Cellular Organelles, Tonicity, and Cell Division: Key Concepts in Anatomy & Physiology

Cellular Organelles

Overview of Major Organelles

Cells contain specialized structures called organelles that perform distinct functions necessary for cellular survival and activity. Understanding these organelles is fundamental in Anatomy & Physiology.

• Nucleus: The control center of the cell, containing genetic material (DNA) and responsible for regulating gene expression and cell division.

• Mitochondria: Known as the "powerhouse" of the cell, mitochondria generate ATP through cellular respiration.

• Ribosome: Small structures, either free in the cytoplasm or attached to the rough endoplasmic reticulum, that synthesize proteins by translating mRNA.

• Rough Endoplasmic Reticulum (RER): A network of membranes studded with ribosomes; involved in protein synthesis and modification.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; functions in lipid synthesis, detoxification, and calcium storage.

• Golgi Apparatus (Body): Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Centrioles / Centrosome: Structures involved in organizing microtubules and facilitating chromosome movement during cell division.

• Lysosome: Contains digestive enzymes to break down waste materials and cellular debris.

• Peroxisome: Contains enzymes for breaking down fatty acids and detoxifying harmful substances.

• Vesicle: Small membrane-bound sacs that transport substances within or outside the cell.

Example: The mitochondria in muscle cells are abundant to meet high energy demands during contraction.

Tonicity

Definition and Types

Tonicity refers to the relative concentration of solutes dissolved in solution which determines the direction and extent of diffusion of water across a semipermeable membrane.

• Hypotonic: The solution has a lower concentration of solutes compared to the cell; water enters the cell, which may cause it to swell or burst.

• Isotonic: The solution has the same concentration of solutes as the cell; water moves in and out at equal rates, so the cell maintains its shape.

• Hypertonic: The solution has a higher concentration of solutes than the cell; water leaves the cell, causing it to shrink (crenate).

Example: Red blood cells placed in a hypertonic solution will lose water and shrink.

Cellular Division

Phases of Mitosis

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets, resulting in two daughter cells. It is essential for growth, repair, and maintenance in multicellular organisms.

• Prophase: Chromatin condenses into visible chromosomes; the nuclear envelope begins to disintegrate; spindle fibers start to form.

• Metaphase: Chromosomes align at the cell's equatorial plate (metaphase plate).

• Anaphase: Sister chromatids are pulled apart toward opposite poles of the cell by spindle fibers.

• Telophase: Chromatids reach the poles; nuclear envelopes reform around each set of chromosomes; chromosomes begin to decondense.

• Cytokinesis: Division of the cytoplasm, resulting in two separate daughter cells. In animal cells, this is achieved by a cleavage furrow.

Example: Skin cells undergo mitosis regularly to replace cells lost due to abrasion.

Summary Table: Major Organelles and Their Functions

Organelle	Main Function
Nucleus	Stores genetic material; controls cell activities
Mitochondria	Produces ATP via cellular respiration
Ribosome	Protein synthesis
Rough ER	Protein synthesis and modification
Smooth ER	Lipid synthesis, detoxification
Golgi Apparatus	Modifies, sorts, and packages proteins/lipids
Centrioles/Centrosome	Organizes microtubules during cell division
Lysosome	Digests waste and cellular debris
Peroxisome	Breaks down fatty acids, detoxifies substances
Vesicle	Transports materials within the cell