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

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

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)

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

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

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)

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) Some Functions of Membrane Proteins

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 O 2 by cell performing respiration ­Elimination of CO 2 from cell

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

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

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.

Water balance of living cells 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).

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. 1.Glucose? 2.Fructose? 3.Hypotonic/Hyp ertonic? 4.Water?

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 + insideoutside Na +

An Example of Active Transport: The Sodium-Potassium Pump

Passive and Active Transport

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”

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

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

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

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

Codons Codes for amino acids 64 codons can code for 20 different amino acids

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.