1. The Cell Membrane and Movement Across The Membrane

2. Semi-permeable membrane
Separates living cell from aqueous environment
Cell membrane controls what gets in or out
semi-permeable
only allows some material to get in or go out of the cell
GOAL: to maintain HOMEOSTASIS
So what needs to get across the membrane?
sugar
lipids aa O2
H2O
salt
waste

3. Building a membrane
How do you build a barrier that keeps the watery contents of the cell separate from the watery environment?
What substance do you know that doesn’t mix with water?

4. Aaaah, one of those structure–function
Phospholipids
Phosphate
“attracted to water”
Phosphate head
hydrophilic
Fatty acid tails
hydrophobic
Arranged as a bilayer
Fatty acid
“repelled by water”
Aaaah, one of those structure–function
examples

5. Arranged as a Phospholipid bilayer
Serves as a cellular barrier / border
sugar
salt
starch
H2O O2
lipids
polar
hydrophilic
heads O2
H2O
lipids
Permeable to lipid soluble molecules & small molecules: water, oxygen, carbon dioxide
nonpolar
hydrophobic
tails
lipids
H2O O2
polar
hydrophilic
heads
lipids

6. Permeability to polar molecules?
Membrane becomes semi-permeable via transport proteins
specific channels allow specific material across cell membrane
protein channels in lipid bilyer membrane
inside cell
H2O aa
sugar
salt
outside cell
NH3

7. Nonpolar areas of protein
Why are proteins the perfect molecule to build structures in the cell membrane?
Polar areas
of protein
Nonpolar areas of protein

8. Membrane Proteins Proteins determine membrane’s specific functions
cell membrane & organelle membranes each have unique collections of proteins
Classes of membrane proteins:
peripheral proteins
loosely bound to surface of membrane
ex: cell surface identity marker (antigens)
integral proteins
penetrate lipid bilayer, usually across whole membrane
transmembrane protein
ex: transport proteins
channels, carriers, pumps

9. Many Functions of Membrane Proteins
“Channel”
Outside
Plasma
membrane
Inside
Transporter
Enzyme activity
Cell surface receptor
“Antigen”
Signal transduction - transmitting a signal from outside the cell to the cell nucleus, like receiving a hormone which triggers a receptor on the inside of the cell that then signals to the nucleus that a protein must be made.
Cell surface identity marker
Cell adhesion
Attachment to the cytoskeleton

10. Signal molecule binds to receptor Activation of proteins
Overview of Cell Surface Receptors
EXTRACELLULAR
FLUID
Receptor
Signal
molecule
Relay molecules in a signal transduction pathway
Plasma membrane
CYTOPLASM
Activation
of cellular
response
Figure 11.5
Reception 1
Transduction 2
Response 3
Growth
Turn on an enzyme
Make a protein
Signal molecule binds to receptor
Activation of proteins

11. Membrane carbohydrates
Play a key role in cell-cell recognition
ability of a cell to distinguish one cell from another
antigens
important in organ & tissue development
basis for rejection of foreign cells by immune system
The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

12. Filaments of cytoskeleton
Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer
Glycoprotein
Extracellular fluid
Glycolipid
Transmembrane
proteins
The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.
Phospholipids
Filaments of cytoskeleton
Cholesterol
Peripheral
protein
Cytoplasm
1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model

13. CHECKPOINT
Draw and explain what semi-permeable means, and how this applies to the cell membrane.

14. Movement through a membrane
Why do molecules move through a membrane?
HIGH ?
To maintain
HOMEOSTASIS
(equilibrium or steady state)
Have class demonstrate moving through a membrane to maintain equilibrium
LOW ?

15. Transport through a membrane
PASSIVE TRANSPORT
No cellular energy is used to move substances into or out of a cell
Ex:
Three types of passive transport
SIMPLE DIFFUSION
FACILITATED DIFFUSION
OSMOSIS
CO2 O2
H2O

16. Molecules move from high to low
Simple Diffusion
move from HIGH to LOW concentration
Concentration Gradient: difference between high and low concentrations
Molecules evenly distributed
Movement from high concentration of that substance to low concentration of that substance.
Molecules become evenly distributed throughout the available space. Once they are distributed, no further change in concentration occurs.
The molecules are still in motion, but over time the same amount of molecules move into an area as move out of an area. – called equilibrium.
Diffusion ONLY occurs if there is a CONCENTRATION GRADIENT

17. Simple Diffusion Example
Move from HIGH to LOW O2 O2 O2
Which way will oxygen move?
inside cell O2 O2 O2
LOW
HIGH O2
outside cell O2 O2 O2 O2 O2 O2 O2

18. Simple Diffusion Explained

19. Simple diffusion is defined as the movement of…
Molecules from areas of higher concentration to areas of lower concentration.
Molecules from areas of lower concentration to areas of higher concentration.
Water molecules across a membrane
Gas or water molecules across a membrane

20. When sugar is mixed with water , equilibrium is reached when…
Molecules of sugar stop moving
Water and sugar molecules are moving at the same speed
The dissolved sugar molecules are evenly distributed throughout the solution
There are more sugar molecules than water molecules

21. Diffusion is one of the processes whereby materials are exchanged between a cell and its environment.
True
False

22. CHECKPOINT
Explain how molecules move through the cell membrane through the process of diffusion.
Be specific

23. Then what are cell membrane channels for?
Need to make “doors” through membrane
Each “door” is specific to the substance it carries
inside cell
H2O aa
sugar
salt
outside cell
waste

24. Facilitated Diffusion
Move from HIGH to LOW concentration
Passive Transport
no energy needed
Channel protein:
corridor for specific ion or molecule can pass
Carrier protein:
undergo a subtle change in shape that translocates the solute – binding site
HIGH
LOW

25. Facilitated Diffusion
Move from HIGH to LOW through a channel
Ex: Glucose into cells of your body
sugar
sugar
sugar
inside cell
sugar
sugar
LOW
Which way will sugar move?
HIGH
outside cell
sugar
sugar
sugar
sugar
sugar
sugar
sugar

26. Facilitated Diffusion Explained

27. Facilitated diffusion requires…
Enzymes
Carrier proteins
Lipid carriers
Carbohydrate carriers

28. Facilitated diffusion occurs…
Into the cell only.
Out of the cell only.
In either direction depending on the temperature
In either direction depending on the concentration gradient of the molecule

29. Unlike simple diffusion, facilitated diffusion required energy expenditure by the cell.
True
False

30. Facilitated diffusion requires a specific transporter for a specific molecule.
True
False

31. CHECKPOINT
COMPARE and CONTRAST simple diffusion and facilitated diffusion.
Provide an example for each to support your answer

32. Osmosis Movement of Water Across Cell Membrane

33. Osmosis Water is very important, so we talk about water separately
diffusion of water from HIGH concentration of water to LOW concentration of water
across a semi-permeable membrane
Is affected by the concentration gradient of the dissolved substances called the solution’s tonicity
The ability of solution to gain or lose water
Great impact on cells without walls

34. Vocabulary
A water solution is a mixture of water molecules and molecules of dissolved substances.
Solute: material that is dissolved in a liquid (sugar)
Solvent: the liquid the solute is dissolved in (water)
Solution: combination of the solute and solvent (sugar and water)

35. How Osmosis Works

36. Vocabulary Concentration: amount of water present in a given volume
*highest concentration of water is in pure water (highest water potential)
The more particles dissolved in water the fewer the water molecules present
Water
Which way
will the water
diffuse?
Where is the
highest
concentration
of water
(highest water
potential)?
Sugar +
Water

37. Keeping water balance
Cell survival depends on balancing water uptake & water loss
Hypotonic
Isotonic
Hypertonic

38. 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.

39. Water Balance in 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

40. 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.

41. Paramecium (protist) removing excess water videoB C

42. A B C

43. DO NOW
Draw an arrow to show which way the water would move by osmosis.

45. ACTIVE TRANSPORT
Cells may need molecules to move against concentration “hill”
need to pump “uphill”
from LOW to HIGH using energy
protein pump
requires energy
ATP
Plants have nitrate & phosphate pumps in their roots.
Why?
Nitrate for amino acids
Phosphate for DNA & membranes
Not coincidentally these are the main constituents of fertilizer.
ATP

46. Types of Active Transport
Sodium Potassium Pumps
Types of Active Transport
1. Protein Pumps:
Example: Sodium / Potassium Pumps are important in nerve responses.
Protein changes shape to move molecules: this requires energy!

47. Types of Active Transport
2. Endocytosis: taking material into a cell
Cell membrane forms food vacuole “cell eating”
This is how white blood cells eat bacteria!
Pinocytosis - liquids
Phagocytosis - solids

48. Types of Active Transport
3. Exocytosis: Forces material out of cell
Vesicle fuses with cell membrane
EX: Hormones or wastes released from cell
Endocytosis & Exocytosis animations

49. 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
ATP