1. Membrane Transport

2. Surface Area and Volume of the Cell
The surface area of a cell is important for carrying out the cell’s functions
A small cell has more surface area relative to its cell volume and is more efficient
A cell has to be big enough to house DNA, proteins and other structures to survive and reproduce
Size of a cell is limited by
Having enough surface area to obtain sufficient nutrients and oxygen and to get rid of wastes
Also limited by the distance these materials must diffuse within a cell

3. 10 µm 30 µm 10 µm 30 µm Surface area Total surface area
Figure 4.2B Effect of cell size on surface area.
Surface area
of one large cube
= 5,400 µm2
Total surface area
of 27 small cubes
= 16,200 µm2

4. Large cells have more surface areas than smaller cells, BUT larger cells have less surface area relative to their volume than small cells of the same shape.
So larger cells have much smaller surface area relative to its volume when compared to smaller cells.
The need for a surface area large enough to service a cell’s volume explains the microscopic size of cells.

5. Watch this! Movie
So what are we saying?
Smaller cells have a larger surface area to volume ratio
This means that it is easier for them to move nutrients in and wastes out.

6. Membrane Structure
The membrane is a fluid mosaic of phospholipids and proteins.
It is a mosaic because of the diverse protein molecules embedded in the phospholipids.
The membrane is fluid because molecules can drift in the membrane.
Fluidity due to the double bond of the phospholipid tails which prevents them from packing tightly together
Also from cholesterol found between the tails in animal cells.

7. Phospholipid bilayer Hydrophobic regions of protein Hydrophilic
Campbell, Neil, and Jane Reece, Biology, 8th ed., Figure 7.3 The fluid mosaic model for membranes.
Hydrophobic regions
of protein
Hydrophilic
regions of protein

8. WATER Hydrophilic head Hydrophobic tail WATER
Campbell, Neil, and Jane Reece, Biology, 8th ed., Figure 7.2 Phospholipid bilayer (cross section).
Hydrophobic
tail
WATER

9. Functions of the Membrane
Helps with the framework of the cell
Controls what goes in and out of the cell – selectively permeable
The membrane is selective due to its hydrophobic interior
Nonpolar molecules (carbon dioxide and oxygen) cross easily
Polar molecules (glucose and other sugars) do not cross easily

10. Parts of the Membrane Phospholipid bilayer Proteins Cholesterol
Glycoproteins and Glycolipids

11. Carbohydrate of glycoprotein Glycoprotein Glycolipid Integrin
Figure 5.1A The plasma membrane and extracellular matrix of an animal cell.
Phospholipid
Microfilaments
of cytoskeleton
Cholesterol

12. Phospholipids The membrane is made up of two layers of phospholipids
The heads interact with the extracellular fluid and the cytoplasm
The tails are sandwiched in between because they are hydrophobic.
Allows for small hydrophobic molecules to pass right through

13. Proteins There are 2 types
Peripheral Proteins – These are found only on one side
Integral Proteins – Span the membrane. Also referred to as transmembrane proteins and integrins.
These proteins have to have both hydrophilic and hydrophobic parts

14. Integral Proteins Help with many activities of the cell Enzymes
Cell to Cell Recognition
Intercellular Junctions
Transport
Signal Transduction
Helps to attach the cytoskeleton and the ECM

15. Enzymes
Figure 5.1B Enzyme activity.

16. Messenger molecule Receptor Activated molecule
Figure 5.1C Signal transduction.
Activated
molecule

17. Figure 5.1D Transport.

18. Cholesterol Found in between the phospholipid tails of animal cells
Helps to keep the fluidity of the cell
What else helps with this?

19. Glycoproteins and Glycolipids
Are made up of carbohydrate chains
These chains can attach to proteins = glycoproteins
Or to lipids = glycolipid
Help with cell – cell recognition
Serve as identification tags that are specifically recognized by membrane proteins of other cells
Enables cells of the immune system to recognize and reject foreign cells (bacteria)

20. Formation of Membranes
Phospholipids were probably one of the first organic molecules to form on Earth
They can spontaneously self-assemble to form membranes
All cells have membranes

21. Water-filled bubble made of phospholipids
Figure 5.2 Artificial membrane-bound sacs.

22. Water
Figure 5.2 Diagram of a section of a membrane sac.
Water

23. Transport Across A Membrane

24. Transport Across a Membrane
Passive – no energy is needed
Diffusion
Osmosis
Facilitated diffusion
Active - energy is required
Sodium Potassium Ion Exchange Pump

25. Passive Transport

26. Diffusion Molecules tend to spread out evenly
Molecules will move from where there is a high concentration to where there is a lower concentration
Molecules vibrate and move randomly due to heat
Molecules tend to move down their concentration gradient

27. Molecules will tend to move until they are about equal on both sides – dynamic equilibrium
Molecules will still move back and forth but there will be NO net change
Diffusion is how most molecules move across a membrane.

28. Molecules of dye Membrane Equilibrium
Figure 5.3A Passive transport of one type of molecule.

29. Two different substances Membrane Equilibrium
Figure 5.3B Passive transport of two types of molecules.

30. Osmosis It is crucial for cells that water moves across their membrane
Water moves across membranes in response to solute concentration inside and outside of the cell by a process called osmosis
Osmosis will move water across a membrane down its concentration gradient until the concentration of solute is equal on both sides of the membrane
Osmosis is the diffusion of water across a selectively permeable membrane

31. Osmosis
Water moves from the solution with the lower solute concentration and more free water molecules to that with the higher solute concentration and fewer free water molecules

32. cluster of water molecules
Lower
concentration
of solute
Higher
concentration
of solute
Equal
concentration
of solute
H2O
Solute
molecule
Selectively
permeable
membrane
Water
molecule
Figure 5.4 Osmosis, the diffusion of water across a membrane.
Note that osmosis is a force that is actually able to cause a differential in water levels in the two arms of the U-tube shown in Figure 5.4.
Solute molecule with
cluster of water molecules
Net flow of water

33. Tonicity is a term that describes the ability of a solution to cause a cell to gain or lose water
Tonicity is dependent on the concentration of a nonpenetrating solute on both sides of the membrane
Isotonic indicates that the concentration of a solute is the same on both sides
Cells lose and gain water at the same rate
Volume in animal cells will stay the same
Plants will be limp (flaccid)

34. Hypertonic indicates that the concentration of solute is higher outside the cell
Cell can shrivel and die
Plants cells go through plasmolysis because the cell shrivels up and pulls away from the cell wall.
Hypotonic indicates a higher concentration of solute inside the cell
Animal cells gain water and may lyse
Plant cells are turgid (firm). Cell wall prevents the cell from bursting.

35. Isotonic solution Hypotonic solution Hypertonic solution Animal cell
(A) Normal
(B) Lysed
(C) Shriveled
Plasma
membrane
Plant
cell
Fimgure 5.5 How animal and plant cells behave in different solutions.
(D) Flaccid
(E) Turgid
(F) Shriveled
(plasmolyzed)

36. Watch this! Osmosis animation
Use the information in the video to explain what happened to the cells in the plasmolysis lab

37. Osmoregulation – a process by which organisms are able to maintain water balance within their cells
This process prevents excessive uptake or excessive loss of water
Plant, prokaryotic, and fungal cells have different issues with osmoregulation because of their cell walls

38. Problems? Naah.
You place a dialysis bag with 20% sucrose in a beaker of 15 % NaCl. What direction will the water move? What about the sugar? The salt? Which solution is hypertonic and which is hypo?

39. More problems.
You place a piece of celery into salt water and into distilled water. What will happen to the weight of the celery in each case? Why? Explain which conditions are hypotonic and which are hypertonic.

40. Best Guess
What happens to a cell in a hypertonic solution? What kind of solution would be good to have if you have to get an IV? Why does Gatorade taste like orange-flavored sweat?

41. Facilitated Diffusion
Diffusion of molecules across a membrane through transport proteins
Does not require energy because the molecules move down their concentration gradient
Helps with the movement of polar and charged molecules across the membrane
Driving source – concentration gradient
Seen with sugars, amino acids, ions, and water (aquaporins – protein for water movement)

42. Facilitated Diffusion
Transport proteins
Are specific for the solutes they transport
Some are hydrophilic channels that are used like a tunnel
Others bind the molecules, change shape and release the molecule on the other side
Video

43. Solute molecule Transport protein
Figure 5.6 Transport protein providing a channel for the diffusion of a specific solute across a membrane.
Transport
protein 43

44. Active Transport

45. Active Transport
Energy (ATP) is used because molecules move against their concentration gradient.
Cells have a mechanism for moving a solute against its concentration gradient
The mechanism alters the shape of the membrane protein through phosphorylation using ATP
Na – K Ion Exchange Pump

46. Transport protein Solute 1 Solute binding
Figure 5.8 Active transport of a solute across a membrane. 1
Solute binding

47. Transport protein Solute 1 Solute binding 2 Phosphorylation
Figure 5.8 Active transport of a solute across a membrane. 1
Solute binding 2
Phosphorylation

48. Transport protein Protein changes shape Solute 1 Solute binding 2
Figure 5.8 Active transport of a solute across a membrane. 1
Solute binding 2
Phosphorylation 3
Transport

49. Transport protein Protein changes shape Phosphate detaches Solute 1
Figure 5.8 Active transport of a solute across a membrane. 1
Solute binding 2
Phosphorylation 3
Transport 4
Protein reversion

50. Requires no energy Requires energy Passive transport Active transport
Diffusion
Facilitated
diffusion
Osmosis
Higher solute
concentration
Higher water
concentration
Higher solute concentration
Solute
Water
Lower solute
concentration
Lower water
concentration
Lower solute
concentration

51. Transport of Large Molecules
A cell uses two mechanisms for moving large molecules across membranes
Exocytosis is used to export bulky molecules, such as proteins or polysaccharides
Endocytosis is used to import substances useful to the livelihood of the cell
In both cases, material to be transported is packaged within a vesicle that fuses with the membrane
Animation

52. There are three kinds of endocytosis
Phagocytosis is engulfment of a particle by wrapping cell membrane around it, forming a vacuole
Pinocytosis is the same thing except that fluids are taken into small vesicles
Receptor-mediated endocytosis is where receptors in a receptor-coated pit interact with a specific protein, initiating formation of a vesicle Cholesterol – goes through this
Video

53. Phagocytosis EXTRACELLULAR Food CYTOPLASM FLUID being ingested
Pseudopodium
“Food” or
other particle
Figure 5.9 Three kinds of endocytosis.
Food
vacuole

54. Pinocytosis Plasma membrane Vesicle Plasma membrane
Figure 5.9 Three kinds of endocytosis.
Plasma membrane

55. Receptor-mediated endocytosis
Plasma membrane
Receptor-mediated endocytosis
Coat protein
Receptor
Coated
vesicle
Coated
pit
Coated
pit
Specific
molecule
Figure 5.9 Three kinds of endocytosis.
Material bound
to receptor proteins