Christine has a PhD in biomedical engineering.
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.
The transporter needs fuel to move glucose across the membrane. This fuel is not really a substance, but is actually the difference between the concentration of glucose inside and outside of the cell. Because the concentration outside the cell is typically higher, it is easy for glucose to find the transporter on the cell surface and bind to it. Once bound, the transporter can open up and allow the glucose to enter the cell where it is used up to provide energy for the cell. The difference in concentration is called the concentration gradient. Molecules naturally want to move from areas of high to low concentration; this is called diffusion. Because the glucose transporter works with the concentration gradient, its process of moving glucose across the cell membrane is called facilitated diffusion.
Let's review what we've learned. Glucose, a sugar molecule used by most living things for energy, needs to get into the cell because it is a major source of energy. But the cell membrane stands in its way, as it should; remember that it protects the cell from invasion, marks its boundaries and controls what can get in and out, similar to a walled medieval city. However, it would take too long for glucose to cross the cell membrane on its own through a double layer of lipids called a bilayer. This is because the hydrophobic region of the cell membrane makes it really hard for glucose to cross without a little help.
The glucose transporter is a membrane protein that allows molecules like glucose to cross into the cell and it goes through something called conformational change, which is when the protein changes its shape to get through easier when glucose is bound to it. But remember that it's not necessarily that easy. The difference in glucose concentration, called the concentration gradient can affect how much fuel the cell has. Glucose tends to move from an area of high concentration to an area of low concentration, a process called diffusion. Because the glucose transporter works with the concentration gradient, its process of moving glucose across the cell membrane is called facilitated diffusion.
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