1. Bio Today: Cell Membrane Notes & Diffusion Lab
Unit 3 Test Wed 11/15 (A), Th 11/16 (B)

2. Cell Membrane Lab Set Up
Obtain a plastic tube with starch water.
Fill your beaker halfway with water.
Ask teacher to add iodine to beaker.
Put the plastic bag into a beaker of water + iodine.
Complete the “initial” part of data table
Return to seats

3. A closer look at the cell membrane…
The cell membrane is sometimes known as the “gatekeeper of the cell” because it controls what can enter and leave the cell
The cell membrane is known to be selectively permeable. This means only some things can enter and leave the cell.
Ex: think of it acting like a strainer or filter.

4. Cell Membrane Structure
The structure of the cell membrane is made up of a phospholipid bilayer
Phospho = phosphate head.
Hydrophilic head of phospholipid molecule
Hydrophilic = “water loving”
Lipid = fatty acid
hydrophobic tails of phospholipid molecule
Hydrophobic = “water fearing”
Bilayer = 2 layers

5. Phospholipid Bilayer
The phospholipid bilayer is folded in a way so that the hydrophilic phosphate heads face the inside and outside of the cell where the environment is “watery”.
Phosphate Heads
Lipid Tails
(Outside of Cell) Water
(Inside of Cell)
Water
Two layers back to back

6. Movement Across the Membrane
In order for substances to actually enter and leave the cell, the cell membrane must be fluid, or allow for movement
This is accomplished by the fact that the hydrophobic tails are made up of unsaturated hydrocarbons, which allow the structure to have kinks and prevent them from packing together (allowing for movement)

7. Cell Membrane Additions
In addition to the phospholipids, there are many proteins that are either on the surface of the membrane or span the entire membrane. There are 3 main general types of proteins:
Transport Proteins: regulate which molecules enter and leave the cell.
Proteins for identification: stick out from the cell surface and help cells identify each other
Proteins for anchoring: located on the inside and attach the membrane to internal support structures. Provide flexibility.

8. Concentration & Transport of Molecules Across the Membrane
In our bodies we care about maintaining the balance of the concentration of certain ions and macromolecules inside and outside our cells. Concentration is the relative amount of a substance that is located in a specific/defined area

9. Passive Transport
Substances will naturally move from an area of high concentration to an area of low concentration.
Diffusion is the general term referring to the passive transport of substance/molecules across a membrane.
Osmosis is the specific term referring to the passive transport of water across a membrane.

10. Diffusion Lab Part 2 and Cell membrane WS
Answer observations/predictions questions
Carefully take starch bags out of beaker and observe.
Clean up, answer lab questions and turn in
Complete Cell Membrane questions