Biology (9201) | Transport in cells

Welcome to Transport in Cells!

Hello future Biologists! Don't worry if cell transport sounds complicated—it's just about how cells get the 'stuff' they need (like food and oxygen) and get rid of the 'stuff' they don't want (like waste). Think of it as the cell's essential delivery and rubbish removal service.

This topic is vital because if cells can't control what moves in and out, they can't survive! We will look at three main ways things move: Diffusion, Osmosis, and Active Transport.

The Gatekeeper: The Cell Membrane

Before we discuss movement, we must talk about the barrier. Every cell is surrounded by a cell membrane.

• It keeps the contents of the cell separate from the outside environment.
• Its most important feature is that it is partially permeable (sometimes called selectively permeable).
• What does partially permeable mean? It means the membrane acts like a security guard, allowing some substances (like water and small molecules) to pass through easily, but blocking others (like large molecules).

1. Diffusion: The Easy Way In (Passive Transport)

What is Diffusion?

Diffusion is the simplest way substances move. It is a passive process, meaning it does NOT require energy from the cell.

Definition:

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

Think of it this way: Imagine you spray deodorant or perfume in one corner of a room. Initially, the smell is very strong there (high concentration). Over time, the deodorant particles move randomly until they are spread out evenly throughout the whole room (low concentration). This is diffusion!

The Concentration Gradient

The difference between the high concentration area and the low concentration area is called the concentration gradient. Particles move down the concentration gradient until the concentration is equal everywhere—this state is called equilibrium.

Real-Life Examples of Diffusion:

• Gas Exchange in Lungs: Oxygen moves from the air in the alveoli (high concentration) into the blood (low concentration). Carbon dioxide moves from the blood (high concentration) into the alveoli (low concentration).
• Digestion: Simple sugars and amino acids move from the high concentration in the small intestine into the bloodstream (low concentration).

Factors Affecting the Rate of Diffusion

How quickly diffusion happens depends on four main things:

1. Temperature: Higher temperature means particles have more kinetic energy and move faster, so diffusion is quicker.
2. Concentration Difference: The steeper the concentration gradient (the bigger the difference between high and low), the faster the diffusion.
3. Surface Area: A larger surface area (like the folded walls of the small intestine or the many alveoli in the lungs) allows more particles to move across at once, speeding up the process.
4. Distance: The shorter the distance the particles have to travel (e.g., the very thin walls of capillaries and alveoli), the faster the diffusion.

2. Osmosis: The Water Movement (Passive Transport)

Osmosis is really just a special, important type of diffusion.

What is Osmosis?

Definition:

Osmosis is the net movement of water molecules across a partially permeable membrane from an area of high water concentration (dilute solution) to an area of low water concentration (concentrated solution).

Don't worry if this sounds confusing! Let's simplify the language:

The Easy Way to Remember Water Movement:
Water moves from the place where there are lots of free water molecules (i.e., a very watery solution, like pure water) to the place where there are fewer free water molecules (i.e., a solution with lots of solutes, like sugar or salt).

Analogy: Imagine two rooms separated by a screen door (the partially permeable membrane). Room A has 10 people and 90 empty seats (lots of free water). Room B has 50 people and 50 empty seats (less free water). The empty seats (water) will move from Room A to Room B until the ratio is balanced, even though the people (solutes) cannot pass through the screen door.

The Effect of Osmosis on Cells

The outcome of osmosis depends on whether the cell has a strong cell wall (plants) or not (animals).

A. Animal Cells (e.g., Red Blood Cells)

Animal cells are flexible and have no cell wall.

• In a very dilute solution (lots of water outside): Water moves into the cell. The cell swells up and may burst (lyse) because the membrane cannot handle the pressure.
• In a concentrated solution (lots of salt/sugar outside): Water moves out of the cell. The cell shrivels up (crenates). This is why drinking seawater is dangerous—it pulls water out of your cells!

B. Plant Cells

Plant cells have a strong cell wall surrounding the cell membrane, which provides support.

• In a very dilute solution (lots of water outside): Water moves into the cell, pushing the cell membrane against the cell wall. The cell becomes very firm and swollen. We call this turgid. This is the ideal state for plants, as it provides support.
• In a concentrated solution (lots of salt/sugar outside): Water moves out of the cell. The cell membrane shrinks away from the rigid cell wall. The cell becomes flaccid (limp). If too much water is lost, the cell becomes plasmolysed (the cell contents shrivel completely).

3. Active Transport: Swimming Against the Current

Sometimes, a cell needs to grab hold of specific substances, even if the concentration of that substance is already higher inside the cell than outside.

What is Active Transport?

Definition:

Active Transport is the movement of particles across a cell membrane against the concentration gradient (from an area of low concentration to an area of high concentration).

Since particles are moving uphill (against the flow of diffusion), this process requires work.

• Active Transport is an active process, meaning it requires energy.
• The energy is supplied by respiration, usually in the form of ATP.
• Special proteins embedded in the cell membrane act as 'pumps' to move the substances.

Analogy: Imagine you are trying to clean up ping pong balls scattered outside a large stadium. You already have 500 balls inside (high concentration) but need to collect the last 10 balls from the outside car park (low concentration). You have to use energy to carry them against the flow of what nature wants!

Why is Active Transport Necessary?

Cells use active transport when they need to absorb vital materials very efficiently, even if the external supply is low.

Real-Life Examples of Active Transport:

• Root Hair Cells: These cells absorb mineral ions (like nitrates and magnesium) from the soil. The concentration of these ions in the soil is often much lower than inside the root cell, so they must be pumped in using active transport.
• Small Intestine: After digestion, glucose and amino acids may be actively transported from the low concentration in the gut into the bloodstream where the concentration is already high, ensuring maximum absorption.

Summary of Transport Mechanisms

Understanding the difference between the three transport types is crucial for exam success! Here is a simple comparison:

Key Difference Trick:

• If it's High to Low, it's Passive (Diffusion or Osmosis).
• If it's Low to High, it's Active (Active Transport).

Did you know? The amount of energy a human body uses every day just for active transport across cell membranes is enormous—it accounts for a significant chunk of your daily calorie requirement!

Congratulations! You've mastered how cells move materials. Remember to focus on the direction of the concentration gradient and whether or not energy is needed!