Topic 4 - Transport across membranes

Question 1

What are the functions of the cell-surface membrane?

Answer 1

Separate cell contents from the outside environment.
Allows different conditions to be established inside and outside the cell.
Regulating the transport of materials in and out of the cell - selectively permeable.
Cell signalling and recognition.

Question 2

Describe the phospholipids are arranged in the phospholipid bilayer.

Answer 2

The hydrophilic heads of both phospholipid layers point to the outside of the cell surface membrane, attracted by water on both sides.
The hydrophobic tails of both phospholipid layers point into the centre of the cell membrane, repelled by water on both sides.

Question 3

What molecules cross the cell-surface membrane by simple diffusion/osmosis?

Answer 3

Water
Alchohol
Gases
Non-polar molecules

Question 4

What are the differences between intrinsic and extrinsic proteins?

Answer 4

Intrinsic proteins span the whole width of the membrane.
Extrinsic proteins are confined to the inner or outer surface of the membrane.

Question 5

State the importance of cholesterol in the cell surface membrane.

Answer 5

Reduces lateral movement of molecules.
Make membrane less fluid at high temperatures.
Prevents leakage of water/ions.

Question 6

State the importance of glycolipids in the cell surface membrane.

Answer 6

Act as a recognition site.
Help maintain stability of membrane.
Helps cells attach to one another to form tissues.

Question 7

State the importance of glycoproteins in the cell surface membrane.

Answer 7

Act as recognition sites.
Help cells attach to one another and form tissues.
Allows cells to recognise one another.

Question 8

Why is membrane fluidity important?

Answer 8

For diffusion of substances across the membrane.
Allows membranes to fuse (e.g. a vesicle fusing with the cell membrane during exocytosis).
Allows cells to move and change shape.

Question 9

Name the functions of membranes within cells.

Answer 9

Control entry and exit of materials in discrete organelles such as mitochondria.
Separates organelles from cytoplasm.
Provides an internal transport system.
Isolates potentially damaging enzymes.
Provides a surface for reactions to occur.

Question 10

What is the definition of diffusion?

Answer 10

The net movement of molecules or ions from a region of high concentration to low concentration, until an equilibrium is achieved.

Question 11

How does increasing the temperature increase the rate of diffusion?

Answer 11

Increases the kinetic energy of molecules and ions and speeds up the diffusion rate.

Question 12

How does the concentration gradient affect the rate of diffusion?

Answer 12

The larger the difference in concentration, the faster diffusion occurs.

Question 13

How does the surface area affect the rate of diffusion?

Answer 13

The larger the surface ares, the faster diffusion occurs because of the greater area for diffusion to occur.

Question 14

What is Fick’s law?

Answer 14

Rate of diffusion is proportional to:
(surface area x difference in concentration) / length of diffusion of pathway.

Question 15

How does the length of diffusion pathway affect the rate of diffusion?

Answer 15

The thinner the exchange surface, the faster diffusion will occur.

Question 16

Why can small, non-polar, lipid-soluble molecules travel by simple diffusion?

Answer 16

The movement of phospholipid molecules in the plasma membrane creates gaps for them to slip through.

Question 17

Why can water pass straight through the phospholipid membrane even though it is a polar molecule?

Answer 17

It is a very small molecule

Question 18

What does facilitated diffusion involve?

Answer 18

Channel and carrier proteins

Question 19

Describe how channel proteins can help control the entry and exit of molecules and ions?

Answer 19

These channels are hydrophilic. They are selective and open and open at the presence of a specific ion. If the particular ion isn’t present, it remains closed.

Question 20

Describe how a carrier protein works.

Answer 20

When a molecule thats trying to cross binds to it, this causes the protein to change shape in a way that means the molecule is released inside of the membrane. No energy is required.

Question 21

What is the definition of osmosis?

Answer 21

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

Question 22

What has the highest water potential and what is the value for this?

Answer 22

Distilled water - 0 kPa

Question 23

What happens when there are dissolved solutes in water?

Answer 23

There are no longer 100% water molecules, so the water potential is more negative or ‘lower’.

Question 24

What does a low solute concentration mean?

Answer 24

High water potential

Question 25

What is the definition of active transport?

Answer 25

The movement of molecules or ions into or out of a cell from a region of low concentration to a region of high concentration using energy released from the hydrolysis of ATP and carrier molecules.

Question 26

Where is the energy for active transport produced?

Answer 26

Mitochondria produces ATP.
Hydrolysis of ATP releases energy.

Question 27

What is a similarity and some differences between active transport and facilitated diffusion?

Answer 27

Similarity: Both use carrier proteins.
Differences: Active transport requires energy. Diffusion goes down concentrated gradient, active transport against.

Question 28

What does the sodium-potassium pump involve?

Answer 28

The moving of sodium and potassium ions across the plasma membrane involving the hydrolysis of ATP to ADP and a phosphate, which provides the necessary energy.
This process maintains the large excess of Na+ ions outside the cell and the large excess of K+ ions on the inside.

Question 29

What is digestion?

Answer 29

The hydrolysis of large insoluble molecules into smaller soluble ones.

Question 30

How does a large surface area of villi help maximise the exchange of materials across their surface?

Answer 30

More channel and carrier proteins.

Question 31

How does the thin walls of villi help maximise the exchange of materials across their surface?

Answer 31

Short diffusion pathway

Question 32

How does a good supply of blood vessels in the villi help maximise the exchange of materials across their surface?

Answer 32

Maintains steep concentration gradient

Question 33

What else helps maintain the concentration gradient?

Answer 33

Muscle contractions, which moves the villi into and away from the gut lumen.

Question 34

By what process does glucose get absorbed?

Answer 34

Co transport

Question 35

What is co transport a combination of?

Answer 35

Active transport, simple and facilitated diffusion.

Question 36

Describe the process of co transport in the lumen of the small intestine.

Answer 36

Active transport of sodium ions out from the epithelial cells, (lining the gut), into the blood.
This decreases the concentration of sodium ions inside the cell, so sets up a concentration gradient.
Sodium ions diffuse down their concentration gradient, through carrier proteins in the epithelial cell membrane. These carrier proteins transport glucose molecules and sodium ions together into the cell and are called co-transport proteins.
This increases the concentration of glucose molecules inside the epithelial cell, which then diffuse down their concentration gradient into the blood through carrier proteins by facilitated diffusion.
This process ultimately moves glucose molecules moving against their concentration gradient from the lumen of the small intestine into the cell and down their concentration gradient out of the cell and into the blood.