Cells: Exchange Of Materials Across A Membrane

Question 1

Diffusion definition

Answer 1

The movement of particles from a region of high concentration to an area of low concentration down a concentration gradient until evenly spread. The particles that pass through the membrane must be lipid soluble, small and uncharged: it is a passive process

Question 2

Concentration gradient definition

Answer 2

The path from an area of higher concentration to an area of lower concentration - therefore particles can diffuse down (in this case) or up a concentration gradient

Question 3

Factors that affect the rate of diffusion

Answer 3

1. The concentration gradient
2. Surface area (large S.A, increased diffusion)
3. Length of diffusion distance

Question 4

Facilitated diffusion definition

Answer 4

The movement of particles down a concentration gradient, using the kinetic energy of the molecules themselves, passing through a protein channels or carrier proteins. It is still passed down a concentration gradient: it is a passive process

Question 5

Carrier protein function

Answer 5

• Move large molecules into or out of cells down their concentration gradient
• Specific proteins move specific molecules
• When a molecule is present it will attach to the carrier protein, changing the proteins shape and causing the molecule to be released from the protein to the opposite side of the membrane

Question 6

Protein channel function

Answer 6

• Form pores for water soluble ions and molecules i.e glucose, amino acids, Na+, K+
• Each channel is selective and remains closed until the specific molecule is present

Question 7

Osmosis definition

Answer 7

The movement of water molecules from a solution with a higher water potential to a solution with a lower water potential through a partially permeable membrane

Question 8

Active Transport definition

Answer 8

• The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins: ATP is produced during respiration (metabolic energy)

Question 9

ATP function in active transport

Answer 9

The molecule used for energy: produced in the mitochondria during respiration, which is when energy is released from glucose. ATP is comprised of a molecule of sugar and 3 phosphate groups bound to it

In primary active transport ADP and Pi combine when energy is released

Question 10

Carrier proteins change shape using energy from…

Answer 10

…ATP

Question 11

Co-transporters are…

Answer 11

… A type of carrier proteins change shape, but they bind to two molecules at the same time. A concentration gradient of one molecule is used to move the other molecule against its concentration gradient

Question 12

Uniport

Answer 12

1 molecule moved in 1 direction

Question 13

Symport

Answer 13

2 molecules moved in 1 direction

Question 14

Antiport

Answer 14

2 molecules moved in opposite directions

Question 15

Chyme

Answer 15

The soup-like mixture made by the digested food when it reaches the small intestine

Question 16

Facilitated diffusion in the absorption of glucose in the ileum: What happens?

Answer 16

• This occurs by transporting glucose from the lumen of the intestine, into an epithelial cell and then into the blood
• This occurs via different channel and carrier proteins
• When the carbohydrate molecules are first broken down there is a higher concentration of them in the intestinal lumen, so glucose moves down its concentration gradient from the intestinal lumen into the cell, and then into the blood stream
• When equilibrium is reached then indirect active transport takes over

Question 17

What is the difference between direct and indirect active transport?

Answer 17

Primary (direct) active transport – Involves the direct use of metabolic energy (e.g. ATP hydrolysis) to mediate transport. Secondary (indirect) active transport – Involves coupling the molecule with another moving along an electrochemical gradient. This uses the downhill flow of an ion to pump some other molecule or ion against the gradient

Question 18

What is the role of the sodium-potassium pump?

Answer 18

It helps to maintain osmotic equilibrium and membrane potential in cells. The sodium and potassium move against the concentration gradients

Question 19

Different factors that can affect the rate of glucose absorption

Answer 19

• The concentration of glucose
• temperature
• Amount of protein channels / carrier proteins
• Amount of co-transporter proteins

Question 20

What is simple diffusion?

Answer 20

This is the net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached. This process does not require ATP. For molecules to diffuse across the membrane they must be lipid soluble and small.

Question 21

What is facilitated diffusion?

Answer 21

This is a passive process (it does not require ATP) but it differs from simple diffusion as proteins are used to transport molecules. The movement of ions and polar molecules, which cannot simply diffuse, can be transported across membranes by facilitated diffusion using protein channels and carrier proteins.

Question 22

What are channel proteins?

Answer 22

Channel proteins form pores in the membrane for charged particles to diffuse through (down their concentration gradient). Different channel proteins facilitate the diffusion of different charged particles. They are embedded in the cell membrane.

Protein channels form tubes filled with water and this enables water-soluble ions to pass through the membrane. This is still selective, as the channel proteins only open in the presences of certain ions when they bind to the protein.

Question 23

What are carrier proteins and how do they work?

Answer 23

Carrier proteins move large molecules across the membrane, down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules. Here’s how they work:
• First, a large molecule attaches to a carrier protein in the membrane.
• Then, the protein changes shape.
• This releases the molecule on the opposite side of the membrane

Question 24

What is cholesterols function in the cell membrane?

Answer 24

• Cholesterol gives the membrane stability.
• It is a type of lipid that’s present in all cell membranes (except bacterial cell membranes). Cholesterol fits between the phospholipids
• Cholesterol binds to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This restricts the movement of the phospholipids, making the membrane less fluid and more rigid.
• Cholesterol helps to maintain the shape of animal cells (which don’t have cell walls). This is particularly important for cells that aren’t supported by other cells, e.g. red blood cells, which float free in the blood.
• Cholesterol also has hydrophobic regions, so it’s able to create a further barrier to polar substances moving through the membrane.

Question 26

What is osmosis?

Answer 26

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

Question 27

What is water potential?

Answer 27

Water potential is the pressure created by water molecules and is measured in kPa. Pure water has a water potential of zero, so when solutes are dissolved in water the water potential will become negative. The more negative the water potential, the more solute must be dissolved in it.

Question 28

What does it mean if a solution is isotonic?

Answer 28

An isotonic solution is when the water potential is the same in the solution and the cell within the solution.

Question 29

What does it mean if a solution is hypertonic?

Answer 29

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

Question 30

What does it mean if a solution is hypotonic?

Answer 30

Hypotonic is when the water potential of a solution is more positive (closer to zero) than the cell.

Question 31

What is active transport?

Answer 31

The movement of molecules and ions from an area of lower concentration to an area of higher concentration (against the concentration gradient) using ATP and carrier proteins. The carrier proteins act as pumps to move substances across the membrane. This is very selective, as only certain molecules can bind to the carrier proteins to be pumped.

Question 32

How does a carrier protein transport molecules across a membrane?

Answer 32

Certain molecules can bind to the receptor site on carrier proteins. ATP will bind to the protein on the inside of the membrane and is hydrolysed into ADP and Pi. This causes the protein to change shape and open towards the inside of the membrane. This causes the molecule to be released on the other side of the membrane. The Pi molecule is then released from the protein, and this results in the protein reverting to its original shape. This is how ATP and carrier proteins are used in active transport.

Question 33

What is the process of active transport?

Answer 33

I. Transport is through carrier proteins spanning the cell membrane 2. Molecule binds to a receptor complementary in shape on the protein 3. ATP binds to the carrier protein from the inside of the cell and it is hydrolysed 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 then released and the protein returns to its original shape

Question 34

What is co-transport?

Answer 34

Co-transport is a type of active transport. Co-transport is how glucose and amino acids are absorbed from then ileum into the blood stream.

Question 35

How does co-transport work in the ileum?

Answer 35

I. Sodium ions are actively transported out of the epithelial cell into the blood 2. This reduces the sodium ion concentration in the epithelial. cell. 3. Sodium ions can then diffuse from the lumen down their concentration gradient into the epithelial cell 4. The protein the sodium ions diffuse through is a co-transported protein, so either glucose or amino acids also attach and are transported into the epithelial cell against their concentration gradient 5. Glucose then moves by facilitated diffusion from the epithelial cell to the blood.