Cellular Organelles: Mitochondria Overview

Cellular Organelles and Non-Cellular Organelles

• Mitochondria (sing., mitochondrion) are found in all eukaryotic cells, including plant and algae cells.
• Mitochondria are surrounded by a double membrane; an outer membrane that is smooth and determines the shape of the mitochondria and an inner membrane that folds into cristae.
• The cristae project into the interior of the organelle, resembling shelves.
• The matrix within the mitochondria contains enzymes, ribosomes, and circular mitochondrial DNA.
• Cristae contain respiratory chain enzymes that produce ATP (adenosine triphosphate).
• ATP is produced when carbohydrates are broken down, releasing energy.
• ATP produces high-energy molecules within the cytoplasm
• During the Krebs cycle, most energy is stored as ATP, and some is released as heat to maintain body temperature.
• Mitochondria have diverse shapes (spherical, oval, or elongated) but have the same basic structure.
• Number of mitochondria depends on cell function.
• Mitochondria have outer and inner membranes, cristae, and a matrix.
• Ribosomes and mitochondrial DNA are also present within the mitochondria.

Centrosome

• Located near the nucleus.
• Composed of a matrix and two small, cylindrical structures called centrioles, which lie perpendicular to each other.
• Each centriole has nine tubules arranged in a circle; each tubule contains three microtubules.
• Microtubules run along a longitudinal axis and are parallel to each other.

Centrosome and Centrioles

• The centrosome matrix, centrioles, and microtubules are all depicted in diagrams.

Cross Section of Centriole

• Micrographs of centriole cross-sections are shown .

Longitudinal Section of Centriole

• A micrograph of a longitudinal centriole section is shown.

Centriole

• Functions in cell division by forming spindle fibers (microtubules).
• Centers for mitotic spindles, distributing chromosomes to daughter cells during cell division.
• Plays a role in cilia and flagella formation.

Cytoplasmic Inclusions

• Temporary structures that include metabolic products.
• Non-living components of the cell.
• Classified into three groups:

Stored Food

• Glycogen and lipid droplets.

Glycogen

• Appears as granules not bounded by membranes, arranged in groups within cells.
• A storage form of carbohydrate that serves as an energy source (particularly prominent in liver and muscle cells).

Lipids

• Stored in fat cells as lipid droplets.
• Lipid droplets in electron micrographs (EM) are not bounded by membranes and vary in size.
• Serve as energy resources.

Pigments

• Viewed in electron micrographs (EM) as small granules, not membrane-bound, and scattered.
• Two types.

Exogenous Pigments

• Come from outside the cell, like carotene (pigment in carrots).

Endogenous Pigments

• For example, melanin produced by melanocytes (cells in skin and eyes).

Hemoglobin

• Pigmented protein in red blood cells (RBCs).

Crystalloids

• Mineral salts, such as calcium carbonate, found in cells like those in the testes.