Chemical Elements and Water

Question 1

What are the most frequently occurring chemical elements in living things?

Answer 1

Carbon, hydrogen, oxygen and nitrogen are the most frequently occurring chemical elements in living things.

Question 2

What are the most important elements needed by living organisms?

Answer 2

A variety of other elements are needed by living organisms, including sulfur, calcium, phosphorus, iron and sodium.

Question 3

What is sulfur needed for?

Answer 3

Needed for the synthesis of two amino acids.

Question 4

What is calcium needed for?

Answer 4

Acts as a messenger by binding to calmodulin and a few other proteins which regulate transcription and other processes in the cell

Question 5

What is phosphorous needed for?

Answer 5

Is part of DNA molecules and is also part of the phosphate groups in ATP.

Question 6

What is iron needed for?

Answer 6

Is needed for the synthesis of cytochromes which are proteins used during electron transport for aerobic cell respiration.

Question 7

What is sodium needed for?

Answer 7

When it enters the cytoplasm, it raises the solute concentration which causes water to enter by osmosis.

Question 8

Draw and label a diagram showing the structure of water molecules to show their polarity and hydrogen bond formation.

Answer 8

DRAW

Question 9

What are the thermal, cohesive and solvent properties of water?

Answer 9

Thermal properties of water include heat capacity, boiling and freezing points and the cooling effect of evaporation.

Water has a large heat capacity which means that a considerable amount of energy is needed to increase it’s temperature. This is due to the strength of the hydrogen bonds which are not easily broken. This is why the temperature of water tends to remain relatively stable. It is beneficial for aquatic animals as they use water as a habitat.

Water has a high boiling and freezing point. It boils at 100 C because the strong hydrogen bonds. All these hydrogen bonds between the water molecules need to break for the liquid to change to gas. Water becomes less dense as it gets closer to the freezing point and so ice always forms on the surface first. The high boiling point of water is vital for life on earth as if water boiled at a lower temperature the water in living organisms would start to boil and therefore these organisms would not survive.
The fact that water becomes less dense as it freezes is beneficial to organisms as ice will always form at the surface of lakes or seas and by doing so it insulates the water underneath, maintaining a possible habitat for organisms to live in.

Water can evaporate at temperatures below the boiling point. Hydrogen bonds need to break for this to occur.

Cohesion is the effect of hydrogen bonds holding the water molecules together. Water moves up plants because of cohesion. Long columns of water can be sucked up from roots to leaves without the columns breaking. The hydrogen bonds keep the water molecules sticking to each other.

The solvent properties of water mean that many different substances can dissolve in it because of its polarity.

Question 10

Explain the relationship between the properties of water and its uses in living organisms as a coolant, medium for metabolic reactions and transport medium.

Answer 10

Water can evaporate at temperatures below the boiling point. Hydrogen bonds need to break for this to occur. The evaporation of water cools body surfaces (sweat) and plant leaves (transpiration) by using the energy from liquid water to break the hydrogen bonds. The solvent properties of water mean that many different substances can dissolve in it because of its polarity. This allows substances to be carried in the blood and sap of plants as they dissolve in water. It also makes water a good medium for metabolic reactions.

Question 11

How are organic and inorganic compounds distinct?

Answer 11

Organic compounds are compounds that are found in living organisms and contain carbon. Inorganic compounds are the ones that don’t contain carbon. Although, there are a few compounds found in living organisms which also contain carbon but are considered as inorganic compounds. These include carbon dioxide, carbonates and hydrogen carbonates.

Question 12

Identify amino acids, glucose, ribose and fatty acids from diagrams showing their structure.

Answer 12

DRAW

Question 13

What are three monosaccharides?

Answer 13

Glucose, galactose and fructose are all monosaccharides.

Question 14

What are three disaccharides?

Answer 14

Maltose, lactose and sucrose are all disaccharides.

Question 15

What are three polysaccharides?

Answer 15

Starch, glycogen and cellulose are all polysaccharides.

Question 16

What is one function of glucose, lactose and glycogen in animals?

Answer 16

In animals, glucose is used as an energy source for the body and lactose is the sugar found in milk which provides energy to new borns until they are weaned. Finally, glycogen is used as an energy source (short term only) and is stored in muscles and the liver.

Question 17

What is one function of fructose, sucrose and cellulose in plants?

Answer 17

In plants, fructose is what makes fruits taste sweet which attracts animals and these then eat the fruits and disperse the seeds found in the fruits. Sucrose is used as an energy source for the plant whereas cellulose fibers is what makes the plant cell wall strong.

Question 18

Outline the role of condensation and hydrolysis in the relationships between monosaccharides, disaccharides and polysaccharides; between fatty acids, glycerol and triglycerides; and between amino acids and polypeptides.

Answer 18

EMPTY

Question 19

What are three functions of lipids?

Answer 19

Lipids can be used for energy storage in the form of fat in humans and oil in plants.
Lipids can be used as heat insulation as fat under the skin reduces heat loss.
Lipids allow buoyancy as they are less dense than water and so animals can float in water.

Question 20

Compare the use of carbohydrates and lipids in energy storage

Answer 20

Carbohydrates and lipids can both be used as energy storage however carbohydrates are usually used for short term storage whereas lipids are used for long term storage. Carbohydrates are soluble in water unlike lipids. This makes carbohydrates easy to transport around the body (from and to the store). Also, carbohydrates are a lot easier and more rapidly digested so their energy is useful if the body requires energy fast. As for lipids, they are insoluble which makes them more difficult to transport however because they are insoluble, lipids do not have an effect on osmosis which prevents problems within the cells in the body. They also contain more energy per gram than carbohydrates which makes lipids a lighter store compared to a store of carbohydrates equivalent in energy.

Question 21

Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and phosphate.

Answer 21

A nucleotide is made of the sugar deoxyribose, a base (which can be either adenine, guanine, cytosine or thymine) and a phosphate group. Below is a representation of a nucleotide.

Question 22

What are the names of the four bases in DNA?

Answer 22

Adenine, Guanine, Cytosine and Thymine.

Question 23

How are nucleotides linked together in a single strand?

Answer 23

nucleotides are linked to one another to form a strand. A covalent bond forms between the sugar of one nucleotide and the phosphate group of another nucleotide.

Question 24

How is a DNA double heix formed?

Answer 24

DNA is made up of two nucleotide strands. The nucleotides are connected together by covalent bonds within each strand. The sugar of one nucleotide forms a covalent bond with the phosphate group of another. The two strands themselves are connected by hydrogen bonds. The hydrogen bonds are found between the bases of the two strands of nucleotides. Adenine forms hydrogen bonds with thymine whereas guanine forms hydrogen bonds with cytosine. This is called complementary base pairing. Below is a digram showing the molecular structure and bonds within DNA.

Question 25

Draw and label a simple diagram of the molecular structure of DNA.

Answer 25

DRAW

Question 26

Explain DNA replication in terms of unwinding the double helix and separation of the strands by helicase, followed by formation of the new complementary strands by DNA polymerase.

Answer 26

DNA replication is semi-conservative as both of the DNA molecules produced are formed from an old strand and a new one. The first stage of DNA replication involves the unwinding of the double strand of DNA (DNA double helix) and separating them by breaking the hydrogen bonds between the bases. This is done by the enzyme helicase. Each separated strand now is a template for the new strands. There are many free nucleotides around the replication fork which then bond to the template strands. The free nucleotides form hydrogen bonds with their complimentary base pairs on the template strand. Adenine will pair up with thymine and guanine will pair up with cytosine. DNA polymerase is the enzyme responsible for this. The new DNA strands then rewind to form a double helix. The replication process has produced a new DNA molecule which is identical to the initial one.

Question 27

Explain the significance of complementary base pairing in the conservation of the base sequence of DNA.

Answer 27

Complementary base pairing is very important in the conservation of the base sequence of DNA. This is because adenine always pairs up with thymine and guanine always pairs up with cytosine. As DNA replication is semi-conservative (one old strand an d one new strand make up the new DNA molecules), this complementary base pairing allows the two DNA molecules to be identical to each other as they have the same base sequence. The new strands formed are complementary to their template strands but also identical to the other template. Therefore, complementary base pairing has a big role in the conservation of the base sequence of DNA.

Question 28

How is DNA pairing semi conservative?

Answer 28

EMPTY

Question 29

Compare the structures of DNA and RNA

Answer 29

DNA and RNA both consist of nucleotides which contain a sugar, a base and a phosphate group. However there are a few differences. Firstly, DNA is composed of a double strand forming a helix whereas RNA is only composed of one strand. Also the sugar in DNA is deoxyribose whereas in RNA it is ribose. Finally, both DNA and RNA have the bases adenine, guanine and cytosine. However DNA also contains thymine which is replaced by uracil in RNA.

Question 30

Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase.

Answer 30

DNA transcription is the formation of an RNA strand which is complementary to the DNA strand. The first stage of transcription is the uncoiling of the DNA double helix. Then, the free RNA nucleotides start to form an RNA strand by using one of the DNA strands as a template. This is done through complementary base pairing, however in the RNA chain, the base thymine is replaced by uracil. RNA polymerase is the enzyme involved in the formation of the RNA strand and the uncoiling of the double helix. The RNA strand then elongates and then separates from the DNA template. The DNA strands then reform a double helix. The strand of RNA formed is called messenger RNA.

Question 31

What is the genetic code composed of?

Answer 31

A triplet of bases (3 bases) forms a codon. Each codon codes for a particular amino acid. Amino acids in turn link to form proteins. Therefore DNA and RNA regulate protein synthesis. The genetic code is the codons within DNA and RNA, composed of triplets of bases which eventually lead to protein synthesis.

Question 32

How does translation work? (explain the process)

Answer 32

Translation is the process through which proteins are synthesized. It uses ribosomes, messenger RNA which is composed of codons and transfer RNA which has a triplet of bases called the anticodon. The first stage of translation is the binding of messenger RNA to the small subunit of the ribosome. The transfer RNA’s have a specific amino acid attached to them which corresponds to their anticodons. A transfer RNA molecule will bind to the ribosome however it’s anticodon must match the codon on the messenger RNA. This is done through complementary base pairing. These two form a hydrogen bond together. Another transfer RNA molecule then bonds. Two transfer RNA molecules can bind at once. Then the two amino acids on the two transfer RNA molecules form a peptide bond. The first transfer RNA then detaches from the ribosome and the second one takes it’s place.The ribosome moves along the messenger RNA to the next codon so that another transfer RNA can bind. Again, a peptide bond is formed between the amino acids and this process continues. This forms a polypeptide chain and is the basis of protein synthesis.

Question 33

What is the relationship between one gene and one polypeptide?

Answer 33

A polypeptide is formed by amino acids liking together through peptide bonds. There are 20 different amino acids so a wide range of polypeptides are possible. Genes store the information required for making polypeptides. The information is stored in a coded form by the use of triplets of bases which form codons. The sequence of bases in a gene codes for the sequence of amino acids in a polypeptide. The information in the genes is decoded during transcription and translation leading to protein synthesis.

Question 34

Define enzymes

Answer 34

Globular proteins which act as catalysts of chemical reactions.

Question 35

Define active site

Answer 35

Region on the surface of an enzyme to which substrates bind and which catalyses a chemical reaction involving the substrates.

Question 36

Explain enzyme-substrate specificity

Answer 36

The active site of an enzyme is very specific to its substrates as it has a very precise shape. This results in enzymes being able to catalyze only certain reactions as only a small number of substrates fit in the active site. This is called enzyme-substrate specificity. For a substrate to bind to the active site of an enzyme it must fit in the active site and be chemically attracted to it. This makes the enzyme very specific to it’s substrate. The enzyme-substrate complex can be compared to a lock and key, where the enzyme is the lock and the substrate is the key.

Question 37

How does temperature affect enzymes?

Answer 37

Enzyme activity increases with an increase in temperature and usually doubles with every 10 degrees rise. This is due to the molecules moving faster and colliding more often together. However at a certain point the temperature gets to high and the enzymes denature and stop functioning. This is due to the heat causing vibrations within the enzyme destroying its structure by breaking the bonds in the enzyme.

Question 38

How do variable pH levels affect enzymes?

Answer 38

Enzymes usually have an optimum pH at which they work most efficiently. As the pH diverges from the optimum, enzyme activity decreases. Both acid and alkali environments can denature enzymes.

Question 39

How does substrate concentration affect enzyme activity?

Answer 39

Enzyme activity increases with an increase in substrate concentration as there are more random collisions between the substrate and the active site. However, at some point, all the active sites are taken up and so increasing the substrate concentration will have no more effect on enzyme activity. As long as there are active sites available, an increase in substrate concentration will lead to an increase in enzyme activity.

Question 40

Define denaturation

Answer 40

Denaturation is changing the structure of an enzyme (or other protein) so it can no longer carry out its function.

Question 41

Explain the use of lactase in the production of lactose-free milk

Answer 41

Lactose is the sugar found in milk. It can be broken down by the enzyme lactase into glucose and galactose. However some people lack this enzyme and so cannot break down lactose leading to lactose intolerance. Lactose intolerant people need to drink milk that has been lactose reduced. Lactose-free milk can be made in two ways. The first involves adding the enzyme lactase to the milk so that the milk contains the enzyme. The second way involves immobilizing the enzyme on a surface or in beads of a porous material. The milk is then allowed to flow past the beads or surface with the immobilized lactase. This method avoids having lactase in the milk.

Question 42

Define cell respiration

Answer 42

Cell respiration is the controlled release of energy from organic compounds in cells to form ATP.

Question 43

How is glucose broken down in cell respiration?

Answer 43

In cell respiration, glucose in the cytoplasm is broken down by glycolysis into pyruvate with a small yield of ATP.

Question 44

What happens with pyruvate during anaerobic cell respiration?

Answer 44

In anaerobic cell respiration the pyruvate stays in the cytoplasm and in humans is converted into lactate which is the removed from the cell. In yeast the pyruvate is converted into carbon dioxide and ethanol. In either case, no ATP is produced.

Question 45

What happens with pyruvate during aerobic cell respiration?

Answer 45

If oxygen is available, the pyruvate is taken up into the mitochondria and is broken down into carbon dioxide and water. A large amount of ATP is released during this process.

Question 46

What fundamental energy conversion occurs during photosynthesis?

Answer 46

Photosynthesis involves the conversion of light energy into chemical energy.

Question 47

How is light from the sun diverse?

Answer 47

The light from the sun is composed of a range of wavelengths (colours).

Question 48

What is the main photosynthetic pigment?

Answer 48

Chlorophyll is the main photosynthetic pigment.

Question 49

How does chlorophyll absorb different wavelengths of light?

Answer 49

Chlorophyll can absorb red and blue light more than green. Chlorophyll cannot absorb green light and so instead reflects it making leaves look green.

Question 50

How is light energy used in photosynthesis?

Answer 50

Light energy is used to produced ATP and to split water molecules (photolysis) to form oxygen and hydrogen.

Question 51

What happens to the ATP and hydrogen of the light reactions of photosynthesis?

Answer 51

ATP and hydrogen derived from photolysis of water are used to fix carbon dioxide to make organic molecules.

Question 52

How can the rate of photosynthesis be measured?

Answer 52

Photosynthesis can be measured in many ways as it involves the production of oxygen, the uptake of carbon dioxide and an increase in biomass. For example, aquatic plants release oxygen bubbles during photosynthesis and so these can be collected and measured. The uptake of carbon dioxide is more difficult to measure so it is usually done indirectly. When carbon dioxide is absorbed from water the pH of the water rises and so this can be measured with pH indicators or pH meters. Finally, photosynthesis can be measured through an increase in biomass. If batches of plants are harvested at a series of times and the biomass of these batches is calculated, the rate increase in biomass gives an indirect measure of the rate of photosynthesis in the plants.

Question 53

How does temperature affect the rate of photosynthesis?

Answer 53

As temperature increases, the rate of photosynthesis increases more and more steeply until the optimum temperature is reached. If temperature keeps increasing above the optimum temperature then photosynthesis starts to decrease very rapidly.

Question 54

How does light intensity affect the rate of photosynthesis?

Answer 54

As light intensity increases so does photosynthesis until a certain point. At a high light intensities photosynthesis reaches a plateau and so does not increase any more. At low and medium light intensity the rate of photosynthesis is directly proportional to the light intensity.

Question 55

How does carbon dioxide concentration affect the rate of photosynthesis?

Answer 55

As the carbon dioxide concentration increases so does the rate of photosynthesis. There is no photosynthesis at very low levels of carbon dioxide and at high levels the rate reaches a plateau.