Lesson 3 - Yeast Morphology and Evaluation SAQ

* Describe the main functions of the following yeast organelles:

a)    Periplasm

b)    Golgi body

c)    Ribosomes

d)    Plasmamembrane

e)    Cell wall

Response:

a)    The periplasm is not just the space between the cell wall and the cell membrane. It has a gel-like consistency, which probably helps to protect the relatively fragile cell membrane. It does this by providing a buffer layer between the membrane and cell wall. The periplasm contains a number of enzymes, including invertase, which converts sucrose into glucose and fructose.

b)    The Golgi body is often compared to the sorting room in a post office. It takes molecules (usually proteins) that have been  synthesised in the endoplasmic reticulum. It then modifies their structures, places them within a membrane-bound vesicle, and then transports them to chosen locations (either inside or outside the cell). The structural modifications act as labels that indicate the molecules’ destination (hence why we liken this to a post office!).

c)    Ribosomes are responsible for protein synthesis. They comprise two sub-units, one large and one small, made up of proteins and RNA. They are responsible for synthesising the proteins that correspond to the genetic code held in the nuclear chromosomes. In the process called transcription, which occurs in the nucleus, a molecule of messenger RNA (mRNA) is made whose structure is complementary to the DNA sequence which makes up genes. mRNA is transported to the smaller sub-unit of the ribosome. Here, in the process called translation, the genetic code, now held in the sequence of RNA is read and using transfer RNA (tRNA) the smaller sub-unit receives amino acids and builds protein molecules that correspond to the original DNA template.

d)    The plasmamembrane (cell membrane) lies underneath the cell wall and is the real barrier between the cell interior and the external environment. It has a double-layered structure, consisting mainly of fatty materials. These fatty materials are collectively called lipids; proteins are embedded within them. The plasmamembrane controls what enters and leaves the cell. Many of the proteins found in the plasmamembrane help to transport specific compounds into and out of the cell.  

e)    The cell wall gives the cell its shape and protects the much more fragile cell membrane. It prevents periplasmic enzymes from escaping. Bud scars mark the locations where daughter cells are excised from the mother. The external surface of the wall carries the mannans and projecting zymolectin proteins that are responsible for flocculation.