1. Aim: How can we describe membrane structure and function
Aim: How can we describe membrane structure and function? Do Now: Are membranes important? Why? Explain the purpose of a raincoat. Homework: Read chapter 5 pages Complete chapter questions and complete multiple choice questions 1-6.

2. Chapter 5: Membrane Structure and Function

3. Plasma Membrane:
Is the boundary that separates the living cell from its nonliving surroundings.
Selectively Permeable (chooses what may cross the membrane)
Fluid mosaic of lipids and proteins: The membrane is not static.
Lipid bilayer: contains embedded proteins and a 2x layer of phosphates and lipids.

5. Phospholipids: Are the most abundant lipid in the plasma membrane.
Are amphipathic: both hydrophilic (head) and hydrophobic regions (tails).
Head composed of phosphate group attached to one carbon of glycerol is hydrophilic.
Two fatty acid tails are hydrophobic.

6. Hydrophilic
head
Hydrophobic tail
WATER
Phospholipid Bilayer

7. Fluid Mosaic Model
A membrane is a fluid structure with a “mosaic” of various proteins embedded in it when viewed from the top

8. The Fluidity of Membranes
Phospholipids in the plasma membrane Can move within the bilayer two ways
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)

9. The Fluidity of Membranes
The type of hydrocarbon tails in phospholipids affects the fluidity of the plasma membrane.
Fluid
Viscous
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
Carbon tails

10. Membrane Proteins and Their Functions
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer.
Fibers of
extracellular
matrix (ECM)

11. Types of Membrane Proteins
1. Integral/Transmembrane Proteins:
Penetrate the hydrophobic core of the lipid bilayer spanning the membrane.
EXTRACELLULAR
SIDE

12. Types of Membrane Proteins
2. Peripheral proteins:
Are appendages loosely bound to the surface of the membrane.

13. Six Major Functions of Membrane Proteins
Figure 7.9
Transport. (left) A protein that spans the membrane
may provide a hydrophilic channel across the
membrane that is selective for a particular solute.
(right) Other transport proteins shuttle a substance
from one side to the other by changing shape. Some
of these proteins hydrolyze ATP as an energy source
to actively pump substances across the membrane.
Enzymatic activity. A protein built into the membrane
may be an enzyme with its active site exposed to
substances in the adjacent solution. In some cases,
several enzymes in a membrane are organized as
a team that carries out sequential steps of a
metabolic pathway.
Signal transduction. A membrane protein may have
a binding site with a specific shape that fits the shape
of a chemical messenger, such as a hormone. The
external messenger (signal) may cause a
conformational change in the protein (receptor) that
relays the message to the inside of the cell.
(a)
(b)
(c)
ATP
Enzymes
Signal
Receptor

14. Six Major Functions of Membrane Proteins
Cell-cell recognition. Some glyco-proteins serve as
identification tags that are specifically recognized
by other cells.
Intercellular joining. Membrane proteins of adjacent cells
may hook together in various kinds of junctions, such as
gap junctions or tight junctions
Attachment to the cytoskeleton and extracellular matrix
(ECM). Microfilaments or other elements of the
cytoskeleton may be bonded to membrane proteins,
a function that helps maintain cell shape and stabilizes
the location of certain membrane proteins. Proteins that
adhere to the ECM can coordinate extracellular and
intracellular changes
(d)
(e)
(f)
Glyco-
protein

15. Aim: How can we describe ideal cellular conditions
Aim: How can we describe ideal cellular conditions? Do Now: Describe the terms hypertonic, isotonic, and hypotonic.

16. The Role of Membrane Carbohydrates in Cell-Cell Recognition
Is a cell’s ability to distinguish one type of neighboring cell from another.
Membrane carbohydrates:
Interact with the surface molecules of other cells, facilitating cell-cell recognition.

17. Synthesis and Sidedness of Membranes
Membranes have distinct inside and outside faces.
This affects the movement of proteins synthesized in the endomembrane system (Golgi and ER).

18. Synthesis and Sidedness of Membranes
Membrane proteins and lipids are made in the ER and Golgi apparatus. ER

19. Permeability of the Lipid Bilayer
1. Hydrophobic molecules:
Are lipid soluble and can pass through the membrane rapidly
2. Polar molecules:
Do NOT cross the membrane rapidly

20. Transport Proteins Transport proteins:
Allow passage of hydrophilic substances across the membrane.

21. Passive Transport
Passive transport is diffusion of a substance across a membrane with no energy investment.
CO2, H2O, and O2 easily diffuse across plasma membranes.
Diffusion of water is known as Osmosis.

22. Simple Diffusion Diffusion:
-Is the tendency for molecules of any substance to spread out evenly into the available space.
-Move from high to low concentration
-Down the concentration gradient

23. Effects of Osmosis on Water Balance
Is the movement of water across a semipermeable membrane
Is affected by the concentration gradient of dissolved substances called the solution’s tonicity

24. Water Balance of Cells Without Walls
Tonicity:
Is the ability of a solution to cause a cell to gain or lose water.
Has a great impact on cells without walls.

25. Three States of Tonicity

26. Isotonic Solutions If a solution is isotonic:
The concentration of solutes is the same as it is inside the cell
There will be NO NET movement of WATER

27. Hypertonic Solution If a solution is hypertonic:
The concentration of solutes is greater than it is inside the cell
The cell will lose water (PLASMOLYSIS)

28. Hypotonic Solutions If a solution is hypotonic:
The concentration of solutes is less than it is inside the cell
The cell will gain water

29. Water Balance in Cells Without Walls
Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water.

30. Water Balance of Cells with Walls
Cell Walls:
Help maintain water balance.
Turgor pressure:
Is the pressure of water inside a plant cell pushing outward against the cell membrane.
If a plant cell is turgid:
-It is in a hypotonic environment.
-It is very firm, a healthy state in most plants.
If a plant cell is flaccid:
-It is in an isotonic or hypertonic environment.

31. Water Balance in Cells with Walls
Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell.

32. How Will Water Move Across Semi-Permeable Membrane?
Solution A has 100 molecules of glucose per ml
Solution B has 100 molecules of fructose per ml
How will the water molecules move?
There will be no net movement of water since the concentration of solute in each solution is equal

33. How Will Water Move Across Semi-Permeable Membrane?
Solution A has 100 molecules of glucose per ml
Solution B has 75 molecules of fructose per ml
How will the water molecules move?
There will be a net movement of water from Solution B to Solution A until both solutions have equal concentrations of solute

34. How Will Water Move Across Semi-Permeable Membrane?
Solution A has 100 molecules of glucose per ml
Solution B has 100 molecules of NaCl per ml
How will the water molecules move?
Each molecule of NaCl will dissociate to form a Na+ ion and a Cl- ion, making the final concentration of solutes 200 molecules per mil. Therefore, there will be a net movement of water from Solution A to Solution B until both solutions have equal concentrations of solute

35. Facilitated Diffusion
Is a type of Passive Transport Aided by Proteins.
In facilitated diffusion:
-Transport proteins speed the movement of molecules across the plasma membrane.

36. Aim: How can we explore active and passive transport
Aim: How can we explore active and passive transport? Do Now: Describe the difference between a mouse and an elephant passing through the door opening.

37. Facilitated Diffusion & Proteins
Channel proteins:
Provide corridors that allow a specific molecule or ion to cross the membrane
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
A channel protein (purple) has a channel through which
water molecules or a specific solute can pass.

38. Facilitated Diffusion & Proteins
Carrier proteins
Undergo a subtle change in shape that translocates the solute-binding site across the membrane
A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.

39. Active Transport Active transport:
Uses energy to move solutes against their concentration gradients.
Requires energy, usually in the form of ATP.

40. Active Transport
The sodium-potassium pump is one type of active transport system.
Na+ binding stimulates
phosphorylation by ATP. 2
Na+
Cytoplasmic Na+ binds to
the sodium-potassium pump. 1
K+ is released and Na+
sites are receptive again;
the cycle repeats. 3
Phosphorylation causes the
protein to change its conformation, expelling Na+ to the outside. 4
Extracellular K+ binds to the
protein, triggering release of the
Phosphate group. 6
Loss of the phosphate
restores the protein’s
original conformation. 5
CYTOPLASM
[Na+] low
[K+] high P
ATP
ADP K+
[Na+] high
[K+] low
EXTRACELLULAR
FLUID P
P i

41. Comparison of Passive & Active Transport
Passive transport. Substances diffuse spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.
Diffusion. Hydrophobic
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse through the lipid bilayer.
Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins,
either channel or carrier proteins.
ATP

42. Maintenance of Membrane Potential by Ion Pumps
Is the voltage difference across a membrane
An electrochemical gradient:
Is caused by the concentration electrical gradient of ions across a membrane
An electrogenic pump:
Is a transport protein that generates the voltage across a membrane

43. Proton Pump
EXTRACELLULAR
FLUID + H+
Proton pump
ATP
CYTOPLASM –

44. Cotransport Cotransport:
Occurs when active transport of a specific solute indirectly drives the active transport of another solute
Involves transport by a membrane protein.
Driven by a concentration gradient.

45. Example of Cotransport
Cotransport: active transport driven by a concentration gradient

46. Bulk Transport
Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
Large proteins
Cross the membrane by different mechanisms

47. Exocytosis & Endocytosis
In exocytosis:
Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents
In endocytosis:
The cell takes in macromolecules by forming new vesicles from the plasma membrane

48. Endocytosis: Formation of new vesicles from the plasma membrane that aid in taking in new molecules.

49. Exocytosis: secretes a vesicle containing biological molecules, outside of the cell. Ex: Pancreatic cells that produce insulin, secrete it into extracellular fluid via exocytosis.