Module 2: Section 2 - Biological Molecules

Question 1

What are five important functions of water, inside and outside of cells?

Answer 1

1) water is a reactant in loads of important chemical reactions, including hydrolysis reactions
2) water is a solvent, which means some substances dissolve in it. Most biological reactions take place in solution, so water’s pretty essential
3) water transports substances. The fact that it’s a liquid and a solvent means it can easily transport all sorts of materials, like glucose and oxygen, around plants and animals
4) water helps with the temperature control because it has a high specific heat capacity and a high latent heat of evaporation
5) water is a habitat. The fact that it helps with temperature control, is a solvent and becomes less dense when it freezes means many organisms can survive and reproduce in it.

Question 2

Explain the structure of a water molecule in 6 steps and draw the structure of a water molecule. See pg 20

Answer 2

1) A molecule of water is one atom of oxygen joined to two atoms of hydrogen by shared electrons
2) because the shared negative hydrogen electrons are pulled towards the oxygen atom, the other side of each hydrogen atom is left with a slight positive charge
3) the unshared negative electrons on the oxygen atom give it a slight negative charge
4) this makes water a polar molecule - it has a delta negative charge on one side and a delta positive charge on the other
5) the slightly negatively-charged oxygen atoms attract the slightly positively-charged hydrogen atoms of other water molecules
6) this attraction is called hydrogen bonding and it gives water some useful properties

Question 3

How do hydrogen bonds give water a high specific heat capacity and why does this make water a good habitat?

Answer 3

1) specific heat capacity is the energy needed to raise the temperature of 1 gram of a substance by 1 degrees C
2) the hydrogen bonds between water molecules can absorb a lot of energy. So water has a high specific heat capacity - it takes a lot of energy to heat up
3) this means water doesn’t experience rapid temperature changes, which is one of the properties that makes it a good habitat - the temperature under water is likely to be more stable that it is on land

Question 4

Hydrogen bonds give water a high latent heat of evaporation. Why is this useful for living organisms?

Answer 4

1) It takes a lot of energy to break the hydrogen bonds between water molecules
2) So water has a high latent heat of evaporation - a lot of energy is used up when water evaporates
3) This is useful for living organisms because it means water’s great for cooling things. This is why some mammals sweat when they’re too hot. When sweat evaporates, it cools the surface of the skin.

Question 5

Water’s polarity makes it very cohesive. Why and how is this useful?

Answer 5

1) cohesion is the attraction between molecules of the same type. Water molecules are very cohesive because they are polar
2) this helps water to flow, making it great for transporting substances. It also helps water to be transported up plant stems in the transpiration stream

Question 6

Water’s polarity also makes it a good solvent. Why and how is this useful?

Answer 6

1) a lot of important substances in biological reactions are ionic (like salt, for example). This means they’re made from one positively-charged atom or molecule and one negatively-charged atom or molecule
2) because water is polar, the slightly positive end of a water molecule will be attracted to the negative ion, and the slightly negative end of a water molecule will be attracted to the positive ion
3) this means the ions will get totally surrounded by water molecules - on other words, they’ll dissolve
4) water’s polarity is useful as a solvent in living organisms. e.g. in humans, important ions can dissolve in the water in blood and then be transported around the body

Question 7

Draw a diagram of a positive ion dissolved in water and a negative ion dissolved in water

Answer 7

see pg. 21

Question 8

Water’s less dense when it’s solid. Why and how is this useful?

Answer 8

1) at low temperatures water freezes - it turns from a liquid to a solid
2) water molecules are held further apart in ice than they are in liquid water because each water molecule forms four hydrogen bonds to other water molecules, making a lattice shape. This makes ice less dense than liquid water - which is why ice floats
3) this is useful for living organisms because, in cold temperatures, ice forms an insulating layer on top of water - the water below doesn’t freeze. So organisms that live in water, like fish, don’t freeze and can still move around

Question 9

What are most carbohydrates?

Answer 9

Most carbohydrates are polymers. A polymer is a molecule made up of many similar, smaller molecules (called monomers) bonded together.

Question 10

What are the monomers that make up carbohydrates called?

Answer 10

Monosaccharides

Question 11

Draw the structure of ribose

Answer 11

see pg 22

Question 12

All carbohydrates are made up of the same three chemical elements, which are?

Answer 12

Carbon, hydrogen and oxygen. For every carbon atom in the carbohydrate there are usually two hydrogen atoms and one oxygen atom

Question 13

There are two forms of glucose - alpha and beta. They both have a ring structure. What is glucose’s structure related to?

Answer 13

Glucose’s structure is related to its function as the main energy source in animals and plants. Its structure makes it soluble so it can be easily transported. Its chemical bonds contain lots of energy

Question 14

What is ribose?

Answer 14

Ribose is a monosaccharide with five carbon atoms - this means it’s a pentose monosaccharide

Question 15

What are monosaccharides joined together by?

Answer 15

Monosaccharides are joined together by glycosidic bonds

Question 16

Explain a condensation reaction and a hydrolysis reaction

Answer 16

1) during synthesis, a hydrogen atom on one monosaccharide bonds to a hydroxyl (OH) group on the other, releasing a molecule of water. This is called a condensation reaction.
2) the reverse of this synthesis reaction is hydrolysis. A molecule of water reacts with the glycosidic bond, breaking it apart

Question 17

When is a disaccharide formed and give a few examples?

Answer 17

A disaccharide is formed when two monosaccharides join together:

• for example, two alpha glucose molecules are joined together by a glycosidic bond to form maltose
• other disaccharides are formed in a similar way. Sucrose is a disaccharide formed when alpha glucose and fructose join together. Lactose is a disaccharide formed by the joining together of beta glucose and galactose

Question 18

When is a polysaccharide formed and give an example?

Answer 18

A polysaccharide is formed when more than two monosaccharides join together:

-lots of alpha glucose molecules are joined together by glycosidic bonds to form amylose

Question 19

What are the three polysaccharides you need to know about?

Answer 19

Starch - the main energy storage material in plants

Glycogen - the main energy storage material in animals

Cellulose - the major component of cell walls in plants

Question 20

Why is starch good for storage?

Answer 20

Starch is insoluble in water, so it doesn’t cause water to enter cells by osmosis which would make them swell. This makes it good for storage.

Question 21

Cells get energy from glucose. How do plants store excess glucose?

Answer 21

Plants store excess glucose as starch. When a plant needs more glucose for energy it breaks down starch to release the glucose.

Question 22

Starch is a mixture of two polysaccharides of alpha-glucose. What are these two polysaccharides and what are there properties?

Answer 22

1) Amylose - a long, unbranched chain of alpha-glucose. The angles of the glycosidic bonds give it a coiled structure, almost like a cylinder. This makes it compact, so it’s really good for storage because you can fit more in a small space.
2) Amylopectin - a long, branched chain of alpha-glucose. Its side branches allow the enzymes that break down the molecule to get at the glycosidic bonds easily. This means that the glucose can be released quickly.

Question 23

How do animal cells store excess glucose?

Answer 23

Animal cells store excess glucose as glycogen - another polysaccharide of alpha-glucose

Question 24

What are the properties of glycogen and how are these properties useful?

Answer 24

Its structure is very similar to amylopectin, except that it has loads more side branches coming off it. Loads of branches means that stored glucose can be released quickly, which is important for energy release in animals.

It’s also a very compact molecule, so it’s good for storage.

Question 25

Describe the structure of cellulose. What properties does cellulose have as a result of

Answer 25

1) cellulose is made of long, unbranched chains of beta-glucose
2) when beta-glucose molecules bond, they form straight cellulose chains
3) the cellulose chains are linked together by hydrogen bonds to form strong fibres called microfibrils. The strong fibres mean cellulose provides structural support for cells (e.g. in plant cell walls)

Question 26

What are triglycerides and what is their structure?

Answer 26

1) triglycerides are macromolecules - they’re complex molecules with a relatively large molecular mass
2) like all lipids, they contain the chemical elements carbon, hydrogen and oxygen
3) triglycerides have one molecule of glycerol with three fatty acids attached to it

Question 27

Why are lipids insoluble in water?

Answer 27

Because… Fatty acid molecules have long ‘tails’ made of hyrocarbons. The tails are hydrophobic. These tails make lipids insoluble in water. All fatty acids have the same basic structure, but the hydrocarbon tail varies.

Question 28

What is the process in which triglycerides are synthesised called?

Answer 28

Esterification

Question 29

What reaction breaks down triglycerides?

Answer 29

Triglycerides break down when the ester bonds are broken. Each ester bond is broken in a hydrolysis reaction (in which a water molecule is used up).

Question 30

How are triglycerides synthesised?

Answer 30

Triglycerides are synthesised by the formation of an ester bond between each fatty acid and the glycerol molecule.

Each ester bond is formed by a condensation reaction (in which a water molecule is released).

Question 31

Define saturated and unsaturated fats

Answer 31

Saturated fatty acids don’t have any double bonds between their carbon atoms. The fatty acid is ‘saturated’ with hydrogen.

Unsaturated fatty acids have at least one double bond between carbon atoms, which cause the chain to kink.

Question 32

What is a phospholipid?

Answer 32

1) phospholipids are also macromolecules. They’re pretty similar to triglycerides except one of the fatty acid molecules is replaced by a phosphate group.
2) the phosphate group is hydrophilic and the fatty tails are hydrophobic.

Question 33

In animals and plants, triglycerides are mainly used as energy storage molecules. Some bacteria (e.g. mycobacterium tuberculosis) use triglycerides to store both energy and carbon. What are the properties of triglycerides and how does this make them good for storage?

Answer 33

Triglycerides are good for storage because:

1) the long hydrocarbon tails of the fatty acids contain lots of chemical energy - a load of energy is released when they’re broken down. Because of these tails, lipids contain about twice as much energy per gram as carbohydrates.
2) they’re insoluble, so they don’t cause water to enter the cells by osmosis which would make them swell. The triglycerides bundle together as insoluble droplets in cells because the fatty acid tails are hydrophobic - the tails face inwards, shielding themselves from water with their glycerol heads.

Question 34

Phospholipids are found in the cell membranes of all eukaryotes and prokaryotes. They make up what’s known as the phospholipid bilayer. Cell membranes control what enters and leaves a cell. How do the properties of phospholipids relate to their function?

Answer 34

1) phospholipid heads are hydrophilic and their tails are hydrophobic, so they form a double layer with their heads facing out towards the water on either side
2) the centre of the bilayer is hydrophobic, so water-soluble substances can’t easily pass through it - the membrane acts as a barrier to those substances

Question 35

What are the properties of cholesterol and how do these properties relate to its function?

Answer 35

Cholesterol is another type of lipid - it has a hydrocarbon ring structure attached to a hydrocarbon tail. The ring structure has a polar hydroxyl (OH) group attached to it. In eukaryotic cells, cholesterol molecules help strengthen the cell membrane by interacting with the phospholipid bilayer.

1) cholesterol has a small size and flattened shape - this allows cholesterol to fit in between the phospholipid molecules in the membrane
2) they bind closely to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This helps to make the membranes less fluid and more rigid

Question 36

What are proteins made up of?

Answer 36

1) Proteins are polymers
2) amino acids are the monomers in proteins
3) a dipeptide is formed when two amino acids join together
4) a polypeptide is formed when more than two amino acids join together
5) proteins are made up of one or more polypeptides

Question 37

What is the basic composition of all amino acids?

Answer 37

1) all amino acids have the same general structure - a carboxyl group and an amino group attached to a carbon atom. The difference between different amino acids is the variable group they contain
2) all amino acids contain the chemical elements carbon, oxygen, hydrogen and nitrogen. Some also contain sulfur.

Question 38

What reaction joins together and breaks apart amino acids and what is the name of the bond that holds amino acids together?

Answer 38

1) amino acids are linked together by peptide bonds to form dipeptides and polypeptides
2) a molecule of water is released during the reaction - it’s a condensation reaction
3) the reverse of this reaction adds a molecule of water to break the peptide bond - it’s a hydrolysis reaction

Question 39

Define the primary structure of a protein

Answer 39

Primary structure - this is the sequence of amino acids in the polypeptide chain. Different proteins have different sequences of amino acids in their primary structure. A change in just one amino acid may change the structure of the whole protein

Question 40

Define the secondary structure of a protein

Answer 40

Secondary structure - the polypeptide chain doesn’t remain flat and straight. Hydrogen bonds form between nearby amino acids in the chain. This makes it automatically coil into an alpha helix or fold into a beta pleated sheet - this is the secondary structure

Question 41

Define the tertiary structure of a protein

Answer 41

Tertiary structure - the coiled or folded chain of amino acids is often coiled and folded further. More bonds form between different parts of the polypeptide chain. For proteins made from a single polypeptide chain, the tertiary structure forms their final 3D structure

Question 42

Define the quaternary structure of a protein

Answer 42

Quaternary structure - some proteins are made up of several different polypeptide chains held together by bonds. The quaternary structure is the way these polypeptide chains are assembled together. E.g. haemoglobin is made of four polypeptide chains, bonded together. For proteins made from more than one polypeptide chain, the quaternary structure is the protein’s final 3D structure

Question 43

What bonds hold together the primary structure?

Answer 43

Primary structure - held together by the peptide bonds between amino acids

Question 44

What bonds hold together the secondary structure?

Answer 44

Secondary structure - held together by hydrogen bonds

Question 45

What bonds hold together the tertiary structure?

Answer 45

Tertiary structure - this is affected by a few different kinds of bonds:

• ionic groups. These are attractions between negatively charged R groups and positively charged R groups on different parts of the molecule
• disulfide bonds. Whenever two molecules of the amino acid cysteine come close together, the sulfur atom in one cysteine bonds to the sulfur in the other cysteine, forming a disulfide bond
• hydrophobic and hydrophilic interactions. When hydrophobic R groups are close together in the protein, they tend to clump together. This means that hydrophilic R groups are more likely to be pushed to the outside, which affects how to protein folds up into its final structure
• hydrogen bonds - these weak bonds form between slightly positively-charged and slightly negatively-charged atoms in other R groups on the polypeptide chain

Question 46

What bonds hold together the quaternary structure?

Answer 46

Quaternary structure - this tends to be determined by the tertiary structure of the individual polypeptide chains being bonded together. Because of this, it can be influenced by all the bonds that are involved in protein bonding.

Question 47

What is the general structure of globular proteins and what property does this give them?

Answer 47

1) In a globular protein, the hydrophilic R groups on the amino acids tend to be pushed to the outside of the molecule. This is caused by the hydrophobic and hydrophilic interactions in the protein’s tertiary structrure
2) this makes globular proteins soluble, so they’re easily transported in fluids
3) globular proteins have a range of functions in living organisms

Question 48

Name three globular proteins

Answer 48

Haemoglobin, insulin and amylase

Question 49

What is the function and structure of Hb?

Answer 49

Hb is a globular protein that carries oxygen around the body in red blood cells. It’s known as a conjugated protein - this means it’s a protein with a non-protein group attached. The non-protein part is called a prosthetic group. Each of the four polypeptide chains in Hb has a prosthetic group called haem. A haem group contains iron, which oxygen binds to.

Question 50

What is the function and structure of insulin?

Answer 50

Insulin is a hormone secreted by the pancreas. It helps to regulate the blood glucose level. Its solubility is important - it means it can be transported in the blood to the tissues where it acts. An insulin molecule consists of two polypeptide chains, which are held together by disulfide bonds.

Question 51

What is the function and structure of amylase?

Answer 51

Amylase is an enzyme that catalyses the breakdown of starch in the digestive system. It is made of a single chain of amino acids. Its secondary structure contains both alpha-helix and beta-pleated sheet sections.

Question 52

What are most enzymes?

Answer 52

Most enzymes are globular proteins.

Question 53

Give me some general properties about fibrous proteins

Answer 53

Fibrous proteins are insoluble and strong. They’re structural proteins and are fairly unreactive (unlike many globular proteins)

Question 54

What are the properties of collagen and where is it found?

Answer 54

Collagen - found in animal connective tissues, such as bone, skin and muscle. It is a very strong molecule. Minerals can bind to the protein to increase its rigidity. e.g. in bone

Question 55

What are the properties of keratin and where is it found?

Answer 55

Keratin - found in many of the external structures of animals, such as skin, hair, nails, feathers and horns. It can either be flexible (as it is in skin) or hard and tough (as it is in nails)

Question 56

What are the properties of elastin and where is it found?

Answer 56

Elastin - found in elastic connective tissue, such as skin, large blood vessels and some ligaments. It is elastic, so it allows tissues to return to their original shape after they have been stretched

Question 57

Define a cation, anion and an inorganic ion

Answer 57

1) an ion with a positive charge is called a cation
2) an ion with a negative charge is called an anion
3) an inorganic ion is one which doesn’t contain carbon

Question 58

What’re the roles of calcium in biological processes?

Answer 58

Involved in the transmission of nerve impulses and the release of insulin from the pancreas. Acts as a cofactor for many enzymes e.g. those involved in blood clotting. It is important for bone formation.

Question 59

What’re the roles of sodium in biological processes?

Answer 59

Important for generating nerve impulses, for muscle contraction and for regulating fluid balance in the body.

Question 60

What’re the roles of potassium in biological processes?

Answer 60

Important for generating nerve impulses, for muscle contraction and for regulating fluid balance in the body. Activates essential enzymes needed for photosynthesis in plant cells.

Question 61

What’re the roles of hydrogen in biological processes?

Answer 61

Affects the pH of substances. Also important for photosynthesis reactions that occur in the thylakoid membranes inside chloroplasts.

Question 62

What’re the roles of ammonium in biological processes?

Answer 62

Absorbed from the soil by plants and is an important source of nitrogen (which is then used to make for example, amino acids, nucleic acids)

Question 63

What’re the roles of nitrate in biological processes?

Answer 63

Absorbed from the soil by plants and is an important source of nitrogen (which is then is then used to make, for example, amino acids, nucleic acids)

Question 64

What’re the roles of hydrogencarbonate in biological processes?

Answer 64

Acts as a buffer, which helps to maintain the pH of the blood

Question 65

What’re the roles of chloride in biological processes?

Answer 65

Involved in the ‘chloride shift’ which helps to maintain the pH of the blood during gas exchange. Acts as a cofactor for the enzyme amylase. Also involved in some nerve impulses

Question 66

What’re the roles of phosphate in biological processes?

Answer 66

Involved in photosynthesis and respiration reactions. Needed for the synthesis of many biological molecules, such as nucleotides (including ATP), phospholipids, and calcium phosphate (which strengthens bones)

Question 67

What’re the roles of hydroxide in biological processes?

Answer 67

Affects the pH of substances