Cellular Organelles: Lysosomes, Mitochondria, Chloroplasts, and Peroxisomes part 8

Lysosomes contain a unique collection of enzymes that enable them to perform specific functions within the cell.
These enzymes are called hydrolases, specifically acid hydrolases, because they function in an acidic environment.
The interior of a lysosome has a pH of around 5, which is significantly more acidic than the cytosol with a pH of about 7.2. This difference in PH is more than 100 times more acidic.

Hydrolases break down various molecules:
- Nucleases: Degrade DNA and RNA.
- Proteases: Break down proteins.
- Glycosidases: Break down sugars.
- Lipases: Break down lipids.
The acidic environment and the compartmentalization of these enzymes within the lysosome serve as a protective measure. If these enzymes were to escape into the cytosol, they could digest cellular macromolecules.
Lysosomes fuse with other organelles or vesicles to digest their contents. This process requires maintaining the integrity of the lysosome and controlling the activity of its enzymes.

Lysosomes maintain their acidic interior by actively pumping protons (H+) into the organelle.
This process is powered by the hydrolysis of ATP, indicating that it is an energy-dependent process that moves protons against their concentration gradient.

There are over 50 types of lysosomal storage diseases.
These diseases result from genetic defects in genes that code for lysosomal enzymes.
When an enzyme is defective or missing, the corresponding macromolecule cannot be broken down and accumulates within the lysosome.
This buildup leads to storage, disrupting cellular function.

When a substance is taken into the cell from the outside, such as through a food vacuole, it is contained within a membrane-bound vesicle.
To digest the contents of this vesicle, it fuses with a lysosome.
After fusion, the hydrolytic enzymes within the lysosome, activated by the acidic environment, break down the contents of the vesicle.
If the vesicle contains food particles, they are digested, and the resulting nutrients are released into the cell for utilization.

Autophagy, or "self-eating," is a process by which cells recycle their own components.
When organelles become old or damaged, a membrane forms around them, creating an autophagic body or autophagosome.
This autophagosome then fuses with a lysosome, and the hydrolytic enzymes digest the damaged organelle.
The resulting component parts are then recycled to create new cellular structures.

Phagocytosis: A cell, such as a macrophage, engulfs bacteria or other pathogens from the outside.
The resulting vesicle (phagosome) fuses with a lysosome to break down and eliminate the ingested material.
Endocytosis: Smaller molecules are brought into the cell via vesicles that fuse with lysosomes for digestion.
The pH of these vesicles is gradually lowered through fusion with early and late endosomes until they ultimately fuse with a lysosome.

Mitochondria are organelles responsible for oxidative phosphorylation, a process that uses oxygen to generate ATP.
This is achieved by setting up a proton gradient across the inner mitochondrial membrane, which then drives ATP production from ADP and phosphate.

Mitochondria have two membranes: an outer membrane and an inner membrane.
The outer membrane is more permeable than the inner membrane.
The inner membrane is highly impermeable, especially to ions like protons, and is folded into cristae to increase surface area.
$Surface Area = \sum{i=1}^{n} Area{cristae_i}$
The folding of the inner membrane increases the surface area for embedding proteins of the electron transport chain.
The space between the two membranes is called the intermembrane space, while the interior of the inner membrane is called the matrix.
A pH differential is maintained between the matrix and the intermembrane space.

Mitochondria are dynamic organelles that can move around the cell, often attached to microtubules.

Mitochondria contain their own circular DNA, separate from the linear chromosomes in the nucleus.
The mitochondrial genome is small, containing only about 37 genes.
These genes primarily encode proteins involved in oxidative phosphorylation.
Mitochondria also have unique ribosomes that are structurally different from those in the cytosol and endoplasmic reticulum.

Chloroplasts are organelles found in plant cells that carry out photosynthesis, converting light energy into chemical energy in the form of sugars.

The pH differential that drives ATP synthesis in chloroplasts is across the thylakoid membrane.
There is a pH gradient between the stroma (the interior of the chloroplast) and the interior of the thylakoid.

Chloroplasts also have their own circular genome.
The genes on this genome encode proteins involved in photosynthesis.
The presence of their own DNA and unique ribosomes supports the endosymbiotic theory, which suggests that mitochondria and chloroplasts were once free-living bacteria that were engulfed by eukaryotic cells.

Peroxisomes are organelles involved in various metabolic processes, including oxidation reactions.
They oxidize molecules using molecular oxygen $O2$ , producing hydrogen peroxide $H2O2$ . $R-H + O2 \rightarrow R + H2O2$
Some peroxisomes are involved in the synthesis of certain lipids, such as cholesterol and bile salts.
The number of each type of organelle varies depending on the cell's location and function.