1. The Plasma Membrane
SGN 7

2. The plasma membrane is a semipermeable barrier that allows for a managed internal environment (cellular homeostasis) separate from the external environment The plasma membrane encloses the cellular contents

3. The lipid based structure is produced by the EMS and fused into place, often with specialized imbedded proteins
Independent organelle /endomembrane system membranes share many characteristics with PM or cell membrane

4. Membrane structure
The membrane is composed primarily of lipids, but also significantly proteins and to a lesser degree carbs
Components of the membrane are generally amphipathic molecules (for example phospholipids) or molecules with several different hydrophilic and hydrophobic regions (for example many types of embedded proteins)

5. The structure of the membrane is called a fluid mosaic phospholipid bilayer A bilayered arrangement of phospholipids with integral or peripheral proteins associated with it (“mosaic”)

6. A phospholipid is composed of two fatty acid tails (hydrophobic) and a third region that includes a charged phosphate group (hydrophilic), making the molecule amphipathic

7. In each layer the tails are oriented towards the middle of the membrane, creating a hydrophobic interior region and hydrophilic regions facing outward from the cell and inward to the cell’s aqueous interior
The membrane is held together primarily by hydrophobic interactions between tail regions

8. Membranes are fluid and dynamic (fluid mosaic)
Phospholipids and some proteins float about within their layer
Cholesterol in the membrane increases fluidity at low temps and decreases fluidity at high temps
Unsaturated fatty acid tails keep lipids from packing together too tightly increase fluidity and permeability

9. Portions of membrane pinch off and reform to add new membrane or digest old membrane, and also in regard to secreting or taking in substances

10. But membranes are stabilized and organized due to…
Internal anchoring – membrane anchored to matrix of cytoskeleton within cell
External anchoring – attachment to fibers of cell wall or in animals attachment to the extracellular matrix
But membranes are stabilized and organized due to…

11. Membrane proteins serve many functions and are associated with the membrane in different ways
Two general classes of membrane protein
Integral proteins Peripheral proteins

12. Integral proteins – primarily embedded within membrane, often extending beyond the membrane on the inside and/or outside of the cell Transport, anchoring, signal reception and transduction, etc.
Receptors extending through the membrane
Chloride pump – defective in individuals with Cystic fibrosis

13. Hydrophobic domains of protein are oriented towards the hydrophobic interior of the membrane, while hydrophilic domains are oriented towards outside or inside of cell, or towards lumen of channel

14. Peripheral proteins
Typically on the internal side of the membrane and loosely associated with integral proteins or phospholipids
Anchoring, internal transduction of signal, enzymes, etc.

15. Membrane carbohydrates
Found only on the outer surface of the membrane
Typically oligosaccharides (10 or fewer monosaccharides) as glycolipids and glycoproteins
Often serve as self markers , play a role in cell adhesion, and also as part of signal receptors

16. Traffic across the membrane
The tightly packed phospholipid bilayer, with a hydrophobic interior, allows passage of some but inhibits passage of most compounds, which can only pass with some assistance from the membrane

17. The smaller and more neutral a compound, the easier it can diffuse directly across the bilayer; increased size and charge or polarity inhibit direct diffusion
Therefore the membrane is said to be selectively permeable, with mechanisms regulating whether compounds can pass or not, and at what rate
Can diffuse freely through the bilayer
Amino acid
Cannot diffuse freely through the bilayer
Glucose
Carbon dioxide

18. Molecules and ions move across the membrane in three principle ways
Passive diffusion
Facilitated diffusion
Active transport

19. Passive diffusion
Movement by diffusion through the phospholipids themselves
Diffusion – random movement of substances leading to their movement from high concentration to low concentration
Some compounds can diffuse through the membrane according to the concentration gradient on either side of the membrane (diffusion is movement down the concentration gradient)
No metabolic energy required

20. Passive transport and gas exchange in animals4. 3. 1. 2.

21. Involves movement of small, typically nonpolar, noncharged molecules (O2, CO2) and also moderately sized predominantly nonpolar molecules (testosterone, estradiol)
H2O is so small and in such overwhelming concentration it is able to move by passive transport at a limited rate; movement of H2O determined by solute concentration on either side of the membrane

22. Facilitated diffusion
Movement by diffusion but through channel proteins
Channel proteins are transport proteins that form pores/channels and are highly specific for substances that they transport
Diffusion - No metabolic energy required

23. Can be open channels (ex. aquaporins) or gated channels (ex
Can be open channels (ex. aquaporins) or gated channels (ex. calcium channels) Gated channels – stimulus required to open or to induce operation Stimulus can be substance being transported or can be another compound or signal, such as voltage change in nerve cells
Gated channel
Open channel
Aquaporins – H2O specific open channels

24. Active transport Movement of compounds against the concentration gradient (from low to high concentration) Energy allows protein pumps to move compound against concentration gradient, and maintain concentrations on either side of the membrane against concentration gradient

25. Electrogenic pumps move ions against electrochemical gradient (combines concentration and charge separation) Keeps inside of the cell more negative than outside by unequal distribution of anions and cations This “separation of charge/concentration” is a type of potential energy, which can be used in different ways

26. Proton pumps move H+ out of cell or across organelle membrane, creating electrochemical gradient
Proton pumps are instrumental in cellular respiration and photosynthesis
In cellular respiration the energy of electrochemical gradients is converted into a molecular form of energy that is available to the cell, called ATP

27. Cotransporters move two compounds
Cotransproters can be antiport or symport
In plants proton pumps are involved in
cotransport with sucrose
Cell establishes high H+ concentration outside of
cell with proton pump
A cotransport protein binds to H+ and sucrose on
and diffusion of H+ into cell powers import of
sucrose (combines active and facilitated transport)
Allows cells to import sucrose against concentration
gradient
Absorption of glucose into cells from intestines starts with energy requiring antiport to create an electrochemical gradient; then uses facilitated diffusion and symport to import glucose

28. Sodium - potassium pump in animal nerve cells uses antiport
Three Na+ out for every 2 K+ in produces interior negative potential that is used to do cellular work, including transmiting nerve impulses

29. Mass or bulk transport across the membrane
Involves the movement of very large particles or even cells, as well as movement of smaller molecules in high concentration
Requires energy to manipulate chunks of membrane

30. Exocytosis – loaded vesicles from the Golgi apparatus fuse with membranes and spill substances outside of cell (insulin, neurotransmitters)

31. Endocytosis – compounds bind to cell surface receptors or cell engulfs material; membrane then invaginates and pinches off internally, forming vesicle, which migrates to specific location within cell, for example, a lysosome, where material will be digested Phagocytosis – taking in large particles or cells (ex. microorganisms engulfed by white blood cells)) Pinocytosis – engulfing extracellular fluid, in which are dissolved desired solutes (ex. egg in ovary takes in nutrients secreted by surrounding cells)

32. Receptor mediated endocytosis (ex. cholesterol uptake)

33. Review of basic concepts of water and solute movement across membranes
Students should understand diffusion, osmosis and dynamic equilibrium

34. Students should understand conditions of tonicity: hypertonic, isotonic, hypotonic Students should understand the different responses of cells with cell walls versus cells without cell walls in solutions of different tonicity
Refers to the solute concentration of the aqueous environment of the cell
Indirectly tells us about water concentration
“Hyper” – high solute/low water outside, so water flows out of the cell