Chapter 2

Question 1

Where is the water in organisms stored?

Answer 1

Two thirds is contained inside cells and the remainder is found in extracellular biofluids, such as plasma in animals and phloem sap in plants

Question 2

Describe the structure of water (including the charges of each end)

Answer 2

Water is a polar molecule made of an oxygen atom and two hydrogen atoms. The oxygen atom has a slightly negative charge and the hydrogen atoms have a slightly positive charge. The water molecule is a bent molecule (rather than linear) and so the two hydrogen atoms are on one side of the molecule and form the positive pole whilst the oxygen atom is on the other, forming the negative pole.

Question 3

Why are water molecules attracted to each other?

Answer 3

Because the slight positive charges are attracted to the slight negative charges. These forces of attraction are called hydrogen bonds

Question 4

What type of force is a hydrogen bond? How strong is it?

Answer 4

Strictly speaking a hydrogen bond is a weak intermolecular force rather than an actual bond. Individually they are very weak but if there are a lot of hydrogen bond in a given volume they are collectively quite strong

Question 5

Give 4 properties of water

Answer 5

1. Water is a solvent
2. High specific heat capacity
3. High latent heat of vaporisation
4. Cohesive and adhesive properties

Question 6

why is water a good solvent?

Answer 6

water molecules are attracted to other polar and charged particles, which makes water a good solvent for substances with these properties. The water molecules form a ‘shell’ around charged ions and other molecules that possess a slight charge on their surface. This prevents the ions and molecules from clumping together and so causes the ions and other molecules to dissolve.

Question 7

give 3 things that water being a good solvent is important for

Answer 7

1. for transporting substances around the body in the plasma, for example, glucose insulin and lymph
2. in removing metabolic waste, for example urea in urine
3. in allowing chemical reactions to take place inside cells, for example respiration and protein synthesis

Question 8

why can’t non-polar ions be transported in the plasma? What happens to get around this barrier? give an example.

Answer 8

non-polar ions cannot dissolve in water and therefore cannot be transported in the plasma. Instead they have to be combined with molecules that are soluble, for example, cholesterol is insoluble in water so it is attached to lipoproteins to enable it to be transported in the plasma.

Question 9

what are ions dissolved in water within the body called?

Answer 9

electrolytes

Question 10

why does water have a high specific heat capacity?

Answer 10

hydrogen bonds restrict the movement of water molecules, which increases the amount of energy needed to break the hydrogen bonds.

Question 11

why is water having a high specific heat capacity important?

Answer 11

• A large amount of energy is needed to make bodies of water change temperature. Water must lose a large amount of energy to cool down. This means that the temperature of water bodies is relatively stable in comparison to air and land. This makes water an ideal habitat for aquatic organisms
• it is also important as it prevents our internal body temperature changing quickly as a result of changes in the environment

Question 12

why is water having a high latent heat of vaporisation important?

Answer 12

as it takes a large amount of heat to break the hydrogen bonds, this means that when water evaporates it has a cooling effect. This property of water is important since it allows our bodies to lose heat through sweating.

Question 13

why is water viscous?

Answer 13

as a result of its cohesive and adhesive properties

Question 14

why are the cohesive and adhesive properties of water important? Give examples.

Answer 14

• as water can be used as a lubricant in the form of pleural fluid (to minimise friction between the lungs and thoracic rib cage) and mucus (e.g. to allow the passage of faeces down the colon)
• cohesion and adhesion of water molecules is also critical to transport in xylem vessels in plants

Question 15

what is pleural fluid?

Answer 15

a fluid that is used to minimise friction between the lungs and thoracic rib cage

Question 16

how is the bacteria which causes cholera (vibrio cholerae) transmitted between infected and non-infected people?

Answer 16

through the ingestion of water or food contaminated with faeces containing the bacteria

Question 17

how many deaths is it estimated that cholera causes per year?

Answer 17

120 000

Question 18

what are enterocytes?

Answer 18

simple columnar epithelial cells found in the small intestines, colon, and appendix, which posses projections called microvilli on their surface

Question 19

what is the cholera toxin composed of, which cells does it target, and how does it enter cells?

Answer 19

the cholera toxin is composed of six protein subunits - a single copy of the A subunit and 5 copies of the B subunit. The B subunit binds to the surface of target cells - enterocytes (simple columnar epithelial cells found in the small intestines, colon, and appendix, which posses projections called microvilli on their surface). Once bound to the enterocyte, the entire cholera toxin complex enters the cell by endocytosis.

Question 20

how does the cholera toxin cause harm to the body?

Answer 20

once inside the cell (by endocytosis) the toxin is transported to the Golgi apparatus, where the A subunit is then unfolded. The A subunit causes an increase in adenylate cyclase enzyme activity, which increases the intracellular concentration of the molecule cAMP to more than 100 times the normal level. This leads to the phosphorylation of (attachment of a phosphate group to) the CTFR (cystic fibrosis transmembrane regulator) chloride channel proteins. As a consequence, chloride ions leave the cell resulting in the secretion of H2O, Na+, K+ and HCO-3 into the intestinal lumen. The absorption of Na+ and water into enterocytes is also decreased. The combined effect is the rapid loss of fluids from the intestine. In serious cases up to two litres per hour can be lost, leading to severe dehydration and other symptoms such as diarrhoea.

Question 21

what can the symptoms of cholera lead to? What is this called? Why can’t you just combat this by drinking water?

Answer 21

the symptoms of cholera can lead to low electrolyte concentrations in the blood within hours. This is called hyponatraemia and it can be life threatening. It is therefore vital that the sodium ions and water are replaced. However, the use of drinking water alone is not fully effective because water is lost from the cytoplasm of cells and is not being properly absorbed in the large intestine. Also, drinking water does not replace the ions that have been lost from the epithelial cells of the intestine.

Question 22

give some oral rehydration solutions for cholera

Answer 22

oral rehydration solutions contain water, sodium chloride, glucose, potassium, and other electrolytes, such as chloride and citrate ions. Commercially available preparations can be bought over the counter but it is also possible to prepare a homemade solution. This can be made from eight level teaspoons of sugar and one level teaspoon of table salt dissolved in one litre of boiled drinking water.

Question 23

give one thing that the hydrogen bonds that form between water molecules are responsible for

Answer 23

water’s high surface tension, which allows small insects such as pont skaters to move on its surface without sinking.

Question 24

does water have a high or low viscosity?

Answer 24

a relatively low visocosity

Question 25

give 4 things that water as a liquid is useful for

Answer 25

1. transporting substances
2. a medium for life
3. a solvent
4. maintaining temperature

Question 26

give two reactions that water can be used as a reactant of

Answer 26

1. photosynthesis
2. hydrolysis

Question 27

which 3 molecules is water particularly useful in the digestion of? What is it also used in the digestion of?

Answer 27

carbohydrates, proteins, and lipids. Water is also used in the digestion of large biological molecules.

Question 28

what percentage of the cell cytoplasm is water? why is this useful?

Answer 28

over 90% - this is useful as glucose and oxygen can move around and react together. It also means molecules and ions dissolved in the water can be transported.

Question 29

define latent heat of vaporisation

Answer 29

the energy required to convert 1 kg water into a gas

Question 30

Why is ice less dense than water?

Answer 30

due to the cohesive nature of the water molecules: ice has an open lattice with hydrogen bonds holding the water molecules apart, when ice melts the rigid hydrogen bonds collapse, allowing the water molecules to move closer together. The distance the molecules are held apart is what makes ice less dense than water.

Question 31

why is the density of ice useful?

Answer 31

because it ensures that organisms have a habitat in ponds and lakes

Question 32

when is the cohesive property of water particularly useful?

Answer 32

in transpiration

Question 33

when is surface tension useful?

Answer 33

as a habitat for living organisms