The Cell: Plasma Membrane, Nucleus, and Cytoplasm – Structure and Function

The Cell: Structure and Function

Overview

The cell is the basic structural and functional unit of all living organisms. Understanding its major regions and the roles of its components is fundamental in Anatomy & Physiology.

Major Regions of a Generalized Cell

1. Plasma Membrane

• Definition: The plasma membrane forms the outer boundary of the cell, separating the internal environment from the external environment.

• Functions:

  • Physical barrier: Encloses the cell, separating cytoplasm from the extracellular fluid (ECF).

  • Selective permeability: Controls which substances can enter and exit the cell.

  • Communication: Interacts with chemical messengers and relays messages to the cell's interior.

  • Cell recognition: Carbohydrates on the membrane surface allow cells to recognize each other.

• Divides body fluids into:

  • Intracellular fluid (ICF): Fluid inside cells.

  • Extracellular fluid (ECF): Fluid outside cells.

2. The Nucleus

• Definition: The nucleus is the control center of the cell, containing the cell's DNA.

• Functions:

  • Stores genetic material (DNA).

  • Controls cell activity and function by regulating gene expression.

3. Cytoplasm

• Definition: The cytoplasm is the intracellular fluid and organelles found between the nucleus and plasma membrane.

• Components:

  • Cytosol: The fluid portion containing dissolved substances.

  • Organelles: Specialized structures with specific functions (e.g., mitochondria, ribosomes).

  • Inclusions: Non-living substances (e.g., glycogen, lipid droplets).

Plasma Membrane Structure

Composition

• 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 stiff or too fluid.

• Proteins:

  • Integral (transmembrane) proteins: Span the bilayer and are involved in transport, signal reception, enzymatic activity, and cell joining.

  • Peripheral proteins: Attached to the membrane surface; involved in signaling, maintaining cell shape, and forming part of cell junctions.

• Carbohydrates: Attached to proteins and lipids on the extracellular surface, important for cell recognition.

Membrane Junctions

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

• Desmosomes: Anchor cells together, providing mechanical strength and resistance to pulling forces.

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

Membrane Transport Mechanisms

Passive Transport

Passive transport does not require cellular energy (ATP). Substances move down their concentration gradients.

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

• Facilitated Diffusion: Movement of water-soluble (polar) molecules via membrane proteins (channels or carriers).

• Osmosis: Diffusion of water across a selectively permeable membrane, often through aquaporin channels.

Osmosis and Tonicity

• Osmolarity: Total solute concentration of a solution.

• Tonicity: The ability of a solution to change the shape of a cell by altering its internal water volume.

Solution Type	Effect on Cell
Isotonic	No net water movement; cell shape unchanged.
Hypertonic	Water leaves the cell; cell shrinks (crenation).
Hypotonic	Water enters the cell; cell swells and may burst (lysis).

Active Transport

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

• Primary Active Transport: Uses ATP directly. Example: Na+-K+ pump moves 3 Na+ out and 2 K+ into the cell per ATP hydrolyzed.

  • Equation: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Secondary Active Transport: Uses energy stored in concentration gradients (often Na+) created by primary active transport. Cotransporters move another substance against its gradient as Na+ moves down its gradient.

Vesicular Transport

• Endocytosis: Cell engulfs large molecules or particles from the ECF into vesicles.

• Exocytosis: Cell releases large molecules to the ECF by fusing vesicles with the plasma membrane.

• Both processes require ATP.

The Nucleus and Cytoplasm

The Nucleus

• Structure: Surrounded by a double membrane (nuclear envelope) with nuclear pores; contains chromatin (DNA and proteins) and nucleolus (site of ribosome assembly).

• Function: Stores genetic information, directs protein synthesis, and regulates cell activities.

Cytoplasm Components

• Cytosol: Fluid portion containing dissolved substances.

• Inclusions: Non-dissolved substances (e.g., glycogen, lipid droplets).

• Organelles: Specialized structures performing specific functions.

Organelles

• Ribosomes: Sites of protein synthesis; can be free in cytosol or bound to rough ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins for export or membrane insertion.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies chemicals, stores Ca2+.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes; break down waste, damaged organelles, and foreign substances.

• Peroxisomes: Detoxify harmful substances and neutralize free radicals; abundant in liver and kidney cells.

• Mitochondria: Powerhouse of the cell; site of aerobic respiration and ATP production.

  • Equation for aerobic respiration: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Cytoskeleton

• Microfilaments: Composed of actin; support cell shape, enable movement, and form microvilli.

• Intermediate Filaments: Rope-like proteins (e.g., keratin, desmin); provide mechanical strength and stabilize cell structure.

• Microtubules: Hollow tubes of tubulin; maintain cell shape, serve as tracks for organelle movement, form cilia and flagella.

Cellular Extensions

• Microvilli: Finger-like projections that increase surface area for absorption; supported by microfilaments.

• Cilia: Hair-like structures that move substances across cell surfaces; composed of microtubules.

Summary Table: Cell Junctions

Junction Type	Main Function	Key Structure
Tight Junction	Seals cells together, prevents leakage	Interlocking proteins
Desmosome	Provides mechanical strength	Linker proteins, intermediate filaments
Gap Junction	Allows communication between cells	Channel proteins (connexons)

Key Terms and Definitions

• Phospholipid bilayer: Double layer of phospholipids forming the basic structure of the plasma membrane.

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

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

• Endocytosis: Process of taking substances into the cell via vesicles.

• Exocytosis: Process of releasing substances from the cell via vesicles.

Example Applications

• Na+-K+ Pump: Maintains electrochemical gradients essential for nerve impulse transmission and muscle contraction.

• Osmosis in Red Blood Cells: Placing cells in hypotonic solution causes swelling; in hypertonic solution, cells shrink.