1. Cells: The Living Units: Part A3
Cells: The Living Units: Part A

2. (f) Cell-cell recognition
Some glycoproteins (proteins bonded
to short chains of sugars) serve as
identification tags that are specifically
recognized by other cells.
Glycoprotein
Figure 3.4f

3. Membrane Junctions
Three types:
Tight junction
Desmosome
Gap junction

4. Membrane Junctions: Tight Junctions
Prevent fluids and most molecules from moving between cells
Where might these be useful in the body?

5. (a) Tight junctions: Impermeable junctions prevent molecules
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Interlocking
junctional proteins
Intercellular
space
(a) Tight junctions: Impermeable junctions prevent molecules
from passing through the intercellular space.
Figure 3.5a

6. Membrane Junctions: Desmosomes
“Rivets” or “spot-welds” that anchor cells together
Where might these be useful in the body?

7. (b) Desmosomes: Anchoring junctions bind adjacent cells together
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Intercellular space
Plaque
Intermediate
filament (keratin)
Linker glycoproteins
(cadherins)
(b) Desmosomes: Anchoring junctions bind adjacent cells together
and help form an internal tension-reducing network of fibers.
Figure 3.5b

8. Membrane Junctions: Gap Junctions
Transmembrane proteins form pores that allow small molecules to pass from cell to cell
For spread of ions between cardiac or smooth muscle cells

9. (c) Gap junctions: Communicating junctions allow ions and small mole-
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Intercellular
space
Channel
between cells
(connexon)
(c) Gap junctions: Communicating junctions allow ions and small mole-
cules to pass from one cell to the next for intercellular communication.
Figure 3.5c

10. Membrane Transport
Plasma membranes are selectively permeable
Some molecules easily pass through the membrane; others do not

11. Types of Membrane Transport
Passive processes
No cellular energy (ATP) required
Substance moves down its concentration gradient
Active processes
Energy (ATP) required
Occurs only in living cell membranes

12. Passive Processes
What determines whether or not a substance can passively permeate a membrane?
Lipid solubility of substance
Channels of appropriate size
Carrier proteins
PLAY
Animation: Membrane Permeability

13. Passive Processes
Simple diffusion
Carrier-mediated facilitated diffusion
Channel-mediated facilitated diffusion
Osmosis

14. Passive Processes: Simple Diffusion
Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through the phospholipid bilayer
PLAY
Animation: Diffusion

15. (a) Simple diffusion of fat-soluble molecules
Extracellular fluid
Lipid-
soluble
solutes
Cytoplasm
(a) Simple diffusion of fat-soluble molecules
directly through the phospholipid bilayer
Figure 3.7a

16. Passive Processes: Facilitated Diffusion
Certain lipophobic molecules (e.g., glucose, amino acids, and ions) use carrier proteins or channel proteins, both of which:
Exhibit specificity (selectivity)
Are saturable; rate is determined by number of carriers or channels
Can be regulated in terms of activity and quantity

17. Facilitated Diffusion Using Carrier Proteins
Transmembrane integral proteins transport specific polar molecules (e.g., sugars and amino acids)
Binding of substrate causes shape change in carrier

18. (b) Carrier-mediated facilitated diffusion via a protein
Lipid-insoluble
solutes (such as
sugars or amino
acids)
(b) Carrier-mediated facilitated diffusion via a protein
carrier specific for one chemical; binding of substrate
causes shape change in transport protein
Figure 3.7b

19. Facilitated Diffusion Using Channel Proteins
Aqueous channels formed by transmembrane proteins selectively transport ions or water
Two types:
Leakage channels
Always open
Gated channels
Controlled by chemical or electrical signals

20. (c) Channel-mediated facilitated diffusion
Small lipid-
insoluble
solutes
(c) Channel-mediated facilitated diffusion
through a channel protein; mostly ions
selected on basis of size and charge
Figure 3.7c

21. Passive Processes: Osmosis
Movement of solvent (water) across a selectively permeable membrane
Water diffuses through plasma membranes:
Through the lipid bilayer
Through water channels called aquaporins (AQPs)

22. (d) Osmosis, diffusion of a solvent such as
Water
molecules
Lipid
billayer
Aquaporin
(d) Osmosis, diffusion of a solvent such as
water through a specific channel protein
(aquaporin) or through the lipid bilayer
Figure 3.7d

23. Passive Processes: Osmosis
Water concentration is determined by solute concentration because solute particles displace water molecules
Osmolarity: The measure of total concentration of solute particles
When solutions of different osmolarity are separated by a membrane, osmosis occurs until equilibrium is reached

24. Solute and water molecules move down their concentration gradients
(a) Membrane permeable to both solutes and water
Solute and water molecules move down their concentration gradients
in opposite directions. Fluid volume remains the same in both compartments.
Left
compartment:
Solution with
lower osmolarity
Right
compartment:
Solution with
greater osmolarity
Both solutions have the
same osmolarity: volume
unchanged
H2O
Solute
Solute
molecules
(sugar)
Membrane
Figure 3.8a

25. (b) Membrane permeable to water, impermeable to solutes
Solute molecules are prevented from moving but water moves by osmosis.
Volume increases in the compartment with the higher osmolarity.
Both solutions have identical
osmolarity, but volume of the
solution on the right is greater
because only water is
free to move
Left
compartment
Right
compartment
H2O
Solute
molecules
(sugar)
Membrane
Figure 3.8b

26. When osmosis occurs, water enters or leaves a cell
Importance of Osmosis
When osmosis occurs, water enters or leaves a cell
Change in cell volume disrupts cell function
PLAY
Animation: Osmosis

27. Tonicity
Tonicity: The ability of a solution to cause a cell to shrink or swell
Isotonic: A solution with the same solute concentration as that of the cytosol
Hypertonic: A solution having greater solute concentration than that of the cytosol
Hypotonic: A solution having lesser solute concentration than that of the cytosol

28. Figure 3.9 (a) Isotonic solutions (b) Hypertonic solutions
(c) Hypotonic solutions
Cells retain their normal size and
shape in isotonic solutions (same
solute/water concentration as inside
cells; water moves in and out).
Cells lose water by osmosis and
shrink in a hypertonic solution
(contains a higher concentration
of solutes than are present inside
the cells).
Cells take on water by osmosis until
they become bloated and burst (lyse)
in a hypotonic solution (contains a
lower concentration of solutes than
are present in cells).
Figure 3.9

29. Summary of Passive Processes
Energy Source
Example
Simple diffusion
Kinetic energy
Movement of O2 through phospholipid bilayer
Facilitated diffusion
Movement of glucose into cells
Osmosis
Movement of H2O through phospholipid bilayer or AQPs
Also see Table 3.1