1. Bio. 12 Chapter 4 Membrane Structure and Function

2. Overview 4.1 – Plasma Membrane Structure & Function
Structure and function of the PM
Major functions of proteins
4.2- Permeability of the PM
Types of transport and their requirements
Diffusion vs. osmosis
Facilitated vs active transport
Exocytosis vs Endocytosis

3. 4.1 Plasma Membrane What is the plasma membrane?
-The boundary that separates the internal environment from the external environment
-Regulates the entrance and exit of molecules

4. Phospholipid Bilayer
The plasma membrane is composed mainly of phospholipids
There are two layers:
Hydrophilic (water loving) polar heads of the phospholipids face the outside and inside of the cell
Hydrophobic (water hating) non-polar tails face each other
Hydrophilic
head
Hydrophobic tail

5. Fluid Mosaic Model
What does Fluid Mosaic Model mean? - Fluid= consistency of light oil - Mosaic= contains embedded proteins scattered throughout

6. The Fluidity of Membranes
Cholesterol (another type of lipid) Has different effects on membrane fluidity at different temperatures
Figure 7.5
Cholesterol

7. Attached to the membrane
Carbohydrate (sugar) chains can be found attached to phospholipids and proteins on the outer surface of the membrane
on phospholipids = glycolipid
on proteins = glycoprotein

8. Two Types of Membrane Proteins
1. Integral proteins
-Penetrate the hydrophobic core of the lipid bilayer
-Are often transmembrane proteins, completely spanning the membrane
-Can move around latterally

9. Two Types of Membrane Proteins
2. Peripheral proteins
Are attached to one side of the membrane

10. Label the following in your notes

11. Five Major Functions of Membrane Proteins

12. 1. Channel Proteins
Provides a channel for molecules to pass through passively (energy is not required)
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM

13. 2. Carrier Protein
Combine with a substance and help it pass through the membrane.
Energy is required (active transport)

14. 3. Cell Recognition Protein
Some glycoproteins serve as
identification tags that are specifically recognized by other cells
Glyco-
protein

15. 4. Receptor Protein
Has 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 change in shape of the protein (receptor) relaying the message to the inside of the cell

16. 5. Enzymatic Protein
Protein is an enzyme with an active site
Speeds up a metabolic reaction
In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway.

17. 4.2 Membrane Permeability
Differentially permeable (selectively permeable) = certain substances can move across the membrane while others cannot
Some substances can pass freely though the membrane (passive transport) and others require energy (active transport)

18. Part 1:Passive Transport A) Diffusion
-Moves down the concentration gradient (goes from high to low concentration)
-H2O and small, noncharged molecules(CO2, O2, glycerol, alcohol) easily diffuse across plasma membranes
-Diffusion of water is known as Osmosis

19. A Look at Diffusion
Diffusion= the tendency for molecules of any substance to spread out evenly into the available space

20. Some material can’t diffuse through the membrane… instead they need Transport Proteins
Allow passage of hydrophilic substances across the membrane
Needed for facilitated and active transport

21. Passive Transport: B) Facilitated Transport
Channel or carrier proteins and a concentration gradient are needed (no energy required)
Some sugars and amino acids
Transport proteins speed the movement of molecules across the plasma membrane

22. 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

23. 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

24. Three States of Tonicity

25. 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

26. 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)

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

28. Water Balance in Cells without Walls
Animal cell. An animal cell fares best in an isotonic environment

29. 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

30. Water Balance in Cells with Walls
Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environment

31. 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

32. 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

33. Part 2:Active Transport
Uses energy to move solutes against their concentration gradients
Requires energy, usually in the form of ATP

34. Active Transport The sodium-potassium pump
Is one type of active transport system P
P i

35. Comparison of Passive & Active Transport
Passive transport.
Substances diffuse down their concentration gradients
no energy required
The rate of diffusion can be greatly increased by transport proteins
Active transport.
Against concentration gradient
Energy for this work is usually supplied by ATP
Sodium-potassium pump.
Diffusion
Facilitated diffusion
ATP

36. 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

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

38. Bulk Transport Bulk transport across the plasma membrane occurs by:
1.Exocytosis
2.Endocytosis
a) phagocytosis
b) pinocytosis
c) receptor-mediated endocytosis

39. 1. Exocytosis In exocytosis
Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents

40. Exocytosis

41. 2. Endocytosis In endocytosis
The cell takes in macromolecules by forming new vesicles from the plasma membrane

42. Endocytosis

43. Three Types of Endocytosis
Phagocytosis
-large particles like food is taken into the cell
-“cell eating”
- Seen in ameobas and human white blood cells

44. Three Types of Endocytosis
2. Pinocytosis
Liquid or very small particles are brought into the cell
“cell drinking”
Seen in blood cells and plant roots

45. Three Types of Endocytosis
3. Receptor-mediated endocytosis
Specific type of pinocytosis which uses receptor proteins shaped for specific molecules
Selective and more efficient
Seen in genetic disorders

46. Questions?