Cell Membranes

Question 1

How do ionic substances, polar molecules and non-polar molecules behave in water?

Answer 1

• Polar and charged things dissolve in water
• Amphipathic things partially dissolve in water (form micelle or bilayer)
• Nonpolar things are excluded by water and do not mix with it

Question 2

What are phospholipids/bilayers for? What jobs do they do?

Answer 2

1) Isolate cell’s content from outside environment
2) Regulate exchange of substances between inside / outside cell
3) Communicate with other cells
4) Create attachments within / between cells
5) Regulate biochemical reactions

Question 3

What are proteins for in a cell membrane? What jobs do they do?

Answer 3

1) Integrins — give membrane stronger framework
2) Receptor — trigger cell activity when molecule from outside environment binds to protein
3) Recognition — allow cells to recognize one another
4) Enzymes
5) Attachment — anchor membrane to internal framework and external surface of neighboring cells 6) Transport — move molecules and ions (hydrophilic) across the membrane

Question 4

What is cholesterol for in a cell membrane? What jobs does it do?

Answer 4

• Decreases/increases the fluidity of the plasma membrane (the colder a cell, the more cholesterol)
• Not just Cold, cholesterol increases in a cell in all temperature extremes (ie: Hot spring, Antartica)
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Question 5

What are carbohydrates for in a cell membrane? What jobs do they do?

Answer 5

• Allow cellular adhesion to occur (for cells to link together)
• They are the ID tags on lipids and proteins

Question 6

How/why do phospholipids form a bilayer in water?

Answer 6

• This happens because phospholipids are amphipathic, and their polar heads are attracted to water while the non-polar tails are excluded by water
• The excluded tails are kept separate from the water by the formation of a spherical bilayer with the tails facing each other
• This is important because these forces are the same ones that keep fluid cell membranes intact

Question 7

What would happen if you put a cell in a hydrophobic environment?

Answer 7

• Amphipathic phospholipids in a membrane would “flip” and their polar heads would cluster together (or towards the cell’s watery interior)
• The hydrophobic tails would mix with the hydrophobic environment
• This would essentially dissolve the cell membrane’s bilayer

Question 8

What factors affect the fluidity of membranes?

Answer 8

• Temperature
• Lipid tail length
• The degree of unsaturation
• Ratio of cholesterol to phospholipid

Question 9

How can organisms, like plants, fish and bacteria, which lack the ability to regulate their body temp keep membranes fluid in cold temperatures? (more than one way)

Answer 9

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Question 10

How can organisms, like plants, fish and bacteria, which lack the ability to regulate their body temp keep membranes together (less fluid) in hotter temperatures? (more than one way)

Answer 10

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Question 11

Cell membranes are selectively permeable (What does this mean?)

Answer 11

-

Question 12

What types of molecules can move freely across the lipid bilayer? Why can they get across?

Answer 12

• Small
• Nonpolar

Question 13

What types of molecules cannot easily cross the lipid bilayer? Why can’t they get across easily? How do these molecules cross the cell membrane?

Answer 13

• Large, polar, ionic

Question 14

Concentration

Answer 14

-The amount of molecules in a given volume

Question 15

Diffusion

Answer 15

• The tendency of a substance to move down its concentration gradient from where it is more concentrated to less concentrated
• Spread out evenly in available space
• Requires no work

Question 16

Net diffusion

Answer 16

-

Question 17

“random” motion

Answer 17

-

Question 18

Collision

Answer 18

-

Question 19

Dynamic equilibrium

Answer 19

-Molecules still move back and forth, but there is no net change in concentration on either side of the membrane

Question 20

Facilitated diffusion

Answer 20

-The passage of a substance through a specific transport protein across a biological membrane down its concentration gradient -Passive tranport

Question 21

Passive transport

Answer 21

-The diffusion of a substance across a biological membrane without any input or energy

Question 22

Osmosis (aquaporins)

Answer 22

-The diffusion of water across a selectively permeable membrane -Possible with the existence of aquaporins (responsible for entry and exit of water through the membrane)

Question 23

Solution, solute, solvent

Answer 23

• A liquid consisting of a homogeneous mixture of two or more substances, consisting of a solute and solvent
• Solute: substance that is dissolved in a solution
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Question 24

Concentration gradient

Answer 24

-The net movement of molecules from the more concentrated to less concentrated side of a membrane.

Question 25

Hypotonic

Answer 25

-A solution with a solute concentration lower than that of a cell -Cell gains water, swells, and may burst (lyse)

Question 26

Hypertonic

Answer 26

-A solution with a solute concentration higher than that of a cell -Cells shrivels and can die from water loss

Question 27

Isotonic

Answer 27

-A solution with an equal solute concentration than that of a cell

Question 28

Active processes

Answer 28

* Allow for transport from areas of low conc. to areas of high conc. (against concentration gradient) * Require energy use by cell, in form of ATP * ATP phosphorylates protein pump or motor protein, producing ADP and Pi * ADP is “renewed” using energy from cell respiration: ADP and Pi bonded again, forming ATP * Includes: * Active Transport (protein pump) * Endocytosis: phagocytosis, Pinocytosis, and Receptor-mediated endocytosis * Exocytosis

Question 29

Active Transport

Answer 29

-The movement of a substance across a biological membrane up its concentration gradient using ATP

Question 30

Endocytosis

Answer 30

-The process of a cell taking in substances

Question 31

Exocytosis (ex)

Answer 31

* A process of a cell to export bulky materials such as proteins or polysaccharides * Crying -- export salty solution containing proteins

Question 32

How do trans-membrane proteins and cholesterol stay in membrane?

Answer 32

* The ends of the proteins are polar and the middle is nonpolar, so the polar tips are attracted to the polar phospholipid heads (and the nonpolar section is excluded to the middle where the nonpolar phospholipid tails are) *

Question 33

How does temperature affect membrane fluidity?

Answer 33

* Increasing = increasing fluidity * Cold -- membrane freezes

Question 34

How does lipid tail length affect membrane fluidity?

Answer 34

* Decreasing length = increasing fluidity * Longer tails = more drag, more LDf, less fluid

Question 35

How does the degree of unsaturation affect membrane fluidity?

Answer 35

* Increasing = increasing fluidity * Unsaturated = not packed as tightly, less LDF, more fluidity

Question 36

How does the ratio of cholesterol to phospholipid affect membrane fluidity?

Answer 36

* Inserted into membrane, bulky structure causes more LDF * **Low temps: increases fluidity** -- bulky structure prevents phospholipids from getting too close and freezing * **Moderate to high temps: decreases fluidity** -- more LDF

Question 37

Passive processes

Answer 37

* Occur from areas of high concentration to areas of low concentration only (down concentration gradient) * No extra energy use needed by cell (driven by random movement of particles) * Include: * Diffusion * Osmosis (via lipid bilayer and Aquaporins) * Facilitated Diffusion (channel protein)

Question 38

Phagocytosis

Answer 38

-“Cellular eating” The cell engulfs a particle by wrapping extensions around it and packaging it into a vacuole

Question 39

Pinocytosis

Answer 39

-“Cellular drinking” The cell gulps droplets of fluid into tiny vesicles (not specific)

Question 40

Receptor-mediated endocytosis

Answer 40

-Receptor proteins for specific molecules are embedded in a region of a membrane in coated pits, which pinches to form a vesicle (highly specific)

Question 41

Cholesterol levels in hot, cold, and temperate environments

Answer 41

**Low Temps:** * Cholesterol (bulky and mostly hydrophobic) helps keep phospholipids spread out and thus keeps the cell membrane fluid (increases fluidity) **High Temps:** * The phospholipids are spread out due to more rapid molecular motion * Cholesterol’s bulk does not play a significant role in increasing spacing between phospholipids but rather it’s bulk increases LDF between hydrophobic regions, thereby decreasing fluidity. * Cholesterol’s small hydrophilic group reduces fluidity through attractive forces (hydrogen bonding) with phospholipid heads