Lecture 17: Transport

Question 1

The necessity for transport in eukaryotic cells

Answer 1

• Metabolism needs fuel, produces waste products.
• Transport between cytosol and organelles.
• Proteins secreted via the secretory system.
• Signalling within and between cells.
• Proteins targeted to different organelles.

Question 2

Physical communication between animal cells:

Answer 2

Gap junctions

Question 3

Gap junctions:

Answer 3

• exchange of metabolites and signalling between cells.
• channel proteins: connexins in vertebrates, innexins in invertebrates.
• rapid exchange of ions and metabolites < ~1 kDa.
• ~ 20 connexins in human genome, mutations lead to muscular dystrophy, deafness, skin diseases, cataracts.

Question 4

connexins vs innexins

Answer 4

connexin in vertebrates

innexin in invertebrates

Question 5

Physical communication between plant cells:

Answer 5

Plasmodesmata

Question 6

Plasmodesmata :

Answer 6

• Exclusion limit (~1 KDa), but can dilate to allow passage of molecules >20 kDa, (proteins and RNA)
• Enable cytoplasmically interconnected fields of cells known as symplasm

Question 7

Membrane structure:

Answer 7

Phospholipid bilayer:

• phospholipid molecules (hydrophobic tails, hydrophilic heads)
• integral proteins
• carbohydrates
• surface proteins
• glycoprotein
• glycolipid
• peripheral protein
• cholesterol
• protein channel (transport protein)

Question 8

The lipid bilayer :

—WHO

Answer 8

Gorter + Grendel 1925.
noticed must be a ‘bi’ layer because surface area occupied by lipids of erythrocytes was double that of total surface area of erythrocytes.

Question 9

Gorter + grendel how did they test:

Answer 9

erythrocytes –> extraction of membrane lipid in solvent –> lipid film on water

Question 10

Inherent transport properties of lipid membranes: Hydrophobic molecules

Answer 10

O2, CO2, N2, steroid hormones.

All pass through lipid bilayer.

Question 11

Inherent transport properties of lipid membranes: Small uncharged polar molecules

Answer 11

H2O, urea, glycerol

Some passes through, more rejected to pass through lipid bilayer

Question 12

Inherent transport properties of lipid membranes: Large uncharged polar molecules

Answer 12

glucose, sucrose

little passes through, majority rejected

Question 13

Inherent transport properties of lipid membranes: ions

Answer 13

e.g. H+, Na+, HCO-3, K+, Ca2+, Cl-, Mg2+

ALL REJECTED

Question 14

Time taken for 1 ml water to pass through 1 cm2 of membrane under 1 atm pressure: Red blood cell

Answer 14

2 days

Question 15

Time taken for 1 ml water to pass through 1 cm2 of membrane under 1 atm pressure: Zoothamnium (a colonial protist)

Answer 15

6 weeks

Question 16

Time taken for 1 ml water to pass through 1 cm2 of membrane under 1 atm pressure: Amoeba

Answer 16

9 months

Question 17

Aquaporins

Answer 17

membrane channels for transport of water, glycerol CO2 etc

Question 18

Aquaporins what do they carry out:

Answer 18

Facilitate diffusion of small uncharged molecules (water, glycerol, urea, CO2)

Question 19

occurrence of aquaporins:

Answer 19

correlates with high water fluxes (e.g. kidney, eye)

Question 20

how man aquaporins genes in man and in Arabidopsis

Answer 20

10 genes in man

30 genes in Arabidopsis

Question 21

What can aquaporins activity be regulated by/

Answer 21

by PTMs (phosphorylation)

Question 22

Passive transport:

Answer 22

of an ion or molecule facilitated by favourable concentration gradient.

Question 23

Active Transport::

Answer 23

Active transport against a concentration gradient requires energy.

Question 24

Passive transport uses which channels?

Answer 24

channel protein and transporter protein

Question 25

active transport uses

Answer 25

just transporter proteins & requires energy

Question 26

Passive and active transport:

Answer 26

Transport of a charged ion or molecule is facilitated by a favourable membrane potential. Transport against an unfavourable membrane potential requires energy.

Question 27

Three ways of driving active transport:

Answer 27

- coupled transporter - ATP-driven pump - Light-driven pump All go against electrochemical gradient

Question 28

ATP-driven membrane transport in cells: P-type pump

Answer 28

P-ATPases: transport specific ions e.g. H+, Na+, K+, Mg2+, Ca2+. One or two polypeptides (< than rotary ATPases).

Question 29

ATP-driven membrane transport in cells: H+-PPases

Answer 29

In plants (not animals and fungi) H+-PPAses also couple PPi hydrolysis to H+ movement ATP --> ADP + Pi Pi used in pump

Question 30

ATP-driven membrane transport in cells: ATP-Binding Cassette transporters

Answer 30

(ABC) transporters. Substrates include lipids and sterols, ions and small molecules, drugs and large polypeptides. Cystic fibrosis transmembrane conductance regulator (CFTR) is an ABC Cl‐ transporter. ATP --> Pi + ADP

Question 31

ATP-driven membrane transport in cells: Rotary ATPases family of membrane protein complexes

Answer 31

(F‐ATPase, V‐ATPase and A‐ATPase) are H+ pumps with role in energy conversion. Pi + ADP --> ATP

Question 32

ion channels

Answer 32

- Ions pass down their electrochemical gradient (a function of ion concentration and membrane potential) without input of metabolic energy. - Water‐filled pores allow only ions of a certain size and/or charge to pass.

Question 33

how many genes for ion channels cloned in man?

Answer 33

>100. Opened by different stimuli, including voltage, temperature, pH, stretch and ligands

Question 34

patch clamping:

Answer 34

used to study small patch of membrane. Glass pipette (v.small) touches cell membrane, causes tight seel to form between pipette &membrane. Ions that flow through when single ion channel opens must flow through pipette. Resulting electrical current can be measured. Record of current can show whether channels open/closed

Question 35

Patch clamping once collected:

Answer 35

- alter voltage across patch | - measure current (ions) which flows

Question 36

connexins are arranged into

Answer 36

connexons

Question 37

Transporters are often linked: Two types of glucose carriers and Na+/K+ ATPase enable...

Answer 37

epithelial cells to transport glucose from the gut

Question 38

Transporters often linked: HCl secretion in gastric parietal cells:

Answer 38

- The H+/K+‐ATPase in luminal membrane drives H+ ions into lumen in exchange for K+, raising [H+] against a concentration gradient of about 3 million to 1 (pH 1.5 to 3.5, 100 mM HCl). + - K+ taken cycles via K+ channels. - For each H+ ion secreted, one HCO3- ion (from CO2 & OH- catalyzed by carbonic anhydrase, CA) enters the blood in exchange for a Cl– ion via a coupled anion antiporter. Intracellular Cl– enters lumen via Cl– channels. + Immunolocalisation of H /K ATPase in COUPLED TRANSPORTER (e.g. Na+/H+; Cl‐/HCO3‐) - H+/K+‐ATPase inhibitors (benzimidazole derivatives) act as ant‐acids (ulcers, heartburn).

Question 39

Transporters often linked: Sucrose + sieve tube elements

Answer 39

Sucrose enters sieve‐tube elements of phloem by active transport (SUT1; sucrose/H+ symporter)