Cells: The Living Units – Structure and Function in Anatomy & Physiology

Chapter Overview: Cells – The Living Units

This chapter introduces the fundamental unit of life, the cell, and explores its structure, function, and role in multicellular organisms. Understanding cell biology is essential for comprehending the organization and physiology of the human body.

Objectives

• Learn about cell theory.

• Understand the primary animal cell and the function of its organelles.

• Examine selective permeability and methods of cellular transport across membranes.

• Recognize specialized cell projections.

• Understand the basics of gene expression.

Levels of Biological Organization

Biological systems are organized hierarchically, from the simplest to the most complex:

• Chemical (atoms, molecules)

• Cellular (cells)

• Tissue (groups of similar cells)

• Organ (structures composed of different tissues)

• Organ System (groups of organs working together)

• Organism (the complete living being)

• Population (group of organisms of the same species)

• Biosphere (all ecosystems on Earth)

The Cell Theory

• All organisms are composed of one or more cells.

• The cell is the basic unit of structure and function in living things.

• All cells arise from pre-existing cells.

Cells in Multicellular Life

• Cells are the smallest units that can carry out all vital physiological processes.

• Multicellular organisms have specialized cells for different functions.

• Types of cells differ in shape, size, and subcellular differences, which contribute to their specific functions.

• In multicellular organisms, all cells (except gametes) contain the same DNA, but express different genes depending on their function.

Structure of the Generalized Animal Cell

The animal cell contains various organelles, each with specialized functions. Major components include:

• Plasma membrane

• Nucleus

• Cytoplasm (containing organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, etc.)

Plasma Membrane

• Composed of a phospholipid bilayer with embedded proteins.

• Functions as a selective barrier, regulating the entry and exit of substances.

• Contains cholesterol for fluidity and glycoproteins for cell recognition.

Cell Junctions

• Tight junctions: Prevent leakage of extracellular fluid between cells.

• Desmosomes: Anchor cells together, providing mechanical strength.

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

Cellular Projections

• Microvilli: Fingerlike extensions that increase surface area for absorption.

• Cilia: Hair-like extensions that move substances along the cell surface.

• Flagella: Longer extensions used for cell movement (e.g., sperm cells).

Nucleus

• Bound by a double membrane (nuclear envelope).

• Contains DNA organized as chromatin or chromosomes.

• Site of DNA replication and transcription (gene expression).

• Contains the nucleolus, where ribosomal RNA is synthesized.

How DNA is Organized in the Nucleus

• DNA wraps around histone proteins to form chromatin.

• During cell division, chromatin condenses into visible chromosomes.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

• Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

Ribosomes

• Small, non-membrane-bound organelles composed of rRNA and proteins.

• Sites of protein synthesis.

• Can be free in the cytoplasm or bound to rough ER.

Golgi Body (Golgi Apparatus)

• Modifies, sorts, and packages proteins and lipids from the ER.

• Produces vesicles for transport within or outside the cell.

Lysosome

• Membrane-bound organelles containing digestive enzymes.

• Break down waste materials and cellular debris.

Peroxisomes

• Membrane-bound organelles that contain enzymes for breaking down fatty acids and detoxifying harmful substances.

• Produce hydrogen peroxide as a byproduct, which is then broken down by catalase.

Vesicles and Vacuoles

• Vesicles: Small membrane-bound sacs for transport and storage.

• Vacuoles: Larger storage organelles (more prominent in plant cells).

Mitochondrion

• Double-membraned organelle known as the "powerhouse" of the cell.

• Site of aerobic respiration and ATP production.

• Contains its own DNA and can replicate independently.

Centrioles

• Non-membrane-bound organelles involved in organizing microtubules during cell division.

• Help form the spindle apparatus during mitosis and meiosis.

Cytoplasm

• Includes the cytosol (fluid) and all organelles except the nucleus.

• Site of many metabolic reactions.

Cytoskeleton and Structural Support

• Network of protein filaments (microfilaments, intermediate filaments, microtubules) that provide structural support, shape, and movement.

• Involved in intracellular transport and cell division.

Extracellular Materials

• Substances found outside the cell, including extracellular fluid and extracellular matrix (ECM).

• ECM provides structural support and mediates cell signaling.

Extracellular Matrix (ECM)

• Composed of proteins (e.g., collagen, elastin) and polysaccharides.

• Provides structural support and regulates cell behavior.

• Example: Collagen fibers in connective tissue.

Permeability of Plasma Membrane

• The plasma membrane is selectively permeable, allowing some substances to pass while restricting others.

• Transport mechanisms include passive and active processes.

Types of Transport Across the Plasma Membrane

Type of Transport	Energy Requirement	Example
Passive Transport	No (does not require ATP)	Diffusion, Osmosis
Active Transport	Yes (requires ATP)	Sodium-potassium pump
Bulk Transport	Yes (requires energy)	Endocytosis, Exocytosis

Key Equations

• Fick's Law of Diffusion:

$J = -D \frac{dC}{dx}$

• Where J is the rate of diffusion, D is the diffusion coefficient, and \frac{dC}{dx} is the concentration gradient.

Summary Table: Major Cell Organelles and Their Functions

Organelle	Structure	Main Function
Nucleus	Double membrane, contains DNA	Genetic control center, site of transcription
Ribosome	No membrane, rRNA and proteins	Protein synthesis
Rough ER	Membrane with ribosomes	Protein synthesis and modification
Smooth ER	Membrane without ribosomes	Lipid synthesis, detoxification
Golgi apparatus	Stack of membranes	Protein and lipid modification, packaging
Lysosome	Membrane-bound vesicle	Digestion of macromolecules
Mitochondrion	Double membrane, own DNA	ATP production
Peroxisome	Membrane-bound vesicle	Breakdown of fatty acids, detoxification

Additional info: Some details, such as the full list of organelles and their functions, were expanded for completeness and clarity.