1. Chapter 5 A Closer Look At Plasma Membranes
Honors Biology

2. HOMEOSTASIS……….. It Isn’t Easy Being Single? (why not)
Concentrations of ions & other substances outside the cell may rapidly become too high or low. A mechanism is needed to selectively permit substances to enter or leave the cell.
What is homeostasis?
Homeostasis is the name given to the dynamic processes that enable optimum conditions to be maintained for constituent cells, in spite of continual changes taking place both internally and externally.
What mechanism is needed to selectively permit substances to enter or leave the cell?
Phospholipid bi-layer with embedded proteins.
How does a cell maintain homeostasis?
By controlling what enters and leaves the cell.
What are 2 examples of homeostasis with the human body?
It Isn’t Easy Being Single: (Concentrations on ions and other substances outside the cell may rapidly become too high or low; a mechanism is needed to selectively permit substances to enter or leave the cell.
Cell involved in constant chemical activity.
Take in: oxygen and nutrients to maintain metabolism (aerobic respiration)
Release: Waste take could to toxic to the cell. (carbon dioxide, sodium, enzymes, end products of digestion)

3. Membrane Structure & Function
Phospholipid molecules form a bilayer.
Phosphate/glycerol head is polar & hydrophilic, two fatty acid tails are nonpolar & hydrophobic.
phospholipid molecule
Note: Do hydrophilic substances have an easier time passing thru the membrane? NO
lipid bilayer
water
Cholesterol (right) is another lipid molecule that’s commonly found in the plasma membrane.
Question: What is the arrangement of the phospholipids?
H – T - T - H

4. What is the ‘Fluid Mosaic Model’ of the plasma membrane structure?
The lipid molecules are free to move and protein molecules float independently in the lipid bi-layer.
open channel
proteins
transport protein
gated channel
proteins
lipid
bilayer

5. oligosaccharide groups
A wide variety of protein molecules float around in the lipid bi-layer and perform most of the functions of the membrane.
oligosaccharide groups
cholesterol
phospholipid
EXTRACELLULAR ENVIRONMENT
cytoskeletal
proteins (beneath
the plasma
membrane)
RECEPTOR
PROTEIN
RECOGNITION
PROTEIN
ADHESION
PROTEIN
open
channel
protein
gated
channel
protein
(open)
gated
channel
protein
(closed)
active
transport
protein
LIPID
BILAYER
area of
enlargement
TRANSPORT
PROTEINS

6. 7 Types of Membrane Proteins (Note: Most of the plasma membrane functions are carried out by these proteins)
1. Cytoskeletal Proteins - composed of microtubules. Needed for internal support and attachment.
2. Adhesion Proteins - composed of glycoproteins attached to oligosaccharides. Serve as the “glue” to hold cells together.
3. Open-Channel Proteins- needed for passive transport and facilitated diffusion.
4. Gated Channel Proteins - help control the directional flow or transport of ions across the membrane.

7. 7 types of membrane proteins..cont.
5. Carrier Proteins - carry on active transport requiring energy (ATP) to “actively pump” their cargo across the membrane.
6. Receptor proteins - these proteins have binding sites for hormones and enzymes, which allow them to do their work .
7.Recognition Proteins -these proteins are the “fingerprints” of the cells. These self-recognizing proteins identify their own body cells or recognize invaders.

8. What will happen in the example above?
What causes the reaction?
What energy creates the reaction below?
When will the reaction stop?

9. Concentration Gradients
Molecules are in constant motion, due to their own kinetic energy.
If there are more molecules of a substance in one area than another, a concentration gradient exists.
Random collisions of molecules cause them to move from an area where they’re highly concentrated to an area of low concentration.
Once the molecules have arranged themselves equally throughout two adjoining regions, it’s called “dynamic equilibrium.”
At this point, a concentration gradient no longer exists.

10. Molecular size – smaller molecules move faster than larger ones.
Diffusion
The net movement of like molecules down their concentration gradient is known as diffusion.
The rate at which diffusion occurs can be affected by the following factors:
Molecular size – smaller molecules move faster than larger ones.
Temperature – heat energy causes molecules to move more rapidly & to collide more frequently.
Electrical gradients – the negative side of a membrane will attract positive ions & repel negative ions.
Pressure gradients – applying pressure can speed up the rate at which molecules move.

11. selectively permeable membrane between two compartments
Osmosis
selectively permeable membrane
between two compartments
The movement of water across a selectively permeable membrane in response to concentration gradients, fluid pressure, or both is known as osmosis.
water
molecules
protein
molecules

12. Osmosis in a Plant Cell

13. Tonicity
The relative concentrations of solutes in two fluids is known as tonicity.
Water tends to move to areas where solute concentrations are greater.
When solute concentrations in two fluids are equal, we say the two fluids are isotonic.
In an isotonic solution, there is no net movement of water in either direction.

14. Water moves from a hypotonic solution to a hypertonic solution.
compartment 1
compartment 2
HYPOTONIC
SOLUTION
HYPERTONIC
SOLUTION
membrane permeable to
water but not to solutes
fluid volume increases
In compartment 2

15. Tonicity
2M sucrose
solution
A hypotonic solution has fewer solutes dissolved in it than an adjoining solution does.
A hypertonic solution has more solutes dissolved in it than an adjoining solution does.
As a result, water molecules will move from a hypotonic solution to a hypertonic solution.
HYPOTONIC
CONDITIONS
HYPERTONIC
CONDITIONS
ISOTONIC
CONDITIONS
1 liter of
distilled water
10M sucrose
solution
2M sucrose
solution

16. Plasma membranes are selectively permeable.
(This means that some substances are able to enter the cell and others are not.)
O2, CO2, other small
nonpolar molecules,
as well as H2O
C6H12O6, other large, polar
water-soluble molecules,
ions (such as H+, Na+, K+,
Ca++, CI-) along with H2O X

17. Passive Transport vs. Active Transport
concentration gradient
Passive Transport vs. Active Transport
high
low
DIFFUSION ACROSS
LIPID BILAYERS
lipid-soluble
substances as
well as water
diffuse across
PASSIVE
TRANSPORT
Water-soluble substances, and water,
diffuse through interior of transport
proteins. No energy boost required.
Also called facilitated diffusion
ACTIVE
TRANSPORT
Specific solutes are
pumped through interior
of transport proteins.
Requires energy boost

18. SPECIALIZED TYPE OF VACUOLE:
CONTRACTILE VACUOLES: PROTISTS HAVE THESE ORGANELLES - THEY COLLECT EXCESS WATER AND EXCRETE IT SO THE CELL DOESN’T BURST. (cytolysize)
contractile
vacuole
(emptied)
contractile
vacuole (filled)

19. PLASMOLYSIS: LOSS OF WATER FROM A CELL RESULTING IN A DROP IN TURGOR PRESSURE.
Turgor pressure: Water pressure inside the plant cell. (turgidity)
PASSIVE TRANSPORT:
REQUIRES NO ENERGY TO MOVE SUBSTANCES ACROSS A MEMBRANE = DIFFUSION , EXAMPLE: WATER, NON-POLAR MOLECULES, AND LIPID-SOLUBLE SUBSTANCES.
LIPID BILAYER KEEPS MOST SUBSTANCES OUT BECAUSE THEY’RE REPELLED OR TOO LARGE.

20. FACILITATED DIFFUSION: THIS SPEEDS UP THE DIFFUSION RATE!
PROTEINS PROVIDE CONVENIENT OPENINGS FOR PARTICLES TO PASS THROUGH - SUGARS AND AMINO ACIDS ARE MOVED THIS WAY...
THIS SPEEDS UP THE DIFFUSION RATE!

21. Facilitated Diffusion

22. ACTIVE TRANSPORT (Requires energy)
FROM LOW CONC. TO HIGHER CONC.
In active transport, SOLUTES MOVE AGAINST THE CONCENTRATION GRADIENT!
Ex- Sodium-Potassium Pump-helps nerve cells transfer electrical impulses by maintaining an electrical gradient on the plasma membrane.
Ex- Calcium Pump - helps keep the calcium concentration high in cells.

23. ENDOCYTOSIS: a form of active transport in which the cell uses energy to bring materials into the cell by engulfing.
A. PHAGOCYTOSIS: (“CELL EATING”) Cell takes in large molecules by pinching in the plasma membrane.
B. PINOCYTOSIS: (“CELL DRINKING”) Cell takes in fluid by pinching in the plasma membrane.

24. Exocytosis

25. Active Transport
EXOCYTOSIS: a form of active transport in which the cell excretes wastes or secretes substances needed elsewhere in the organism. These substances do not move through the plasma membrane itself.
Slide 19
EXOCYTOSIS
Vesicle in cytoplasm moves to plasma membrane,
fuses with it; contents released to the outside