Cellular Level of Organization: Fundamentals of Anatomy & Physiology (Chapter 3)

Introduction to Cells

Cell Theory

The cell theory is a foundational concept in biology, describing the properties and functions of cells in living organisms.

• Cells are the building blocks of all organisms: Every living thing is composed of one or more cells.

• All cells come from the division of preexisting cells: New cells are produced by the division of existing cells, ensuring continuity of life.

• Cells are the smallest units that perform all vital physiological functions: Cells carry out essential processes such as metabolism, growth, and response to stimuli.

• Each cell maintains homeostasis at the cellular level: Cells regulate their internal environment to maintain stable conditions necessary for survival.

Types of Cells

Cells are classified based on their function and role in reproduction.

• Sex cells (germ cells or reproductive cells): Specialized for reproduction.

  • Sperm (male gametes)

  • Oocytes (female gametes; develop into ova)

• Somatic cells: All body cells except sex cells; responsible for forming tissues and organs.

Cytology is the branch of cell biology that studies cells, their structure, and function.

Cell Structure and Anatomy

Model Cell Anatomy

A typical eukaryotic cell contains various organelles, each with specialized functions. The following are key components:

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

• Cytosol: Fluid portion of the cytoplasm containing dissolved nutrients, ions, and proteins.

• Cytoskeleton: Network of protein filaments providing structural support.

• Organelles: Specialized structures such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, ribosomes, and centrioles.

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

• Microvilli: Extensions that increase surface area for absorption.

• Secretory vesicles: Transport and release cellular products.

Additional info: The diagram (Fig. 3-1) visually represents the spatial arrangement of these organelles within a cell.

Nucleus and Nucleoplasm

The nucleus is the largest organelle and serves as the cell's control center.

• Nuclear envelope: Double membrane surrounding the nucleus, containing nuclear pores for communication.

• Nucleoplasm: Fluid inside the nucleus, containing nucleotides, enzymes, nucleoproteins, and chromatin.

• Nucleolus: Site of ribosomal RNA synthesis and ribosome assembly.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

Functions of the nucleus:

• Control of metabolism

• Storage and processing of genetic information

• Control of protein synthesis

Plasma Membrane

Structure and Function

The plasma membrane, also known as the cell membrane, separates the cytoplasm from the extracellular fluid (interstitial fluid).

• Physical isolation: Acts as a barrier to protect cellular contents.

• Regulation of exchange: Controls entry of ions and nutrients, and exit of wastes and cellular products.

• Sensitivity: Detects changes in extracellular fluid composition and chemical signals.

• Structural support: Anchors cells and tissues, maintaining integrity.

Membrane Composition

• Phospholipid bilayer: Composed of hydrophilic heads facing outward and hydrophobic tails facing inward, forming a selective barrier.

• Membrane proteins:

  • Anchoring proteins: Stabilize the membrane by attaching to internal or external structures.

  • Recognition proteins: Identify cells as normal or abnormal.

  • Enzymes: Catalyze chemical reactions at the membrane surface.

  • Receptor proteins: Bind and respond to ligands (e.g., hormones, ions).

  • Carrier proteins: Transport specific solutes across the membrane.

  • Channels: Regulate water flow and solute passage; may be gated.

• Membrane carbohydrates: Proteoglycans, glycoproteins, and glycolipids extend outside the membrane, forming the glycocalyx.

  • Functions of glycocalyx: Lubrication, protection, anchoring, locomotion, specificity in binding, and immune recognition.

Summary Table: Major Cell Structures and Functions

Structure	Main Function	Location/Notes
Plasma Membrane	Barrier, regulates exchange	Outer boundary of cell
Nucleus	Genetic control, protein synthesis	Largest organelle, contains DNA
Cytoskeleton	Structural support, movement	Throughout cytoplasm
Microvilli	Increase surface area	Cell surface
Organelles (e.g., mitochondria, ER, Golgi)	Specialized cellular functions	Within cytoplasm

Additional info: This summary table provides a quick reference for the main cell structures discussed in the chapter introduction and their primary functions.