Cell Structure and Membrane Specializations: Study Notes for Anatomy & Physiology

Chapter 3: Cells

Introduction to Cells and Tissues

Cells and tissues are fundamental units of life, responsible for all chemical activities required to sustain living organisms. Understanding their structure and function is essential in Anatomy & Physiology.

• Cells are the building blocks of all living things.

• Tissues are groups of similar cells that perform specific functions.

• All cells share a general structure, but may differ in specialized features.

• Each cell contains three main regions: nucleus, cytoplasm, and plasma membrane.

Cell Structure

• Nucleus: Contains genetic material (DNA), which directs cellular activities.

• The nucleus has three regions:

  • Nuclear envelope (membrane): Double membrane barrier of the nucleus, containing nuclear pores for exchange of material with the rest of the cell.

  • Nucleolus: Dense region within the nucleus, site of ribosome synthesis. Ribosomes migrate to the cytoplasm where they begin protein production.

  • Chromatin: Unwound genetic material composed of DNA and protein. Condenses to form chromosomes during cell division.

• Nuclear pores: Allow RNA to leave the nucleus.

• Cytoplasm: Material outside the nucleus and inside the plasma membrane. Consists largely of water and contains cell organelles.

Plasma Membrane

The plasma membrane is the outer boundary of the cell, providing protection and regulating the movement of substances in and out of the cell.

• Structure: Composed of a double phospholipid layer with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Contains proteins for structure, attachment, and transport; cholesterol for stability; and glycoproteins for cell identification.

Plasma Membrane Specializations

Certain cells have specialized membrane structures to enhance their function, especially in absorption and movement.

• Microvilli: Finger-like projections that increase surface area for absorption (e.g., in digestive tract).

• Cilia: Hair-like projections that move substances rhythmically (e.g., in respiratory tract, also in fallopian tubes).

Membrane Junctions

Membrane junctions are specialized connections between cells that help maintain tissue integrity and facilitate communication.

• Tight junctions: Impermeable junctions that bind cells together into leakproof sheets.

• Desmosomes: Anchoring junctions that prevent cells from being pulled apart.

• Hemidesmosomes: Anchoring junctions that attach cells to a basement membrane.

• Gap junctions: Allow direct communication between adjacent cells through channels.

Table: Types of Membrane Junctions

Junction Type	Main Function	Location Example
Tight Junctions	Seal adjacent cells, prevent leakage	Intestinal lining
Desmosomes	Anchor cells together, resist mechanical stress	Skin epidermis
Hemidesmosomes	Attach cells to basement membrane	Basal layer of skin
Gap Junctions	Allow passage of ions and small molecules	Cardiac muscle

Key Terms and Definitions

• Organelle: Specialized structure within a cell that performs a specific function.

• Chromosome: Condensed form of chromatin, visible during cell division.

• Phospholipid bilayer: Fundamental structure of the plasma membrane.

Example: Cell Membrane Function

In the small intestine, microvilli on epithelial cells increase the surface area for nutrient absorption, while tight junctions prevent leakage of digestive enzymes between cells.

Additional info: The notes have been expanded to include definitions, examples, and a table for clarity and completeness.