1. Transport across membranes

2. Learning Outcomes
explain what is meant by passive transport (diffusion and facilitated diffusion including the role of membrane proteins), active transport, endocytosis and exocytosis;

3. Exchange across the plasma membrane
The membrane provides an effective barrier against the movement of substances, however some exchange between the cell and the environment is essential.

4. Transport across membranes
Materials can move across cell membranes:
Passively
Diffusion (simple or facilitated)
Osmosis
Actively
Active transport
Bulk transport

5. Diffusion
Net movement of molecules or ions from a region of high concentration to a region of low concentration
Occurs along a concentration gradient
Result = equilibrium (molecules or ions evenly spread out within a given space or volume)

6. Factors affecting the rate of diffusion
Concentration gradient
Greater the difference in concentration the greater the rate of diffusion
Temperature
At higher temperature kinetic energy particles increases
Diffusion is faster
Surface area
Greater the surface area, more particles can cross
Increases rate of diffusion

7. Factors affecting the rate of diffusion
Nature of molecules or ions
Large molecules diffuse slower
Non-polar molecules diffuse more easily
The respiratory gases (CO2 and O2) are small enough to diffuse quickly through the membrane.
Large, polar molecules (glucose and amino acids) and ions (Na+ and Cl-) cannot diffuse through the phospholipid bilayer

8. Facilitated Diffusion
Protein molecules exist in membranes to facilitate diffusion.
2 type of protein molecule
Channel protein
transmembrane protein that forms a tunnel through the bilayer.
Carrier proteins
change shape to help molecules move into and out of cells.

9. Facilitated Diffusion

10. Active Transport
Energy consuming transport of molecules or ions across a membrane against a concentration gradient, made possible by transferring energy from respiration.
Energy makes the carrier proteins change shape, transferring ions across the membrane.

11. Examples of active transport
Reabsorption in kidneys
Digestion in gut
Helps absorb glucose from our intestines
Load sugars into phloem
Inorganic ion uptake in root hairs
Magnesium ions are in short supply in the soil but are needed for photosynthesis

12. Bulk transport
This is the method of transporting large quantities of materials into cells (endocytosis) or out of cells (exocytosis)
Endocytosis - Engulfing of material by cell membrane to form a endocytic vacuole.
2 forms
Phagocytosis the uptake of solid material
Pinocytosis the uptake of liquid
Exocytosis - Process by which materials are removed from cells

13. Examples of bulk transport
Hormones released into bloodstream from endocrine glands
White blood cells engulf invading microorganisms by phagocytosis
In plant cells materials to build the cell wall are carried outside in vesicles.

14. OSMOSIS Special type of diffusion involving water molecules Example:
Two solutions are separated by a partially permeable membrane. Solute molecules are too large to pass through pores in the membrane, but water molecules are small enough.

15. What would happen if the membrane were not present?

16. Net movement of solute molecules from B to A by diffusion
Net movement of water molecules from A to B by diffusion
Equilibrium – concentrations of water molecules and solute molecules in A would equal that in B.

17. What will happen if the membrane is present?

18. What will happen if the membrane is present?

19. Solute molecules too large to pass through membrane
Water molecules pass easily from A to B
Net movement of water from A to B until equilibrium is reached, i.e. solution A has the same concentration of water molecules as solution B.
The level of liquid A will fall and the level of liquid B will rise
Equilibrium is brought about by the movement of water molecules alone.

20. Definition of osmosis Water potential Ψ Osmosis
Tendency of water molecules to diffuse from one place to another.
Measured in kPa
Pure water has a water potential of 0kPa
Osmosis
Is the net movement of water molecules from a region of high water potential to a region of low water potential (down a water potential gradient) across a partially permeable membrane.

21. Water potential Highest water potential 0kPa Pure water No solute
Lower water potential
-50kPa
Dilute solution
Small amount of solute dissolved
Very low water potential
-500kPa
Concentrated solution
Large amount of solute dissolved
Decreasing water potential

22. Some Important Terms Hypotonic Hypertonic Isotonic a region of
higher water potential.
Lower solute concentration
Hypertonic
lower water potential
Higher solute concentration
Isotonic
a region where there are equal water potentials on either side of a membrane.

23. Determining Water Potential in Potato tubers
Salt Soluntion
(mol-1)
Starting Mass (g)
Finishing mass (g)
Change in mass (g)
%age change in mass
0.1
0.2
0.3
0.4
0.5

24. Osmosis in Red Blood Cells

25. Osmosis in Plant Cells

26. Important Terms Turgid Plasmolysed
the term used to describe a plant cell where the protoplast exerts a pressure on the cell wall.
Plasmolysed
the term used to describe a plant cell where the protoplast has shrunk away from the cell wall due to loss of water by osmosis.

27. Osmosis in red onion cells