Cellular Anatomy and Physiology

Introduction to Cells

Cells are the fundamental structural and functional units of all living organisms. The human body contains approximately 50 to 100 trillion cells, each specialized for particular functions essential to life.

• Cells: Basic units of life in all living things.

• Cellular diversity: Over 200 different cell types in the human body.

The Cell Theory

The cell theory provides the foundation for understanding biological structure and function.

1. Cells are the basic structural and functional unit of living organisms.

2. The activity of an organism depends on the collective activities of its cells.

3. According to the principle of complementarity, biochemical activities of cells are determined by their structure (anatomy), which determines their function (physiology).

4. Continuity of life has a cellular basis.

Anatomy of a Generalized Cell

Main Regions of a Cell

Most cells have three main regions:

1. Plasma membrane (cell membrane or cytoplasmic membrane)

2. Nucleus

3. Cytoplasm

Cell Structure Overview

The generalized cell contains various organelles, each with specialized functions. The diagram below identifies key components:

• Nucleus: Contains genetic material (DNA).

• Plasma membrane: Surrounds the cell, controlling entry and exit of substances.

• Cytoplasm: Contains organelles and cytosol.

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

The Plasma Membrane

Structure and Function

The plasma membrane is a transparent barrier that separates cell contents from the surrounding environment.

• Composition:

  • Two layers of phospholipids (polar molecules with hydrophilic heads and hydrophobic tails).

  • Cholesterol and proteins scattered among phospholipids.

  • Sugar groups attached to proteins and lipids (glycoproteins, glycolipids).

• Hydrophilic heads face outward; hydrophobic tails face inward.

• Role:

  • Acts as a selective barrier.

  • Responsible for specialized membrane functions:

    • Enzymes

    • Receptors for hormones or chemical messengers

    • Transport as channels or carriers

  • Glycoproteins serve as cell recognition molecules.

The Nucleus

Structure and Function

The nucleus is the control center of the cell, containing genetic material (DNA) necessary for cell reproduction and protein synthesis.

• Nuclear envelope: Double membrane with nuclear pores for exchange of material.

• Nucleolus: Site of ribosome assembly.

• Chromatin: DNA wound around histones; condenses into chromosomes during cell division.

Cytoplasm

Components

The cytoplasm is the cellular material outside the nucleus and inside the plasma membrane. It is the site of most cellular activities.

• Cytosol: Fluid that suspends organelles and contains nutrients/electrolytes.

• Inclusions: Chemical substances such as stored nutrients or cell products.

• Organelles: Metabolic machinery of the cell, often membrane-bound.

Organelles

Mitochondria

Known as the "powerhouse" of the cell, mitochondria carry out reactions in which oxygen is used to break down food into ATP molecules.

Ribosomes

• Made of protein and ribosomal RNA.

• Site of protein synthesis.

• Found free in cytoplasm or attached to rough endoplasmic reticulum.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins and transports them within the cell.

• Smooth ER: Lacks ribosomes; functions in lipid metabolism, detoxification of drugs and pesticides.

Golgi Apparatus

• Stack of flattened membranes; modifies and packages proteins arriving from the rough ER into transport vesicles.

Lysosomes

• Membranous "bags" containing digestive enzymes.

• Enzymes digest worn-out or unusable cell structures and foreign substances.

Peroxisomes

• Membranous sacs of oxidase enzymes.

• Detoxify harmful substances such as alcohol and formaldehyde.

Cytoskeleton

• Network of protein structures that extend throughout the cytoplasm.

• Provides cell shape, support, and machinery for intracellular transport.

• Three types:

  • Microfilaments (largest)

  • Intermediate filaments

  • Microtubules (smallest)

Centrioles

• Rod-shaped bodies made of nine triplets of microtubules.

• Generate microtubules and direct formation of mitotic spindle during cell division.

Cell Diversity

Types and Functions

Cells vary in size, shape, and function. The human body contains over 200 different cell types, each specialized for particular roles.

• Fibroblast: Secretes cable-like fibers.

• Erythrocyte (red blood cell): Carries oxygen in the bloodstream.

• Epithelial cell: Packs together in sheets; intermediate filaments resist tearing.

• Muscle cells: Skeletal and smooth muscle cells contract to shorten forcefully.

• Fat cells: Store nutrients as lipid droplets.

• White blood cells: Fight disease; digests infectious microorganisms.

• Nerve cells: Receive and transmit messages.

• Oocyte (female): Largest cell; built for fertilization.

• Sperm (male): Built for swimming to the egg; flagellum acts as a motile whip.

Cell Physiology

Cellular Functions

Cells possess the ability to:

• Metabolize

• Digest food

• Dispose of wastes

• Reproduce

• Grow

• Move

• Respond to stimuli

Cell Division

Overview

The cell life cycle consists of changes the cell experiences from formation to division. It has two major phases:

1. Interphase: Metabolic phase; cell grows and carries on metabolic processes.

2. Cell division: Cell reproduces itself.

Preparations: DNA Replication

• Genetic material is duplicated and readied for division into two cells.

• Occurs toward the end of interphase.

Process of DNA Replication

• DNA uncoils into two nucleotide chains; each side serves as a template.

• Nucleotides are complementary:

  • Adenine (A) pairs with Thymine (T)

  • Guanine (G) pairs with Cytosine (C)

• For example, a TAGTGC sequence generates a new strand with ATACGC.

Events of Cell Division

• Mitosis: Division of the nucleus; results in two daughter nuclei.

• Cytokinesis: Division of the cytoplasm; results in two daughter cells.

Phases of Mitosis

• Prophase:

  • Chromatin coils into chromosomes; chromatids held together by centromere.

  • Centrioles direct mitotic spindle assembly.

  • Nuclear envelope and nucleoli break down.

• Metaphase:

  • Chromosomes align at the cell's center (metaphase plate).

  • Straight line of chromosomes is visible.

• Anaphase:

  • Centromeres split; chromatids move apart to opposite ends.

  • Anaphase ends when chromosomes stop moving.

• Telophase:

  • Reverse of prophase; chromosomes uncoil to become chromatin.

  • Spindles break down and disappear.

  • Nuclear envelope re-forms; nucleoli appear in daughter nuclei.

Cytokinesis

• Division of the cytoplasm.

• Begins during late anaphase and completes during telophase.

• Cleavage furrow forms to pinch the cell into two parts.

• Results in two daughter cells.

Protein Synthesis

DNA as a Blueprint

DNA segments carry instructions for building proteins or polypeptide chains. Protein synthesis is essential for cell structure and function.

• Genes: DNA segments that carry blueprints for building proteins.

• Proteins have many functions:

  • Fibrous (structural) proteins: Building materials for cells.

  • Globular (functional) proteins: Act as enzymes (biological catalysts).

• Information is encoded in a sequence of bases (triplets).

• Each triplet codes for an amino acid. Example: A DNA sequence of AAA specifies the amino acid phenylalanine.

Table: Major Cell Organelles and Their Functions

Key Equations and Concepts

DNA Base Pairing

DNA replication relies on complementary base pairing:

• Adenine (A) pairs with Thymine (T)

• Guanine (G) pairs with Cytosine (C)

For a DNA template strand:

Cell Cycle Phases

• Interphase: Cell growth and DNA replication

• Mitosis: Nuclear division

• Cytokinesis: Cytoplasmic division

Summary Table: Phases of Mitosis

Additional info:

• Cellular processes such as metabolism, growth, and division are tightly regulated and essential for tissue development and maintenance.

• Protein synthesis involves transcription (DNA to RNA) and translation (RNA to protein), though only the DNA blueprint is described here.