Bi 1a Cell Membrane Structure and Function

 Cells need an inside & an outside…  separate cell from its environment  cell membrane is the boundary IN food - sugars - proteins - fats salts O2O2 H2OH2O OUT waste - ammonia - salts - CO 2 - H 2 O products - proteins cell needs materials in & products or waste out

 How do you build a barrier that keeps the watery contents of the cell separate from the watery environment? What substance do you know that doesn’t mix with water?  FATS   LIPIDS  Remember: oil & water don’t mix!!

 Membrane is made of special kind of lipid  phospholipids  “split personality”  Membrane is a double layer  phospholipid bilayer inside cell outside cell lipid “repelled by water” “attracted to water” phosphate

 Cell membrane controls what gets in or out  Need to allow some materials — but not all — to pass through the membrane  semi-permeable ▪ only some material can get in or out aa H2OH2O sugar lipidssalt waste So what needs to get across the membrane? O2O2

 What molecules can get through the cell membrane directly?  fats and oils can pass directly through inside cell outside cell lipid salt aa H2OH2O sugar waste but… what about other stuff?

 Need to make “doors” through membrane  protein channels allow substances in & out ▪ specific channels allow specific material in & out ▪ H 2 O channel, salt channel, sugar channel, etc. inside cell outside cell sugaraa H2OH2O salt waste

 Channels are made of proteins  proteins both “like” water & “like” lipids bi-lipid membrane protein channels in bi-lipid membrane

 Proteins act as doors in the membrane  channels to move specific molecules through cell membrane HIGH LOW

 Why do molecules move through membrane if you give them a channel? ? ? HIGH LOW

 Diffusion  move from HIGH to LOW concentration

 Move from HIGH to LOW concentration  passive transport  no energy needed diffusion

 2nd Law of Thermodynamics governs biological systems  universe tends towards disorder (entropy)  Diffusion  movement from HIGH  LOW concentration  Diffusion  movement from HIGH  LOW concentration

 Move from HIGH to LOW inside cell outside cell Which way will fat move? fat LOW HIGH

 Move from HIGH to LOW through a channel inside cell outside cell sugar Which way will sugar move? sugar LOW HIGH

 Move from HIGH to LOW concentration  directly through membrane ▪ simple diffusion ▪ no energy needed  help through a protein channel ▪ facilitated diffusion (with help) ▪ no energy needed HIGH LOW

inside cell outside cell lipid inside cell outside cell H2OH2O simple diffusionfacilitated diffusion H2OH2O protein channel

 Random motion drives diffusion  Movement is based on kinetic energy (speed), charge, and mass of molecules  Equilibrium is reached when there is an even distribution of solute molecules (water)

 Cells may need molecules to move against concentration “hill”  need to pump “uphill” ▪ from LOW to HIGH using energy  protein pump  requires energy ▪ ATP ATP

“The Doorman” conformational change  Cells may need to move molecules against concentration gradient  conformational shape change transports solute from one side of membrane to other  protein “pump”  “costs” energy = ATP ATP LOW HIGH

symportantiport  Many models & mechanisms ATP

simple diffusion facilitated diffusion active transport ATP

 Moving large molecules into & out of cell  through vesicles & vacuoles  REQUIRES ENERGY  endocytosis ▪ phagocytosis = “cellular eating” ▪ pinocytosis = “cellular drinking”  exocytosis exocytosis

phagocytosis pinocytosis receptor-mediated endocytosis fuse with lysosome for digestion non-specific process triggered by molecular signal

 Water is very important, so we talk about water separately  Osmosis  diffusion of water from HIGH concentration of water to LOW concentration of water ▪ across a semi-permeable membrane

 Direction of osmosis is determined by comparing total solute concentrations  Hypertonic - more solute, less water  Hypotonic - less solute, more water  Isotonic - equal solute, equal water hypotonichypertonic water net movement of water

 Diffusion of water through a semi-permeable membrane  Semi-permeable: permeable to solvents (WATER), but not to large molecules  High [water] to low [water]  Dissolved molecules (i.e. glucose, starch) are called solutes  REMEMBER: Water = solvent Glucose, Starch = solutes

freshwaterbalancedsaltwater  Cell survival depends on balancing water uptake & loss

 Hypotonic  a cell in fresh water  high concentration of water around cell ▪ problem: cell gains water, swells & can burst ▪ example: Paramecium ▪ ex: water continually enters Paramecium cell ▪ solution: contractile vacuole ▪ pumps water out of cell ▪ ATP  plant cells ▪ turgid = full ▪ cell wall protects from bursting freshwater ATP 1 No problem, here KABOOM!

 Saltwater ( lots of salt)  HYPERTONIC  a cell in salt water  low concentration of water around cell ▪ cell loses water  example: shellfish  problem: cell loses water ▪ in plants: plasmolysis ▪ in animals: shrinking cell  solution: take up water saltwater I will survive! I’m shrinking, I’m shrinking! 2

 Balanced conditions (ISOTONIC )  no difference in concentration of water between cell & environment ▪ cell in equilibrium ▪ example: blood ▪ problem: none ▪ water flows across membrane equally, in both directions ▪ volume of cell doesn’t change balanced I could be better… That’s better! 3

 Hypertonic Environment  High [solute], low [water]  Hypotonic Environment  High [water], low [solute]  Isotonic Environment  [water] = [solute] Isotonic HypotonicHypertonic Part 3 pg. 85

Isotonic Hypotonic Hypertonic Sheep red blood cells 0.9% saline 10% NaCl Distilled water Predictions? Crenation

Elodea leaves 10% NaCl Distilled water Hypertonic Hypotonic Predictions? Plasmolysis

WHY ARE PASSIVE TRANSPORT ( diffusion, facilitated diffusion, osmosis) and ACTIVE TRANSPORT IMPORTANT TO THE CELL? HOW DOES THE CELL MAINTAIN ITS HOMEOSTASIS?