Cell Organelles and Their Functions

Mitochondria ("Power House of the Cell")

Mitochondria are double-membraned organelles responsible for producing most of the cell's ATP supply through cellular respiration. They are abundant in cells with high energy demands, such as muscle and kidney cells.

• Structure: Enclosed by two membranes; the outer membrane is smooth, while the inner membrane is folded into cristae, increasing surface area for ATP production.

• Function: Site of aerobic cellular respiration, where nutrients are broken down and energy is captured to form ATP.

• Key Reaction:

• Example: Muscle cells contain hundreds of mitochondria to meet energy needs.

Ribosomes

Ribosomes are small complexes of RNA and protein that synthesize proteins. They can be free in the cytosol or attached to the endoplasmic reticulum.

• Structure: Composed of a large and small subunit.

• Function: Translate mRNA into polypeptide chains (proteins).

• Types: Free ribosomes (cytosol) and membrane-bound ribosomes (attached to ER).

• Example: Secretory cells have abundant ribosomes for protein production.

Endoplasmic Reticulum (ER)

The Endoplasmic Reticulum is a network of membranes involved in protein and lipid synthesis. It is divided into two types:

Rough Endoplasmic Reticulum (RER)

• Structure: Studded with ribosomes.

• Function: Synthesizes proteins for secretion or membrane insertion.

• Example: Pancreatic cells have extensive RER for enzyme production.

Smooth Endoplasmic Reticulum (SER)

• Structure: Lacks ribosomes; consists of tubules arranged in a looping network.

• Function: Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs, and stores calcium.

• Example: Liver cells have abundant SER for detoxification.

Golgi Apparatus

The Golgi apparatus consists of stacked, flattened cisternae and is responsible for modifying, sorting, and packaging proteins and lipids for secretion or delivery to other organelles.

• Function: "Traffic director" for cellular proteins; concentrates and packages proteins and lipids made at the RER.

• Example: Secretory vesicles in glandular cells originate from the Golgi apparatus.

Lysosomes

Lysosomes are spherical organelles containing hydrolytic (digestive) enzymes. They digest cellular debris and foreign material.

• Function: Digest particles by endocytosis, remove dead cells, and perform metabolic functions.

• Structure: Membrane-bound, with a stable but fragile membrane.

• Example: Abundant in phagocytic cells like macrophages.

Peroxisomes

Peroxisomes are small organelles containing enzymes that detoxify harmful substances and play a role in energy metabolism.

• Function: Detoxification in liver and kidney cells; breakdown of fatty acids.

• Example: Peroxisomes neutralize hydrogen peroxide () via catalase.

The Endomembrane System

The endomembrane system is a group of organelles that work together to produce, degrade, store, and export biological molecules, and degrade potentially harmful substances. It includes the ER, Golgi apparatus, secretory vesicles, lysosomes, and the nuclear envelope.

• Function: Coordination of synthesis, modification, and transport of cellular products.

• Example: Secretory pathway for protein export.

Cytoskeleton and Cell Movement

Microfilaments

Microfilaments are the thinnest elements of the cytoskeleton, composed of actin. They are involved in cell movement and changes in cell shape.

• Function: Cell motility, cytokinesis, and muscle contraction.

• Example: Actin filaments pinch the cell in two during cell division.

Intermediate Filaments

Intermediate filaments are tough, rope-like protein fibers that provide structural support and resist tension.

• Function: Maintain cell shape and anchor organelles.

• Example: Keratin filaments in epithelial cells.

Microtubules

Microtubules are hollow tubes made from tubulin proteins. They are the largest cytoskeletal elements and help maintain cell shape, enable intracellular transport, and form structures like centrioles, cilia, and flagella.

• Function: Cell shape, transport of organelles, and formation of mitotic spindle.

• Example: Microtubules form the core of cilia and flagella.

Centrosome, Centrioles, Cilia, and Flagella

Centrosome and Centrioles

The centrosome is a microtubule organizing center near the nucleus, containing paired centrioles arranged at right angles. Centrioles help organize the mitotic spindle during cell division and form the bases of cilia and flagella.

• Function: Organize microtubules and facilitate cell division.

• Example: Centrioles duplicate before mitosis to help form the spindle apparatus.

Cilia and Flagella

Cilia are motile extensions that move substances across the cell surface, while flagella are longer projections that propel cells.

• Cilia Function: Move mucus and debris in respiratory tract.

• Flagella Function: Propel sperm cells.

• Example: Ciliated epithelial cells in the trachea; sperm cell flagellum.

Summary Table: Major Cell Organelles and Their Functions