The Cell: Plasma Membrane Structure and Function

The Cell: Plasma Membrane Structure and Function

Overview of Cell Structure

Cells are the basic units of life and are composed of three major regions, each with distinct structures and functions. Understanding these regions is fundamental to the study of anatomy and physiology.

• Plasma Membrane: Forms the outer boundary of the cell and separates the internal environment from the external environment.

• Nucleus: Contains the cell’s genetic material (DNA) and controls cellular activities.

• Cytoplasm: The region between the plasma membrane and nucleus, containing cytosol and organelles.

Key Terms

• Intracellular Fluid (ICF): Fluid inside the cell.

• Extracellular Fluid (ECF): Fluid outside the cell.

Functions of the Plasma Membrane

The plasma membrane is a dynamic structure that plays several critical roles in cell physiology.

• Physical Barrier: Encloses the cell, separating the cytoplasm from the ECF.

• Selective Permeability: Regulates which substances can enter or exit the cell.

• Communication: Interacts with chemical messengers and relays signals to the cell’s interior.

• Cell Recognition: Carbohydrates on the membrane surface allow cells to identify and interact with each other.

Plasma Membrane Composition

The plasma membrane is primarily composed of lipids, proteins, and carbohydrates, each contributing to its structure and function.

• Phospholipids: Form a bilayer with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward. This arrangement creates a semi-permeable barrier.

• Cholesterol: Inserts between phospholipid tails, stabilizing membrane fluidity and preventing it from becoming too rigid or too fluid.

• Proteins:

  • Integral (Transmembrane) Proteins: Span the membrane and are involved in transport, signaling, enzymatic activity, and cell junction formation.

  • Peripheral Proteins: Attached to the membrane surface, assisting in signal transduction, cell shape maintenance, and movement.

• Carbohydrates: Attached to proteins and lipids on the extracellular surface, playing a role in cell recognition and communication.

Cell Junctions

Cell junctions are specialized structures that connect adjacent cells and facilitate communication and structural integrity.

• Tight Junctions: Seal adjacent cells together, preventing leakage of substances between cells.

• Desmosomes: Anchor cells together, providing resistance to mechanical stress.

• Gap Junctions: Allow direct communication between cells through channels that permit the passage of ions and small molecules.

Membrane Transport Mechanisms

Substances move across the plasma membrane via passive or active transport processes.

Passive Transport

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

• Facilitated Diffusion: Movement of polar or charged molecules (e.g., glucose, ions) via membrane proteins (channels or carriers).

• Osmosis: Diffusion of water across the membrane, often through aquaporin channels.

Active Transport

• Primary Active Transport: Uses ATP to move substances against their concentration gradient via pumps (e.g., Na+-K+ pump).

• Secondary Active Transport: Uses energy stored in ion gradients created by primary active transport to move other substances against their gradient.

• Endocytosis: Uptake of large molecules into the cell via vesicles.

• Exocytosis: Release of large molecules from the cell via vesicles.

Osmosis and Tonicity

Osmosis is the movement of water across a selectively permeable membrane. Tonicity describes how a solution affects cell volume:

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

• Hypertonic: Water moves out of the cell; cell shrinks.

• Hypotonic: Water moves into the cell; cell swells.

Summary Table: Cell Regions and Functions

Region	Main Components	Primary Functions
Plasma Membrane	Phospholipids, proteins, cholesterol, carbohydrates	Barrier, selective transport, communication, recognition
Nucleus	DNA, nuclear envelope, nucleolus	Genetic control, ribosome synthesis
Cytoplasm	Cytosol, organelles	Metabolic activities, structural support

Example: Na+-K+ Pump Equation

The sodium-potassium pump uses ATP to transport ions:

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

Review Questions

• What are the three main regions of a human cell?

• Why do phospholipids form a bilayer in water?

• What are the functions of transmembrane proteins?

• Which cell junction aids in cell-to-cell communication?

• How does facilitated diffusion differ from simple diffusion?

• What happens to a cell in a hypertonic solution?

• What is the difference between endocytosis and exocytosis?

Additional info: Expanded explanations and table added for academic completeness.