Cell Organelles: Mitochondria

Cell Organelles

• Cell organelles perform specific functions within the cell
• Mitochondria are the powerhouses of the cell, responsible for energy production through aerobic respiration (glycolysis, Krebs cycle, electron transport chain)

Mitochondria (MT)

• Found in animal and plant cells (excluding red blood cells), not bacteria
• Number varies based on the cell's metabolic demands (algae and fungi have one to a few, liver cells have 500-1000 per cell)
• Located throughout the cytoplasm, concentrated where energy is needed most, among muscle myofibrils

Mitochondrial Structural Features

• Vary in shape (spherical, oval, rod-like, elongated)
• Typically 0.5-1 µm in diameter and 1-2 µm in length (second largest organelle in the cell)
• Similar in size to a bacterium
• Double-membraned organelle
  • Outer membrane: single continuous layer, relatively low protein content than plasma membrane, has large protein channels (porins) for transporting small molecules and ions
  • Intermembrane space: space between the outer and inner membranes. Contains common ions and sugars similar to cytoplasm, but with different protein composition (e.g., cytochrome c)
  • Inner membrane: highly folded (cristae) to increase surface area, highly permeable to small molecules like O2, CO2, and H2O but impermeable to most ions. Has high protein content (~75%), contains specific transport proteins to regulate movement of materials in and out of the matrix and perform oxidative phosphorylation, generating ATP (energy molecule)
  • Matrix: semifluidic, contains ~30% of all mitochondrial proteins, including enzymes for ATP production (e.g., Krebs cycle), oxidation of pyruvate and fatty acids, ribosomes, tRNA, and mitochondrial DNA genome

Cellular Respiration

• One glucose molecule can generate 38 ATP molecules
• Glycolysis (cytoplasm), Krebs Cycle (matrix), Electron Transport Chain (inner membrane)

Endoplasmic Reticulum (ER)

• Internal network of membranes within the cytoplasm
• Originates from the outer nuclear membrane
• Eukaryotic cells, important for protein processing.
• Rough ER (RER): studded with ribosomes; site for protein synthesis, folding and modification, and transport (protein secretory pathway) proteins destined for secretion
• Smooth ER (SER): lacks ribosomes; synthesizes lipids, stores calcium, involved in detoxification

Golgi Apparatus

• Highly curved, tubular structure
• Modifies, sorts, and packages proteins and lipids received from the ER for secretion or use within the cell.
• Involved in secretion, a prominent structure in cells with secretory functions (e.g., epithelial cells)
• Plays a key role in modifying molecules previously synthesized in the ER (e.g., modification of carbohydrate groups using enzymes at each cisternae)

Lysosomes

• Membrane-bound organelles containing hydrolytic enzymes
• Involved in digestion, recycling of cellular material, and breakdown of captured particles (e.g. phagocytosis, endocytosis)
• Enzymes are activated in the Golgi body, and enclosed in a lipid membrane with glycosylated transport proteins

Importance of Golgi apparatus

• Golgi body (apparatus) interacts mostly with the endoplasmic reticulum (ER), plasma membrane (PM), and lysosomes of the cell
• Glycosylation in the Golgi (in addition to in ER) is the mechanism by which proteins are targeted to their correct locations within the cell
• Cell death occurs without Golgi bodies, as there are no lysosomes for breaking down substances
• Proteins can't travel to their correct locations without Golgi apparatus

Cytoskeleton

• Network of proteins (microtubules, actin filaments, intermediate filaments) in the cytoplasm
• Provides structural support and framework
• Allows movement of molecules and organelles
• Functions include:
  • Cellular shape, supporting structure, motility
  • Mechanical strength and integrity

Differences in Cytoskeletal components

• Microfilaments (actin): involved in cell shape, muscle contraction, cell division

• Intermediate filaments (keratin, vimentin): maintain cell shape and anchor nucleus

• Microtubules: involved in cell shape, intracellular transport, cell motility, and cell division (e.g. chromosome movement, cilia, flagella movement)