Predicting Substance Movement Across Cell Membranes

Hello. In the previous lesson, you practised calculating actual size and magnification from micrographs and scale bars. This lesson moves from what cells look like to how they keep their internal conditions stable: by controlling movement across the cell membrane.

By the end, you should be able to examine a membrane scenario and predict:

• which transport process is occurring;
• which substance has a net movement;
• whether it moves into or out of the cell;
• whether cellular energy is required; and
• the likely effect on the cell.

These are common Biology exam questions, especially when a diagram gives concentrations inside and outside a cell.

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The central idea: membranes control exchange

The cell membrane is selectively permeable. It allows some substances to cross relatively easily, restricts others, and contains protein channels or carrier proteins that assist particular substances. This selectivity helps maintain homeostasis: internal conditions suitable for cellular reactions.

A prediction about membrane transport always begins with two questions:

1. Which substance is moving?
2. Is it moving from an area of higher concentration to lower concentration, or the reverse?

A concentration gradient is a difference in the concentration of a substance between two regions. Each substance has its own gradient. For example, a cell might have a higher concentration of oxygen outside than inside, while having a higher concentration of carbon dioxide inside than outside. Oxygen and carbon dioxide therefore have different net directions of movement.

The word net matters. Molecules move randomly in all directions all the time. However, when more particles move in one direction than the other, there is a net movement in that direction.

At equilibrium, particles still move randomly, but equal numbers move in both directions over time. There is therefore no net movement.

Watch this short explanation from Science Shorts to establish the concentration-gradient model before applying it to cells.

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Diffusion: particles move down their concentration gradient

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. It is a form of passive transport, meaning it does not require ATP from the cell.

For a substance to diffuse directly through the phospholipid bilayer, it generally needs to be small and nonpolar. Oxygen and carbon dioxide are the standard biological examples.

Suppose a body cell is using oxygen during cellular respiration. Its oxygen concentration becomes lower than the oxygen concentration in surrounding capillary blood. Oxygen therefore has a net movement into the cell by diffusion. At the same time, cellular respiration produces carbon dioxide, so carbon dioxide is often at a higher concentration inside the cell. Carbon dioxide therefore has a net movement out of the cell by diffusion.

The key rule is:

A substance undergoing diffusion moves from its own high concentration to its own low concentration.

Do not write that “particles move to where there are more particles.” That describes the wrong direction. Diffusion reduces a concentration difference.

Facilitated diffusion: still passive, but assisted

Some substances cannot cross the hydrophobic centre of the membrane directly. Charged ions, such as sodium ions, and larger polar substances, such as glucose, often require a membrane protein.

If they still move from high to low concentration without ATP, the process is facilitated diffusion.

Transport process	Direction relative to concentration gradient	Protein needed?	ATP needed?
Simple diffusion	High to low	No	No
Facilitated diffusion	High to low	Yes, channel or carrier	No
Active transport	Low to high	Yes, pump or carrier	Yes

A carrier protein by itself does not prove active transport. The deciding factor is whether the substance moves against its concentration gradient and requires energy.

For example, if sodium ion concentration is higher outside a nerve cell than inside, and sodium ions enter through open channel proteins, their movement is facilitated diffusion. The ions move down their gradient; the channel provides a suitable pathway but does not use ATP to push them.

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Osmosis: diffusion of water

Osmosis is the net movement of water molecules across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.

In exam questions, it is usually quicker to state the same rule in terms of solutes:

Water moves from a solution with lower solute concentration to a solution with higher solute concentration, provided the solute cannot cross the membrane.

More dissolved solute means less free water relative to the volume of solution. Water therefore moves towards the more concentrated solution, diluting it.

For osmosis to occur, three conditions are needed:

1. There must be a selectively permeable membrane.
2. Water must be able to cross the membrane.
3. There must be a difference in concentration of solutes that cannot freely cross the membrane.

Consider two sides of a membrane. Side A contains a $0.2\mathrm{M}$ sugar solution and Side B contains a $0.6\mathrm{M}$ sugar solution. If sugar cannot cross the membrane but water can, water has a net movement from Side A to Side B. Side B has the higher solute concentration and is therefore lower in free water concentration.

Tonicity: describing the surrounding solution relative to a cell

The terms hypotonic, isotonic, and hypertonic compare the solution outside a cell with the cell’s cytoplasm. They are never absolute labels: a solution is hypertonic or hypotonic relative to something else.

Surrounding solution relative to cell	Relative solute concentration outside	Net water movement	Effect on an animal cell
Hypotonic	Lower outside	Into the cell	Swells; may burst
Isotonic	Equal inside and outside	No net movement	Remains normal size
Hypertonic	Higher outside	Out of the cell	Shrinks

In a hypotonic solution, the fluid outside the cell contains fewer solutes than the cytoplasm. Water enters the cell by osmosis. Animal cells, including red blood cells, have no cell wall, so excessive water entry may cause them to burst, or lyse.

In a hypertonic solution, the fluid outside has a higher solute concentration than the cytoplasm. Water leaves the cell, causing it to shrink. In red blood cells, this is called crenation.

In an isotonic solution, solute concentration is equal on both sides of the membrane. Water still crosses in both directions, but at equal rates, so there is no net movement and no overall change in cell volume.

The following short video section applies this reasoning to intravenous fluids and red blood cells.

Plant cells respond differently

Plant cells also gain and lose water by osmosis, but their cell wall changes the visible result.

• In a hypotonic environment, water enters a plant cell. The vacuole expands and presses the cell membrane against the cell wall. The cell becomes turgid, which helps support non-woody plants.
• In a hypertonic environment, water leaves the plant cell. The cell membrane and cytoplasm pull away from the cell wall. This is plasmolysis, and it contributes to wilting.
• A plant cell generally does not burst in a hypotonic solution because the rigid cell wall resists further expansion.

The movement is still osmosis in each case. The cell wall changes the outcome, not the direction of water movement.

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Active transport: moving substances against a gradient

Cells sometimes need to move substances from a lower concentration to a higher concentration. This is movement against the concentration gradient, so it cannot happen through diffusion alone.

Active transport moves substances across a membrane against their concentration gradient using energy from ATP and a specific carrier protein or pump.

A standard example is mineral-ion uptake by root hair cells. The concentration of some mineral ions in the soil can be lower than inside the root hair cell. Yet plants need these ions for processes such as protein synthesis and chlorophyll production. The ions are transported from the dilute soil solution into the more concentrated root hair cell using ATP.

A strong exam explanation includes all three features:

Mineral ions are moved from a lower concentration in the soil to a higher concentration in the root hair cell. This is against the concentration gradient, so carrier proteins use energy from ATP to transport the ions into the cell.

Active transport depends on cellular respiration because respiration supplies ATP. If a cell cannot release enough energy, active transport is reduced. This is one reason why cells with high transport demands often contain many mitochondria.

Do not confuse these processes

A useful way to distinguish them is to focus first on the substance and direction.

If the question states…	The likely process is…	Reason
Oxygen moves from a high concentration outside a cell to a lower concentration inside	Diffusion	Oxygen moves down its gradient without ATP
Glucose moves through a carrier protein from high concentration to low concentration	Facilitated diffusion	It is protein-assisted but still down the gradient
Water enters a cell because the cytoplasm has more dissolved solutes	Osmosis	Water moves across a membrane towards higher solute concentration
Mineral ions enter root hair cells even though concentration is higher inside	Active transport	Ions move against the gradient using ATP
A red blood cell shrinks in salty fluid	Osmosis	The surrounding solution is hypertonic, so water leaves

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A reliable method for membrane-transport questions

When a diagram includes concentrations inside and outside a cell, avoid guessing based on whether the arrows point into or out of the cell. Use this sequence instead:

1. Name the substance. Is it water, a gas, glucose, an ion, or another solute?
2. Compare concentrations of that same substance. Identify where its concentration is higher and lower.
3. Check membrane information. Can the substance pass directly, through a protein, or not at all?
4. Decide whether movement is down or against the gradient.
5. Name the process and state energy use.
6. For water, predict the effect on cell volume.

Worked prediction 1: gas exchange

A muscle cell has a lower oxygen concentration than the blood surrounding it.

• Oxygen concentration is higher in the blood.
• Oxygen concentration is lower in the muscle cell.
• Oxygen has a net movement into the cell.
• The process is diffusion because oxygen moves down its concentration gradient.
• ATP is not required for this membrane crossing.

A concise exam response would be:

Oxygen diffuses from the blood into the muscle cell because oxygen concentration is higher in the blood than in the cell. It moves down its concentration gradient and does not require ATP.

Worked prediction 2: red blood cell in concentrated salt solution

A red blood cell is placed in a solution with a higher solute concentration than its cytoplasm. The salt ions cannot freely enter the cell.

• The surrounding solution is hypertonic relative to the red blood cell.
• Water concentration is relatively higher inside the cell.
• Water has a net movement out of the cell by osmosis.
• The red blood cell shrinks or crenates.

A complete response would be:

The solution is hypertonic to the red blood cell. Water moves out of the cell by osmosis, from the region with lower solute concentration inside the cell towards the higher solute concentration outside. The red blood cell loses water and crenates.

Worked prediction 3: mineral uptake by a root hair cell

The concentration of nitrate ions is lower in the soil than in the root hair cell, but the plant takes up nitrate ions.

• Nitrate ions move into the root hair cell.
• Their movement is from lower concentration in the soil to higher concentration inside the cell.
• This is against the concentration gradient.
• The process is active transport.
• ATP and carrier proteins are required.

Notice that “the plant needs nitrates” is not enough as an explanation. The marker needs the scientific mechanism: against the concentration gradient, using ATP.

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Key takeaways

• Diffusion is net movement of particles from high to low concentration; it is passive.
• Facilitated diffusion also moves substances down a gradient, but it uses channel or carrier proteins.
• Osmosis is the net movement of water across a selectively permeable membrane towards higher solute concentration.
• A hypotonic solution causes net water entry; a hypertonic solution causes net water loss; an isotonic solution causes no net movement of water.
• Active transport moves substances from low to high concentration using ATP and membrane proteins.
• For every exam response, identify the substance, compare concentrations, state the direction of net movement, name the process, and explain whether ATP is required.

Next, you will use surface-area-to-volume ratio to predict how efficiently cells exchange substances with their environment.