Metabolism & Metabolic Pathways

Question 1

Structure of carbohydrates

Answer 1

Composed of carbon, hydrogen and oxygen - C(H2O)n

Classified as Monosaccharide (1 unit), Disaccharide (2), Oligosaccharides (3-10), Polysaccharides (>10)

Question 2

Monosaccharide isomers e.g. glucose isomers

Answer 2

D-Isomers - a stereoisomer which rotates light that is polarized in a clockwise direction

L-Isomers: a stereoisomer that has a non superimposable mirror image counterpart

Question 3

Roles of carbohydrates

Answer 3

(1) Source of energy for chemical reactions
(2) inhibits breakdown of proteins for energy use
(3) Fatty acid breakdown and prevention of ketosis

Question 4

Structure of proteins

Answer 4

Amino acids joined together by peptide bonds to form polypeptide chains

(1) Primary structure: the unique sequence of amino acids that make up a protein or polypeptide chain
(2) Secondary structure: The way in which the primary structure of a polypeptide chain folds. e.g. beta sheets and alpha helixes
(3) Tertiary structure: The final 3D structure of a protein, entailing the shaping of a secondary structure. e.g. antibodies and keratin
(4) Quaternary structure: formed when two or more polypeptide chains join together to form a protein e.g. haemoglobin

Question 5

Roles of proteins

Answer 5

(1) transport e.g. albumin
(2) hormone signalling
(3) immune functions e.g. antibodies
(4) gene transcription and translation

Question 6

Structure of lipids

Answer 6

• Made of hydrogen and carbon atoms linked by neutral covalent bonds
• non-polar due to the covalent bond, therefore insoluble in water

Question 7

Fatty acids

Answer 7

Cis fatty acids: 2 Hydrogen atoms on the same side of the = bond that make membranes more fluid

Trans fatty acid: the 2 hydrogen atoms are on opposite side of = bond

Question 8

Triglycerides

Answer 8

• 3-C glycerol linked to 3 fatty acids
• A carboxyl group is attached to the hydroxyl group of glycerol
• Unsaturated fatty acids have double bonds (C=C)
• polyunsaturated fatty acids have more than 1 double bone (C-C)

Question 9

Phospholipids

Answer 9

• Modified triglycerides with one fatty acid replaced by a phosphate group
• Electrically charged phosphate group creates a polar region
• Present in the bilayer of eukaryote cell membrane

Question 10

Steroids

Answer 10

• 4 interconnected rings of carbon
• Insoluble in water
• Includes sex hormones

Question 11

Glycolipids

Answer 11

• A carbohydrate attached to one of more fatty acid chains

- Play a key role in cell surface recognition

Question 12

Ketone bodies

Answer 12

• Formed from oxidation of fatty acids in the liver
• Excessive formation of ketone bodies found in Type I diabetes
• Acetone formed from breakdown of ketone bodies

Question 13

Roles of Lipids

Answer 13

• Chemical messengers (e.g. glycolipids)
• Energy storage
• Membrane lipid bilayer (e.g. phospholipids)

Question 14

Glucose and Fatty Acid interactions

Answer 14

• Fatty acids can be formed from glucose, but not vice versa
• glucose can be stored as a lipid
• White adipose tissue is specialised for triglyceride storage
• storage of carbohydrate as glycogen is limited

Question 15

Glucose Transporters

Answer 15

• Glucose is a polar molecule that is impermeable to the cell membrane

Sodium-Glucose Cotransporters (SLGTs) 1-4

• found in specialised epithelial cells of the small intestine and PCT of kidney
• actively transports glucose by coupling glucose with Na+
• the Na+ gradient is maintained by active transport of Na+ across the Na+K+ ATPase channel

Facilitative Glucose Transporters (GLUTS) 1-5
- mediates an energy independent transport of glucose by facilitated diffusion

Question 16

Glycolysis

Answer 16

• Breakdown of glucose to form pyruvate and 2 ATP molecules

- Pyruvate can be used in anaerobic respiration or by TCA cycle in presence of oxygen

Question 17

Glucose Phosphorylation

Answer 17

• Occurs inside the cell by hexokinases
• Hexokinases have higher affinity for glucose transporters therefore utilised more in brain, RBCs and muscles
• Glucokinases have lower affinity for glucose transportes therefore utilised more in liver and pancreatic B cells

Question 18

Anaerobic respiration

Answer 18

• Glucose is converted to Glucose-6-Phosphate by hexokinase
• Glucose-6-P is converted into Frucose-6-P by phosphofructokinase
• A Pi is added to glyceraldehyde by dehydrogenase
• Pyruvate Kinase allows formation of pyruvate, which can be reduced to lactic acid
• 2 ATP is produce

Question 19

Tricarboxylic Acid Cycle

Answer 19

• A 2C Acetyl-CoA is combined with oxaloacetate to form citrate
• FADH2 and NADH produced are sent to the electron transport chain
• 4 ATP is produced

Question 20

Electron Transport Chain

Answer 20

• NADH and FADH2 produced from TCA and Fatty acid B-oxidation are used to produce ATP
• Energy from NADH and FADH2 are used to pump protons across the ATP-ADP carrier channels in the mitochondrial membrane down a concentration gradient
• 36 ATP produced in the presence of oxygen
• 2 ATP produced in the absence of oxygen
• By products include H2O and CO2
• Uncoupling can lead to excessive proton influx and heat generation that may result in cell damage, decrease ATP production and ageing

Question 21

Energy Substrate Requirement

Answer 21

Glucose: obligatory for brain cells and RBCs
Fatty acids: most tissues
Ketones: most tissues, except in the liver
Amino Acids/Proteins: minimal usage, except in fast dividing cells e.g. in cancer

Question 22

Amino Acid Metabolism

Answer 22

• AA are used in synthesis of proteins and peptides
• Sources of energy during fasting, trauma and sepsis
• Deamination: removal of an amine group by deaminase enzymes
• Transmination: transfer of an amine group to a ketoacid to make new amino acids
• Urea Formation: removal of the nitrogenous base