Organelles

Organelles are membrane-bound structures within a cell that perform a specialized function. They are similar to organs, just on a smaller scale.

Organelles are almost unique to eukaryotes, however, some prokaryotic organelles have been known to exist.

Nucleus
The nucleus stores the genetic information of a cell, mostly in the form of chromatin. A subregion within the nucleus, the nucleolus, produces RNA responsible for constructing ribosomes.

Mitochondria
The mitochondria are primarily responsible for converting stored energy to ATP (adenosine triphosphate), a form of chemical energy. ATP powers all metabolic processes within a cell.

The internal membrane (cristae) of a mitochondrion is folded to increase the surface area for the processes that create ATP. Mitochondria are more numerous in cells that have a high demand for energy, such as muscle cells.

Ribosomes
The ribosomes play a role in protein synthesis. They do not have a membrane. Cytoplasmic ribosomes float freely in the cytoplasm, while the other ribosomes are attached to the surface of the rough endoplasmic reticulum.

Endoplasmic reticulum
The ER is a set of folded membranes which create a surface for chemical reactions to occur, isolated from the rest of the cell. It might be harmful for a cell to have too high a concentration of insulin, or any other substance it produces, within it at any moment. The rough ER is made rough by the presence of ribosomes. These ribosomes rely on the ER's enzymes present on its inner surface.

The smooth ER has enzymes present on its inner surface for processing macromolecules (highly complex molecules). The smooth ER can also perform specialized tasks in certain cells, such as secreting testosterone in testicular cells. The smooth ER can also transport macromolecules within the cell by forming and detaching vesicles.

Golgi apparatus
The Golgi apparatus accepts incoming vesicles sent by the endoplasmic reticulum. It finalizes the production of any macromolecules made in the ER and activates them. Afterwards, it can dispatch macromolecules to different locations within and outside of the cell, all while keeping them separate from the rest of the cell.

The Golgi apparatus itself uses vesicles to move its products. These vesicles can travel to the cell membrane and secrete their contents into the extracellular fluid. It has a constant supply of membrane to form vesicles by recycling incoming vesicle material from the ER.

Lysosomes
Lysosomes are responsible for breaking down old organelles, macromolecules, or "larger structures". Removing aging parts keeps the cell functioning efficiently.

Cytoskeleton
The cytoskeleton is comprised of the structural elements of the cell. There are three components to the cytoskeleton: actin filaments, intermediate filaments, and microtubules. Actin filaments are stretchy and form a dense web underneath the cell membrane. Intermediate filaments restrict organelles to a general region of the cell. Microtubules are tracks along which organelles can move.

Centrosome
The centrosome oversees the movement of spindle fibres during mitosis.

Movement organelles
In prokaryotic organisms, ciliae and flagellae are common forms of mobility. The cilia are small hairlike projections that wave. The flagella is a single tail that whips back and forth, propelling the organism.

Cell wall
Unlike animal cells, which are surrounded by a membrane, plant cells are protected by a layer of cellulose fibres. The cell wall is highly permeable, unlike the cell membrane.

Central vacuole
The central vacuole of a plant cell is a massive structure which maintains turgor pressure within the cell and stores water. It is responsible for the stiffness of a plant that is well-watered, and conversely, the flaccidity of a parched one.

Chloroplast
Chloroplast is a plastid (stacked internal membranes) that enables autotrophy via photosynthesis in plants. It is a green pigment, and are composed of thylakoids, the centres of photosynthesis. Thylakoids are stacks of smaller disks called grana (singular: granum).