Module 2.1.2 - Biological molecules

Question 1

What are the different types of biological molecules?

Answer 1

• Water
• Proteins
• Carbohydrates
• Lipids
• Inorganic ions
• Nucleic acids

Question 2

What are the different important functions of water?

Answer 2

• it’s a reactant in lots of important chemical reactions such as photosynthesis as well as hydrolysis reactions
• substances can dissolve in it as it is a solvent. This is important as most biological processes take place in solution (e.g. in the cytoplasm of eukaryotic and prokaryotic cells)
• it can transport substances such as glucose and oxygen around plants and animals as it is a liquid and a solvent
• it has a high specific heat capacity and high latent heat of evaporation so it helps with temperature control
• due to its properties, it is a habitat so many organisms can survive and reproduce in it

Question 3

what is the structure of a molecule of water?

Answer 3

• one oxygen atom covalently bonded to two hydrogen atoms

Question 4

what is a covalent bond?

Answer 4

• A covalent bond is one in which two atoms share the same pair of electrons

Question 5

Which atom in a water molecule is delta negative?

Answer 5

• oxygen

Question 6

which atom in a water molecule is delta positive?

Answer 6

• hydrogen atoms

Question 7

what is a polar molecule?

Answer 7

• a molecule with a slight negative charge on one side and a slight positive charge on the other

Question 8

why does the structure of water make it a polar molecule?

Answer 8

• the electrons in the covalent bonds of water are not equally shared as they are pulled towards the oxygen nucleus
• this is because the negative electrons are more attracted to the oxygen nucleus as it has more positively charged protons than the hydrogen atom nuclei
• the other side of the hydrogen atom is then left with a partial positive charge and the unshared negative electrons (at the bottom)on the oxygen atom give it a partial negative charge

Question 9

what is a hydrogen bond?

Answer 9

• a hydrogen bond is a weak bond between a slightly positively charged hydrogen atom in one water molecule and a slightly negatively charged atom in another molecule
• the slightly negatively charge atom doesn’t necessarily have to be water all the time
• hydrogen bonds are weaker than covalent bonds but stronger than most intermolecular forces
• hydrogen bonding gives water some of its useful properties

Question 10

List the properties of water

Answer 10

• high specific heat capacity
• high latent heat of evaporation
• very cohesive
• lower density when solid
• good solvent

Question 11

what does cohesion mean and is water cohesive?

Answer 11

• cohesion is the attraction between molecules of the same type
• water molecules are very cohesive and tend to stick to each-other as they are polar and attracted to each-other due to the hydrogen bonds

Question 12

Give the explanation and function of the property of cohesion in water

Answer 12

• due to cohesion, water molecules tend to stick together which helps water to flow (e.g. if water is taken out from the end of a tube the other water molecules will follow behind)
• function: good for transporting substances (transportation medium)
• this is important as it helps water to be transported up plant stems in the transpiration stream
  example= allows plants to be able to draw water up xylem vessels in a continuous stream against the pull of gravity.
• cohesion of water creates surface tension which allows the surface of the water to act as a habitat for insects

Question 13

why does water have a high surface tension?

Answer 13

• at the surface of air an water, the cohesion between water molecules produces surface tension
• water molecules at the surface are more attracted to the water molecules below than molecules in the air
• this uneven attraction causes the water molecules to be pulled inwards towards the water below, creating surface tension

Question 14

what is adhesion and why is water adhesive?

Answer 14

• adhesion is the attraction between non-alike molecules, such as water molecules sticking to other molecules
• water is adhesive as it can be attracted to other polar or charged surfaces

Question 15

Give the explanation and function of the property of a high specific heat capacity in water

Answer 15

• The hydrogen bonds can absorb a lot of energy so water takes a lot of energy to heat it up
• function: good habitat
• water doesn’t experience rapid temperature changes

Question 16

Give the explanation and function of the property of high latent heat of evaporation in water

Answer 16

• Due to cohesion, it takes a lot of energy to break the bonds between the water molecules in order for it to change state
• so a lot of energy is used up when water evaporates
• function: coolant
• when sweat evaporates(changes from a liquid to a gas on the surface of the skin) it uses a lot of heat energy so cools the skin.

Question 17

Give the explanation and function of water being a lower density when it is a solid rather than a liquid

Answer 17

• in ice, water molecules are held more further apart than they are in liquid water which makes ice less dense.
• this explains why ice floats
• function: good habitat
• in cold temperatures, ice can form an insulating layer on top of water so the water below doesn’t freeze = helps aquatic organisms

Question 18

Give the explanation and function of water being a good solvent

Answer 18

• in biological reactions, a lot of substances are ion (made up of positively and negatively charged atoms)
• so the slightly positive side of the water molecule will be attracted to the negative ion and the slightly negative side of the water molecule will be attracted to the positive ion
• so the ions will get totally surrounded by water molecules (dissolve)

summary: polarity of water will separate ions due to attraction by surrounding them.

function: reaction medium, good habitat
- in humans, important ions can dissolve in the water and then be transported around the body.