Lesson 1.7 · Unit 1 · HS-LS1-2, HS-LS1-4
Getting things across the membrane, and making more cells
A membrane that let everything through would be useless and one that let nothing through would be fatal. The cell's solution is a barrier with a set of specific doors, some of which are free and some of which cost ATP. Knowing which is which explains a large fraction of physiology, including why your cells do not burst in fresh water and why a third of your resting energy budget goes to a single pump.
- Passive transport requires no ATP and moves substances down a gradient. Simple diffusion for small nonpolar molecules such as oxygen and carbon dioxide, facilitated diffusion through a channel or carrier for glucose and ions, and osmosis for water.
- Osmosis is the diffusion of water across a selectively permeable membrane from where solute is less concentrated to where it is more concentrated. Water follows solute, and that sentence resolves most osmosis questions.
- Tonicity describes what a solution does to a cell. In an isotonic solution there is no net movement. In a hypotonic solution water enters and the cell swells and may burst, which is hemolysis in a red cell. In a hypertonic solution water leaves and the cell shrinks, called crenation.
- Active transport requires ATP and moves substances against a gradient. The sodium-potassium pump moves three sodium ions out and two potassium ions in per ATP, which both maintains the gradients and leaves the inside slightly more negative.
- That pump is expensive and it is worth it. It consumes a substantial share of resting energy, and it underwrites the resting membrane potential of unit 5, the secondary active transport of glucose in the gut and kidney, and the volume control of every cell.
- Bulk transport moves large material in vesicles. Endocytosis brings material in (phagocytosis for solids, pinocytosis for fluid, receptor-mediated for specific molecules); exocytosis sends it out.
- The cell cycle is interphase then mitotic phase. Interphase divides into G1 growth, S phase DNA replication and G2 preparation. Mitosis proceeds prophase, metaphase, anaphase, telophase, followed by cytokinesis.
- Mitosis produces two genetically identical diploid cells and is used for growth and repair. Meiosis, which produces gametes, is a different process and is taken up in lesson 11.6.
Where students lose marks: saying water moves "from high to low concentration" without saying of what. Water moves from where water is more concentrated to where it is less, which is the same as from low solute to high solute. State which substance you mean, every time, or half your osmosis answers will read backwards.
The problem. A hospital mistakenly infuses pure distilled water into a patient's vein instead of the isotonic saline that was ordered. Predict what happens to the red blood cells, explain the mechanism, and calculate roughly how the situation differs from the correct infusion.
Step one: establish the reference. Normal saline is 0.9 percent sodium chloride by mass, which is isotonic to blood plasma: a red cell placed in it neither swells nor shrinks, because the water concentration inside and outside are effectively equal.
Step two: compare the infused fluid to that reference. Distilled water contains no solute at all. Relative to the cell's interior, which is full of proteins, ions and organic molecules, it is extremely hypotonic.
Step three: state the direction water moves and why. Water moves toward the higher solute concentration, which is inside the cell. So water enters the red cells. Say it in terms of solute to keep the direction straight.
Step four: predict the structural consequence. A red blood cell has no cell wall and its membrane has limited stretch. Water entering has nowhere to go, so the cell swells from its normal biconcave disc toward a sphere, and then the membrane fails. The cell ruptures and releases its hemoglobin. This is hemolysis.
Step five: identify the secondary problem. Free hemoglobin in the plasma is not merely useless; it is filtered at the kidney and can obstruct the tubules, so a large hemolytic event threatens kidney function as well as oxygen transport. The primary error causes a second failure in a different organ system.
Step six: contrast with the correct infusion. With 0.9 percent saline there is no net water movement across the red cell membrane, so cell volume is unchanged and the fluid simply expands plasma volume, which is the point of giving it.
Step seven: extend the reasoning to the opposite error. Infusing a strongly hypertonic solution instead would drive water out of the red cells, shrinking and crenating them, and would pull water out of tissues into the blood. Both errors are dangerous, in opposite directions, and both follow from the same one-line rule: water follows solute.
Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.
- Define osmosis.
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The diffusion of water across a selectively permeable membrane from a region of lower solute concentration to one of higher solute concentration
- Name the three types of passive transport.
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Simple diffusion, facilitated diffusion and osmosis
- State the ion ratio moved by the sodium-potassium pump.
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Three sodium ions out for every two potassium ions in, per ATP
- List the stages of mitosis in order.
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Prophase, metaphase, anaphase, telophase
- During which phase of interphase is DNA replicated?
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S phase
- A red blood cell is placed in a 5 percent salt solution. Predict what happens and name
the process.
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Five percent salt is far more concentrated than the roughly 0.9 percent that is isotonic to blood, so the solution is hypertonic. Water moves toward the higher solute concentration, which is outside the cell, so water leaves. The cell loses volume and its membrane puckers. The cell shrinks and crenates, because water leaves it by osmosis
- Explain why oxygen crosses a membrane by simple diffusion but glucose requires a
carrier.
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The membrane interior is made of phospholipid tails, which are nonpolar. Oxygen is a small nonpolar molecule, so it dissolves in and passes through that interior freely. Glucose is a much larger molecule and is polar, carrying several hydroxyl groups, so the nonpolar interior repels it and its size makes passage slow regardless. It therefore needs a protein carrier that provides a hydrophilic route, though it still moves down its gradient and still costs no ATP. Oxygen is small and nonpolar so it dissolves through the lipid interior; glucose is large and polar so it needs a carrier protein
- A cell is treated with a poison that stops ATP production. Predict what happens to its
sodium and potassium gradients and to its volume.
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The sodium-potassium pump requires ATP, so it stops. The leak channels do not require ATP and keep working, so sodium continues to leak in and potassium to leak out, and with nothing restoring them the gradients run down toward equilibrium. As sodium accumulates inside, the total solute concentration inside rises, so water follows it in by osmosis and the cell swells and eventually bursts. This is why cells in ischemic tissue swell. The gradients dissipate, sodium accumulates inside, water follows by osmosis, and the cell swells and may lyse
- A white blood cell engulfs a bacterium. Name the process and explain how it differs
from facilitated diffusion.
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The process is phagocytosis, a form of endocytosis in which the membrane extends around the particle and pinches off to form a vesicle inside the cell. It differs from facilitated diffusion in three ways: it moves a large particle rather than a dissolved molecule, it requires ATP rather than being free, and the material never passes through the membrane at all, since it is enclosed by a piece of membrane instead. Phagocytosis; it is active, handles large particles, and encloses rather than conducts the material through the membrane
- Explain why a drug that blocks a cell from completing S phase would arrest division,
and predict which tissues in the body would be affected first.
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S phase is when DNA is replicated. Without replication each daughter cell would receive only half a genome, so the cell cannot proceed to mitosis and the cycle halts. The effect appears first in tissues whose cells divide most frequently, because those cells enter S phase soonest and most often. In the body that means the lining of the digestive tract, the bone marrow producing blood cells, and hair follicles, which is why these three are the classic sites of side effects for drugs that interfere with cell division. DNA cannot be replicated so mitosis cannot proceed; rapidly dividing tissues such as gut lining, bone marrow and hair follicles are affected first