AS1 Chapter 4 - Cell Physiology

Question 1

Give a definition of diffusion

Answer 1

Diffusion can be defined as the net movement of a substance from where it is in higher concentration to where it is in a lower concentration, down the concentration gradient.

Question 2

What factors affect diffusion across membranes?

Answer 2

1. The concentration gradient
2. The size of the molecule
3. The temperature
4. The thickness of the exchange surface
5. The surface area of the membrane

Question 3

What substances can easily pass through the cell membrane by simple diffusion?

Answer 3

Non- polar molecules (such as lipid soluble oxygen and carbon dioxide)
Very small polar molecules such as water can pass through (due to their small size)
Water soluble (polar) molecules generally cannot pass through the phospholipid bilayer due to the hydrophobic centre.

Question 4

Definition of osmosis

Answer 4

Osmosis can be defined as the net movement of water through a selectively (differentially) permeable membrane, from a solution of high water potential to a solution of lower (more negative) water potential, down the water potential gradient.

Question 5

Definition of water potential

Answer 5

The water potential of a solution may be regarded as its tendency to take in water by osmosis from pure water across a selectively permeable membrane.

Question 6

Water potential is measured in …

Answer 6

Kilopascals (kPa)

Question 7

Pure water has a water potential of …

Answer 7

0 kPa

Question 8

What symbol denotes ‘water potential’?

Answer 8

Psi Cell

Question 9

What symbol denotes ‘solute potential’?

Answer 9

Psi S

Question 10

What symbol denotes ‘pressure potential’?

Answer 10

Psi P

Question 11

Definition of solute potential

Answer 11

The solute potential of a solution may be regarded as its potential to take in water by osmosis from pure water across a selectively permeable membrane.

Extra: The potential may or may not be the same as the tendency to take in water (water potential). The potential relates to the solute concentration only but the tendency (water potential) is affected by other factors, such as the space available within a cell. For example, a turgid cell will still have the potential to take in water, as it is still more concentrated than pure water, but because it is turgid it may be unable to take in water, as there is simply no space.

Question 12

Definition of pressure potential

Answer 12

This is the effect of pressure on the solution.

Extra: A plant cell that is turgid will exert considerable pressure on its cell wall, whereas one that is not will exert much less pressure. This pressure influences the ability of the cell to take in or lose water by osmosis. The pressure potential is usually positive (although it can be 0).

Question 13

What is the equation for water potential?

Answer 13

Water potential = Solute potential + Pressure potential

Psi Cell = Psi S + Psi P

Question 14

Sugar molecules and water molecules in solution will form …

Answer 14

Hydration shells

Question 15

In solutions, some of the water molecules are not ‘free’, as they form (blank) around the solutes.

Answer 15

Hydration shells

Question 16

What are the two ways in which water can move through the selectively or differentially permeable phospholipid membrane?

Answer 16

1. Simple diffusion (osmosis)

2. Facilitated diffusion (osmosis) - special channel proteins called aquaporins

Question 17

When animal cells swell up and burst this is known as …

Answer 17

Cell lysis

Question 18

When animal cells lose water, shrink and shrivel up this is known as …

Answer 18

Cell crenation

Question 19

How and why does cell lysis occur in animal cells?

Answer 19

Animal cells do not have a cell wall and therefore there is noting to stop the expansion of the cell membrane until it bursts (due to excess water moving into the cell by osmosis).

Question 20

Crenation (in animal cells) and plasmolysis (in plant cells) occurs when cells are placed in (blank) solutions.

Answer 20

Hypertonic

Question 21

Lysis (in animal cells) and turgidity (in plant cells) occurs when cells are placed in (blank) solutions.

Answer 21

Hypotonic

Question 22

The point at which the cell membrane just begins to lose contact with the cell wall in plants is called the point of …

Answer 22

Incipient plasmolysis

Question 23

What is the point of incipient plasmolysis?

Answer 23

The point at which the cell membrane just begins to lose contact with the cell wall.

Question 24

What is plasmolysis?

Answer 24

Plasmolysis occurs when water moves out of the plant cell in excess. The vacuole will shrink and the cell membrane will pull away from the cell wall. (i.e. The protoplast will pull away from the cell wall, except at points where plasmodesmata connect adjacent protoplasts). The cell will lose all turgor (pressure).

Question 25

If plant tissue suffers a shortage of water, the cells will cease to be turgid and the tissue is described as being (blank1). If a large number of cells are (blank1), the phenomenon of (blank2) occurs.

Answer 25

Flaccid

Wilting

Question 26

What substances can and cannot permeate the phospholipid bilayer unaided?

Answer 26

Can:
• Hydrophobic (non-polar) molecules [O2, N2]
• Small uncharged hydrophilic (polar) molecules [H2O, CO2, Urea, Glycerol]

Cannot:
• Large uncharged hydrophilic (polar) molecules [Glucose, Sucrose]
• Ions [Na+, K+, Cl-, Ca2+, Mg2+]

Question 27

Give examples of diffusion

Answer 27

• The spread of smoke in a room

* The diffusion of dye in a beaker of water

Question 28

Name four mechanisms of transport through the phospholipid bilayer

Answer 28

1. (Simple) diffusion
2. Facilitated diffusion
3. Active transport
4. Cytosis
   - Endocytosis
   - Exocytosis

Question 29

Name the two types of protein involved in facilitated diffusion

Answer 29

1. Carrier protein

2. (Ion) channel protein

Question 30

The rate of facilitated diffusion is affected by …

Answer 30

1. The number of carrier or channel proteins in the membrane

2. Whether or not the central pores of the channel proteins are open or closed

Question 31

Facilitated diffusion

What is a carrier protein?

Answer 31

Carrier proteins - They take in the diffusing molecule, such as glucose, change shape and release the molecule on the other side of the membrane. These protein carriers have binding sites that match specific molecules and they assist the movement of these molecules across the membrane.

Question 32

Facilitated diffusion

What are channel proteins?

Answer 32

(Ion) channel proteins - These channels are formed by proteins with a central pore that enables charged particles (ions) to pass through. Some of these ion channels are permanently open but others are gated. Gated channels can open or close allowing control of ion movement.

Question 33

Both (simple) diffusion and facilitated diffusion are …

Answer 33

Passive processes

Question 34

What is simple diffusion?

Answer 34

Simple diffusion is the net movement of a substance from where it is in a higher concentration to where it is in a lower concentration, down the concentration gradient, across the phospholipid bilayer.

Question 35

What is facilitated diffusion?

Answer 35

Facilitated diffusion is the net movement of a substance from where it is in a higher concentration to where it is in a lower concentration through the phospholipid bilayer, with the support (facilitation) of carrier and channel proteins

Question 36

What is the type of protein involved in active transport?

Answer 36

Carrier proteins (sometimes called pumps)

Question 37

What are the similarities between (simple) diffusion and facilitated diffusion?

Answer 37

Both diffusion and facilitated diffusion only transport molecules down the concentration gradient and neither involves the expenditure of metabolic energy (i.e. they are passive processes).

Question 38

What is active transport?

Answer 38

Active transport is the movement of a substance from where it is in a low concentration to where it is in a higher concentration (against the concentration gradient) through the cell surface membrane, with the assistance of carrier proteins (pumps) which expend metabolic energy.

Question 39

How do protein carriers transport material across the cell surface membrane in active transport/facilitated diffusion?

Answer 39

1. The substance to be transported binds to the carrier protein.
2. The carrier changes shape and releases the transported substance on the other side of the membrane.

Question 40

Carrier proteins and channel proteins are (blank) to a particular type of molecule or ion.

Answer 40

Specific

Question 41

What are the two key differences between facilitated diffusion and active transport?

Answer 41

1. In active transport, substances are moved against the concentration gradient.
2. Metabolic energy in the form of ATP is required in active transport.

Question 42

Cells that carry out a lot of active transport usually have a large number of …

Answer 42

Mitochondria

Question 43

What is the role of mitochondria in cells which carry out a lot of active transport?

Answer 43

The mitochondria supply the ATP (energy) needed for active transport.

Question 44

What is cytosis?

Answer 44

Cytosis is the process by which substances are transported into or out of the cell without having to pass through the membrane itself.

Question 45

The process of cytosis is important in transporting …

Answer 45

1. Large molecules that are too big for the carriers.

2. The bulk transport of smaller molecules (for example, water).

Question 46

Definition of endocytosis

Answer 46

Endocytosis is the movement of substances into the cell

Question 47

Definition of exocytosis

Answer 47

Exocytosis is the movement of substances out of the cell

Question 48

Describe the steps which occur in endocytosis

Answer 48

1. The cell surface membrane invaginates (infolds) around substances entering the cell from the outside to form a membrane-bound sac or vesicle.
2. This membrane-bound sac/vesicle then pinches off on the inside of the cell surface membrane.
3. The fluid nature of the cell surface membrane allows it to reform and close the gap created by cytosis.

Question 49

What are the two types of endocytosis?

Answer 49

Phagocytosis

Pinocytosis

Question 50

What is phagocytosis?

Answer 50

Phagocytosis involves the transport of solid material into the cell.

Question 51

Give an example of phagocytosis

Answer 51

The engulfing of bacteria by phagocytes

Question 52

What is pinocytosis?

Answer 52

Pinocytosis (‘cell drinking’) involves the transport of fluid into the cell.

Question 53

Describe the steps which occur in exocytosis

Answer 53

1. Secretory vesicles (possibly having budded off from the Golgi apparatus) move to and fuse with the cell surface membrane.
2. The contents of the vesicle are then released outside the cell.
3. The fluid nature of the cell surface membrane allows it to reform and close the gap created by cytosis.

Question 54

What role does exocytosis play in the body?

Answer 54

Exocytosis is important in the secretion of many proteins from cells, including digestive enzymes and many hormones.

Question 55

What are ‘aquaporins’?

Answer 55

Aquaporins are specialised channel proteins for water

Question 56

A solution always has a (blank) water potential

Answer 56

Negative

Question 57

Substances which can permeate/diffuse through the phospholipid bilayer unaided are usually …

Answer 57

Fat-soluble (i.e. Hydrophobic, non-polar)

With the exception of some small, uncharged polar (hydrophilic) molecules such as water, due to their small size

Question 58

What is a protoplast?

Answer 58

The protoplast is the cytoplasm (including the vacuole) and the surrounding cell membrane of a plant cell. (i.e. A plant cell without its cell wall).

Question 59

Give two examples of situations where plasmolysis can occur in nature

Answer 59

1. Plants growing in a field that has been given too much fertiliser.
2. A seed from a woodland tree being carried to a salt marsh and starting to germinate in this environment.

Question 60

Substances which require hydrophilic channel proteins and carrier proteins to permeate the cell surface membrane are usually …

Answer 60

Water-soluble

Question 61

Past Paper Question June 2017 Q6 a)i) (Revised Spec. + Legacy Material)
Cholera is a disease caused by the bacterium Vibrio cholerae and is transmitted through drinking contaminated water.
In the ileum, V. cholerae produces a toxin which activates a type of membrane glycoprotein called CFTR.
The presence of the toxin causes the CFTR glycoprotein to pump excessive levels of chloride ions out of the cells and into the gut lumen. This in turn affects the osmotic balance.

a)i) Describe how the structure of a glycoprotein differs from a typical protein. [1]

Answer 61

Q6 a)i) Has a carbohydrate/sugar chain attached. [1]

Question 62

Past Paper Question June 2017 Q6 b)i) (Revised Spec. + Legacy Material)
Cholera is a disease caused by the bacterium Vibrio cholerae and is transmitted through drinking contaminated water.
In the ileum, V. cholerae produces a toxin which activates a type of membrane glycoprotein called CFTR.
The presence of the toxin causes the CFTR glycoprotein to pump excessive levels of chloride ions out of the cells and into the gut lumen. This in turn affects the osmotic balance.

b)i) One effect of V. cholerae infection is dehydration of the cells lining the ileum.
Using the information provided and your knowledge, suggest an explanation for this. [4]

Answer 62

b)i) Excessive;
Chloride ions are pumped out of cells/ into lumen (due to action of the toxin);
Results in more negative/lower water potential in the lumen of the intestine;
Water follows by osmosis;

Question 63

Past Paper Question June 2017 Q6 b)ii) (Revised Spec. + Legacy Material)
Cholera is a disease caused by the bacterium Vibrio cholerae and is transmitted through drinking contaminated water.
In the ileum, V. cholerae produces a toxin which activates a type of membrane glycoprotein called CFTR.
The presence of the toxin causes the CFTR glycoprotein to pump excessive levels of chloride ions out of the cells and into the gut lumen. This in turn affects the osmotic balance.
One effect of V. cholerae infection is dehydration of the cells lining the ileum.

b)ii) Treatments such as Dioralyte can help replace the chloride ions lost from the cells lining the ileum. Suggest how this will reduce dehydration of these cells. [2]

Answer 63

b)ii) The water potential of the cell is decreased/becomes more negative;
so water is drawn in/less water leaves the cells;

Question 64

Past Paper Question June 2017 Q6 c) (Revised Spec. + Legacy Material)
Cholera is a disease caused by the bacterium Vibrio cholerae and is transmitted through drinking contaminated water.
In the ileum, V. cholerae produces a toxin which activates a type of membrane glycoprotein called CFTR.
The presence of the toxin causes the CFTR glycoprotein to pump excessive levels of chloride ions out of the cells and into the gut lumen. This in turn affects the osmotic balance.

The CFTR glycoprotein is also involved in the condition cystic fibrosis. Cystic fibrosis affects several systems in the body, including the respiratory system.
In individuals with cystic fibrosis the CFTR glycoprotein does not form correctly and therefore does not function normally. The mucus in the respiratory system is normally thin and watery. However, in patients with cystic fibrosis, it becomes thick and sticky.
(c) Using the information provided (including the diagram at the start of this question), suggest how the mucus becomes thick and sticky in individuals with cystic fibrosis.

Answer 64

c) Fewer chloride ions move out of the cell/more remain in the cells of the lung epithelium;
so less water moves out of the cell/into the mucus layer;

Question 65

Past Paper Question - June 2017 Q6 a)ii) AS1 (Revised Spec. + Legacy Material)
Q6 a)ii) Define the term ‘osmosis’. [1]

Answer 65

Q6 a)ii) The net movement of water molecules from an area of higher water potential to an area of lower water potential, through a semi-permeable membrane. [1]

Question 66

Past Paper Question - June 2016 Q4 b)ii) AS1 (Legacy Material)
Q4 In order to complete its life cycle, a virus must enter a host cell via the cell surface membrane. Several types of molecules are found in the cell membranes of animal cells, including cholesterol and glycoproteins.
b) In 2014, the Ebola virus was responsible for an outbreak of Ebola Virus Disease in West Africa. Like all viruses, the Ebola virus must complete its life cycle inside a host cell.
Infection of a cell by Ebola virus is a two stage process:
• Stage 1 - The virus enters the cell by forming a vesicle.
• Stage 2 - The viral genetic material moves out of the vesicle into the cytoplasm of the cell.
It has been suggested that stage 2 involves a transporter protein in the vesicle membrane, called NPC1.
Scientists investigated the importance of the NPC1 transporter protein in Ebola virus infection in mice. They used mice with different levels of the NPC1 protein. Their results are summarised in the table below.

NPC1 protein level Percentage of mice surviving 15 days
after infection/ %
Normal 10
Reduced 80
Zero 100

ii) Describe the relationship between the level of NPC1 protein and the percentage of mice surviving 15 days after infection by Ebola. Using the information provided, suggest a possible explanation for this relationship. [3]

Answer 66

Q4 b)ii) There is negative correlation between level of NPC1 protein and survival/ as the level of NPC1 protein increases survival decreases;
NPC1 protein allows the genetic material to enter cytoplasm/ leave the vesicle;
Once in the cytoplasm virus can replicate/ can go on to infect other cells; [3]

Question 67

Past Paper Question - June 2016 Q7 a)i) AS1 (Legacy Material)
Q7 Water relations between cells and solutions can be described using the concept of water potential.
a)i) For a plant cell at the point of incipient plasmolysis:

State the value of the pressure potential (Psi p) for the cell ____________

State the relationship between the water potential (Psi cell) and the solute potential (Psi s) of the cell ____________________________ [2]

Answer 67

Q7 a)i) 0 kPa;

They are equal/ Psi cell = Psi s [2]

Question 68

Past Paper Question - June 2016 Q7 a)ii) AS1 (Legacy Material)
Q7 Water relations between cells and solutions can be described using the concept of water potential.

The water potential (Psi cell) of a different plant cell is -1900 kPa and the pressure potential (Psi p) is 400 kPa.
a)ii) Calculate the solute potential (Psi s) of the cell.

(Show your working). [2]

Answer 68

Q7 a)ii) Water potential = Solute potential + Pressure potential
-1900 = Solute potential + 400
Solute potential = -2300 kPa [2]

Question 69

Past Paper Question - June 2016 Q7 c) AS1 (Legacy Material)
Q7 c) If erythrocytes (red blood cells) are placed in pure water for five minutes, the water will change from colourless to pale red. Explain this change in colour. [2]

Answer 69

Q7 c) Water enters the cells by osmosis making the cells swell and lysis occurs;
So haemoglobin is released making solution pale red/due to absence of cell wall. [2]

Question 70

Past Paper Question - June 2016 Q7 d) AS1 (Legacy Material)
Q7 d) Amoeba proteus is a single-celled organism, lacking a cell wall, that lives in damp soil or water. When viewed under the microscope, structures called contractile vacuoles can be seen within the cell. These vacuoles can be observed to enlarge in size, move towards the cell membrane and then rapidly shrink in size.

It has been found that the higher the water potential of the solution the A. proteus is found in, the greater the rate of enlarging and shrinking of contractile vacuoles.
Suggest the probable function for contractile vacuoles in A. proteus. [2]

Answer 70

Q7 d) Contractile vacuoles remove water (that enters by osmosis);
So preventing lysis; [2]

Question 71

Past Paper Question - June 2015 Q1 AS1 (Legacy Material)
Q1 The descriptions in the following table relate to transport across a cell membrane.
Complete the table, identifying the mechanism of transport described.

Description Mechanism of transport

Substances moved across
a cell membrane against a
concentration gradient.

Process by which substances 
taken into a cell cause the cell 
membrane to enclose the 
material in a vesicle.
 
Removal or secretion of 
substances from a cell by 
the fusion of a vesicle with 
the cell membrane.
 
Movement of substances 
across a cell membrane from 
higher to lower concentration, 
using protein carriers in the 
membrane.
 
Movement of substances 
across a cell membrane from 
higher to lower concentration, 
directly through the 
phospholipid layer. 
[5]

Answer 71

Q1 active transport;

endocytosis/phagocytosis/pinocytosis;

exocytosis;

facilitated diffusion;

(simple) diffusion; [5]

Question 72

Past Paper Question - January 2014 Q8 a) Section B AS1 (Legacy Material)
Q8 The cell membrane consists of a phospholipid bilayer with various proteins embedded in it. This structure enables different substances to travel through the membrane by either simple diffusion, facilitated diffusion or active transport.

(a) Describe the similarities and differences between simple diffusion, facilitated diffusion and active transport. [6]

Answer 72

Q8 a) Any six points from:
• both simple and facilitated diffusion are passive/require no energy
• and movement is down the concentration gradient
• simple diffusion is possible between the phospholipid molecules
• while facilitated diffusion requires transmembrane proteins
• active transport carries substances against the concentration gradient which requires ATP for energy
• active transport also requires specific carriers
• the carriers undergo a change of shape to move the substance across the membrane
• use of protein carriers in active transport and facilitated diffusion confers selectivity

Question 73

Past Paper Question - January 2014 Q8 b) Section B AS1 (Legacy Material)
Q8 The cell membrane consists of a phospholipid bilayer with various proteins embedded in it. This structure enables different substances to travel through the membrane by either simple diffusion, facilitated diffusion or active transport.

b) Large and small molecules, as well as ions, must be able to travel through the cell membrane. Explain why the different methods of transport named in part (a) (simple diffusion, facilitated diffusion and active transport) are necessary to allow each of these substances to pass through and also how they may allow the membrane to be selective. [7]
Quality of written communication [2]

Answer 73

Q8 b) Any seven points from:
• some molecules are hydrophobic/non polar/very small (e.g. oxygen and carbon dioxide)
• and so can pass directly between the phospholipid molecules in the
bilayer
• ions are polar/charged/hydrophilic
• and therefore cannot pass between the phospholipid molecules
• therefore they need hydrophilic pores/channel proteins through which to pass
• transport through bilayer/pore/channel is non-selective/but channel
selective if gated
• larger polar molecules (such as glucose) depend on protein carriers
in the membrane
• which have specific receptor sites [not active sites]
• that are complementary to the molecule being carried
• these carriers are therefore selective in what they can carry
• also the relative abundance of different carriers will influence the relative amount of different substances able to cross the membrane [7]
Written communication [2]

Question 74

Past Paper Question - June 2014 Q6 d) AS1 (Legacy Material)
Q6 Proteins comprise a large group of organic molecules with a wide variety of functions. The specific function of a protein depends on its shape which is determined by its sequence of amino acids.

d) Some proteins are secreted out of the cell in which they are produced, via vesicles which fuse with the plasma membrane.
Name this process of secretion out of the cell. [1]

Answer 74

Q6 d) Exocytosis; [1]

Question 75

Past Paper Question - June 2014 Q8 Section B AS1 (Legacy Material)
Q8 Give an account of the process of osmosis in cells and explain the effect of changing external solute concentration on both animal and plant cells. [13]
Quality of written communication [2]

Answer 75

Q8 Any thirteen points:
• osmosis is the net movement of water across a partially (selectively) permeable membrane
• from an area of higher water potential to an area of lower water potential
• pure water has a water potential of zero
• addition of solutes decreases the solute potential / creates a negative solute potential
• by restricting the movement of free water molecules / by creating hydration shells
• so they decrease the water potential of a solution
• all cells contain a range of dissolved solutes
• if a cell has a lower water potential than its environment (neighbouring cells), then water will move in / if a cell has a higher water potential than its environment (neighbouring cells), then water will move out
• in animal cells, only the dissolved solutes contribute to the water potential
• if animal cells take in water, they may swell and lyse (burst)
• because they have no cell wall
• if animal cells lose water, they will crenate
• the presence of a cell wall in plant cells creates a pressure potential
• thus the water potential of a plant cell = solute potential + pressure potential (Ψcell = Ψs + Ψp)
• when plant cells absorb water, they become turgid / the wall resists the inward movement of excess water
• when plant cells lose water, the cell becomes accid / the cell membrane begins to pull away from the cell wall / the cell begins to plasmolyse
• however, the membrane remains attached at the plasmodesmata
• if the cell can gain water, it can recover
• further loss of water will result in complete plasmolysis / will result in the plasmodesmata breaking the connection with neighbouring cells
• other appropriate response

Question 76

Past Paper Question - January 2013 Q5 c)i) AS1 (Legacy Material)
Q5 Albumen is a globular protein which is an important soluble constituent of blood plasma. It contributes significantly to the solute potential of the plasma. This, in turn, has an effect on the movement of water in and out of body cells.

Albumen is produced in the liver and people suffering from liver disease are unable to produce sufficient amounts of albumen.

(c) (i) How would a lack of albumen change the solute potential of blood plasma? [1]

Answer 76

Q5 c)i) Solute potential would be increased/made less negative; [1]

Question 78

Past Paper Question - January 2013 Q5 c)ii) AS1 (Legacy Material)
Q5 Albumen is a globular protein which is an important soluble constituent of blood plasma. It contributes significantly to the solute potential of the plasma. This, in turn, has an effect on the movement of water in and out of body cells.

Albumen is produced in the liver and people suffering from liver disease are unable to produce sufficient amounts of albumen.

c)ii) Describe and explain the effect this change would have on the red blood cells. [2]

Answer 78

Q5 c)ii) Red blood cells would absorb water/swell/may burst (haemolyse); because the cell cytoplasm would have a lower water potential than the plasma; [2]

Question 80

Past Paper Question - January 2013 Q5 c)iii) AS1 (Legacy Material)
Q5 Albumen is a globular protein which is an important soluble constituent of blood plasma. It contributes significantly to the solute potential of the plasma. This, in turn, has an effect on the movement of water in and out of body cells.

Albumen is produced in the liver and people suffering from liver disease are unable to produce sufficient amounts of albumen.

One symptom of liver disease is the accumulation of watery fluid in body cavities such as the abdominal cavity.

iii) Explain the accumulation of water in the abdominal cavity of patients with liver disease. [1]

Answer 80

Q5 c)iii) Water potential is higher in the blood plasma than within the cavity (tissue fluid)/water is not being (osmotically) moved into the blood because of the lack of albumen in the blood; [1]

Question 82

Past Paper Question - January 2012 Q4 a)i)+ii) AS1 (Legacy Material)
Q4 a) The diagram below represents 3 adjacent plant cells. The solute potential (Psi s) and pressure potential (Psi p) for each cell are indicated.

i) State the name of the strands of cytoplasm which interconnect cells 1 and 2. [1]
ii) Describe and explain the net flow of water between cells 2 and 3. [2]

(Go do this past paper question)

Answer 82

Q4 a)i) Plasmodesmata; [1]

ii) There is no net flow of water between cells 2 and 3;
they have equal cell water potentials/both have a cell water potential
of –600 kPa;
Insist on the term water potential. [2]

Question 84

Past Paper Question - January 2012 Q4 b) AS1 (Legacy Material)
Q4 b) Osmosis involves the movement of water through a selectively permeable membrane.
Describe the pathway by which water molecules move through the cell-surface membrane and contrast this with the movement of small hydrophilic molecules such as glucose. [2]

Answer 84

Q4 b) Water moves/diffuses through the phospholipid bi-layer;
small hydrophilic molecules require a (protein) carrier/channel
or
Water moves via aquaporins;
small hydrophilic molecules require a different/specific protein carrier/ channel; [2]

Question 86

Past Paper Question - June 2012 Q4 a) AS1 (Legacy Material)
Q4 When cells are bathed in solutions with a water potential that is different to their cell water potential, their appearance may change due to osmosis. Some cells may be able to take corrective action to counteract the effects of osmosis.

(a) Red blood cells and onion cells behave differently when immersed in dilute (hypotonic) solutions. For each cell type, describe and explain fully the appearance of the cells after 10 minutes immersion.

● a red blood cell
_______________________________________________________
_______________________________________________________

● an onion cell
_______________________________________________________
_______________________________________________________ [3]

Answer 86

Q4 a) Any three from:
• cell lysed/burst
• as cell membrane cannot withstand the increased volume (due to
water uptake)
• turgid/full turgor/cell swollen
• due to protoplasm pushing against the cell wall/presence of wall
prevents further uptake [3]

Question 88

Past Paper Question - June 2012 Q4 b)i) AS1 (Legacy Material)
Q4 When cells are bathed in solutions with a water potential that is different to their cell water potential, their appearance may change due to osmosis. Some cells may be able to take corrective action to counteract the effects of osmosis.

b) Paramecium (shown below) is a single-celled organism, which lives in freshwater environments. Such environments cause Paramecium to take in water through the cell membrane by osmosis.

i) How does the water potential inside the Paramecium cell compare with that of the freshwater environment? [1]

Answer 88

Q4 b)i) It would be lower (inside the Paramecium cell)/higher outside the cell; [1]

Question 90

Past Paper Question - June 2012 Q4 b)ii) AS1 (Legacy Material)
Q4 When cells are bathed in solutions with a water potential that is different to their cell water potential, their appearance may change due to osmosis. Some cells may be able to take corrective action to counteract the effects of osmosis.

b) Paramecium (shown below) is a single-celled organism, which lives in freshwater environments. Such environments cause Paramecium to take in water through the cell membrane by osmosis.

To counteract this osmotic uptake of water, Paramecium has structures called contractile vacuoles. Each contractile vacuole consists of a circular membrane-bound sac, surrounded by tubules. Solutes are pumped from the cytoplasm into the contractile vacuole and water is subsequently absorbed.

ii) Explain how this pumping of solutes into the contractile vacuole causes water to move into the vacuole. [2]

Answer 90

Q4 b)ii) The presence of more solutes will decrease the water potential inside the vacuole/removal of solutes from cytoplasm increases its water potential;
as the vacuole water potential becomes lower than that of the cytoplasm water is drawn in (by osmosis)/water leaves the cytoplasm (enters vacuole) because cytoplasm has a higher water potential; [2]

Question 92

Past Paper Question - June 2012 Q4 b)iii) AS1 (Legacy Material)
Q4 When cells are bathed in solutions with a water potential that is different to their cell water potential, their appearance may change due to osmosis. Some cells may be able to take corrective action to counteract the effects of osmosis.

b) Paramecium (shown below) is a single-celled organism, which lives in freshwater environments. Such environments cause Paramecium to take in water through the cell membrane by osmosis.

To counteract this osmotic uptake of water, Paramecium has structures called contractile vacuoles. Each contractile vacuole consists of a circular membrane-bound sac, surrounded by tubules. Solutes are pumped from the cytoplasm into the contractile vacuole and water is subsequently absorbed.

iii) Contractile vacuoles are found in many single-celled organisms, but only in those which do not possess a cell wall. Suggest why this is the case. [2]

Answer 92

Q4 b)iii) If these organisms do not regulate water content, they are likely to burst;
where a cell wall is present, entry of water is limited (by pressure potential)/contractile vacuoles allow excess water to be removed; [2]

Question 93

Past Paper Question - June 2011 Q8 a)i) AS1 (Legacy Material)
Q8 a) The water potential of a cell (ψcell) has two components, the solute potential (ψs) and the pressure potential (ψp).

(i) Explain why the solute potential of a cell’s contents is always less than zero. [1]

Answer 93

Q8 a)i) There is always some solute in the cell contents/only water with no solutes has a potential of zero; [1]

Question 93

Past Paper Question - June 2011 Q8 a)ii) AS1 (Legacy Material)
Q8 a) The water potential of a cell (ψcell) has two components, the solute potential (ψs) and the pressure potential (ψp).

ii) State the term which is used to describe a plant cell when it’s pressure potential (ψp) is zero. [1]

Answer 93

Q8 a)ii) Incipient plasmolysis (or plasmolysed?) [1]

Question 93

Past Paper Question - June 2011 Q8 b)i)+ii)+iii)
Q8 b) The solute and pressure potentials of carrot tissue were determined after immersion in five external solutions (differing in their solute potential). These are shown in the graph below.

i) The water potential of the carrot tissue was calculated using the values for solute and pressure potentials shown in the graph opposite. Three of the values are shown in the table below.

Complete the table by calculating the two missing values. [2]

Solute potential of the Water potential of the
external solution /kPa carrot tissue /kPa

          –200                                                           0
          –400                                                       –150
          –600                                                       –370
          –800
         –1000

ii) Plot the water potential values, including those you have calculated, on the graph opposite and draw an appropriate line of best fit. [2]

Full turgor occurs when no more water can enter the tissue.

iii) Using the graph, determine the solute potential of the external solution at the point of full turgor. [1]

(Go do this past paper question)

Answer 93

Q8 b)i) -490;
-600; [2]

ii) Points accurately plotted;
line of best fit appropriate;
[Including points consequential to those given (i)]
[Line of best fit does not simply join first and last points] [2]
iii) Point of intersection between line plotted and x-axis (at y = 0) [1]

Question 93

Past Paper Question - June 2011 Q8 c)i) AS1 (Legacy Material)
Q8 c) A weighing method can be used to determine the average water potential (ψcell) of plant tissues, such as potato.

i) Briefly describe the procedure for the weighing method. [3]

Answer 93

Q8 c)i) Any three from
• core of potato is cut to a specific length and sliced/the same cork borer is used, for all samples
• samples are (surface dried and) weighed
• the samples are immersed in a series of sucrose solutions
(0 to 0.5 M sucrose)
• after 24 hours the samples are surface dried by a standard
method and reweighed
• the percentage change in mass is calculated for each sample [3]

Question 93

Past Paper Question - June 2011 Q8 c)ii) AS1 (Legacy Material)
Q8 c) A weighing method can be used to determine the average water potential (ψcell) of plant tissues, such as potato.

ii) Explain how you would analyse the results to obtain an estimate of the water potential of potato cells. [2]

Answer 93

Q8 c)ii) The percentage change in mass is plotted against the molarity of the sucrose solution/solute potential of the sucrose solution;
the water potential is determined by the point where the line of best fit crosses the x-axis/where there is no % change in mass (% change in mass = 0); [2]

Question 93

Past Paper Question - June 2011 Q8 c)iii) AS1 (Legacy Material)
Q8 c) A weighing method can be used to determine the average water potential (ψcell) of plant tissues, such as potato.

iii) Explain the biological basis of the weighing method as a means of determining the average water potential of the cells in a plant tissue. [2]

Answer 93

Q8 c)iii) The water potential of the potato is equivalent to the solute potential of the bathing solution/ cell 5  external;
[insist on term “water potential” – do NOT allow reference to “water concentration”]
when there is no change in mass/as there is no net movement of water in or out of the tissue; [2]

Question 93

Past Paper Question - January 2011 Q2 a)+b)+c) AS1 (Legacy Material)
Q2 The diagram below represents a plant cell immersed in a bathing solution with a solute potential of –1200kPa (ψexternal). The solute potential (ψs) and pressure potential (ψp) of the cell are also shown.

a) Calculate the water potential (ψcell) of the cell. [1]
b) Describe and explain the movement of water between the cell and its bathing solution. [2]
c) Draw a diagram of the cell to show its final appearance in the bathing solution. [2]

(Go do this past paper question)

Answer 93

Q2 a) Water potential (ψcell) of the cell = -1400 + 500 = -900kPa; [1]

b) Water moves out of the cell/into the solution/external medium water moves from a region of higher water potential to a region of lower water potential/as the cell has a higher (less negative) water potential (allow converse)
Insist on the term water potential. [2]
c) Error :/

Question 93

Past Paper Question - January 2011 Q5 b)i)+ii)+iii) AS1 (Legacy Material)
Q5 b) The table below shows the effect of changing conditions on three different mechanisms of membrane transport.

Effect of changing conditions on rate of movement

Change in Mechanism 1 Mechanism 2 Mechanism 3
conditions

Increased Rate increases No effect on rate No effect on rate
oxygen levels significantly

Addition of
cyanide Rate decreases No effect on rate No effect on rate
(a respiratory significantly
poison)

Increased
numbers of Rate increases Rate increases No effect on rate
membrane
carriers

Using the information in the table, identify each mechanism of membrane transport and, in each case, give a reason for your identification.

(i) Mechanism 1

Identification ____________________________________________

Reason __________________________________________________
_______________________________________________________ [2]

(ii) Mechanism 2

Identification ____________________________________________

Reason __________________________________________________
_______________________________________________________ [2]

(iii) Mechanism 3

Identification ____________________________________________

Reason __________________________________________________
_______________________________________________________ [2]

Answer 93

Q5 b)(i) Active transport;
increased oxygen increases ATP production/addition of cyanide decreases ATP production; [2]

(ii) Facilitated diffusion;
involves carrier proteins without ATP requirement; [2]
The reason must distinguish this from the other two mechanisms.

(iii) (Simple) diffusion;
no use of carrier proteins; [2]

Question 93

Past Paper Question - June 2013 Q6 b)i)+ii)+iii) AS1 (Legacy Material)
Q6 The Venus Flytrap plant (Dionaea muscipula) possesses modified leaves which form two plates of a trap. When triggered, small invertebrates are caught and digested. The mechanism of trap closure is believed to involve osmotic changes and the flow of water between the mesophyll layers within the plates.
A whole view of the plates is shown in the following diagrams together with a cross-sectional view showing the mesophyll layers.

The plate consists of two mesophyll layers, the upper and lower mesophyll. The water potentials of the cells of these layers is different and information regarding this is shown in the table below.

Potential/ kPa Cells of upper mesophyll Cells of lower mesophyll
ψcell 0
ψs -250 -250
ψp 100

i) Make a copy of the table above. Calculate the missing values and present these in the empty spaces in the table. [2]

When the trap is open, water movement between the mesophyll layers is not possible. However, when triggered to close, water will move from one layer to the other.

ii) Determine the direction in which water will flow when the trap is triggered to close. Explain your answer. [2]
iii) Explain what causes the change in the shape of the plates during closure of the trap. [2]

(Go do this past paper question)
[Textbook Chapter 4 - Exam Question 2, page 88-89]

Answer 93

Q6 b)i) [2]

Potential/ kPa Cells of upper mesophyll Cells of lower mesophyll
ψcell 0 -150
ψs -250 -250
ψp 250 100

ii) Water will flow from upper mesophyll cells to lower mesophyll cells; since osmosis takes place from higher to lower water potential;
[Must make reference to water potential, not concentration] [2]
iii) The cells of the lower mesophyll become more turgid;
lower mesophyll increases in size (and becomes more convex); [2]

Question 94

Past Paper Question - June 2018 Q1 a)i+ii) AS1 (Revised Spec.)
Q1 The process of osmosis is the movement of water from a region of higher water potential to a region of lower water potential through a selectively permeable membrane.

a) The diagram below represents a plant cell immersed in a solution with a water potential of -1180 kPa (ψexternal). The initial solute potential (ψs) and pressure potential (ψp) of the cell are also shown.

i) Calculate the water potential (ψcell) of the cell. [1]
ii) Draw an arrow on your diagram to show the direction of water movement. [1]

Answer 94

Q1 a)i) -1230 kPa [1]

ii) Arrow to show water movement into the cell; [1]

Question 95

Past Paper Question - June 2018 Q1 b) AS1 (Revised Spec.)
Q1 The process of osmosis is the movement of water from a region of higher water potential to a region of lower water potential through a selectively permeable membrane.

b) For each of the statements described below, indicate the net movement of water by placing a (tick) in the correct box. [3]

                                                     Net movement of water Statement                     Into the cell    Out of the cell   No net movement

Carrot tissue does not
gain or lose mass after
20 minutes in a sugar
Solution

An animal cell (ψcell =
-825 kPa) is immersed
in a solution (ψs = -974
kPa)

An animal cell shows
evidence of crenation
when immersed in a
salt solution

Answer 95

Q1 b) [3]
Net movement of water
Statement Into the cell Out of the cell No net movement

Carrot tissue does not
gain or lose mass after (Tick)
20 minutes in a sugar
Solution

An animal cell (ψcell =
-825 kPa) is immersed (Tick)
in a solution (ψs = -974
kPa)

An animal cell shows
evidence of crenation (Tick)
when immersed in a
salt solution

Question 96

Past Paper Question - June 2018 Section B Q8 a) AS1 (Revised Spec.)

Q8 All eukaryotic cells, and some organelles, are surrounded by selectively permeable membranes. The composition of the membranes is important in controlling the movement of molecules directly across the membranes.

Substances which cannot pass directly across the membrane may be transported by endocytosis or exocytosis.

a) Describe the structure of cell membranes and explain how their composition makes the membrane selectively permeable. [9]

Answer 96

Q8 a) • Fluid mosaic model structure.
• Phospholipid bilayer.
• Phospholipids consist of fatty acid chains and a phosphate head.
• Fatty acid chains are hydrophobic, while phosphate heads are hydrophilic (oriented towards cytoplasm).
• Allows passage of hydrophobic/non-polar substances across the bilayer.
• Can allow small hydrophilic/polar molecules such as water to pass through/large polar molecules cannot cross.
• Cholesterol within the cell surface membrane of animals influences the fluidity of the membrane.
• (Intrinsic and extrinsic) proteins occur throughout membrane.
• Some are receptors for specific molecules.
• Glycocalyx is comprised of carbohydrate attached to proteins or lipids.
• Proteins may be carriers for specific molecules.
• (Gated) channel proteins allow passage of polar/water-soluble substances.
• Some cells/organelles contain more protein to act as carriers/receptors depending on their role. [9]

Question 97

Past Paper Question - June 2018 Section B Q8 b) AS1 (Revised Spec.)

Q8 All eukaryotic cells, and some organelles, are surrounded by selectively permeable membranes. The composition of the membranes is important in controlling the movement of molecules directly across the membranes.

Substances which cannot pass directly across the membrane may be transported by endocytosis or exocytosis.

b) Describe the processes of endocytosis and exocytosis. [5]

Answer 97

Q8 b) • Exocytosis involves transport out of the cell, endocytosis involves transport into the cell;
• In exocytosis, a substance is packaged within a vesicle;
• Vesicle moves to and fuses with the plasma membrane;
• Endocytosis may be phagocytosis or pinocytosis;
• Phagocytosis involves the transport of solid material (into the cell);
• Pinocytosis involves the transport of fluid (into the cell);
• Cell surface membrane invaginates/infolds around the substance to form a vesicle (in endocytosis); [5]