Diffusion, osmosis and active transport

Introduction to Transport in Cells

In this chapter, we are looking at how substances get in and out of cells. Think of a cell like a busy factory: it needs raw materials (like oxygen and glucose) delivered, and it needs to get rid of waste products (like carbon dioxide and urea). There are three main ways these "deliveries" and "pick-ups" happen: diffusion, osmosis, and active transport. Understanding these processes is vital because without them, cells simply couldn't function!

1. Diffusion

Diffusion is the movement of particles from an area of high concentration to an area of low concentration.

Imagine someone spraying perfume in the corner of a room. At first, the scent is only in that one spot (high concentration). Eventually, the scent particles spread out until they are evenly distributed throughout the room (low concentration). This happens because the particles move randomly and bump into each other.

Key features of diffusion:

• It is a passive process, meaning it requires no energy from the cell.
• Particles move down a concentration gradient.
• It happens in liquids and gases because the particles are free to move.

Example in Biology: Oxygen diffusing from the air in your lungs into your blood, or carbon dioxide diffusing out of a cell.

Quick Summary: Diffusion = High to Low (No energy needed).

2. Osmosis

Osmosis is a special type of diffusion. It is the movement of water molecules from a region of high water concentration to a region of low water concentration across a partially permeable membrane.

Wait, what is a partially permeable membrane?
It is a very thin layer (like the cell membrane) that has tiny holes in it. It allows small molecules like water to pass through, but blocks larger molecules like sugar or starch.

Common Mistake Alert: Students often get confused between the concentration of the solution and the concentration of the water. If a solution is "concentrated" (lots of sugar), it actually has a "low water concentration." Water will always move toward the area where there is less water (the more sugary/salty side) to try and balance things out.

Core Practical: Investigating Osmosis in Potatoes

You will likely perform a practical (1.16) where you place potato cylinders into sugar solutions of different concentrations. To see if osmosis happened, you measure the change in mass.

The Math Skill: Calculating Percentage Change
To compare results fairly, we calculate the percentage change in mass because the potato pieces might not have started at the exact same weight.

\( \text{Percentage Change} = \frac{\text{Change in Mass}}{\text{Initial Mass}} \times 100 \)

Note: If the potato loses mass, the change is a negative number.

3. Active Transport

Sometimes, a cell needs to pull in substances even when there is already a high concentration inside the cell. To do this, it must move particles against the concentration gradient (from low to high concentration).

Active transport requires energy. This energy comes from respiration. Cells that do a lot of active transport usually have many mitochondria to provide this energy.

Real-world Examples:

• Plant Roots: Root hair cells use active transport to pull mineral ions from the soil (where they are in low concentration) into the plant (where they are in high concentration).
• Human Gut: Sometimes glucose needs to be absorbed from the small intestine into the blood even when the blood already has a higher glucose level.

Analogy: Diffusion is like sliding down a slide (easy, no effort). Active transport is like climbing back up the slide (requires energy!).

4. Factors Affecting the Rate of Movement

The speed at which particles move in and out of cells depends on several factors. (Note: These factors are especially important for Higher Tier and Biology Only content).

Surface Area to Volume Ratio (\( SA:V \))

The larger the surface area of a cell compared to its volume, the faster substances can move in and out. This is why small organisms (like bacteria) can rely on diffusion alone, but large organisms (like humans) need specialized exchange surfaces like lungs.

Fick's Law (Higher Tier/Biology Only)

Fick’s Law describes the relationship between the variables that affect the rate of diffusion. The law states:

\( \text{Rate of Diffusion} \propto \frac{\text{Surface Area} \times \text{Concentration Gradient}}{\text{Thickness of Membrane}} \)

What this means:

• If you double the surface area, you double the rate of diffusion.
• If you double the concentration difference, you double the rate of diffusion.
• If you double the thickness of the membrane, the rate of diffusion is halved (it is inversely proportional).

Key Comparison Table

Quick Review: Check your understanding

• Can you define diffusion, osmosis, and active transport?
• Do you know which process requires energy from respiration?
• Can you calculate the percentage change in mass for a potato in an osmosis experiment?
• Higher Tier: Can you explain how doubling the thickness of a membrane affects the rate of diffusion using Fick's Law?

Don't worry if the math parts feel tricky! Just remember the golden rule for percentage change: it's always "the difference divided by the starting value." Practice a few examples, and you'll have it down in no time.