Chapter 5 Homeostasis & The Plasma Membrane

 It’s all about balance!  Failure to adjust….death  Cells maintain balance by controlling materials entering/leaving

 Concentrations outside the cell change  Plasma membrane must be able to maintain balance  Selective Permeability-property that allows some materials to pass through while keeping others out

Membrane Structure  Made of two layers made up of sheets of lipid molecules-lipid bilayer

Membrane Structure  Proteins are embedded in the bilayer  Think of raisins in raisin bread

Fluid Mosaic Model  The phospholipid bilayer acts more like a liquid than a solid.

The Phospholipid Bilayer

 Most lipids have two fatty acids attached to glycerol  The third fatty acid is replaced by a phosphate group-phospholipid  Phospholipids-polar, water-soluble head + long, nonpolar, insoluble tail  Align to form double layer with “heads” on outside and “tails” on the inside

Diffusion  Cells maintain homeostasis by controlling the movement of substances across the membrane  Cells must use NRG to transport some substances…..Ex- Active Transport!  Other substances move across the membrane with no NRG required

Diffusion  Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.  Goal of diffusion=Dynamic Equilibrium  Continuous movement with no concentration change  Concentration gradient-difference in concentration across space

Dynamic Equilibrium

Diffusion!!

Osmosis-Diffusion of Water  Osmosis is the diffusion of water molecules through a selectively permeable membrane  Movement from high to low concentration  Type of passive transport!  Isotonic solution-concentration of dissolved substances equal to concentration inside the cell…..NO NET MOVEMENT!

Isotonic Solution

Hypotonic vs Hypertonic  Hypotonic-solution where concentration of dissolved substances is lower than inside the cell  Water moves into cell…swelling!  Increases cell pressure-Turgor Pressure  Hypertonic-solution where concentration of dissolved substances is higher than inside the cell  Water moves out of cell…Plasmolysis

Dealing with Osmosis  Cells must compensate for the water that enters or exits the cell.  Animal Cells- Must maintain isotonic environment  Plant Cells- Prefer a hypotonic environment

Plants vs. Animals

Protists  Have contractile vacuoles that actively remove water from the cell.

Crossing the Membrane  Diffusion through ion channels:  Ion Channels- transport proteins with polar pores that ions can pass through.  Pore is the thickness of the membrane  Ion does not have touch the nonpolar interior of the bilayer.

Ion Channel

Facilitated Diffusion  Carrier proteins allow needed substances to move through membrane by binding, carrying, and releasing substances.  This movement is called facilitated diffusion

Facilitated Diffusion  Carrier proteins change shape using chemical energy to move particle through the membrane  Once particle is passed, original protein shape is restored

Facilitated Diffusion

Active Transport  In order to move particles from a lower concentration to higher concentration a cell must use energy…Active Transport  Proton pumps- specialized proteins for pushing different substances against the concentration gradient.  Need ATP for energy!!!!!

Active Transport

Sodium-Potassium Pump  Four Steps to the Pump:  1. Three Na ions attach to the pump.  2. Pump changes shape, transporting the ions across the membrane.  3. Two K ions outside bind to the pump  4. Two K ions are transported are released inside the cell.

Transport of Large Particles  Endocytosis-process in which cell surrounds and takes in material from its environment  Engulfed and enclosed by portion of cell membrane  Exocytosis-expelling wastes from interior to exterior environment

Endocytosis Types

Exocytosis