I. Plasma Membrane Structure

1. I. Plasma Membrane Structure • Plasma membrane • – Boundary that separates living cells • from their nonliving surroundings. • - Apprx. 8 nm thick • Composed chiefly of lipids and proteins • Surrounds the cell and controls chemical traffic in/out of cell • Is semi-permeable Enables cells to maintain internal environment different from external environment

2. Phospholipid bilayer Composed of 2 layers of phospholipids Heads are hydrophillic Tails are hydrophobic

3. Membrane Structure (Fluid Mosaic Model) • Membrane proteins embedded in phospholipid bilayer • Give membrane ‘fluidity’ similar to salad oil • Phospholipids & proteins can drift laterally (2 um / sec)

4. Membranes must be fluid to work properly ! - solidification causes changes in permeability and enzyme deactivation How do cells control membrane fluidity ? 1. Unsaturated hydrocarbon tails 2. Adding cholesterol makes membrane: • Decreases fluidity at low temps by restraining phospholipid movement • Increases fluidity at high temps by preventing close packing of phospholipids

5. So, how do the plant overcome the winter? • Increase thepercentage of cholesterol in phospholipids • Prevents membrane from solidifying in cold weather winter wheat

6. Proteins in Plasma Membrane • Mosaic of proteins ‘bobbing’ in a fluid lipid bilayer • Proteins determine a membrane’s specific function: • Two types • 1. Integral proteins (‘transmembrane’, or embedded) • 2. Peripheral proteins (bound to surface of membrane)

7. Some Functions of Membrane Proteins Transport – protein provides channel across membrane for particular solutes Enzymatic activity – proteins may be enzymes that catalyze steps in metabolic pathway Signal transduction – protein is a receptor for chemical messenger (hormone). Conformational change in protein relays message to inside of cell Intercellular joining – membrane proteins of adjacent cells join together for strength (epithelium) Cell-cell recognition – glycoproteins act as I.D. tags that are recognized by other cells (e.g. RBCs)

8. Regulating Traffic Across Membranes II. Passive Transport: Diffusion and Facilitated diffusion • Diffusion : net movement • of a substance down • a concentration gradient. • Solutes diffuse from high to low concentration. • Continues until a dynamic equilibrium is reached. • No requirement for energy expense (passive) • Examples: • Uptake of O2 by cell performing respiration • Elimination of CO2 from cell

9. Diffusion of solutes across a membrane Each dye diffuses down its own concentration gradient.

10. Passive transport • Transport proteins speed the movement of molecules across the plasma membrane. • Channel protein and Carrier protein required Facilitated diffusion • Channel protein : aquaporins, ion channels • Carrier protein

11. Osmosis • Diffusion (passive transport) of water across a selectively permeable membrane • Direction of water movement is determined by the difference in total solute concentration, regardless of type or diversity of solutes. • Water moves always from high concentration solution to low concentration solution.

12. Tonicity : the ability of a solution to cause a cell to gain or lose water • Isotonic: no net movement of water across the membrane (normal). • Hypertonic : the cell loses water to its environment (crenation). • Hypotonic : the cell gains water from its environment (lysis). Water balance of living cells

13. 0 Questions An artificial cell consisting of an aqueous solution enclosed in a selectively permeable membrane has just been immersed in a beaker containing a different solution. The membrane is permeable to water and to the simple sugars glucose and fructose but completely impermeable to sucrose. • Glucose? • Fructose? • Hypotonic/Hypertonic? • Water?

14. Active Transport • Requires the cell to expend energy: ATP • Transport proteins pump molecules across a membrane against their concentration gradient. • “Uphill” transport • Maintain steep ionic gradients across the cell membrane (Na+ , K+ , Ca++ , Mg++ , Cl-) Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ inside outside

15. An Example of Active Transport: The Sodium-Potassium Pump

16. Passive and Active Transport

17. More examples of active transport • Exocytosis • Removing large particles out of the cell with a vesicle • Endocytosis • Ingesting large particles • Pinocytosis: “Cell drinking” • Phagocytosis: “Cell eating”

18. Protein Synthesis • The process of using DNA to form proteins • Involves two steps: • Transcription • Translation

19. Genetic Information • Uses 2 main forms of genetic information: • DNA Deoxyribonucleic Acid • Double stranded • Sugar: Deoxyribose • Stays in the nucleus • Bases: A T G C • RNA Ribonucleic Acid • Single stranded • Sugar: Ribose • Can leave the nucleus • Bases: A U G C

20. Transcription • DNA unwinds • One strand of the double helix is used as a template • Nucleotides line up along the DNA and form a copy, called mRNA • Once completed, DNA winds back up and mRNA leaves

21. mRNA must be spliced before it leaves the nucleus ( immature RNA) • Enzymes remove noncoding areas called introns, and coding regions called exons are spliced back together • The result is a shorter, coding strand of mRNA • Every 3 bases on mRNA is a codon

22. Codons • Codes for amino acids • 64 codons can code for 20 different amino acids

23. Translation • mRNA binds to a ribosome • tRNA binds to ribosome along the codon and reads which amino acid it codes for • tRNA finds the specific amino acids • For every codon, the tRNA brings the amino acids • Amino acids link together forming a proteins • Peptide bonds link each amino acid together.