Movement Across Membranes

Question 1

how can materials cross the cell membrane?

Answer 1

1. by moving between phospholipids

2. by moving through/being pumped by transmembrane proteins

Question 2

what kinds of material do phospholipids allow to move between them? give example

Answer 2

• small, nonpolar (hydrophobic, non charged) molecules

- eg. CO2, O2

Question 3

what kinds of materials do transport proteins allow to move across the membrane? give example

Answer 3

• allow ions, polar molecules, and/or larger molecules to cross
• e.g. water and glucose, amino acids, potassium ions

Question 4

what are the 2 types of transport proteins? are transport proteins general or specific?

Answer 4

1. Channel proteins
2. Carrier proteins
   - Transport proteins are specific for the solute they transport

Question 5

how do channel proteins work?

Answer 5

they form a hydrophilic channel through which ions and polar molecules can move across the membrane
-ie. they give a molecule a channel to flow through

Question 6

how do carrier proteins work?

Answer 6

they bind molecules and undergo a conformational change that moves the molecule across the membrane
-ie. they “carry” a molecule through

Question 7

what are the different ways a substance can move across a cell membrane?

Answer 7

1. diffusion –> simple or facilitated
2. osmosis
3. active transport

Question 8

what is diffusion? does it require energy?

Answer 8

Movement of a molecule from an area of higher solute concentration to an area of lower solute concentration
-does not require energy

Question 9

what is the difference between simple diffusion and facilitated diffusion?

Answer 9

in simple diffusion molecules simply diffuse between phospholipids, whereas in facilitated diffusion molecules use transport proteins to move through phospholipids.

• simple diffusion moves small, non polar molecules
• facilitated diffusion moves large/charged molecules/ions

Question 10

what are the similarities between simple diffusion and facilitated diffusion?

Answer 10

• neither require energy

- both move from area of high solute concentration to area of lower solute concentration

Question 11

what is osmosis? which direction does the material move? is energy required? does it require a transport protein?

Answer 11

-Movement of water across a selectively permeable
membrane
-Water moves from an area of lower [solute] to an
area of higher [solute]
-no energy required
-requires Channel protein - aquaporin - required because water is polar

Question 12

what is active transport?is energy required? does it require a transport protein?

Answer 12

-Movement of a molecule across a membrane from an
area of lower solute concentration to an area of higher solute concentration
-“building of a gradient”
-energy is required to pump
-transport protein required to pump

Question 13

how are very large molecule transported across the cell membrane? what is it when they enter? what is it when this leave?

Answer 13

by being packaged in vesicles
Enter the cell by endocytosis
Leave the cell by exocytosis

Question 14

what is endocytosis?

Answer 14

cell takes in molecules by forming a vesicle from the plasma membrane

Question 15

what is exocytosis?

Answer 15

cell releases molecules by the fusion of vesicles with the plasma membrane
-when vesicle and plasma membrane fuse, the contents of the vesicle are released outside of the cell
-Mechanism used by cells to release substances
they have produced (e.g. hormones)

Question 16

what is membrane potential? which way do ions want to move/why?

Answer 16

• the difference in electrical charge across a cell’s plasma membrane due to the differential distribution of ions
• affects the transmembrane movement of all charged substances
• Because the inside of the cell is negative compared with the outside, the membrane potential favours the passive transport of cations (positively charged ion) into the cell and anions (negatively charged ion) out of the cell

Question 17

what is an electrochemical gradient?

Answer 17

these 2 forces drive the diffusion of a molecule across
the membrane

• The diffusion gradient of an ion, which is affected by both:
• the concentration difference of an ion across a membrane (a chemical force)
• and the ion’s tendency to move relative to the membrane potential (an electrical force) (difference in electrical charge across membrane)

Question 18

what is an electrogenic pump?

Answer 18

transport protein that generates an electrochemical gradient across a membrane

• eg Sucrose-H+ cotransporter and proton pump
• An active transport protein that generates voltage across a membrane while pumping ions.

Question 19

what can electrogenic pumps store?

Answer 19

Electrogenic pumps store energy that can be used

to do work in the cell

Question 20

all cells have a separation of opposite charges –> The cytoplasmic side of the membrane is … in charge relative to the extracellular side because of …

Answer 20

• negative

- an unequal distribution of anions and cations on the two sides.

Question 21

how do ions move through passive transport?

Answer 21

An ion diffuses not simply down its concentration gradient but, more exactly, down its electrochemical gradient.