lecture7 for BGYA01

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Biological Sciences
Clare Hasenkampf

BGYA01 L ECTURE 7 O CTOBER 2, 2007 Last class we began our discussion of eukaryotic cell structure. We talked about the double membrane organelles- the nucleus the mitochondria and the plastids. Clicker question What is the minimum number of times that a ribosomal protein passes through a nuclear pore, before it can participate in translation? a) 0 b) 1 c) 2 *** d) 3 We began to see how extensively eukaryotes use membranes to make internal compartments. Now we will talk about the endomembrane system, and we will see yet again how extensively eukaryotic cells use membranes. B) THE ENDOMEMBRANE SYSTEM Now lets talk about the major organelle-system of the cytoplasm - the endomembrane system. The endomembrane system is shown on page 76 as the brown sac system plus the blue sac system. It is also shown on page 77 brown sacs and blue sacs. It is also shown on page 101, figure 5.5- without the other cellular components. What is the endomembrane system? It is a collection of interconnected organelles that collectively are called the endomembrane system. The components of the endomembrane system are: the endoplasmic reticulum(ER) the rough ER and the smooth ER] the Golgi apparatus, and lysosomes and secretory vesicles and vesicles that travel between these components. You can think of the endomembrane system as a branching, (usually) one way transportation system along which proteins are shipped. Also within the endomembrane system, the lipids are manufactured, as are many types of proteins. Lets look at the starting point of the system- the endoplasmic reticulum. 1 www.notesolution.comOverhead Figure 4.10, page 80. The beginning of the ER usually begins at the nuclear membrane. The endoplasmic reticulum (ER) is a complex membranous system that consists of a branching network of tubules and sacs. The interior of the system is known as the lumen. Usually the ER starts right next to the nucleus (Figure 4.10, page 80). The ER can make up as much as to 10% of the volume of the cell. Also because of its many folds, it has a membrane surface area may times greater than that of the cells plasma membrane. The are two different regions of ER- the smooth ER and the rough ER. Figure 4.10, page 80. The smooth ER is the site of the synthesis of lipids- phospholipids, steroids and fatty acids. Some of these lipids are needed to make material for membranes. Membrane proteins are made in the rough ER, and then move to the smooth ER where they can combine with the components of the membranes. Then membrane material is shipped to where it is needed as membrane vesicles. (the places that need membrane material from the smooth ER are the plasma membrane, the nuclear envelope, and the or some other membrane bound organelle.) These membrane vesicles pinch off of the smooth ER, go to the relevant organelle and then fuse with existing membrane material of the organelle for which they are destined, to allow the relevant membrane to grow. What about the rough ER? Why is the rough ER rough? The rough ER is rough because it has ribosomes attached to it. The attachment of ribosomes to the ER, gives the ER a rough appearance in pictures and so this portion of the ER is called the rough ER. Figure 4.10, page 80. Why are ribosomes attached to the ER? It is because the rough ER is a major site of protein synthesis. (translation). Certain categories of proteins need to enter the endomembrane system, this happens as they are made. As they are made they are pushed into the lumen of the ER, as we will see in a minute. Some proteins of the cell are made on a ribosome attached to the ER and some proteins are made on free ribosomes (ones not attached to the ER). 2 www.notesolution.com
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