1. The Plasma Membrane and Transport across it

2. Cell Membrane Controls what enters and leaves the cell
Like water, nutrients and waste
The membrane is Selectively Permeable
Membrane selects which molecules can permeate (pass through) into the cell.
Like a window screen – allows what you want in and keeps others out.

3. Structure of Plasma Membrane
Lipid Bilayer (2 layers of phospholipids)
Lipids with phosphate “heads” attached
Head of phospholipid is polar
Hydrophilic (points outward, likes watery environment)
Fatty acids (tails) are nonpolar
Hydrophobic (points inward, avoids water)

4. Other Membrane Molecules
Membrane Proteins - (transport)
Allow substances/waste to move in and out of the cell.
Creates a tunnel that can be closed or open.
Carbohydrates/ Other Proteins-
Stick out of cell surface (id card for other cells to see)

5. Cell Membrane Structures
Carb/Protein (ID Cards)
Membrane Proteins (tunnels)
Fatty Acid Tails
Polar Heads

6. How stuff gets into the cell
2 ways cell gets nutrients it needs and gets rid of waste
Passive Transport
Requires no energy
Active Transport
Requires an input of energy

7. Passive Transport Occurs due to concentration gradient
Molecules naturally move from areas of higher concentration to lower concentration (no energy required)
Diffusion – natural movement of molecules
Facilitated Diffusion – through protein channels
Osmosis – diffusion of water molecules

8. Diffusion
In nature, molecules ALWAYS move from areas of higher concentration to areas of lower concentration, as is shown in this picture.
They move to reach equilibrium
This also happens across the cell membrane!
These odor molecules are diffusing from the bottle (area of high concentration) to the surrounding air (lower concentration)

9. Facilitated Diffusion
Diffusion of molecules through protein channels in the cell membrane
Used for substances that the cell needs but cannot pass through the membrane
Large molecules
Charged (ions)

10. Osmosis The diffusion of water molecules is called osmosis
Just like diffusion
Water molecules move from areas of higher conc. to areas of lower water conc.
They will move until there are equal concentrations of water (and solute) molecules everywhere
Called equilibrium
Molecules still diffuse, but at equal rates

11. Osmosis The movement of water molecules across a membrane
Water moves from areas of high water conc. to areas of lower water conc.
This is how cells maintain a stable internal environment
Homeostasis – reacting to environment
Blue – water molecules, easily pass through membrane
Red – large (or charged) molecules, can’t pass through membrane

12. Passive Transport Video

13. Isotonic Solutions - “Iso” equal
Cells that are in equilibrium with the surrounding solution do not experience osmosis.
These cells have the same concentration of solute (and water) as the surrounding solution

14. Hypotonic Solutions – “hypo” lower
[solute] (solute concentration) is lower outside the cell
More water outside, so water moves into the cell, to try to reach equilibrium
The cell swells with the extra water.
Animal cells may swell so much, they may burst (especially in pure water)
Plant cells – cell swells and membrane pushes against the cell wall
Like celery in water – crisp/ rigid from pressure on cell walls

15. Hypertonic Solutions - “hyper” higher
[solute] is higher outside of the cell.
Less water outside the cell, more inside. Water moves out of the cell.
Animal cells shrivel because of water loss.
Plant cell membranes move away from cell wall as water moves out.
Why plants wilt.

16. Other Forms of Transport
Active Transport – (needs energy)
Large particle transport
Endocytosis – cell surrounds molecule(s), which are then engulfed by cell
doesn’t pass through membrane
Exocytosis - expulsion/secretion of materials. Opposite of endocytosis

17. Active Transport (con’t)
Movement of molecules against (up) the concentration gradient
From areas of lower concentration to areas of higher concentration
For instance, if a cell needs a high concentration of something (that would normally diffuse OUT of cell)

18. Summary!