1 - Membrane Transport

Question 1

What is the difference between passive and active transport?

Answer 1

Active transport requires energy (ATP), while passive transport doesn’t

Question 2

How does molecular size affect rate of diffusion

Answer 2

• ROD is inversely related to its molecular radius
• Small molecules bounce further following a collision.
• So, smaller the molecule, quicker the diffusion

Question 3

Describe simple diffusion via protein channels

Answer 3

• Small water-soluble substances are repelled by phospholipids, so they use protein channels to allow them to traverse the membrane
• Only small ions (H+, Na+, K+, Ca2+ )
• Ion selective: only permit the movement of one ion species
• May be gated

Question 4

Describe facilitated diffusion

Answer 4

• For substances too large/lipid soluble to pass through protein channels (glucose, some amino acids)
• Carrier protein with specific binding site is used
• Substance binds to carrier protein → protein undergoes conformational change → substance gets transferred
• Carrier protein is normally specific for particular substance, and sub. can move both directions depending on conc gradient

Question 5

Describe active transport

Answer 5

• membrane protein acts as a molecular pump to move substances across membrane
• E.g: Na2+, K+, Ca2+, sugars, amino acids
• High affinity binding site for particular substance on low conc side of membrane
• After the substance binds, protein undergoes a conformational change
• Goes to the opposite side of the membrane and is expelled
• Powered by ATP
• Many move more than 1 substance at a time

Question 6

What are the basic structural components of plasma membrane? How do they regulate movment of materials between ECF and ICF?

Answer 6

• Double layer of phospholipid molecules arranged parallel: hydrophobic tails means it can act as a barrier to prevent the movement of water-soluble substances across the membrane.
• Membrane Proteins: control the movement of substances (e.g: channels, receptors, pumps)

Question 7

What is diffusion? How does it occur?

Answer 7

• Tendency for substances to distribute themselves evenly in the environment by moving down their concentration gradient
• Occurs bc of random molecule motion, due to thermal energy of the system
• This motion causes particles to collide, ensuring that they become evenly distributed.

Question 8

How does distance effect ROD

Answer 8

greater the distance, slower the rate of diffusion

Question 9

How does cell size affect ROD?

Answer 9

• Bigger the cell, larger area of plasma membrane available for diffusion, therefore greater ROD

Question 10

ROD formula

Answer 10

Question 11

Describe exocytosis

Answer 11

• Macromolecules that are synthesized within the cell and stored in membrane-bound vesivles are secreted into extracellular space
  1. Migration of vesicle → plasma mem. where it fuses
  2. Plasma mem. adjacent to vesicle breaks down
  3. Contents are released into extracellular space

Question 12

What is the significance of the plasma membrane for physiological systems?

Answer 12

Acts as an effective barrier between ECF and ICF

Question 13

Define solvent, solute and aqueous solution

Answer 13

• Solvent: substance that dissolves the solute
• Solute: substance being dissolved
• Aq Soln: solute has been dissolved in water

Question 14

Define concentration gradient

Answer 14

Substances moving from an area of high concentration to low.

Question 15

How does conc gradient affect ROD?

Answer 15

1. Lots of molecules in high conc
2. Higher frequency of collisions
3. Progressive migration of molecules from high to low conc until molecules are evenly distributed

Question 16

Describe simple diffusion via lipid bilayer

Answer 16

• Substances with high lipid solubility (O2, CO2, N2, alcohols, fatty acids)
• Able to dissolve in phospholipids and diffuse directly through plasma membrane
• Higher lipid solubility, faster diffusion

Question 17

Describe an experiment that would allow you to determine whether a substance moved across the plasma membrane of a cell by simple or facilitated diffusion.

Answer 17

Saturation Kinetics
* Change solute conc outside of cell then measure ROD
* Simple: fairly linear relationship
* Facilitated: plateus because once all carrier proteins are occupied, there can be no more increase in ROD

Question 18

What is bulk transport?

Answer 18

Large substances (e.g: macromolecules) enter / leave cells enclosed in membrane-bound vesicles. Powered by ATP.

Question 19

Basic description of endocytosis

Answer 19

• Transfer of large particles, macromolecules and ECF into the cell
• A fold of the plasma membrane forming around substance to be ingested, forms a membrane bound vesicle
• 3 ways

Question 20

Hypotonic?

Answer 20

• Lower solute conc in ECF than ICF
• Water moves into cell
• Cell swells

Question 21

Hypertonic?

Answer 21

Higher solute conc in ECF than ICF
Water moves out of the cell down conc gradient
Cell shrinks

Question 22

What is an isotonic solution?

Answer 22

Soln with no net movement of water

Question 23

Osmolarity formula:

Answer 23

Molarity x no. molecules

Question 24

What is tonicity?

Answer 24

The ability of a solution to affect the fluid volume and pressure in a cell

Question 25

What is osmolarity?

Answer 25

• A measure of the no. of particles in a solution and reflects osmotic pressure more accurately
• This is because: osmotic pressure is more dependent on total no. rather than total weight
• Some solutes dissociate into their consituent ions when in soln, so contributes a larger number of solute particles than substances
• Provides us with a measure of osmotic effectiveness of a solution

Question 26

What is osmotic pressure?

Answer 26

Total conc of non-penetrating solutes (driving force for osmosis)

Question 27

Explain the difference between diffusion and osmosis

Answer 27

• Diffusion of solutes doesn’t change volume but osmosis does
• Membrane is impermeable so water moves down conc gradient by osmosis instead
• Therefore, increasing the volume

Question 28

What is osmosis?

Answer 28

• movement of water across plasma membrane
• mem is highly water permeable
• H2O molecules are small and have sufficient kinetic energy to pass straight through lipid bilayer and protein channels (despite hydrophobic core)
• Move down conc gradient

Question 29

Describe receptor-mediated endocytosis

Answer 29

• Very selective bc the membrane expresses extracellular receptors to which a particular macromolecule binds
• Then triggers endocytosis of the portion of mem containing receptor/macromolecule complex
• E.g: cholesterol, iron and insulin

Question 30

Describe pinocytosis

Answer 30

• Most cells in the body perform this
• Small volume of ECF being incorporated into a membrane-bound vesicle formed by small fold in plasma mem

Question 31

Describe phagocytosis

Answer 31

Few specialised cells (e.g: macrophages) can ingest large solid particles (e.g: cellular debris)
1. Extension of a portion of plasma mem around particle
2. Fusion of these extensions to form a particle-containing vesicle (phagosome)
3. Phagosome fuses with lysosome (contains digestive enzymes)
4. Enzymes break down the ingested debris and release produces into cell’s cytoplasm