Lecture 3: Membrane Transport

Question 1

Composition of the Cell Fluids… inside o outside

Answer 1

INSIDE TO OUTSIDE

1. Intracellular fluid
2. Cell Membrane
3. EXTRA CELLULAR FLUID
   — 4. INTERSTITIAL FLUID = is the liquid outside cells
   — 5. Capillary Wall
   — 6. Plasma

Question 2

Body Fluid Compositions = 3

Answer 2

1 * Humans 55% water

2 * 1/3 Total body water outside cells- extracellular fluid

3 * 2/3 inside cells- intracellular fluid

Question 3

Body Fluids - minerals, lipids and proteins amounts in it.

Answer 3

Element. Inside Outside
Na+. 10 mM 140mM
K+. 140 mM. 5 mM
Cl- 5 mM. 110 mM
HCO3- 10 mM 28 mM
Proteins 16 g/L 1 g/L

Question 4

What is Inside and Outside the PhosphoLipid Bilayer of cells? = 3

Answer 4

1 * Lipid bilayer divides intra- and extracellular spaces

2 * Many large organic anions inside cells

3 * Sodium Chloride outside Potassium inside

Question 5

Purpose of the Phospholipid bilayer? What can is trapped and what can’t move?

Answer 5

1 * Lipid bilayer divides intra- and extracellular spaces

2 * LAREG OR CHARGED SOLUTES are TRAPPED by the lipid layer

3 * ATP, Glucose-6-Phosphate, Acetyl-CoA, Proteins, etc are all charged and CANNOT DIFFUSE OUT.

Question 6

Purpose of the Membrane Transport: Transmembrane Proteins. (2)
Example?

Answer 6

1 * Transmembrane proteins allow movement across lipid layer

2 * Controls permeability to ions, molecules etc

example = Glucose uptake controlled by insulin

Question 7

Membrane Transport: Types of ways materials pass through the membrane? 5

Answer 7

1 * DIFFUSE ACROSS LIPID LAYER

2 * DIFFUSE through a CHANNEL/PORE

3 * DIFFUSION USING A CARRIER

4 * ACTIVE TRANSPORT using a CARRIER

5 * Endocytosis/Exocytosis

Question 8

Membrane transport: types - regulation, active, passive, energy use?, regulated? selective? 4

Answer 8

1 * Active transport and endocytosis/exocytosis use CELLULAR ENERGY FROM ATP.

2 * Diffusion through the lipid, a channel/pore or carrier protein uses ENERGY FROM DIFFUSION

3 * Carrier mediated transport can be ACTIVE OR PASSIVE , it requires BINDING OF MOLECULE TO THE CARRIER

4 * Channels, carriers and endocytosis/exocytosis are SELECTIVE AND CAN BE REGULATED.

Question 9

Understanding VESICLE TRANSPORT: process, pinocytosis, phagocytosis. when used? 4

Answer 9

1 * Endocytosis
- Pinocytosis: smaller stuff
- Phagocytosis: bigger stuff

2 * Binds to a RECEPTOR PROTEIN on CELL SURFACE

3 * Binding trigger invagination into CLATHRIN coated pit

4 * ACTIN AND MYOSIN pinch off a vesicle

Question 10

When does Exocytosis occur?

Answer 10

• Exocytosis of neurotransmitters from nerves
• Exocytosis of hormones from endocrine cells

Question 11

Explain Pinocytosis of IRON

Answer 11

1 *Iron transported in blood bound TO PROTEIN-TRANSFERRIN

2 *Transferrin binds to RECEPTOR on cell surface.

3 * Transferrin bound iron endocytose into cell.

Question 12

PINOCYTOSIS VS PHAGOCUTOCIS

Answer 12

1. — Pinocytosis primarily refers to the uptake of extracellular fluids and small molecules by a cell
   —pinocytosis is the ingestion of SURROUNDING FLUIDS
2. — phagocytosis is a process by which the cell membrane invaginate around large macromolecular structures (e.g., proteins and viruses) that are otherwise unable to diffuse into the cell.
   **phagocytosis involves the ingestion of solid material

** Phagocytosis of BACTERIA BY MACROPHAGES

Question 13

Vesicle transport and ACTIVE transport —

Uses energy or not?

Answer 13

Vesicle transport and active transport use ATP and can transport AGAINST A CONCENTRATION GRADIENT.

Question 14

Overall Summary of Vesicle Transport:

Answer 14

• Exocytosis of neurotransmitters from nerves
• Exocytosis of hormones from endocrine cells
• Pinocytosis of iron:
• Iron transported in blood bound to protein-transferrin - Transferrin binds to receptor on cell surface.
• Transferrin bound iron endocytosed into cell.
• Phagocytosis of bacteria by macrophages
• Vesical transport and active transport use ATP and can transport against a concentration gradient.

Question 15

Understanding Channels; what can pass through, selective, what is it? ungated vs gated

6

Answer 15

1 * Small NONPOLAR molecules can DIFFUSE through lipid eg O2, CO2.

2 * Small POLAR MOLECULES eg Na+, HCO3- use PROTEIN CHANNELS

3 * Channels are TRANSMEMBRANE PROTEINS that form a WATER FILLED PORE.

4 * Channels are often SELECTIVE,
EG. sodium channels do not allow potassium through

5 * Channels are often “GATED”
- gated channle fall in 3 classes
( PORES= are UNGATED channels)

6 * Gate turns ON AND OFF TRANSPORT through channel

Question 16

Gated channel fall in 3 classes.

What are they?

Answer 16

1. Voltage,
2. Ligand,
3. Stretch/mechanical.

Question 17

Characteristics of the 3 classes of Gated Channels:

Answer 17

1. Voltage gated channels:
   * in nerves and muscle cells.
   * Depolarisation of heart opens calcium channels.
   * Calcium entry triggers contraction
2. Ligand gated channels :
   *Neurotransmitters open
   * Nicotinic receptor for Acetylcholine.
3. Mechanically gated channels (stretch gated)
   * Touch and hearing rely

Question 18

Gap Junctions: what, role, found, structure? Connexon? 5

Answer 18

1 * Gap junctions are pores between cells.

2 * Allow small polar molecules to diffuse directly between cells.

3 * Common in Heart, gastric smooth muscle.

4 * Formed from transmembrane protein Connexon

5 * Conenexon binds connexon of next cell forming tube.

Question 19

Summary of Channels:

Selective?, Permeable?, Channel Usage? Aquaporins? Water movement? Osmosis?
7

Answer 19

1 * Cell membranes are selectively permeable.

2 * Selectivity governed by transmembrane proteins that
form channels/pores.

3 * Permeability can be changed by opening or closing channels.

4 * Lipid soluble small molecules don’t need or use channels.

5 * Almost all cells have aquaporins water channels.

6 * Water can diffuse into or out of cells

7 * Osmosis is the diffusion of water

Question 20

What is an AQUAPORIN?

Answer 20

Aquaporin: A water channel.

Aquaporins form pores in the membranes of cells and selectively conduct water molecules through the membrane, while preventing the passage of ions (such as sodium and potassium) and other small molecules.

Question 21

Understanding OSMOSIS: 5

Answer 21

1 * Solutes dilute the water in solution

2 * About 56 mole of water per Kg

3 * Molality of pure water ~56N.

4 * Mole/L of solution molarity (M), mole/Kg of solvent molality (N)

5 * Difference small for dilute solutions.

Question 22

What is Osmolarity?

Answer 22

Osmolarity is inverse of water concentration

Question 23

Osmosis: Understanding Osmoles

Answer 23

• 1 Osmole is one mole of osmotically active stuff.
• 1 mole glucose = 1 mole glucose i.e. 1 Osmole
• 1 mole NaCl = 1 mole Na+ + 1 mole Cl- i.e. 2 Osmoles
• 1 mole MgCl2 = 1 mole Mg2+ + 2 moles Cl- i.e. 3 Osmoles
• 1 mole Al(OH)3 = ??? Depends on how much it disassociates
• 1 Osmole /L = 1 osmolar = 1 Osmol/L

Question 24

Understanding osmolarity: 3

Answer 24

1 * Osmolarity of proteins and macromolecules hard to predict
depends on disassociation.

2 * Proteins that bind ions can have negative osmolarity

3 * Osmolarity of intracellular fluid ~300 mOsmol/L

Question 25

What is Osmotic Pressure? How does it work? 4

Answer 25

1. Osmotic pressure is defined as the pressure that must be applied to the solution side to stop fluid movement when a semipermeable membrane separates a solution from pure water.

2 * Selectively permeable membrane stops solute moving out lets water in

3 * Osmotic difference drives water in

4 * Pressure increases until pressure drives water back out

Question 26

Osmotic Pressure at Equilibrium? formula? 4

Answer 26

• Osmotice pressure is given by: pie =RT * change in C

1. At equilibrium pressure difference equal and opposite to osmotic drive.
2. Osmotic Pressure is the pressure needed to stop osmosis happening
3. • Assumes perfectly permeable to water and completely impermeable to solutes.

4 * 1 mOsmol/L ~ 19.3 mmHg

Question 27

Cells Osmolarity?

Hyper-osmotic vs Hypo -osmotic

Hypotonic?

isomotic? Isotonic?

Answer 27

1 * Cells osmolarity ~ 300 mOsmole/L

2 * Hyperosmotic cells shrink - hypertonic

3 * Hyposmotic cells swell- hypotonic Lysis is burst cells

4 * 150 mM NaCl is isomotic – isotonic no change in volume

Question 28

Osmosis: Tonicity

Answer 28

Tonicity is a change in cell volume by osmosis.

Question 29

Osmosis: Isotonic solutions…

Answer 29

Isotonic solutions do not cause osmotic pressures on cells.

Question 30

• Isosmotic is not isotonic if ….

Answer 30

• Isosmotic is not isotonic if the solute can move

Isosmotic solutions are not isotonic if the solute can enter cells

Question 31

Diffusion of urea into cells adds to the internal osmolarity…4

Answer 31

1 * Diffusion of urea into cells adds to the internal osmolarity

2 * Water follows the urea

3 * In general water will follow salt and solute movements

4 * If the solute is impermeable isosmotic is isotonic

Question 32

Osmosis of Materials:

Answer 32

1 * A- here are large anions, proteins, ATP, organic phosphate

2 * Anions in cells cannot get out

3 * Diffusion gradients for Na+ & Cl- in and K+ out

4 * If sodium moved in water would follow and swell the cell

5 * Cells are permeable to potassium what stops the cell shrinking?

Question 33

Summary of this Lecture: 9

Answer 33

1 * Lipid membrane separates extra from intra- cellular fluid

2 * Most macromolecules and cell component cannot get out.

3 * Diffusion across cell membrane through lipid, through channel/pore or using a carrier.

4 * Active transport by carriers and vesicle transport use ATP.

5 * Channels are selective and can be gated.

6 * Water diffuses by osmosis

7 * Osmotic pressure are very large for small concentration changes

8 * Isosmotic solutions have the same osmolarity

9 * Isosmotic solutions are not isotonic if the solute can enter cells

Question 34

Isosmotic solutions osmolarity?

Answer 34

Isosmotic solutions have the same osmolarity