1. Transport Across Cell Membranes

2. Cell Membrane Structure
The cell membrane functions to:
form a barrier between the internal and external environment of the cell.
Regulate what is allowed to enter and exit the cells internal environment.
Provides protection and support.

3. Fluid mosaic model
The fluid mosaic model was developed to explain how the cell membrane is structured.
Phospholipid bilayer with proteins floating in and on it.

4. Proteins in the cell membrane
Water is able to travel through pores in proteins found in the cell membrane
Proteins have polar and non-polar regions, this way they can span the membrane
There are both peripheral (on the outside) proteins and integral (part of the membrane) proteins

5. Some proteins within the membrane have carbohydrates attached
Some proteins within the membrane have carbohydrates attached. These are called glycoproteins
Lipids in the bilayer can also have carbohydrates attached. These are called glycolipids.
Carbohydrate chains only occur on the outside of cells and function to identify the cell

6. Proteins perform several functions in the cell membrane:
Channel proteins: allow a specific molecule to cross the membrane freely
Carrier protein: combine with a specific molecule to move it across the membrane
Recognition proteins: allow the cell to be recognized by other cells (glycoproteins)
Ex. Tissue rejection in organ donation
Receptor protein: have a special shape to be able to bind with other molecules
Enzymatic proteins: carry out metabolic reactions

7. Selective Permeability
Permeable- the ability to pass through a membrane
The cell membrane is a selectively permeable membrane.
This means that it allows some things through and prevents others from entering or exiting.
Transport across a membrane is classified as either passive or active
Why would cells be selectively permeable?

8. Solution- a liquid with one or more substances dissolved in it
Solvent- the liquid that the solute is dissolved in
Solute- the substance dissolved in a solution
Concentration- how strong it is… the solute/volume (percentage)

9. Jigsaw Activity Form groups of 3
Each member of the group find information on one type of passive transport (p.71-74)
1. Diffusion
2. Osmosis (ignore “osmosis in cells” section)
3. Facilitated transport

10. Transport through the Membrane
Simple Diffusion
Facilitated
Diffusion
Active
Transport

11. Diffusion
Particles moving from an area of greater concentration toward an area of lesser concentration until it is equally distributed
Diffusion refers to the process by which the molecules of two different substances intermingle (mix) as a result of their kinetic energy of random motion.
Does not use a transport protein
Moves with the concentration gradient (high to low)
Does not require additional cell energy
Passive
Examples:
Water
Carbon dioxide
oxygen

13. The rate of diffusion can be increased by:
Increased temperature
Increased surface area
Change in shape of molecules
Change in concentration
(Why for each?)

14. Osmosis
The net movement of water molecules from the area of greater concentration of water to the area of lesser concentration of water until it is evenly distributed
Must be across a selectively permeable membrane
Water passes through pores in membrane proteins
Osmotic pressure can work against physical pressure

15. Review

16. Facilitated Transport
An assisted form of diffusion
Moves molecules that would not be able to cross the membrane alone
Moves toward the concentration gradient
Requires carrier proteins
No energy is needed

17. Tonicity
Water molecules travel freely through protein channels to try to maintain an environmental solute concentration.
Osmotic pressure is a measure of the number of collisions water molecules make with the cell membrane surface.
If there is a high concentration difference between the inside and outside of the cell there will be a high osmotic pressure… more collisions as water tries to get across the membrane

18. Turgor pressure is the force the cytoplasm exerts against the inside of the cell membrane.
Large and prolonged changes in turgidity can have effects on cell function.
What the solution concentration outside the cell looks like has effect on the cells turgor pressure.

19. Isotonic
If the cell is placed in a solution that has equal solute concentration to inside the cell the solution is called isotonic
The cell will not shrink or swell because the osmotic pressure on each side of the cell membrane is equal
The concentrations of solution on both sides of the membrane are equal

20. Hypertonic
When a cell is exposed to an environment with a greater solute concentration, water will leave the cell to try to bring the inside and the outside solutions into equilibrium.
The cell will shrivel as water leaves and the solute inside the cell will concentrate so that it is more similar to the concentration outside the cell.
The outside solution is called hypertonic (hyper=high). It has a higher concentration of solute than the cytoplasm.
The shrinking of a cell in a hypertonic solution is called plasmolysis. If it is a red blood cell it is called crenation.

21. Hypotonic
When a cell is placed in a solution that has a lower concentration of solute compared to the cell cytoplasm it is called hypotonic (hypo=low)
Water moves from outside the cell to inside the cell to try to lower the solute concentration inside the cell
The cell swells and may even burst if the osmotic pressure is great enough.
When a red blood cell bursts it is called hemolysis.

24. Practice Question
Which set of conditions would cause the fluid levels to change as shown after 10 minutes?
Solution in Side A
Solution in Side B
Distilled water
5% salt
2% glucose
5% glucose
5% protein
2% protein
2% salt

25. Practice Question Is the solution hypertonic, isotonic, or hypotonic?
Which way will water move?
What will happen to the cell?

26. Active Transport
Ions or molecules move across the membrane and collect either inside or outside the cell
works in the opposite direction as diffusion from low concentration to high concentration.
Requires carrier proteins and energy to move molecules across the membrane

27. Active transport uses carrier proteins like one form of facilitated diffusion.
Energy (in the form of ATP) is required for the protein to bind with the molecule to move it across the membrane.

28. Active transport is used by cells of the thyroid gland to collect iodine from the body. Iodine moves against the concentration gradient toward the more concentrated area inside the cell.
Up to 40% of a cells energy supply can be used up to perform active transport. Cells that primarily gather certain molecules by active transport have lots of mitochondria close to their cell membrane to produce energy.
Proteins involved in
active transport are of
ten called pumps

29. Sodium-Potassium pump
All animal cells, especially nervous cells, contain sodium-potassium pumps.
This protein moves sodium to the outside of the cell and potassium into the cell.
Both molecules are moved by the same protein: the sodium-potassium pump.

30. The attachment and detachment of a phosphate group from ATP causes a shape change in the protein that allows it to bind sodium

31. NaCl Salt is very important in the functions of cells.
Sodium is pumped across the membrane by carrier proteins
Chloride ions flow across the membrane through protein channels because they are attracted by the positively charged sodium.

32. Exocytosis and Endocytosis
The cell needs a way to get very large or macromolecules across the cell membrane and these do not fit through cell membrane proteins.
These large molecules are transported by vesicle formation.
Vesicle formation requires
energy, so this is a form of
Active Transport.

33. Exocytosis
Is a process of the cell to get molecules out of the cell (exo=exit)
Vesicles are like small packages of molecules that need to be released encased in cell membrane
The contents are released to outside the cell when the vesicle membrane fuses with the cell membrane.

34. Vesicles are often formed by the Golgi apparatus and contain proteins that the cell needs in it’s membrane or is getting out of the cell.
Vesicles are used by the cell as a way to get membrane bound proteins from Golgi apparatus membrane to the cell membrane surface where they adhere to the surface and become part of the membrane.

35. Some cells are specialized to produce molecules and excrete them out of the cell
Ex. Pancreatic cells excrete digestive enzymes.
These cells wait for a signal from outside the cell before they exocytosis takes place.

36. Endocytosis
Endocytosis is the process cells use to get macromolecules from outside the cell membrane to inside the cell membrane
The cell membrane
Pushes in toward the cell
In one are
The macromolecule is
Surrounded by the cell
Membrane and the
Membrane pinches off
Forming a vesicle inside the cell.

37. Phagocytosis
When endocytosis is used to take in a large object like a food particle or cell the process is called phagocytosis and the vesicle formed is called a vacuole.
Single cell organisms like amoebas often perform phagocytosis to obtain food.
Some human white blood cells also do phagocytosis to engulf objects in the body that they see as foreign

38. Pinocytosis
Pinocytosis is when vesicles form around a liquid or very small particles.
Blood cells, kidney cells, and intestinal wall cells perform pinocytosis constantly
To balance out the cell membrane lost by endocytosis, there is exocytosis also happening.

39. Receptor-Mediated Endocytosis
This is a form of very specific pinocytosis.
The cell wall is lined with receptors in one location called a coated pit, that bind a specific molecule. When that molecule is bound the vesicle forms and is moved into the cell.