1. Cell Membranes and Transport
Ch. 5

2. Structure of membranes
Bilayer membrane ~7 nm wide
Contains embedded proteins

3. Fluid Mosaic Model
Membrane described as fluid b/c both phospholipids and proteins can move about by diffusion
Bilayer has fluidity we associate with olive oil
Move sideways in their own layers
Proteins move like icebergs in sea

4. Fluid Mosaic Model
Some phospholipid tails are saturated, some are unsaturated
More unsaturated = more fluid
WHY???

5. Factors affecting fluidity
Longer tail = less fluid membrane
Lower temp. = less fluid membrane

6. Phospholipids
Tails of phospholipids are nonpolar, so it is difficult for polar molecules (water soluble) to pass through the membrane
Ex: sugars, amino acids, and proteins cannot leak out of the cell

7. Cholesterol hydrophilic heads + hydrophobic tails
Fits in between phospholipids
Animal cells: amount cholesterol= amount phospholipids in cell membrane
Plant cells: little to no cholesterol; Prokaryotes = none!
Regulates fluidity, stabilize membranes
Hydrophobic regions: prevent ions/polar molecules from passing through membrane
Important in myelin sheath around nerve cells: ion leaks would slow signals

8. Cholesterol

9. Two types of proteins in Cell membraneS:
Intrinsic (aka integral): found in inner layer, outer layer, or most commonly spanning the entire membrane (transmembrane proteins- cross both sides of membrane)
Extrinsic (aka peripheral): found on either the inner or outer surface of the membrane

10. Intrinsic Proteins
Stay in membrane due to its hydrophobic and hydrophilic regions
Most float like mobile icebergs although some are fixed like islands to structures inside or outside the cell and do not move about

11. Extrinsic proteins Many are bound to intrinsic proteins
Some are held in place by binding to molecules inside or outside the cell

12. Proteins’ roles in membranes
Transport proteins: hydrophilic channels
Moving Ions, polar molecules
Enzymes: on plasma membrane on small intestine surface hydrolyze disaccharides
Proteins for photosynthesis + cell respiration: E- transport chains in mitochondria membrane and thylakoid membrane of chloroplast
Receptors: bond to antigen and begin communication pathway

13. Glycolipids and Glycoproteins
Many proteins and lipids in membrane have carbohydrate chains attached that face the outside of the membrane
Glycolipids = carb. (polysaccharide) attached to lipid
Glycoproteins = carb. attached to protein

14. Glycolipids and Glycoproteins
Form H-bonds with water to stabilize the membrane
Form sugary coating on membrane called glycocalyx
Act as receptor molecules
for cell-cell recognition
(immune cells)

15. Signaling Receptors Coordinate activities of animal cells
Recognize messenger molecules like hormones and neurotransmitters
When molecule binds with receptor, it triggers a series of chemical reactions in the cell

16. Example: cell Insulin Receptors

17. Endocytosis receptors
Bind to molecules that are parts of the structures that are to be engulfed by cell

18. Cell Marker Receptors Aka antigens; Allow cell-cell recognition
Each cell type has its own specific antigen (similar to how different countries have different flags)
Ex: ABO blood types

19. Transport Proteins
Provide channels or passageways for ions and polar molecules to pass through cell membrane
Two main types:
Channel proteins – Facilitated diffusion (NO ATP!)
Carrier proteins – Active transport (ATP REQUIRED!)

20. Review and practice!
KAHOOT!

21. Transport across the CelL MEmBRANe
Ch. 5

22. Cell Surface Membrane (CSM) transport
Phospholipid bilayer creates effective barrier against water soluble molecules and ions
Prevents aqueous contents from escaping
Some essential transport is achieved through:
Diffusion
Facilitated diffusion
Osmosis
Active transport and bulk transport

23. Diffusion (simple diffusion)
Diffusion: net movement, as a result of random motion of its molecules or ions, of a substance from a region of high concentration to a region of low concentration
Particles move down a concentration gradient

24. Factors that affect Rates of Diffusion
‘steepness’ of concentration gradient: greater difference = higher rate
Temperature: higher temperature = higher rate
Surface area of diffusion: greater surface area = higher rate (more contact space)
Nature of molecules/ions: small molecules = greater rate

26. Respiratory gases cross membrane by diffusion (uncharged and nonpolar)

27. Facilitated diffusion
Diffusion that takes place with the help “assistance” of a certain protein molecule
Channel proteins
Carrier proteins

28. Channel proteins Water-filled pores Fixed shape
Allow charged substances (usually ions) to diffuse through membrane
Gated to allow for selectivity and control of movement

29. Carrier proteins
Flips between shapes to alternately open binding sites on interior/exterior of membrane
Direction of movement depends on concentration gradient inside and outside of cell

31. Osmosis
Type of diffusion involving water molecules only

32. Water potential
Water potential: tendency of water molecules to move from one place to another
Measured by Greek symbol psi Ψ

33. Water Potential Problems
In beaker A, which has a higher water potential, distilled water or the beet core?
Where will water flow in the diagram B? Explain why.

34. Water potential
Water potential for pure water (distilled) is 0
Solute potential - amount that solutes lower the water potential
Solutes make water potential less than 0, the more solute, the more negative the water potential!
Pressure potential (symbol Ψp) Increasing pressure increases water potential
Pressure potential makes water potential less negative (more positive)

35. Water potential

36. Water potential in animal cells

37. Osmosis in plant cells

38. Active Transport (ATP needed)
Active transport moves molecules/ions against concentration gradient
Achieved by carrier proteins (specific for structure of particular ion/molecule)
ATP supplied by cellular respiration
Can occur into or out of the cell

39. Sodium-potassium pump
Found in CM of all animal cells
Use ~30% cell energy and ~70% neuron cell energy
Pump 3 Na+ ions out of the cell and 2 K+ ions into the cell
Net result: inside of cell become more negative than the outside

40. Active transport
Important in re-absorption in the kidneys, where certain useful molecules and ions have to be re-absorbed into the blood after filtration into the kidney tubules
Involved in
absorption of
products of
digestion in the
gut

41. Active transport
Used to load sugar from photosynthesizing cells of leaves into phloem tissue for transport around the plant

42. Bulk transport (ATP Required!)
Bulk transport involves the mechanism of moving large quantities of molecules into the cell (endocytosis) or out of the cell (exocytosis)
Large molecules such as proteins or polysaccharides, part of cells, or even whole cells may be transported across the membrane

43. phagocytosis ‘cell eating’ Bulk up take of solid material
Cells specializing in this are called phagocytes and the vacuoles are called phagocytic vacuoles
Ex: engulfing of bacteria by immune cells

44. Pinocytosis ‘cell drinking’ – bulk uptake of liquid into cell
Vacuoles ( vesicles) formed are often extremely small, in which the process is called micro-pinocytosis
Human egg cells take up nutrients from cells that surround it by pinocytosis

45. exocytosis
Ex: how plants transport materials out of CM to make cell wall
Ex: secretion of digestive enzymes of the pancreas
Secretory vesicle from the Golgi carry the enzymes to the CSM and release their contents