1. Membrane Transport Nestor T. Hilvano, M.D., M.P.H.
(Images Copyright Discover Biology, 5th ed., Singh-Cundy and Cain, Textbook, 2012.)

2. Learning Objectives Describe plasma membrane and give its functions.
Differentiate the types of transport of molecules across the membrane.
Compare and contrast diffusion and osmosis.
Discuss briefly what happens when a red blood
cells is placed in hypotonic solution? In
hypertonic solution?

3. Plasma Membrane Functions:
Regulates passage of ions and macromolecules in and out of the cell
Allows some substances to cross more easily than others
Allows information to be transferred across the membrane
Possesses devices for cell attachment
Receptors for hormones
Ion pumps for regulating the internal environment

4. Components of Plasma Membrane
The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it
Selectively permeable
Composed mainly of Bilayer of phospholipids (selective barriers), and some proteins, cholesterol, carbohydrates

5. Fig. 7-7 Fibers of extracellular matrix (ECM) Glyco- Carbohydrate
protein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Figure 7.7 The detailed structure of an animal cell’s plasma membrane, in a cutaway view
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE

6. Transport of Molecules Across the Plasma Membrane
Passive transport- move from [high] to [low]; requires no ATP
Active transport- move from [low] to [high]; needs ATP
Facilitated transport- via carrier protein
Exocytosis and endocytosis

7. Passive Transport Cell does not expend energy
Diffusion – particles spread out from area of high concentration to an area of low concentration gradient.
i.e. O2 and CO2 are small and nonpolar molecules that easily cross the membrane.
Osmosis – is the net diffusion of water across a selectively permeable membrane

8. Molecules of dye Membrane Pores Net diffusion Net diffusion
Figure 5.3A
Molecules of dye
Membrane
Pores
Figure 5.3A Passive transport of one type of molecule
Net diffusion
Net diffusion
Equilibrium 8

9. p. 76 Lower concentration of solute Higher concentration of solute
Figure 5.4
Lower concentration of solute
Higher concentration of solute
Equal concentrations of solute
H2O
Solute molecule
Selectively permeable membrane
Water molecule
Figure 5.4 Osmosis, the diffusion of water across a membrane
Solute molecule with cluster of water molecules
Osmosis
p. 76 9

10. Tonicity
Tonicity is important in maintaining cell volume (ability to cause a cell to loss or gain water)
Isotonic solution – solute concentration is the same both intracellular and extracellular
An animal cell (RBC) placed in:
Hypotonic – will gain water, swell and lyse
Hypertonic – will lose water and crenate

11. Figure 7.5 Water Moves into and out of Cells by Osmosis
Osmosis is the diffusion of water across a selectively permeable membrane. Cells lose water in a hypertonic solution and gain water in a hypotonic solution. Our cells are bathed in an isotonic fluid to prevent osmotic swelling and bursting.

12. Facilitated Transport
Uses specific transport proteins in the membrane
Does not requires energy
Substances move down its concentration gradient
Channel proteins- ions channels (gated channel) that open or close in response to stimulus
Carrier proteins- binds to protein and changes it’s shape

13. Channel protein Solute (a) A channel protein Solute Carrier protein
Fig. 7-15
EXTRACELLULAR FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein
Figure 7.15 Two types of transport proteins that carry out facilitated diffusion
Solute
Carrier protein
(b) A carrier protein

14. Figure 7.7 Passive Carrier Proteins
The facilitated diffusion of glucose into our cells is mediated by a class of passive carriers called GLUT proteins. No energy is required for the transport of glucose by carrier proteins. The sugar moves from the side of the membrane where it is at high concentration to the side where it is at lower concentration.

15. Active Transport Requires expenditure of energy (ATP)
Molecules move against the concentration gradient
i.e. Na+ - K+ pump in nerve impulse transmission

16. Figure 7.8 Active Carrier Proteins
Active carrier proteins use energy to move materials from regions of low concentration to regions of high concentration. When food arrives, proton pumps secrete hydrogen ions (H+) into your stomach.
Faciliatated diffusion and active transport:

17. Figure 7.6 (part 1) The Plasma Membrane Controls What Enters and Leaves the Cell
Proteins that span the plasma membrane (b, c) play an important role in moving materials into and out of cells. The flowchart summarizes the different types of processes that move materials across biological membranes.

18. Endocytosis and Exocytosis
Transport of large molecules
Exocytosis – export bulky materials
i.e. tearing, insulin release into the blood
stream
Endocytosis – cells take in macromolecules by forming vesicles/vacuoles from its plasma membrane.
a) phagocytosis, b) pinocytosis

19. Figure 7.9 Cell Contents Are Exported through Exocytosis
Exocytosis exports materials from the cell.

20. Figure 7.10a Extracellular Substances Are Imported through Endocytosis
(a) Endocytosis brings material from the outside of the cell to the inside, wrapped in membrane vesicles. (d) Pinocytosis is nonspecific endocytosis of external fluid. (b) Receptor-mediated endocytosis is a highly selective process in which only certain extracellular molecules are recognized by, and bound to, special plasma membrane receptors. (c, e) Phagocytosis is endocytosis on a large scale. A macrophage (blue) engulfing an invading yeast cell (yellow) is seen in (e). Macrophages are part of the body’s defense system.

21. Homework
Define – diffusion, osmosis, endocytosis, exocytosis, channel proteins, carrier proteins, isotonic solution, hypotonic solution, and hypertonic solution.
Describe the structure of plasma membrane and give its functions. What is it’s primary component?
Compare and contrast passive transport, facilitated transport, and active transport across a membrane as to concentration gradients, energy expenditure, use of carrier/transport proteins, and cite an example of substances.
Discuss the effect of hypotonic and hypertonic solution to red blood cells.