Cell Membranes: Structure, Function, and Chemistry

Functional Groups in Cell Chemistry

Key Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In cell biology, seven functional groups are especially important:

• Hydroxyl group (-OH): Found in alcohols; increases solubility in water.

• Carbonyl group (C=O): Found in aldehydes and ketones; important in sugars.

• Carboxyl group (-COOH): Characteristic of acids; can donate a proton.

• Amino group (-NH2): Found in amino acids; acts as a base.

• Sulfhydryl group (-SH): Important in protein structure (disulfide bonds).

• Phosphate group (-PO4): Key in energy transfer (ATP).

• Methyl group (-CH3): Affects gene expression and protein function.

Macromolecules and Their Formation

Major Linkages in Macromolecules

Macromolecules are formed by the joining of smaller units (monomers) through specific linkages:

• Proteins: Linked by peptide bonds between amino acids.

• Carbohydrates: Linked by glycosidic bonds between monosaccharides.

• Nucleic Acids: Linked by phosphodiester bonds between nucleotides.

Lipids

Structure and Function

Lipids are hydrophobic molecules that play key roles in energy storage, membrane structure, and signaling.

• Fatty acids: Long hydrocarbon chains; building blocks of many lipids.

• Triglycerides: Three fatty acids linked to glycerol; main energy storage form.

• Phospholipids: Major component of cell membranes; amphipathic nature (hydrophilic head, hydrophobic tails).

Phospholipids and Membrane Structure

Phospholipid Bilayer

Phospholipids spontaneously form bilayers in aqueous environments, creating the fundamental structure of biological membranes.

• Hydrophilic heads face outward toward water.

• Hydrophobic tails face inward, away from water.

Cell Membrane Functions

Compartmentalization and Selectivity

Cell membranes separate the cell from its environment and regulate the movement of substances.

• Selective permeability: Allows certain molecules to pass while blocking others.

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

• Transport: Import and export of nutrients and waste.

The Fluid Mosaic Model

Membrane Structure

The fluid mosaic model describes the cell membrane as a dynamic structure composed of a phospholipid bilayer with embedded proteins.

• Lipids: Provide fluidity and flexibility.

• Proteins: Serve as channels, receptors, and enzymes.

• Carbohydrates: Attached to lipids (glycolipids) and proteins (glycoproteins); involved in cell recognition.

Major Classes of Membrane Lipids

Phospholipids, Glycolipids, and Steroids

• Phospholipids: Main structural component; amphipathic.

• Glycolipids: Lipids with carbohydrate groups; important for cell recognition.

• Steroids: Cholesterol is the main steroid in animal membranes; modulates fluidity.

Regulating Membrane Fluidity

Factors Affecting Fluidity

• Fatty acid composition: Unsaturated fatty acids increase fluidity; saturated decrease it.

• Cholesterol: Buffers membrane fluidity by preventing extremes.

• Temperature: Higher temperatures increase fluidity.

Studying Membrane Fluidity

FRAP (Fluorescence Recovery After Photobleaching)

FRAP is a technique used to measure the lateral mobility of membrane components.

• Fluorescently labeled molecules are bleached in a region.

• Recovery of fluorescence indicates movement of unbleached molecules into the area.

Membrane Proteins

Types and Functions

• Integral (transmembrane) proteins: Span the membrane; involved in transport and signaling.

• Peripheral proteins: Attached to membrane surface; involved in signaling and structural support.

• Glycoproteins: Proteins with carbohydrate groups; important for cell-cell recognition.

Protein Structure in Membranes

Quaternary Structure and Complexes

• Many membrane proteins form complexes for function (e.g., channels, receptors).

• Quaternary structure refers to the assembly of multiple polypeptide chains.

Experimental Techniques

Freeze Fracture and Hydropathy Analysis

• Freeze fracture: Technique to visualize membrane structure by splitting the bilayer.

• Hydropathy analysis: Predicts transmembrane regions based on hydrophobicity of amino acids.

Summary Table: Major Membrane Components

Component	Function	Example
Phospholipid	Structural framework	Phosphatidylcholine
Glycolipid	Cell recognition	Ganglioside
Steroid	Fluidity regulation	Cholesterol
Integral Protein	Transport, signaling	Ion channel
Peripheral Protein	Support, signaling	Spectrin
Glycoprotein	Recognition	MHC complex

Key Equations

Membrane Fluidity (Diffusion)

The rate of lateral diffusion in membranes can be described by:

$D = \frac{kT}{4\pi\eta r}$

Where D is the diffusion coefficient, k is Boltzmann's constant, T is temperature, η is viscosity, and r is the radius of the diffusing particle.

Additional info:

• These notes cover material relevant to Ch. 2 (The Chemistry of the Cell), Ch. 3 (The Macromolecules of the Cell), Ch. 7 (Membranes: Their Structure, Function, and Chemistry), and Ch. 8 (Transport Across Membranes: Overcoming the Permeability Barrier).