Membrane Structure and Function: Study Guide for General Biology

Chapter 7: Membrane Structure and Function

Overview

The plasma membrane is a fundamental structure in all cells, controlling the movement of substances in and out and facilitating communication. This chapter explores the fluid mosaic model, membrane components, transport mechanisms, and the role of proteins and carbohydrates in membrane function.

Concept 7.1: Fluid Mosaic Model of Membranes

The fluid mosaic model describes the structure of the plasma membrane as a dynamic combination of lipids and proteins. This model explains how membranes maintain flexibility and selective permeability.

• Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions. This property allows them to form bilayers in aqueous environments.

• The fluid mosaic model refers to the membrane's structure, where proteins float in or on the fluid lipid bilayer like boats on a pond.

• Membrane fluidity is influenced by temperature, the composition of fatty acids (saturated vs. unsaturated), and the presence of cholesterol.

• Increasing the number of saturated hydrocarbon tails decreases fluidity, while unsaturated tails increase fluidity.

Membrane Proteins:

• Integral proteins penetrate the hydrophobic core of the lipid bilayer and are often involved in transport or signaling.

• Peripheral proteins are loosely bound to the surface of the membrane and often function in cell signaling or maintaining cell shape.

Major Functions of Membrane Proteins

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
Cell-cell recognition	Allow cells to identify each other
Intercellular joining	Connect adjacent cells
Attachment to cytoskeleton and ECM	Anchor the membrane to internal and external structures

Concept 7.2: Membrane Structure and Selective Permeability

Selective permeability allows the membrane to control which substances enter or leave the cell. This is achieved through the specific arrangement of lipids and proteins.

• Channel proteins form pores that allow specific molecules to pass through.

• Carrier proteins bind to molecules and change shape to shuttle them across the membrane.

• Transport proteins are often specific for the substances they move.

• Aquaporins are channel proteins that facilitate water transport; discovered by Peter Agre.

Transport Methods for Common Materials

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

Concept 7.3: Diffusion, Osmosis, and Facilitated Diffusion

Transport across membranes can occur passively or actively, depending on the energy requirements and the nature of the substance.

• 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: Equal solute concentration inside and outside the cell.

• Hypertonic: Higher solute concentration outside the cell; water leaves the cell.

• Hypotonic: Lower solute concentration outside the cell; water enters the cell.

• Flaccid: Limp cell due to water loss.

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

Facilitated diffusion uses transport proteins to move substances down their concentration gradient without energy input.

Concept 7.4: Active Transport

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

• Sodium-potassium pump is a classic example, moving Na+ out and K+ into the cell.

• ATP provides the energy for the pump to change shape and transport ions.

Summary of Sodium-Potassium Pump Steps:

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

2. ATP is hydrolyzed, phosphorylating the pump.

3. Pump changes shape, releasing Na+ outside.

4. K+ binds from the extracellular fluid.

5. Pump dephosphorylates, returning to original shape.

6. K+ is released into the cytoplasm.

Equation for ATP hydrolysis:

$ATP + H_2O \rightarrow ADP + P_i + \text{energy}$

Concept 7.5: Bulk Transport (Exocytosis and Endocytosis)

Bulk transport moves 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 material by forming a vesicle from the plasma membrane.

Membrane Potential and Transport Forces

Membrane potential is the voltage difference across the membrane, created by ion gradients. The inside of the cell is usually negative relative to the outside.

• Two forces drive ion diffusion: chemical (concentration gradient) and electrical (membrane potential).

• The combination is called the electrochemical gradient.

Cell Recognition and Membrane Carbohydrates

Membrane carbohydrates, such as glycolipids and glycoproteins, play key roles in cell-cell recognition.

• Examples: Blood group antigens, immune cell recognition.

Labeling Membrane Components

• Glycolipid: Lipid with a carbohydrate attached, involved in cell recognition.

• Glycoprotein: Protein with a carbohydrate attached, also involved in recognition.

• Integral protein: Embedded in the membrane, often spanning it.

• Peripheral protein: Attached to the membrane surface.

• Cholesterol: Stabilizes membrane fluidity.

• Phospholipid: Forms the bilayer structure.

• ECM fibers: Part of the extracellular matrix, providing structural support.

• Cytoskeleton microfilaments: Provide internal support and shape.

• Integrins: Connect the cytoskeleton to the ECM.

Comparisons of Transport Types

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	Na+/K+ pump
Bulk transport	Yes	Exocytosis, endocytosis

Additional info:

• Membrane fluidity is crucial for proper function, affecting protein mobility and cell signaling.

• Transport proteins are highly specific, often only allowing one type of molecule to pass.

• Plant cells do not burst in hypotonic solutions due to the rigid cell wall, while animal cells can lyse.