Key Concept The cell membrane is a barrier

Key Concept: The cell membrane is a barrier that separates a cell from the external environment. Michigan Standard: 2. 5 h- Explain the role of cell membranes as a highly selective barrier (diffusion, osmosis and active transport)

I. Cell membranes are composed of two phospholipid layers. A. Cell membrane is made up of phospholipids (phosphorous + fats)

1. a polar head and two nonpolar tails (likes water) (dislikes water) Draw this

Draw this on the left side of your paper This represents a very small portion of a cell membrane a cell

Remember a cell membrane is a 3 -D structure that completely surrounds a cell (kind of like the skin of an apple)

B. Other molecules are embedded within the “bi-layer” 1. Cholesterol- for strength 2. Proteins- for moving things in and out of the cell 3. Carbohydrates- for identification

C. The cell membrane has been described by a fluid mosaic model 1. “fluid”- pieces moves around (like a soap bubble) 2. “mosaic”- made of many pieces

D. The cell membrane has the property of selective permeability 1. allows some, but not all materials to move through it 2. semipermeable or selectively permeable mean the same thing example: waterproof clothing, a window screen

II. Chemical signals are transmitted across the cell membrane A. Receptors help cells communicate and also help to move moleculues 1. receptor- a protein that detects a signal molecule and performs an action in response 2. “lock and key” B. Receptors are usually very specific and only react to

Key Concept: Materials move across membranes because of concentration differences. Michigan Standard: 2. 5 h- Explain the role of cell membranes as a highly selective barrier (diffusion, osmosis and active transport)

I. Diffusion and Osmosis are types of passive transport. A. Passive transport- the movement of molecules across a cell membrane without energy input from the cell B. Diffusion- movement of molecules in a fluid or gas from a region of higher concentration to a region of lower concentration

C. Concentration- number of molecules of a substances in a given volume (space/ area) ++ + + ++ +++ ++ + + + High or Low? High. Low or Low?

D. concentration gradient- difference in concentration of a substance from one location to another 1. molecules diffuse down their concentration gradient 2. High concentration Low concentration

High Concentration Equal Concentration Low Concentration + + + + + + +

E. Equilibrium- equal concentrations in all areas

F. Osmosis- diffusion of water molecules across a semipermeable membrane from an area of higher water concentration to an area of lower water concentration Remember- the cell membrane is a semipermeable membrane (it lets certain things through it and keeps other things out)

This is a solution. Remember. Solvent + Solute= Solution Solvent is water Solute is salt

Which one has a greater concentration of solutes (salt)? Low solutes High solutes vs.

Which one has a greater concentration of solvent (water)? Low solutes High water High solutes Low water vs.

F. There are three types of solutions 1. Hypertonic solutions 2. Hypotonic solutions 3. Isotonic solutions

1. Hypertonic solution- has a higher concentration of solutes (dissolved particles) than a cell Solution (outside of cell) Inside of cell High concentration solutes Low concentration of water High concentration of water

2. Hypotonic solution- has a lower concentration of solutes (dissolved particles) than a cell Solution (outside of cell) Inside of cell low concentration solutes high concentration of water low concentration of water

3. Isotonic solution- has the same concentration of solutes (dissolved particles) as a cell Solution (outside of cell) Inside of cell equal concentration solutes equal concentration of water

So, what’s going to happen? Osmosis!!

But first, this cell is in what kind of a solution? Hypertonic Solution High solutes outside Low water outside Low solutes inside High water inside

How will osmosis happen? Remember water moves from areas of high water concentration to areas of low water concentration Will water Out of the move into the Cell!! cell or out of the cell? The cell shrinks! Low water High water outside inside

Now, what kind of solution is this cell in? Hypotonic Solution Low solutes outside High water outside High solutes inside Low water inside

Will water go into this cell or out of the cell? Hypotonic Solution High water outside Into the cell!! The cell expands! Low water inside

Now, what kind of solution is this cell in? Isotonic Solution Equal water and solutes inside and outside

Will water go into this cell or out of the cell? Isotonic Solution Equal water and solutes inside and outside Both!! The cell remains the same size.

II. Some molecules diffuse through transport proteins. A. Some molecules cannot slip through the cell membrane very easily. B. Facilitated diffusion- the diffusion of molecules across a membrane through transport proteins

B. Facilitated diffusion- the diffusion of molecules across a membrane through transport proteins

Movement of Molecules Passive Transport Diffusion Osmosis Facilitated Diffusion Active Transport Section 3. 5

Key Concept: Cells use energy to transport materials that cannot diffuse across a membrane.

I. Proteins can transport materials against a concentration gradient. A. Active transport drives molecules across a membrane from a region of lower concentration to a region of high concentration. 1. High Concentration Low Concentration

A protein uses energy to move molecules against the concentration gradient. Low Concon. High Concon.

II. Endocytosis and exocytosis transport materials across the membrane in vesicles. A. Sometimes a cell needs to move big molecules into or out of a cell with a vesicle.

B. Endocytosis- the process of taking liquids or fairly large molecules into a cell by engulfing them in a membrane. The cell membrane folds and a pocket is created The pocket pinches off and a vesicle is created The vesicle and its contents are broken down

C. Phagocytosis- a type of endocytosis in which the cell membrane engulfs large particles. “cell eating” 1. Very important process to fighting infection!

D. Exocytosis- the process of releasing substances out of a cell by the fusion of a vesicle with the cell membrane. A vesicle is created around what needs to be removed The vesicle moves to the cell membrane The vesicle fuses with the cell membrane and releases the contents