Welcome to "Transport Across Cell Membranes"!

In this chapter, we are going to explore how cells act as tiny, exclusive clubs. Not just anyone can get in! The cell-surface membrane is the "bouncer" at the door, deciding what enters and what leaves. Understanding this is vital because, without controlled transport, your cells couldn't get the glucose they need for energy or get rid of toxic waste.

Don't worry if this seems like a lot of detail at first. We will break it down piece by piece using simple analogies and clear steps.

1. The Structure of the Membrane: The Fluid-Mosaic Model

The AQA syllabus describes the membrane as a fluid-mosaic model. Imagine a sea of oil with different objects floating in it.

What makes up the "Mosaic"?

  • Phospholipids: These form a bilayer (two layers). They have "heads" that love water and "tails" that hate it. This bilayer acts as a barrier to most water-soluble substances.
  • Proteins: These are scattered throughout. Some go all the way through (intrinsic) and act as "tunnels" or "gates," while others sit on the surface (extrinsic).
  • Cholesterol: These molecules fit between phospholipids. They restrict movement of other molecules, making the membrane less fluid and more stable at high temperatures.
  • Glycoproteins and Glycolipids: These are proteins or lipids with carbohydrate "chains" attached. Think of them as antennas used for cell recognition and signaling.

Quick Review: Why is it called "Fluid-Mosaic"?
It is Fluid because the individual phospholipids can move around relative to each other. It is a Mosaic because the proteins embedded in the bilayer vary in shape and size, just like the tiles in a mosaic artwork.

Key Takeaway: The membrane is a dynamic, flexible barrier made of lipids and proteins that controls what enters the cell.

2. Simple Diffusion

Simple diffusion is the passive (energy-free) net movement of molecules from an area of high concentration to an area of low concentration.

How it works:

Molecules move directly through the phospholipid bilayer. However, only certain molecules can do this. To get through, a molecule must be:

  1. Small: So it can fit between the phospholipids.
  2. Non-polar (Lipid-soluble): Because the middle of the membrane is made of fatty "tails" that repel anything with a charge.

Example: Oxygen (\(O_2\)) and Carbon Dioxide (\(CO_2\)) move this way.

Common Mistake to Avoid: Don't forget that diffusion is passive. It does NOT require ATP (energy) from the cell!

3. Facilitated Diffusion

Some molecules, like Glucose or Ions (\(Na^+\), \(K^+\)), are too big or carry a charge (polar), so they can't go through the bilayer. They need "help" from proteins.

Two types of "Help":

  • Channel Proteins: These form water-filled pores or "tunnels" through the membrane. They are specific to certain ions.
  • Carrier Proteins: When a specific molecule (like glucose) binds to the protein, it changes shape and releases the molecule on the other side.

Did you know? This is still passive transport! The molecules are still moving down their concentration gradient (from high to low), so no energy is needed.

Key Takeaway: Facilitated diffusion uses specific proteins to move large or charged molecules without using energy.

4. Osmosis

Osmosis is a special type of diffusion. It is the net movement of water from a region of higher water potential to a region of lower water potential through a selectively permeable membrane.

What is Water Potential (\(\Psi\))?

Think of water potential as the "pressure" exerted by water molecules.
- Pure water has the highest possible water potential, which is \(0\).
- As you add solutes (like salt or sugar), the water potential becomes more negative (e.g., \(-20\) or \(-100\)).

Memory Aid: Water always flows from Less Negative (closer to zero) to More Negative. It’s like water "trying" to dilute the saltier side.

Quick Review Box:
- Hypotonic: Solution with higher \(\Psi\) (more water). Cell might swell.
- Isotonic: \(\Psi\) is the same. No net movement.
- Hypertonic: Solution with lower \(\Psi\) (less water). Cell might shrivel.

5. Active Transport

Sometimes, the cell needs to pull in molecules even when there is already a high concentration inside. This is like trying to cram more people into a crowded elevator. It takes energy.

Key Features:

  • Moves substances against the concentration gradient (from Low to High).
  • Requires Carrier Proteins (acting as "pumps").
  • Requires the hydrolysis of ATP to provide the energy for the protein to change shape.

Analogy: Diffusion is like sliding down a slide (easy, no effort). Active transport is like climbing up the ladder (takes work and energy!).

Key Takeaway: Active transport uses ATP and carrier proteins to move substances from low to high concentrations.

6. Co-transport (The Big Exam Topic!)

This is a clever way the cell moves two things at once. The most important example in the AQA syllabus is the absorption of sodium ions and glucose in the ileum (small intestine).

The 3-Step Process:

  1. The Setup: Sodium ions (\(Na^+\)) are actively transported out of the epithelial cells into the blood by the sodium-potassium pump. This creates a concentration gradient (low \(Na^+\) inside the cell).
  2. The Co-transport: \(Na^+\) ions want to diffuse back into the cell. They move through a co-transporter protein and "bring" a glucose molecule with them against its gradient.
  3. The Exit: The glucose now moves from the cell into the blood by facilitated diffusion.

Mnemonic: P.C.F. (Pump out sodium, Co-transport glucose in, Facilitated diffusion into blood).

7. Factors Affecting the Rate of Transport

If you get a question asking how a cell is adapted for rapid transport, look for these three things:

  1. Surface Area: More membrane (like microvilli) means more space for transport to happen.
  2. Number of Proteins: More channel or carrier proteins means more "doors" for molecules to walk through.
  3. Steepness of Gradient: A bigger difference in concentration (or water potential) makes movement faster.

Key Takeaway: Cells adapt by increasing surface area or the number of available protein "pumps" and "tunnels."

Required Practical Tips

In this chapter, you study Required Practicals 3 and 4. Keep these points in mind for your exams:

  • RP3 (Water Potential): You create a dilution series (different concentrations of sucrose) to find the water potential of plant tissue. Where the line crosses the X-axis on your graph (zero change in mass), the water potential of the solution equals the water potential of the plant.
  • RP4 (Membrane Permeability): You investigate how variables like temperature or alcohol concentration affect the leakage of pigment from beetroot cells. High temperatures denature membrane proteins and increase lipid fluidity, making the membrane more "leaky."

Don't give up! Transport is the foundation of almost everything else in Biology. Once you master the difference between "Passive" and "Active," the rest will fall into place!