Exchange Across Cell Membrane - Diffusion

Question 1

What is an example of passive transport?

Answer 1

• Facilitated diffusion
• Simple diffusion

Question 2

What does ‘passive’ mean?

Answer 2

Energy comes from the natural, inbuilt motion of particles (kinetic energy)

rather than the external sources such as ATP

net movement of particles from high to low concentration

Question 3

Properties of diffusion particles

Answer 3

• All particles are constantly in motion due to the kinetic energy they possess
• Motion is random - no set pattern in which the particles move around
• Particles are constantly bouncing off one another as well as objects e.g side of vessel they are contained

Question 4

What is the definition of diffusion?

Answer 4

Net movement of molecules/ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed

Question 5

What molecules can diffuse straight through the cell membrane?

(simple diffusion)

Answer 5

Small and non-polar molecules (makes them soluble in lipids - dissolve in hydrophobic bilayer)

e.g oxygen

Question 6

Why do charged ions and polar molecules do not diffuse easily?

Answer 6

Hydrophobic nature of the fatty acid tails of the phospholipids in the membrane

Question 7

What factors affect facilitated diffusion?

Answer 7

• Concentration gradient
• Number of channel or carrier proteins

Question 8

The concentration gradient is one of these factors – how does the concentration gradient affect the rate of diffusion?

(e.g Ventilation of air in lungs)

Answer 8

The steeper the concentration gradient the higher the rate of diffusion.

Question 9

How does the concentration gradient affect diffusion?

Answer 9

• The higher it is the faster the rate of diffusion
• As diffusion takes place the difference between two sides decreases until it reaches an equilibrium (both sides are equal)
• Diffusion slows down over time

Question 10

How does surface area affect the rate of diffusion?

(Microvilli , Alveoli , cristae , thylakoids,

Answer 10

Larger surface area the faster rate of diffusion

Question 11

How does concentration gradient affect facilitated diffusion?

Answer 11

Higher concentration gradient , faster rate

As equilibrium reached , rate of diffusion will level off

Question 12

How does thickness of the exchange surface affect the rate of diffusion?

Answer 12

The thinner the exchange surface (i.e shorter distance the particles have to travel) the faster the rate of diffusion

Question 13

How does microvilli increase the rate of diffusion?

Answer 13

Some cells e.g epithelial cells have microvilli (projections in cell surface membrane folding up on itself)

It gives cell a large surface area - increase rate of diffusion

Question 14

What is facilitated diffusion?

Answer 14

The passive movement of molecules crosses the cell membrane in a specific transmembrane protein

Question 15

What molecules are used for facilitated diffusion?

Answer 15

• Charged ions
• Polar molecules

Question 16

Where does diffusion occur with concentration gradient?

Answer 16

Down a concentration gradient

Question 17

What two proteins are involved in facilitated diffusion?

Answer 17

• Carrier proteins
• Channel proteins

Question 18

What do channel proteins form?

Answer 18

Water-filled hydrophobic channels across the membrane

Question 19

What type of substances do channel proteins pass through?

Answer 19

Water-soluble susbtances

small charged or polar molecules e.g ions Na + , K + , Ca²⁺

Question 20

How are channel proteins selective in the opening of the presence of a specific ion?

Answer 20

If the particular ion is not present , channel remains closed - control entry and exit of ions

Question 21

How do carrier proteins transport large molecules across cell membrane?

Answer 21

Large molecule bind with the protein causing it to change shape

Releases molecule on the opposite side of the membrane

Question 22

What type of molecules are used with carrier proteins?

Answer 22

e.g glucose specific to carrier protein

Causes to change shape and molecule released to the inside of the membrane

Question 23

What factors does simple diffusion depend on?

Answer 23

• Concentration gradient
• Thickness of the exchange surface
• Surface area
• Microvilli - increase diffusion

Question 24

With reference to the key factors that affect the rate of diffusion discussed in question one, discuss how the alveoli is adapted for efficient gas exchange.

Answer 24

– Short diffusion pathway

– each alveolus is surrounded by a dense capillary network.

-Steep concentration gradient

– dense capillary network means that blood is rapidly transported away, maintaining the concentration gradient.

• Large surface area

– alveolar walls provide a large SA/ large SA: Volume ratio

Question 25

People who suffer from emphysema, a disease usually caused by smoking or exposure to air pollution, have a greatly reduced amount of elastin in their alveolar walls. Why do people who suffer from emphysema often suffer from an increased rate of breathing?

Answer 25

– Destroyed elastin reduces the surface area of the alveoli - Destroyed elastin reduces efficiency of ventilation (higher residual volume in lungs) - gas exchange is less efficient - Less oxygen is transported into the blood and to the respiring tissues. - Increased breathing rate is required to provide sufficient oxygen

Question 26

Why would ions diffuse slowly in the cell membrane?

Answer 26

Ions and polar molecules are water soluble centre of bilayer is hydrophobic

Question 27

What is carrier proteins used?

Answer 27

Move large molecules down their concentration gradient different carrier proteins facilitate the diffusion of different molecules

Question 28

Why would channel proteins be moved?

Answer 28

Facilitate the diffusion of charged particles

Question 29

Why increasing channel proteins help to increase the rate of facilitated diffusion in kidney cells?

Answer 29

* Aquaporins in a special type of channel protein allow facilitated diffusion of water through the cell membrane * Some kidney cells are adapted to have lots of aquaporins - allow to reabsorb a lot of water that would otherwise excreted by body

Question 30

How does a number of channel or carrier proteins affect facilitated diffusion?

Answer 30

Once all of proteins in use , faciliated diffusion can't happen faster even if you increase concentration gradient Increase: faster rate

Question 31

Diffusion of oxygen:

Answer 31

Simple diffusion through the cell membrane Diffuse straight through Random movement of oxygen molecules down the concentration gradient

Question 32

Facilitated diffusion of glucose

Answer 32

Using carrier proteins Attach to protein in membrane causing it change shape Release towards opposite side of membrane

Question 33

Describe the difference between facilitated diffusion and active transport

Answer 33

* Facilitated - use channel and carrier * Active - only use carrier * Active transport - Active process - requires ATP to move substances against the concentration gradient * Facilitated diffusion requires no ATP - 'passive' requires own kinetic energy for it

Question 34

Give two differences between facilitated diffusion and osmosis

Answer 34

* Facilitated diffusion requires the use of channel or carrier proteins * Facilitated diffusion moves larger molecules compared with osmosis with molecules diffusing through the partially permeable membrane

Question 35

Explain why molecules of oxygen and carbon dioxide is able to diffuse through the membranes?

Answer 35

Small and non-polar Use simple diffusion to diffuse through phospholipid bilayer

Question 36

Explain why ventilation of the lungs increase the efficiency of gas exchange

Answer 36

Alveoli causing the greater concentration of diffusion between alveoli and blood capillaries

Question 37

One similarity between active transport and facilitated diffusion

Answer 37

Involve transport proteins

Question 38

Give one evidence why it is entering the cell by facilitated diffusion and not diffusion

Answer 38

Concentration levels off above external concentration

Question 39

Give two functions of proteins in plasma membrane

Answer 39

* Channel protein : Is able to transport ion/water by facilitated diffusion * Carrier protein helps to transport two molecules

Question 40

Exam Questions Ventilation and Circulation of air and blood

Answer 40

* Maintain a high concentration gradient * Alvelo and capillary have thin walls ('one cell thick) * Reduces diffusion distance * Alveoli - (folded) Large Surface Area

Question 41

What do both protein channel and carrier proteins have in common?

Answer 41

* High to low concentration * Specific - each molecule has its own transport protein * Same protein can be used for active transport (against the concentration gradient using ATP)