Cell Biology: Peroxisomes and Mitochondria

Peroxisomes

• Similar to lysosomes, but formed by self-replication (possibly budding from smooth ER)
• Contain oxidases, not hydrolases
• Use oxidases to combine oxygen with hydrogen ions to create hydrogen peroxide (H₂O₂)
• Hydrogen peroxide is a powerful oxidizer, used with catalase (another oxidase) to break down harmful substances
• Many cell functions involve secretion of special chemicals
• These are made by the endoplasmic reticulum-Golgi apparatus system
• Stored in vesicles called secretory vesicles (or secretory granules)
• These vesicles contain proenzymes (inactive enzymes)
• Proenzymes become activated later, performing digestion in the duodenum

Mitochondria

• Called the "powerhouses" of the cell
• Essential for extracting energy from nutrients
• Essential for most cellular functions
• Variable numbers in each cell, depending on energy needs
• Predominantly located in regions of high energy use
• Vary in size and shape (globular, elongated, branching)
• Comprised of two lipid bilayer membranes (outer and inner)
• Inner membrane has infoldings (shelves) with oxidative enzymes
• Matrix (inner cavity) contains enzymes for extracting energy from nutrients
• Enzymes of the matrix work with oxidative enzymes on the shelves, forming water and carbon dioxide

Cytoskeleton (Filaments and Tubules)

• Fibrillar proteins organize into filaments or tubules
• Created by precursor protein molecules that polymerize
• Actin filaments form an elastic support for the cell membrane
• Important in muscle cells, where they're organized into contractile machines for muscle contraction (discussed in more detail in a later chapter)
• Microtubules are stiff filaments composed of polymerized tubulin molecules
• Form structural support for cells
• Part of cilia, flagella, centrioles, and mitotic spindles (discussed later in the chapter)

Nucleus

• Control center of the cell
• Contains DNA (genes) that determine cellular characteristics and proteins (including structural and enzymes)
• Controls cell replication
• DNA similar to that in the nucleus, controlling mitochondrial replication
• Genes direct cell reproduction via mitosis (discussed more fully in a later chapter)

Nuclear Membrane

• Consists of two separate bilayer membranes
• Continuous with the endoplasmic reticulum
• Contains numerous nuclear pores
• Pores allow passage of molecules up to 44,000 molecular weight

Nucleolus

• Stains heavily in most cell nuclei
• Made of RNA and proteins, types of those in ribosomes
• Nucleolus grows larger when cell actively synthesizes proteins
• Contains RNA for ribosome production (transported into cytoplasm)