L11: Biological Membranes: Structure, Function, and Transport Processes

Question 1

Membranes are selectively permeable. What does this mean?

Answer 1

It influences which substances pass, how they pass, and how much pass the narrow span.

Question 2

Which membrane components underlie the selective permeability of membranes?

Answer 2

phospholipid bilayer with proteins

Question 3

Why is membrane selective permeability important to cells?

Answer 3

For cells to remain alive their interior must differ from the environment.

Question 4

Which dominates the span of a membrane: hydrophilic area or hydrophobic area?

Answer 4

hydrophobic

Question 5

How do non-polar substances move across a membrane?

Answer 5

non-polar solutes (e.g., O2, CO2) freely pass through the phospholipid bilayer

Question 6

1. How do O2 and CO2 gases move across a membrane?
2. Why is it important that these gases move so freely?

Answer 6

1. From the ECF to the cytosol
2. Cells need O2 to make ATP and just rid toxic CO2 made while making ATP.

Question 7

Why can’t polar and charged substances move across a membrane in the same way that non-polar substances do? How do polar substances move across a membrane?

Answer 7

Passage of polar (e.g., H2O, sugars, some a.a.) or charged solutes are inhibited due to zone of hydrophobicity.

Transmembrane proteins called channels (tube-like) and carriers (shape-changers) permit polar and charged solutions to pass

Question 8

Which type of biomolecule do cells use to make membrane channels and carriers?

Answer 8

proteins

Question 9

How do channels and carriers differ in solute movement across a membrane?

Answer 9

channels (tube-like) and carriers (shape-changers) permit polar and charged solutes to pass.
Channels may be gated; i.e., open/close due to voltage changes or ligands (e.g., drugs, Rx).
Carriers called pumps need energy to pump solutes across.

Question 10

What do channel gates do?

Answer 10

open and close

Question 11

What are two signals that open/close channels?

Answer 11

voltage changes or ligands (e.g., drugs, Rx).

Question 12

What is a pump and what does it do?

Answer 12

pump solutes across

Question 13

What does the fluid mosaic model say about membrane organization? How do we know that membranes and membrane proteins are organized this way?

Answer 13

Proteins comprise a mosaic within a fluid phospholipid bilayer. Moving phospholipids move proteins. We know this from cellular hybridization experiments.

Question 14

Which three factors influence membrane fluidity/permeability?

Answer 14

1. temperature
2. degree of saturation of phospholipid tails (saturated less permeable, unsaturated)
3. whether or not cholesterol stabilizes membrane.

Question 15

How do the aforementioned factors influence membrane fluidity/permeability? Which of these factors influence fluidity/permeability in your cells?

Answer 15

1. degree of saturation of phospholipid tails
2. amount of cholesterol

Question 16

How does membrane permeability change if temperature cools/warms?

Answer 16

warmer: increases membrane fluidity as well as its permeability (proteins may denature and potentially dangerous molecules can pass through membrane)
cold: strengthens intermolecular interactions and decreases membrane fluidity and permeability (potentially preventing essential molecules such as oxygen and glucose from passing through the membrane into the cell)

Question 17

How does membrane permeability change with changes in phospholipid saturation?

Answer 17

unsaturated fats increase: more fluid
more saturated fats: strong and stacked tightly

Question 18

How does membrane permeability change with the presence/absence of cholesterol?

Answer 18

Animals add cholesterol to plasma membranes so they are not too fluid at warmer temperatures but fluid enough to remain permeable at colder temperatures. helper that helps maintain cell permeability

Question 19

Which biota make cholesterol and use it to stabilize membranes?

Answer 19

animals

Question 20

Which six protein functions are associated with proteins embedded within membranes?

Answer 20

1) transport- Take in nutrients; excrete waste. Pump ions; generate gradients.
2) enzymatic activity - Catalyze reactions such as those in cellular respiration.
3) signal transduction- Change cell function to help organism maintain homeostasis.
4) cell-cell recognition- Important during development and during immune function.
5) Intercellular joining - Recall desmosomes and tight junctions?
6) attachment to the cytoskeleton and extra cellular matrix (ECM)

Question 21

Discern among endocytosis and exocytosis. Which other terms were used to refer to these bulk cellular transport processes?

Answer 21

During endocytosis (phagocytosis) cells take in bulk materials via vacuoles.
During exocytosis (secretion) cells release bulk materials via vesicles.

Question 22

In which process are contents transported via vesicles and which process are contents transported via vacuoles?

Answer 22

During endocytosis (phagocytosis) cells take in bulk materials via vacuoles.
During exocytosis (secretion) cells release bulk materials via vesicles.

Question 23

What is diffusion?

Answer 23

The movement of a solute from an area of greater concentration to lesser concentration is called diffusion

Question 24

Which types of solutes freely diffuse across cellular membranes?

Answer 24

non-polar solutes diffuse from greater to less [solute] directly across the phospholipid bilayer.

Question 25

What are the two driving forces behind diffusion?

Answer 25

diffusion results from the intrinsic kinetic motion of molecules and chemical gradients

Question 26

What is a chemical gradient?

Answer 26

A chemical gradient occurs when a solute is unequally distributed such that one region has more of the solute than another region. A chemical gradient will dissipate as solutes move from an area where they are more concentrated to areas where they are less concentrated

Question 27

What does it mean when we say that a chemical gradient dissipates towards equilibrium?

Answer 27

The inward movement and the outward movement will occur at the same rate. (not always true)

Question 28

Does [O2] inside/outside of a cell reach equilibrium? Why or why not?

Answer 28

oxygen gas is expected to diffuse inwards through simple diffusion because of its gradient and because its a non-polar solute. Solutes move inwards until the solute reaches equilibrium.
[O2] atm > [O2] ECF > [O2] cytosol

Question 29

Does [CO2] inside/outside of a cell reach equilibrium? Why or why not? Which cellular process underlies the answer to the previous questions.

Answer 29

CO2 is released through cellular respiration and will diffuse out of cells till equilibrium because the chemical gradient is reversed. [CO2] atm < [CO2] ECF < [CO2] cytosol

Question 30

What is facilitated diffusion?

Answer 30

Polar/charged solutes diffuse from greater to lesser [solute] through transport proteins

Question 31

How does facilitated diffusion differ from diffusion?

Answer 31

simple diffusion requires no energy and small non-polar can pass freely.
facilitated diffusion is for polar/charged solutes to pass through requires ATP and transport proteins

Question 32

Which types of solutes must be transported across cellular membranes via facilitated diffusion?

Answer 32

complex solutes like glucose

Question 33

Glucose can cross cellular membranes via facilitated diffusion. Why does glucose diffuse into most cells as opposed to out of them?

Answer 33

glucose is diffused into cells because it is one of the body’s main source of energy

Question 34

What is osmosis?

Answer 34

water diffusion

Question 35

1. How does water diffuse across cellular membranes via osmosis in relation to the free water gradient?
2. How does water diffuse across cellular membranes via osmosis in relation to the solute gradient?

Answer 35

1. water diffuses from greater to lesser [free water] concentration through special proteins called aquaporins. channels are always open so water can freely move across plasma membranes
2. water diffuses from an area of lesser solute concentration to greater concentration [solute]

Question 36

What are aquaporins?

Answer 36

proteins that serve as channels to let water freely pass through plasma membranes

Question 37

How does osmosis impact cell tone (volume)?

Answer 37

[Solute] inside/outside of a cell influences the net direction of water diffusion. Net water diffusion changes cell volume (called cell tone).

Question 38

Discern among isotonic, hypotonic, and hypertonic environs.

Answer 38

isotonic - equal solute concentration on both sides of the plasma membrane.
hypotonic - a lower solute concentration surrounding the outside of the cell and as higher concentration inside the cell.
hypertonic - there is a higher solute concentration surrounding the outside of the cell and a lower solute concentration inside the cell.

Question 39

How does the cell tone of a Red Blood Cell (RBC) (or other non-walled cell) and a plant cell (or other walled cell) change in response to being immersed in these environs?

Answer 39

isotonic - RBC normal; plant flaccid (wimpy)
hypotonic - RBC lyses (swells & ruptures); plant turgid (desired bc it pushes plants into turgid state that increases SA)
hypertonic - RBC crenates (shrivels); plant plasmolyze (shrivel)

Question 40

Discern among passive and active transport.

Answer 40

passive transport: substance moves across a membrane with its gradient (high to low) no investment of cellular energy required; the energy comes from the solutes themselves.

active transport: solutes are moved across membrane through a carrier against their gradient (i.e., pumping them in a direction opposite they want to go naturally). Cells must invest energy (e.g., ATP) to overcome gradient

Question 41

What are the three examples of passive transport processes?

Answer 41

• diffusion
• osmosis
• facilitated diffusion

Question 42

Which type of transport requires an investment of cellular energy such as ATP? Which type of transport is carried out by a pump?

Answer 42

active transport

Question 43

What is the Na+/K+ pump? Where within a cell do you find this pump? How many of each ion are pumped in each direction during a single pump cycle? For this pump to work (and most pumps for that matter) the protein carrier must change its shape (aka conformation). What must be added/removed to/from the protein to trigger this shape change?

Answer 43

The sodium-potassium (Na+/K+) (embedded in the plasma membrane) pump transports 3 Na+ out of a cell while transporting 2 K+ into the cell.

Phosphoryl-ate pump; it changes shape ejecting Na+ to the ECF. terminal phosphate group gon from atp attaches to pump giving off a little bit of chemical energy that changes shape so it pumps 3 sodium ions out)

Dephosphor-ylate pump; it returns to original shape pumping K+ inside cytosol. (phosphate group breaks off and the pump change back into original shape and allows 2 potassium’s inside cell)

Question 44

Neurons and muscle cells are especially effective at transforming resting membrane potentials into action potentials to do their work.
1. Which form of cellular transport occurs during the depolarization phase of an action potential?
2. Which form of cellular transport occurs during re-polarization of the membrane back to a resting membrane potential?

Answer 44

1. facilitated diffusion - depolarization to action potential (electrical impulse) (gates open sequentially); [ion] reach equilibrium
2. active diffusion - re-polarization: (gates closed); NA+/K+ pump re-establishes ion gradients; brings cell back in resting membrane potential

Question 45

What role does the Na+/K+ pump play during this process?

Answer 45

re-establishes ion gradients; cell in resting membrane potential

allows a wave of electrical activity “impulse” to move down length of the axon so the signaling system can be used over and over

Question 46

During which state, resting potential or action potential, are voltage-gates open? -closed?

Answer 46

action potential

Question 47

What is a proton pump?

Answer 47

active transport: membrane proteins that require beefy to shuttle H+ across a membrane

Question 48

How can proton pumps be used to indirectly pump other solutes across a membrane?

Answer 48

H+ gradients can be discharged through a co-transporter protein co-transporting other solutes.

a facilitated pump will move solute against gradient into cell and use proton as source of energy. proton pump will then use a source of energy from the cell that will pump the proton against the gradient back out of the cell.

Question 49

Which type of transport has occurred when H+ is pumped against its gradient?

Answer 49

active transport

Question 50

Which type of transport has occurred when H+ diffuses across a membrane through another co-transporter protein?

Answer 50

facilitated diffusion

Question 51

Where do you find ATP Synthase within a cell, what role does it play, and how is it energized to carry out its role?

Answer 51

Proton pumps are common in plants, fungi, and prokaryotes.

Proton pumps are also common in the mitochondria of all biota (including animals). H+ gradients are discharged through ATP Synthase to energize the recharging ADP ➟ ATP