Human metabolism: carbohydrates, fats and proteins

Question 1

Dietary mono and di-saccharides offer readily available glucose and causes a rapid raise in blood glucose.

Name an example of a
I) Monosaccaride
II) Disaccaride

Answer 1

Mono: Glucose, fructose (honey/friuts)
Di: Sucrose (glucose and fructose) Lactose (glucose, galactose)

Question 2

Are plant and animal starches broken down into glucose?

Answer 2

Yes but they take much longer to be digested

Question 3

Why is glucose a versatile fuel?

Answer 3

Can be consumed by all tissues

Can be respired anaerobically (unlike fats and proteins)

Question 4

What is the initial pathway for all glucose metabolism in the cytoplasm?

Answer 4

Glycolysis - starts with anaerobic pathway whereby glucose (6 carbon) is converted into 2 x pyruvate (3 carb) and 2 moles of ATP by substrate level phosphorylation.

Question 5

Under anaerobic conditions what happens to the pyruvate

Answer 5

Reduced to lactate and regeneration of NAD+ form NADH (NAD+ needed in glycolysis)

Question 6

Under aerobic conditions what happens to pyruvate?

Answer 6

Oxidative phosphorylation in the mitochondrion, pyruvate (3 carbon) is converted into the acetyl coenzyme A (CoA).

Acetyl coenzyme A is then consumed into the krebs cycle

Question 7

What does the krebs cycle create? Why is this significant?

Answer 7

1. 3 x NADH
2. 1 X FADH2
3. 1 X GTP
4. 2 x CO2

Reduced 1 & 2 pass electrons does transport chain to produce ATP

Question 8

How much ATP is made from each mole go glucose + ATP in

1) Aerbic respiration
2) Anaeorbic respiration

Answer 8

Aerobic 30-32moles (16 times more, due to electron transport chain), 20 ATP
Anaerobic 2 moles, 5 ATP

Question 9

Why are red blood cells reply on anaerobic respiration

Answer 9

They continue no intracellular organelle such as mitochondria so they ca squeeze through capillaries.

Therefore no oxidative phosphrylation

Question 10

What substrates fuels can be used for anaerobically respiting muscle?

Answer 10

Blood glucose (usually from lover glycogen)
Muscle glycogen

Question 11

Why can fat stores never be converted to glucose?

Answer 11

Fat stores can be broken down to acetyl coenzyme A (for the aerobic krebs) but it cannot be converted back to pyruvate

Question 12

Which organ needs a lot of sugar?

Answer 12

The brain 100g/day

Question 13

In starvation where us glucose obtained from?

Answer 13

Muscle protein and amino acids

Cause muscle wasting

Question 14

For a non-diabetic what is the normal range of blood glucose levels

Answer 14

4-8 mol/l

Question 15

Below which blood glucose level is a person at risk of hypoglycaemic coma?

Answer 15

2mmol/l

Question 16

How is fat stored in the body?

Answer 16

Triacylglycerol which forms liquid droplets in the cytosol of the adipocyte.

Question 17

How is energy produced from triacylglycerol (TAG)

Answer 17

Fatty acids side chains are long with many hydrogen ions, there act as a reservoir for reducing co factors NADH and FADH2 along the electron transport chain.

Fat has higher energy yield than carbohydrates

Question 18

What hormones catalyse the HSL enzyme which breaks down TAG to free fatty acids y

Answer 18

Noradrenaline
Adrenaline
Glucagon

Therefore blood fatty acids raised in starvation, stress, exercise, diabetic coma and GDM

Question 19

Which pathway in high aerobic tissue causes oxidation of fatty acids

Answer 19

beta-oxidation pathway - Fatty acid combines with co-enzyme A - fatty acyl coenzyme A, then the carbon fatty acid is nippled away producing FADh2 and NADH

Question 20

What is the main difference between fat & carbohydrate storage

Answer 20

Carbohydrate are limited and only stored for 24 hours

Fat can be stored in limitless quantities

Question 21

What is the issue with starvation and brain function

Answer 21

Brain has high glucose requirement. After 24 hours, bodies predominant fuel is fats. Fats limited as fat cannot be converted into glucose. Brain cannot consume fatty acids as lipoproteins do not cross the BBB.

Question 22

As fat acid can be converted into acetyl co enzyme which substrate of the krebs cycle must increase and where does it come from

Answer 22

Oxaloacetate (amino acids, muscle protein)

Question 23

What drives ketogensis in the liver in starvation

Answer 23

Oxaloacetate is withdrawn from the krebs cycle for gluconeogenesis but rates of fatty acid uptake and beta-oxidation remain high.

Therefore the liver channels excess acetyl coenzyme A into ketogenesis

Ketones are then released into the bloods which can then be used by the brain

Question 24

How does the response to insulin change in pregnancy?

Answer 24

Initial increase in insulin sensitivity
Mild insulin resistance develops can be reduced by 50-70% by 3rd trimester

• if become more severe → GDM

Question 25

In GDM what happens to serum levels of

• insulin
• fatty acids
• ketones

Answer 25

• Normal or raised
• high
• high (increased ketonuria

Question 26

Why is GDM associated with macrosomia

Answer 26

High levels of glucose uptake in innate. Glucose uptake is predictive of birth weight

Question 27

What the 2 situations proteins are broken down into amino acid

Answer 27

1. High oral intake (cannot be stored)

2. Starvation - amino acids for glucose synthesis.

Question 28

How is the nitrogenous amino group excreted from amino acid?

Why is this important?

Answer 28

Amino acid metabolism is initiated with the removal of the nitrogenous amino group which is then incorporated into urea in the hepatic urea cycle.

Important to prevent the build up of toxic nitrogen compounds such as ammonia (NH4+)

Question 29

How are amino acids converted into ammonium

Answer 29

Tranamiantion and deamination of glutamate

1. Deamination amino group transferred to alpha-ketoglutarate to glutamate
2. Deamination Amino acid released as NH4+

**many other tranaminases for individual amino acids

Question 30

What is the purpose of the urea cycle and where does it occur?
Where is urea excreted

Answer 30

Detoxify NH4+ (2 moles of nitrogen to 1 mole of urea)
Cytoplasm and mitochondria of liver cells
Urea is excreted by the kidneys

Question 31

What is the urea equation

Answer 31

NH4+ + Bicarb + Aspartate → (using 2 ATP) urea + fumarate

Fumarate → Krebs intermediate

Question 32

How do high levels of ammonia present

Answer 32

swelling of brain, lethargy, mental restrication

High levels push equation to left (high levels of

Question 33

Can amino acids be converted into glucose in starvation

Answer 33

Only the gluocgenic amino acids which links in with krebs intermediates
Most muscle tissue releases
alanine → Pyruvate
glutamine → Alpha ketoglutame

Question 34

What is ketogensis

Answer 34

Ketogenesis is the biochemical process through which organisms produce ketone bodies by breaking down fatty acids and ketogenic amino acids.

Question 35

How does the liver manage low glucose levels in starvation

Answer 35

Gluconeogenesis (glucagon and cortisol)
Ketogenesis
Protein metabolism

Question 36

What is phenylketonuria?

Answer 36

Rare condition (1 in 10000) are born without the enzyme phenylalanine hydroxylase, leading to high levels of phenylalanine.

If untreated competitive for amino acid transporters and effect the developing brain leading to gross mental restriction.

Question 37

When is phenylketonuria test for? Treatment?

Answer 37

Day 6-14 after birth, guthrie test

Diet with low phenylketonuria until age 11 & in pregnancy whilst brain is developing

Question 38

As the glucose requirements of the foetus increase what energy fuel is used instead

Answer 38

Fatty acids and ketone bodies

insulin resistance in mother

Question 39

In breast feeding which metabolic pathway is slow to develop?

Answer 39

Lipogenesis (Acetyl CoA converts to triglyercaide for fat storage)
High levels of fat from breast milk, lipoprotein lipase used in lots of tissue so fat is used as energy sorce

Question 40

Why is brown fat adipose tissue important in newborn?

Answer 40

Newborns cannot shiver

Brown adipose can produce heat from fatty acids - non shivering thermogenesis