Cell membranes use both passive and active transport to regulate what passes in and out. Passive transport relies on diffusion and osmosis to move molecules down concentration gradients, while active transport requires ATP and uses carrier proteins to move molecules against gradients. Membranes are selectively permeable due to these transport mechanisms, which maintain homeostasis and allow for important cellular functions.

1. BIOLOGY 189
Cell Membranes and
Signaling

2. Biological Membranes
• General structure: fluid mosaic model.
• Phospholipids form a bilayer
• Two regions:
 Hydrophilic regions—“heads” that
associate with water molecules
 Hydrophobic regions—nonpolar fatty
acid “tails” that do not dissolve in water

3. Membrane Molecular Structure

4. Biological Membranes
• May differ in lipid composition as
there are many types of
phospholipids.
• Phospholipids may differ in:
Fatty acid chain length
Degree of saturation
Kinds of polar groups present

5. Biological Membranes
• Two important factors in membrane
fluidity:
Lipid composition—types of fatty
acids can increase or decrease
fluidity
Temperature—membrane fluidity
decreases in colder conditions

6. Biological Membranes
• Membranes contain proteins
Peripheral membrane proteins
lack hydrophobic groups and
are not embedded in the
bilayer.
Integral membrane proteins
are partly embedded in the
bilayer.

7. Biological Membranes
• Anchored membrane proteins
have lipid components that anchor
them in the bilayer.
• Transmembrane proteins extend
through the bilayer on both sides

8. Biological Membranes
• Plasma membrane carbohydrates
located on the outer membrane
serve as recognition sites.
Glycolipid—a carbohydrate
bonded to a ________
Glycoprotein—a carbohydrate
bonded to a ______

9. Selective Permeability
• Biological membranes allow
some substances to pass or not.
Passive transport does not
require energy.
Active transport requires
energy.

10. Diffusion
Passive transport
• Two types of diffusion:
Simple diffusion
Facilitated diffusion through
channel proteins or aided by
carrier proteins

11. Diffusion
• Speed of diffusion depends on
 Diameter of the molecules—smaller
molecules diffuse faster
 Temperature of the solution—higher
temperatures lead to faster diffusion
 The concentration gradient the
greater the concentration, the faster a
substance will diffuse

12. Diffusion
Higher concentration inside
the cell causes the solute to
diffuse out, and a higher
concentration outside
causes the solute to diffuse
in, for many molecules.

13. Osmosis
• Diffusion of water across
membranes.
• Depends on the concentration of
solute molecules on either side of
the membrane.
• Water passes through special
membrane channels.

14. Hypertonic, Isotonic, Hypotonic
When comparing two solutions separated
by a membrane:
• A hypertonic solution has a higher
solute concentration
• Isotonic solutions have equal solute
concentrations
• A hypotonic solution has a lower solute
concentration

15. Hypotonic

16. Isotonic

17. Hypertonic

18. Hypotonic Isotonic Hypertonic
solution solution solution
H2O H2O H2O H2O
Animal
cell
Lysed Normal Shriveled
H2O H2O H2O H2O
Plant
cell
Turgid (normal) Flaccid Plasmolyzed
Funny animation

19. Facilitated Diffusion
Diffusion aided by
• Channel proteins. Integral
membrane proteins that form
channels across the membrane.
• Substances can also bind to carrier
proteins to speed up diffusion.

20. A Ligand-Gated Channel Protein Opens in Response to a Stimulus

21. Diffusion
• Water crosses membranes with the help
of Aquaporins (specific channels)
• They allow large amounts of water to
move against its concentration gradient
• I t may “hitchhike” with ions such as Na+
as they pass through channels.

22. Carrier Proteins Facilitate Diffusion

23. Active transport
• requires energy to move substances
against their concentration gradients.
• energy source: ATP.
• A substance moves in the direction of
the cell’s needs, usually by means of a
specific carrier protein.

24. Active transport
Two types
• Primary active transport involves
hydrolysis of ATP for energy.
• Secondary active transport uses
the energy from an ion
concentration gradient, or an
electrical gradient.

25. Active Transport
• The sodium–potassium (Na+–K+)
pump is an integral membrane
protein that pumps Na+ out of a cell
and K+ in.
• One molecule of ATP moves two
K+ and three Na+ ions.

26. EXTRACELLULAR
[Na+] high Na+
FLUID [K+] low Na+
Na+ Na+ Na+
Na+ Na+
Na+
[Na+] low ATP
Na+ P
CYTOPLASM [K+] high P
ADP
1 2 3
K+
K+
K+
+
K+
K
P
K+ P
6 5 4

27. Endocytosis and Exocytosis
• Macromolecules are too large or
too charged to pass through
biological membranes and instead
pass through vesicles.
• Cells use endocytosis or
exocytosis

28. Endocytosis
Three types:
• Phagocytosis, (“cellular eating”)
• Pinocytosis, (“cellular drinking”)
• receptor mediated endocytosis

29. Endocytosis

30. Receptor-Mediated Endocytosis

31. Exocytosis
Exocytosis moves materials out of the cell
in vesicles.
Summary of cell membrane
transport