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From MyMCAT

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Contents 1 The Endoplasmic Reticulum 2 The Golgi Apparatus 3 Lysosomes and Endosomes 4 Peroxisomes 5 The Nucleus, nucleolus, and the nuclear envelop 6 Mitochondria

The Endoplasmic Reticulum

The endoplasmic reticulum (ER) consists of an extensive network of interconnected saccules, tubules, and vesicles. Often, the ER is divided into two types, the rough and the smooth ER. The amount of each ER is dependent on the specific tasks that the cell must carry out.

Rough endoplasmic reticulum is named after the fact that under a microscope, its texture looks "rough" because of the numerous ribosomes scattered across its surface. As such, high levels of mRNA translation (protein synthesis) occur in the rough ER, specifically for proteins that are destined to become membrane bound or secreted out inside vesicles.

The smooth endoplasmic reticulum is much less abundant in cells and while continuous with the rER, it is devoid of ribosomes. Unlike the rough ER, the smooth ER is prominently used in the detoxification of toxic molecules and in lipid metabolism, steroid, cholesterol, and triglyceride synthesis. In muscles, the smooth ER is modified and instead serves as storage for calcium ions (which is used in muscle contraction signaling).

The Golgi Apparatus

The Golgi apparatus consists of organized stacks of squished vesicles called golgi cisternae. The Golgi primarily acts as a hub for vesicle transport receiving lipids and proteins from the ER and dispatching them to various destinations. The Golgi also is the site where carbohydrates are attached to some proteins.

Lysosomes and Endosomes

Lysosomes are specially organized vesicles loaded with hydrolytic enzymes enabling them to used for the degradation of macromolecules. Unlike the cytosol with a pH of 7.3, their pH is a much lower 5, which further aids in the breakdown of molecules. Lysosomes are essential in turning over cellular debris and killing of phagocytosed microorganisms. Lysosomes typically receive material which has been internalized (endocytosis) into endosomes by fusing with these vesicles and degrading all of their contents.

Peroxisomes

Peroxisomes are small, membrane-bounded structures rich in oxidative enzymes. Hydrogen peroxide is the primary chemical produced and is used to oxidize a variety of alcohols and toxic materials. Thus, peroxisomes are found in abundance in liver and kidney cells. Peroxisomes also have a second function in the breakdown of long fatty acids which cannot normally be broken down in the mitochondria.

The Nucleus, nucleolus, and the nuclear envelop

The nucleus is the largest organelle of the cell and it is separated from the cytosol by the nuclear envelope. The nuclear envelope provides a compartment for the cell's DNA and separates it from the cytosol. The nuclear envelope is similar to and in fact connects with the endoplasmic reticulum, but posses only a modest number of ribosomes and only on the outer surface. Unlike the endoplasmic reticulum, which exists as long, wave sheets, the nuclear envelop wraps the nucleus in a roughly spherical shape and contains periodically spaced nuclear pores, which allow for the selective exchange of material between the nucleoplasm and the cytoplasm.

Within the nucleus lies chromatin, the nucleolus and nucleoplasm. Chromatin is the natural state of DNA in a complex web of proteins and RNA scaffolds. The nucleolus is a transient structure that is found in certain stages of the cell cycle. They are rich in rRNA and protein and their main role is the assembly of ribosomes.

Mitochondria

Mitochondria are the membrane-bound organelles that serve to convert various forms of energy to ATP through the electron transport chain. The citric acid cycle, the electron transport chain, and fatty acid metabolism (beta oxidation) all occur inside the mitochondria, and various shuttles and transporters are used to selectively move molecules from the cytosol to the inner mitochondria.