Title: InkBiology / Membranes and transport across cells
On every note of this deck, unless a note says otherwise:
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Front: DEFINE
Define: Diffusion.
Back: The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random motion.

Front: DEFINE
Define: Passive.
Back: Requiring no energy from ATP; driven by the kinetic energy of the particles themselves.

Front: DEFINE
Define: Water potential (ψ).
Back: The tendency of water molecules to move out of a solution, measured in kPa. Pure water at atmospheric pressure has a water potential of 0 kPa; adding any solute makes it negative.

Front: DEFINE
Define: Denaturation.
Back: The loss of a protein's tertiary structure when hydrogen and ionic bonds break, so its shape and therefore its function is lost.

Front: DEFINE
Define: Phospholipid.
Back: A lipid in which one of the three fatty acids of a triglyceride is replaced by a phosphate group, giving a hydrophilic head and two hydrophobic tails.

Front: DEFINE
Define: Turgid.
Back: A plant cell whose contents press against the wall, giving a positive pressure potential.

Front: DEFINE
Define: Permeability.
Back: How readily a membrane allows a substance to cross it.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Osmosis is the movement of water from a region of high concentration to a region of low concentration.
Back: Concentration of what? Of solute, and then the sentence runs backwards. Osmosis is the movement of water from a higher water potential to a lower one through a partially permeable membrane, and water potential exists because concentration of water does not survive a question about pressure.

Front: DEFINE
Define: Simple diffusion.
Back: Diffusion directly through the phospholipid bilayer, available to small non-polar molecules such as oxygen and carbon dioxide.

Front: DEFINE
Define: Co-transport.
Back: The transport of two substances across a membrane by the same carrier protein at the same time, in which one moves down its concentration gradient and provides the means for the other to move against its own.

Front: DEFINE
Define: Glycolipid.
Back: A phospholipid with a carbohydrate chain attached, acting in cell recognition and adhesion.

Front: DEFINE
Define: Intrinsic protein.
Back: A protein embedded within the bilayer, often spanning it completely.

Front: CHECK YOURSELF
Cells lining the proximal convoluted tubule of the kidney reabsorb glucose from the filtrate even when the concentration of glucose in the filtrate is lower than in the cell. These cells have many mitochondria and their surface facing the filtrate is covered in microvilli. Explain how glucose is reabsorbed, and account for the two features described.
Back: Sodium-potassium pumps in the membrane facing the blood actively transport sodium ions out of the cell, hydrolysing ATP to do it. This keeps the concentration of sodium ions inside the cell low.
A sodium gradient therefore exists between the filtrate and the cell, and sodium ions diffuse in down it through a co-transporter protein in the membrane facing the filtrate. The same protein carries glucose at the same time, so glucose is brought in with the sodium against its own concentration gradient. No ATP is used at this protein: the energy came from the gradient the pump established.
Glucose then leaves the cell into the blood by facilitated diffusion through a carrier protein, down its concentration gradient, and the blood carries it away, which keeps that gradient in place.
The many mitochondria supply ATP by aerobic respiration for the sodium-potassium pumps, which run continuously and account for a large share of the cell's energy budget. The microvilli increase the surface area of the membrane facing the filtrate, so more co-transporter proteins can be held there and the rate of uptake is higher, which is the surface area term of the same relationship that governs diffusion.

Front: DEFINE
Define: Carrier protein.
Back: An intrinsic protein with a binding site specific to one substance, which changes shape to move it across the membrane.

Front: DEFINE
Define: Bilayer.
Back: Two layers of phospholipid with the hydrophobic tails facing inwards and the hydrophilic heads facing the watery solutions on each side.

Front: DEFINE
Define: Incipient plasmolysis.
Back: The point at which the pressure potential of a plant cell has just fallen to zero, so its water potential equals its solute potential.

Front: DEFINE
Define: Crenation.
Back: The shrinking and puckering of an animal cell when water leaves it by osmosis.

Front: CHECK YOURSELF
A student places identical discs of beetroot in a range of ethanol concentrations from 0% to 100% at room temperature and measures the absorbance of the surrounding liquid after 30 minutes. Absorbance increases as ethanol concentration increases. Explain this result, and explain why the same student sees almost no pigment released when the discs are left in water at 20 °C.
Back: Ethanol is an organic solvent and the bilayer is lipid, so ethanol dissolves phospholipids out of the membrane. The more concentrated the ethanol, the more phospholipid is removed and the more disrupted the remaining arrangement is.
Once the bilayer is incomplete, the hydrophobic core no longer forms a continuous barrier. Betalain is water-soluble and would normally be held in the vacuole; with the tonoplast and the cell surface membrane both breached, it diffuses out into the surrounding liquid and the absorbance rises.
In water at 20 °C neither thing has happened. The phospholipids have relatively little kinetic energy and stay closely packed, and the intrinsic proteins are well below the temperature at which they denature, so their tertiary structure is intact and no gaps open around them.
The membrane is therefore still selectively permeable at 20 °C. A large polar molecule like betalain cannot cross the hydrophobic core unaided, and nothing in the membrane transports it, so it stays inside and the water stays clear.

Front: DEFINE
Define: Plasmolysis.
Back: The pulling away of the cell surface membrane from the cell wall as water leaves a plant cell.

Front: SPOT THE ERROR
Say what is wrong with this claim:
A carrier protein is involved, so this must need energy.
Back: The carrier protein is not what costs ATP: facilitated diffusion uses carriers and needs none. What costs energy is the direction, moving a substance from where there is less of it to where there is more.

Front: DEFINE
Define: Hydrophobic.
Back: Repelled by water; non-polar.

Front: DEFINE
Define: Endocytosis.
Back: The bulk transport of material into a cell by invagination of the cell surface membrane to form a vesicle; requires ATP.

Front: DEFINE
Define: Hydrophilic.
Back: Attracted to water; polar or charged.

Front: DEFINE
Define: Osmosis.
Back: The net movement of water molecules from a solution of higher water potential to a solution of lower water potential, through a partially permeable membrane.

Front: DEFINE
Define: Haemolysis.
Back: The bursting of a red blood cell when water enters it by osmosis.

Front: CHECK YOURSELF
A plant cell has a solute potential of −1400 kPa and a pressure potential of +500 kPa. It is placed in a sucrose solution of water potential −1100 kPa. Calculate the cell's water potential, state the direction of net water movement, and explain what will happen to the cell's pressure potential as a result.
Back: Water potential is solute potential plus pressure potential: ψ = −1400 + 500 = −900 kPa.
The cell is at −900 kPa and the solution is at −1100 kPa. The cell has the higher (less negative) water potential, so water moves by osmosis out of the cell into the solution, through the partially permeable cell surface membrane.
As water leaves, the volume of the cell contents falls, so they press less hard against the cellulose wall and the pressure potential drops below +500 kPa. The solute potential becomes slightly more negative too, since the same solutes are now dissolved in less water, but the fall in pressure potential is the larger effect.
Water continues to leave until the cell's water potential has fallen to −1100 kPa and matches the solution. If the pressure potential reaches zero before that happens, the cell is at incipient plasmolysis, and any further loss of water pulls the membrane away from the wall.

Front: DEFINE
Define: Extrinsic protein.
Back: A protein on one surface of the bilayer that does not enter the hydrophobic core.

Front: DEFINE
Define: Glycoprotein.
Back: A membrane protein with a carbohydrate chain attached, acting in cell recognition, adhesion or as a receptor.

Front: SPOT THE ERROR
Say what is wrong with this claim:
Something must be holding the bilayer together: bonds between the phospholipids, or a frame of protein underneath.
Back: No covalent bond joins one phospholipid to the next, and there is no frame underneath. Each molecule slides past its neighbours millions of times a second. The sheet holds because the hydrophobic tails have nowhere better to be than beside each other.

Front: DEFINE
Define: Active transport.
Back: The movement of a substance across a membrane against its concentration gradient, using a carrier protein and energy from the hydrolysis of ATP.

Front: DEFINE
Define: Exocytosis.
Back: The bulk transport of material out of a cell by fusion of a vesicle with the cell surface membrane; requires ATP.