1. Cell Membrane Structure and Function
Chapter 3
Cell Membrane Structure and Function

3. Figure 3.12 Phospholipases in venoms can destroy cells

4. 3.1 What Does The Plasma Membrane Do?
The cell plasma membrane separates the cell contents from the external environment.
The membrane acts as a gatekeeper, regulating the passage of molecules into and out of the cell.
Cell membranes are complex structures that contain many different components.

5. 3.2 What Is The Structure Of The Plasma Membrane?
Plasma membranes are “fluid mosaics”.
Membranes are composed of a double lipid layer that is highly fluid without breaking.
Proteins are embedded in this double lipid layer and give the membrane its mosaic character .

6. 3.2 What Is The Structure Of The Plasma Membrane?
The plasma membrane is a fluid mosaic.
extracellular fluid (outside)
binding site
Cholesterol
Stronger
Flexible
Less soluble
to water
soluble
substances
phospholipid bilayer
carbohydrate
cholesterol
phospholipid
receptor protein
transport protein
protein filaments
recognition protein
cytoplasm (inside)
Fig. 3-1

7. 3.2 What Is The Structure Of The Plasma Membrane?
The phospholipid bilayer is the fluid portion of the membrane.
Phospholipid molecules have a polar head group and a pair of nonpolar tails.
CH3
CH2
CH2
CH2
CH2
CH2
CH2
CH3 – O +
CH2
H3C N
CH2
CH2 O P O
CH2 O CH
CH3 O HC O C
CH2
CH2
CH2
CH2
CH2
CH2
CH2 CH O
H2C O C
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH3
tails (hydrophobic): separate interior from surrounding
head (hydrophilic)
Fig. 3-2

8. 3.2 What Is The Structure Of The Plasma Membrane?
The head groups are hydrophilic and point toward the aqueous environment on both sides of the membranes.
The tail groups are hydrophobic and point away from the aqueous environments and toward each other.
extracellular fluid (watery environment)
phospholipid
hydrophilic heads
hydrophobic tails
bilayer
hydrophilic heads
cytoplasm (watery environment)
Fig. 3-3

9. 3.2 What Is The Structure Of The Plasma Membrane?
A mosaic of proteins is embedded in the membrane.
Some of the membrane proteins are anchored to a network of protein filaments within the cytoplasm.
Other proteins are free to move around in the lipid matrix.
Many proteins have carbohydrate groups that stick outside of the cell.

10. 3.3 How Does The Plasma Membrane Play Its Gatekeeper Role?
The phospholipid bilayer blocks the passage of most molecules.
The embedded proteins selectively transport (hydrophilic molecules), respond to, and recognize molecules.
There are three types of membrane proteins—transport proteins, receptor proteins, and recognition proteins.

11. Membrane Proteins Fig. 3-1 extracellular fluid (outside) binding site
phospholipid bilayer
carbohydrate
cholesterol
phospholipid
receptor protein
transport protein
protein filaments
recognition protein
cytoplasm (inside)
Fig. 3-1

12. Phospholipid bilayer blocks passage to most molecules
Polar H20 soluble: Salts, AA, sugars
Pass freely: Small molecules: H20, uncharged lipid soluble molecules

13. 3.3 How Does The Plasma Membrane Play Its Gatekeeper Role?
Membrane proteins
Transport proteins: allow the movement of water-soluble molecules through the plasma membrane by forming channels or by carrying them across
Receptor proteins: possess a binding site on the outer surface for binding specific chemicals that may alter overall cell function
Recognition proteins: with sugar groups attached to the exterior of the cell, are used by the immune system to identify cells as belonging to “self”

14. 3.3 How Does The Plasma Membrane Play Its Gatekeeper Role?
Animation—Plasma Membrane Structure

15. Characteristics of molecules in fluids
3.4 What Is Diffusion?
Characteristics of molecules in fluids
Concentration: the number of molecules in a given unit of volume
Gradient: a physical difference (concentration, pressure, electrical charge) between two regions of space that causes molecules to move from one region to another

16. Molecules in fluids move in response to gradients.
3.4 What Is Diffusion?
Molecules in fluids move in response to gradients.
Diffusion: the movement of molecules from regions of high molecular concentration to low molecular concentration
A drop of dye in water illustrates diffusion.

17. Diffusion of a dye in water
3.4 What Is Diffusion?
Diffusion of a dye in water
The dye molecules diffuse into the water; the water molecules diffuse into the dye 2
Both dye molecules and water molecules are evenly dispersed 3
A drop of dye is placed in water 1
drop of dye
water molecule
Fig. 3-4

18. Summing up: the principles of diffusion
3.4 What Is Diffusion?
Summing up: the principles of diffusion
Diffusion is the movement of molecules down a gradient from high concentration to low concentration.
The greater the concentration gradient, the faster the rate of diffusion.
If no other processes intervene, diffusion will continue until the concentration gradient is eliminated.

19. 3.5 What Is Osmosis?
Osmosis: the diffusion of water molecules from a high water concentration to a low water concentration across a biological membrane
Pure water has the highest water concentration.
The addition of dissolved solutes to pure water reduces the number of water molecules and thus lowers the water concentration.

20. 3.5 What Is Osmosis?
Osmotic water flow across a membrane takes place across selectively permeable membranes that allow water to pass, but not certain small impermeable molecules, such as sugars.

21. A selectively permeable membrane
3.5 What Is Osmosis?
A selectively permeable membrane
water
selectively permeable membrane
Water molecule: can fit through the pore
sugar
pore
Sugar with water molecules clustered around it: cannot fit through the pore
Fig. 3-5

22. 3.5 What Is Osmosis?
Example of osmosis: a water-permeable bag that is impermeable to sugar and has sugar inside of it
Water flows into the bag, down a water concentration gradient.
The bag swells and eventually bursts from the additional water.

23. 3.5 What Is Osmosis? Osmosis selectively permeable membrane Bag bursts
sugar molecule
Water flows in
water molecule
Fig. 3-6

24. 3.5 What Is Osmosis?
PLAY
Animation—Osmosis

25. Summing up: the principles of osmosis
3.5 What Is Osmosis?
Summing up: the principles of osmosis
Osmosis is the diffusion of water across a selectively permeable membrane.
Dissolved substances reduce the concentration of water molecules in a solution.
Water moves across a membrane down its concentration gradient from a high concentration of water molecules to a low concentration of water molecules.

26. There are two kinds of transport across cell membranes:
3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
There are two kinds of transport across cell membranes:
Passive transport
Energy-requiring transport

27. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?

28. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Passive transport: movement of molecules across a membrane, down a concentration gradient, without the use of energy
The phospholipid bilayer and transport proteins regulate which molecules can cross the membrane down concentration gradients.
Membranes are selectively permeable, and only allow some molecules to cross and not others.

29. Examples of passive transport
3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Examples of passive transport
Simple diffusion: the transfer of gases, water, and lipid-soluble substances—such as ethyl alcohol and vitamin A—across the phospholipid bilayer

30. lipid-soluble molecules and O2, CO2, and H2O
3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Simple diffusion through the phospholipid bilayer
(extracellular fluid)
lipid-soluble molecules and O2, CO2, and H2O O2
(cytoplasm)
(a)
Simple diffusion through the phospholipid bilayer
Fig. 3-7a

31. Examples of passive transport (continued)
3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Examples of passive transport (continued)
Facilitated diffusion: the diffusion of water-soluble molecules through a channel or carrier protein down a concentration gradient
Channel protein: pores in the lipid bilayer through which ions or molecules can diffuse
Carrier protein: membrane protein that grabs a specific molecule on one side of the membrane and carries it to the other side

32. Facilitated diffusion via a channel protein
3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Facilitated diffusion via a channel protein
H2O,
ions
Proteins form a hydrophilic channel
Cl–
Cl–
Cl–
Cl–
channel protein
Cl–
(cytoplasm)
(b)
Facilitated diffusion through a channel protein
Fig. 3-7b

33. amino acids, sugars, small proteins
3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Facilitated diffusion via a carrier protein
(extracellular fluid)
amino acids, sugars, small proteins
carrier protein
A carrier protein has a binding site for a molecule 1
A molecule enters the binding site 2
The carrier protein changes shape, transporting the molecule across the membrane 3
The carrier protein resumes its original shape 4
(cytoplasm)
(c)
Facilitated diffusion through a carrier protein
Fig. 3-7c

34. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
PLAY
Animation—Movement Across a Membrane

35. Examples of passive transport (continued)
3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Examples of passive transport (continued)
Water crosses membranes in response to molecular concentration differences on each side of the membrane.

36. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Isotonic extracellular fluid: the same molecular concentration outside the cell as inside the cell
There is equal movement of water across the cell membrane in each direction under this condition; there is no net water movement.

37. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Isotonic solution
10 micrometers
Equal movement of water into and out of cells
(a)
Isotonic solution has the same salt concentration as the cytoplasm
Fig. 3-8a

38. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Hypertonic extracellular fluid: the molecular concentration outside the cell is greater than the molecular concentration inside the cell
Net water movement out of the cell; the cell shrinks.

39. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Hypertonic solution
Net water movement out of cells; cells shrivel
(b)
Hypertonic solution has a higher salt concentration than the cytoplasm
Fig. 3-8b

40. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Hypotonic extracellular fluid: the molecular concentration outside the cell is less than the molecular concentration inside the cell
Net water movement into the cell; the cell swells.

41. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Hypotonic solution
Net water movement into cells; cells swell and burst
(c)
Hypotonic solution has a lower salt concentration than the cytoplasm
Fig. 3-8c

42. 3.7 How Do Molecules Move Against A Concentration Gradient?
Energy-requiring transport processes
During active transport, the cell uses energy to move substances against a concentration gradient.
Membrane proteins regulate active transport.
Adenosine triphosphate (ATP) donates energy to the active transport processes.

43. 3.7 How Do Molecules Move Against A Concentration Gradient?
Active transport
One binding site on a protein binds a transported molecule and a second binding site binds ATP.
Energy from ATP moves the other molecule up a concentration gradient.

44. 3.7 How Do Molecules Move Against A Concentration Gradient?
Active transport
(extracellular fluid)
The transport protein binds both ATP and CA2+ 1
Energy from ATP changes the shape of the transport protein and moves the ion across the membrane 2
The protein releases the ion and the remnants of ATP (ADP and P) and closes 3
ADP
ATP binding site
recognition site
ATP P
ATP
Ca2+
(cytoplasm)
Fig. 3-9

45. 3.7 How Do Molecules Move Against A Concentration Gradient?
Endocytosis: moves fluid droplets or large particles across cell membranes
Pinocytosis moves water into a cell.
Phagocytosis moves solid material into a cell.
Receptor-mediated endocytosis transports specific molecules across membranes.

46. 3.7 How Do Molecules Move Against A Concentration Gradient?
During endocytosis, a portion of the plasma membrane engulfs the extracellular fluid or particle and pinches off into the cytoplasm as a membranous sac, called a vesicle.

47. 3.7 How Do Molecules Move Against A Concentration Gradient?
Pinocytosis: movement of water into a cell
(extracellular fluid) 1 3 2
vesicle containing extracellular fluid
(cytoplasm)
A dimple forms in the plasma membrane, which deepens and surrounds the extracellular fluid The membrane encloses the extracellular fluid, forming a vesicle. 1 2 3
(a)
Pinocytosis
Fig. 3-10a

48. 3.7 How Do Molecules Move Against A Concentration Gradient?
Phagocytosis: movement of solid material into a cell
(extracellular fluid)
food particle
pseudopods 1 2
vesicle containing the particle
(cytoplasm) 3
The plasma membrane extends pseudopods toward an extracellular particle (food, for example) The ends of the pseudopods fuse, encircling the particle A vesicle that contains the engulfed particle is formed. 1 2 3
(b)
Phagocytosis
Fig. 3-10b

49. 3.7 How Do Molecules Move Against A Concentration Gradient?
Receptor-mediated endocytosis: transports only specific molecules across membranes
The process depends on the many receptor proteins on the outside surface of a cell.
Receptors can be in depressions in the plasma membrane, called coated pits.
The transported molecule binds to receptors in the coated pits, starts the formation of a membrane vesicle that surrounds the bound molecule, and the vesicle enters the cell.

50. 3.7 How Do Molecules Move Against A Concentration Gradient?
Receptor-mediated endocytosis
nutrients
(extracellular fluid)
receptors 1
coated pit 2 3 4
coated vesicle
(cytoplasm)
Receptor proteins for specific molecules or complexes of molecules are localized at coated pit sites The receptors bind the molecules and the membrane dimples inward The coated pit region of the membrane encloses the receptor-bound molecules A vesicle (“coated vesicle”) containing the bound molecules is released into the cytosol. 1 2 3 4
(c)
Receptor-mediated endocytosis
Fig. 3-10c

51. 3.7 How Do Molecules Move Against A Concentration Gradient?
Exocytosis: moves material out of the cell, including the waste products of digestion and secreted materials, such as hormones
During exocytosis, a vesicle carrying material to be expelled moves to the cell surface, where the vesicle fuses with the plasma membrane.
Following fusion, the vesicle opens to the extracellular fluid and its contents diffuse out.

52. 3.7 How Do Molecules Move Against A Concentration Gradient?
Exocytosis
(extracellular fluid)
secreted material
plasma membrane
plasma membrane
vesicle
Material is enclosed in a vesicle that fuses with the plasma membrane, allowing its contents to diffuse out
(cytoplasm)
0.2 micrometer
Fig. 3-11

53. 3.7 How Do Molecules Move Against A Concentration Gradient?
Some plasma membranes are surrounded by cell walls.
Cell walls occur around the plasma membranes of plants, fungi, and some bacteria.
Cell walls provide support for the cells, making them capable of resisting gravity and blowing winds.

54. 3.7 How Do Molecules Move Against A Concentration Gradient?
Some plasma membranes are surrounded by cell walls (continued).
Cell walls are porous to small molecules, which can pass across these barriers to the plasma membrane.
Plasma membranes of these cells regulate the transport of molecules by the same processes as those that occur in other cells without cell walls.