Transport mechanisms I

Question 1

What is the difference between the capillary wall and the cell membrane/plasma membrane?

Answer 1

The capillary wall separates the interstitial fluid and the plasma, and it surrounds all the vessels in the body. The cell membrane/plasma membrane surrounds all the cells in our body and is not always the same depending on the cell.

Question 2

Are the following substances highly permeable, less permeable, or impermeable to the cell membrane?
H2O, larger molecules, very large molecules, O2, CO2, charged particles, small uncharged particles.

Answer 2

Highly permeable, less permeable, impermeable, highly permeable, highly permeable, less permeable, highly permeable

Question 3

Describe the structure of the cell membrane.

Answer 3

It consists of an amphipathic phospholipid bilayer, with polar, hydrophilic heads facing towards the extracellular and intracellular fluid and nonpolar, hydrophobic tails pointing inwards.

Question 4

What proportion of the plasma membrane’s weight do phospholipids comprise? Name 3 other components of it.

Answer 4

40-50%. It also consists of cholesterol, proteins, and glycocalyx.

Question 5

What is the role of cholesterol in the lipid bilayer at normal temperatures?

Answer 5

At cooler, more physiological temperatures, it orients itself to separate the lipid tails. By reducing interactions between lipid tails, it resists the packing of the tails and their aggregation. This keeps the membrane more fluid than it would be otherwise. It can also be involved in the formation of vesicles and lipid rafts.

Question 6

What is the role of cholesterol in the lipid bilayer at high temperatures?

Answer 6

It serves to stabilize the cell membrane and adds firmness.

Question 7

Is cholesterol hydrophobic, hydrophillic, or amphipathic?

Answer 7

It is slightly amphipathic.

Question 8

What types of proteins are found in the cell membrane? Explain where they are found.

Answer 8

Integral and peripheral proteins. Integral proteins span across the membrane (those that span all the way across are transmembrane proteins). Peripheral proteins are found mostly along the cytoplasmic side and are less strongly embedded.

Question 9

Describe the difference between integral and peripheral proteins in terms of a) polarity, b) ease of removal

Answer 9

Integral proteins are amphipathic and are hard to remove without disrupting the membrane. Peripheral proteins are polar and are easier to remove.

Question 10

What is the glycocalyx?

Answer 10

It is a layer of carbohydrate formed by a chain of monosaccharides that extrend from the extracellular surface of the cell membrane bound to proteins. It is also made of glycoproteins and glycoplipids.

Question 11

Name 3 functions of the glycocalyx.

Answer 11

It provides protection from infection, enables cells to identify each other, and is a site of interaction between cells.

Question 12

The model used to describe the structure of the cell membrane is called the _________.

Answer 12

Fluid mosaic model

Question 13

Name 6 functions of plasma membrane proteins.

Answer 13

Selective transport channel, enzyme function to catalyze membrane reactions, cell surface receptor for chemical signals, cell surface identity marker, cell adhesion, attachment to cytoskeleton.

Question 14

What are the three main forms of transmembrane transportation? Which ones involve proteins?

Answer 14

Via the phospholipid bilayer, via interaction with a protein channel, or via a protein carrier. The latter 2 involve a protein cluster.

Question 15

What are the two categories of transport mechanisms across the cell membrane? What is the difference between them?

Answer 15

Passive and active transport. Passive transport is energy independent, while active transport depends on an input of energy.

Question 16

3 types of passive transport

Answer 16

1. Diffusion
2. Carrier-mediated facilitated diffusion
3. Osmosis

Question 17

3 forms of active transport.

Answer 17

1. Carrier-mediated primary active transport
2. Carrier-mediated secondary active transport
3. Pino/phagocytosis

Question 18

What is simple diffusion?

Answer 18

It is the movement of molecules from one location to another as a result of random thermal motion.

Question 19

What is flux?

Answer 19

The amount of particles crossing a surface per unit time.

Question 20

What is net flux?

Answer 20

It is the movement of particles from high to low concentration.

Question 21

At equilibrium after diffusion, what is the value of flux and net flux?

Answer 21

Both are equal to zero because the net movement is equal in all directions.

Question 22

If you have two compartments of equal size separated by a membrane and then put 20 mml/l of solution into one of them, what will happen?

Answer 22

Over time, the concentration between both will even out such that they both have a concentration of 10 mmol/l.

Question 23

What does downhill mean?

Answer 23

It means movement from high concentration to low concentration (down the concentration gradient)

Question 24

In the case of diffusion into a single cell from the surrounding extracellular fluid compartment, how do their respective concentrations change?

Answer 24

The concentration of the cell rises until it is equal to the extracellular concentration. The extracellular volume is so huge that it makes no difference to its concentration.

Question 25

What law allows us to calculate the rate of diffusion?

Answer 25

Fick’s law

Question 26

What is the formula for Fick’s law? Define the variables.

Answer 26

J = PA(Co-Ci)
J: net flux: rate of diffusion
P: Permeability coefficient
A: Surface area of the membrane
Co-Ci: Concentration gradient of the diffusing molecule across the membrane

Question 27

To change the flux, what variable in the Fick’s law equation can we change?

Answer 27

The concentration gradient.

Question 28

Explain the relationship between diffusion time and distance.

Answer 28

Diffusion time increases in proportion to the square of the distance travelled by the solute molecules.

Question 29

What would happen if we were to move our capillaries further away from our cells? Why?

Answer 29

Diffusion from the capillaries into the cells wouldn’t be nearly as effective, because diffusion is only effective at very short distances.

Question 30

Name the 5 factors that affect diffusion across the cell membrane.

Answer 30

1. Mass of the molecule
2. Concentration gradient across the cell membrane
3. Lipid solubility
4. Electrical charge
5. Availability of selective ion channels or membrane carriers.

Question 31

Explain the difference between how nonpolar molecules and ions diffuse.

Answer 31

Non-polar molecules diffuse by dissolve in the lipid component of the membrane, while ions diffuse through channels.

Question 32

Describe the structure of ion channels.

Answer 32

They usually consist of clusters of proteins with transmembrane domains that also extend into the extra- and intracellular fluid. They cluster together to form a channel in the middle.

Question 33

Ion channels show selectivity based on _____ and ____.

Answer 33

Their diameter, the distribution of charges in the channel

Question 34

What is the electrochemical gradient?

Answer 34

It is the simultaneous existence of an electrical gradient caused by the membrane potential (extra negative charge in the cell) and by the concentration gradient of the molecule. These forces form an equilibrium that determines the direction of diffusion.

Question 35

What are the three types of ion channels?

Answer 35

Ligand-gated, voltage-gated, mechnically-gated.

Question 36

What is a ligand-gated channel?

Answer 36

It is a channel with a receptor to which a molecule binds and changes the shape of the channel.

Question 37

What is a voltage-gated ion channel?

Answer 37

The voltage across the cell membrane determines whether the channel will open or close.

Question 38

What is a mechanically-gated channel?

Answer 38

It is regulated by the stress on the membrane caused by stretching or pain.

Question 39

Ion channels can exist in ____ or ____ state as they undergo ______ changes.

Answer 39

Open, closed, conformational

Question 40

What are the 4 most popular ions that flow through voltage-gated ion channels?

Answer 40

Na+, K+, Ca+, Cl-