Physio: Cell Structure and Membrane Transport

The Cell

Basic Unit of Structure

The cell is the fundamental unit of structure and function in the human body. All physiological processes in organs are derived from the specialized activities of cells.

• Specialized cells include cardiac muscle cells, skeletal muscle cells, nerve cells, and adipose (fat) cells.

• Each cell type is adapted to perform specific functions essential for organ physiology.

• Examples:

  • Cardiac muscle cells: contract to pump blood.

  • Skeletal muscle cells: enable voluntary movement.

  • Nerve cells: transmit electrical signals.

  • Adipose cells: store energy as fat.

Structure of a Typical Cell

A typical eukaryotic cell contains several organelles, each with distinct functions.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Nucleolus: Site of ribosome synthesis.

• Plasma membrane: Encloses the cell, regulating entry and exit of substances.

• Mitochondrion: Produces ATP through cellular respiration.

• Golgi complex: Modifies, sorts, and packages proteins and lipids.

• Lysosome: Contains digestive enzymes for breaking down waste.

• Endoplasmic reticulum (rough and smooth): Synthesizes proteins and lipids.

• Ribosome: Site of protein synthesis.

• Centrioles: Involved in cell division.

• Secretory vesicle: Transports materials out of the cell.

Additional info: The cytoskeleton (microtubules, actin filaments) provides structural support and facilitates intracellular transport.

Cell Communication with the Exterior

Role of the Plasma Membrane

The plasma membrane enables cells to interact with their environment while maintaining internal conditions.

• Exchange: Allows selective exchange of substances with the extracellular environment.

• Isolation: Maintains the integrity of the cell by keeping its contents concentrated and protected.

The Plasma Membrane

Composition and Structure

The plasma membrane is primarily composed of phospholipids, which form a bilayer structure.

• Phospholipids: Molecules with a hydrophilic (polar) head and hydrophobic (non-polar) tails.

• Amphipathic nature: Each phospholipid contains both a polar and a non-polar component, allowing the formation of a bilayer in aqueous environments.

• Additional info: The bilayer is interspersed with proteins (integral and peripheral), cholesterol, and carbohydrates, contributing to membrane fluidity and function.

Phospholipid Structure

• Polar head: Contains a phosphate group and is hydrophilic (water-attracting).

• Non-polar tails: Composed of fatty acid chains, hydrophobic (water-repelling).

• Amphipathic: This dual nature is critical for membrane formation and function.

Example: In water, phospholipids spontaneously arrange into bilayers, with hydrophobic tails facing inward and hydrophilic heads facing outward.

Functions of the Plasma Membrane

• Barrier: Separates the cell from its environment, controlling the movement of molecules and ions.

• Communication: Contains receptors for signaling molecules.

• Transport: Facilitates the movement of substances via passive and active mechanisms.

• Fluid mosaic model: Describes the dynamic arrangement of lipids and proteins, allowing lateral movement and clustering.

Key Terms

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

• Amphipathic: Having both hydrophilic and hydrophobic regions.

• Integral proteins: Embedded within the membrane, often functioning as channels or receptors.

• Peripheral proteins: Attached to the membrane surface, involved in signaling and structural support.

Summary Table: Major Cell Organelles and Functions

Organelle	Main Function
Nucleus	Genetic control center
Mitochondrion	ATP production
Golgi complex	Protein and lipid modification, sorting
Lysosome	Intracellular digestion
Endoplasmic reticulum	Protein and lipid synthesis
Ribosome	Protein synthesis
Plasma membrane	Selective barrier, communication
Secretory vesicle	Transport of cellular products

Summary Table: Plasma Membrane Components

Component	Function
Phospholipids	Form bilayer, barrier function
Proteins (integral & peripheral)	Transport, signaling, structural support
Cholesterol	Regulates fluidity
Carbohydrates	Cell recognition, signaling

Key Formula: Fluid Mosaic Model

The fluid mosaic model is a conceptual framework, not a mathematical formula, but the arrangement can be described as:

• Phospholipid bilayer + Proteins + Cholesterol + Carbohydrates = Dynamic, functional plasma membrane

Additional info: The amphipathic nature of phospholipids is essential for the selective permeability of the membrane, allowing the cell to maintain homeostasis.