1. Cell Transport Chapter 3 (Sections 3.3 – 3.5)

2. Cell Membrane • Function – regulates what enters and exits the cell (selectively permeable) How does the membrane do that? Let’s look at… • Structure – double layer (bilayer) of phospholipids • Phosphate head is POLAR – attracted to water (hydrophilic) • Fatty acid tails are NONPOLAR – repelled by water (hydrophobic)

3. Both the extracellular fluid and the cytoplasm are watery, so… • The polar phosphate heads face the outside of the membrane • The nonpolar fatty acid tails are on the inside of the membrane

4. Fluid Mosaic Model • Fluid: the phospholipids can move side to side • Mosaic: there are a variety of proteins embedded into the membrane

5. Passive Transport • Passive transport = movement of molecules across a cell membrane WITHOUT the input of energy from the cell • Types of passive transport: • Diffusion • Osmosis • Facilitated diffusion • Concentration gradient – the difference in the concentration of a substance from one location to another

6. DIFFUSION • Diffusion = the movement of molecules from high concentration to low concentration (DOWN the concentration gradient) • Why do molecules diffuse? • Molecules are constantly moving and colliding • Collisions make them spread out randomly • Results in movement from high concentration to low concentration

7. Which molecules can diffuse across the cell membrane? • Small (Why? They can squeeze between phospholipids.) • Nonpolar (Why? Because the nonpolar fatty acid tails will let them through the membrane.)

8. OSMOSIS • Osmosis = diffusion of water molecules • Water molecules are attracted to dissolved substances  can’t move around • Free water – not attracted to a dissolved substance  can move • Best definition of osmosis? H2O travels from an area of highfree water concentration to an area of lowfree water concentration

9. Example of Osmosis Lots of solute Low free H2O Little solute High free H2O Equal free H2O Concentration H2O moves

10. Cells must maintain a balance of water or they could shrivel or explode • How do cells deal with this problem? • Plant cells are constantly taking in water but they have rigid cell walls • Paramecia (single-celled organisms that live in fresh water) are constantly taking in water so they use contractile vacuoles to pump water out • Our body cells are surrounded by extra cellular fluid (rather than pure water) which has approximately the same concentration of water as our cells

11. FACILITATED DIFFUSION • Some molecules cannot easily diffuse across the membrane • If the substance is polar, the nonpolar fatty acid tails in the center of the membrane won’t let it through

12. So small, polar molecules get help from transport proteins • Transport proteins… • Are SPECIFIC (They only let certain molecules through) • Some are channel proteins (tunnel) • Some are carrier proteins (change shape) • Still move from high to low with NO energy used!

13. Transport Processes that NEED Energy • If a substance cannot cross the membrane by diffusion/osmosis/facilitated diffusion, the cell will use ENERGY to transport the substance ACTIVE TRANSPORT • Active transport = cells use transport proteins to pump materials from LOW to HIGH • Low to high = AGAINST the concentration gradient • Energy supplied by ATP (the ready-to-use energy molecule)

14. Facilitated diffusion vs. Active transport Uses transport proteins Moves substances from LOW to HIGH concentration NEEDS energy (ATP) • Uses transport proteins • Moves substances from HIGH to LOW concentration • Does NOT need energy

15. What if a transport protein can’t be used? (The substance it too big or there is too much) • Use endocytosis or exocytosis! ENDOCYTOSIS • Endocytosis = the membrane engulfs a large amount of material and brings it INTO the cell • When the membrane surrounds the material, the membrane pocket pinches off to form a vesicle

16. Types of endocytosis • Phagocytosis = the cell membrane engulfs large particles • The term means “cell eating” • Used by white blood cells to fight infection • Pinocytosis = the cell membrane engulfs a large amount of extracellular fluid • The term means “cell drinking”

17. EXOCYTOSIS • Exocytosis = a vesicle fuses with the cell membrane to transport materials OUT of the cell • Used to… (Just a couple of examples) • Send nerve signals from one neuron to another • Release cellular products (example: pancreas cells releasing insulin)