1. Cells
Cell Boundaries

2. What Are We Learning?
Define the function of the cell membrane and cell wall.
Illustrate the arrangement of phospholipids in a lipid bilayer.
Describe two characteristics of a lipid bilayer.
Describe the functions of proteins in the cell membrane.
Describe what happens during diffusion.
Explain the processes of osmosis, facilitated diffusion, and active transport.

3. Cell Boundaries
All cells are surrounded by a thin, flexible barrier known as the cell membrane.
Regulates what enters and leaves the cell.
Provides protection and support.
Many cells also produce a strong supporting layer around the membrane known as a cell wall.
The main function of the cell wall is to provide support and protection for the cell.

4. Cell Boundaries
Cell walls are present in plants, algae, fungi, and many prokaryotes.
Plant cell walls
composed mostly of cellulose, a tough carbohydrate fiber.

5. Water and the Cell Membrane
All cells are surrounded by water.
Water is present inside the cell too.
All cell organelles perform their functions in water.

6. Water and the Cell Membrane
What happens when oil is poured in water?
Water and oil will separate because water is a polar molecule and oil is nonpolar.
Solubility: the ability of a substance to dissolve in a solution
Rule of Solubility: Polar dissolves in Polar & Non-polar dissolves in Non-polar
This is why water and oil do not mix!

7. Cell Membrane Architecture
The cell membrane is made up mostly of a phospholipid bilayer
lipid= fats and oils (nonpolar)
Phosphate (polar molecule)
Short “head” is joined to two long “tails.”
The head contains phosphorous and nitrogen, which make it polar.
Hydrophilic (water-loving) head
The long tails of fatty acids are nonpolar.
Hydrophobic (does NOT like water) tails

8. Cell Membrane Architecture
Water interacts with the polar heads and repels the nonpolar lipids tails.
The polar heads of the phospholipids point toward the water inside and outside the cell.
The tails are buried in the interior of the membrane, directing themselves away from the water.
This double layer of phopholipids forms a flexible lipid bilayer.

9. How does water get through?
Water is present inside and outside of the cell
It is important for water to move in and out of cell for regulation
But if water is polar, how does it do that?
Water molecules are SMALL
The lipid molecules vibrate, creating temporary spaces for water molecules to slide through

10. Cell Membrane Architecture
Lipids bilayers stop polar molecules. How?
Polar molecules cannot interact with the nonpolar tails of the phospholipids.
However, most food molecules and other substances needed by the cell are polar (uh-oh).
Why could this be a problem for the cell?
How does a cell solve that problem?
Later in the lecture.

11. Cell Membrane Architecture
Lipid bilayers also contain PROTEINS that assist in molecule transport.
The lipid bilayer is fluid because proteins move about like icebergs floating on the surface of the ocean.
Allows phospholipids and protein molecules to shift from one region of the cell membrane to another.
Very important because cell membranes can be structure to fit the needs of different cell types.

12. Cell Membrane Proteins
The smooth lipid bilayer is interrupted by proteins sticking out from the surface.
Proteins that protrude from the cell membrane may serve as channels, receptors, or markers.

13. Cell Membrane Proteins
Proteins channels are formed by doughnut-shaped proteins.
Act as passageways through which only certain molecules can pass.
Each channel will admit only certain molecules

14. Cell Membrane Proteins
Receptor proteins in the cell membrane are shaped like boulders.
Transmit information into the cell by reacting to certain other molecules from the external molecules.
The part of the receptor that sticks out from the cell membrane has a specific shape.
Only molecules with the right shape can fit into these receptors.
When a molecule and a receptor are locked together, it triggers responses inside the cell.

15. Cell Membrane Proteins
Cell surface markers are elongated proteins, often with shorth chains of carbohydrates attached.
Function as the “name tags” of cells, giving each an identity.

16. Cell Membrane, A Fluid Mosaic
Lipid bilayer gives cell membranes a flexible structure that forms a strong barrier.
Contains proteins and carbohydrates
Proteins form channels and pumps that help to move material across the cell membrane.
Carbohydrates molecules are attached to many of these proteins.
Act like chemical identification cards, allowing cards to identify one another.

17. Cell Membrane, A Fluid Mozaic
Outside cell
The cell membrane regulates the movement of dissolved molecules from the liquid on one side of the membrane to the liquid on the other side.
Inside cell (cytoplasm)

18. When you think SOLUTES think STUFF
A Quick Review
A solution is a mixture of two or more substances.
The substances dissolved in the solution are called solutes.
Example- salt would be the SOLUTE which is dissolved in water which is the SOLVENT
The concentration of a solution is the mass of solute in a given volume of solution, (Concentration = mass/volume).
When you think SOLUTES think STUFF
In a solution, particles move constantly. They collide with one another and tend to spread out randomly.

19. Diffusion
Particles in a solution tend to move from an area where they are more concentrated to an area where they are less concentrated by a process called diffusion.
When the concentration of the solute is the same throughout a system, the system has reached equilibrium.
Passive diffusion:
Requires NO energy
Think about what happens when you spray perfume in a room

20. How Stuff Moves Across the Membrane
Passive Diffusion
Solutes move from high concentration to low concentration
Requires NO Energy
Includes:
Osmosis
Facilitated diffusion
Active Transport
Solutes move AGAINST the concentration gradient (From Low to High)
Requires Energy
Requires specific protein channels

21. Osmosis
Most biological membranes are selectively permeable, meaning that some substances can pass across them and others cannot.
Osmosis is the diffusion of water through a selectively permeable membrane.

22. How Osmosis Works
If the concentration of H2O is lower inside the cell than outside, there will be a net movement of H2O to the inside until equilibrium is reached.
In isotonic solutions the concentration of the two solutions is the same.
A hypertonic solution is the solution with the greater concentration of solutes.
A hypotonic is the solution with the lesser concentration of solutes.

24. Osmotic Pressure
Osmosis exerts a pressure known as osmotic pressure on the hypertonic side of the selective permeable membrane.
Cells are filled with salts, sugars, proteins, and other molecules, so they are almost always hypertonic to fresh water.
As a consequence, osmotic pressure should produce a net movement of water into a typical cell that is surrounded by fresh water.
Most animal cells are bathed in fluids that are isotonic with concentrations of dissolved materials roughly equal to those in the cell. In plant and bacteria cells the cell walls prevent the cells from expanding.

25. Example Which Beaker has a solution with more solutes than the cell?
Where will the water molecules move, in or out of the cell?
Is the solution Hypertonic, Hypotonic, or Isotonic to the cell?
What will happen to the cell? (lyse, shrivel, or stay the same?)

26. Facilitated Diffusion
Cell membranes have protein channels that make it easy for certain molecules to cross the membrane in a process known as facilitated diffusion.
Allows for faster molecule transport
Facilitated diffusion does not require the use of cell’s energy.

27. Active Transport
During active transport cells move materials against a concentration difference.
Requires energy.
Carried out by transport proteins found in the cell membrane.
Moves molecules in only one direction.
Many cells use such proteins to move Ca2+, K+, and Na+.
Enables cells to concentrate substances in a particular location, even against the forces of diffusion.

28. Sodium-Potassium Pump
The Na+-K+ pump moves sodium out of the cells and potassium into cells.
This flow of ions is vital for restoring a balance needed to keep a nerve cell functioning.
It also helps transport sugars and amino acids into your cells.
Every second, each channel can transport more than 300 sodium ions out of the cell.

29. Endocytosis
Endocytosis is the process of taking material into the cell by means of in-foldings, or pockets, of the cell membrane.
The resulting pocket breaks loose from the outer portion of the cell membrane and form a vacuole within the cytoplasm.
In phagocytosis extensions of cytoplasm surround a particle and package within a food vacuole.
In pinocytosis, many cells take up liquid from the surrounding environment.

30. Exocytosis
Many cells also release large amounts of material from the cell, a process known as exocytosis.
During exocytosis, the membrane of the vacuole surrounding the material fuses with the cell membrane, forcing the contents out of the cell.
Removal of water by means of contractile vacuole.

31. Endocytosis and Exocytosis