Ece 593-728

1. Ece 593-728 Cell Structures Chapter three

2. Cells and Viruses • There are basically three type of biological units: prokaryotic cells, eukaryotic cells and viruses. • Eukaryotic cells: these cells make up the human body and plants. This cells are distinguished by the presence of a nuclear membrane surrounding the cell nucleus and the presence of numerous membrane bound organelles within the cell.

3. Cont… • Prokaryotic cells: unlike eukaryotic cells, these cells lack membranous structures; bacteria make up the largest class of prokaryotic cells. • Viruses: viruses differ from both eukaryotic and prokaryotic cells in the sense that they can exist as living and non living entities. They consist of only a nucleic acid molecule surrounded by a protein shell.

4. Cell Compartments • The cell interior is divided into a number of compartments, these compartments are surrounded by membranes. These membrane bound compartments are known as cell organelles. • Each cell organelle performs a specific function that contributes to the cell’s survival. • The interior of a cell is divided into two regions:

5. Cont… • (a) Nucleus: this is a spherical structure usually located near the center of the cell. • (b) Cytoplasm: this region makes up all the interior of the cell except the nucleus. The cytoplasm contains two components: cell organelles and the cytosol ( fluid surrounding the organelles).

6. Cont… • Membranes: these form a major structural element in human cells. They are found surrounding the entire cell (plasma membrane) and enclosing most of the cell organelles. • Functions of the membrane • They act as a selective barrier to the passage of molecules, allowing some molecules to cross while excluding others.

7. Cont… • One of the advantages of restricting the movements of molecules across membranes is that the products of chemical reactions can often be confined to specific organelles. • As we shall discover in later chapters, the hindrance offered by a membrane to the passage of substances can be altered to allow increased or decreased flow of molecules or ions across the membrane.

8. Membrane Structure • All membranes consist of a double layer of lipid molecules in which proteins are embedded. The lipid layer prevents the movement of most polar or ionized molecules through the membrane, whereas the proteins provide pathways for the selective transfer of these substances through the lipid barrier. • The major membrane lipids are PHOSPHOLIPIDS.

9. Cont… • Phospholipids: These are amphipathic molecules. By this we imply that one end has a charged polar region, and the remainder of the molecule consists of two long fatty acid chains which is non polar. The phospholipids in cell membranes are organized into bimolecular layer with the non polar fatty acids chains located in the middle and the polar regions located toward the surface.

10. Membrane proteins • There are basically two classes of membrane proteins. They are; integral and peripheral membrane proteins. • Integral membrane proteins • Closely associated with the membrane lipids and cannot be extracted from the membrane without disrupting the lipid bilayer • They are amphipathic, thus possessing a polar and non polar region.

11. Membrane Proteins • Peripheral membrane proteins • These proteins are not amphipathic. • They are located at the membrane surface where they are bound to the polar regions of the integral membrane proteins

12. Cell Organelles • The cell consists of units called organelles, these organelles perform specific functions that aid the survival of the cell. The cell organelles include; nucleus, ribosome, endoplasmic reticulum, golgi apparatus, mitochondria, lysosomes. • Nucleus: • The primary function of the nucleus is the storage, transmission, and expression of genetic information used to synthesize the proteins that determine the structure and function of the cell.

13. Cell Organelles • Ribosomes: • These are the sites were protein molecules are synthesized using genetic information sent by RNA messenger molecules from DNA in the nucleus. • These ribosomes can be either found bound to an organelle called endoplasmic recticulum or found free in the cytoplasm. • Free ribosomes are released into the cytosol, whereas those bound are transferred to the golgi apparatus.

14. Cont… • Endoplasmic Reticulum: • Two forms can be distinguished, the rough surfaced and smooth surfaced endoplasmic reticulum. • The rough endoplasmic reticulum is involved in the packaging of proteins that are to be secreted by cells. While the smooth endoplasmic reticulum is the site at which lipid molecules are synthesized. The SER also stores and releases calcium ions involved in controlling various cell activities.

15. Cont… • Golgi Apparatus: • This is a series of closely opposed, flattened membranous sacs that are slightly curved, forming a cup shaped structure. • The Golgi apparatus sorts the different types of proteins received from the rough endoplasmic reticulum into vesicles that will be delivered to various parts of the cell.

16. Cont… • Mitochondria: • These are primarily concerned with the chemical processes by which energy is made available to cells in the form of molecules of adenosine triphosphate (ATP). • Most of the ATP used by the cells is formed in the mitochondria by a process that consumes oxygen and produces carbon dioxide. Mitochondria possess small amounts of DNA that contain the genes for the synthesis of some of the mitochondrial proteins.

17. Cont… • Lysosomes: • These are spherical organelles that are surrounded by a single membrane • The fluid within a lysosome is highly acidic and contains digestive enzymes • They act as cellular stomachs breaking down bacteria , they may also break down cell organelles that have been damaged and are no longer functioning normally.

18. Diagram showing an animal cell