BIO 211 Anatomy & Physiology: Chapter 03 - Cells

Cell Theory and Cell Diversity

Cell Theory

The cell theory is a foundational concept in biology, describing the properties and functions of cells as the basic units of life.

• The cell is the smallest unit of life: All living organisms are composed of cells, which perform all vital functions.

• All organisms are made of one or more cells: The activities of an organism depend on the combined activities of its cells. Structure and function are complementary; biochemical functions are dictated by cell shape and subcellular structures.

• Cells arise only from other preexisting cells: New cells are produced by the division of existing cells, through processes such as mitosis and meiosis.

Key Contributors: Schleiden, Schwann, and Virchow.

Cell Diversity

Human bodies contain over 250 different types of cells, which vary in size, shape, and subcellular components, leading to differences in function.

• Examples: Muscle cells (long and contractile), nerve cells (branched for communication), red blood cells (biconcave for gas transport).

• Application: Cell specialization allows for complex functions and tissue formation.

Generalized Cell Structure

Basic Parts of a Cell

All cells share common structures and functions, typically organized into three main components:

• Plasma membrane (PM): Flexible outer boundary that separates the cell from its environment.

• Cytoplasm: Intracellular fluid containing organelles.

• Nucleus: DNA-containing control center.

Extracellular Materials

Materials outside the cell play important roles in cell function and communication.

• Extracellular fluids (ECF): Includes interstitial fluid (surrounds cells), blood plasma, and cerebrospinal fluid.

• Cellular secretions: Aid in digestion and act as lubricants (e.g., saliva, mucus).

• Extracellular matrix: Substance that acts as glue to hold cells together and provides structural support.

Cell Plasma Membrane

Fluid Mosaic Model

The plasma membrane is a dynamic barrier that separates intracellular fluid (ICF) from extracellular fluid (ECF), controlling the entry and exit of substances.

• Phospholipids: Form a flexible lipid bilayer; heads are hydrophilic (water-loving), tails are hydrophobic (water-fearing).

• Cholesterol: Provides structural integrity and rigidity.

• Proteins: Serve as channels for transport and receptors for communication.

• Glycocalyx: Surface sugars that act as identity markers for cell recognition.

• Cell junctions: Structures that help hold cells together and facilitate communication.

Functions of the Plasma Membrane

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

• Selective permeability: Determines which substances can enter or exit the cell.

• Communication: Membrane proteins interact with chemical messengers to relay messages to the cell interior.

• Cell recognition: Glycocalyx allows cells to recognize each other and interact appropriately.

PM Lipid Bilayer Composition

• 75% phospholipids: Phosphate heads (polar, charged, hydrophilic) and fatty acid tails (nonpolar, hydrophobic).

• 20% cholesterol: Increases membrane stability.

• 5% glycolipids: Lipids with sugar groups on outer membrane surface, functioning with glycocalyx for cell recognition.

Diagram Description

The lipid bilayer consists of two layers of phospholipids with hydrophilic heads facing outward toward water and hydrophobic tails facing inward, away from water. Proteins are interspersed throughout the bilayer, and carbohydrates are attached to proteins and lipids on the extracellular surface.

Key Terms and Definitions

• Hydrophilic: Water-loving; describes molecules or parts of molecules that interact well with water.

• Hydrophobic: Water-fearing; describes molecules or parts of molecules that do not interact well with water.

• Selective permeability: The ability of the plasma membrane to allow certain substances to pass while blocking others.

• Glycocalyx: A layer of carbohydrates on the cell surface involved in cell recognition and protection.

Example: Red Blood Cell Membrane

Red blood cells have a plasma membrane rich in proteins and carbohydrates, which help them maintain their shape and allow for recognition by the immune system.

Additional info: The fluid mosaic model describes the plasma membrane as a flexible, dynamic structure with proteins and lipids that can move laterally within the layer, allowing for membrane fluidity and adaptability.