Chemistry For Life

Question 1

What is the single base unit of all elements?

Answer 1

The atom

Question 2

What do atoms of two or more elements form when they react together?

Answer 2

A compound

Question 3

What is an atom made up of?

Answer 3

A nucleus containing positive protons and neutral neutrons surrounded by negative electrons arranged in electron shells

Question 4

What are the two ways atoms can get a stable (full) outer shell?

Answer 4

• Ionic bonding: the atoms involved in the reaction donate or receive electrons. The atom or part of the molecule, gains one or more electrons and becomes a negative ion (anion). The other atom or part of the molecule loses one or more electrons and becomes a positive ion (cation). Strong forces of attraction called ionic bonds hold the oppositely charged ions together
• covalent bonding: the atoms involved in the reaction share electrons. Covalent bonds are very strong and the molecules formed are usually neutral. However in some covalent compounds the molecules are slightly polarised. The electrons in the covalent bonds are not evenly shared. This means the molecule has a part that is slightly negative and a part that js slightly positive. This seperation of charge is called a dipole, and the tiny charges are represented as delta + and delta -. The molecule is described as a polar molecule

Question 5

What happens when ionic substances are dissolved in water?

Answer 5

The ions seperate

Question 6

Why in living organisms do most ionic substances exist as positive and negative ions?

Answer 6

Because cells are 60-70% water and ions dissociate in water

Question 7

What are the important anions and their formula?

Answer 7

• nitrate ions (NO3-)
• phosphate ions (PO43-)
• chloride ions (Cl-)
• hydrogen carbonate ions (HCO3-)

Question 8

What are nitrate ions needed for?

Answer 8

Needed in plants for the formation of amino acids and therefore proteins from the products of photosyntheis, and also for the formation of DNA

Question 9

What are phosphate ions needed for?

Answer 9

Needed in all living organisms including plants and animals in the formation of ATP and ADP as well as DNA and RNA

Question 10

What are chloride ions needed for?

Answer 10

Needed in nerve impulses and mang secretory systems

Question 11

What are hydrogen carbonate ions needed in?

Answer 11

Needed for buffering the blood to prevent it from becoming too acidic

Question 12

What are theimportanrt cations and their formulas?

Answer 12

• sodium ions (Na+)
• calcium ions (Ca2+)
• hydrogen ions (H+)
• magnesium ions (Mg2+)

Question 13

What are sodium ions used for?

Answer 13

Needed in nerve impulses and many secretory systems

Question 14

What are calcium ions used for?

Answer 14

Needed for the formation of calcium pectate for the middle lamella between two cell walls in plants and fornbone formation and muscle contraction in animals

Question 15

What are hydrogen ions used for?

Answer 15

Needed in cellular respiration and photosynthesis and in numerous pumps and systems in organisms as well as pH balance

Question 16

What are magnesium ions used for?

Answer 16

Needed for the production of chlorophyll in plants

Question 17

What is the structure of water molecules?

Answer 17

• they have two atoms of hydrogen joined to one atom of oxygeb
• the electrons are held closer to the oxygen than hydrogen so water is a polar molecule

Question 18

Water has hydrogen bonding. What is this and what does this mean for the water molecule?

Answer 18

• the slightly negative oxygen atom of one water molecule will attract the slightly positive hydrogen atoms of other water molecules in a weak electrostatic attraction called a hydrogen bond
• this means the water molecules ‘stick together’ more than you might expect
• water has a relatively high melting and boiling point compared to other substances that have molecules of a similar size as it takes a lot of energy to overcome the attractive forces of all the hydrogen bonds

Question 19

The properties of water make it very important in biological systems for several reasons. What are these reasons?

Answer 19

• water is a polar solvent. Because water is a polar molecule many ionic substances will dissolve in it. However many polar covalently bonded substances will also dissolve in it although they do not often dissolve in other covalently bonded solvents. As a result most of the chemical reactions in cells occur in water
• water is an excellent transport medium because so many substances will dissolve in it. Water also carries other substances such as starch that form colloids rather than solution
• as water cools further than 4°c it’s molecules become more widely spread and it becomes less dense than water and floats: forming an insulated layer and helping to protect the water underneath from freezing. It also melts quickly as it is at the top. It means that organisms can continue to live under water
• water has a high specific heat capacity and is slow to absorb and release heat. The hydrogen bonds between molecules take a lot of energy to seperate. This means that the temperature of large bodies of water doesn’t change through the year making them good habitats
• water is a liquid and so it cannot be compressed. This is an important factor in many hydraulic mechanisms in lving organisms
• water molecules are cohesive: the forces between the molecule mean they stick together. This is very important in the transpiration stream
• water molecules are adhesive: they are attracted to other different molecules. This is also important in plant transport systems and in surface tension
• water has a very high surface tension because the attraction between the water molecules including the hydrogen bonds) is greater than the attraction between the water molecules and the air. As a result the water molecules hold together forming a thin skin of surface tension. Surface tension is important in plant transport systems and also affects the life at the surfaces of water masses