Cell Structure and Function: Foundations of Cytology

Cellular Structure and Function (Cytology)

General Classes of Human Cells

Cells are the basic units of life, and the human body contains two general classes:

• Sex cells (gametes): reproductive cells such as sperm and egg.

• Somatic cells: all other body cells that make up tissues and organs.

Basic Components of Most Cells

Most cells share three fundamental components:

• Cytoplasm

• Nucleus

• Cell Membrane (Plasma Membrane)

These components are essential for maintaining cellular structure, function, and homeostasis.

Main Cell Components and Their Functions

Cell Component	Function (Definition)
Cytoplasm	Material between the plasma membrane and the membrane surrounding the nucleus Contains the cell contents Provides a medium for chemical reactions and supports the organelles Contributes to cell structure, growth, and metabolism Subdivided into: cytosol, organelles, and inclusions
Nucleus	Located near the center of the cell Control center of cellular operations Controls metabolism Stores and processes genetic information Controls protein synthesis
Cell Membrane	Outer boundary of the cell Separates internal and external environments Regulates what enters and leaves the cell

Intracellular and Extracellular Fluid

Definitions

• Extracellular Fluid (ECF): Fluid outside cells, surrounds the cell; contains sodium, chloride, and bicarbonate ions.

• Intracellular Fluid (ICF): Fluid within the cell; contains potassium and phosphate ions in higher concentrations.

Interstitial Fluid is a component of ECF, found in the tissues and fills the spaces between cells, helping maintain the balance of ions and water between ECF and ICF.

Ionic Composition Differences

• Sodium (Na+): Higher in ECF

• Potassium (K+): Higher in ICF

There is also a slight charge difference across the cell membrane: the inside is negatively charged, and the outside is positively charged.

Cell Membrane (Plasma Membrane)

Functions of the Cell Membrane

Function	Description
Physical Isolation	Separates the inside of the cell from the surrounding extracellular fluid; acts as a barrier.
Regulation of Exchange	Controls entry of ions and nutrients, elimination of wastes, and release of products.
Sensitivity	Detects changes in the environment (e.g., pH, ion concentration).

Structure of the Cell Membrane

• Phospholipid Bilayer: The cell membrane is primarily composed of a double layer of phospholipids. Each phospholipid has a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails.

• Proteins: Make up about 55% of the membrane's weight. There are two main types:

  • Integral (Intrinsic) Proteins: Embedded within the membrane and cannot be easily separated. Function as channels, carriers, or receptors.

  • Peripheral (Extrinsic) Proteins: Attached to the inner or outer surface and can be easily separated. Often function in signaling or structural support.

• Cholesterol: A type of lipid that stabilizes the membrane and affects its fluidity.

Functions of Membrane Proteins

• Transporters: Facilitate movement of substances across the membrane.

• Receptors: Bind specific extracellular compounds and initiate cellular responses.

• Enzymes: Catalyze reactions within the cytosol.

• Anchors: Stabilize the cell's position by attaching the membrane to the cytoskeleton or extracellular structures.

Additional Cell Membrane Structures

• Microvilli: Small extensions that increase surface area for absorption. Common in cells lining the digestive tract.

• Cell Junctions: Structures that hold cells together in tissues.

  • Tight Junctions: Prevent water and solutes from passing between cells; found in epithelial regions.

  • Desmosomes: Provide strong adhesion and resist stretching/twisting; maintain tissue stability.

Summary Table: Key Cell Membrane Components

Component	Structure	Main Function
Phospholipid Bilayer	Double layer of phospholipids with hydrophilic heads and hydrophobic tails	Barrier, selective permeability
Integral Proteins	Embedded within bilayer	Transport, receptor, enzyme
Peripheral Proteins	Attached to membrane surface	Signaling, structural support
Cholesterol	Lipid interspersed within bilayer	Membrane stability, fluidity
Microvilli	Membrane extensions	Increase surface area for absorption
Tight Junctions	Interlocking membrane proteins	Prevent passage between cells
Desmosomes	Protein complexes between cells	Strong adhesion, tissue stability

Key Terms and Definitions

• Cytoplasm: The material within a cell, excluding the nucleus.

• Cytosol: The fluid portion of the cytoplasm where organelles are suspended.

• Nucleus: The control center of the cell, containing genetic material.

• Cell Membrane: The outer boundary that regulates entry and exit of substances.

• Phospholipid: A lipid molecule with a hydrophilic head and hydrophobic tails, forming the bilayer.

• Integral Protein: A protein embedded within the membrane.

• Peripheral Protein: A protein attached to the membrane surface.

• Microvilli: Extensions that increase surface area.

• Tight Junction: A cell junction preventing passage between cells.

• Desmosome: A cell junction providing strong adhesion.

Example Application

Cells lining the digestive tract have abundant microvilli to maximize absorption of nutrients. Tight junctions in these cells prevent leakage of digestive enzymes and acids, maintaining compartmentalization.

Additional info: The study notes above expand on the original content by providing full academic definitions, structured tables, and context for each cell component and membrane structure. This guide is suitable for exam preparation and foundational understanding in Anatomy & Physiology.