1. Transport Across the Cell Membrane

2. What is the Function of a Cell Membrane?
Acts as a barrier from the external environment
Controls substances that enter and exit the cell
Selectively permeable: movement across the cell membrane depends on the chemical composition of a substance
EX: molecule size, structure, charge, etc.

3. Transport Across the Cell Membrane
Passive Transport
Does not require input of energy
Active Transport
Requires input of energy

4. Passive Transport Does NOT require the input of energy from the cell
Involves movement down a concentration gradient.
Difference in concentration (# of molecules) of a given substance between adjoining areas.
Number of substances that can pass through using this method is limited due to low energy requirements.
Ex: Oxygen, Nitrogen, Carbon Dioxide

5. Passive Transport Types of Passive Transport Diffusion Osmosis
Facilitated Diffusion

6. Diffusion What is diffusion? What types of molecules can do this?
Spontaneous movement of molecules from an area of high concentration to an area of low concentration
What types of molecules can do this?
Small uncharged molecules
Ex: oxygen, carbon dioxide, nitrogen, water
How do they do it?
By passing through the phospholipid bilayer or through channels formed by transmembrane proteins

7. Osmosis Movement of WATER across a selectively permeable membrane
Water moves across the membrane to maintain equilibrium
Concentration of solute (proteins, minerals…) is equal on both sides of the membrane
Once equilibrium is obtained, water will continue to move across the membrane but there is no net gain on either side.

8. Osmosis

9. Osmosis HYPOtonic solution
A solution where the concentration of solute molecules is lower outside the cell than inside the cell
There is a higher concentration of water outside of the cell
Water moves into the cell and the cell expands (or bursts)
Called hemolysis

10. Osmosis ISOtonic solution
A solution where the concentration of solute molecules outside the cell is equal to the concentration of solutes inside the cell
Concentration of water is the same on both sides of the membrane

11. Osmosis HYPERtonic solution
A solution where the concentration of solute molecules is higher outside of the cell than inside
There is a higher concentration of water inside the cell
Water moves out of the cell and the cell shrinks
Called crenation

12. Osmosis

13. Facilitated Diffusion
Some proteins cannot pass through the cell membrane’s hydrophobic middle layer.
Carrier proteins can help to pass some molecules through the membrane.
Molecules that use this mode of transport include:
Large polar molecules
Large ions

14. Simple Vs. Facilitated Diffusion
Simple Diffusion
Facilitated Diffusion
Molecules pass through the phospholipid bilayer or protein channels
Molecules pass through the cell membrane via carrier proteins
Carrier proteins typically only carry 1 type of molecule

15. Active Transport Types of Active Transport Bulk Transport Endocytosis
Exocytosis
Transport through Gates or Channels

16. Active Transport
Used to move substances in or out of a cell AGAINST the concentration gradient
Requires ENERGY in order to move substances across the membrane
Energy is used to open a channel or a gate in the protein of the membrane to assist in the binding of a substance to the protein
Energy can also be used to assist in a shape change that pushes the substance through the membrane (bulk transport)

17. Bulk Transport
Some molecules are too large to cross the membrane through proteins. They must travel in and out of cells through bulk transport.
2 Types
Endocytosis
Exocytosis

18. Endocytosis Process by which cells ingest large materials
The cell membrane engulfs material from outside of the cell.
This portion of the membrane pinches off, forming a membrane bound vesicle inside the cell called a vacuole.

19. Exocytosis Reverse process of Endocytosis
Molecules are carried by vacuoles inside the cell and expelled into the extracellular space (outside of the cell).

20. videos
Passive and Active Transport
Endocytosis and Exocytosis