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

2. 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:
____________________
Energy-requiring transport

3. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?

4. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Passive transport: movement of molecules across a ____________ , _________ a concentration gradient, without the use of energy
The _____________ and transport proteins regulate which molecules can cross the membrane down concentration gradients.
Membranes are________________ , and only allow some molecules to cross and not others.

5. 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 (O2 and CO2, water, and lipid-soluble substances—such as ethyl alcohol (ethanol/ alcohol) and vitamin A—across the phospholipid bilayer
Rate of diffusion
Concentration gradient
__________________
Lipid solubility

6. 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

7. 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 (Na+, K+, Ca2+, AA, simple sugars) through a ___________ or carrier protein down a concentration gradient

8. Facilitated diffusion
Channel protein: _________ in the lipid bilayer through which ions or molecules can diffuse
Specialized e.g. nerve cells : potassium ion, sodium ion and calcium ions.
H20 channels: aquaporins (faster)
Carrier protein: membrane protein that ___________ a specific molecule on one side of the membrane and carries it to the other side
AA, sugar, small protein

9. 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

10. 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

11. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
PLAY
Animation—Movement Across a Membrane

12. 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 _____________ on each side of the membrane.

13. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Isotonic extracellular fluid: the _________ 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.

14. 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

15. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Hypertonic extracellular fluid: the molecular concentration outside the cell is ________ than the molecular concentration inside the cell
Net water movement out of the cell; the cell___________.

16. 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

17. 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane?
Hypotonic extracellular fluid: the molecular concentration outside the cell is _______ than the molecular concentration inside the cell
Net water movement ________ the cell; the cell swells.

18. 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

19. 3.7 How Do Molecules Move Against A Concentration Gradient?
Energy-requiring transport processes
During active transport, the cell uses energy to move substances _________ a concentration gradient.
Membrane proteins regulate active transport.
______________ (ATP) donates energy to the active transport processes.

20. 3.7 How Do Molecules Move Against A Concentration Gradient?
Active transport (pumps)
One binding site on a protein binds a transported molecule and a second binding site binds ATP.
Energy from ATP (adenosine triphosphate) moves the other molecule up a concentration gradient. (ADP + Phosphate)

21. 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

22. 3.7 How Do Molecules Move Against A Concentration Gradient?
Endocytosis: moves __________ or large particles across cell membranes
Pinocytosis moves ________ into a cell.
Phagocytosis moves __________ material (molecule, particle) into a cell.
__________________ endocytosis transports specific molecules across membranes.

23. 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 ________________ sac, called a vesicle.

24. 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

25. 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

26. 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 ________ proteins on the outside surface of a cell.
Receptors can be in depressions in the plasma membrane, called____________.
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.

27. 3.7 How Do Molecules Move Against A Concentration Gradient?
Receptor-mediated endocytosis
nutrients
(extracellular fluid)
receptors
E.g. remove
LDL from
blood 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

28. 3.7 How Do Molecules Move Against A Concentration Gradient?
Exocytosis: moves material out of the cell, including the ___________ of digestion and secreted materials, such as___________
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 ________________ and its contents diffuse out.

29. 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

30. 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_________ , fungi, and some bacteria.
Cell walls provide support for the cells, making them capable of resisting gravity and blowing winds.

31. 3.7 How Do Molecules Move Against A Concentration Gradient?
Some plasma membranes are surrounded by cell walls (continued).
Cell walls are ___________ to small molecules (O2, CO2, water, AA, sugars) 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.