Lipids and Cell Membranes: Structure, Function, and Transport

Lipids and Cell Membranes

Introduction

Lipids are a diverse group of hydrophobic biological molecules that play essential roles in energy storage, cellular structure, and signaling. Cell membranes are primarily composed of lipids, which contribute to their selective permeability and dynamic nature.

Types of Lipids

Major Classes of Lipids

Lipids include several major classes, each with distinct structures and functions.

• Steroids

• Fats

• Phospholipids

• Waxes

General Structure of Lipids

• Nonpolar and Hydrophobic: Lipids are primarily nonpolar, making them insoluble in water.

• Mainly Composed of Carbon and Hydrogen: Their structure consists of long hydrocarbon chains or rings, with nonpolar covalent bonds.

Functions of Lipids

• Energy Storage: Lipids store energy efficiently due to their high caloric content.

• Structural Component: Lipids are key components of cell membranes, providing barrier and fluidity.

Steroids / Sterols

Structure and Function

Steroids are a class of lipids characterized by a core structure of four fused carbon rings.

• Structure: Four interlocking carbon rings with various functional groups attached.

• Functions:

  • Serve as hormones (e.g., estrogen, testosterone)

  • Act as vitamins (e.g., vitamin D)

  • Assist in fat digestion (e.g., bile acids)

  • Commonly found in cell membranes (e.g., cholesterol)

Examples: Cholesterol, Vitamin D

Fats (Triglycerides)

Structure

Fats, also known as triglycerides, are composed of one glycerol molecule bonded to three fatty acids.

• Glycerol: A three-carbon alcohol.

• Fatty Acids: Long hydrocarbon chains with a carboxyl group at one end.

Saturated vs. Unsaturated Fatty Acids

• Saturated Fatty Acids: All carbon atoms are single-bonded (saturated with hydrogen).

• Unsaturated Fatty Acids: Contain one or more double bonds (monounsaturated: one double bond; polyunsaturated: multiple double bonds).

Functions of Fats

• Energy Storage: Fats store energy for long-term use.

• Insulation and Cushioning: Adipose tissue protects organs and insulates the body.

• Waterproofing: Fats help prevent water loss in organisms.

Triglyceride Synthesis

Triglycerides are formed by dehydration synthesis, joining glycerol and three fatty acids.

• Equation: $\text{Glycerol} + 3\ \text{Fatty Acids} \rightarrow \text{Triglyceride} + 3\ \text{H}_2\text{O}$

Trans Fats

Trans fats are produced by hydrogenating unsaturated fats, making them solid at room temperature. They are associated with increased risk of heart disease.

• Byproduct of Hydrogenation: Alters the shape of fatty acid tails.

• Health Impact: Raises risk for heart attack.

Phospholipids

Structure

Phospholipids are major components of cell membranes, consisting of a glycerol backbone, two fatty acids, and a phosphate group.

• Glycerol: Forms the backbone.

• Fatty Acids: Hydrophobic tails.

• Phosphate Group: Hydrophilic head.

Phospholipid Bilayer

The cell membrane is a bilayer of phospholipids, with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Hydrophilic Heads: Face the extracellular and intracellular environments.

• Hydrophobic Tails: Face each other, forming the interior of the membrane.

Comparison: Phospholipids vs. Other Lipids

Phospholipids interact with water differently than other lipids due to their amphipathic nature (having both hydrophilic and hydrophobic regions).

• Phospholipids: Form bilayers in water, essential for membrane structure.

• Other Lipids (e.g., fats, steroids): Do not form bilayers; generally hydrophobic.

Cell Membranes

Organization and Function

Cell membranes define cell borders and organize cellular activities. The phospholipid bilayer contains enzymes and helps organize cell reactions within organelles.

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

• Control of Traffic: Regulates movement of molecules in and out of the cell.

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a mosaic of various molecules that can move laterally within the layer.

• Mosaic: Composed of phospholipids, proteins, and steroids.

• Fluid: Components drift laterally, allowing flexibility.

• Cholesterol: Stabilizes membrane and maintains fluidity at different temperatures.

Membrane Proteins

Types of Membrane Proteins

Membrane proteins are responsible for most membrane functions and are classified based on their association with the membrane.

• Integral Proteins: Permanently anchored within the membrane; may span the bilayer (transmembrane).

• Peripheral Proteins: Temporarily attached to one side of the membrane (extracellular or intracellular).

Functions of Membrane Proteins

• Attachment Proteins: Anchor the membrane to the cytoskeleton and extracellular matrix (ECM).

• Junction Proteins: Form intercellular junctions to attach adjacent cells.

• Receptor Proteins: Receive chemical signals from other cells and initiate signal transduction.

• Enzymes: Catalyze chemical reactions at the membrane.

• Glycoproteins: Identify cell type, individual, and species.

• Transporter Proteins: Move substances across the membrane, either passively (no energy) or actively (requires ATP).

Transport Across the Plasma Membrane

Overview

Cells maintain differences between their internal and external environments by controlling the movement of molecules across the plasma membrane.

• Import: Nutrients, ions, and signal molecules.

• Export: Wastes, signal molecules, excess ions.

Types of Transport

• Diffusion: Movement of molecules down their concentration gradient (high to low concentration); requires no energy.

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

• Facilitated Transport: Membrane proteins help hydrophilic molecules cross the membrane.

• Active Transport: Uses energy (ATP) to move substances against their concentration gradients.

• Bulk Transport: Movement of large molecules or small organisms across the membrane.

Diffusion

Diffusion is the passive movement of molecules from areas of high concentration to low concentration until equilibrium is reached.

• Brownian Motion: Random movement of molecules.

• Equilibrium: No net movement once concentrations are equal.

Factors Affecting Diffusion Rate

• Size: Smaller particles diffuse faster.

• Temperature: Higher temperatures increase diffusion rate.

• Concentration Gradient: Greater difference increases rate.

• Charge: Charged ions may be attracted or repelled.

• Pressure: Molecules move from high to low pressure.

Selectively Permeable Membranes

Cell membranes allow some molecules to pass more easily than others, depending on size, charge, and hydrophobicity.

• High Permeability: Small, nonpolar molecules (e.g., O2, CO2).

• Low Permeability: Large, polar molecules (e.g., glucose, sucrose).

Factors Influencing Permeability

• Saturation of Fatty Acid Tails: Unsaturated chains (with double bonds) increase permeability by preventing close packing.

• Length of Fatty Acid Tails: Longer chains reduce permeability due to stronger hydrophobic interactions.

• Cholesterol Content: Cholesterol decreases membrane permeability and increases density.

• Temperature: Lower temperatures decrease permeability by reducing fluidity.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane, moving from areas of high water concentration to low water concentration (toward higher solute concentration).

• Equilibrium: Water movement continues until solute concentrations are equal.

Tonicity

Tonicity describes the ability of a solution to cause water movement due to solute concentration.

• Isotonic: Equal solute concentration inside and outside the cell; no net water movement.

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

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

Osmosis in Plant vs. Animal Cells

Solution Type	Animal Cell	Plant Cell
Hypotonic	Lysed (bursts)	Turgid (normal)
Isotonic	Normal	Flaccid
Hypertonic	Shriveled	Plasmolyzed

Review Questions

• What are the structure and function of fats, phospholipids, and steroids?

• Compare and contrast how phospholipids and other lipids interact with water.

• Explain the fluid mosaic model.

• Describe the different functions of membrane proteins.

• How do proteins stay in place within the cell membrane?

• What is diffusion?

• How do different factors influence the speed of diffusion?

• Explain selective permeability of the membrane and how different factors influence permeability.

• What is osmosis?

• How does tonicity (hyper-, hypo-, iso-) influence water movement?