Movement into and out of cells - Science - Combined (0653) - Cambridge IGCSE

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Movement Into and Out of Cells (Biology B3)

Welcome to one of the most fundamental chapters in Biology! Every single living cell—from a tiny bacterium to the cells in your brain—needs to control what moves in and out. This movement is how cells get food, remove waste, and communicate. Understanding these three processes—Diffusion, Osmosis, and Active Transport—is key to understanding life itself. Let's master them together!

The Gatekeeper: The Cell Membrane

Remember the structure of the cell? All movement we discuss happens across the cell membrane. This membrane is partially permeable (or selectively permeable).

It's like a security door or a fine sieve.
It lets small particles (like water, oxygen, and carbon dioxide) pass through easily.
It blocks or carefully controls the movement of larger particles (like proteins or starch) and charged ions.

Part 1: Diffusion (Passive Movement Downhill)

Diffusion is the easiest way for things to move across the cell membrane because it doesn't require the cell to use any energy.

What is Diffusion? (Core Concept)

Imagine you spray perfume in one corner of a room. After a few minutes, you can smell it everywhere, even though nobody waved the air around. This spreading is diffusion!

Definition: Diffusion is the net movement of particles (atoms, ions, or molecules) from a region where they are in higher concentration to a region where they are in lower concentration.

This movement is said to be down a concentration gradient.

Net Movement: This means that while particles are moving randomly in all directions, more particles move from the crowded area (higher concentration) to the less crowded area (lower concentration) until they are evenly spread out.
Random Movement: The particles possess kinetic energy, causing them to constantly bump into each other and move randomly.

The Importance of Diffusion in Living Organisms (Core)

Diffusion is vital for the survival of all living things, especially for gas and solute exchange:

Gas Exchange in Lungs/Gills: Oxygen moves by diffusion from the air (high concentration) into the blood (low concentration). Carbon dioxide moves from the blood (high concentration) into the air in the lungs (low concentration).
Gas Exchange in Leaves: Carbon dioxide diffuses from the air into the leaf cells (for photosynthesis), and oxygen diffuses out.
Absorption of Digested Food: Small soluble molecules like glucose and amino acids diffuse from the high concentration in the small intestine into the bloodstream (low concentration).

Factors Affecting the Rate of Diffusion (Supplement)

How quickly diffusion happens is controlled by several factors:

Concentration Gradient: The bigger the difference between the high and low concentration areas, the faster the rate of diffusion. (Think of a steeper hill—a ball rolls faster.)
Temperature: Higher temperature means particles have more kinetic energy, so they move and collide faster, increasing the rate of diffusion.
Surface Area: A larger surface area allows more particles to move across simultaneously. (This is why your lungs have millions of alveoli and plant roots have root hairs!)
Distance: The shorter the distance the particles have to travel, the faster the rate. (Gas exchange surfaces like alveoli are extremely thin.)

Quick Takeaway for Diffusion

Diffusion is Passive (no energy needed) and always moves particles down the concentration gradient until equilibrium is reached.

Part 2: Osmosis (The Diffusion of Water)

Osmosis is a special type of diffusion, focusing only on the movement of water.

What is Osmosis? (Core and Supplement)

Definition (Core): Osmosis is the diffusion of water through a partially permeable membrane.

To get a full mark definition, especially in Extended papers, you need to use the term water potential.

Definition (Supplement): Osmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution), across a partially permeable membrane.

Water Potential: This is the term used to describe how 'free' the water molecules are to move.

Analogy: Pure water is the "happiest" water—it has the highest water potential. If you add solute (like sugar), the water molecules get busy clinging to the sugar, making the solution more concentrated and lowering its water potential.
Water always moves from where it is most free (dilute solution) to where it is least free (concentrated solution).

Did You Know?

The cell membrane acts as the partially permeable membrane, allowing water to move freely in and out, but restricting most dissolved solutes.

Osmosis in Plant Cells: The Crucial Terms (Supplement)

Plant cells have a strong cell wall outside the cell membrane, which makes their response to osmosis different from animal cells.

Scenario 1: Cell in Pure Water / Very Dilute Solution (High Water Potential Outside)

Water moves into the cell by osmosis.

The vacuole and cytoplasm swell, pushing the cell membrane against the rigid cell wall.
The cell becomes turgid. This creates an outward force called turgor pressure, which helps keep non-woody plants stiff and upright.

Scenario 2: Cell in Concentrated Solution (Low Water Potential Outside)

Water moves out of the cell by osmosis.

The vacuole shrinks, and the cytoplasm pulls away from the cell wall.
The cell membrane becomes wrinkled. This process is called plasmolysis.
If the cell loses only a small amount of water, it is called flaccid (limp).

Common Mistake Alert!

Do NOT say the cell is "plasmolysed" when it is only slightly floppy. Use the term flaccid for a limp cell, and plasmolysed only when the cell membrane has visibly pulled away from the cell wall.

Osmosis in Animal Cells

Animal cells (like red blood cells) do not have a cell wall.

In dilute solutions, water rushes in, but since there is no wall to resist the pressure, the cell swells up and bursts (lysis).
In concentrated solutions, water leaves the cell, causing it to shrink and become crinkled (crenated).

Quick Takeaway for Osmosis

Osmosis is the diffusion of water across a partially permeable membrane. It is vital for plant support (turgor) and keeping tissues hydrated.

Part 3: Active Transport (Movement Uphill)

Sometimes, a cell needs to gather a substance even if it is already highly concentrated inside. This requires special effort and energy, known as active transport.

Describing Active Transport (Supplement)

Active transport is the complete opposite of diffusion and osmosis.

Definition: Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (i.e., against a concentration gradient).

Because it is moving particles "uphill" against the natural gradient, it requires energy supplied by the cell, typically from respiration.

Think of it like cycling up a hill—you need to use energy (from respiration) to make the journey.

The Importance of Active Transport (Supplement)

Active transport allows cells to absorb nutrients or ions even when they are scarce in the environment.

The classic example is ion uptake by root hairs:

Plant roots need mineral ions (like nitrates) for growth.
These ions are often in low concentration in the soil water compared to the high concentration already present inside the root hair cells.
To absorb these vital ions, the root hair cells use active transport, expending energy (ATP) produced by mitochondria to pump the ions inwards, against the concentration gradient.

Quick Review: Diffusion vs. Active Transport

Chapter Summary: Key Takeaways

The movement of substances is essential for cell function and depends on the concentration of particles and the energy available.

Diffusion: Movement of particles down the concentration gradient (High to Low). Passive.
Osmosis: Special case of diffusion for water molecules, moving across a partially permeable membrane from Higher Water Potential (dilute) to Lower Water Potential (concentrated).
Active Transport: Movement of particles against the concentration gradient (Low to High). Requires energy from respiration.

Keep these three processes clear in your mind, and you will ace this topic!