What pays for glucose uptake, followed across both faces of one intestinal cell, is the question the Unit 2 cell transport explanation for SC121 answers, pump and cotransporter included. Searches like "sc 121 unit 2 assignment example", "sc121 unit 2 sample" and "sc121 unit 2 example" land here.
What a finished SC121 Unit 2 cell transport explanation looks like
A three-to-four-page explanation built around one annotated diagram of an absorptive cell with its apical surface facing the lumen and its basolateral surface facing the interstitial fluid. The text follows glucose in order. At the apical membrane, a sodium-glucose cotransporter moves two sodium ions down their gradient and one glucose molecule up its gradient in the same cycle. Inside, glucose concentration climbs above that of the blood, so a carrier on the basolateral membrane lets it leave by facilitated diffusion with no energy spent. The explanation then asks why sodium stays low inside the cell and answers with the sodium-potassium pump, three sodium out and two potassium in per ATP hydrolyzed, on the basolateral side. A short paragraph adds the electrical pull of a negative interior to the chemical gradient.
How a SC121 Unit 2 example is structured
The opening paragraph states the question the paper answers, where the energy comes from, and defines the three transport categories the explanation will use: simple diffusion, facilitated diffusion and active transport, with active transport split into primary and secondary. The body follows the molecule rather than the categories, so each category is introduced at the point glucose meets it. A worked accounting section follows the path, listing each crossing, its direction relative to the gradient, and its energy source, ending with one ATP spent for every three sodium ions exported. A contrast paragraph sets this route beside a red blood cell, which takes up glucose by facilitated diffusion alone because its cytoplasm keeps the concentration low. Many samples close with oral rehydration solution as an everyday demonstration that sodium and glucose travel together.
Two faces, two jobs
The cell is drawn with apical and basolateral membranes labeled separately, since the argument depends on different transporters sitting on opposite sides of the same epithelial cell.
Secondary active transport, defined by its payer
The cotransporter is called secondary active because it spends no ATP itself. The paper names the pump as the true payer and the sodium gradient as the stored energy.
An energy ledger
With two sodium ions per glucose and three per ATP, the ledger arrives at roughly two thirds of an ATP for each glucose absorbed, a figure few submissions ever compute.
Electrochemical, not only chemical
The negative resting potential of the cell interior pulls sodium inward alongside its concentration gradient, and the explanation counts both forces rather than treating the gradient as concentration alone.
A contrast cell
Red blood cells import glucose without any sodium coupling, which shows that the intestinal route needs a payer only because glucose must move against its gradient there.
Where marks go in SC121 Unit 2
Transport vocabulary without an energy source is the most common loss, a paper that names facilitated diffusion and active transport correctly and never says what powers the uphill step. Calling the cotransporter an ATPase is the next, because it confuses the payer with the beneficiary, the exact distinction the unit is built to test. Direction errors follow, glucose described as moving down its gradient at the apical membrane, or sodium described as pumped inward. Diagrams with transporters drawn on the wrong face of the cell lose accuracy marks in many sections even when the prose is right. Osmosis appended as an unrelated fourth topic tends to dilute the argument. The remaining deductions go to unlabeled arrows, missing units on concentrations, and sources that are study-card websites rather than physiology texts.
Get a SC121 Unit 2 example written to your instructions
The Unit 2 prompt and rubric your SC121 section posted are what we need, plus the molecule it names if there is one; glucose, calcium and water each have their own payer. The first custom explanation is free, follows your molecule across every membrane it meets, and arrives in 24-48h with the diagram positions marked.
SC121 Unit 2 questions, answered
How do primary and secondary active transport differ?
Primary active transport spends ATP directly at the transporter, as the sodium-potassium pump does. Secondary active transport uses a gradient that a primary transporter built, so the cotransporter moving glucose never touches ATP itself. The energy still comes from ATP, one step removed. Explanations that make this chain explicit, pump first, gradient second, uphill movement third, usually earn the mechanism marks.
Does the explanation need numbers?
A few help. Stoichiometry, three sodium out and two potassium in per ATP, and typical concentrations, around 145 millimolar sodium outside the cell against roughly a tenth of that inside, make the gradient concrete. Nobody expects exact values, but an explanation with some quantities in it reads as physiology rather than vocabulary, and many sections reward that difference.
Can I choose a different molecule?
Often, when the prompt leaves the molecule open. Water through aquaporins shows movement paid for by an osmotic gradient, and calcium pumped out of the cytosol shows primary active transport doing a signaling job. Glucose is popular because its route uses nearly every category at once, but any molecule works if the paper names what pays at each membrane crossing.