# Membranes, Transport and the Energy Currency

Biology I · Life, Its Origin and Its Diversity · https://tryals.app/learn/biology-i/membranes-transport-and-the-energy-currency

## Choosing What Gets In

A membrane is a **fluid mosaic**: a lipid bilayer with proteins floating in and through it. It is **selectively permeable**: small nonpolar molecules such as oxygen and carbon dioxide slip through the lipid core unaided, while ions and large polar molecules cannot and need a protein.

Transport divides by whether the cell pays:

| Type | Direction | Energy | Example |
|---|---|---|---|
| Simple diffusion | Down the gradient | None | Oxygen, carbon dioxide |
| Facilitated diffusion | Down the gradient | None | Glucose via GLUT |
| Osmosis | Water, down its gradient | None | Water via aquaporins |
| Active transport | Against the gradient | ATP | The sodium-potassium pump |

**Osmosis** is the movement of water across a semipermeable membrane from lower to higher solute concentration. A cell in a **hypotonic** solution gains water and swells; in a **hypertonic** one it loses water and shrinks; in an **isotonic** one there is no net movement. Plant cells resist bursting because their wall builds up turgor pressure, animal cells have no such protection, which is why intravenous fluids must be isotonic.

**ATP** is the cell's energy currency. Hydrolysing its terminal phosphate releases about 30.5 kJ/mol under standard conditions, and that release is coupled to work the cell needs done. Cells hold only seconds' worth of ATP and continually regenerate it, a human turns over roughly their own body mass in ATP each day, not by holding it but by cycling it.

Energy is extracted through **redox** reactions, where oxidation is the loss of electrons and reduction the gain. Electron carriers such as NAD⁺ and FAD shuttle electrons to the chain that ultimately makes ATP. The mnemonic worth keeping is OIL RIG: oxidation is loss, reduction is gain.

> **Common pitfall:** describing osmosis as water moving "to where there is less water". True but unhelpful, the useful statement is that water moves *toward the higher solute concentration*, because that is the quantity you are given in a problem.

## Practice questions

6 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. A cell is placed in a solution and its volume does not change. If the cell’s internal solute concentration is 0.30 mol/L, what is the solute concentration of the solution, in mol/L?

**Answer:** 0.3 (within ±0.005)

**Why:** No net movement means the solution is **isotonic**, so its concentration equals the cell’s own 0.30 mol/L. This is exactly why intravenous saline is prepared at the concentration of blood plasma.

Page: https://tryals.app/practice/biology-i/membranes-transport-and-the-energy-currency/a-cell-is-placed-in-a-solution-and-its-volume-does-not-change-if-the

### 2. Which forms of transport require the cell to spend energy?

A. Endocytosis engulfing a large particle
B. Glucose moving down its gradient through a carrier
C. The sodium-potassium pump moving ions against their gradient
D. Oxygen diffusing into the cell

**Answer:** A. Endocytosis engulfing a large particle; C. The sodium-potassium pump moving ions against their gradient

**Why:** Pumping against a gradient and bulk transport both cost ATP. Oxygen diffusion and facilitated glucose transport both run *down* a gradient and are free, facilitated diffusion uses a protein but no energy, which is the distinction that catches people out.

Page: https://tryals.app/practice/biology-i/membranes-transport-and-the-energy-currency/which-forms-of-transport-require-the-cell-to-spend-energy

### 3. Why can oxygen cross a membrane unaided while a sodium ion cannot?

A. Oxygen is actively pumped inward by dedicated transport proteins
B. Oxygen carries a net positive charge that attracts it into the cell
C. Sodium ions are physically much larger than diatomic oxygen molecules
D. Oxygen is small and nonpolar, so it dissolves through the lipid core

**Answer:** D. Oxygen is small and nonpolar, so it dissolves through the lipid core

**Why:** The bilayer’s core is hydrophobic, so small nonpolar molecules dissolve straight through. A sodium ion is actually *smaller* than an oxygen molecule but carries a charge, and a charged particle cannot enter a hydrophobic core, size is not the obstacle, charge is.

Page: https://tryals.app/practice/biology-i/membranes-transport-and-the-energy-currency/why-can-oxygen-cross-a-membrane-unaided-while-a-sodium-ion-cannot

### 4. Sort each solution by its effect on an animal cell placed in it.

**Answer:**

- Cell swells: Hypotonic, more dilute, Pure distilled water
- Cell shrinks: Hypertonic, more concentrated, Concentrated salt solution
- No net change: Isotonic, matching the cell

**Why:** Water moves toward the higher solute concentration. A hypotonic environment drives water in and can burst an animal cell, which is why cells in fresh water need pumps or walls to survive.

Page: https://tryals.app/practice/biology-i/membranes-transport-and-the-energy-currency/sort-each-solution-by-its-effect-on-an-animal-cell-placed-in-it

### 5. Facilitated diffusion and active transport both rely on membrane-spanning proteins. What distinguishes the work these proteins do, and why does this distinction dictate the cell's energetic cost?

A. Active carriers create new solute gradients whereas channel proteins only dissipate them
B. Facilitated carriers bind their substrate whilst active transport pumps move ions freely
C. Facilitated diffusion relies on ATP synthesis whereas active transport consumes stored ATP
D. Active transport moves solutes up a gradient whereas facilitated diffusion runs down one

**Answer:** D. Active transport moves solutes up a gradient whereas facilitated diffusion runs down one

**Why:** Proteins cannot bypass thermodynamics: moving cargo against an established gradient demands metabolic coupling via ATP hydrolysis. Channels and facilitated carriers merely provide a path down an existing electrochemical gradient, so the cell pays no direct metabolic surcharge.

Page: https://tryals.app/practice/biology-i/membranes-transport-and-the-energy-currency/facilitated-diffusion-and-active-transport-both-rely-on

### 6. Oxidation is the gain of electrons and reduction is the loss.

**Answer:** False

**Why:** False, it is exactly reversed. OIL RIG: **o**xidation **i**s **l**oss, **r**eduction **i**s **g**ain. The naming is genuinely confusing, since "oxidation" no longer requires oxygen and "reduction" refers to reducing the charge, not the size.

Page: https://tryals.app/practice/biology-i/membranes-transport-and-the-energy-currency/oxidation-is-the-gain-of-electrons-and-reduction-is-the-loss