substances against their concentration gradients, requiring energy input, usually from ATP.

Definition: The movement of ions or molecules across a membrane against their concentration gradient using energy.
Example: The sodium-potassium pump ( out, in) maintains electrochemical gradients in animal cells.

Bulk Transport: Exocytosis and Endocytosis

Large molecules and particles are transported across membranes via vesicles in processes called exocytosis and endocytosis.

Exocytosis: The process by which cells export materials by fusing vesicles with the plasma membrane.
Endocytosis: The process by which cells import materials by engulfing them in vesicles.
Types of Endocytosis:
- Phagocytosis: "Cell eating"; uptake of large particles or cells.
- Pinocytosis: "Cell drinking"; uptake of extracellular fluid and dissolved solutes.
- Receptor-Mediated Endocytosis: Uptake of specific molecules via receptor proteins.