Anatomy 8-24

Anatomy and Physiology, Grades 11 and 12

Membrane Transport

Monday  |  Module 1  |  40 Minutes  |  Mr. Gibbs

At a Glance

• 0:00 to 0:08  Quiz, five questions

• 0:08 to 0:14  Membrane highlights, and the problem transport solves

• 0:14 to 0:24  Passive transport

• 0:24 to 0:33  Active and vesicular transport

• 0:33 to 0:36  Wrap and the decision tree

• 0:36 to 0:39  Tuesday lab briefing

• 0:39 to 0:40  Close

Materials: whiteboard, printed quizzes. Draw a bilayer across the board during the review and add every transport mechanism to that same drawing rather than starting new ones.

1. Quiz

8 minutes. Collect promptly. No review of answers today, tell them you will go over it next class.

Five questions covering last week. Straightforward recall of the load bearing ideas, nothing tricky.

1.  In one sentence each, what is the difference between transcription and translation? Include where each happens.

2.  How many bases make up a codon, and what does the codon AUG do?

3.  Name the four phases of mitosis in order.

4.  In the fluid mosaic model, what does the word fluid describe?

5.  Name two functions of the plasma membrane.

Answer key

• 1. Transcription copies DNA into mRNA and happens in the nucleus. Translation reads mRNA to build a polypeptide and happens at a ribosome. Accept any answer with both product and location correct.

• 2. Three bases. AUG is the start codon and also codes for methionine. Full credit requires both.

• 3. Prophase, metaphase, anaphase, telophase.

• 4. That the phospholipids and many proteins drift laterally within the membrane rather than being fixed in place.

• 5. Any two of: physical boundary, selective permeability, communication and reception, cell identity, cell adhesion, establishing electrochemical gradients.

Adjust these to match what you actually emphasized. Question 4 is the one most likely to separate the class, since students often answer that the membrane is made of liquid rather than that its components move.

2. Membrane Highlights, and the Problem

6 minutes. Review only what today depends on. Draw the bilayer and leave it up.

• Cold call four things: what makes a phospholipid amphipathic, why the bilayer forms without energy input, what fraction of the membrane is protein by mass, and the difference between integral and peripheral proteins.

• Then rebuild the one fact that generates the entire lecture: the middle of the membrane is a hydrophobic core. Anything charged or water soluble cannot pass through it on its own.

• Draw the two lists on the board and keep them there: crosses freely, meaning oxygen, carbon dioxide, small nonpolar molecules, and lipid soluble substances including steroid hormones. Cannot cross alone, meaning ions, glucose, amino acids, and water in any useful quantity.

• State the problem as a problem: every one of those things in the second list is something a cell absolutely must move. So the membrane has to be a barrier and a gateway at the same time. Everything today is how it does both.

3. Passive Transport

10 minutes. No ATP. Always down a concentration gradient.

Simple diffusion  (2 min)

• Molecules move randomly. Net movement runs from high concentration to low, and continues until the gradient is gone.

• Correct the common error: movement does not stop at equilibrium. Molecules keep moving. Net movement is what reaches zero.

• What crosses this way: oxygen and carbon dioxide, which is the entire basis of gas exchange in the lungs and at every tissue.

• Rate depends on: steepness of the gradient, temperature, molecular size, surface area, and distance to travel. Note that every one of those is a design feature somewhere in the body, and the alveolus is the obvious example.

Facilitated diffusion  (3 min)

• Still passive, still down the gradient, still no ATP. The only difference is that it needs a protein.

• Channel mediated: a protein pore. Ions move through ion channels, water moves through aquaporins. Some channels are always open, called leak channels, and some open only on a signal, called gated channels. Flag that gated channels are the mechanism behind Module 6.

• Carrier mediated: the protein binds the solute, changes shape, and releases it on the other side. Glucose entering most cells uses a GLUT carrier.

• The distinction worth testing: carriers can be saturated. When every carrier is occupied, the rate stops increasing no matter how steep the gradient gets. This is called the transport maximum, and channels do not show it in the same way.

Osmosis and tonicity  (5 min, the clinical heart of the lecture)

• Define it precisely: osmosis is the diffusion of water across a selectively permeable membrane. Water moves toward the side with more solute, which is the side with less free water.

• Say the confusing part out loud: water follows solute. Students reverse this constantly. Write it on the board.

• Aquaporins: water does cross the bilayer slowly on its own, but cells that move serious volumes of water have dedicated channels. Kidney tubule cells are packed with them.

• Tonicity, and walk a red blood cell through all three: isotonic means equal solute concentration and no net water movement, so the cell keeps its normal shape. Hypotonic means the outside has less solute, so water enters and the cell swells and can burst, which is hemolysis. Hypertonic means the outside has more solute, so water leaves and the cell shrivels, which is crenation.

• The clinical anchor, and this is what makes it an anatomy course: intravenous fluid has to be isotonic to blood. Normal saline is 0.9 percent sodium chloride for exactly this reason. Ask what would happen if a patient received pure water intravenously, and let someone work out that their red cells would lyse.

Filtration, one minute

• Include it because your text will: filtration is driven by hydrostatic pressure rather than a concentration gradient. Pressure forces water and small solutes through a membrane while larger items stay behind. This is how capillary beds work and how the kidney glomerulus begins making urine. Module 15 is built on it.

4. Active and Vesicular Transport

9 minutes. Now ATP is involved, and movement can run against the gradient.

Primary active transport  (3 min)

• Definition: a pump protein uses ATP directly to move a substance against its concentration gradient.

• The one they must know: the sodium potassium pump. Three sodium ions out and two potassium ions in, for each ATP spent. It runs continuously in every cell in the body.

• Connect to last lecture explicitly: this is membrane function six, establishing electrochemical gradients. The pump is what creates the resting membrane potential, and that stored difference is what nerve and muscle cells spend when they fire and contract.

• Give the cost, because it lands: this single pump consumes on the order of twenty to twenty five percent of a resting cell’s energy budget, and considerably more in neurons. Ask why a cell would spend a quarter of everything it has maintaining a gradient, and let them get to the fact that the gradient is itself a form of stored energy.

Secondary active transport  (2 min)

• The idea: no ATP is used directly. Instead the transporter rides the gradient that primary active transport already built, using sodium flowing back in to drag something else along.

• Two directions: symport, where both move the same way, as with sodium and glucose in the small intestine. Antiport, where they move opposite ways, as with the sodium and calcium exchanger.

• Ask the sharp question: if no ATP is used here, is this really active transport? Yes, indirectly. Somebody paid for the gradient. Tell them this distinction shows up on exams.

Vesicular transport  (4 min)

• Why it exists: some things are simply too large for any channel or carrier. Whole bacteria, cell debris, large proteins.

• Endocytosis, three kinds: phagocytosis, where the cell engulfs a large particle, which is what macrophages and neutrophils do to bacteria. Pinocytosis, where the cell takes in droplets of extracellular fluid nonselectively. Receptor mediated endocytosis, where specific receptors gather a specific substance before the membrane invaginates.

• The clinical example for the third one: cholesterol travels as LDL and enters cells by receptor mediated endocytosis. In familial hypercholesterolemia the LDL receptor is defective, so cholesterol is not taken up and accumulates in the blood instead. Another protein shape problem with a whole body consequence.

• Exocytosis: a vesicle fuses with the membrane and releases contents outward. Hormone secretion, enzyme secretion, neurotransmitter release. Connect back to the organelle lecture, since this is where the rough ER and Golgi pathway finally delivers.

5. Wrap, the Decision Tree

3 minutes. Put this on the board and have them copy it.

• Question 1: does it require ATP? No means passive. Yes means active.

• Question 2, if passive: does it need a protein? No means simple diffusion. Yes means facilitated diffusion, either a channel or a carrier. If the substance is water, it is osmosis.

• Question 3, if active: is ATP used directly by the transporter? Yes means primary. No, it rides a gradient someone else built, means secondary. If it is packaged in a membrane sac, it is vesicular.

• Then test it: give three substances and have them route each one. Oxygen crossing into a capillary. Glucose entering a muscle cell. A bacterium being taken in by a neutrophil.

6. Tuesday Lab Briefing

3 minutes. Calm and specific. They should be writing this down.

DO NOT PREVIEW THE TISSUE TYPES.  The histology lab is deliberately built so that students meet each tissue and its name at the same moment, under the scope. The lab guide says plainly that students who arrive with a memorized list retrieve a label from memory instead of reading the evidence in front of them. Naming the four tissue types today, or assigning a reading that lists them, quietly breaks the lab. Tell them what they will DO tomorrow, not what they will SEE.

• 1. The question, in one sentence: "Tomorrow you find out what your body is actually made of, by looking at it. Eighty five minutes, five stations, real slides."

• 2. What they will do, three sentences: they will rotate through five microscope stations, draw what they see at each one, and finish by identifying three unlabeled slides they have never been shown. Say that the last part is graded on the reasoning, not the answer.

• 3. What to bring, and be specific: a sharpened pencil, and a second one. Every drawing is pencil only, no pens, no color. A student with only a pen cannot complete the lab.

• 4. What to wear: closed shoes, and long hair tied back before they get to a scope.

• 5. Groups: assigned by you, announced tomorrow, not negotiable. Say that now so nobody arrives having planned a table with friends.

• 6. The one safety item, and make it this one: coarse focus is used at 4x only. At 10x and 40x, fine focus alone. Coarse adjustment at high power drives the objective into the slide, and that cracks a slide and scratches a lens that costs more than the slide. Say that tomorrow you will demonstrate it and then hold them to it.

• 7. One line on the prepared slides: these are permanent prepared slides, not something they make. They are not to be forced, wiped with anything but lens paper, or carried more than one at a time.

If you want to assign anything to read tonight, assign the microscope handling section rather than the tissue content. Arriving competent with the instrument is what protects the lab clock. Arriving with a memorized tissue list is what undermines its design.

7. Close

1 minute.

• One question to leave open: if the sodium potassium pump is running in every cell in your body every second of your life, what happens to those gradients when a cell runs out of ATP? Do not answer it. That is what cell death looks like at the molecular level, and it is worth having them wonder about.

• Name the reading. Dismiss by procedure.

Objectives

• Students will explain why the hydrophobic core of the membrane necessitates transport proteins.

• Students will distinguish simple diffusion, facilitated diffusion, and osmosis, and identify what crosses by each.

• Students will predict the effect of isotonic, hypotonic, and hypertonic solutions on a red blood cell and explain why IV fluids must be isotonic.

• Students will distinguish primary from secondary active transport and describe the sodium potassium pump.

• Students will describe phagocytosis, pinocytosis, receptor mediated endocytosis, and exocytosis.

• Students will classify an unfamiliar transport example using ATP requirement and protein involvement.