Week 5 - Oct 3

Lipids, Membranes, and Membrane Transport

Introduction to Biological Membranes

Biological membranes are essential structures that define cell boundaries and regulate the movement of substances into and out of cells. They are primarily composed of lipids and proteins, forming a dynamic and selectively permeable barrier.

• Flexible: Membranes allow cells to change shape, which is crucial for processes like movement and division.

• Repairable: Lipids can move to reform a continuous surface if the membrane is disrupted.

• Expandable: Cells can increase their surface area by adding new membrane lipids.

Example: During cell growth, new phospholipids are inserted into the membrane to accommodate increased surface area.

Structure and Fluidity of the Cell Membrane

The cell membrane is a phospholipid bilayer with embedded proteins. Phospholipids are in constant lateral motion, contributing to membrane fluidity, but rarely flip-flop between layers.

• Fluid Mosaic Model: Describes the membrane as a mosaic of lipids and proteins that move laterally within the layer.

• Selective Permeability: Some substances cross the membrane more readily than others due to its structure.

Factors Affecting Membrane Fluidity and Permeability

Several factors influence how fluid and permeable a membrane is:

• Ratio of Saturated vs. Unsaturated Fatty Acids: Unsaturated fatty acids (with double bonds) introduce kinks, preventing tight packing and increasing fluidity. Saturated fatty acids (no double bonds) pack closely, decreasing fluidity.

• Chain Length of Fatty Acid Tails: Shorter tails increase fluidity; longer tails decrease it.

• Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.

• Cholesterol: Modulates membrane fluidity and permeability (see below).

Factor	Effect on Fluidity	Effect on Permeability
High Unsaturated Fatty Acids	Increases	Increases
High Saturated Fatty Acids	Decreases	Decreases
Short Fatty Acid Tails	Increases	Increases
Long Fatty Acid Tails	Decreases	Decreases
High Temperature	Increases	Increases
Cholesterol (high temp)	Decreases	Decreases
Cholesterol (low temp)	Maintains	Maintains

Saturated vs. Unsaturated Fatty Acids

• Saturated Fatty Acids: No double bonds; straight chains; found in butter; pack tightly.

• Unsaturated Fatty Acids: One or more double bonds; kinked chains; found in oils; pack loosely.

Example: Butter (solid at room temperature) is high in saturated fats, while safflower oil (liquid at room temperature) is high in unsaturated fats.

Regulation of Membrane Fluidity

Cells can regulate membrane fluidity by adjusting the levels of unsaturated fatty acids. This is achieved by controlling the activity of desaturase enzymes, which introduce double bonds into fatty acid chains.

• Desaturase Enzymes: Catalyze the formation of double bonds, creating unsaturated fatty acids.

• Homeoviscous Adaptation: Organisms adjust fatty acid composition to maintain optimal membrane fluidity under varying temperatures.

Role of Cholesterol in Membranes

Cholesterol is an important component of animal cell membranes, affecting both fluidity and permeability.

• At high temperatures, cholesterol decreases fluidity by restricting phospholipid movement.

• At low temperatures, cholesterol prevents tight packing of phospholipids, maintaining fluidity.

Selective Permeability of the Membrane

The cell membrane is selectively permeable, allowing some molecules to cross more easily than others. Permeability depends on size, polarity, and charge.

Type of Molecule	Permeability	Examples
Small, nonpolar	High	O2, CO2, N2
Small, uncharged polar	Moderate	H2O, glycerol
Large, uncharged polar	Low	Glucose, sucrose
Ions	Very low	Na+, K+, Cl-

Key Point: The hydrophobic core of the membrane is a barrier to polar and charged molecules.

Why Membranes are Impermeable to Ions

Ions are surrounded by a hydration shell of water molecules, making it energetically unfavorable for them to cross the hydrophobic core of the membrane without assistance.

Membrane Transport Mechanisms

Types of Movement Across the Membrane

• Simple Diffusion (Passive): Movement of molecules from high to low concentration without energy input.

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

• Facilitated Diffusion (Passive): Movement of molecules down their concentration gradient with the help of membrane proteins.

• Active Transport: Movement of molecules against their concentration gradient, requiring energy (usually ATP).

Simple Diffusion

Simple diffusion is the net movement of substances from an area of high concentration to an area of low concentration. It is a spontaneous process, driven by entropy.

• Thermodynamics: $\Delta G < 0$, $\Delta S > 0$

• Example: Oxygen and carbon dioxide gas exchange in the lungs occurs by simple diffusion.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane due to differences in solute concentration.

• Water moves from regions of low solute concentration (hypotonic) to high solute concentration (hypertonic).

• The membrane allows water to pass but not solute molecules.

Tonicity and Water Movement

Condition	Water Movement	Effect on Cell
Hypertonic (outside > inside)	Out of cell	Cell shrinks
Hypotonic (outside < inside)	Into cell	Cell swells or bursts
Isotonic (outside = inside)	No net movement	No change

Membrane Proteins and Transport

Membrane proteins play critical roles in transport, enzymatic activity, signal transduction, and cell recognition.

• Transport Proteins: Facilitate movement of substances across the membrane.

• Enzymatic Activity: Some membrane proteins catalyze reactions at the membrane surface.

• Signal Transduction: Receptors transmit signals from outside to inside the cell.

• Cell-Cell Recognition: Glycoproteins serve as identification tags.

Types of Membrane Proteins

• Integral Proteins: Span the membrane; often amphipathic and include transmembrane proteins.

• Peripheral Proteins: Attached to one side of the membrane; do not interact with the hydrophobic core; easily detached.

Transmembrane Proteins

• Amphipathic: Contain both hydrophobic and hydrophilic regions.

• Structure: Alpha helices or beta sheets traverse the hydrophobic core.

• Multipass proteins can form channels or carriers for facilitated diffusion.

Example: Opsin is a multipass transmembrane protein involved in vision.

Facilitated Diffusion

Facilitated diffusion is the passive movement of molecules across the membrane with the help of transport proteins, such as channels or carriers. Substances move along their concentration gradient without energy input.

• Channel Proteins: Form pores for specific molecules (e.g., aquaporins for water).

• Carrier Proteins: Undergo conformational changes to transport molecules.

Example: Glucose transport into cells via GLUT transporters.

Additional info: Active transport (not detailed here) requires energy, often from ATP hydrolysis, to move substances against their concentration gradient.