The Cell Membrane

Essential Question

How does the cell membrane control what enters and leaves?

Every cell is surrounded by a membrane that is only a few molecules thick, yet it controls what the cell takes in and gives off. In this lesson, you’ll learn how the membrane is built from phospholipids and proteins, why some substances cross easily and others cannot, and what the proteins in the membrane do.

A Selective Barrier

• The membrane is selectively permeable
• Small nonpolar molecules cross easily
• Charged ions and large polar molecules need proteins
• Water crosses slowly through the lipids

A cell membrane is selectively permeable, which means it lets some substances cross while it limits others. Small nonpolar molecules such as oxygen and carbon dioxide slip through the lipid part of the membrane. Charged ions such as sodium and large polar molecules such as glucose cannot cross the lipids, so they need proteins to cross. Water is polar, but it is small, so some of it crosses the lipids slowly, and in many cells water also crosses through water channel proteins.

The Fluid Mosaic

• Phospholipids form a double layer (bilayer)
• Heads are hydrophilic and face the water
• Tails are hydrophobic and face each other
• Proteins are embedded in the bilayer

In Unit 5 you learned that a phospholipid has a hydrophilic head and two hydrophobic tails. In water, phospholipids arrange themselves into a bilayer: the heads face the watery fluid on both sides, and the tails face each other in the middle, away from the water. Proteins are embedded in the bilayer. The lipids and many of the proteins can drift sideways, so scientists call this the fluid mosaic model: fluid because the parts move, and a mosaic because it is made of many different parts.

Membrane Proteins

• Channel proteins form tunnels for ions
• Carrier proteins bind and move a substance
• Receptor proteins bind signal molecules
• Marker proteins identify the cell

Proteins carry out most of the membrane’s special jobs. A channel protein forms a tunnel that lets certain ions or small molecules pass through. A carrier protein binds a specific substance, changes shape, and releases it on the other side. A receptor protein binds a signal molecule from outside the cell and passes the message into the cell. Marker proteins, often with sugar chains attached, identify a cell so that other cells can recognize it. A cell can change which proteins it has, which changes what it lets in and out.

Keeping Conditions Steady

• Nonpolar molecules cross much faster than polar ones
• Charged ions cross a plain bilayer hardly at all
• Proteins let the cell control the rest
• Control of entry and exit helps keep inside conditions steady

The graph shows how quickly different substances cross a bilayer that has no proteins (sample values, on a log scale). Nonpolar oxygen and carbon dioxide cross fastest, water is slower, and glucose and ions barely cross at all. Because most of a cell’s needs are polar or charged, the cell must use membrane proteins to bring them in, and it can adjust those proteins. This control lets the cell keep conditions inside steady even when conditions outside change, which is a form of homeostasis.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Name the Membrane Proteins

The diagram shows four proteins in a cell membrane, with a clue about each one. Name each protein: channel, carrier, receptor, or identity marker.

Show the solution

1. A: a tunnel that ions pass through is a channel protein.
2. B: a protein that binds glucose, changes shape, and releases it is a carrier protein.
3. C: a protein that binds a signal molecule from outside is a receptor protein.
4. D: a sugar chain that other cells recognize is an identity marker.
5. Answer: A: channel; B: carrier; C: receptor; D: identity marker.

Example 2: Which Substances Cross Easily?

The graph shows the relative rate at which substances cross a plain phospholipid bilayer (sample values, log scale). Write a claim, evidence, and reasoning about which kinds of substances cross easily.

Show the solution

1. Claim: small nonpolar molecules cross easily, and charged ions hardly cross at all.
2. Evidence: oxygen (100) and carbon dioxide (90) cross far faster than glucose (0.001) and sodium ions (0.00001).
3. Reasoning: the middle of the bilayer is hydrophobic and nonpolar, so it lets nonpolar molecules through but blocks charged and large polar substances.
4. Answer: Small nonpolar molecules (oxygen and carbon dioxide) cross easily; ions and glucose do not.

Example 3: Predict How Each Substance Crosses

A cell is placed in a solution that contains oxygen, glucose, and sodium ions. Use the model to predict how each substance can cross the membrane.

Show the solution

1. Oxygen: small and nonpolar, so it passes through the lipid bilayer.
2. Sodium ions: charged, so they are blocked by the lipids and must cross through a channel protein.
3. Glucose: large and polar, so it needs a carrier protein.
4. Answer: Oxygen crosses the bilayer; sodium ions use a channel; glucose uses a carrier.

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Practice

Label the parts of the membrane, predict how substances cross it, read data on how fast substances cross a bilayer, and explain why ions are blocked. Try it on your own first, then check your work with the answer key.

• The Cell Membrane Practice Worksheet (PDF)
• The Cell Membrane Practice Answer Key (PDF)

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Lesson Slides

• 6.5 The Cell Membrane Slides (PDF)

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Big Ideas

• The membrane is selectively permeable
• Phospholipids form a bilayer with hydrophobic tails inside
• Small nonpolar molecules cross; ions and polar molecules need proteins
• Channel, carrier, receptor, and marker proteins do special jobs

Back to the essential question: the membrane is a phospholipid bilayer with proteins embedded in it. Its hydrophobic middle lets small nonpolar molecules cross and blocks ions and large polar molecules, which need proteins to cross. By controlling what enters and leaves, the membrane helps the cell keep stable conditions inside. Next, you’ll see how substances move across the membrane without using energy.

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