1 Chapter 5 Homeostasis & Transport Topics: Passive & Active Transport

2 Transport: Movement of substances across the cell membrane Two types of transport – Passive transport – transport requiring no energy input Diffusion Osmosis Facilitated Diffusion Diffusion thru Ion Channels – Active transport – transport requiring energy input Pumps Endocytosis Exocytosis

3 Diffusion – Passive Transport Molecule movement from HI to LO conc – Particles may go in or out depending on conc gradient KE of molecules drives this process Stops when equilibrium reached; dynamic state Dependent upon: – Particle size – Particle type – Nature of phospholipid bilayer So, a) small particles may pass thru, b) nonpolar particles may pass thru, & c) lipid-soluble particles may pass thru

4 Osmosis – Passive Transport Diffusion of WATER molecules thru membrane due to conc gradient Direction of flow from HI to LO conc of the solutes (solution = solute + solvent); movement stops when equil is reached Consider the cell  H 2 O molecules + solutes in solution inside cell & outside the cell with cell membrane separating the 2 solutions H 2 O will move into or out of the cell depending upon the conc of solute molecules in relation to each other Reference solution is always OUTSIDE cell solution

5 Osmosis and Diffusion Tutorials smosis1.html smosis1.html ffusion1.html ffusion1.html

6 Solutions Hypotonic – solute conc outside cell is less than inside; so H 2 O flows into cell until equil is reached Hypertonic – solute conc outside cell is more than inside; so H 2 O flows out of cell until equil is reached Isotonic – solution conc same on both sides of cell membrane; equil reached

7 How does osmosis effect cells? Many cells in hypotonic environment; so H 2 O diffuses in Plant cells – have rigid cell wall; H 2 O stored in vacuole; excess H 2 O causes swelling to cell wall; this is turgor pressure; loss of tugor pressure called plasmolysis (loss of H 2 O) Paramecium – unicellular fresh-H 2 O eukaryote – Water environment hypotonic, if too much H 2 O enters organism the organism will burst/die – Contractile Vacuole – organelle that pumps excess H 2 O out of cell; expends E to do this RBCs – no coping mechanism for excess or loss of H 2 O; too much H 2 O causes cell to swell and cause cytolysis (bursting); too little causes cell to shrivel

8 Facilitated Diffusion – Passive Transport Diffusion assisted by carrier proteins No E required; driven by HI to LO conc Carrier proteins specific to particle they carry Particle & carrier protein bind; shape altered; as particle/protein combo move across membrane it is guarded from lipid inner portion of membrane Particle released from protein at other side Glucose transported this way; molecule too big to simply diffuse on its own – it needs help

9 Ion Channels – Passive Transport Ions not soluble in lipid portion of membrane; so cannot diffuse thru on their own Membrane has proteins specific to an ion & these allow that ion to pass thru the hydrophobic lipid portion Driving force is conc gradient (HI to LO); no E Some channels open always; some require a stimulus – Stretching of the cell membrane (mechanical signal) – Electrical signal – Chemical signal

10 Pumps – Active Transport What happens when a cell needs a specific ion, but there is no conc gradient to get it via an ion channel? Or, vice versa? Answer – the cell must expend E to transport the ion; remember ions cannot diffuse normally b/c they cannot pass thru the lipid inner portion of the membrane Carrier protein specific to an ion moves the ion thru the membrane against a conc gradient Na + -K + Pump is good example

11 Na + -K + Pump Most animal cells require a high [Na + ] outside cell and a higher [K + ] inside cell; so pumping mechanism necessary to maintain this Process: (see diagram next slide) – 3 Na + located in cytosol bind to a carrier protein – ATP molecule splits off a P group, which attaches to the pump carrier protein (that also has the 3 Na + ). This complex changes shape. – This shape change releases the 3 Na + ions from the inside of the cell – While carrier protein still at outside of cell, it picks up 2 K + – This releases the P group, which changes the carrier protein shape & the 2 K + ions released into the cell

12 Cell Membrane & Na + /K + Pump

13 Websites for Na+/K+ Pump video lates/student_resources/shared_resources/a nimations/ion_pump/ionpump.html lates/student_resources/shared_resources/a nimations/ion_pump/ionpump.html /animations/membrane_transport/ membrane_transport.htm /animations/membrane_transport/ membrane_transport.htm

14 Endocytosis Process by which a cell digests external fluid & particles How it works: – Cell membrane moves to engulf substance – Membrane surrounds substance and pinches shut – Forms a membrane-bound organelle (vesicle) Two types – Pinocytosis – ingestion of fluid only – Phagocytosis – ingestion of solids – res/phagocitosis.swf res/phagocitosis.swf Phagocytes – term for a cell that ingests

15 Exocytosis Process is reverse of endocytosis Vesicle contents (liquid/solid) released from cell to outside Process – Vesicle moves to cell membrane and fuses with it – Portion of cell membrane opens & contents ejected from cell

16 Review Questions 1. What is the cell membrane composed of? 2. The cell membrane is completely permeable. T/F 3. What types of substances can enter the cell? 4. Why can’t all substances enter the cell? 5. What is passive transport? 6. What is active transport?

17 Review Questions, con’t 7. Passive transport includes what processes? 8. Active transport includes what processes? 9. How is diffusion different from osmosis? 10. How do big molecules get into the cell? 11. How do substances get out of the cell? 12. What cell organelles are involved in transport of substances into/out of the cell?