1. Membranes and Diffusion

2. Cell Membrane Structure
Phospholipids have two regions:
Polar head is hydrophilic, meaning it “plays well” with water
Nonpolar tails (fatty acid chains) are hydrophobic, meaning they avoid water at all costs

3. Cell Membrane Structure
Phospholipids are arranged in a bilayer that surrounds the cell
The heads are on the outside and inside of the cell
The tails are buried between the heads

4. What makes up membranes? (Cell Membrane Function)
Phospholipids – create the main barrier
Proteins – channels and receptors (to receive signals from other cells)
Carbohydrates – ID tags (to say what kind of cell it is and who the cell belongs to)

5. Membranes aren’t stiff! (Cell Membrane Function)
The membrane components move around, so we say they’re fluid!

6. Food Oxygen What do membranes do? Nutrients Carbon Dioxide
Keep bad things out
Let needed things in
Let wastes out
Maintain homeostasis (what is that again??)
Food
Oxygen
Carbon Dioxide
Nutrients

7. How do things get through the membrane?
Diffusion – the movement of molecules from an area of high to low concentration
Concentration gradient – a difference in concentration from one place to another

8. Types of Diffusion
Simple diffusion – movement of a particle through the membrane (O2, CO2, nonpolar things)
Facilitated diffusion – movement of a particle through a protein channel (polar things)

9. Types of Diffusion Osmosis – movement of water through the membrane
When the particle (solute) can’t move through the membrane, water moves instead

10. Osmosis Terms to compare external and internal environment of the cell
Isotonic solutions – same concentration of solute as inside the cell
Water moves equally in both directions
5% NaCl
95% water
5% NaCl
95% water

11. Osmosis
Hypertonic solutions – higher concentration of solute, lower concentration of water than cell
More water moves towards the hypertonic solution (to dilute the high salt??)
20% NaCl
80% water
5% NaCl
95% water

12. Osmosis
Hypotonic solutions – lower concentration of solute, higher concentration of water than cell
More water moves away from the hypotonic solution (to dilute the high salt??)
0% NaCl
100% water
5% NaCl
95% water

13. Let’s try something…
The Problem: Imagine the plastic bag is a cell and the candy is the food. You must get the candy into the bag, using the following rules:
The candy must enter through a solid part of the bag
The inside of the bag may not be directly open to the external environment
The candies entering the bag must remain clustered together
You may work with your hands inside the bag to act as the inside of the cell
The candy may only be eaten if it enters the bag “cell” under these conditions

14. Active Transport Requires energy
Particles do not necessarily move from high to low concentration
Can be through a protein pump

15. Active Transport
Phagocytosis – membrane reaches out and grabs food (larger particles)
Pinocytosis – membrane folds inward (smaller particles/water)
Exocytosis – vesicle fuses with outer membrane, pushing particles out