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High school biology
Course: High school biology > Unit 2
Lesson 3: The cell membraneThe cell membrane review
Key terms
| Term | Meaning |
|---|---|
| Cell membrane | Specialized structure that surrounds the cell and its internal environment; controls movement of substances into/out of cell |
| Hydrophobic | Molecule that repels water (“water-fearing”) |
| Hydrophilic | Molecule that is attracted to water (“water-loving”) |
| Amphipathic | Molecule that contains both a hydrophobic and a hydrophilic end |
| Phospholipid | Amphipathic lipid made of glycerol, two fatty acid tails, and a phosphate group |
| Phospholipid bilayer | A biological membrane involving two layers of phospholipids with their tails pointing inward |
| Semipermeable membrane | Membrane that allows certain substances to pass through |
Structure and function of the cell membrane
The cell membrane is semipermeable (or selectively permeable).
It is made of a phospholipid bilayer, along with other various lipids, proteins, and carbohydrates.
Each phospholipid is amphipathic, with two hydrophobic tails and a hydrophilic head. The hydrophobic tails face inward towards one another, and the hydrophilic heads face outwards.
The unique structure of the cell membrane allows small substances (like oxygen or carbon dioxide) to easily pass through.
Common mistakes and misconceptions
- Hydrophobic tails face inward and hydrophilic heads face outward. If you get these two ends mixed up, think of the root word “phobia” which means “fear.” Hydrophobic tails fear the water, so they will always try to be as far as possible from the water solutions in and out of the cell.
- The cell membrane contains a phospholipid bilayer, but the terms are not interchangeable. Part of the cell membrane is a phospholipid bilayer, made of two layers of phospholipid molecules. However, the cell membrane also contains other macromolecules like membrane proteins, and carbohydrates. Therefore, we can say that the cell membrane is made of a phospholipid bilayer, but it is not only made of it.
Want to join the conversation?
Does the phospholipid bilayer of the cell membrane, make the membrane "water resistant," due to inward hydrophobic tails and the outward hydrophilic heads?(10 votes)- Yes, it does, even though water can still get through the bilayer into the cytoplasm via the integral proteins in the bilayer(24 votes)
- why is a cell membrane semipermeable?(3 votes)
The cell is kinda like an exclusive concert. It can't let just anyone in. There are all sorts of free radicals and harmful metals that could invade the cell, not to mention those bacteria and viruses that constantly try to attack the cell. The cell needs a membrane to protect itself - to some degree - and establish an environment where it can do its job. You wouldn't want some stranger to randomly pop into your house, and neither does the cell. It does need some things from the outside world, though. That is why it allows certain particles to pass through the membrane. Hope this helps!(28 votes)
How can water pass through the phospholipid bilayer if it has aquaphobic tails? Won't the water molecules get repelled out?(6 votes)
I think another article said that some water molecules can sneak past the hydrophobic tails because they are very small (the water molecules, that is).(6 votes)
- What is the big difference between plasma membrane and cell membrane.(3 votes)
there is no big difference, it is the same thing. another name for a plasma membrane (or a cell membrane) is a fluid mosiac model. again there is no big difference between these names, they are the same thing.(5 votes)
- So, is the plasma membrane the same thing as the cell membrane?(3 votes)
Yes, the plasma membrane and cell membrane are the same things.(4 votes)
- How can a water pass through the phospholipid?(3 votes)
Water passes through channel proteins by osmosis, thus avoiding the hydrophobic centre of the lipid bilayer (the fatty acid tails are non-polar are therefore water cannot pass through them)(3 votes)
- how can a phospholipid be both hydrophilic and hydrophobic?(3 votes)
- The head is hydrophilic, the tails are hydrophobic.(3 votes)
- i have a question i do not understand the word phobia can anyone comment to explain a bit more thanks.(3 votes)
phobia is greek for fear >(2 votes)
- How does the water in phospholipid?(3 votes)
- I don't understand what you mean. Can you work your question a little differently? Do you mean "how does the water get past the lipids"?(2 votes)
- What Cannot pass through the cell membrane?(2 votes)
- Large molecules, polar molecules, and water on its own cannot pass through the cell membrane through the lipids. They need a channel.(3 votes)