Transport across cell membranes

Question 1

How do movement across cell membranes occur by?

Answer 1

• Simple and facilitated diffusion
• Osmosis
• Active transport
• Co-transport

Question 2

What is simple diffusion?

Answer 2

The net movement of molecules down a concentration gradient from an area of high concentration to an area of low concentration.

Question 3

What point does diffusion occur till?

Answer 3

Dynamic equilibrium, until the concentration in both substances are equal

Question 4

Why is simple diffusion passive?

Answer 4

Because it does not require energy in the form of ATP

Question 5

How do molecules move with no chemical energy.

Answer 5

Kinetic energy

Question 6

What must the molecules be like in order for them to diffuse across the membrane using simple diffusion?

Answer 6

Lipid-soluble and small

Question 7

What are the factors that affect the rate of simple diffusion and how?

Answer 7

1. Concentration gradient.
   The higher the concentration gradient, the increase in rate of simple diffusion
2. Diffusion distance.
   The larger the diffusion distance, the more decrease there will be in rate of simple diffusion. It is only effective in transporting substances over a short distance
3. Surface area.
   The larger the surface area the increase in rate of simple diffusion
4. Molecular size.
   Smaller molecules will diffuse faster
5. Temperature
   The higher the temperature, the more kinetic energy the particles will have which means there will be an increase in the rate of simple diffusion
6. Stirring
   This applies to experiments which will increase the rate of simple diffusion

Question 8

Why does the membrane have to be thin?

Answer 8

To provide the shortest pathway for the substances to diffuse across.

Question 9

What is facilitated diffusion? Is it passive?

Answer 9

The process where ions and molecules are transported across the membrane using protein channels and carrier proteins since they cannot be transported by simple diffusion. It is passive since it does not require ATP.

Question 10

What kind of molecules pass through facilitated diffusion?

Answer 10

Large molecules that cannot diffuse by simple diffusion.

Question 11

What is osmosis?

Answer 11

The movement of water molecules from an area of high water potential to an area of low water potential across a partially permeable membrane.

Question 12

What is water potential and its units?

Answer 12

The pressure created by water molecules and is measured in kPa.

Question 13

What is the water potential of pure water?

Answer 13

0

Question 14

When solutes are dissolved in water, does the water potential become more negative or more positive?

Answer 14

Negative

Question 15

What is an isotonic solution?

Answer 15

When the water potential is the same in the solution and the cell within the solution.

Question 16

What is a hyper-tonic solution?

Answer 16

When the water potential of a solution is more negative than the cell.

Question 17

What is a hypo-tonic solution?

Answer 17

When the water potential of a solution is more positive than the cell.

Question 18

What happens if animal cells are placed in solutions that are:
- Isotonic
- Hypotonic
- Hypertonic

Answer 18

Isotonic:
No loss or gain of water.

Hypertonic:
More water leaves the cell causing it to shrivel

Hypotonic:
More water enters the cell causing it to burst

Question 19

Why is the plasma membrane referred to as fluid-mosaic?

Answer 19

Mosaic - different components including phospholipids and glycoproteins
Fluid - movement of these components.

Question 20

What are the molecules in the plasma membrane that make up the partially permeable membrane?

Answer 20

Phospholipids, proteins, glycoproteins and glycolipids, cholesterol.

Question 21

How is the phospholipid bilayer arranged.

Answer 21

Two layers of phospholipids. Hydrophilic heads face outside hydrophobic tails face inside.

Question 22

What is the difference between hydrophobic tails and hydrophilic heads?

Answer 22

Hydrophobic tails repel water
Hydrophilic heads attract water

Question 23

What is the purpose of cholesterol and how is this useful?

Answer 23

• Restricts the lateral movement of other molecules in the membrane.
• This prevents the membrane from being too fluid which causes gaps in the membrane and causes water and dissolved ions to leak out of the cell causing the cell to burst.

Question 24

Where are proteins found and what are the 2 types?

Answer 24

They are embedded across the cell membrane.
Peripheral - embedded completely across the membrane
Integral - embedded from one side of the bilayer to the other.

Question 25

What is the function of peripheral and integral proteins?

Answer 25

Peripheral - they are connected to proteins to make glycoproteins or glycolipids.

Integral - carrier proteins or channel proteins involved in the transport of molecules across the membrane

Question 26

What is the function of glycolipids and glycoproteins?

Answer 26

Cell recognition

Question 27

What are protein channels?

Answer 27

Tubes filled with water that allow water-soluble ions to diffuse through the channel.

Question 28

Why are protein channels selective?

Answer 28

They only open to certain ions when they bind to the protein.

Question 29

What are carrier proteins?

Answer 29

Proteins that bind onto larger molecules (e.g. glucose) and change shape to transport them.

Question 30

Why is it referred to as a partially permeable membrane?

Answer 30

Only certain molecules can pass through the plasma membrane. These include small molecules (CO2, H2O, O2) and lipid-soluble substances

Question 31

What substances cannot pass through the partially permeable membrane?

Answer 31

Water soluble substances and ions that are polar (e.g. sodium ions) and large molecules (e.g. glucose)

Question 32

What is active transport

Answer 32

The movement of substances from an area of low concentration to an area of high concentration, using ATP and a carrier protein.

Question 33

How does a molecule pass through the membrane via active transport

Answer 33

1. carrier proteins are found spanning the cell membrane
2. The molecule binds to the receptor on the carrier protein, which is complementary to the shape of the protein
3. ATP will bind to the carrier protein from inside the cell and hydrolyses into ADP + Pi.
4. This causes the carrier protein to change shape and release the molecule to the other side.
5. The phosphate ion is released and the protein returns to its original shape.

Question 34

What is the function of ATP hydrolase?

Answer 34

Releases energy to allow ions to be moved against a concentration gradient.

Question 35

Explain co transport in the ileum.

Answer 35

1. Sodium ions are actively transported out of the epithelial cell and into the blood within the capillaries by a carrier protein
2. This means there is a low concentration of sodium ions in the epithelial cells
3. Sodium ions attach to a co transport protein which either glucose or amino acids attach to and move from the lumen, down the concentration gradient and into the epithelial cell

Question 36

Describe the mechanism for the absorption of amino acids in the ileum