Cell Structure and Function: Plasma Membrane, Cytoplasm, and Organelles

Ultrastructure and Function of the Cell

Overview

The cell is the fundamental structural and functional unit of all living organisms. It is composed of three major parts: the plasma membrane, the cytoplasm, and the nucleus. Each part plays a distinct role in maintaining cellular integrity and function.

• Plasma Membrane

• Cytoplasm

• Nucleus

The Plasma Membrane

Functions

The plasma membrane (also called plasmalemma) forms the outer boundary of the cell and serves several critical functions:

• Selective Permeability: Acts as a gate that allows certain substances to enter or exit the cell, maintaining homeostasis.

• Electrochemical Gradient: Maintains a difference in charge and chemical composition between the internal and external environments.

• Proteins: Contains peripheral and integral proteins that are essential for cell communication, defense, and transport.

Structure

According to the "Fluid Mosaic Model Theory" (Singer & Nicolson), the plasma membrane is primarily composed of phospholipids, proteins, and carbohydrates.

• Phospholipids: Form a fluid "sea" with a bilayer structure. Hydrophobic fatty acid chains (tails) face inward, while hydrophilic phosphate heads face outward.

• Proteins:

  • Peripheral Proteins: Attached to the surface; function as transporters or receptors.

  • Integral Proteins: Embedded within the membrane; act as anchors or channels.

• Carbohydrates: Glycolipids and glycoproteins form the glycocalyx, which acts as an adhesive, immune marker, and cell identity marker.

• Cholesterol: Interspersed among phospholipids; regulates membrane fluidity.

Movement Across the Plasma Membrane

Molecular transport across the plasma membrane occurs via two main mechanisms: passive movement and active movement.

Passive Movement

Passive movement occurs along the concentration gradient (from high to low concentration) and does not require energy (ATP). Types include:

Passive Movements
Diffusion
Facilitated Diffusion
Osmosis
Filtration
Dialysis

• Diffusion: Random movement of molecules from areas of high concentration to low concentration until equilibrium is reached. Example: Oxygen entering cells, inhaled spray dispersing in air.

• Facilitated Diffusion: Movement of molecules via protein channels or carriers.

• Osmosis: Movement of water molecules through a selectively permeable membrane from high to low water concentration. Osmotic Pressure: The force exerted by a solution to prevent water flow across a membrane.

Types of Solutions (Osmosis Context)

Type	Description	Effect on RBCs
Hypotonic	Lower solute concentration outside the cell than inside	Hemolysis (cell swells and bursts)
Isotonic	Equal solute concentration inside and outside the cell	No net movement; cell remains normal
Hypertonic	Higher solute concentration outside the cell than inside	Crenation (cell shrinks)

• Filtration: Movement of small particles across a membrane due to hydrostatic pressure (e.g., kidney glomeruli).

• Dialysis: Separation of solutes by a semipermeable membrane (e.g., artificial kidney machine).

Active Movement

Active movement requires energy (ATP) to transport substances against their concentration gradient (from low to high concentration).

Active Movements
Active Transport
Exocytosis
Endocytosis

• Active Transport: Uses energy to move substances across the membrane against the gradient (e.g., sodium-potassium pump).

• Exocytosis: Fusion of secretory vesicles with the plasma membrane to expel contents from the cell.

• Endocytosis: Engulfing of particles by the cell membrane to form vesicles within the cytoplasm. Types include:

  • Pinocytosis: "Cell drinking"; uptake of fluids and small molecules.

  • Receptor-mediated Endocytosis: Uptake of specific molecules via receptors.

  • Phagocytosis: "Cell eating"; engulfment of large particles (e.g., bacteria) by cells like neutrophils.

The Cytoplasm

Overview

The cytoplasm is the part of the cell between the plasma membrane and the nucleus. It consists of an aqueous phase and a particulate phase.

• Aqueous Phase (Fluid Phase): Contains water (75-90%), proteins, carbohydrates, lipids, nucleic acids, and inorganic substances.

• Particulate Phase: Consists of organelles and inclusions.

Organelles

• Endoplasmic Reticulum (ER): Network of tubes and sacs for substance transport. Two types:

  • Smooth ER: No ribosomes; involved in lipid synthesis, hormone production, and detoxification.

  • Rough ER: Studded with ribosomes; involved in protein synthesis.

• Ribosomes: Nonmembranous organelles for protein synthesis. Found free in cytoplasm or attached to ER.

• Golgi Apparatus (Golgi Complex): Series of flattened sacs for processing, storing, and packaging proteins.

• Lysosomes: Membrane-bound organelles containing digestive enzymes for breaking down waste and cellular debris.

Additional info: The notes do not cover the nucleus in detail, but it is the control center of the cell, containing genetic material (DNA) and regulating cell activities.