Chapter 3: Cells – The Living Units

3.1 Cells: The Smallest Living Units

Cells are the fundamental structural and functional units of all living organisms. Understanding their diversity, structure, and function is essential for studying anatomy and physiology.

• Definition: A cell is the smallest unit of life capable of carrying out all vital processes.

• Cell Theory:

  • All organisms are composed of one or more cells.

  • The cell is the basic unit of structure and organization in organisms.

  • Cells arise from pre-existing cells.

• Cell Diversity: Cells vary in size, shape, and function, reflecting their specialized roles in the body.

Example: Muscle cells are elongated for contraction, while nerve cells have long extensions for communication.

Structure of the Generalized Cell

Most cells share common structural components, although their specific features may differ according to function.

• Plasma Membrane: Outer boundary of the cell, regulates entry and exit of substances.

• Cytoplasm: Contains organelles and cytosol.

• Nucleus: Control center containing genetic material (DNA).

Extracellular Materials

Extracellular materials are substances found outside the cells, contributing to tissue structure and function.

• Examples: Interstitial fluid, blood plasma, and extracellular matrix.

• Function: Provide nutrients, remove wastes, and facilitate cell communication.

3.2 Structure of the Plasma Membrane

Plasma Membrane Overview

The plasma membrane is a dynamic structure that separates the cell from its environment and controls the movement of substances in and out.

• Fluid Mosaic Model: Describes the membrane as a flexible layer with proteins embedded in or attached to a bilayer of phospholipids.

Membrane Lipids

Lipids are the primary structural components of the plasma membrane.

• Phospholipids: Form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

• Cholesterol: Stabilizes membrane fluidity.

• Glycolipids: Contribute to cell recognition and interaction.

Membrane Proteins

Proteins embedded in the plasma membrane perform various essential functions.

• Integral Proteins: Span the membrane and are involved in transport and signaling.

• Peripheral Proteins: Attached to the membrane surface, often involved in cell signaling or maintaining cell shape.

Functions of Membrane Proteins

• Transport: Move substances across the membrane.

• Receptors: Bind signaling molecules and initiate cellular responses.

• Enzymatic Activity: Catalyze reactions at the membrane surface.

• Cell Recognition: Allow cells to identify each other.

• Cell Joining: Facilitate cell-to-cell adhesion and communication.

Glycocalyx

The glycocalyx is a carbohydrate-rich area on the cell surface, important for cell recognition and protection.

• Function: Enables cells to recognize each other and provides protection from mechanical and chemical damage.

Cell Junctions

Cell junctions are specialized structures that connect adjacent cells, allowing communication and maintaining tissue integrity.

• Tight Junctions: Prevent leakage of extracellular fluid between cells.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap Junctions: Allow direct communication between cells via channels.

3.3 Passive Membrane Transport

Overview of Passive Transport

Passive transport is the movement of substances across the plasma membrane without the use of cellular energy (ATP). Substances move down their concentration gradient.

• Types: Diffusion, facilitated diffusion, and osmosis.

Diffusion

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration.

• Simple Diffusion: Direct movement of small, nonpolar molecules (e.g., oxygen, carbon dioxide) through the lipid bilayer.

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

Equation:

Where: J = flux D = diffusion coefficient dC/dx = concentration gradient

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane.

• Water moves from areas of low solute concentration to areas of high solute concentration.

• Osmotic pressure: The pressure required to prevent water movement across the membrane.

Equation:

Where: = osmotic pressure i = van 't Hoff factor M = molarity R = gas constant T = temperature (Kelvin)

Influence of Membrane Permeability

Membrane permeability affects the rate and extent of diffusion and osmosis.

• Permeable membranes: Allow free movement of substances.

• Semipermeable membranes: Restrict movement based on size, charge, or solubility.

Tonicity and Effects on Cells

Tonicity describes the ability of a solution to cause a cell to gain or lose water.

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

• Hypertonic: Water leaves the cell; cell shrinks (crenation).

• Hypotonic: Water enters the cell; cell swells and may burst (lysis).

Example: Placing red blood cells in a hypotonic solution causes them to swell and potentially burst.

Additional info: These notes expand upon the original slides by providing definitions, equations, and examples for key concepts in cell structure and membrane transport, suitable for college-level Anatomy & Physiology students.