Exam style questions on Transport across membranes

Question 1

Why can lipid-soluble molecules diffuse through the phospholipid bilayer while polar substances cannot?

Answer 1

Lipid-soluble molecules interact with the hydrophobic tails of the bilayer, but polar substances are repelled by the hydrophobic core.

Question 2

What is simple diffusion?

Answer 2

Simple diffusion is the net movement of molecules from an area of high concentration to low concentration without requiring energy or ATP.

Question 3

What conditions are necessary for simple diffusion to occur?

Answer 3

Molecules must be small, non-polar, and lipid-soluble.

Question 4

How does facilitated diffusion differ from simple diffusion?

Answer 4

Facilitated diffusion uses carrier or channel proteins for transport, allowing polar or charged molecules to cross, while simple diffusion transports small, non-polar molecules directly through the bilayer.

Question 5

How are carrier proteins involved in facilitated diffusion?

Answer 5

Carrier proteins bind to specific molecules, change shape, and allow the molecule to cross the membrane.

Question 6

What is osmosis?

Answer 6

Osmosis is the movement of water from an area of higher water potential to lower water potential through a partially permeable membrane.

Question 7

What are the effects of isotonic, hypotonic, and hypertonic solutions on animal cells?

Answer 7

-Isotonic: No net water movement; cell remains the same.
-Hypotonic: Water enters, causing the cell to swell and potentially burst
-Hypertonic: Water leaves, causing the cell to shrivel

Question 8

What determines the direction of water movement in osmosis?

Answer 8

The water potential gradient across the membrane

Question 9

How does active transport differ from diffusion?

Answer 9

-Moves molecules from low to high concentration, against the gradient, requiring ATP and carrier proteins.

Question 10

How is ATP used in active transport?

Answer 10

ATP is hydrolyzed into ADP and Pi, releasing energy. This energy changes the shape of the carrier protein, allowing it to pump molecules across the membrane.

Question 11

Why do plant cells not burst in hypotonic solutions, unlike animal cells?

Answer 11

Plant cells have a rigid cell wall that withstands pressure, while animal cells lack this structure and burst

Question 12

What happens to a red blood cell placed in a solution with a higher water potential than its interior?

Answer 12

Water enters the cell by osmosis, causing it to swell and potentially burst due to the lack of a cell wall.

Question 13

What is active transport? How does it differ from facilitated diffusion?

Answer 13

-Moves substances from low to high concentration.
-Requires energy in the form of ATP.
-Facilitated diffusion does not require energy and occurs down the concentration gradient.

Question 14

What is the role of ATP in active transport?

Answer 14

-ATP binds to the carrier protein.
-ATP is hydrolysed into ADP and a phosphate group.

Question 15

What structural changes does ATP cause to the carrier protein?

Answer 15

-The phosphate group attaches to the carrier protein, altering its tertiary structure.
-This change enables transport across the membrane.

Question 16

How is the specificity of carrier proteins determined during active transport?

Answer 16

-Carrier proteins have a receptor site with a shape complementary to the transported molecule.
-Only molecules with the complementary shape can bind and be transported.

Question 17

What happens to the carrier protein after the phosphate group detaches? Why is this important?

Answer 17

-The phosphate group detaches, restoring the carrier protein’s original shape.
-Essential for the protein to transport more molecules.

Question 18

Why is co-transport necessary for glucose absorption in the ileum?

Answer 18

-Glucose concentration is higher in epithelial cells than in the lumen.
-This prevents glucose from diffusing into epithelial cells.
-Co-transport allows glucose to move against its concentration gradient.

Question 19

What are the steps involved in the co-transport of glucose with sodium ions?

Answer 19

-Sodium ions are actively transported out of the epithelial cell into the blood using ATP.
-This lowers sodium ion concentration in the epithelial cell.
-Sodium ions move into the cell by facilitated diffusion via a co-transporter protein.
-Glucose binds to the same co-transporter protein and is transported into the epithelial cell with sodium ions.
-Glucose moves into the blood by facilitated diffusion.

Question 20

Why does sodium need to be actively transported from the epithelial cell to the blood before co-transport can occur?

Answer 20

-Active transport of sodium into the blood creates a low sodium concentration in the epithelial cell.
-This establishes the gradient necessary for sodium ions to move into the epithelial cell from the lumen.

Question 21

What is the role of microvilli in glucose absorption?

Answer 21

Increase the surface area of epithelial cells.

Question 22

Why are microvilli beneficial for co-transport?

Answer 22

-This allows more co-transporter proteins to be embedded in the membrane.
-The increased number of proteins enhances glucose and sodium absorption.

Question 23

Blood flow in capillaries maintains a concentration gradient for glucose. How does this help optimise glucose absorption?

Answer 23

-Continuous blood flow removes glucose from the capillaries.
-This maintains a steep concentration gradient for glucose to move into the blood by facilitated diffusion.

Question 24

A mutation prevents the production of a functional sodium-potassium pump in epithelial cells. How would this affect glucose absorption in the ileum?

Answer 24

-Sodium cannot be actively transported out of the epithelial cell into the blood.
-This prevents the creation of a low sodium concentration in the epithelial cell.
-Sodium ions from the lumen cannot enter the epithelial cell via facilitated diffusion.
-As a result, glucose absorption into the epithelial cell and subsequently the blood is impaired.

Question 25

How might a reduction in the number of co-transporter proteins on epithelial cells impact glucose absorption?

Answer 25

-Fewer co-transporter proteins reduce the rate of glucose and sodium absorption into epithelial cells.
-Less glucose enters the bloodstream.
(reduced energy supply to body cells, leading to fatigue or malnutrition)

Question 26

Features found in a cell specialized for absorption

Answer 26

• microvilli - large surface area
• large number of co transport / carrier proteins - fast rate of absorption
• Large number of mitochondria - make more ATP by respiration
• Membrane bound enzymes - maintain concentration gradient for faster absorption

Question 27

How is the movement of substances across cell membranes affected by membrane structure?

Answer 27

• Phospholipid bilayer allows movement of non- polar molecules
• Carrier proteins allow active transport
• Carrier/ channel proteins allow facilitated diffusion
• number of channels/carriers determine how much movement
• cholesterol affects fluidity/rigidity/permeability