Cell Membrane Structure and Function: Study Notes for Anatomy & Physiology

The Cell Membrane

Introduction

The cell membrane, also known as the plasma membrane, is a fundamental structure in all living cells. It serves as a barrier, regulates the movement of substances, and facilitates communication between the cell and its environment.

Learning Outcomes

• Understand cell theory and the scientific discoveries that led to it.

• Understand variation in and limits on cellular morphology.

• Know the three basic components of cells and the terminology used to describe them.

• Describe how lipids, carbohydrates, and proteins are distributed in a cell membrane and explain their respective functions.

• Understand passive transport mechanisms across cell membranes.

• Understand active transport mechanisms across cell membranes.

Cell Theory

Historical Development

Cell theory is a cornerstone of biology, describing the properties and functions of cells. Key scientific discoveries contributed to its development:

• Robert Hook (1633): First observed cells in cork tissue.

• Anton Van Leeuwenhoek (1677): Improved microscopes and observed living cells.

• Schleiden & Schwann (1838): Formulated the basic cell theory.

• Louis Pasteur (1859): Demonstrated that cells arise from preexisting cells.

Original Cell Theory

• All living organisms are composed of cells.

• The cell is the most basic unit of life.

• All cells come from preexisting cells.

Modern Cell Theory

Modern cell theory expands on the original statements:

• The cell contains hereditary information (DNA) passed from cell to cell during division.

• All cells are basically the same in chemical composition and metabolic activities.

• All basic chemical and physiological functions are carried out inside the cells (e.g., movement, digestion).

• Cell activity depends on the activities of sub-cellular structures (organelles, nucleus, plasma membrane).

Cellular Morphology

Variation and Limits

Cells vary widely in shape and size, which affects their function and efficiency.

• Most human cells are 10–15 μm in diameter.

• Surface area to volume ratio is crucial; as cells grow, volume increases faster than surface area, limiting cell size.

• Examples of cell shapes: Bacterial cells (small, simple), Plant cells (rectangular), Egg cells (large, spherical), Nerve cells (long, thin extensions).

Additional info: Large cells may have specialized adaptations (e.g., elongated shape in neurons) to overcome surface area limitations.

Basic Components of Cells

Major Structures

• Plasma membrane: Outer boundary of the cell.

• Cytoplasm: Internal fluid containing organelles.

• Nucleus: Contains genetic material (DNA).

• Organelles: Specialized structures (e.g., mitochondria, endoplasmic reticulum).

• Cytoskeleton: Network of protein filaments providing structural support.

Structure of the Cell Membrane

Fluid Mosaic Model

The cell membrane is described by the fluid mosaic model, which depicts a dynamic bilayer of lipids interspersed with proteins and carbohydrates.

• Lipid bilayer: Composed mainly of phospholipids (75%), cholesterol (20%), and glycolipids (5%).

• Proteins: Integral (span the membrane) and peripheral (attached to one side).

• Carbohydrates: Attached to lipids and proteins, forming the glycocalyx.

Distribution and Functions of Membrane Components

• Phospholipids: Amphipathic molecules forming the bilayer; provide fluidity and barrier function.

• Cholesterol: Stabilizes and stiffens the membrane.

• Glycolipids: Contribute to the glycocalyx, involved in cell recognition.

• Proteins:

  • Integral proteins: Transporters, channels, receptors.

  • Peripheral proteins: Enzymes, structural support.

• Glycocalyx: Carbohydrate-rich area on the cell surface; functions in cell recognition, adhesion, and immune response.

Functions of Membrane Proteins

• Transport: Move substances across the membrane.

• Receptors: Bind signaling molecules and initiate cellular responses.

• Enzymes: Catalyze chemical reactions.

• Cell adhesion: Help cells stick to each other and to the extracellular matrix.

• Cell identity: Glycoproteins serve as markers for cell recognition.

Passive Transport Mechanisms

Overview

Passive transport moves substances across the cell membrane without energy input, relying on concentration gradients.

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

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

• Facilitated diffusion: Movement of substances via membrane proteins.

Factors Affecting Diffusion Rate

• Temperature: Higher temperature increases kinetic energy and diffusion rate.

• Molecule size: Smaller molecules diffuse faster.

• Concentration gradient: Steeper gradients increase diffusion rate.

• Membrane surface area: Larger area allows more diffusion.

• Membrane permeability: More permeable membranes facilitate diffusion.

Osmosis and Tonicity

• Osmosis: Movement of water from low solute concentration to high solute concentration.

• Tonicity: Describes the effect of a solution on cell volume (isotonic, hypotonic, hypertonic).

• Aquaporins: Channel proteins that facilitate water movement.

Additional info: In hypotonic solutions, cells may swell; in hypertonic solutions, cells may shrink.

Active Transport Mechanisms

Overview

Active transport requires energy (usually ATP) to move substances against their concentration gradients.

• Primary active transport: Direct use of ATP to transport molecules (e.g., sodium-potassium pump).

• Secondary active transport: Uses the gradient created by primary transport to move other substances.

• Vesicular transport: Movement of large molecules via vesicles (endocytosis, exocytosis, transcytosis).

Types of Membrane Transport Proteins

Type	Direction	Example
Uniport	One substance, one direction	Calcium pump
Symport	Two substances, same direction	Sodium-glucose transporter
Antiport	Two substances, opposite directions	Sodium-potassium pump

Sodium-Potassium Pump

• Maintains high sodium concentration outside and high potassium concentration inside the cell.

• Essential for cell signaling, muscle contraction, and osmotic balance.

• Uses ATP to transport ions against their gradients.

$\text{Na}^+/\text{K}^+ \text{ pump:} \, 3\, \text{Na}^+ \text{ out}, \, 2\, \text{K}^+ \text{ in}, \, \text{per ATP hydrolyzed}$

Vesicular Transport

• Endocytosis: Uptake of large molecules into the cell.

• Phagocytosis: "Cell eating" of large particles.

• Pinocytosis: "Cell drinking" of fluids.

• Receptor-mediated endocytosis: Specific uptake via receptors.

• Exocytosis: Discharge of substances out of the cell.

• Transcytosis: Transport of molecules across the cell.

Membrane Potential and Electrophysiology

Introduction

The separation of charges across the plasma membrane creates an electrical gradient, known as the membrane potential.

• Resting membrane potential is typically negative inside the cell relative to the extracellular fluid.

• Generated by ion transport (mainly Na+ and K+).

• Essential for nerve impulse transmission and muscle contraction.

$\text{Membrane potential} = \text{Voltage difference across plasma membrane}$

Summary Table: Membrane Transport Mechanisms

Transport Type	Energy Required	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Osmosis	No	Down gradient	Water via aquaporins
Primary Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Secondary Active Transport	Indirect (gradient)	Against gradient	Sodium-glucose symport
Vesicular Transport	Yes (ATP)	Bulk movement	Endocytosis, exocytosis

Clinical Application: Hydration and Sports Drinks

During intense exercise, water and electrolytes are lost through sweating, affecting extracellular fluid (ECF) tonicity. Sports drinks, which are mildly hypotonic, help restore normal cell hydration. Overhydration with plain water can lead to cellular swelling and water poisoning.