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There was a lot of excitement today at 

the Biology School because today they 

were going on a field trip across the 

membrane! Wally the water molecule, Olivia 

the oxygen molecule and George the glucose 

molecule were all excited because they have 

never been through the membrane before! 

When they got to the membrane, Wally, Olivia and 

George were met by Mr. Transport to explain to them 

more about the membrane and how each of them was 

going to be able to enter it. The first thing that he 

explained to them was about the phospholipid bilayer. 

This is a double layer of phospholipids. It has to be 

a double layer because phospholipids consist of 

hydrophilic heads because of their phosphate 

group and hydrophobic tails that consists of 

unsaturated fatty acids. The hydrophilic heads 

protect the hydrophobic tails from polar substance. 

This bilayer tends to move and resemble a fluid.

 “Is that all that is in the membrane?” asked 

George.

      “Why of course not”, replied Mr. Transport, 

“Inside the membrane with the hydrophobic tails 

are cholesterol. Cholesterol is vital to the 

membrane and acts differently to protect it at 

different temperatures. When the membrane is 

getting too warm, the cholesterol has the power to 

restrain the movement of phospholipids to not 

make it too fluid. What the cell is getting too cool, 

cholesterol has the power to keep the hydrocarbon 

tails apart and maintain the fluidity of the 

membrane.”

     Next Mr. Transport told them about the proteins 

in the bilayer membrane. 

     “Do you kids know the 3 different kinds of 

proteins embedded in the fluid matrix of the lipid 

bilayer?” asked Mr. Transport.

     “I know the peripheral protein.” said Wally. “It 

is the proteins that are just on the surface if the 

membrane.”

     “That is correct!” exclaimed Mr. Transport. 

“I know there is something called integral 

proteins”, said Olivia. “They are the proteins that 

go through the entire membrane including the 

hydrophobic core. The hydrophobic regions of an 

integral protein are made out of stretches of 

nonpolar amino acids that are often in coils are 

alpha helices.”

“Wow you kiddos are so smart! Do any of you 

know what the last protein is called?”

“I think I do”, said George. “It is transport proteins. 

These are the proteins that allow the passage of 

hydrophilic substances across the membrane. There 

are two types. The first one is channel proteins that 

have a hydrophilic channel that certain molecules or 

ions can use as a tunnel. The next one is carrier 

proteins that are able to bind to molecules and 

change shape to shuttle them across the membrane. 

Transport proteins are specific for the substance it 

moves.”

“Wow your teacher has definitely taught you a lot 

about the plasma membrane!” said Mr. Transport in 

amazement.

“What are those things on top of the proteins?” 

asked Wally. 

“Ohhhh! I know!” exclaimed Olivia, “Those are 

carbohydrate bonded to the proteins. They are 

called glycoproteins. They are used in cell-to-cell 

recognition. Essentially they are the name tags of 

the cell and tell other cells what kind they are.”

“That is correct!” said Mr. Transport, “Now who is 

ready for the fun part?!”

“I am!” yelled Wally, Olivia, and George.

“We are all going to get to go through the 

membrane and enter the cell but because we are 

all different we have to enter the cell by different 

ways. There are two basic ways that each of you 

will enter. The first way is called passive transport. 

Passive transport requires no energy and it is when 

substances move along the concentration gradient. 

Right now there are less oxygen and water in the 

cells so Wally and Olivia will be able to move 

through the cell using passive transport.

Olivia is an oxygen molecule. This means that she 

is small and hydrophobic. She is able to enter the 

cell by a process called simple diffusion. This 

means she is able to go right through the bilayer of 

the membrane without any outside help.

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