Cell Membrane Structure and Transport Mechanisms: Study Guide

Cell Membrane Structure and Transport Mechanisms

Concept 5.1: Cellular Membranes are Fluid Mosaics of Lipids and Proteins

The plasma membrane is a dynamic structure composed primarily of lipids and proteins. Its fluid mosaic model describes the arrangement and movement of these molecules, which is essential for membrane function.

• Phospholipids are amphipathic molecules forming a bilayer, with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins are embedded within or attached to the membrane, serving various functions such as transport, signaling, and structural support.

• Cholesterol modulates membrane fluidity and stability.

• Fluid mosaic model: Describes the membrane as a mosaic of protein molecules drifting in a fluid bilayer of phospholipids.

Example: The movement of proteins and lipids within the membrane allows for cell signaling and transport of materials.

Membrane Proteins: Integral and Peripheral

Membrane proteins are classified based on their association with the lipid bilayer.

• Integral proteins: Span the membrane or are deeply embedded; involved in transport, signaling, and cell recognition.

• Peripheral proteins: Loosely attached to the membrane surface; often involved in signaling or maintaining cell shape.

Major Functions of Membrane Proteins

Function	Description
Transport	Move substances across the membrane via channels or carriers.
Enzymatic activity	Catalyze chemical reactions at the membrane surface.
Signal transduction	Transmit signals from outside to inside the cell.
Cell-cell recognition	Allow cells to identify and interact with each other.
Intercellular joining	Connect adjacent cells via junctions.
Attachment to cytoskeleton and ECM	Anchor the membrane to internal and external structures.

Other Membrane Components

• Glycolipids: Lipids with carbohydrate chains; involved in cell recognition.

• Glycoproteins: Proteins with carbohydrate chains; play roles in cell-cell interactions.

• ECM fibers: Extracellular matrix components that provide structural support.

Concept 5.2: Membrane Structure Results in Selective Permeability

Biological membranes are selectively permeable, allowing certain substances to cross more easily than others. This property is crucial for maintaining cellular homeostasis.

• Selective permeability: The ability of the membrane to regulate the passage of substances.

• Transport proteins: Facilitate the movement of specific molecules across the membrane.

• Aquaporins: Specialized channel proteins for water transport.

Methods of Transport for Key Materials

Material	Method of Transport
CO2	Simple diffusion
Glucose	Facilitated diffusion via carrier protein
H+	Active transport via pump
O2	Simple diffusion
H2O	Osmosis via aquaporin

Concept 5.3: Passive Transport is Diffusion Across a Membrane with No Energy Investment

Passive transport involves the movement of substances down their concentration gradient without the use of cellular energy (ATP).

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated diffusion: Passive movement of molecules via transport proteins.

Key Terms and Definitions

• Concentration gradient: Difference in concentration of a substance across a space.

• Passive transport: Movement of substances without energy input.

• Osmosis: Diffusion of water.

• Isotonic: Solution with equal solute concentration as the cell.

• Hypertonic: Solution with higher solute concentration than the cell.

• Hypotonic: Solution with lower solute concentration than the cell.

• Flaccid: Limp cell due to water loss.

• Plasmolysis: Shrinking of the cell membrane from the cell wall due to water loss.

Example: Red blood cells in a hypotonic solution swell and may burst, while plant cells become turgid due to their cell wall.

Concept 5.4: Active Transport Uses Energy to Move Solutes Against Their Gradients

Active transport requires energy (usually ATP) to move substances against their concentration gradients, often via specific transport proteins.

• Active transport: Movement of molecules from low to high concentration using energy.

• Sodium-potassium pump: Transports Na+ out and K+ into the cell, maintaining electrochemical gradients.

Equation:

$\text{Na}^+/\text{K}^+ \text{ pump:} \quad 3\,\text{Na}^+ \text{ out},\ 2\,\text{K}^+ \text{ in per ATP hydrolyzed}$

Summary: Sodium-Potassium Pump Steps

1. Na+ binds to the pump from the cytoplasm.

2. ATP is hydrolyzed, phosphorylating the pump.

3. Conformational change releases Na+ outside.

4. K+ binds from the extracellular fluid.

5. Phosphate group is released, returning the pump to its original shape.

6. K+ is released inside the cell.

Example: Nerve impulse transmission relies on the sodium-potassium pump to maintain membrane potential.

Concept 5.5: Bulk Transport Across the Plasma Membrane Occurs by Exocytosis and Endocytosis

Bulk transport mechanisms move large molecules or particles across the membrane via vesicles, requiring energy.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell.

• Endocytosis: The cell engulfs external materials by forming vesicles from the plasma membrane.

• Phagocytosis: "Cell eating"; uptake of large particles.

• Pinocytosis: "Cell drinking"; uptake of fluids and dissolved solutes.

Example: White blood cells use phagocytosis to ingest bacteria.

Comparison of Transport Mechanisms

Type	Energy Required?	Direction	Example
Passive Transport	No	Down gradient	O2 diffusion
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Bulk Transport	Yes (ATP)	Vesicle-mediated	Exocytosis of hormones

Additional info:

• Membrane fluidity is affected by temperature, cholesterol, and fatty acid composition.

• Transport proteins can be channels (form pores) or carriers (change shape to move molecules).

• Facilitated diffusion is passive but requires a protein; active transport always requires energy.