Biology 5-6 A-Level Topic 6 of 14

Cell Membranes and Transport

Structure of plasma membranes and mechanisms of material transport across membranes.

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Topic Summary

Cell membranes are semi-permeable barriers that regulate the movement of molecules and ions between cells. Understanding how cell membranes transport materials is crucial for understanding cellular processes, such as nutrient uptake and waste removal.

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Key Revision Points

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Cell membranes regulate what enters and leaves a cell.
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Ion channels allow ions to pass through the membrane.
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Carrier-mediated transport helps move larger molecules across the membrane.
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Facilitated diffusion is an energy-independent process.
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The structure of plasma membranes affects their permeability.

💡 Real-World Application

Understanding how cells regulate what enters and leaves them is crucial for developing treatments for diseases like cancer, where abnormal cell growth can be influenced by changes in cellular transport mechanisms. For example, researchers have developed targeted therapies that exploit the altered membrane properties of cancer cells to deliver chemotherapy agents.

⚠️ Common Exam Misconception

Myth: Cell membranes are impermeable and only allow certain substances to pass through.

Reality: While cell membranes do regulate what enters and leaves a cell, they are not completely impermeable and can be influenced by various factors such as temperature, pH, and the presence of carrier proteins.

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Study Guide & Exam Tips

Simple Explanation

Think of your cell membrane like a security checkpoint. It controls who gets in and out, making sure only the right stuff makes it into or out of the cell.

Step-by-Step Method

Step 1: Identify the type of transport (passive or active).
Step 2: Determine if the substance is hydrophilic or hydrophobic.
Step 3: Consider the role of carrier proteins and ion channels in facilitating transport.
Step 4: Evaluate the energy requirements for each type of transport.

🎯 Examiner Tips

Make sure to understand the differences between passive and active transport.
Pay attention to the specific mechanisms used by cells to regulate what enters and leaves.

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Key Terms & Definitions

Term	Definition
Plasma membrane	The thin layer of lipid and protein molecules surrounding a cell.
Semi-permeable	Allowing certain substances to pass through while restricting others.
Ion channel	A protein structure that facilitates the movement of ions across the plasma membrane.
Carrier-mediated transport	The process by which molecules are transported across the plasma membrane with the help of carrier proteins.
Facilitated diffusion	The passive movement of molecules down their concentration gradient with the assistance of carrier proteins.

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Detailed Subtopics

Plasma membranes are semi-permeable barriers that surround cells and regulate the movement of molecules, ions, and water. The membrane is composed of a phospholipid bilayer with embedded proteins, cholesterol, and other lipids. Phospholipids have hydrophilic heads and hydrophobic tails, allowing them to form a barrier between the cell's interior and exterior. Proteins embedded in the membrane can be receptors, enzymes, or transport molecules.

Passive transport is the movement of molecules across the plasma membrane without the use of energy. This process occurs through diffusion, osmosis, and facilitated diffusion. Diffusion is the random movement of particles from an area of higher concentration to one of lower concentration. Osmosis is the movement of water molecules through a selectively permeable membrane from an area of higher concentration to one of lower concentration. Facilitated diffusion involves the use of transport proteins to facilitate the movement of specific molecules.

Active transport is the movement of molecules across the plasma membrane using energy. This process occurs through primary active transport, secondary active transport, and facilitated transport. Primary active transport uses ATP to pump ions or molecules against their concentration gradients. Secondary active transport uses the energy from one ion's movement to drive the movement of another ion in the same direction.

Ion channels are proteins that allow specific ions to pass through the plasma membrane. These channels can be selective, allowing only certain ions to pass, or non-selective, allowing all ions to pass. Ion pumps use energy from ATP to pump ions across the membrane against their concentration gradients. Examples of ion pumps include sodium-potassium pumps and calcium pumps.

The regulation of cellular transport is crucial for maintaining proper cell function. The movement of molecules across the plasma membrane can be regulated by various mechanisms, including changes in temperature, pH, or the presence of hormones. Additionally, some cells have specialized transport systems that allow them to regulate the movement of specific molecules.

Disruptions in cellular transport can lead to various diseases. For example, cystic fibrosis is caused by mutations in the CFTR gene, which codes for a chloride channel protein. This disruption leads to impaired ion transport and thickened mucus production. Other diseases related to cellular transport include kidney stones, which are caused by an imbalance of ions in the kidneys.

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Interactive Practice Questions

Quiz Progress: 0 / 10 answered

What is a characteristic of cell membranes?

Which type of transport helps move larger molecules across the plasma membrane?

What is the function of ion channels in cell membranes?

What is an example of a disease related to cellular transport?

Which type of transport does not require energy?

What is the structure of plasma membranes composed of?

What is the role of carrier proteins in facilitated diffusion?

What is an example of a mechanism used by cells to regulate what enters and leaves?

What is the difference between passive and active transport?

Q10

What is the function of ion pumps in cell membranes?

Essay 1 20 Marks

Describe the role of cell membranes in regulating what enters and leaves a cell.

Examiner Marking Scheme Requirements:

Cell membranes regulate what enters and leaves a cell
Ion channels allow ions to pass through the membrane
Carrier-mediated transport helps move larger molecules across the membrane

Essay 2 20 Marks

Explain how cells use energy-dependent mechanisms to transport molecules across their plasma membranes.

Examiner Marking Scheme Requirements:

Cells use primary active transport to pump ions or molecules against their concentration gradients
Secondary active transport uses the energy from one ion's movement to drive the movement of another ion in the same direction

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