Anatomy of the Cell and Cell Division: Structure, Organelles, and the Cell Cycle

Introduction to Cells

Overview of Cellular Structure

Cells are the smallest living units in the human body, with a typical diameter of about 0.1 mm. The invention of the microscope in the 17th century enabled the study of cells, and Robert Hooke coined the term 'cell' in 1665 due to their resemblance to small rooms in monasteries or prisons.

• Cells: Fundamental units of life in all organisms.

• Cytology: The study of cellular structure, integrating biology, chemistry, and physics.

• The human body contains trillions of cells.

• Two general classes of cells:

  • Sex cells (reproductive cells)

  • Somatic cells (all other body cells)

Types of Human Cells

• Cells that connect body parts, form linings, or transport gases (e.g., fibroblasts, epithelial cells, erythrocytes)

• Cells that move organs and body parts (e.g., skeletal muscle cells, smooth muscle cells)

• Cells that store nutrients (e.g., fat cells)

• Cells that fight disease (e.g., macrophages)

• Cells that gather information and control body functions (e.g., nerve cells)

• Cells of reproduction (e.g., sperm cells)

Cell and Extracellular Fluid

Components of the Cell

The cell is surrounded by extracellular fluid, which is the watery medium outside the cell. The cell itself is formed of three main components:

• Nucleus

• Cytoplasm

• Plasma membrane (cell membrane): Separates cytoplasm from extracellular fluid

Cytoplasm

• Cytosol: The liquid portion of the cytoplasm

• Organelles: Intracellular structures with specific functions

Functions

• Cell membrane: Isolation, protection, sensitivity, support, entrance and exit regulation

• Cytosol: Distributes material by diffusion

Organelles and the Cytoplasm

Classification of Organelles

• Nonmembranous organelles:

  • No membrane; direct contact with cytosol

  • Includes: cytoskeleton, microvilli, centrioles, cilia, ribosomes, proteasomes

• Membranous organelles:

  • Covered with plasma membrane; isolated from cytosol

  • Includes: endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, mitochondria

Nonmembranous Organelles

• Cilia: Movement of materials over the cell surface

• Microvilli: Increase surface area for absorption

• Centrosome: Movement of chromosomes during cell division; organization of microtubules and cytoskeleton

• Ribosomes: Protein synthesis

• Proteasomes: Breakdown and recycling of damaged or abnormal intracellular proteins

Membranous Organelles

• Mitochondria: Produce 95% of ATP (energy)

• Endoplasmic Reticulum (ER):

  • Rough ER: Production of proteins

  • Smooth ER: Lipids and carbohydrates synthesis

• Golgi apparatus: Storage, alteration, and packaging of secretory products and lysosomal enzymes

• Lysosomes: Intracellular removal of damaged organelles or pathogens

• Peroxisome: Catabolism of fats; neutralization of toxic compounds

• Nucleus: Control of protein synthesis, cell metabolism, storage and processing of genetic information

• Nucleolus: Site for rRNA synthesis and assembly of ribosomal subunits

• Flagellum (only in sperm): Movement of the cell

Anatomy of a Cell

Major Structures and Their Functions

Organelle	Main Function
Cilia	Move materials over cell surface
Microvilli	Increase surface area for absorption
Centrosome/Centriole	Organize microtubules, chromosome movement
Mitochondria	ATP production
Rough ER	Protein synthesis
Smooth ER	Lipid and carbohydrate synthesis
Golgi apparatus	Packaging and modification of proteins
Lysosome	Digestion and recycling
Peroxisome	Detoxification
Nucleus	Genetic control
Nucleolus	rRNA synthesis
Ribosome	Protein synthesis

A Cell's Life Cycle

Phases of the Cell Cycle

Most of a cell's life is spent in a nondividing state called interphase. Body (somatic) cells divide in three main stages:

• DNA replication: Duplicates genetic material exactly

• Mitosis: Divides genetic material equally

• Cytokinesis: Divides cytoplasm and organelles into two daughter cells

Interphase

• G0 phase: Specialized cell functions only

• G1 phase: Cell growth, organelle duplication, protein synthesis

• S phase: DNA replication and histone synthesis

• G2 phase: Finishes protein synthesis and centriole replication

Cell Structure and Mitosis

Mitosis: Nuclear Division

Mitosis is the process of nuclear division in which DNA is separated into two equal sets, ensuring each daughter cell has the same genetic makeup as its predecessor.

• Chromosome: Structure containing DNA and genes

• Chromatid: Identical copies of a chromosome

• Centromere: Holds two chromatids together

• Kinetochore: Protein complex around centromere

Stages of Mitosis

Stage	Main Events
Prophase	Nucleoli disappear, centrioles move to poles, spindle fibers form, nuclear envelope disappears
Metaphase	Chromosomes align at metaphase plate, spindle fibers attach to kinetochores
Anaphase	Microtubules pull chromosomes apart, daughter chromosomes group near centrioles
Telophase	Nuclear membranes reform, chromosomes uncoil, nucleoli reappear, cell has two nuclei

Cytokinesis

• Cleavage furrow forms around metaphase plate

• Membrane closes, producing two daughter cells

Summary Table: Cell Cycle Phases

Phase	Description
Interphase	Cell grows, DNA replicates, prepares for division
Prophase	Chromosomes condense, spindle forms
Metaphase	Chromosomes align at cell center
Anaphase	Chromatids separate to opposite poles
Telophase	Nuclear envelope reforms, chromosomes decondense
Cytokinesis	Cytoplasm divides, two daughter cells form

Key Equations

• DNA Replication: $ext{DNA}_{ ext{parent}} ightarrow 2 imes ext{DNA}_{ ext{daughter}}$

• ATP Production (Mitochondria): $ext{C}_6 ext{H}_{12} ext{O}_6 + 6 ext{O}_2 ightarrow 6 ext{CO}_2 + 6 ext{H}_2 ext{O} + ext{ATP}$

Example

During mitosis, a human somatic cell with 46 chromosomes will produce two daughter cells, each with 46 chromosomes, ensuring genetic continuity.

Additional info: The notes include visual models and diagrams to aid in understanding cell structure and division, which are essential for practical lab identification and exam preparation.