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Membrane Transport: Diffusion, Facilitated Diffusion, and Active Transport, Lecture notes of Physiology

European Carolus Magnus UniversityPhysiology

An in-depth analysis of various models explaining how substances move in and out of cells through membranes. It covers topics such as simple diffusion, selectively permeable membranes, facilitated diffusion, and active transport. questions and answers to help understand the concepts, making it an essential resource for students studying cell biology or related fields.

Typology: Lecture notes

2021/2022

Uploaded on 08/05/2022

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Chapter 3.4 - Membrane Structure and Function
How do substances move in and out of cells?
Why?
An advertisement for sports drinks, such as Gatorade, PowerAde, and Vitaminwater, etc. seem to be everywhere. All of
these drinks are supposed to help your body recover and replenish lost electrolytes, fluids, and vitamins after exercise.
But how do the essential molecules contained in these drinks get into your cells quickly to help you recover after
exercise?
Model 1 – Simple Diffusion
1. How many different types of molecules are shown in Model 1? Two
2. Count and record the number of triangles and circles found on each side of the membrane.
Triangles — 14 on left, none on right; Triangles — 14 on left, none on right
3. Which shape is larger? Triangle
4. Describe the direction of the movement of the molecules in Model 1? Triangle
5. Which molecules are able to pass through the semi-permeable membrane? Justify your answer.
The dots, because they are small and can fit though the gaps, and because they are shown as equally distributed
on both sides of the membrane
Model 2 – The Selectively Permeable Cell Membrane
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Chapter 3.4 - Membrane Structure and Function How do substances move in and out of cells? Why? An advertisement for sports drinks, such as Gatorade, PowerAde, and Vitaminwater, etc. seem to be everywhere. All of these drinks are supposed to help your body recover and replenish lost electrolytes, fluids, and vitamins after exercise. But how do the essential molecules contained in these drinks get into your cells quickly to help you recover after exercise? Model 1 – Simple Diffusion

  1. How many different types of molecules are shown in Model 1? Two
  2. Count and record the number of triangles and circles found on each side of the membrane. Triangles — 14 on left, none on right; Triangles — 14 on left, none on right
  3. Which shape is larger? Triangle
  4. Describe the direction of the movement of the molecules in Model 1? Triangle
  5. Which molecules are able to pass through the semi-permeable membrane? Justify your answer. The dots, because they are small and can fit though the gaps, and because they are shown as equally distributed on both sides of the membrane

Model 2 – The Selectively Permeable Cell Membrane

  1. What two major types of biological molecules compose the majority of the cell membrane in Model 2? Phospholipids and proteins
  2. How many different protein molecules are found in Model 2? Four—two types of surface proteins and two types of membrane -spanning proteins
  3. What is the difference between the position of the surface proteins and the membrane-spanning proteins?

Surface proteins do not reach across the membrane, while membrane - spanning proteins do

  1. When a carbohydrate chain is attached to a protein, what is the structure called? Glycoprotein
  2. When a carbohydrate is attached to a phospholipid, what is the structure called? Glycolipid
  3. What types of molecules are shown moving across the membrane? Small polar and nonpolar molecules
  4. Where exactly in the membrane do these molecules pass through? Between the phospholipids
  5. How does the concentration of the small molecules inside the cell compare to that outside the cell? More small molecules are outside the cell compared to inside, so the concentration is higher outside compared to inside
  6. Because particles move randomly, molecules tend to move across the membrane in both directions. Does the model indicate that the molecules are moving in equal amounts in both directions? Justify your answer using complete sentences. No, more molecules are moving into the cell compared to moving out of the cell. The arrows show the movement

Read This! When there is a difference in concentration of a particular particle on either side of a membrane, a concentration gradient exists. Particles move along the concentration gradient from high to low concentration until a state of equilibrium is reached. At that point, there is no more net movement in one direction, although the particles continue to move randomly across the membrane, often called dynamic equilibrium. The net movement of particles along the concentration gradient is called diffusion.

  1. Look back at Models 1 and 2. Which particles are moving by diffusion across the membranes shown? Dots in Model 1 and small polar and nonpolar molecules (also represented by dots) in Model 2
  2. Using all the information from the previous models and questions circle the correct response to correctly fill in each blank. a. Diffusion is the net movement of molecules from an area of (low/ high ) concentration to an area of ( low /high) concentration. b. The molecules will continue to move along this (semi-permeable membrane/ concentration gradient ) until they reach (diffusion/ equilibrium ). c. Once equilibrium is reached, molecules will continue to move across a membrane ( randomly /in one direction).

Model 3 – Facilitated Diffusion

  1. In which direction is the transported substance moving—from an area of high concentration to low or from an area of low concentration to high? Support your answer. The transported substance is moving from an area of low concentration to high concentration. More particles are seen in the area toward which the arrow is pointing
  2. Is the substance being moved along (down) a concentration gradient? Justify your answer. The transported substance is moving from an area of low concentration to high concentration. More particles are seen in the area toward which the arrow is pointing
  3. ATP is a type of molecule that can provide energy for biological processes. Explain how the energy is being used in Model 4. Energy is used to change the shape of the protein (open the channel).
  4. What happens to the ATP after it binds to the protein? It changes to ADP
  5. The type of transport shown in Model 4 is called active transport, while diffusion and facilitated diffusion are called passive transport. Given the direction of the concentration gradient in active and passive transport examples, explain why active transport requires energy input by the cell. Active transport moves molecules against (up) a concentration gradient, whereas in passive transport molecules move down a concentration gradient. To move molecules against a gradient requires energy.
  6. 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 X

Molecules move along (down) a concentration gradient

X X

Moves molecules against (up) a concentration gradient

X

Always involves channel (membrane-spanning)proteins X^ X Molecules pass between the phospholipids

X

Moves ions like Na +^ and K +

X (though students might not know this from the activity)

X

Moves large molecules X X

Moves small nonpolar and polar molecules X

EXTRACREDIT

  1. Given the information in the graph, which type of cell transport would be best to move substances into or out of the cell quickly? Active transport
  2. Which type of transport would be the best if the cell needs to respond to a sudden concentration gradient difference? Diffusion—the rate increases as the concentration difference increases
  3. 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? Facilitated diffusion relies on proteins—the number of proteins will limit how fast the substance can be moved. Even if the concentration continues to increase, the number of protein channels will limit the rate of facilitated diffusion
  4. Why does active transport, on the same graph, start off with such a high initial rate compared to diffusion and facilitated diffusion? Active transport does not depend on a concentration gradient, only a supply of energy.