Membrane Structure and Function (Chapter 7): Study Notes

Membrane Structure and Function

Overview

The plasma membrane is a fundamental structure in all living cells, acting as a boundary that separates the cell from its environment. Its unique composition and organization allow it to regulate the movement of substances, maintain homeostasis, and facilitate communication and recognition between cells.

Cellular Membranes: Fluid Mosaics of Lipids and Proteins

Plasma Membrane

• Definition: The plasma membrane is the boundary that separates the living cell from its surroundings.

• Thickness: Approximately 8 nm thick (about 1/8000 the thickness of a page).

• Selective Permeability: Allows some substances to cross more easily than others.

• Evolutionary Conservation: One of the earliest structures formed during evolution; its basic structure is highly conserved across species.

Plasma Membrane Contents

• Phospholipids: The most abundant lipid in the plasma membrane; they are amphipathic molecules (contain both hydrophobic and hydrophilic regions).

• Other Components: Proteins, carbohydrates, and cholesterol are also present.

Discovery and Models of Membrane Structure

Discovery of the Fluid Mosaic Model

• 1915: Membranes were found to be made of proteins and lipids.

• 1925: Scientists proposed the membrane must be a phospholipid bilayer.

The Sandwich Model

• 1935: Davson and Danielli proposed a model with proteins coating the outside of the lipid bilayer.

• Later, it was discovered that membrane proteins are amphipathic and can be embedded within the bilayer.

The Fluid Mosaic Model

• 1972: Singer and Nicolson proposed that the membrane is a mosaic of proteins embedded in a fluid phospholipid bilayer.

• Hydrophobic regions of proteins interact with fatty acid tails; hydrophilic regions interact with phosphate heads and aqueous environments.

• Freeze-fracture technique: Used to split membranes and study their structure via electron microscopy.

Membrane Dynamics

Fluidity of the Membrane

• Phospholipids and some proteins can move laterally within the bilayer.

• Held together mainly by hydrophobic interactions.

• Lateral movement is frequent (~107 times per second); flip-flop (transverse movement) is rare (~once per month).

Membrane Fluidity and Lipid Composition

• Unsaturated hydrocarbon tails (with double bonds) increase fluidity, especially at low temperatures.

• Saturated hydrocarbon tails pack tightly, making the membrane more viscous.

• Cholesterol: Acts as a fluidity buffer, restraining movement at high temperatures and preventing tight packing at low temperatures.

• Adaptations: Fishes in cold environments have more unsaturated phospholipids; winter wheat increases unsaturated phospholipids in autumn.

Membrane Proteins

Types of Membrane Proteins

• Peripheral proteins: Bound to the surface of the membrane.

• Integral proteins: Penetrate the hydrophobic core; those that span the membrane are called transmembrane proteins.

• Structure: Hydrophobic regions of integral proteins consist of stretches of nonpolar amino acids, often forming alpha helices.

• Some proteins interact with the extracellular matrix (ECM) or the cytoskeleton, providing structural support and facilitating communication.

Functions of Membrane Proteins

• Transport: Move substances across the membrane (e.g., channels, carriers).

• Enzymatic activity: Catalyze reactions at the membrane surface.

• Signal transduction: Relay signals from outside to inside the cell.

• Cell-cell recognition: Allow cells to identify each other (important for immune response).

• Intercellular joining: Connect adjacent cells.

• Attachment: Anchor the membrane to the cytoskeleton and ECM.

Membrane Carbohydrates

Role and Diversity

• Usually short, branched chains of fewer than 15 sugar monomers.

• Can be covalently bonded to lipids (glycolipids) or proteins (glycoproteins).

• High diversity among species, individuals, and cell types (e.g., ABO blood types).

Synthesis and Sidedness of Membranes

• Membranes have distinct inside (cytoplasmic) and outside (extracellular) faces.

• Asymmetrical distribution of proteins, lipids, and carbohydrates is established during membrane synthesis in the ER and Golgi apparatus.

Selective Permeability and Transport

Selective Permeability

• Cells must exchange materials with their environment (intake: sugars, amino acids, O2; expel: waste, CO2).

• Regulate concentrations of inorganic ions (Na+, K+, Ca2+, Cl-).

• Small nonpolar molecules cross easily; large polar molecules and ions require protein assistance.

Transport Proteins

• Channel proteins: Provide hydrophilic channels for specific molecules or ions (e.g., aquaporins for water).

• Carrier proteins: Bind and change shape to shuttle molecules across the membrane.

• Transport proteins are specific for the substances they move.

Membrane Fluidity: Table

Condition	Phospholipid Tail Type	Membrane State
High Temperature	Unsaturated	Fluid
Low Temperature	Saturated	Viscous
Presence of Cholesterol	Both	Buffer (restrains movement at high temp, prevents packing at low temp)

Example Applications

• Blocking HIV Entry: Drugs like maraviroc mask CCR5 receptors, preventing HIV from entering cells.

• Cell-Cell Recognition: Immune cells use membrane proteins and carbohydrates to distinguish self from non-self.

• Adaptation: Organisms adjust membrane lipid composition in response to environmental temperature changes.

Key Terms

• Amphipathic: Molecules with both hydrophobic and hydrophilic regions.

• Phospholipid Bilayer: Double layer of phospholipids forming the core of the membrane.

• Integral Protein: Protein embedded within the membrane.

• Peripheral Protein: Protein attached to the membrane surface.

• Glycoprotein/Glycolipid: Protein/lipid with covalently attached carbohydrate.

• Selective Permeability: Ability of the membrane to allow some substances to pass while blocking others.

Equations

• Membrane Potential: $V_m = V_{inside} - V_{outside}$

• Diffusion Rate (Fick's Law): $J = -D \frac{dC}{dx}$

Additional info:

• Membrane proteins are classified by their location and function; transmembrane proteins typically have hydrophobic amino acids in their membrane-spanning regions.

• Membrane carbohydrates play a critical role in cell-cell recognition and immune response.

• Adaptations in membrane lipid composition are examples of evolutionary responses to environmental pressures.