Membrane Structure and Transport: Study Notes for General Biology

Membrane Structure and Transport

Introduction

The cell membrane, also known as the plasma membrane, is a fundamental structure in all living cells. It serves as a selective barrier, separating the intracellular fluid (ICF) from the extracellular fluid (ECF), and plays a dynamic role in cellular activity by regulating the movement of substances into and out of the cell.

Phospholipid Structure

Amphipathic Nature of Phospholipids

Phospholipids are the primary lipid components of the plasma membrane. They possess an amphipathic structure, meaning each molecule has both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

• Polar (hydrophilic) head: Composed of choline, phosphate, and glycerol.

• Nonpolar (hydrophobic) tails: Consist of fatty acid chains.

This arrangement allows phospholipids to form bilayers, with hydrophobic tails facing inward and hydrophilic heads facing outward toward the aqueous environment.

Phospholipid Diversity in Red Blood Cells (RBCs)

The plasma membrane of human red blood cells contains various types of phospholipids, such as phosphatidylserine, phosphatidylcholine, and sphingomyelin. Glycolipids, which have carbohydrate groups attached, are also present and play roles in cell recognition.

• Approximately 5 x 106 lipid molecules are found in a 1 μm x 1 μm area of the membrane.

• Glycolipids are identified by blue hexagons in diagrams.

Functions of the Cell Membrane

Barrier and Selective Permeability

The cell membrane acts as an active barrier, maintaining the distinct environments of the ICF and ECF. Its selective permeability allows certain substances to pass while restricting others.

• Controls entry and exit of ions, nutrients, and waste products.

• Facilitates communication and signaling between cells.

Selective permeability is essential for maintaining homeostasis and proper cellular function.

Membrane Proteins

Types and Functions

Membrane proteins are crucial for the specialized functions of the plasma membrane. They are categorized as integral (embedded within the membrane) or peripheral (attached to the membrane surface).

• Transport proteins: Channels and carriers that facilitate movement of substances.

• Enzymes: Catalyze reactions at the membrane surface.

• Motor proteins: Involved in cell division and movement.

• Cell-to-cell connection proteins: Help form cell junctions and maintain tissue integrity.

Glycocalyx

Structure and Biological Importance

The glycocalyx is a carbohydrate-rich layer on the cell surface, formed by glycoproteins and glycolipids. It serves as a biological marker for cell recognition and immune response.

• Enables the immune system to distinguish "self" from "nonself" cells.

• Patterns of glycocalyx can change, especially in cancer cells, affecting immune recognition.

Fluid Mosaic Model

Membrane Structure and Dynamics

The plasma membrane is described by the fluid mosaic model, which depicts the membrane as a dynamic structure with proteins floating in or on a fluid lipid bilayer.

• Membrane lipids form a flexible bilayer.

• Proteins move laterally, creating a mosaic pattern.

• Glycocalyx forms the "sugar coating" on the cell surface.

Experimental evidence, such as the rapid intermixing of proteins in hybrid cells, supports this model.

Membrane Lipid Composition and Adaptation

Role of Cholesterol and Fatty Acid Saturation

Membrane fluidity is influenced by lipid composition, including the presence of cholesterol and the saturation of fatty acid tails.

• Unsaturated fatty acid tails: Increase fluidity by preventing tight packing.

• Saturated fatty acid tails: Decrease fluidity by allowing closer packing.

• Cholesterol: Modulates membrane fluidity and stability, typically comprising 20-40% of membrane lipids.

Organisms can adjust membrane lipid composition in response to environmental temperature changes to maintain optimal fluidity.

Membrane Transport Mechanisms

Overview

Substances move across the plasma membrane via passive or active transport mechanisms. The membrane's selective permeability allows only certain molecules to cross freely.

• Passive transport: No energy required; substances move down their concentration gradient.

• Active transport: Requires energy (ATP); substances move against their concentration gradient.

Passive Transport

Passive transport includes simple diffusion, facilitated diffusion, and osmosis.

• Simple diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2, steroid hormones) directly through the lipid bilayer.

• Facilitated diffusion: Movement of larger or polar molecules (e.g., glucose, amino acids, ions) via carrier or channel proteins.

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

All passive transport processes involve movement from areas of high concentration to low concentration (down the concentration gradient).

Facilitated Diffusion

• Carrier-mediated: Specific solutes bind to carrier proteins, causing a conformational change that transports the solute across the membrane.

• Channel-mediated: Solutes move through channel proteins based on size and charge.

Examples include glucose and amino acid transport.

Osmosis and Tonicity

Osmosis refers to the movement of water across the membrane. Tonicity describes the effect of solution concentration on cell volume.

• Isotonic solution: No net movement of water; cell volume remains stable.

• Hypertonic solution: Water leaves the cell; cell shrinks (crenation in RBCs).

• Hypotonic solution: Water enters the cell; cell swells and may burst (lysis in animal cells).

Active Transport

Primary and Secondary Active Transport

Active transport uses energy to move substances against their concentration gradients.

• Primary active transport: Direct use of ATP to pump ions (e.g., Na+/K+ ATPase).

• Secondary active transport: Uses the energy stored in ion gradients created by primary active transport to move other substances (e.g., glucose uptake via Na+-glucose cotransporter).

Example equation for Na+/K+ ATPase:

$3\text{Na}^+_{\text{out}} + 2\text{K}^+_{\text{in}} + \text{ATP} \rightarrow 3\text{Na}^+_{\text{in}} + 2\text{K}^+_{\text{out}} + \text{ADP} + \text{P}_i$

Summary Table: Types of Membrane Transport

Transport Type	Energy Required	Direction	Examples
Simple Diffusion	No	High to Low	O2, CO2
Facilitated Diffusion	No	High to Low	Glucose, Ions
Osmosis	No	High to Low (water)	Water
Active Transport	Yes (ATP)	Low to High	Na+/K+ pump

Key Terms

• Phospholipid: Amphipathic molecule forming the basic structure of cell membranes.

• Integral protein: Protein embedded within the membrane.

• Peripheral protein: Protein attached to the membrane surface.

• Glycocalyx: Carbohydrate-rich layer on the cell surface.

• Selective permeability: Property allowing some substances to cross the membrane more easily than others.

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a membrane.

• Tonicity: Effect of solution concentration on cell volume.

• Active transport: Movement of substances against their concentration gradient using energy.

Additional info: Academic context and definitions have been expanded for clarity and completeness.