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.