Between October and December, in Year 2 of the liceo scientifico (Italian science-focused high school), science moves from biomolecules to the cell. Almost always, the questions in the test concentrate on the membrane: how it is built, what it lets through and how. The typical sequence is “biomolecules and the cell, membranes and transport”, and the first serious test falls six to nine weeks away. It is a topic where two or three ideas are enough to make everything fit together. Students who learn it by heart, on the other hand, confuse osmosis and diffusion, or get the direction of water movement wrong.
Here is the reasoning in order: the structure, the transport that costs no energy, the transport that does, a summary table and three exercises with a reasoned answer. I do not repeat the overview of organelles and biomolecules, which is in the cell and biomolecules in Year 2 of technical school. That article also compares prokaryotic and eukaryotic cells, which I take as known here.
How the membrane is built: the fluid mosaic model
The plasma membrane is a double layer of phospholipids. Each phospholipid has a hydrophilic head, facing the water inside and outside the cell, and two hydrophobic tails, hidden in the middle of the layer. Inside this double layer there are proteins (some cross the membrane from side to side, others sit on one side only). There are also cholesterol molecules in animal cells, which regulate fluidity, and glycoproteins and glycolipids on the outer side, involved in recognition between cells.
The name “fluid mosaic” says two things: the mosaic is the set of different types of proteins and lipids; fluid means that the components are not fixed but move sideways, as in a very dense liquid. The membrane is not a rigid wall: it is a film that can deform, and this is also what makes endocytosis possible, later on.
Selective permeability: what gets through and what does not
The membrane is selectively permeable: it lets some substances through and blocks others. The main criterion is simple, and follows from the structure:
- small, non-polar molecules (oxygen, carbon dioxide) and fat-soluble molecules pass easily, because they cross the hydrophobic region;
- large polar molecules (glucose, amino acids) pass with difficulty;
- ions (Na, K, Cl) almost never pass, because their charge repels them from the hydrophobic centre. Water, small and polar, passes slowly on its own and much faster through dedicated channel proteins (aquaporins).
Everything that does not cross on its own needs a protein to help it, or a transport process that spends energy.
Passive transport: no ATP, down the gradient
The concentration gradient is the difference in concentration of a substance between the two sides of the membrane. Because of thermal agitation, molecules tend to spread out evenly: on average they move from where they are more concentrated to where they are less concentrated, down the gradient. If no metabolic energy (ATP) is needed to do this, the transport is called passive.
Simple diffusion. The substance crosses the double layer directly. This applies to and , and to fat-soluble molecules. Example: in the lungs, oxygen passes from the alveolus (where it is more concentrated) to the blood (where it is less concentrated) with no proteins and no ATP.
Facilitated diffusion. The substance moves down the gradient, but through a membrane protein: a channel or a carrier (transporter). It is needed for ions and polar molecules such as glucose. It is still passive and uses no ATP; it is “facilitated” because without the protein that molecule could not cross. The proteins are specific and finite in number: if they are all occupied the transport becomes saturated, and raising the concentration no longer speeds it up.
Osmosis. This is the diffusion of water across a semipermeable membrane, that is, one that is permeable to water but not to the solute. Water moves from the more dilute solution (less solute, more free water) to the more concentrated one. Put like that it seems the opposite of diffusion, but it is the same idea: water goes to where it is less concentrated, that is, where there is more solute. The direction of the water is the classic mistake: you reason about the water, not about the solute.
Hypotonic, hypertonic and isotonic solutions
These terms describe one solution relative to another, almost always the outside environment relative to the cytoplasm, and they refer to the concentration of solute:
- hypotonic: less solute than the cell; water enters the cell;
- hypertonic: more solute than the cell; water leaves the cell;
- isotonic: same concentration; water enters and leaves in equal amounts, with no net change.
Animal cell. It has no wall. In a hypotonic environment it swells and can burst (haemolysis, in the case of red blood cells); in a hypertonic environment it loses water and shrivels (crenation). This is why intravenous solutions are isotonic with blood.
Plant cell. It has a rigid cell wall. In a hypotonic environment it takes up water and swells, but the wall limits the expansion and the cell becomes turgid: turgor pressure is what keeps non-woody plants upright. In a hypertonic environment it loses water, the membrane pulls away from the wall and plasmolysis occurs; the plant wilts. This is why salt spread in a vegetable garden, or sea water, kills freshwater plants.
Active transport: ATP, against the gradient
When a cell has to move a substance against the gradient, from the less concentrated side to the more concentrated one, it needs energy, usually supplied by ATP. The transport is carried out by proteins called pumps.
The example you need to know is the sodium-potassium pump ( ATPase). In each cycle, spending one molecule of ATP, it carries 3 sodium ions out of the cell and 2 potassium ions in. The result is that sodium is more concentrated outside and potassium more concentrated inside, and that the cell maintains a difference in electric charge between inside and outside, which is essential for neurons and muscles. If ATP production is blocked, the pump stops and the gradients slowly disperse. This consequence comes up often in questions that ask for a reasoned answer.
There is also secondary active transport, in which a protein uses the gradient of one substance (for example sodium, already created by the pump) to drag another one in against its gradient, like glucose in the intestine. ATP is consumed indirectly, to create the initial gradient.
Endocytosis and exocytosis: transport “in bulk”
Very large molecules or whole particles cannot pass by diffusion or through pumps. The cell carries them in vesicles formed by the membrane itself, and this costs ATP.
- Endocytosis: the membrane folds inwards, wraps around the external material and closes into a vesicle that enters the cytoplasm. If the cell takes in a solid particle (a bacterium, a fragment), this is called phagocytosis; if it takes in liquid and solutes, pinocytosis.
- Exocytosis: a vesicle coming from inside fuses with the membrane and releases its contents outside the cell. This is how hormones, neurotransmitters and secreted proteins are released.
Summary table
| Transport | ATP | Gradient | Protein | What it carries |
|---|---|---|---|---|
| Simple diffusion | No | Down | No | Gases, fat-soluble molecules |
| Facilitated diffusion | No | Down | Channel or carrier | Ions, glucose |
| Osmosis | No | Water towards the side with more solute | Aquaporins (optional) | Water |
| Active transport | Yes | Against | Pump | Ions, small molecules |
| Endo/exocytosis | Yes | Not applicable | No (vesicles) | Particles, macromolecules |
Typical mistakes
Confusing osmosis and diffusion. Diffusion concerns any substance moving down its own gradient; osmosis is a particular case, the diffusion of water across a membrane that does not let the solute through. If the question talks about water and a semipermeable membrane, it is osmosis.
Getting the direction of the water wrong. Water does not go towards “where there is less solute”: it goes towards where there is more solute, because that is where the water is less concentrated. Quick check: the hypertonic solution “pulls” water.
Saying that facilitated diffusion uses ATP. It does not: it uses a protein, but down the gradient.
Reading “hypotonic” on its own. It only makes sense in a comparison: “hypotonic relative to what?”. If the external solution is hypotonic, the cell is hypertonic relative to the outside; always fix your point of view.
Forgetting that facilitated diffusion saturates beyond a certain point. A carrier has a finite number of sites, whereas a channel does not.
Three exercises with a reasoned answer
Exercise 1. A red blood cell (an animal cell) is placed in a NaCl solution more concentrated than its cytoplasm. What happens? And what if the solution were isotonic?
Answer. The external solution is hypertonic relative to the cell: it has more solute, so free water is less concentrated outside. Water leaves the cell by osmosis towards the outside, and the red blood cell loses volume and shrivels (crenation). Without a cell wall there is nothing to hold the shape. In an isotonic solution the same amount of water enters and leaves: the cell stays stable. This is why saline solutions given intravenously are isotonic with blood (about 0.9% NaCl, a value that is quoted but normally you are not asked to calculate).
Exercise 2. A wilted lettuce leaf is put in tap water for half an hour and becomes crisp again. Explain the phenomenon in terms of osmosis.
Answer. Tap water is hypotonic relative to the cytoplasm of the cells, which contains many solutes. Water enters by osmosis, passes through the membrane and fills the central vacuole; the membrane presses against the rigid wall and the cell becomes turgid. The sum of many turgid cells gives a rigid leaf. The process uses no ATP: it is osmosis, passive transport. If the same leaf were put in very salty water, the opposite would happen: plasmolysis and further wilting.
Exercise 3 (quiz). Which of these statements about the sodium-potassium pump are correct? A) It carries sodium and potassium down their gradient. B) In each cycle it carries 3 Na ions out and 2 K ions in. C) It uses ATP. D) It is an example of facilitated diffusion.
Answer. B and C are correct. A is false: the pump works against the gradient, keeping sodium more concentrated outside and potassium inside. D is false because facilitated diffusion is passive and goes down the gradient, whereas the pump is primary active transport. A typical follow-up question is: “what happens if ATP synthesis is blocked?”. The pump stops, the ionic gradients gradually dissipate and the cell loses the charge difference between inside and outside.
How to study it and where to find more
A method that works: draw a cell with the two solutions on either side and mark first where there is more solute, then the arrow for the water. Do it twice and you will never get the direction wrong. The rest is a table to learn by heart, but you will only understand it properly if you are clear on why the structure is a double layer. For the next part of the programme, cell division, there is mitosis and meiosis: differences and exercises. To understand what the cell does with the energy that the pumps consume, there is photosynthesis, respiration and fermentation. The general overview of the subject is on the Science page.
There is no relevant practice test on this topic; for more science tests and the format of the questions you can look at the high school practice tests.
If you want to check together where you stand before the test, the free introductory call is the quickest way.
