Diffusion Across Membranes

Question 1

What is meant by diffusion?

Answer 1

Movement of molecules from area of high concentration of molecule to area of low concentration, not involving metabolic energy (ATP)

Question 2

What is meant by facilitated diffusion?

Answer 2

Movement of molecules from area of high concentration of molecule to area of low concentration, across partially permeable membrane via protein channels/carriers, not involving metabolic energy (ATP)

Question 3

What is meant by a passive process in diffusion?

Answer 3

Substances passing avross a cell membrane without using cell’s metabolic energy as only kinetic energy is used

Question 4

How is adenosine formed?

Answer 4

ATP molecules breaking down after being active for a long time

Question 5

What is an effect of an increase adenosine concentration in the body?

Answer 5

Acts as signal to part of brain, triggering sleepiness

Question 6

What is meant by equilibrium in diffusion?

Answer 6

When molecules have moved down concentration gradient, they’re still moving randomly but remain evenly dispersed, so no net diffusion

Question 7

How can fat-soluble molecules diffuse through cell membranes?

Answer 7

Dissolve in lipid bilayer, still moving down concentration gradient

Question 8

How are water molecules able to significantly diffuse directly across cell membranes?

Answer 8

• Despite being polar and insoluble in lipid, phospholipid bilayer would seem to be impenetrable barrier, but present in such great concentrations
• In membranes where very high rate of water movement is required, may be aquaporins to allow water molecules to cross membrane without moving through lipid environment

Question 9

What are aquaporins?

Answer 9

Specific water channel proteins

Question 10

How is the concentration gradient maintained?

Answer 10

• Many molecules entering cells pass into organelles and used for metabolic reactions
• Oxygen diffusing into cytoplasm of respiring cells then diffuses into mitochondria and used for aerobic respiration
• Carbon dioxide diffusing into palisade mesophyll cells will then diffuse into chloroplasts and be used for photosynthesis

Question 11

How does temperature affect the rate of simple diffusion?

Answer 11

As temperature increases, molecules have more kinetic energy, so rate of diffusion will increase

Question 12

What does simple diffusion rely on?

Answer 12

Molecules’ own kinetic energy

Question 13

How does diffusion distance affect the rate of simple diffusion?

Answer 13

Thicker membrane where molecules have to diffuse, slower rate of diffusion

Question 14

How does surface area affect the rate of simple diffusion?

Answer 14

More diffusion can take place across larger surface area

Question 15

Wjat are microvilli?

Answer 15

Specialised cells for absorption with extensions to cell surface membranes

Question 16

How does the size of the diffusing molecule affect the rate of simple diffusion?

Answer 16

Smaller ions/molecules diffuse more rapidly than larger ones

Question 17

How does the concentration gradient affect the rate of simple diffusion?

Answer 17

Steeper gradient, faster diffusion to side where there are fewer molecules down gradient

Question 18

How does facilitated diffusion happen with small polar molecules?

Answer 18

Insoluble in lipid as they can’t interact with hydrophobic tails of lipid bilayer, meaning they diffuse through pores embedded in membrane, around 0.8 nm in diameter

Question 19

What are pores in a membrane?

Answer 19

Water-filled protein channels

Question 20

How does facilitated diffusion happen with cholesterol molecules within the membrane?

Answer 20

Reduce membrane permeability to small water-soluble molecules

Question 21

What can glucose molecules do instead of diffusing through aquaporins?

Answer 21

Can bind to transmembrane carrier protein, opening to allow glucose to pass out on other side of membrane

Question 22

What allows cells to control molecule types passing in and out of their membranes?

Answer 22

Different cell types have membranes with differing proportions of transmembrane protein channels and transmembrane protein carriers

Question 23

What are neurone plasma membrane channels specific to?

Answer 23

• Either sodium or potassium ions, diffusing into and out neurone axon is crucial for nerve impulse conduction
• At synapses, there are also calcium ion channels, may be chloride ion channels

Question 24

What are epithelial cell plasma membranes specific to?

Answer 24

Line airways with chloride ion channels and play crucial role in regulating composition of mucus to trap particles and pathogens