BS

TheCell7e Ch12 Lecture

Mitochondria, Chloroplasts, and Peroxisomes

Overview

  • All cells require metabolic energy, which is a key activity carried out by specific organelles.

Mitochondria

  • Generate energy by breaking down lipids and carbohydrates.

  • Surrounded by a double-membrane system with inner and outer membranes separated by an intermembrane space.

  • Interior (matrix) contains genetic material and enzymes for oxidative metabolism.

    • Folds in the inner membrane (cristae) increase surface area for energy production.

Structure of Mitochondria

  • Outer membrane: Highly permeable, contains porins for small molecules.

  • Inner membrane: Contains proteins for oxidative phosphorylation.

  • Matrix: Houses enzymes for the citric acid cycle, where complete oxidation of pyruvate occurs to produce ATP.

Mitochondrial Dynamics

  • Mitochondria form an interconnected network through fusion and fission.

  • Fusion facilitates genetic material exchange; fission aids in distribution during cell division.

  • Positioned near high-energy areas, such as nerve cell synapses.

Mitochondrial Genome

  • Contains circular DNA, encoding 13 proteins, rRNAs, and tRNAs necessary for its functions.

  • Mitochondrial genetic code differs from the universal genetic code, influencing protein synthesis.

  • Mutations in mitochondrial DNA can lead to inherited diseases, such as Leber’s hereditary optic neuropathy.

Chloroplasts and Plastids

  • Chloroplasts, analogous to mitochondria, utilize sunlight for energy and carbohydrate synthesis through photosynthesis.

  • Structure: Double-membrane with an internal thylakoid membrane system, forming stacked disks (grana).

    • Compartmental structure: Stroma, intermembrane space, thylakoid lumen.

Functions of Chloroplasts

  • Synthesize carbohydrates, fatty acids, and amino acids, plus engage in nitrite reduction.

  • Electron transport and ATP generation occur in the thylakoid membrane.

Chloroplast Genome

  • Contains circular DNA, larger and more complex than mitochondrial DNA, with genes necessary for gene expression and photosynthesis.

  • Import of proteins into chloroplasts involves transit peptides guiding them through the membranes.

Peroxisomes

  • Single-membrane organelles without their own genomes, involved in various metabolic reactions.

  • Functions:

    • Breakdown of substrates leading to hydrogen peroxide production, which is catalyzed to water.

    • Synthesize lipids such as cholesterol and plasmalogens, which are key components of cell membranes.

    • In germinating seeds, peroxisomes convert fatty acids to carbohydrates via the glyoxylate cycle.

Peroxisome Formation and Protein Import

  • Formed by vesicle budding from the ER or growth/division from existing peroxisomes.

  • Enzymes and proteins are imported as unfolded polypeptides, with targeting mechanisms involving Pex proteins and signals recognized by specific receptors.

  • Genetic disorders related to peroxisome biogenesis can arise from mutations in peroxins, leading to conditions like Zellweger syndrome.