Biological Macromolecules and Membrane Structure & Function: Lipids and Membranes

Biological Macromolecules and Membrane Structure & Function

Introduction

This study guide covers the structure and function of biological membranes, focusing on the role of lipids, especially fatty acids, triglycerides, and phospholipids. Understanding these macromolecules is essential for grasping how membranes maintain cellular integrity and regulate interactions with the environment.

Fatty Acids and Lipid Formation

Structure of Fatty Acids

• Fatty acids are long hydrocarbon chains with a carboxyl group (-COOH) at one end.

• Common chain lengths are 16 or 18 carbons (e.g., palmitic acid is C16:0).

• Fatty acids can be saturated (no double bonds) or unsaturated (one or more double bonds).

• Cis double bonds in unsaturated fatty acids cause kinks or bends in the chain, affecting packing and membrane fluidity.

Example: Palmitic acid (C16:0) is a saturated fatty acid commonly found in animal fats.

Functions of Fatty Acids

• Serve as building blocks for complex lipids.

• Major components of cell membranes.

• Energy storage molecules in the form of triglycerides.

Lipids: Structure and Types

Triglycerides (Triacylglycerols)

• Triglycerides are formed by esterification of three fatty acids to a glycerol backbone.

• They function as energy storage molecules in animals and plants.

• Structure: Three fatty acid chains attached to a single glycerol molecule.

Equation:

$\text{Triglyceride} = \text{Glycerol} + 3 \times \text{Fatty Acid} \rightarrow \text{Triacylglycerol} + 3 \times \text{H}_2\text{O}$

Saturated vs. Unsaturated Fats

• Saturated fats have no double bonds; their straight chains pack tightly, making them solid at room temperature (e.g., butter).

• Unsaturated fats have one or more cis double bonds, causing bends that prevent tight packing, making them liquid at room temperature (e.g., olive oil).

• Cis double bonds are crucial for membrane fluidity.

Example: The presence of cis double bonds in unsaturated fats leads to the characteristic bend in the fatty acid chain.

Phospholipids and Membrane Structure

Phospholipid Structure

• Phospholipids consist of a glycerol backbone, two fatty acid tails, and a phosphate group attached to a polar head (e.g., choline).

• They are amphipathic molecules, having both hydrophobic (tails) and hydrophilic (head) regions.

Example: Phosphatidylcholine is a common phospholipid in cell membranes.

Phospholipid Bilayer Formation

• In water, phospholipids spontaneously arrange into bilayers, with hydrophobic tails facing inward and hydrophilic heads facing outward.

• This bilayer forms the fundamental structure of biological membranes.

Membrane Fluidity and Adaptation

Determinants of Membrane Fluidity

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

• Fatty acid composition: Unsaturated fatty acids increase fluidity due to kinks; saturated fatty acids decrease fluidity.

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

Additional info: Membrane fluidity is an evolutionary adaptation; organisms in cold environments have more unsaturated fatty acids in their membranes.

Importance of Membrane Fluidity

• Fluidity affects membrane permeability and the mobility of membrane proteins.

• Enzyme activity and transport processes depend on proper fluidity.

Types of Membrane Proteins

Classification of Membrane Proteins

• Peripheral membrane proteins: Associated with the outer or inner surfaces of the membrane; do not span the bilayer.

• Integral membrane proteins: Embedded within the membrane; may not span the entire bilayer.

• Transmembrane proteins: Span the entire membrane from the exterior to the interior of the cell.

Additional info: Membrane proteins often have hydrophobic domains (α-helices or β-barrels) that interact with the lipid bilayer.

Transport Across Membranes

Passive Transport

• Simple diffusion: Small, nonpolar molecules (e.g., O2, CO2, hydrocarbons) pass directly through the lipid bilayer.

• Facilitated diffusion: Polar molecules and ions move across membranes via channel or carrier proteins, down their concentration gradient.

Equation:

$\text{Rate} = k[\text{Nucleophile}][\text{Electrophile}]$ Additional info: This equation is a general rate law; for membrane transport, the rate depends on concentration gradients and protein availability.

Osmosis

• Osmosis is the movement of water across a semi-permeable membrane, down its concentration gradient.

• Water moves via specialized channel proteins called aquaporins.

Equation:

$\text{Osmosis}: \text{Water moves from high to low concentration across a semi-permeable membrane}$

Summary Table: Types of Membrane Proteins

Type	Location	Function
Peripheral	Surface of membrane	Cell signaling, structural support
Integral	Embedded in membrane	Transport, enzymatic activity
Transmembrane	Span entire membrane	Transport, cell communication

Key Concepts

• Biological membranes are composed primarily of phospholipids, cholesterol, and proteins.

• Membrane fluidity is regulated by temperature, fatty acid composition, and cholesterol.

• Transport across membranes can be passive (diffusion, osmosis) or facilitated by proteins.

• Membrane proteins are classified by their association with the bilayer and their function.