2.3, 2.4 Transport Across Membranes

Question 1

Describe osmosis in terms of concentration of free water molecules:

Answer 1

osmosis is the net movement of water molecules from a region of higher concentration of free water molecules to a region of lower concentration of free water molecules, through a partially permeable membrane, until equilibrium is reached (both sides are equally concentrated)

Question 2

Describe osmosis in terms of concentration of solute:

Answer 2

osmosis is the net movement of water molecules from a solution with a lower concentration of solute to a solution with a higher concentration of solute, through a partially permeable membrane, until equilibrium is reached (both sides are equally concentrated)

Question 3

What is meant by net movement in osmosis?

Answer 3

free water molecules diffuse both ways through the partially permeable membrane, but the net movement is the overall direction of movement (from an area of high to an area of low concentration of free water molecules)

Question 4

Why does osmosis occur?

Answer 4

• the side with less solute particles will have more free water molecules, as less water molecules will form hydrogen bonds with the solute (there is a greater concentration of free water molecules)
• therefore, due to random movement, more free water molecules are able to collide and move across the membrane to the side with a lower concentration of free water molecules
• when both sides are equally concentrated, the movement of water molecules through the membrane on each side will be the same (there is no net movement of water molecules) and so equilibrium has been reached

Question 5

Why is the partially permeable membrane important in osmosis?

Answer 5

• osmosis is the net movement of water molecules across a partially permeable membrane
• if there was no membrane, there would be diffusion of both water and solute
• the presence of the partially permeable membrane prevents solutes from crossing the membrane, so only free water molecules can pass through
• therefore this type of diffusion is specialised, as only water molecules can cross the membrane

Question 6

What is meant by passive transport?

Answer 6

the movement/transport of substances does not require energy, as the process is driven by the concentration gradient alone

Question 7

What types of transport are passive transport?

Answer 7

• osmosis
• diffusion
• facilitated diffusion

Question 8

What is diffusion?

Answer 8

the net movement of particles from an area of high concentration to an area of low concentration, down the concentration gradient, until equilibrium has been reached (until the conc. of the particles is equal throughout the system and there is no net movement of the particles)

Question 9

What is facilitated diffusion?

Answer 9

the process where hydrophilic (polar) molecules/ions larger than CO2 and larger molecules (like glucose) are assisted across the cell membrane by carrier/channel proteins
(particles move down a concentration gradient, from an area of high to low concentration, like diffusion)

Question 10

Why can’t hydrophilic molecules/ions larger than CO2 diffuse across the cell membrane without help?

Answer 10

• they are insoluble in lipids, so hydrophobic tails of the phospholipid bilayer ensure they cannot diffuse through

Question 11

Even though facilitated diffusion requires proteins, it is still a passive process. True of false?

Answer 11

true, it does not require energy even if it requires the aid of proteins

Question 12

What are channel proteins?

Answer 12

• they span the membrane
• they form water-filled pores that allow polar molecules/ions to diffuse through, down the concentration gradient (as they are only soluble in water and not the lipids of the bilayer)
• different channel proteins facilitate the diffusion of different ions/molecules

Question 13

Why do different channel proteins facilitate the diffusion of different ions/molecules?

Answer 13

each type of channel protein has a specific shape that only permits the passage of only one particular type of ion/molecule

Question 14

What are gated channels (in channel proteins)?

Answer 14

channels that open and close depending on the presence of absence of a signal (the signal could be a specific molecule like a hormone, or it could be a change in potential difference across the membrane)

Question 15

What are carrier proteins (facilitated diffusion)?

Answer 15

• they have a specific site where an ion/molecule binds to
• this causes them to change shape, releasing the ion/molecule on the opposite side of the membrane
• the net movement of the ions/molecules is down their concentration gradient
• different carrier proteins facilitate the diffusion of different ions/molecules

Question 16

Why do different carrier proteins facilitate the diffusion of different ions/molecules (facilitated diffusion)?

Answer 16

only a specific ion/molecule will bind to the specific site on a carrier protein, allowing it to be transported across

Question 17

Why is the net movement of ions/molecules assisted by carrier proteins down the concentration gradient (facilitated diffusion)?

Answer 17

• there will be more frequent binding to the carrier protein on the side with a higher concentration of the specific ion/molecule (as there is more of the ion/molecule)
• so the overall movement of the ion/molecule is from an area of high to an area of low concentration of the ion/molecule, down a concentration gradient

Question 18

What is active transport?

Answer 18

• the movement of molecules/ions from a region of low concentration to a region of high concentration, against the concentration gradient
• the process involves carrier proteins

Question 19

Why is active transport called active?

Answer 19

it requires energy, whereas passive transport doesn’t

Question 20

Describe the process of active transport:

Answer 20

• an ion/molecule binds to the carrier protein
• a phosphate group is removed from ATP, as ATP is hydrolysed (broken down) to form ADP
• the removed phosphate group becomes hydrated, and energy is released from the bonds formed between water and the phosphate
• the energy released changes the shape of the carrier protein, allowing the ion/molecule to be transported across the membrane, moving it against the concentration gradient

Question 21

What is ATP and what is ADP?

Answer 21

• stands for adenosine triphosphate
• it acts as the immediate source of energy for all biological processes in cells
• it is hydrolysed to form ADP - adenosine diphosphate (one phosphate group is lost)

Question 22

How is ATP formed?

Answer 22

by respiration, during the breakdown of energy storage molecules such as carbohydrates and fats

Question 23

What is bulk transport?

Answer 23

moving very large molecules or a very large quantities of a particular molecule through the cell membrane

Question 24

What are vesicles?

Answer 24

small, membrane bound sacs used to transport substances

Question 25

What is endocytosis?

Answer 25

- used for bulk transport of substances too large to be taken into a cell by carrier proteins (e.g. proteins, lipids) - a section of the cell membrane surrounds the substances to be transported - the cell membrane pinches off to form vesicles carrying the ingested substances, transporting the substances into the cell

Question 26

Other than molecules, what do some cells use endocytosis for?

Answer 26

cells such as phagocytes (a type of white blood cell) take in and ingest microorganisms and dead cells by endocytosis to destroy them

Question 27

What is exocytosis?

Answer 27

- used for bulk transport of substances produced by the cell that need to be released from the cell - vesicles (pinched off the sacs of the Golgi apparatus) carrying the substances fuse with the cell surface membrane, releasing their contents outside the cell

Question 28

Other than to secrete molecules out of the cell, what is exocytosis used for?

Answer 28

to transport substances (like integral proteins) that need to be inserted into the cell membrane

Question 29

What are some examples of substances transported out of the cell by exocytosis?

Answer 29

- digestive enzymes - hormones (such as insulin) - neurotransmitters (released from nerve endings) - lipids

Question 30

Is exocytosis and endocytosis active or passive?

Answer 30

active, they both require ATP as an energy source

Question 31

In summary, what substances do each type of transport transport across the cell membrane?

Answer 31

- osmosis: free water molecules - diffusion: small nonpolar molecules such as O2 and CO2, hydrophobic molecules, and small polar molecules such as H2O (but thats osmosis) - facilitated diffusion: hydrophilic molecules and ions larger than CO2 - endocytosis: substances that need to be taken into the cell by bulk transport (e.g. cholesterol) - exocytosis: substances that need to be secreted outside the cell (e.g. digestive enzymes) or substances that need to be inserted into the cell membrane