Biological Membranes - A. N. Ogurtsov 2012

Structure and Functions of Biomembranes
Membrane Structures of the Cell
Lysosomes

During phagocytosis, Cells are capable of engulfing particles larger than 1 µm, whereas pinocytosis involves the uptake of Proteins and other high-molecular-weight substances. The system responsible for digesting extracellular Materials is represented by primary lysosomes.

In the event of Cell death, the lysosomal membrane ruptures, initiating cellular autolysis: released Hydrolases degrade proteins into Amino Acids, Nucleic Acids into NUCLEOTIDES, and Polysaccharides into Monosaccharides. A normally functioning cell is reliably protected from the action of its own internal hydrolases by the intact lysosomal membrane.

Primary lysosomes can fuse with one another as well as with cellular vacuoles, phagosomes, and coated vesicles, thereby forming so-called secondary lysosomes capable of phagocytosis. Secondary lysosomes that arise via autophagy (self-Digestion) and break down internalized Components of the cell itself are termed autolysosomes. In contrast, lysosomes that digest foreign substances taken up by The Cell are called heterolysosomes.

During the first ten minutes following their synthesis, hydrolytic Enzymes reside in the rough Endoplasmic reticulum, for the next ten minutes in the Golgi apparatus, and subsequently for several tens of minutes in primary lysosomes. This is followed by The formation of zymogen granules—dense aggregates of zymogen molecules surrounded by a membrane—which migrate to the cell surface and undergo exocytosis (secretion to the exterior).

To protect Endocrine glands from the local effects of the Hormones they secrete, these substances are stored within the cell inside zymogen granules as prohormones—larger precursor molecules featuring an attached "extra" peptide fragment. Secretion of such a hormone becomes possible only after this "extra" fragment is cleaved off.

Endocrine gland cells accumulate large quantities of hormones in an inactive form within zymogen granules as prohormones. In response to stimulation, the hormone is rapidly released into the bloodstream, resulting in a sharp spike in its concentration, and is carried via Circulation to target cells.

Neurotransmitters (norepinephrine, acetylcholine, dopamine, glutamic acid, and oligopeptides) are synthesized within Nerve Cells and stored in specialized structures known as synaptic vesicles.

METABOLISM/35.html">Selection/41.html">Review Questions and Assignments

1. List the main membranous components of the cell.

2. What are the Similarities and differences in the Organization of Membrane structures between PROKARYOTES AND EUKARYOTES?

3. Define the Cytoplasm and Cytosol of the cell, and characterize the relationship between these elements of cellular architecture.

4. By what factor does the surface area of internal membranes in a Introduction/5.html">Eukaryotic Cell exceed that of its Plasma Membrane? What is the Functional Significance of this ratio?

5. Through what two mechanisms do Biomembranes participate in interactions between the compartments they separate?

6. List and characterize the six Major Groups of Functions associated with biomembranes.

7. Describe the Gorter and Grendel experiment.

8. How can the thickness of a biomembrane be determined knowing its specific capacitance and the Dielectric Constant of the lipid layer?

9. Name and characterize the two principal Properties of the lipid bilayer.

10. Describe the Singer and Nicolson Fluid Mosaic Model of the biomembrane.

11. What are the similarities and differences between the cytosolic and exoplasmic faces of the membrane?

12. What is the functional Significance of the Asymmetry between the cytosolic and exoplasmic faces of a biomembrane?

13. Which cellular Organelles are bounded by a double membrane?

14. What specific functions does The Plasma Membrane perform?

15. What specific functions does The endoplasmic reticulum perform?

16. What specific functions do the membranes of the Golgi apparatus perform?

17. What is the functional difference between the maturing (trans) and forming (cis) poles of the Golgi apparatus?

18. What are the Structural and functional Features of the nuclear membrane?

19. What are the structural and functional features of cell lysosomal membranes?