Cell Structure and Function

1. Cell Structure and Function Biology 155 Krug Fall 2003

2. Definition of a cell: • basic structural and functional unit of life • the smallest units that display the characteristics of life, i.e. reproduction, metabolism, response to stimuli

3. General Subdivisions of a Cell • Plasma Membrane = selectively permeable boundary between the cell and the environment • Nucleus = regulatory center of the cell • Cytoplasm = everything between the plasma membrane and the nucleus (fluid + organelles)

4. Plasma Membrane • Structure = phospholipd bilayer with proteins embedded in, and attached to, the inner (intracellular) and outer (extracellular) surfaces 2. Function • Selectively permeable barrier: controls what enters and leaves the cell • Phospholipids are liquid at body temperature, so proteins float around in the membrane -functions as a Fluid Mosaic

5. Plasma Membrane

6. Plasma Membrane (2) • Function – continued • Main responsibility: ensure the composition of extracellular fluid is not the same as the composition of the intracellular fluid • Water-soluble substances (salts, nutrients) cross membrane with aid of protein channels, which are selective about what can pass through • Lipids can pass directly through bilayer by diffusion (the random walk of molecules) • Attachment site of cytoskeleton, the internal support of the cell

7. Plasma Membrane (3) • Glycocalyx = protein and carbohydrate coat covering the extracellular surface of the plasma membrane • Allows attachment to other cells • Allows the cell to interact with the environment • Gives each person’s cell a distinctive surface; allows your body to recognize foreign tissues as different from your own (i.e., blood type, transplant rejection)

8. Plasma Membrane

9. General Subdivisions of a Cell • Plasma Membrane = selectively permeable boundary between the cell and the environment • Nucleus = regulatory center of the cell • Cytoplasm = everything between the plasma membrane and the nuclear compartment

10. Cell Structure Overview

11. Nucleus • Nuclear Envelope (membrane) • Phospholipid bilayer with nuclear pores b. Controls what enters/leaves the nucleus -- things only go in or out by passing through protein channels, which are selective c. Encloses all the chromosomes

12. Nucleus 2. Chromatin = all the chromosomes, which are long strands of the molecule DNA -- DNA regulates all cell activities, yet never leaves the nucleus; how is this possible? produces RNA, shortmessenger molecules that exit through nuclear pores RNA carries instructions out into the cytoplasm

13. Nucleus 3. Nucleolus – site of ribosome synthesis a. compartment in the nucleus where ribsomes are assembled b. ribosomes are then moved out into cytoplasm through nuclear pores c. ribosomes and RNA work together outside the nucleus, to build all the proteins in the cell

14. Nuclear Compartment

15. Ribosomes = site of protein synthesis --assembled in the nucleolus --exported into the cytoplasm Free – unbound in the fluid cytoplasm, produce proteins for use in the cell Bound – attached to the endoplasmic reticulum (ER), produce proteins for export, or for the plasma membrane Ribosomes

16. Cytoplasm + Organelles • Fluid portion = “cytoplasm” • Water with dissolved salts, nutrients and enzymes (= proteins that do a particular job) • Site of many metabolic reactions • Cytoskeleton = network of protein fibers extending throughout the fluid cytoplasm • Support and structure for the cell • Very dynamic, always remodeling itself • Critical for cells to divide and copy themselves

17. Cell Structure

18. Organelles • Separate compartments within the cytoplasm formed by membranes 2. Mitochondrion = “thread granule”, major source of cell’s energy a. energy is taken from sugar, stored in molecule called ATP (adenosine triphosphate) b. requires oxygen to make this exchange (aerobic metabolism) c. contained within double membrane

19. Mitochondrion

20. Organelles (2) 3. The Cytomembrane System = system of tubes and chambers formed by membranes a. extensively distributed throughout the fluid cytoplasm b. involved in synthesis, modification, processing & packaging of cellular lipids and proteins

21. Cell Structure

22. Cytomembrane System 1. Endoplasmic Reticulum (ER) = “within the cytoplasm network”, a system of tubes and sacs formed by membranes (an enclosed space) a) Rough = with bound ribosomes --modifies proteins produced by the ribosomes b) Smooth = without bound ribosomes -doesn’t modify proteins -functions in lipid synthesis, drug detoxification, carbohydrate metabolism

23. Cytomembrane System (2) 2.Golgi Apparatus = series of flattened sacs formed by membranes, functions in final protein processing prior to use by the cell a. proteins get shuttled from the ER to one end of the Golgi b. in each sac, different modifications are made (proteins get individually tailored) c. proteins get sorted and shipped off to their destination (like the post office of the cell)

24. Cytomembrane System (2) 3.Vesicles = small membrane-bound structures that transport proteins and lipids around the cell a. little transporters that shuttle their contents from one organelle to another b. when they contact the appropriate organelle, they fuse with its outer membrane and dump their contents inside c. same for plasma membrane, allowing the export of materials from the cell (exocytosis)

25. Vesicles move from ER to Golgi

26. Cytomembrane System (2) 4.Lysosome = membrane-bound vesicle that contains digestive enzymes or toxic chemicals a. merges with vesicles containing food particles, invading bacteria b. harsh chemicals and enzymes degrade the food or bacteria, without harming rest of cell