Chapter 3: Cells – The Living Units (Anatomy & Physiology Study Notes)

Cell Theory and the Generalized Cell

Overview of Cell Theory

The cell is the fundamental structural and functional unit of life. Human bodies contain over 250 different types of cells, each varying in size, shape, and subcellular components, which enable them to perform specialized functions.

• Cell Theory: All living organisms are composed of cells. Cells are the basic unit of structure and function in organisms. All cells arise from pre-existing cells.

• Cells differ in size, shape, and subcellular components, leading to diverse functions.

Generalized Cell Structure

Most human cells share several common features, often described as the 'generalized cell.' These include:

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

• Cytoplasm: The water-based intracellular fluid containing organelles.

• Nucleus: The DNA-containing control center of the cell.

Plasma Membrane

Function and Importance

The plasma membrane acts as an active barrier, separating the intracellular fluid (ICF) from the extracellular fluid (ECF). It plays a dynamic role in cellular activity by controlling what enters and leaves the cell. The plasma membrane is also known as the cell membrane.

• Maintains homeostasis by regulating the movement of substances.

• Facilitates communication and signaling between cells.

Plasma Membrane Structure

The plasma membrane is primarily composed of membrane lipids that form a flexible lipid bilayer. Specialized membrane proteins float within this fluid membrane, resulting in a constantly changing pattern known as the fluid mosaic model.

• Lipid Bilayer: Provides the basic structure and barrier function.

• Membrane Proteins: Responsible for various functions such as transport, signaling, and cell recognition.

• Surface Sugars: Help identify cell type (glycocalyx).

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

Key Components of the Plasma Membrane

• Phospholipids: Form the main structure of the bilayer. Each phospholipid has a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails.

• Cholesterol: Interspersed within the bilayer, it increases membrane stability and fluidity.

• Glycolipids: Lipids with attached sugar groups, important for cell recognition.

• Proteins: Integral (span the membrane) and peripheral (attached to the surface) proteins serve as channels, carriers, receptors, enzymes, and structural anchors.

Fluid Mosaic Model

• The membrane is described as a 'fluid mosaic' because of the dynamic arrangement of lipids and proteins.

• Proteins and lipids can move laterally within the layer, allowing flexibility and adaptability.

Cell Junctions

• Tight Junctions: Create an impermeable seal between adjacent cells, preventing passage of substances between them. Example: Blood-brain barrier.

• Desmosomes: Anchoring junctions that provide mechanical stability by linking cells together. Example: Skin, heart.

• Gap Junctions: Allow direct communication between cells through channels. Example: Cardiac muscle, neurons.

Example: The plasma membrane of a neuron contains specialized proteins that allow for the rapid transmission of electrical signals.