Exchange & Concentration Gradients
Now you know every way a substance can cross a membrane, you can predict which route real materials take. Oxygen, glucose, carbon dioxide, salts and wastes each cross differently, and the reason comes down to two things: which way the concentration gradient points, and the physical characteristics of the molecule itself. This lesson ties the whole transport toolkit to the substances cells actually exchange.
Get oriented
Make a prediction, read the goals and the key terms, then clear the trap before you start.
Before You Read
warm-upAn actively respiring cell constantly uses oxygen and produces carbon dioxide.
Before reading: for a respiring cell, predict which way oxygen moves (in or out) and which way carbon dioxide moves, and explain using the idea of a concentration gradient.
Learning Intentions
goalsKnow
- How O₂, CO₂, glucose, salts and wastes each cross the membrane
- The direction of the gradient for each in a typical cell
- Which characteristics decide a substance's route
Understand
- Why small non-polar molecules diffuse but ions need proteins
- Why some substances need active transport
- How a cell's activity sets up the gradients
Can Do
- Relate each material's movement to its gradient and characteristics
- Choose the correct transport process for a given substance
- Justify the route using molecule size, polarity and charge
Scan these before reading
vocabMisconceptions To Fix
watch outBuild the first idea
The core of the dot point, then two checks you can answer straight from the card.
Core Content
Gases: Oxygen and Carbon Dioxide
+5 XPSmall and non-polar, they diffuse straight through
A substance's characteristics decide its route, and small non-polar gases have the easiest path of all.
Oxygen (O₂) and carbon dioxide (CO₂) are small, non-polar molecules, so they dissolve in the lipid core and cross by simple diffusion, no protein and no energy needed. The direction follows the gradient set by respiration: a respiring cell uses O₂ (low inside → O₂ diffuses IN) and produces CO₂ (high inside → CO₂ diffuses OUT). The steeper the gradient, the faster the diffusion, which is why active cells need a constant blood supply to keep gradients steep.
Pause, copy the highlighted point about gases diffusing freely down respiration-set gradients.
Because they are small and non-polar, oxygen and carbon dioxide cross the membrane by simple _____.
Odd one out. Three of these explain why oxygen and carbon dioxide cross a membrane freely. Which one does not?
Take it further
The next move, with the diagram to work from and two more checks.
Glucose and Salts: Needing a Protein
+5 XPPolar and charged, channels, carriers, sometimes pumps
We just saw gases cross freely. That raises a question: why can't glucose and salts do the same? This card answers it → their polarity and charge bar them from the lipid core.
Polar molecules and charged ions are repelled by the hydrophobic core, so they must use membrane proteins, and the direction of their gradient decides whether energy is needed.
Text description
- O₂ and CO₂: small and non-polar; simple diffusion.
- Glucose: large and polar; facilitated diffusion through a carrier protein.
- Salts (ions): charged; through channels, or by active transport.
- Wastes (CO₂, urea): small and soluble; diffuse out, down their gradient.
- The route is decided by the molecule's characteristics and the direction of its gradient.
Glucose is a large polar molecule, so it cannot cross the lipid core; it moves by facilitated diffusion through a carrier protein, down its gradient (e.g. into a respiring cell that is using it up). Salts (ions such as Na⁺, Cl⁻) are charged, so they cross through protein channels by facilitated diffusion when moving down their gradient, or by active transport (with ATP) when a cell must move them against the gradient (e.g. ion uptake in root or kidney cells).
Pause, copy how glucose (carrier) and salts (channels / active transport) cross into your book.
Why does glucose require a carrier protein to enter a cell while oxygen does not?
Match each material to the route it takes across the membrane.
- Oxygen
- Glucose
- Sodium ions moving down their gradient
- Sodium ions moving against their gradient
- Facilitated diffusion through a carrier protein
- Active transport, using a carrier protein and ATP
- Simple diffusion straight through the bilayer
- Facilitated diffusion through a protein channel
Work it yourself
Put the idea on paper in your own book before you read on.
Activity 1
Match Material to Route
Pattern C, Reasoned matching
For each of glucose, oxygen, carbon dioxide, sodium ions (a salt) and urea (a waste), write: (a) its key characteristic (size/polarity/charge), (b) the direction it moves in a typical respiring cell, and (c) the transport process it uses and whether energy is needed.
Finish the picture
The last piece of the dot point, then a true or false check across all three cards.
Wastes Out, and the General Rule
+5 XPPutting gradient and characteristics together
We just placed glucose and salts. That raises a question: how do wastes leave, and is there one rule for all of this? This card answers it → yes, and it combines gradient direction with molecular characteristics.
Wastes build up inside a cell, so they move out down their gradient, and the same two-factor logic explains every exchange.
Wastes such as CO₂ and urea are produced by the cell, so they are at higher concentration inside than outside and diffuse OUT down their gradient (CO₂ as a small non-polar gas crosses freely). The general rule that answers this whole dot point is: a material's movement across a membrane depends on (1) the direction of its concentration gradient, set by the cell's activity, and (2) its characteristics, small non-polar molecules diffuse freely, large polar molecules and ions need proteins, and movement against a gradient needs energy.
Add the two-factor general rule (gradient + characteristics) to your notes before the check.
In a respiring cell, carbon dioxide is at higher concentration inside and diffuses out down its gradient.
Charged ions can dissolve in and cross the hydrophobic lipid core just like oxygen.
Moving a salt against its concentration gradient requires active transport and energy.
Drive It: What Speeds Up Diffusion?
+XP for exploringNet movement is always down the concentration gradient, and a steeper gradient means faster diffusion
Use the simulation. Keep the surface area and distance fixed and double the concentration difference. What happens to the rate of diffusion?
Use the simulation. True or false: when the concentrations on both sides are equal, the molecules stop moving.
See where it goes
Where this shows up outside the classroom, and why the dot point is worth knowing.
Gradients in the lungs and kidneys
Every breath you take depends on these rules. In your lungs, oxygen diffuses from the air sacs (high O₂) into the blood (low O₂), while CO₂ diffuses the other way, both down their gradients, both simple diffusion of small non-polar gases. In your kidneys, glucose is reabsorbed by active transport so none is wasted in urine. The same two factors, gradient direction and molecular characteristics, govern transport in every organ, which you will study in detail in Focus Area 2.
Explain how the movement of oxygen, carbon dioxide, glucose, salts and wastes across the cell membrane is determined by concentration gradients and by the characteristics of the materials themselves.
The question names two determinants, so a full answer needs both for every substance: the characteristics decide which route is available, and the gradient decides the direction and whether energy is needed.
6 marks, five substances, two determinants. Set out the two-factor rule first, then apply it substance by substance rather than describing each process again.
Two properties decide how any substance crosses a membrane. Its characteristics — size, polarity and charge — determine which routes are open to it: small non-polar molecules dissolve in the hydrophobic core, while charged or large polar substances are repelled by it and require a protein. The concentration gradient then determines the direction of net movement and whether the cell must spend energy: movement down a gradient is passive, movement against it requires ATP.
Oxygen and carbon dioxide are small and non-polar, so they dissolve readily in the lipid core and cross by simple diffusion, needing neither protein nor energy. Their gradients are maintained by respiration itself: a respiring cell consumes oxygen, keeping its internal concentration low so oxygen diffuses inwards, and produces carbon dioxide, keeping its internal concentration high so carbon dioxide diffuses outwards. The steeper the gradient the faster the exchange, which is why actively respiring tissues require a dense blood supply to remove carbon dioxide and replenish oxygen continuously.
Glucose is a large polar molecule, so it cannot dissolve in the hydrophobic core and must use a carrier protein. In a respiring cell that is consuming glucose, the internal concentration is lower than the external, so glucose moves down its gradient by facilitated diffusion, requiring no energy. Where a cell must absorb glucose against its gradient, as in the small intestine or the kidney tubule when filtrate concentration has fallen, the same route is used with an ATP-driven carrier, that is by active transport.
Salts dissociate into ions such as sodium and chloride, which are charged and so are strongly repelled by the hydrophobic core; they cross through protein channels by facilitated diffusion when moving down a gradient, or by active transport through carrier proteins when the cell needs to accumulate or expel them against one, as in a root hair cell or a nerve cell restoring its resting potential. Wastes such as carbon dioxide and urea are small and soluble and are continuously produced inside the cell, so they maintain their own outward gradient and leave by simple diffusion without any energy cost.
Across all five materials the same two factors operate: characteristics open or close the direct lipid route and so decide whether a protein is needed, and the gradient decides the direction of movement and therefore whether ATP must be spent. A cell does not possess a separate mechanism for each substance; it possesses a small set of routes, and each substance takes the one its properties and its gradient allow.
Two properties decide how a substance crosses a membrane. Its characteristics of size, polarity and charge determine which routes are available: small non-polar molecules dissolve in the hydrophobic core, whereas charged or large polar substances are repelled and require a protein. The concentration gradient then sets the direction of net movement and whether energy must be spent, movement down a gradient being passive and movement against one requiring ATP. Oxygen and carbon dioxide are small and non-polar, so they cross by simple diffusion through the bilayer with no protein or energy; their gradients are maintained by respiration, which consumes oxygen and so keeps the internal concentration low, and produces carbon dioxide and so keeps it high, and since a steeper gradient gives faster exchange, actively respiring tissue needs a dense blood supply to sustain both. Glucose is large and polar, so it cannot cross the lipid core and must use a carrier protein: in a respiring cell consuming glucose the internal concentration is lower, so it enters by facilitated diffusion at no energy cost, but where it must be absorbed against its gradient, as in the small intestine or a kidney tubule with depleted filtrate, the same protein route is driven by ATP as active transport. Salts dissociate into charged ions such as sodium and chloride which the hydrophobic core repels, so they pass through channel proteins by facilitated diffusion down a gradient, or by active transport when a cell must accumulate or expel them against one, as in root hair uptake or a nerve cell restoring its resting potential. Wastes such as carbon dioxide and urea are small and soluble and are continuously produced within the cell, so they generate their own outward gradient and leave by simple diffusion with no energy cost. Across all five materials the same two factors govern: characteristics decide whether a protein is required, and the gradient decides the direction and whether ATP is spent, so the cell needs not a mechanism per substance but a small set of routes, each substance taking the one its properties and gradient permit.
A newly described drug molecule is small, uncharged and highly lipid-soluble, and is present at a much higher concentration in the bloodstream than inside the target cell. Deduce how it will cross the cell membrane, and justify your answer.
Deduce means reach a conclusion from the information given. The marks are not for knowing the four transport processes; they are for showing which given fact eliminates which process, and what remains.
4 marks. Work through the given properties one at a time, eliminate as you go, state the conclusion, then confirm it against the gradient.
The molecule is uncharged and lipid-soluble, so it is not repelled by the hydrophobic core of the phospholipid bilayer and will dissolve in it. The direct route through the bilayer is therefore open, which means no transport protein is required and facilitated diffusion can be ruled out as unnecessary. Its small size confirms that the bilayer will not obstruct it, and also rules out bulk transport by endocytosis, which is reserved for material too large for any protein route.
The concentration in the bloodstream is much higher than inside the target cell, so a steep concentration gradient runs inwards across the membrane. Net movement will therefore be into the cell, down that gradient, and because the movement follows the gradient rather than opposing it, no energy from the cell is required and active transport can also be ruled out.
The drug will therefore cross the membrane into the cell by simple diffusion, passively and without a transport protein, continuing until the concentrations either side are equal or until the cell removes or metabolises the drug and so maintains the gradient. This conclusion is consistent with the general rule that a substance takes the route its characteristics permit in the direction its gradient dictates, and it predicts that uptake will depend on the dose in the bloodstream rather than saturating, since no protein limits the rate.
Because the molecule is uncharged and highly lipid-soluble, it is not repelled by the hydrophobic core of the bilayer and will dissolve in it, so the direct route through the membrane is open and no transport protein is needed; facilitated diffusion can therefore be ruled out as unnecessary, and its small size both confirms the bilayer will not obstruct it and rules out bulk transport by endocytosis, which is reserved for material too large for any protein. The concentration in the bloodstream is much higher than inside the cell, so a steep gradient runs inwards, net movement will be into the cell down that gradient, and since the movement follows rather than opposes the gradient no cellular energy is required, which rules out active transport. The drug will therefore cross by simple diffusion, passively and without a transport protein, continuing until the concentrations equalise or until the cell metabolises or removes the drug and so maintains the gradient. This follows the general rule that a substance takes the route its characteristics permit in the direction its gradient dictates, and it predicts that uptake will rise with the dose in the bloodstream rather than saturating, because no protein limits the rate.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 12 marks
+5 XPAnalyseBand 6(6 marks) 1. Relate the movement of oxygen, glucose and a salt (e.g. sodium ions) across a cell membrane to their concentration gradients and the characteristics of each material.
2 marks each substance: correct route linked to (a) the substance's characteristics (size/polarity/charge) and (b) the direction of its gradient
ApplyBand 4(3 marks) 2. A muscle cell is respiring rapidly. State the direction in which oxygen and carbon dioxide each move across its membrane, and explain what sets those directions.
1 mark: oxygen moves in · 1 mark: carbon dioxide moves out · 1 mark: explains that the cell's own activity sets each gradient
AnalyseBand 5–6(3 marks) 3. Urea is a small, slightly polar waste molecule produced inside a liver cell. Relate its movement across the membrane to its concentration gradient and its characteristics.
1 mark: identifies the outward gradient and why it exists · 1 mark: relates the molecule's characteristics to its route · 1 mark: concludes no energy is required
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Responses
Q1 (6 marks): Oxygen is a small, non-polar molecule, so it dissolves in the hydrophobic lipid core and crosses by simple diffusion, with no protein or energy needed. In a respiring cell oxygen is constantly used up, so its concentration is lower inside than outside and it diffuses in down its gradient. Glucose is a large, polar molecule that cannot pass through the lipid core, so it crosses by facilitated diffusion through a carrier protein; in a respiring cell that is using glucose, its concentration is lower inside, so it moves in down its gradient without energy. Sodium ions (a salt) are charged, so they cannot cross the lipid core; they move through protein channels by facilitated diffusion when travelling down their gradient, or by active transport using ATP when the cell must move them against their gradient (e.g. ion uptake in kidney cells). In every case, the route depends on the material's characteristics and the direction of its concentration gradient.
Q2 (3 marks): Oxygen moves into the cell [1], because rapid respiration is consuming it, so its concentration inside falls below the concentration outside. Carbon dioxide moves out of the cell [1], because respiration is producing it, so its concentration inside rises above the concentration outside. In both cases the cell's own activity is what creates and maintains the concentration gradient, and each gas then diffuses down that gradient; a steeper gradient gives a faster rate, which is why an active muscle needs a rich blood supply [1].
Q3 (3 marks): Urea is produced inside the liver cell and removed by the blood outside, so its concentration is higher inside than outside and the gradient runs outward [1]. Urea is a small molecule and only slightly polar, so it can cross the phospholipid bilayer directly rather than needing a transport protein, though small channels also assist it [1]. Because the movement follows the gradient rather than opposing it, no ATP is required and the process is passive [1].
Retrieve what stuck
Work the spaced-review set first. It mixes this lesson with earlier ones on purpose.
Check what actually stuck
A short retrieval set drawn from this lesson and the ones before it, so the earlier work keeps coming back rather than fading.
Lock it in
bossYou can now relate the movement of glucose, O₂, CO₂, salts and wastes to their gradients and characteristics. This closes "Cells and their environments". Next lesson begins the biochemical processes, the raw materials a cell needs.
↩ Back to focus-area hubCompare it with where you started
Read your opening prediction again, then mark the lesson complete.
How did your thinking change?
At the start you predicted O₂ and CO₂ movement in a respiring cell. Because the cell uses O₂, it is low inside, so O₂ diffuses in; because the cell makes CO₂, it is high inside, so CO₂ diffuses out, each down its gradient. If you thought direction is fixed regardless of the cell, revisit the misconception box: metabolism sets the gradient.