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Chloroplast Structure

List of Functions of Cell Organelles

Many courses in introductory biology include cell biology and require knowledge of the basic functions of the organelles found in eukaryotic cells. It is useful to be able to summarize the main functions of each type of organelle in just a few words or sentences.

The following table of functions of cell organelles is a list of short summary information for each organelle.
(See the links from some descriptions for further details.)

Membrane-bound Organelles

Organelle Type

Main Functions (not necessarily all functions):

1.

Nucleus

"Control Center" of the cell.
"Contains the cell's DNA (genetic information) in the form of genes.

Re.
Nucleic Acids

  • *Sequestration and *replication of DNA.
  • *Transcription and *modification of RNA.

Contains one or more nucleoli (plural, singular word = nucleolus) whose functions include:

Nucleoli

  • Biosynthesis of ribosomal RNA (rRNA) and production (assembly) of ribosomes.

2.

Rough Endoplasmic Reticulum (RER)

Consists of many interconnected membranous sacs called cisternae, onto whose external surface ribosomes are attached (distinguishing RER from SER on electron micrographs).

Ribosomes

Produce polypeptides that are then either ...

  • inserted into the RER membrane, or
  • moved into the lumen (central region) of the cisternae, or
  • moved to the Golgi complex and probably onwards from there.

In lumen of cisternae

Produce proteins that are then either ...

  • retained within vesicles, or
  • secreted from the cell (via secretory vesicles - see below).

3.

Smooth Endoplasmic Reticulum (SER)

Consists of many interconnected membranous sacs called cisternae (without ribosomes).
Many enzymes are either attached to the surface of the SER or located within its cisternae. Chemical reactions within the SER vary with the type and location of cells. E.g.

  • helps with protein folding and transport of synthesized proteins
  • glycosylation - which involves the attachment of oligosaccharides.
  • disulfide bond formation and rearrangement - to stabilize the tertiary and quaternary structure of many proteins
  • modification of some drugs e.g. by the cytochrome P450 enzymes in liver cells.

4.

Mitochondria

The main function of mitochondria in aerobic cells is the production of energy by synthesis of ATP. However, mitochondria also have many other functions, including e.g.:

  • Processing and storage of calcium ions (Ca2+).
  • Apoptosis, i.e. the process of programmed cell death
  • Regulation of cellular metabolism
  • Synthesis of certain steroids

See also the structure of mitochondria and the functions of mitochondria.

5.

Chloroplasts
(plant cells only)

Chloroplasts are the sites of photosynthesis within plant cells.

6.

Golgi Apparatus

The Golgi apparatus modifies, sorts and packages macromolecules for delivery to other organelles or secretion from the cell via exocytosis - see (9.) below.

7.

Lysosomes

Lysosomes (tiny sacs containing enzymes) are the main sites of intracellular digestion. They enable the cell to make use of nutrients. Their functions can be listed as:

  • Autophagy - digestion of materials from within the cell.
  • Heterophagy - digestion of materials originating from outside the cell.
  • Biosynthesis - recycling unwanted products of chemical reactions to process materials received from outside the cell.

Lysosomes also destroy the cell - usually after it has died.

8.

Peroxisomes
(also called "microbodies" - smaller than lysosomes and contain specific enzymes)

Similar to (but smaller than) lysosomes, the metabolic functions of peroxisomes include:

  • Breakdown of fatty acids by beta-oxidation
  • Breakdown excess purines to urea
  • Breakdown of toxic compounds e.g. in the cells of the liver and kidney.
  • also play a role in the biosynthesis of certain important molecules incl. cholesterol and (in liver cells) bile acids derived from cholesterol.

9.

Secretory vesicles
(sometimes called simply "vesicles")

Transport and delivery of their contents (e.g. molecules such as hormones or neurotransmitters) either into or out of the cell, in both cases via the cell membrane.

  • Exocytosis - movement of the contents of secretory vesicles out of the cell.
  • Endocytosis - movement of the contents of secretory vesicles into the cell.

10.

Vacuole
(plant cells only)

Helps maintain turgor pressure pressure (turgidity) inside the cell - which pushes the plasma membrane against the cell wall. Plants need turgidity to maintain rigidity.


Non-Membranous-bound Organelles

Organelle Type

Main Functions (not necessarily all functions):

1.

Ribosomes

Ribosomes interpret cellular information from the nucleus and synthesize proteins.
There are different types of ribosomes e.g. 80S (eukaryotic), 70S (prokaryotic).

The following structures form part of the cell's cytoskeleton:

2.

Microfilaments
(formed from actin)

Actin has a contractile function in muscle cells.

In non-muscle cells actin microfilaments form part of a web-like layer (called the cell cortex) located immediately below the cell's plasma membrane. This structure helps to define the shape of the cell including the structure of any microvilli. They also facilitate movement of certain particles and structures e.g. macrophages, fibroblasts and nerve growth cones.

3.

Microtubules
(formed from tubulin)

  • As the main "building blocks" forming the cytoskeleton - the cell's framework within which all components of the cell are held in position or allowed restricted movement.
  • Movement of materials and structures within cells e.g. help form the miotic spindle during the "prophase" part of cell division by mitosis.

For further detail see functions of microtubules.

4.

Intermediate Filaments
(formed from intermediate filament proteins, e.g. keratin)

Intermediate filaments are important for maintaining the mechanical structure of cells. There are different types of intermediate filaments that can be identified according to the protein from which they are formed. The different types of intermediate filaments occur in different types of cells and therefore provide structural support (to the cell) in slightly different ways.
E.g. neurofilaments in the axons of neurons are involved in the radial growth of the axon, so determine its diameter as well as contributing strength and rigidity to the cell.

5.

Junctions

"Junctions" are connecting points joining either cells to other cells, or cells to their basement membrane. See the diagram of the cytoskeleton.

6.

Centrosomes

Contain the centrioles, which are involved in the process of mitosis - see cell-division.

7.

Cilia

Some eukaryotic cells have cilia (plural, singular word = cilium) whose function is often to facilitate either movement of the cell or movement of something over the surface of cells e.g. fallopian cells move ova towards the uterus.

8.

Flagella (of spermatozoa differ from prokaryotic flagella)

The main function of the flagellum of a human spermatozoon (sperm cell) is to enable the sperm to move close to the oocyte ("egg" cell) and orient itself appropriately .

The cell membrane is often included in sections about the structure and functions of cell organelles. However, the cell membrane (also known as the plasma membrane) is not within the cell but one of the structures that defines the cell - together with the cell wall in the cases of plant cells and prokaryotic cells. See functions of the cell membrane.

The above list of functions of organelles shows that many, though not all, membrane-bound organelles are sites of biochemical reactions, i.e. where chemicals are made (= "produced", "synthesized" or "biosynthesized") or broken-down (= "degraded') or changed in some way. Such chemical reactions are examples of metabolic processes and often form part of metabolic pathways. Hence knowledge of cell biology is useful when studying metabolism.

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