B4 Transport across Cell Membranes

Question 1

Why is it called the fluid mosaic model?

Answer 1

It shows how the molecules of different substances that make up the cell membrane are arranged in a mosaic and how the molecules move around

Question 2

What is the function of phospholipids in the membrane?

Answer 2

They prevent water-soluble molecules from entering or leaving the cell. They also maintain fluidity for molecules as its tails are flexible

Question 3

What is the function of cholestrol in the membrane?

Answer 3

To add strength to the membrabne as they pull the fatty acid tails close together

Question 4

What is the function of glycolipids and glycoproteins in the membrane

Answer 4

To act as recognition sites for specific chemicals, to allow tissue formation (by allowing cells to attach to each other), maintain stability of the membrane

Question 5

what proteins span the phospholipid bilayer from one side to another

Answer 5

Channel proteins and carrier proteins

Question 6

Describe the permeability of the cell membrane?

Answer 6

large molecules will have difficulty passing through the membrane. Charged particles will also have difficulty passing through the hydrophobic core for the bilayer

Question 7

What is osmosis?

Answer 7

The diffusion of water from a region of higher water potential to a region of lower water potential, through a semi-permeable membrane

Question 8

What is water potential?

Answer 8

The potential (likelihood) of water molecules to diffuse out of or into a system

Question 9

What is the water potential of pure water and how could it be lowered

Answer 9

Pure water has the highest water potential of zero at 0kPa. As the water potential increases, the pressure increases. Adding solutes to pure water lowers its water potential. So the water potential of any solution is always negative

Question 10

What is plasmolysis?

Answer 10

When the cell’s cytoplasm is pulled from its cell wall as the plant cell is placed in a hypertonic solution(one that has a lower water potential inside the cell) due to osmosis moving water out of the cell

Question 11

What is crenation?

Answer 11

When the cell’s cytoplasm is pulled from its cell membrane as the animal cell is placed in a hypertonic solution due to osmosis moving water out of the cell

Question 12

What is lysis?

Answer 12

When the animal bursts as it has no cell wall when placed in a hypotonic solution(one that has a higher water potential inside the cell) due to osmosis moving water inside the cell

Question 13

What is diffusion?

Answer 13

The net movement of particles down a concentration gradient :from a high to a low concentration.

Question 14

What is fick’s law?

Answer 14

rate of diffusion is proportional to surface area x difference in concentration/ length of diffusion path (membrane thickness)

Question 15

What is facilitated diffusion?

Answer 15

It is the diffusion of large and charged molecules (which have difficulty diffusing across the membrane) to diffuse across the bilayer with aid of channel and carrier proteins(which span the membrane).

Question 16

How do carrier proteins work?

Answer 16

A large molecule attaches to it. The protein changes shape, releasing the molecules to the inner side of the membrane (in the cell)

Question 17

How do channel proteins work?

Answer 17

They form pores in the membrane to allow charged particles to diffuse through

Question 18

What is Active Transport?

Answer 18

The movement of molecules and ions against their concentration gradient , so it requires energy in the form of ATP. Carrier proteins are also involved.

Question 19

What is Co-transport?

Answer 19

A type of transport which involves a special type of carrier proteins called co-transporters. Co-transporters bind to two molecules at a time. The concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient

Question 20

Describe the process of Active Transport?

Answer 20

The molecules or ions bind to receptors on the channels of the carrier proteins. On the inside of the cell, ATP binds to the protein , causing it to split into ADP and a phosphate molecule. As a result, the protein molecule changes shape and opens up to the inner side of the membrane(inside the cell). The molecules or ions are then released to the inner side of the membrane. The phosphate molecule is then released frim the protein and recombines with the ADP to form ATP. This causes the protein to revert to its original shape, ready for the process to be repeated

Question 21

Describe the process of Co-transport of glucose from the ileum to the blood?

Answer 21

Na+ ions move out of the epithelial cells in the ileum and into the blood by active transport by the sodium-potassium pump(pumps sodium into the blood while pumping potassium out of the blood and inti the ileum). This creates a concentration gradient - there is now a higher concentration of sodium ions in the lumen of ileum than inside the cell. This causes the Na+ ions in the lumen to diffuse from the lumen of the ileum into the epithelial cells , down their concentration gradient. They do this via the sodium-glucose co-transporter proteins. The co-transporter also carries glucose from the ileum to the cell but against its concentration gradient. As a result, the concentration of glucose inside the cell increases. Glucose can now finally diffuse out of the cell and into the blood, down its concentration gradient through a protein channel by facilitated diffusion