1. The Plasma Membrane IB Biology HL E. McIntyre

2. History of the Plasma Membrane
1665: Robert Hooke
1895: Charles Overton - composed of lipids
’s: must be a phospholipid
1925: E. Gorter and G. Grendel - phospholipid bilayer
1935: J.R. Danielli and H. Davson – proteins also part, proposed the Sandwich Model
1950’s: J.D. Robertson – proposed the Unit Membrane Model
1972: S.J. Singer and G.L. Nicolson – proposed Fluid Mosaic Model

3. Plasma Membrane is made of Phospholipids
Gorter + Grendel
Red Blood Cells analyzed
Enough for Phospholipid bilayer
Polar heads face out and Nonpolar tails face in
Does not explain why some nonlipids are permeable

4. Plasma Membrane Models
Sandwich Model
(Danielli + Davson)
2 layers of globular proteins with phospholipid inside to make a layer and then join 2 layers together to make a channel for molecules to pass
Unit Membrane Model
(Robertson)
Outer layer of protein with phospholipid bilayer inside, believed all cells same composition, does not explain how some molecules pass through or the use of proteins with nonpolar parts, used transmission electron microscopy
Fluid Mosaic Model
(Singer + Nicolson)
Phospholipid bilayer with proteins partially or fully imbedded, electron micrographs of freeze-fractured membrane

5. Which membrane model is correct?
1) Rapidly freeze specimen
2) Use special knife to cut membrane in half
3) Apply a carbon + platinum coating to the surface
4) Use scanning electron microscope to see the surface
According to the electron micrograph which membrane model is correct?
Why?
Fluid-Mosaic Model

6. Fluid-Mosaic Model
Fluid – the plasma membrane is the consistency of olive oil at body temperature, due to unsaturated phospholipids. (cells differ in the amount of unsaturated to saturated fatty acid tails)
Most of the lipids and some proteins drift laterally on either side. Phospholipids do not switch from one layer to the next.
Cholesterol affects fluidity: at body temperature it lessens fluidity by restraining the movement of phospholipids, at colder temperatures it adds fluidity by not allowing phospholipids to pack close together.
Mosaic – membrane proteins form a collage that differs on either side of the membrane and from cell to cell (greater than 50 types of proteins), proteins span the membrane with hydrophilic portions facing out and hydrophobic portions facing in. Provides the functions of the membrane

7. Structure of the Plasma Membrane

8. Structure of the Plasma Membrane
Phospholipid bilayer
Phospholipid
Hydrophilic head
Hydrophobic tails
Cholesterol
Proteins
Transmembrane/
Intrinsic/Integral
Peripheral/Extrinsic
Cytoskeletal filaments
Carbohydrate chain
Glycoproteins
Glycolipids

9. Label the Blank Diagram of the Plasma Membrane
Phospholipid bilayer
Phospholipid
Hydrophilic head
Hydrophobic tails
Cholesterol
Proteins
Transmembrane/
Intrinsic/Integral
Peripheral/Extrinsic
Cytoskeletal filaments
Carbohydrate chain
Glycoproteins
Glycolipids

10. Proteins of the Plasma Membrane Provide 6 Membrane Functions:
1) Transport Proteins
2) Receptor Proteins
3) Enzymatic Proteins
4) Cell Recognition Proteins
5) Attachment Proteins
6) Intercellular Junction
Proteins

11. 1) Transport Proteins
Channel Proteins – channel for lipid insoluble molecules and ions to pass freely through
Carrier Proteins – bind to a substance and carry it across membrane, change shape in process

12. 2) Receptor Proteins
– Bind to chemical messengers (Ex. hormones) which sends a message into the cell causing cellular reaction

13. 3) Enzymatic Proteins
– Carry out enzymatic reactions right at the membrane when a substrate binds to the active site

14. 4) Cell Recognition Proteins
– Glycoproteins (and glycolipids) on extracellular surface serve as ID tags (which species, type of cell, individual). Carbohydrates are short branched chains of less than 15 sugars

15. 5) Attachment Proteins
Attach to cytoskeleton (to maintain cell shape and stabilize proteins) and/or the extracellular matrix (integrins connect to both).
Extracellular Matrix – protein fibers and carbohydrates secreted by cells and fills the spaces between cells and supports cells in a tissue.
Extracellular matrix can influence activity inside the cell and coordinate the behavior of all the cells in a tissue.

16. 6) Intercellular Junction Proteins
– Bind cells together
Tight junctions
Gap junctions

17. How do materials move into and out of the cell?
Materials must move in and out of the cell through the plasma membrane.
Some materials move between the phospholipids.
Some materials move through the proteins.

18. Plasma Membrane Transport
Molecules move across the plasma membrane by:
Active Transport
Passive Transport

19. What are three types of passive transport?
Diffusion
Facilitated Diffusion
Osmosis
ATP energy is not needed to move the molecules through.

20. Passive Transport 1: Diffusion
Molecules can move directly through the phospholipids of the plasma membrane
This is called …
DIFFUSION

21. What is Diffusion?
Diffusion is the net movement of molecules from a high concentration to a low concentration until equally distributed.
Diffusion rate is related to temperature, pressure, state of matter, size of concentration gradient, and surface area of membrane.

22. What molecules pass through the plasma membrane by diffusion?
Gases (oxygen, carbon dioxide)
Water molecules (rate slow due to polarity)
Lipids (steroid hormones)
Lipid soluble molecules (hydrocarbons, alcohols, some vitamins)
Small noncharged molecules (NH3)

23. Why is diffusion important to cells and humans?
Cell respiration
Alveoli of lungs
Capillaries
Red Blood Cells
Medications: time-release capsules

24. Passive Transport 2: Facilitated Diffusion
Molecules can move through the plasma membrane with the aid of transport proteins
This is called …
FACILITATED DIFFUSION

25. What is Facilitated Diffusion?
Facilitated diffusion is the net movement of molecules from a high concentration to a low concentration with the aid of channel or carrier proteins.

26. What molecules move through the plasma membrane by facilitated diffusion?
Ions
(Na+, K+, Cl-)
Sugars (Glucose)
Amino Acids
Small water soluble molecules
Water (faster rate)

27. How do molecules move through the plasma membrane by facilitated diffusion?
Channel and Carrier proteins are specific:
Channel Proteins allow ions, small solutes, and water to pass
Carrier Proteins move glucose and amino acids
Facilitated diffusion is rate limited, by the number of proteins channels/carriers present in the membrane.

28. Specific Types of Facilitated Diffusion
Counter Transport – the transport of two substances at the same time in opposite directions, without ATP. Protein carriers are called Antiports.
Co-transport – the transport of two substances at the same time in the same direction, without ATP. Protein carriers are called Symports.
Gated Channels – receptors combined with channel proteins. When a chemical messenger binds to a receptor, a gate opens to allow ions to flow through the channel.

29. Why is facilitated diffusion important to cells and humans?
Cells obtain food for cell respiration
Neurons communicate
Small intestine cells transport food to bloodstream
Muscle cells contract

30. Passive Transport 3: Osmosis
Water Molecules can move directly through the phospholipids of the plasma membrane
This is called …
OSMOSIS

31. What is Osmosis?
Osmosis is the diffusion of water through a semipermeable membrane. Water molecules bound to solutes cannot pass due to size, only unbound molecules. Free water molecules collide, bump into the membrane, and pass through.

32. Osmosis in action
What will happen in the U-tube if water freely moves through the membrane but glucose can not pass?
Water moves from side with high concentration of water to side with lower concentration of water. Movement stops when osmotic pressure equals hydrostatic pressure.

33. Why is osmosis important to cells and humans?
Cells remove water produced by cell respiration.
Large intestine cells transport water to bloodstream
Kidney cells form urine

34. Osmosis and Tonicity
Tonicity refers to the total solute concentration of the solution outside the cell.
What are the three types of tonicity?
Isotonic
Hypotonic
Hypertonic

35. Isotonic
Solutions that have the same concentration of solutes as the suspended cell.
What will happen to a cell placed in an Isotonic solution?
The cell will have no net movement of water and will stay the same size.
Ex. Blood plasma has high concentration of albumin molecules to make it isotonic to tissues.

36. Hypotonic
Solutions that have a lower solute concentration than the suspended cell.
What will happen to a cell placed in a Hypotonic solution?
The cell will gain water and swell.
If the cell bursts, then we call this lysis. (Red blood cells = hemolysis)
In plant cells with rigid cell walls, this creates turgor pressure.

37. Hypertonic
Solutions that have a higher solute concentration than a suspended cell.
What will happen to a cell placed in a Hypertonic solution?
The cell will lose water and shrink. (Red blood cells = crenation)
In plant cells, the central vacuole will shrink and the plasma membrane will pull away from the cell wall causing the cytoplasm to shrink called plasmolysis.

38. Review: Passive Transport
Diffusion – O2 moves in and CO2 moves out during cell respiration
Facilitated Diffusion – glucose and amino acids enter cell for cell respiration
Osmosis – cell removal or addition of water

39. Review Tonicity
What will happen to a red blood cell in a hypertonic solution?
What will happen to a red blood cell in an isotonic solution?
What will happen to a red blood cell in a hypotonic solution?

40. What are three types of Active transport?
2) Exocytosis
3) Endocytosis
Phagocytosis
Pinocytosis
Receptor-Mediated endocytosis
Active Transport
ATP energy is required to move the molecules through.

41. Active Transport
Molecules move from areas of low concentration to areas of high concentration with the aid of ATP energy.
Requires protein carriers called Pumps.

42. The Importance of Active Transport
Bring in essential molecules: ions, amino acids, glucose, nucleotides
Rid cell of unwanted molecules (Ex. sodium from urine in kidneys)
Maintain internal conditions different from the environment
Regulate the volume of cells by controlling osmotic potential
Control cellular pH
Re-establish concentration gradients to run facilitated diffusion. (Ex. Sodium-Potassium pump and Proton pumps)

43. The Sodium-Potassium Pump
3 Sodium ions move out of the cell and then 2 Potassium ions move into the cell.
Driven by the splitting of ATP to provide energy and conformational change to proteins by adding and then taking away a phosphate group.
Used to establish an electrochemical gradient across neuron cell membranes.

44. Active Transport 2: Exocytosis
Movement of large molecules bound in vesicles out of the cell with the aid of ATP energy. Vesicle fuses with the plasma membrane to eject macromolecules.
Ex. Proteins, polysaccharides, polynucleotides, whole cells, hormones, mucus, neurotransmitters, waste

45. Active Transport 3: Endocytosis
Movement of large molecules into the cell by engulfing them in vesicles, using ATP energy.
Three types of Endocytosis:
Phagocytosis
Pinocytosis
Receptor-mediated endocytosis

46. Phagocytosis
“Cellular Eating” – engulfing large molecules, whole cells, bacteria
Ex. Macrophages ingesting bacteria or worn out red blood cells.
Ex. Unicellular organisms engulfing food particles.

47. Pinocytosis
“Cellular Drinking” – engulfing liquids and small molecules dissolved in liquids; unspecific what enters.
Ex. Intestinal cells, Kidney cells, Plant root cells

48. Receptor-Mediated Endocytosis
Movement of very specific molecules into the cell with the use of vesicles coated with the protein clathrin.
Coated pits are specific locations coated with clathrin and receptors. When specific molecules (ligands) bind to the receptors, then this stimulates the molecules to be engulfed into a coated vesicle.
Ex. Uptake of cholesterol (LDL) by animal cells

49. Review Types of Endocytosis
What is phagocytosis?
What is pinocytosis?
What is receptor-mediated endocytosis?

50. Types of Cell Junctions
In Animal Cells:
Tight Junctions
Desmosomes
Gap Junctions
In Plant Cells:
Plasmodesmata

51. Tight Junctions
Transmembrane Proteins of opposite cells attach in a tight zipper-like fashion
No leakage
Ex. Intestine, Kidneys, Epithelium of skin

52. Desmosomes
Cytoplasmic plaques of two cells bind with the aid of intermediate filaments of keratin
Allows for stretching
Ex. Stomach, Bladder, Heart

53. Gap Junctions
Channel proteins of opposite cells join together providing channels for ions, sugars, amino acids, and other small molecules to pass.
Allows communication between cells.
Ex. Heart muscle, animal embryos

54. Plasmodesmata
Channels between the cell walls of plant cells that are lined with the plasma membranes of adjacent cells and smooth ER runs through.
Allows for the exchange of cytosol between adjacent cells; moving water, small solutes, sugar, and amino acids.
Ex. Xylem and Phloem in Plants

55. Review Types of Cell Junctions
What is the difference between a plasmodesmata, tight junction, gap junction, and desmosome?

56. References
Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. (1999). Biology. Reading: Addison Wesley Longman, Inc.
Mader, Sylvia S. (1996). Biology. Boston: Times Mirror Higher Education Group, Inc.
Raven, Peter H. and Johnson, George B. (1989). Biology. Boston: TimesMirror/Mosby College Publishing.