Cell Organelles

Organelles are found only in eukaryotic cells and are absent from the cells of prokaryotes such as bacteria. The nucleus, the mitochondrion, the chloroplast, the Golgi apparatus, the lysosome, and the endoplasmic reticulum are all examples of organelles.

To see many amazing micrograph images and videos of the organelles we are studying go to http://www.cellimagelibrary.org/.

Non-membrane bound organelles

Cytoskeleton is a structure that helps cells maintain their shape and internal organization, and it also provides mechanical support that enables cells to carry out essential functions like division and movement.

Consists of

Microfilaments 

• Structure : Made of subunits of a protein called actin. Polymers of actin - each microfilament is 7 nm in diameter. 
• Function : Give the cell's shape stability as they are inside the cytoplasm. Give the cell mechanical strength and allow the cell to move. 

Intermediate filaments

• Structure: Made of a variety of proteins. 10nm in diameter. 
• Function: anchor the nucleus in place, extend between some cells allowing cell - cell signalling, and adhere some cells to a basement membrane to stabilise tissues. 

Microtubules

• Structure: Made of protein subunits called tubulin. 18 - 30 nm in diameter. 
• Function: 
• give shape and support to cells.
• form a track within the cell that organelles and chromosomes can be dragged along by cytoskeletal motor proteins. 
• make up cilia, undulipodia and centrioles. 

Cytoskeletal motor proteins - myosins, kinesins and dyneins. 

Amazing enzymes that use the hydrolysis of ATP to move organelles, vessicles and other structures around cells. 

Centrioles (aka centrosome)

• Structure: two separate centriole's at right angles to each other. Each centriole is a bundle of microtubules. 

One centriole

• Functions: 
• form the spindle (threads of tubulin) to which the motor proteins and chromosomes attach in order to move the chromosomes to opposite ends of the cell.  In most animals and fungal cells, spindle microtubules start from centrosomes or spindle pole bodies. Plant cells lack such structured microtubule organizing centers, and some of their microtubules appear to nucleate from near the nuclear envelope, but very little is known about spindle formation in plants. 
• Involved in the formation of cilia and undulipodia. Centrioles line up underneath the cell surface membrane. Microtubules sprout out of the centrioles forming a cilia or undulipodium. 

Cellulose cell wall

• Structure: bundles of cellulose fibres. 

• Function: on the outside of the plant cell membrane, it is a strong wall that prevents the cells from bursting when the cell is turgid. 
• Provide plant cells strength and support
• maintain their cell shape
• keep the entire plant strong and supported
• permeable so allow solutions to pass through. 

Organelles and their role in protein trafficking.

Organelles work together to carry out the specific function of each cell. This is often determined by what proteins are made, how they are modified and where they are taken - to another organelle in the cell or out of the cell to be taken elsewhere in the organism. 

The first organelle that is involved in making and transporting proteins is the nucleus. Here is a video to remind you of the role the nucleus has in transcription. 

The mRNA produced by the process of transcription moves out of the nucleus through a nuclear pore and attaches to a ribosome. The ribosome can be free (not attached to anything) or it can be attached to the rough endoplasmic reticulum. The process of reading the mRNA and ensuring the correct amino acids are added is called translation.

The translated proteins can then be labeled by the golgi body. The proteins are then sent to the correct location into or out of the cell. 

Here is a video that shows how real proteins can be tracked through a cell using confocal microscopy (the proteins have been tagged with fluorescent labels so we can see them).