1. Cell Boundaries
Biology chapter 7.3
Mr. Hines

2. Where does the cell begin and end
Animal cells – the cell membrane separates regions between cells.
All animal cells will have a thin, flexible layer which holds all of the materials inside. (made of a lipid bilayer)
Embedded within the cell membrane are various protein and carbohydrate molecules.

3. Figure 7-12 The Structure of the Cell Membrane
Section 7-3
Outside
of cell
Inside
(cytoplasm)
Cell
membrane
Proteins
Protein
channel
Lipid bilayer
Carbohydrate
chains

4. Plant cells
All plant cells will have a cell membrane AND a second layer called the “cell wall.”
The cell wall adds a layer of protection and adds support (made of carbohydrates).
The term plant cell can be misleading since other forms of life will also contain a cell wall.
Fungi, algae, some protists, and some prokaryotes will have a cell wall too.

6. Diffusion through cell walls
The most important function of the cell membrane is to select what materials can enter the cell and exit the cell.
In other words, the cell membrane acts as a complicated security gate.

7. Measuring concentration
Important terms
Solute
solvent
solution.

8. The solute is measured in grams and the Solvent in measured in Liters.
Therefore a concentration measurement will include these units.
For example –
Solution A has 3grams/Liter of Salt (g/L)
Solution B has 6 grams/Liter of salt. (g/L)
Which is more concetrated?
Which is more dilute?

9. Diffusion
Because all of the molecules within a solution are moving around randomly, they will eventually come to equilibrium.
Equilibrium is when a solution is evenly mixed so that the concentration is the same all over. (equal)
This process is called diffusion.

10. Click the image to play the video segment.
Diffusion
Click the image to play the video segment.
Video 1

11. Diffusion can happen through a membrane if the membrane is permeable to a substance.
Diffusion is important because a cell can get materials in and out of a cell without using energy.

13. If a membrane allows some substances through, but not others, it is said to be “selectively permeable.”
If a membrane allows a substance through, the membrane is said to be “permeable” to the substance.
If a membrane does NOT allow a substance through, the membrane is said to be “impermeable” to the substance.

14. Selectively permeable

15. diffusion
For example: A cell membrane will allow salt molecules through it, but not Carbohydrate molecules.
Therefore the membrane is permeable to salt molecules and impermeable to carbohydrate molecules.
In this case, the salt molecules would move across the membrane until both concentrations are equal (equilibrium)

16. Diffusion What does diffusion have to do with the cell membrane?
Remember that the cell membrane allows materials into the cell.
Diffusion can cause molecules to move through a membrane so that the concentration outside of the membrane is equal to the inside.
This can happen in both directions (inside/ouside)

17. Osmosis
Osmosis is the diffusion of water through a selectively permeable membrane.

18. Click the image to play the video segment.
Osmosis
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Video 2

19. How osmosis works Important terms
Isotonic – condition when concentration are the same on both sides of the membrane.
Hypertonic – name given to the solution on one side of the membrane that is above strength.
Hypotonic – name given to the solution on one side of the membrane that is below strength.
When there is a hypertonic/hypotonic condition, water will move across the membrane until an Isotonic condition is reached.
This is called “osmotic pressure.”

20. Question Too much of any substance can “poison” a living thing.
If you drink too much water, you can die.
Think of how this might happen using the concepts of osmosis.

21. Figure 7-15 Osmosis
Section 7-3

23. Facilitated diffusion
How can molecules such as carbohydrates get across the membrane if they are too large for diffusion?
There are protein molecules embedded within the cell membrane which acts as a doorway (has enlarged entrance)
These proteins are referred to as “protein channels.”
The protein helps the carbohydrate across the membrane – facilitated diffusion.
Facilitated diffusion is still a form of diffusion – even though it is assisted.

25. Active Transport
If substances can not get across the membrane by diffusion, there must be another other methods.
If a cell needs to move materials across the membrane against the concentration difference – it is called active transport.
Active transport requires energy

26. Active transport is carried out by special proteins on the cell membrane.
These proteins are there for the purpose of “pumping” certain chemicals in and out.
The shape of a protein can play an important role in active transport.
Energy can change the shape of a protein which causes release of various chemicals inside/outside a cell.

27. Click the image to play the video segment.
Active Transport
Click the image to play the video segment.
Video 4

28. Figure 7-19 Active Transport
Section 7-3
Molecule to
be carried
Molecule
being carried
Energy

29. Endocytosis
Endocytosis – method for transporting large amounts of materials into the cell by means of infolding of the cell membrane.
There are 2 kinds of Endocytosis
a. Phagocytosis – This is when a cell partially surrounds material with its cell membrane and engulfs it (cell eating).
b. Pinocytosis – pockets of the cell membrane fill with liquid and then pinch of inside of the cell. (cell drinking)

31. Exocytosis
Exocytosis is the process by which cells move materials from the inside of the membrane to the outside (exit).

32. Click the image to play the video segment.
Endocytosis and Exocytosis
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Video 5