Cell membrane introduction | Cells | MCAT | Khan Academy

Summary

The video explains the crucial role of the cell membrane, a semi-permeable structure that protects cells by controlling what enters and exits. It is primarily composed of phospholipids, which have hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails. This forms a phospholipid bilayer that allows small, nonpolar molecules like oxygen to pass through easily, while large, polar, or charged molecules, such as glucose and ions, cannot. The video highlights how this selective permeability enables cell survival in various environments.

Takeaways

• 🔍 The cell membrane is a crucial structure that protects cells and controls what enters and exits, maintaining the cell's integrity.
• 🧫 The cell membrane's main component is the phospholipid, which has a phosphate head, a glycerol backbone, and two fatty acid tails.
• 💧 Phosphate heads are hydrophilic (water-loving) and interact well with water, while fatty acid tails are hydrophobic (water-fearing) and avoid water.
• ⚖️ The cell membrane is semi-permeable, meaning it allows some substances to pass through while blocking others.
• 📏 Small and nonpolar molecules, such as oxygen (O2) and carbon dioxide (CO2), can easily pass through the membrane via passive diffusion.
• 🚫 Large or polar molecules, like glucose or ions, cannot pass through the membrane without assistance due to size and polarity.
• 🔄 The cell membrane forms a phospholipid bilayer structure, where hydrophilic heads face water, and hydrophobic tails are shielded inside.
• 🌐 In real cells, the membrane forms a complete circle, creating a barrier between the internal and external environments.
• 💡 Amphipathic molecules, like phospholipids, have both hydrophilic and hydrophobic regions, allowing them to create this unique bilayer structure.
• 🚪 The cell membrane's structure and properties are key to the survival of cells, as they regulate molecular movement and protect cell contents.

Q & A

What prevents water from entering our cells or substances from leaking out when we swim or shower?

The cell membrane prevents water from entering our cells or substances from leaking out. It acts as a protective barrier between the cell and the outside environment.

What is the structure that forms the outer layer of a cell and regulates what enters and leaves the cell?

The cell membrane forms the outer layer of a cell and regulates what can enter and leave the cell. It has the property of semi-permeability, allowing selective substances to pass through.

What is the main building block of the cell membrane?

The main building block of the cell membrane is phospholipids, which consist of a phosphate head, a glycerol backbone, and two fatty acid tails.

How are phospholipids arranged in the cell membrane, and why is this arrangement important?

Phospholipids are arranged in a bilayer, where the hydrophilic (water-loving) phosphate heads face outward toward water, and the hydrophobic (water-fearing) fatty acid tails face inward, away from water. This arrangement is crucial for forming a barrier that selectively allows substances to pass through.

What does it mean for a molecule to be amphipathic?

An amphipathic molecule has both a hydrophilic (water-loving) and a hydrophobic (water-fearing) section. Phospholipids are amphipathic, which allows them to form the bilayer structure of the cell membrane.

Why can small nonpolar molecules pass through the cell membrane easily?

Small nonpolar molecules, like oxygen (O2) and carbon dioxide (CO2), can pass through the cell membrane easily because they are able to move through the hydrophobic region of the membrane without interference.

Can water and ethanol pass through the cell membrane? If so, how quickly?

Water and ethanol, both small polar molecules, can pass through the cell membrane, but they do so slowly because the hydrophobic region of the membrane does not favor polar molecules.

Why can't large polar molecules like glucose pass through the cell membrane?

Large polar molecules like glucose cannot pass through the cell membrane because they are both large and polar, which makes it difficult for them to move through the tightly packed phospholipids and the hydrophobic region.

What type of molecules are completely blocked from passing through the cell membrane?

Charged molecules, such as ions (e.g., chloride or sodium ions) and amino acids, are completely blocked from passing through the cell membrane because they are highly polar and unable to interact with the hydrophobic interior of the membrane.

What is passive diffusion, and which molecules undergo this process in cells?

Passive diffusion is the movement of molecules across the cell membrane without the use of energy, typically occurring for small, nonpolar molecules like oxygen (O2) and carbon dioxide (CO2).