2.3- Transport Of Substances

Question 1

Phospholipids

Answer 1

Phospholipids
Hydrophilic, polar phosphate head on the outside
Hydrophobic, non-polar fatty acid tails on the inside
Form a bilayer.
• Selectively permeable - allows non-polar molecules to pass through
• Allows cell to maintain different concentrations either side
• Membranes are able to form vesicles
• Provides fluidity

Question 2

Glycoprotein (carbohydrate chain attached to protein)

Answer 2

Important in cell recognition

So that the immune system can tell the difference between body cells & e.g. invading bacteria.

Question 3

Glycolipids

Answer 3

For cell recognition and to provide energy.

Question 4

Extrinsic protein

Answer 4

Receptor sites for hormones. Detect chemicals released form other cells. For cell recognition.

Question 5

Cholesterol

Answer 5

Cholesterol increases stability by reducing movement of other molecules in the membrane.

Question 6

Transport Protein

Answer 6

Can be carrier proteins or channel proteins
The channel proteins create selective hydrophilic pores (for water + water soluble compounds).
Carrier proteins are involved in facilitated diffusion and active transport.
Channel proteins are only used for facilitated diffusion.

Question 7

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

Answer 7

• Molecules within the membrane able to move (fluid)

* Mixture of phospholipids, proteins, cholesterol, glycoproteins, glycolipids (mosaic)

Question 8

1. How does the bilayer formed by phospholipids affect entry and exit of substances into and out of a cell?

Answer 8

• allows movement of small, lipid soluble, non-polar molecules such as gases through the membrane
• prevents movement of large, water soluble, polar molecules such as ions and amino acids
• The membrane is selective and partially permeable

Question 9

Simple diffusion (involving limitations imposed by the nature of the phospholipid bilayer)
1.	Define diffusion

Answer 9

• Diffusion is a passive process.
• It is the NET movement of substances from an area of high concentration, to an area of low concentration, down a concentration gradient.

Question 10

1. Where can diffusion take place?

Answer 10

• Diffusion can occur from one area to another or across a partially permeable membrane

Question 11

1. Which substances can simply diffuse across the phospholipid bilayer?

Answer 11

• Small, non-polar, non-charged, lipid soluble molecules can diffuse across the phospholipid bilayer

Question 12

1. Explain why molecules of oxygen and carbon dioxide are able to diffuse across membranes.

Answer 12

• The hydrophobic fatty acid part of membrane is non-polar
• Oxygen and carbon dioxide are small non-polar molecules;
• Oxygen/carbon dioxide can diffuse through the fatty acid layer
• Down a concentration gradient

Question 13

1. Which factors increase the rate of diffusion and why?

Answer 13

• High concentration gradient
• Increase the temperature
• Increase the surface area
• Increase the pressure
• Decrease the diffusion pathway (reduce membrane width)

Question 14

1. Fick’s law:

Answer 14

Rate of diffusion ∝ concentration gradient x surface area

Diffusion pathway

Question 15

1. Define the process of facilitated diffusion

Answer 15

• The passive movement of substances from high concentration to a low concentration, down a concentration gradient, through a channel or carrier protein.

Question 16

1. What sorts of substances are transported using this method? Why? Examples?

Answer 16

• It is used for substances which are large, polar, charged or water-soluble
• The fatty acid layer of the plasma membrane is hydrophobic and non polar it will not allow polar substances to diffuse though
• Ions, amino acids
 

Question 17

Comparing Chanel vs carrier

Answer 17

Comparing channel and carrier proteins
Selectively transports substances across a membrane (specific)  
The protein forms a pore which does not change shape when a substance travels through it
Cha 
The substance to be transported binds to one side of the protein, this causes it to change shape, so the substance is released on the other side of the membrane
Car
Is made of a polypeptide
 
Can be used for facilitated diffusion  
Can be used for active transport car

Question 18

• What could limit the rate of facilitated diffusion?

Answer 18

• The number of transport proteins available in the membrane
• the concentration gradient
• the surface area

Question 19

1. Define the process of active transport

Answer 19

• Active transport is an active process, it requires energy (which comes from ATP)
• It is the movement of a substance from a low concentration to a high concentration, against the concentration gradient
• It requires ATP which binds to a carrier protein and enables it to change shape.

Question 20

2. How does active transport move substances against their concentration gradient?

Answer 20

• Substance binds to a carrier protein
• ATP binds to carrier protein
• ATP is converted to ADP and releases energy
• This causes the carrier protein to change shape
• The substance is released on the other side of the membrane

Question 21

3. What could limit the rate of active transport?

Answer 21

• The number of carrier proteins
• The amount of oxygen available for respiration. (this affects the amount of ATP available)
• Very High temperature – if the temperature is high it could denature enzymes involved in respiration so ATP cannot be produced.
• If the temperature is moderately high increased rate of active transport  as molecules have more kinetic energy  increased rate of respiration  more ATP ALSO more collisions between carrier proteins and substance to be transported

Question 22

4. Give two ways in which active transport differs from diffusion? (you can simply state the differences)

Answer 22

• In active transport ATP is used;
• and movement is against a concentration gradient

 

Question 23

5. Contrast the processes of facilitated diffusion and active transport. (you must write comparisons here)

Answer 23

• Facilitated diffusion involves channel or carrier proteins whereas active transport only involves carrier proteins;
• Facilitated diffusion does not use ATP whereas active transport uses ATP;
• Facilitated diffusion takes place down a concentration gradient whereas active transport can occur against a concentration gradient.
Since ‘contrast’, both sides of the differences needed

Question 24

1. Write a definition of osmosis:

Answer 24

The net movement of water from an area of high water potential to an area of lower water potential through a partially permeable membrane

Question 25

2. What is the water potential of pure water?

Answer 25

0

Question 26

3. What happens to the water potential of a solution if more solute is added? I

Answer 26

? It decreases (becomes more negative)

Question 27

4. What does it mean if 2 solutions are isotonic? T

Answer 27

They have the same water potential

Question 28

1. When investigating diffusion or osmosis, you might want to make solutions of different concentrations. What is this called?

Answer 28

A dilution series

Question 29

2. Vegetables are made of different types of tissue, in an experiment where do you take a cutting from and why?

Answer 29

• When taking a cutting you must ensure it’s through one tissue only in order for it to be a fair test. As different tissues may affect water potential differently.

Question 30

3. Why are bungs are placed on test tubes when conducting an experiment overnight?

Answer 30

• To stop water from evaporating and therefore changing the water potential of the solution.

Question 31

4. When you need to find the concentration of the solution that has the SAME water potential as the vegetable/fruit you are investigating you:

Answer 31

a. Plot data on a graph (give titles of the x and y axis)
b. Draw a line of best fit
c. From the graph either: (depending on question)
i. Find when the ratio is 1 (there is no change in mass/length)
ii. extrapolate the line of best fit and read off
iii. Find where it crosses the x axis

Question 32

5. Ratio’s or % change in mass are calculated to

Answer 32

• allow a valid comparison when the initial length/mass is not the same.

Question 33

6. Take additional readings/repeat experiments to

Answer 33

• identify any anomalies and calculate a mean

Question 34

7. A graph can be improved if you take additional readings because

Answer 34

• your line of best fit will be more reliable

* AND - depending on question - where the point crosses the x axis is more precise –

Question 35

8. Join dot to dot when

Answer 35

• you cannot predict intermediate values