How Do Sugar Molecules Cross the Cell Membrane?

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  • 0:00 Why Cells Need Sugar
  • 0:37 How the Cell Membrane Works
  • 1:43 Protein Gates Called…
  • 2:25 Concentration Gradients
  • 3:11 Lesson Summary
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Lesson Transcript
Instructor: Christine Kielhorn

Christine has a PhD in biomedical engineering.

Sugar molecules cannot cross the cell membrane on their own. Special proteins embedded in the cell membrane are required to transport sugar across the cell membrane. Read on to learn more about this process and take a quiz.

Why Cells Need Sugar

A cell is kind of like a city. It has several moving parts and jobs that need to be done. And just like a city, a cell needs energy to function. But instead of gas or electricity, cells need sugar. Sugar is typically present outside the cell in the form of glucose, a sugar molecule used by most living things for energy, and it must get into the cell to be used to generate energy. However, the cell membrane is kind of like the wall of a medieval city. It's difficult to cross without special permission, and even a molecule as important as glucose needs help getting across.

How the Cell Membrane Works

Like a city wall, the cell membrane marks the borders of the cell and protects it from invasion. The city wall is studded with towers and gates to allow merchants, messengers and farmers to come and go so that the city can survive. Similarly, the cell membrane also controls what comes in and out of the cell.

One of the ways materials can enter the cell is through special proteins that are embedded in the membrane. These proteins act like gates to allow large molecules, like glucose, to get across the membrane. If glucose tried to cross the membrane without the protein gate, it would take a very long time.

The cell membrane is made of a double layer of lipids, called a bilayer. Lipids are molecules with a hydrophilic head and hydrophobic tail. The hydrophobic tails stick together to create the bilayer, so the hydrophilic heads line the interior and exterior of the cell, but in between is a hydrophobic region. In addition to being a relatively large molecule, glucose is very hydrophilic. It would be very difficult for glucose to move through the hydrophobic portion of the membrane bilayer because hydrophilic and hydrophobic substances do not like to mix.

Protein Gates Called Transporters

Membrane proteins that allow molecules like glucose to cross into the cell are called transporters. These proteins are transmembrane proteins. They have a hydrophobic region that secures the protein inside the membrane, and hydrophilic regions that are located inside and outside the cell.

The glucose transporter has a special binding site for glucose in its hydrophilic region. When a glucose molecule binds to this site, it communicates to the transporter that the glucose is ready to move across the membrane. The protein can then change shape, or undergo a conformational change, to allow the glucose to enter the cell, similar to the opening of a gate. The glucose molecule never has to touch the hydrophobic portion of the cell membrane.

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