Biomed 04

3- Endoplasmic reticulum (ER) Note: All the encircling membranes of membranous organoids has the same structure as the plasma membrane. ER occurs in almost all kinds of nucleated cells. It a system of hollow network of branched and joined tubules . Note: 1 ml (1 cm3) of liver tissue contains about 11 m2 of ER. There are 2 patterns of ER which are: 1) Rough (granular) ER which covered by ribosomes. 2) Smooth (agranular) ER which lacks ribosomes.

3- Endoplasmic reticulum (ER) (continue)

Note: both types may be connected in the same cell. Also, one type may be changed to the other depending on the need of the cell.

Functions of ER: 1- Helps molecules to move from one cell to another (both rough & mooth ER). 2- Makes proteins due to the presence of ribosomes (rough ER). 3- Involved in the synthesis of steroids (smooth ER). 4- Helps to regulate calcium levels in muscle cells smooth ER). 5- Helps with the break down of toxic substances in the cell (smooth ER).

4- Golgi apparatus (Golgi body) (Golgi complex) Camillo Golgi, nerve cells – 1898, 1906. It was found in most eukaryotic cells, but tends to be more prominent where there are proteins are secreted. The Golgi apparatus is made up of: 1- A stack of flattened elongated sacs called cisternae. The cristernae have: i) A cis (immature) face {directed towards the ER and nucleus}, ii) The medial region {in the middle} and iii) The trans (mature) face {directed towards the plasma membrane.

4- Golgi apparatus (Golgi body) (Golgi complex) {continue) 2- Vesicles: Which may be: a) Large rounded vesicles (outgoing transport vesicles) which are detached from the trans face of cisternae. These vesicles are filled with protein. b) Microvesicles (incoming transport vesicles) (transfering vesicles) which are detached from rough ER. They move towards the cis face of cisternae. These vesicles contain the newly synthesized protein. c) Intermediate vesicles which are found in large number close to the periphery of sacs.

Functions of Golgi apparatus: 1) Storge: Proteins that formed by ribosomes migrate as transfering vesicles (microvesicles) to fuse with the membrane of cis face where they are collected, condensed and then enclosed by membranes forming secretory granules. These granules are then move to the plasma membrane where they release their contents by exocytosis. 2) Packing: It forms lipoproteins by bounding both lipids (which migrate from smooth ER) and proteins (which migrate from rough ER) inside a membrane. The formed lipoprotein granules release from trans face of Golgi apparatus. 3) Secretion: Such as hormones (by endocrine glands), enzymes (by exocrine glands), mucous (by goblet cells). 4) It helps in the formation of the acrosome of the sperm which has a secretory ability to penetrate the membrane of the ovum

5- Lysosomes They are saclike structure surrounded by a single membrane. IT contains powerful digesting enzymes such as acid phosphatase, ribonuclease, deoxyribonuclease … etc. The number of them is affected by different physiological and pathological changes. Fasting and ageing decrease their number. Functions of lysosomes: Lysosomes are responsible for digestion of biological compounds. This digestion may be one of the following: i) Intracellular digestion: This takes place inside the cytoplasm which may be:

a) Exogenic origin: They digest the taken substances by endocytosis in a process known as heterophagy. The primary lysosomes fuse with endocytic vesicle to form secondary lysosome or heterophagosome. The engulfed material is then digested by the enzymes into small molecules. b) Endogenic origin : They digest some part of the cytoplasm e.g. mitochondria by a process known as autophagy. In autophagy, the cytoplasmic structure is digested by primary lysosomes forming secondary lysosome or autophagosome.

Heterophagy

Autophagy

Resulting small molecules from autophagy or heterophagy can diffuse into the cytoplasm. If digestion is completed, residual bodies are formed which may be go out by exocytosis or may be remain in the cell. These remaining residuals may be represent an index of cell ageing.

ii) Extracellular digestion: Some cells can discharge lysosomal enzymes outside the cell to destroy some surrounding structures. This explains how the sperm can penetrate the protecting coat of the ovum during fertilization. iii) Autolysis: It is a process in which the cell is self-destructed. When cells approach death, lysosomes rupture in the surrounding cytoplasm causing the digestion of the whole cell. This action is not accidental but it is regulated by signals that scientists do not fully understand.

Heterophagy

Autophagy

6- Peroxisomes (microbodies) They contain enzymes involved in the degradation of fatty acids and amino acids. They also contain the enzyme catalase. Structure of peroxisomes: Plant cells have similar vesicles called gyloxisomes. They are about the same size, or slightly larger than lysosomes. Peroxisome function: Peroxisomes contain enzymes that degrade fatty acids and amino acids. In doing so they produce hydrogen peroxide (H2O2). H2O2 is very toxic because it is unstable and spontaneously degrades to produce compounds called free radicals. Free radicals are very reactive because they have unpaired electrons and will react with a variety of cellular macromolecules and alter their structure. Fortunately peroxisomes contain the enzyme called catalase that degrades hydrogen peroxide to the less dangerous oxygen and water. ()2H2O2

catalase

O2

+

2(H2O)

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