Membrane Structure and Function: Study Guide for General Biology

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Chapter 7: Membrane Structure and Function

Concept 7.1: Cellular Membranes are Fluid Mosaics of Lipids and Proteins

The plasma membrane is a dynamic structure composed of lipids and proteins, forming a fluid mosaic that regulates the movement of substances into and out of the cell. Understanding its components and properties is essential for grasping cellular function.

Fluid Mosaic Model: Describes the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.
Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails, forming the bilayer.
Membrane Fluidity: Refers to the viscosity of the lipid bilayer, allowing lateral movement of components.
Factors Affecting Fluidity:
- Decreasing temperature reduces fluidity.
- Unsaturated hydrocarbon chains increase fluidity.
- Cholesterol acts as a fluidity buffer, stabilizing membrane at various temperatures.
- Increasing saturated hydrocarbon tails decreases fluidity.
Membrane Proteins: Integral and peripheral proteins serve various functions.
- Integral proteins: Penetrate the hydrophobic core; often function as transporters or receptors.
- Peripheral proteins: Loosely bound to the surface; often involved in signaling or maintaining cell shape.

Function	Description
Transport	Move substances across the membrane
Enzymatic activity	Catalyze chemical reactions at the membrane
Signal transduction	Transmit signals from outside to inside the cell
Attachment to cytoskeleton and ECM	Maintain cell shape and stabilize membrane location

Glycolipids vs. Glycoproteins:
- Glycolipids: Lipids with carbohydrate chains; involved in cell recognition.
- Glycoproteins: Proteins with carbohydrate chains; play roles in cell-cell interactions and signaling.

Concept 7.2: Membrane Structure and Selective Permeability

The structure of the plasma membrane allows it to be selectively permeable, controlling the passage of substances.

Selective Permeability: Only certain molecules can cross the membrane freely; others require transport proteins.
Channel Proteins vs. Carrier Proteins:
- Channel proteins: Provide corridors for specific molecules or ions to cross.
- Carrier proteins: Bind to molecules and change shape to shuttle them across.
Transport Proteins: Often specific for the substance they move (e.g., aquaporins for water).

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

Concept 7.3: Diffusion, Osmosis, and Facilitated Diffusion

Cells rely on passive and facilitated transport mechanisms to move substances without energy investment.

Diffusion: Movement of molecules from high to low concentration.
Concentration Gradient: Difference in concentration across a space.
Passive Transport: Movement without energy input (includes diffusion and osmosis).
Osmosis: Diffusion of water across a selectively permeable membrane.
Isotonic Solution: No net water movement; cell remains the same.
Hypertonic Solution: Water leaves the cell; cell shrinks (plasmolysis in plants).
Hypotonic Solution: Water enters the cell; animal cells may burst (lyse), plant cells become turgid.
Turgid: Firm; plant cell in hypotonic solution.
Flaccid: Limp; plant cell in isotonic solution.
Plasmolysis: Plant cell in hypertonic solution; membrane pulls away from wall.

Concept 7.4: Active Transport

Active transport requires energy to move substances against their concentration gradients, often using ATP.

Active Transport: Movement of molecules against their concentration gradient using energy (usually ATP).
Sodium-Potassium Pump: Example of active transport; moves Na+ out and K+ into the cell.
- Uses ATP to change protein shape and move ions.
- Maintains electrochemical gradients essential for cell function.

Concept 7.5: Bulk Transport (Endocytosis and Exocytosis)

Cells use bulk transport mechanisms to move large molecules or particles across the membrane.

Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell (e.g., secretion of neurotransmitters).
Endocytosis: Cell takes in materials by forming vesicles from the plasma membrane.
Receptor-Mediated Endocytosis: Specific molecules are ingested after binding to receptors.
Phagocytosis: "Cell eating"; cell engulfs large particles.
Pinocytosis: "Cell drinking"; cell takes in extracellular fluid.
Bulk transport: Generally requires energy; considered a form of active transport.

Key Components of the Animal Cell Membrane

The animal cell membrane contains various components, each with specific roles:

Glycolipid: Cell recognition and communication.
Glycoprotein: Cell-cell interactions and signaling.
Integral Protein: Transport, signaling, and structural support.
Peripheral Protein: Signaling and maintaining cell shape.
Cholesterol: Modulates membrane fluidity.
Phospholipid: Forms the bilayer structure.
ECM Fibers: Provide structural support outside the cell.
Cytoskeleton Microfilaments: Maintain cell shape and facilitate movement.
Integrins: Connect the cell to the extracellular matrix and transmit signals.

Summary Table: Types of Membrane Transport

Type of Transport	Energy Required?	Example
Simple Diffusion	No	O2, CO2
Facilitated Diffusion (Channel)	No	H2O via aquaporin
Facilitated Diffusion (Carrier)	No	Glucose
Active Transport	Yes (ATP)	Na+/K+ pump
Bulk Transport (Endocytosis/Exocytosis)	Yes (ATP)	Phagocytosis, secretion

Additional info:

Facilitated diffusion is passive and does not require energy, but uses specific transport proteins.
Transport proteins are highly specific, often moving only one type of molecule.
Aquaporins are channel proteins that facilitate rapid water movement across the membrane.
Plant cells do not burst in hypotonic solutions due to the rigid cell wall, which provides structural support.