1. An Interactive Lecture Guide to help you understand THE MOVEMENT OF MATERIALS ACROSS A MEMBRANE

2. Recall the definition of homeostasis
The process of achieving a relatively stable internal environment
CELLS MUST CONSTANTLY RESPOND TO NATURAL FORCES IN THEIR ENVIRONMENT IN ORDER TO MAINTAIN HOMEOSTASIS
Therefore, materials like _Water, O2, ions, nutrients, & hormones are needed to come into a cell and wastes need to be let out of a cell.

3. Recall the structure of the cell membrane:
Two thin layers of phospholipids and proteins.
▪ The membrane is not rigid but fluid.
consistency of “a light grade of machine oil”
▪ This enables it to regulate flow of substances into and out of the cell.

4. SELECTIVE PERMEABILITY:
Similar to selective hearing!
The membrane regulates the exchange of materials into and out of a cell. Polarity, size, and electric charge of molecules determine whether they can pass through a membrane.
SMALL HYDROPHOBIC molecules pass right through (CO2, O2, and N2)
SMALL, UNCHARGED or POLAR molecules pass through as well (H2o, glycerol, ethanol)
LARGER, UNCHARGED or POLAR molecules have a harder time passing through (amino acids, glucose, nucleotides)
IONS can’t pass through (H+, Na+, K+,Ca+2, and Cl-), they need the help of TRANSPORT PROTEINS.

5. WHY DOES THIS MATTER?
Cells survive only if they can maintain homeostasis.
Organelles (little organs) work to maintain this state.
Homeostasis is challenged because molecules move.
In order for cells to maintain internal conditions, they need to be able to control the movement of “stuff” into and out of them.

6. The cell membrane is the biggest STAR of this unit - Why?
It encourages the movement of good stuff in and out.
It discourages the movement of bad stuff in and out.

7. Diffusion is the basic process underlying the movement of molecules into and out of cells
The passive movement of molecules from regions of [high] to [low]*
* [ ] = scientific symbol for concentration

8. Examples of diffusion Student activity
video
Student activity
If I want the concentration of males to be high in our classroom, what can be done?
Dye in water
Perfume or fresh bread!

9. When a difference in concentration exists across a cell membrane a CONCENTRATION GRADIENT exists.
Concentration-
% of molecules in a certain place
In diffusion the rate of movement of molecules from [high] to [low] exceeds the rate from [low] to [high] until the concentration gradient no longer exists. So, rate in = rate out!

10. Osmosis – Diffusion of water!
The movement of water across a selectively permeable membrane.
Down the concentration gradient.
Osmosis in the kitchen video

11. The rate of diffusion (including osmosis) depends on several factors
The size of the concentration gradient
The steeper the gradient, the faster the molecules move
The surface area of the membrane
A greater surface area relative to the enclosed volume results in a great rate of diffusion.

12. PASSIVE TRANSPORT involves diffusion without any input of energy
PASSIVE TRANSPORT involves diffusion without any input of energy. It moves substances down their concentration gradients.
FACILITATED DIFFUSION requires the help of transport proteins in the membrane, but still move down their concentration gradient.

13. The Fluid Mosaic Model
The Cell Membrane is made of a combination of phospholipids, proteins, and other molecules that move around and collectively behave as a “fluid” rather than a solid structure

14. Parts of the Cell Membrane
Phospholipids (bi-layer)
A double layer of lipids that provide a way to separate the outside environment from the interior of the cell
Cholesterol
lipid (steroid) molecules located sporadically I n-between phospholipids that act to adjust the fluidity of the membrane

15. Transport Proteins:
Proteins that form a “pore” for molecules to travel through in either direction through the cell membrane
Receptor Protein
Proteins with specifically-shaped binding sites on the outside of the membrane that function to receive molecular messages from other cells
Ligand: A chemical that bonds with a receptor protein and carries a specific “message” from one cell to another

16. Peripheral Proteins Recognition Proteins
Proteins on the interior of the cell that relay “messages” from the receptor protein to the cell nucleus.
Recognition Proteins
Proteins with carbohydrate “flags” that act to identify the type of cell, and help cells recognize each other.

17. Build a Cell Membrane Activity
Animation
Activity

18. OSMOSIS IN THE CELL MEMBRANE
Water moves towards the hypertonic place
Tonic = dissolved substances
Hyper = high
Hypo = low
Hypertonic Solution:
high concentration of dissolved stuff, low in water - net water loss, cell shrinks

19. Hypotonic Solution
low concentration in dissolved stuff and high in water, net water gain, cell swells

20. Isotonic
equal amount of dissolved material inside and outside of cell- no net loss

21. Review

22. Why not stop at diffusion?
Cells don’t necessarily want to be isotonic, but unless they fight it, they’ll go that way (entropy).
Example:
A frog in a freshwater pond will tend to lose salts from his/her body to their environment by diffusion.
Therefore, the frog has a mechanism to take back salts from the environment- against the [gradient]!
This requires chemical energy which is supplied by ATP.

23. WHEN MOLECULES NEED TO BE IMPORTED OR EXPORTED AGAINST THEIR [GRADIENT], CELLS EXPEND ATP TO MOVE MOLECULES FROM [LOW] to [HIGH]
This is called ACTIVE TRANSPORT _

24. Where might active transport be utilized in our body?
SODIUM - POTASSIUM PUMP-
One of the most widely occurring active transport proteins in eukaryotes.
Used to transport sodium ions out of cells and potassium ions into cells.
Example: nerve cells have 30 times more potassium in them than extracellular fluids.
animation

25. Sodium ions bind to the protein on the inside of the cell membrane;
ATP is hydrolyzed and the phosphate produced is linked to the protein
The shape of the protein is changed in such a way that the sodium ion can be expelled out of the cell
Potassium ions bind to the protein
Phosphate group is removed causing the protein to snap back to its original shape
Potassium ion moves into the cell

26. MOVING BIG STUFF ENDOCYTOSIS-
Large molecular materials are enclosed within invaginations (folding) of the plasma membrane, subsequently pinching off to form cytoplasmic vesicles.
Phagosytosis (cell eating) - cell ingests large particles such as bacteria or pieces of debris
Entrap, engulf, digest, absorb
Pinocytosis (cell drinking) - cell ingests liquid and/or dissolved solutes and small suspended particles.
animation

27. Exocytosis
Helps cells remove larger molecular waste materials