Metabolism 1

Question 1

What is the general molecular formula for carbohydrates?

Answer 1

[C(H2O)]n (‘hydrated carbon’)

n equal to/> 3

Question 2

How are carbohydrates classified?

Answer 2

1. Monosaccharides (1 monomeric unit)
2. Disaccharides (2 monomeric units)
3. Oligosaccharides (3-10 monomeric units)
4. Polysaccharides (>10 monomeric units)

Question 3

What types of carbohydrates are in a typical healthy human diet?

Answer 3

Polysaccharides (complex carbohydrates) e.g. starch which need to be broken down in order to be absorbed

Question 4

What is the suffix ending names of monosaccharides?

Answer 4

‘-ose’

Question 5

What are the ways in which monosaccharides can be classified?

Answer 5

1. No. of C atoms (size) e.g. triose, tetrose, pentose, hexose
2. Isomer (same atoms but bonded in a different 3D arrangement) e.g. D vs L

Question 6

What do glucose, fructose and galactose have in common?

Answer 6

They are all hexoses with different arrangements of the same atoms i.e. isomers

Question 7

What isomer are the most important sugars?

Answer 7

D-isomers

Question 8

What happens to longer-chain monosaccharides? Give some examples.

Answer 8

They form cyclic molecules pentoses + hexoses (e.g. ribose, glucose)

Pentoses normally form 5-membered rings whilst hexoses form 6-membered rings but this is not always the case as O2 is part of the ring

Question 9

What are some examples of important monosaccharides? Why are they important?

Answer 9

Trioses: dihydroxyacetone + G3P (important intermediates in energy metabolism i.e. glycolysis)

Pentoses: ribose + deoxyribose (crucial components of RNA + DNA)

Hexoses: glucose, galactose + fructose (found in important disaccharides/oligosaccharides)

Question 10

How are disaccharides formed?

Answer 10

Reaction between 2 monosaccharides which eliminates H2O + forms a glycosidic bond

Question 11

What are examples of important disaccharides? How are they formed?

Answer 11

Sucrose = glucose + fructose

Lactose = galactose + glucose

Maltose = glucose + glucose (breakdown product of starch/glycogen)

Question 12

Give examples of some important poly/oligosaccharides.

Answer 12

Starch (obtained in diet from plant sources)

Cellulose (linear polymer of glucose that is a component of fibre present in diet from plant sources but not digestible in humans due to lack of cellulase enzyme)

Glycogen (extensively branched polymer of glucose obtained in diet from animal sources)

Dextrin (branched oligomer of glucose that is a breakdown product of starch + glycogen)

Question 13

What is starch made up of?

Answer 13

75% amylopectin (branched polymer of glucose formed by α-1,4 + α-1,6 glycosidic bonds)

25% amylose (linear polymer of glucose formed by α-1,4 glycosidic bonds only)

Question 14

What are the main types of biologically important lipids?

Answer 14

FAs
Triglycerides (TAGs)
Cholesterol
Cholesterol esters

Question 15

What are fatty acids?

Answer 15

Hydrocarbon chains of various lengths

Question 16

How are fatty acids classified?

Answer 16

Considered long chain if >12C + very long chain if >22C

Saturated or unsaturated (at least 1 C=C bond)

Question 17

Fatty acids + ___ = triglycerides

Answer 17

Glycerol

Question 18

What are the different types of fatty acid nomenclature?

Answer 18

1. Most descriptive: C#1:#2(Δ#,#…) - #1 (total no. of Cs), #2 (no. of double bonds) + numbers in brackets (Cs where double bond occurs counting from acidic end)
2. Describing position of final double bound from hydrocarbon end: ω-# (‘omega-#’)

Question 19

What does the cis or trans nomenclature of fatty acids describe?

Answer 19

A form of stereoisomerism at double bounds, around where there is no rotation
(applies to unsaturated FAs)

Question 20

What is the difference between cis and trans fatty acids?

Answer 20

Either isomer can be incorporated into triglycerides + modified lipids but cis FAs pack next to each other less closely than trans ones causing membranes to be more fluid

Question 21

What do phospholipids consist of?

Answer 21

A phosphate group attached to 1 or more FA chains via glycerol or sphingosine, which itself contains a long hydrocarbon chain

Question 22

What do glycolipids consist of?

Answer 22

A carbohydrate element (usually an oligosaccharide) attached to 1 or more FA chains directly or via glycerol or sphingosine

Question 23

What are the functions of phospholipids and glycolipids?

Answer 23

Important components of cellular membranes:

• Phospholipids amphipathic (polar/non-polar) nature integral to membrane structure
• Glycolipids function in cell surface recognition

Question 24

What are ketone bodies? Give a couple of examples of them.

Answer 24

Small (4C) FAs formed by oxidation of FAs in the liver especially during fasting, when they become important energy substrates for the brain

E.G. acetoacetic acid + β-hydroxybutyric acid

Question 25

What can be excessively produced by the liver in type 1 diabetes? What is the consequence of this?

Answer 25

Ketone bodies resulting in dangerously high concentrations in the blood which can cause ketoacidosis Acetone is formed as a breakdown product + is exhaled - pear drop smell on patient's breath

Question 26

What are the roles of lipids?

Answer 26

- Substrates for energy generation in cells e.g. FAs, ketone bodies - Energy storage e.g. triglycerides - Transport between tissues e.g. cholesterol esters, triglycerides - Structural components of cell membranes e.g. phospholipids, cholesterol - Chemical messengers e.g. diglycerides

Question 27

What different types of amino acids exist?

Answer 27

1. Essential: obtained from diet e.g. Leu, Val, Lys + Phe | 2. Non-essential: synthesized endogenously e.g. Ala, Cys, Gly + Ser

Question 28

How is Adenosine Triphosphate (ATP) made?

Answer 28

From ADP using energy released by oxidation

Question 29

What is Adenosine Triphosphate (ATP) used for?

Answer 29

Anabolic processes consume it Catabolic processes produce ATP

Question 30

What is the difference between anabolism and catabolism?

Answer 30

Energy stored in high-energy bond is released in anabolism whereas in catabolism, energy from oxidation of nutrients is stored in high-energy bond

Question 31

Where can glucose be used as an energy source?

Answer 31

For most tissues but obligatory for brain + RBCs

Question 32

Where can fatty acids be used as an energy source?

Answer 32

Most tissues + minimally in neurons

Question 33

Where can ketones be used as an energy source?

Answer 33

Most tissues but not the liver as this is where they are synthesized Important for brain as a partial substitute for glucose when less is available

Question 34

Where can amino acids be used as an energy source?

Answer 34

Not used as fuel by many cell types but used in fast-dividing cells e.g. enterocytes + cancer cells (particularly Glu)

Question 35

What can happen to glucose once it has been taken into the body?

Answer 35

1. Digestive system -> blood where it is taken up by RBCs 2. Lactate is produced + so is energy as a by-product of the reaction 3. If there is lots in the blood, it can be taken to liver to be stored as glycogen (main storage) which can be broken down again when hungry/starving 4. Used as an energy source by the brain + muscle (skeletal + cardiac) too 5. If broken down with insufficient O2 when exerting yourself, lactate is formed which can be recycled in liver to form new glucose

Question 36

How do fatty acids and glucose interact?

Answer 36

Mechanisms exist for minimising utilisation of both glucose + FAs at the same time FAs can be formed from glucose but NOT vice versa so glucose excess can be stored as lipid (triglycerides) but this can only be mobilised/metabolised as FAs

Question 37

Where should triglycerides be stored? What happens if other tissues start to store it?

Answer 37

White adipose tissue (internal or subcutaneous) Spill over into 'ectopic' tissues e.g. skeletal/cardiac muscle + liver has pathological consequences

Question 38

Where are carbohydrates?

Answer 38

Storage as glycogen is limited in extent: - In liver to regulate blood glucose (lasts overnight in humans) - In skeletal muscle for direct energy requirements

Question 39

How is glucose transported into cells?

Answer 39

1. Passive transport via GLUT1-5 | 2. Active transport via SGLT-1 + 2

Question 40

What is the specificity and tissue distribution of GLUT1? What is its main role?

Answer 40

Transports glucose, galactose + mannose Present in RBCs, brain + ubiquitously Involved in basal uptake as it has high affinity for glucose

Question 41

What is the specificity and tissue distribution of GLUT2? What is its main role?

Answer 41

Transports glucose + fructose Present in liver + pancreatic β cells Involved in glucose sensing due to low affinity for glucose

Question 42

What is the specificity and tissue distribution of GLUT3?

Answer 42

Transports glucose only Present in brain, intestine + placenta

Question 43

What is the specificity and tissue distribution of GLUT4? What is its main role?

Answer 43

Transports glucose only Present in muscle (skeletal/cardiac) + adipose tissue Insulin sensitive glucose uptake so good at regulation of glucose levels

Question 44

What is the specificity and tissue distribution of GLUT5?

Answer 44

Transports fructose only Present in intestine

Question 45

What is the specificity and tissue distribution of the SGLT glucose transporters?

Answer 45

SGLT1: transports 2 glucose, Na + galactose & present in intestine/kidney SGLT2: Transports 2 glucose + Na & present in kidney