BI0004 - Lecture 8 - Transport
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At what levels can transport be considered in biology?
- Into and out of cells - across the plasma membrane.
- Within cells - From organelle to organelle, and between organelles and the plasma membrane.
- From cell to cell within a tissue.
- Between tissues.
What examples are there of transported substances?
- Nutrients (e.g. sugars, amino acids)
- Minerals, salts and ions (e.g. Na+, K+, Ca2+, Cl-)
- Liquids (e.g. water)
- Signalling molecules
- Gases (e.g. oxygen, carbon dioxide)
- Metabolic waste products for excretion.
What does the plasma membrane do?
The plasma (cell) membrane separates the cell's interior from the environment.
What is the plasma membrane?
How is it held together?
- The membrane is a lipid bilayer with proteins suspended within it.
- Membranes are primarily held together by noncovalent hydrophobic interactions, and are dynamic and fluid.
- Membrane structure results in selective permeability.
How permeable are phospholipid bilayers?
They are selectively permeable.
Permeable to hydrocarbons and gases (nonpolar hydrophobic molecules)
- Slightly permeable to water (small, polar)
- Impermeable to large polar molecules, whether uncharged (glucose) or charged (amino acids)
Impermeable to ions (charged and hydrated)
What is passive diffusion?
How does oxygen use this?
Small nonpolar hydrophobic molecules dissolve in the lipid bi layer and passively diffuse across.
is spontaneous (no energy required). Net movement is down
the molecule's concentration gradient, from where it is mroe concentrated to where it is less concentrated, until equilibrium is reached.
- Dissolved oxygen diffuses into the cell, where it is consumed in cellular respiration.
- This maintains the concentration gradient so oxygen continues to diffuse in (never reaches equilibrium). Carbon dioxide diffuses out.
Why can water still cross the membrane?
How does this occur?
Water is polar, but it is uncharged and very small, so it can still diffuse across the membrane.
Water diffuses down its concentration gradient, from higher water concentration to lower concentration.
Only free water can cross the membrane. Water that is clustered around hydrophilic solutes is unavailable to move.
Water diffuses across the membrane from the region of lower solute concentration (higher free water concentration) to the region of higher solute concentraiton (lower free water concentration) until the solute concentrations are equal on both sides of the membrane.
Water movement by diffusion across a selectively permeable membrane is termed osmosis.
What is osmosis?
Osmosis is the net movement of water from the solution with less concentrated solute to that with more concentrated solute.
OR in other words, water moves from an area of higher free water concentration to lower free water concentration.
What is tonicity?
- Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water.
- The tonicity of a solution depends on its concentration of nonpenetrating solutes (those that cannot cross the membrane) relative to that inside the cell.
What are hypotonic, isotonic, and hypertonic solutions, and what effect do they have on cells?
A hypertonic solution is one with a higher concentration of solutes outside the cell than inside the cell. - Water will tend to leave the cell.
A hypotonic solution has a lower concentration of solutes outside the cell than inside the cell. - Water will tend to enter the cell.
An isotonic solution is one in which its effective osmole concentration is the same as the solute concentration of a cell. - There is no net movement of water.
What must organisms living in hypotonic environments do?
They must osmoregulate to control their solute and water concentrations.
Pond water is hypotonic to paramecium, so enters the cell by osmosis. Water is collected into a contractile vacuole via a system of canals. When full, the canals and vacuole contract and expel the excess fluid from the cell.
What do channel proteins do?
Facilitated diffusion aided by channel proteins enables water and hydrophilic molecules to cross the membrane.
Channel proteins provide a hydrophilic passageway across the membrane.
Channel proteins are selective for particular ions or small molecules.
Facilitated diffusion through a channel is passive - no energy is required and net movement is down the molecule's concentration gradient.
What are aquaporins?
Aquaporins are protein channels that allow water to diffuse very rapidly across the membrane.
They allow water across the membrane 10X faster (109 molecules s-1)
Water molecules pass through the hydrophilic pore, which is to narrow for ions or other small molecules to enter.
They are abundant in particular cell types, e.g. plant cells, epithelium of kidney.
May be gated in some cell types - opening and closing in response to turgor pressure.
What is the structure of aquaporins?
Aquaporins are tetrameric proteins consisting of four identical subunits each containing a channel.
How can the roles of aquaporins as water channels be demonstrated in frog oocytes?
Frog oocytes are transferred from isotonic to hypotonic solutions and examined at various time intervals.
Oocytes previously microinjected with mRNA encoding aquaporins swell and burst as water enters by osmosis.
Uninjected control oocytes maintain their normal volume, because they do not express aquaporin and so their membranes remain poorly permeable to water.
How does an electrochemical gradient effect the movement of ions?
Ions move through ion channels in response to a combined concentration and electrical gradient - their electrochemical gradient.
The concentration gradient favours from from high concentration to low concentration.
The electrical gradient favours flow of positive ions from from net positive charge to net negative charge (opposite for negative ions)
What does it mean that ion channels are selective and often gated?
It means that they open or close in response to a stimulus.
E.g. When the electrical charge on the membrane becomes positive on the inside relative to the outside, the protein's structure changes to allow K+ through the channel
Movement is down the ion's electrochemical gradient.
What do carrier proteins do?
Facilitated diffusion aided by carrier proteins enables solutes to cross the membrane.
Carrier proteins change shape to transport a solute across the membrane.
Carrier proteins are selective.
Facilitated diffusion through a carrier is passive - no energy is required and net movement is down the molecule's concentration gradient.
What does GLUT-1 do?
GLUT-1 glucose transporter facilitates glucose diffusion down its concentration gradient.
Glucose binding triggers a conformation change in GLUT-1 which transports glucose across the membrane, whereupon it is released into the cell.
What does active transport do?
How does it work?
Active transport uses chemical energy to move solutes against their gradients.
Protein pumps use ATP to transport solutes across the membrane.
Active transport enables cells to maintain internal concentrations that are different from those in the environment. e.g. animal cells maintain higher [K+] and lower [Na+] than their surroundings.
The sodium-potassium-ATPase pumps 3Na+ out of the cell for every 2 K+ pumped into the cell (net transfer of positive charge from cytoplasm to extracellular fluid). ATP powers the shape change in the carrier protein by phosphorylating it.
What is cotransport?
A gradient set up by active transport of one molecule can provide the potential energy to transport another molecule against its gradient.
Cotransport is also termed secondary active transport, because ATP is indirectly providing the energy necessary for cotransport
- The plant cell proton pump uses ATP to actively transport protons out of the cell, setting up a voltage and H+ concentration gradient, storing potential energy.
flows back into the cell, down its concentration gradient, through a cotransporter protein which simultaneously transports sucrose up its concentration gradient.
How can cotransported molecules move?
Cotransported molecules can move in the same or opposite directions through the transporter.
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