Lecture 17: Transport Flashcards
The necessity for transport in eukaryotic cells
- 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.
Physical communication between animal cells:
Gap junctions
Gap junctions:
- 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.
connexins vs innexins
connexin in vertebrates
innexin in invertebrates
Physical communication between plant cells:
Plasmodesmata
Plasmodesmata :
- 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
Membrane structure:
Phospholipid bilayer:
- phospholipid molecules (hydrophobic tails, hydrophilic heads)
- integral proteins
- carbohydrates
- surface proteins
- glycoprotein
- glycolipid
- peripheral protein
- cholesterol
- protein channel (transport protein)
The lipid bilayer :
—WHO
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.
Gorter + grendel how did they test:
erythrocytes –> extraction of membrane lipid in solvent –> lipid film on water
Inherent transport properties of lipid membranes: Hydrophobic molecules
O2, CO2, N2, steroid hormones.
All pass through lipid bilayer.
Inherent transport properties of lipid membranes: Small uncharged polar molecules
H2O, urea, glycerol
Some passes through, more rejected to pass through lipid bilayer
Inherent transport properties of lipid membranes: Large uncharged polar molecules
glucose, sucrose
little passes through, majority rejected
Inherent transport properties of lipid membranes: ions
e.g. H+, Na+, HCO-3, K+, Ca2+, Cl-, Mg2+
ALL REJECTED
Time taken for 1 ml water to pass through 1 cm2 of membrane under 1 atm pressure: Red blood cell
2 days
Time taken for 1 ml water to pass through 1 cm2 of membrane under 1 atm pressure: Zoothamnium (a colonial protist)
6 weeks
Time taken for 1 ml water to pass through 1 cm2 of membrane under 1 atm pressure: Amoeba
9 months
Aquaporins
membrane channels for transport of water, glycerol CO2 etc
Aquaporins what do they carry out:
Facilitate diffusion of small uncharged molecules (water, glycerol, urea, CO2)
occurrence of aquaporins:
correlates with high water fluxes (e.g. kidney, eye)
how man aquaporins genes in man and in Arabidopsis
10 genes in man
30 genes in Arabidopsis
What can aquaporins activity be regulated by/
by PTMs (phosphorylation)
Passive transport:
of an ion or molecule facilitated by favourable concentration gradient.
Active Transport::
Active transport against a concentration gradient requires energy.
Passive transport uses which channels?
channel protein and transporter protein
active transport uses
just transporter proteins & requires energy
Passive and active transport:
Transport of a charged ion or molecule is facilitated by a favourable membrane potential.
Transport against an unfavourable membrane potential requires energy.
Three ways of driving active transport:
- coupled transporter
- ATP-driven pump
- Light-driven pump
All go against electrochemical gradient
ATP-driven membrane transport in cells: P-type pump
P-ATPases: transport specific ions e.g. H+, Na+, K+, Mg2+, Ca2+. One or two polypeptides (< than rotary ATPases).
ATP-driven membrane transport in cells: H+-PPases
In plants (not animals and fungi) H+-PPAses also couple PPi hydrolysis to H+ movement
ATP –> ADP + Pi
Pi used in pump
ATP-driven membrane transport in cells: ATP-Binding Cassette transporters
(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
ATP-driven membrane transport in cells: Rotary ATPases family of membrane protein complexes
(F‐ATPase, V‐ATPase and A‐ATPase) are H+ pumps with role in energy conversion.
Pi + ADP –> ATP
ion channels
- 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.
how many genes for ion channels cloned in man?
>
- Opened by different stimuli, including voltage, temperature, pH, stretch and ligands
patch clamping:
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
Patch clamping once collected:
- alter voltage across patch
- measure current (ions) which flows
connexins are arranged into
connexons
Transporters are often linked: Two types of glucose carriers and Na+/K+ ATPase enable…
epithelial cells to transport glucose from the gut
Transporters often linked: HCl secretion in gastric parietal cells:
- 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).
Transporters often linked: Sucrose + sieve tube elements
Sucrose enters sieve‐tube elements of phloem by active transport (SUT1; sucrose/H+ symporter)
