Biology (9700) | Cell membranes and transport

Cell Membranes and Transport (9700 AS Level Biology)

Hello Biologists! Welcome to one of the most fundamental topics in cell biology. The cell membrane isn't just a boring bag holding the cell contents; it's a dynamic, selective barrier—the cell's ultimate gatekeeper.

In this chapter, we will uncover the structure of this amazing barrier (the fluid mosaic model) and learn the essential ways substances move across it. Understanding transport is key to understanding how every cell, from a tiny bacterium to a human nerve cell, maintains its perfect internal environment (homeostasis). Let's dive in!

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4.1 The Fluid Mosaic Model of Membranes

The structure of the cell membrane is described by the Fluid Mosaic Model, proposed by Singer and Nicholson in 1972.

The name itself gives us the two key properties:

• Fluid: The components (especially phospholipids) are not static; they move laterally (sideways), giving the membrane flexibility.
• Mosaic: Proteins are embedded within the lipid bilayer like scattered tiles in a mosaic pattern.

The Phospholipid Bilayer: The Foundation

The membrane is primarily composed of a double layer of phospholipids. Remember from Topic 2 that phospholipids have two distinct regions:

1. Hydrophilic Head: Contains the phosphate group. It is polar and "water-loving."
2. Hydrophobic Tails: Consist of two fatty acid chains. They are non-polar and "water-hating."

In an aqueous (water-based) environment (like the cytoplasm and tissue fluid), the phospholipids spontaneously arrange themselves into a bilayer. This arrangement is highly stable because:

• The hydrophilic heads face outwards, interacting with the surrounding water.
• The hydrophobic tails face inwards, shielded from the water by the heads (hydrophobic interactions).

Quick Review: This spontaneous formation accounts for the membrane’s basic stability and selective permeability.

Other Key Components and Their Roles

The mosaic part of the model comes from the other molecules embedded within or attached to the bilayer:

1. Proteins

Proteins are crucial for function and provide the ‘mosaic’ look. They are categorized based on their position:

• Intrinsic/Integral Proteins: Span the entire membrane (transmembrane proteins). They are typically involved in transport (as carrier proteins or channel proteins) or act as receptors.
• Extrinsic/Peripheral Proteins: Found only on the surface of the membrane, providing mechanical support or acting as enzymes.

2. Cholesterol

Cholesterol molecules are embedded between the phospholipid tails (found only in animal cells, not plant cells).

• Role in Fluidity: At high temperatures, cholesterol reduces fluidity, preventing the membrane from becoming too liquid.
• Role in Stability: At low temperatures, cholesterol prevents the phospholipids from packing too closely, stopping the membrane from becoming too rigid.

3. Glycocalyx (Glycoproteins and Glycolipids)

These are carbohydrate chains attached to proteins (glycoproteins) or lipids (glycolipids) on the external surface of the membrane.

• Cell Recognition: They act as cell surface antigens, allowing the immune system to recognize the cell as "self" (e.g., blood groups).
• Cell Signalling: They function as receptor sites for hormones or neurotransmitters.

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Cell Signalling (Cell Surface Receptors)

Cells need to communicate with each other, often over long distances. This process relies on the receptors found on the cell membrane.

The Main Stages of Cell Signalling:

1. Secretion: A cell secretes specific chemical molecules called ligands (e.g., hormones like insulin).
2. Transport: The ligand travels (usually via the bloodstream) towards its specific target cells.
3. Binding: The ligand binds specifically to complementary cell surface receptors (which are usually glycoproteins or proteins) on the target cell's membrane.
4. Specific Response: Binding causes a conformational (shape) change in the receptor protein, triggering a specific response inside the target cell (e.g., activating an enzyme or opening an ion channel).

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4.2 Movement Into and Out of Cells

Movement across the membrane is classified based on whether it requires energy (ATP) and whether it goes with or against the concentration gradient.

I. Passive Transport (No ATP Required)

These processes rely on the random kinetic energy of molecules and always move substances down the concentration gradient (from high concentration to low concentration).

1. Simple Diffusion

This is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration.

• Molecules Transported: Small, non-polar molecules like oxygen (\(O_2\)) and carbon dioxide (\(CO_2\)). Lipids and fat-soluble molecules can also pass directly through the hydrophobic tails.
• Mechanism: Moves directly through the phospholipid bilayer.
• Rate Factors: Concentration gradient, temperature, surface area, and distance traveled.

Analogy: Simple diffusion is like walking out of a very crowded room into an empty corridor. It happens naturally.

2. Facilitated Diffusion

This is passive movement that requires the help of specific membrane proteins because the molecule is too large or too polar (charged) to pass through the hydrophobic core.

• Molecules Transported: Polar molecules (like glucose, amino acids) and ions.
• Mechanism: Uses channel proteins (which form water-filled pores for ions) or carrier proteins (which bind to the molecule and change shape).
• Key Difference from Simple Diffusion: The rate of facilitated diffusion can be limited by the number of carrier/channel proteins available (saturation).

3. Osmosis

Osmosis is specifically the diffusion of water molecules across a partially permeable membrane (PPM).

• Definition: Net movement of water from a region of higher water potential ($\Psi$) to a region of lower water potential ($\Psi$).

Don't worry if the term "water potential" seems tricky! For AS Level, just remember:

Pure water has the highest possible water potential ($\Psi = 0$ kPa). Adding solutes lowers the water potential (making it more negative). Water always moves to where there is "less free water" (a lower $\Psi$).

II. Active Transport (ATP Required)

This is the movement of molecules or ions against their concentration gradient (from low concentration to high concentration).

• Energy Source: Requires metabolic energy, usually in the form of ATP.
• Proteins: Requires specific carrier proteins, often called pumps (e.g., the sodium-potassium pump).
• Mechanism: The molecule binds to the carrier protein; ATP hydrolysis releases energy, causing the protein to change shape and push the molecule across the membrane.

Analogy: Active transport is like trying to force water uphill. You need energy (ATP) and a mechanical device (the pump/carrier protein) to do it.

III. Bulk Transport (Moving the Masses)

Bulk transport is used to move large quantities of material, too large for proteins, by changing the shape of the membrane itself. It requires energy (ATP).

1. Endocytosis

The process where the cell takes in materials by wrapping its cell surface membrane around the substance, forming a vesicle inside the cytoplasm.

• Phagocytosis: Taking in solids (e.g., macrophages engulfing bacteria).
• Pinocytosis: Taking in liquids/solution ("cell drinking").

2. Exocytosis

The reverse process, where substances (like hormones, waste products, or enzymes) are packaged into vesicles, which then fuse with the cell surface membrane, releasing the contents outside the cell.

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4.3 Effects of Water Potential on Cells

When a cell is placed in a solution, water moves via osmosis, and the resulting gain or loss of water has very different effects on plant and animal cells due to the presence of the cell wall in plants.

Animal Cells (No Cell Wall)

Animal cells (like red blood cells) are very sensitive to changes in water potential because they have no protective cell wall.

1. Low External Solute Concentration (High $\Psi$):
   • Water moves into the cell.
   • The cell swells.
   • Result: The cell membrane cannot withstand the pressure and bursts (a process called lysis).
2. High External Solute Concentration (Low $\Psi$):
   • Water moves out of the cell.
   • Result: The cell loses volume and the membrane shrivels up (a process called crenation).

Plant Cells (With Cell Wall)

Plant cells are protected by the strong, rigid cell wall, which prevents lysis.

1. Low External Solute Concentration (High $\Psi$):
   • Water moves into the cell (mainly into the vacuole).
   • The cytoplasm pushes against the cell wall, generating turgor pressure.
   • Result: The cell becomes turgid (firm). This is the normal, healthy state for plant cells. The cell wall prevents bursting.
2. High External Solute Concentration (Low $\Psi$):
   • Water moves out of the cell.
   • The cell membrane pulls away from the cell wall.
   • Result: The cell becomes plasmolysed. This is usually irreversible and causes wilting.

Common Mistake Alert: Students often confuse "turgid" and "lysed." Turgid is healthy (plant cells); Lysis is death (animal cells).

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4.4 The Importance of Surface Area to Volume Ratio (SA:V)

For transport processes like diffusion to be effective, cells and organisms rely on maintaining a good balance between their surface area and their volume.

As a cell or organism increases in size, its volume increases much faster than its surface area. This means the SA:V ratio decreases.

Why a high SA:V is essential:

A high SA:V ratio means there is a proportionally large surface area across which substances (like oxygen, nutrients, or waste) can be exchanged relative to the volume of the cell that needs servicing.

• High SA:V = substances can move in and out efficiently, quickly reaching all parts of the cell. (Good for rapid diffusion).
• Low SA:V = transport distance increases, making diffusion slow and inefficient.

Did you know? Many active cells (like cells lining the intestine) are very small or have folds called microvilli to maximize their surface area and maintain a high SA:V ratio for rapid absorption.

Practical Application (Agar Blocks)

You might investigate this using agar blocks containing indicator dye, which changes colour as a diffusing substance (like acid) moves in.

If you compare a large block (low SA:V) and a small block (high SA:V), you will observe that the substance diffuses throughout the small block much faster because the distance required for transport is minimal.