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Membrane Structure and Function 1
Membrane Structure and Function
How do substances move in and out of cells?
Why?
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Model 1 – Simple Diffusion
Semi-permeable membrane
- How many different types of molecules are shown in Model 1?
2 Count and record the number of triangles and circles found on each side of the membrane.
- Which shape is larger?
- Describe the direction of the movement of the molecules in Model 1?
- Which molecules are able to pass through the semi-permeable membrane? Justify your answer.
- If you left this “system” for an extended period of time and then viewed it again, would you ex- pect to find any changes in the concentrations of the molecules on either side of the membrane? Justify your answer.
2 POGIL™^ Activities for High School Biology
Model 2 – The Selectively Permeable Cell Membrane
Outside the cell
Inside the cell
Small nonpolar or small polar molecules Phospholipid Small surface protein Membrane-spanning protein Carbohydrate chain Glycoprotein Glycolipid
- What two major types of biological molecules compose the majority of the cell membrane in Model 2?
- How many different protein molecules are found in Model 2?
- What is the difference between the position of the surface proteins and the membrane-spanning proteins?
- When a carbohydrate chain is attached to a protein, what is the structure called?
- When a carbohydrate is attached to a phospholipid, what is the structure called?
- What types of molecules are shown moving across the membrane?
- Where exactly in the membrane do these molecules pass through?
- How does the concentration of the small molecules inside the cell compare to that outside the cell?
4 POGIL™^ Activities for High School Biology
Model 3 – Facilitated Diffusion
Glucose Hormones
Hormone binding site
Gated channel Channel begins to open
- Which part of the cell membrane is shown in more detail in Model 3?
- What is the gap between the proteins called?
- What type of molecules attach to the protein?
- Explain in detail what happened that allowed the glucose molecules to pass through.
Membrane Structure and Function 5
Read This!
Some molecules, such as glucose, use gated channels as shown in Model 3; however, not all channels are gated. Some channels remain permanently open and are used to transport ions and water across the cell membrane.
- Discuss with your group why the type of protein channel in Model 3 is called a gated channel. Write your group’s responses below.
- To facilitate means to help. Explain why this type of diffusion is called facilitated diffusion.
- The “tails” of phospholipids are nonpolar; therefore, they do not readily interact with charged particles such as ions. How can this explain why facilitated diffusion is necessary for the transport of ions such as Na +^ and K+^ across the cell membrane? In other words, why would these ions not cross by simple diffusion?
Model 4 – Active Transport
ATP ATP
ADP
Ion-binding site
ATP-binding site
Substance to be transported
Membrane Structure and Function 7
- With your group, complete the table below to show the difference between active and passive transport.
Active Transport
Passive Transport Diffusion Facilitated Diffusion Requires energy input by the cell Molecules move along (down) a concentration gradient Moves molecules against (up) a concentration gradient Always involves channel (membrane-spanning)proteins Molecules pass between the phospholipids
Moves ions like Na +^ and K+
Moves large molecules
Moves small nonpolar and polar molecules
- With your group develop a defi nition for active transport.
8 POGIL™^ Activities for High School Biology
Extension Questions
Active transport
Diffusion
Concentration difference
Rate of transport
Facilitated diffusion
- Given the information in the graph, which type of cell transport would be best to move substances into or out of the cell quickly?
- Which type of transport would be the best if the cell needs to respond to a sudden concentration gradient difference?
- Why would the line representing facilitated diffusion level off as the concentration gets higher, while the line representing diffusion continues to go up at a steady rate?
- Why does active transport, on the same graph, start off with such a high initial rate compared to diffusion and facilitated diffusion?
