6.4 Peroxisomes and other organelles

Peroxisomes and Specialized Organelles

Peroxisomes are single-membrane organelles that specialize in oxidative reactions, fatty acid breakdown, and detoxification. They work alongside other compartments like endosomes, lysosomes, and vacuoles to keep the cell's internal environment organized and functional. The cytoskeleton ties it all together by physically positioning and transporting these organelles where they're needed.

Structure and Function of Peroxisomes

Peroxisomes are small, spherical organelles enclosed by a single lipid bilayer membrane. They're packed with oxidative enzymes, most notably catalase and various oxidases, that generate and then safely decompose hydrogen peroxide ($H_2O_2$).

Their core functions include:

• Fatty acid beta-oxidation: Peroxisomes break down fatty acids into acetyl-CoA, which can then enter the citric acid cycle in mitochondria for ATP production. This is especially important for very long chain fatty acids (VLCFAs, C22 and longer), which mitochondria cannot process on their own.
• Detoxification: Catalase converts $H_2O_2$ into water and oxygen ($2H_2O_2 \rightarrow 2H_2O + O_2$), preventing oxidative damage. Peroxisomes also help detoxify substances like ethanol, particularly in liver cells.
• Lipid biosynthesis: They synthesize plasmalogens, a class of phospholipids important for cell membrane function (especially in brain and heart tissue), and contribute to bile acid synthesis in the liver.
• Glyoxylate cycle (plants and some microorganisms): In plant peroxisomes (called glyoxysomes), this pathway converts fatty acids into carbohydrates, which is critical during seed germination when the plant needs glucose but has only stored lipid reserves.

Peroxisomes in Cellular Metabolism

Peroxisomes are a frontline defense against oxidative stress. The oxidative reactions they carry out inevitably produce reactive oxygen species (ROS), but the enzymes inside the organelle immediately neutralize these byproducts. Without functional peroxisomes, ROS and other toxic compounds accumulate and damage proteins, lipids, and DNA.

Because VLCFAs must be shortened in peroxisomes before mitochondria can finish oxidizing them, peroxisomal defects hit lipid metabolism hard. Zellweger syndrome is a genetic disorder in which peroxisomes fail to form properly. Patients accumulate VLCFAs and other toxic metabolites, leading to severe neurological impairment, liver dysfunction, and typically death in infancy. Related peroxisomal disorders include adrenoleukodystrophy (VLCFA buildup damages myelin in the nervous system) and Refsum disease (accumulation of branched-chain fatty acids).

Specialized Organelles and Functions

Endosomes are membrane-bound sorting stations that direct internalized material to the right destination. The pathway works in stages:

1. Early endosomes receive cargo from the plasma membrane via endocytosis (receptor-mediated endocytosis, pinocytosis). They act as the initial sorting hub.
2. Recycling endosomes return reusable components, like receptors and membrane lipids, back to the plasma membrane.
3. Late endosomes mature from early endosomes, becoming more acidic. They fuse with lysosomes, delivering material for degradation by hydrolytic enzymes.

Vacuoles are large, membrane-bound compartments found primarily in plant and fungal cells.

• In plants, the central vacuole can occupy up to 90% of cell volume. It stores water, ions, pigments (like anthocyanins that color flowers), and enzymes. By absorbing water, it generates turgor pressure, which keeps the cell rigid and drives processes like cell elongation and stomatal opening/closing.
• In fungi, vacuoles handle storage (polyphosphates, amino acids), pH regulation, and waste disposal.

Lysosomes contain acid hydrolases that degrade worn-out organelles, macromolecules, and foreign material. They're central to autophagy, the process by which cells recycle their own damaged components to maintain homeostasis.

Cytoskeleton and Cellular Organization

The cytoskeleton is a dynamic network of protein filaments that gives the cell its shape, provides mechanical support, and moves organelles to where they need to be. It has three main components:

Microtubules (tubulin polymers) handle long-range transport and organelle positioning.

• Kinesin motor proteins walk toward the plus end of microtubules (generally away from the cell center, called anterograde transport).
• Dynein motor proteins walk toward the minus end (toward the cell center, retrograde transport).
• This system maintains the spatial organization of the ER, Golgi apparatus, and other organelles.

Microfilaments (actin filaments) are responsible for short-range movement and anchoring.

• Myosin motor proteins interact with actin to drive vesicle trafficking and localized organelle transport near the cell cortex.

Intermediate filaments (e.g., keratin in epithelial cells, vimentin in mesenchymal cells) provide mechanical strength and resistance to shear stress. They don't directly transport organelles but are critical for maintaining cell shape and structural integrity.

The cytoskeleton is also essential during cell division: microtubules form the mitotic spindle, attach to chromosomes at kinetochores, and pull sister chromatids apart during anaphase. Without a functional cytoskeleton, organelle distribution, cell polarity, and division all break down.