Energy Production: Carbohydrate 1

Question 1

What sources are there of Acetyl CoA?

Answer 1

Keto-Acids from amino acids.
Pyruvate from glucose
Fatty acids
Alcohol

Question 2

What happens in stage 1 of catabolism?

Answer 2

Protein, carbohydrates and lipids are broken down in to amino acids, monosaccharides, glycerol and fatty acids.
There is no energy release in this stage.

Question 3

What happens in stage 2 of catabolism?

Answer 3

Amino acids are converted into NH4+, kept-acids and pyruvate.
Monosaccharides such as glucose are converted into pyruvate.
Glycerol is fed into the pathway to form pyruvate.
Some reducing powers are formed such as NAD+ goes to NADH + H+.
Some energy in the form of ATP is produced as well.

Question 4

Where does stage 1 take place?

Answer 4

Extracellularly in the GI-tract.

Question 5

Where does stage 2 take place?

Answer 5

Intracellularly in the cytoplasm and mitochondria.

Question 6

What happens in stage 3 of catabolism?

Answer 6

Keto-acids are fed into the Kreb’s cycle.
Amino acids are converted into acetyl CoA.
Pyruvate is converted into acetyl CoA.
Fatty acids and alcohol are converted into acetyl CoA.
Acetyl CoA starts off the Kreb’s cycle which will then produce some reducing powers and also some ATP. The reducing powers are what’s important here.

Question 7

Where does stage 3 take place?

Answer 7

In the matrix of the mitochondria.

Question 8

What happens in stage 4 of catabolism?

Answer 8

Oxidative phosphorylation with the help of the electron transport chain.
The reducing powers that we have stacked up (NADH+H+ and FADH2) are used and re-oxidised in order to form a large amount of ATP.
This step requires oxygen.

Question 9

What is the general formula of carbohydrates?

Answer 9

(CH2O)n

Question 10

What groups do carbohydrates contain?

Answer 10

Aldehyde (aldose) or keto (ketose) groups.

Question 11

Name three main dietary monosaccharides.

Answer 11

Glucose, fructose and galactose

Question 12

All tissues can metabolise glucose, but which cells have an absolute requirement of glucose? What does this absolute requirement mean?

Answer 12

Red blood cells
Neutrophils
Innermost cells of kidney medulla
Lens of the eye
It means they do not use any of the other ways of getting energy, they only use glucose as their source of energy.

Question 13

What kind of energy source does the brain prefer? Can it use anything else?

Answer 13

It prefers glucose. It can use ketone bodies in times of starvation but then it needs some time to adapt.

Question 14

Give examples of enzymes used to break down carbohydrates.

Answer 14

Amylase
Sucrase
Lactase
Isomaltase

Question 15

Why can cellulose not be digested by humans?

Answer 15

Because cellulose contain beta1-4 bonds. This means bonds that face upwards.
Other carbohydrates have alpha1-4 bonds that face downwards.
The alpha glycosidic linkages can be digested.
The beta glycosidic linkages can’t.

Question 16

What is primary lactase deficiency?

Answer 16

The absence of lactase persistence allele.

Only occurs in adults.

Question 17

What is secondary lactase deficiency?

Answer 17

Caused by injury to small intestine:
Gastroenteritis
Coeliac disease
Crohn's disease
Ulcerative colitis
Occurs in both infants and adults
This is generally reversible

Question 18

How does absorption of sugars work?

Answer 18

They are actively transported into intestinal epithelial cells from the GI-tract.
Then passive transport occurs as the sugars are transported from the epithelial cells to the capillary and into blood stream.
SGLT1 is important in active transport.
GLUT1-GLUT5 are used in facilitated transport.

Question 19

How is glucose taken up by cells from the blood?

Answer 19

Via facilitated diffusion using transport proteins GLUT1-GLUT5.

Question 20

What can GLUTs be regulated by?

Answer 20

insulin

Question 21

Where can you find GLUT2s?

Answer 21

Kidney, liver, pancreatic beta cells and small intestine.

Question 22

Where can you find GLUT4s?

Answer 22

Adipose tissue, striated muscle (insulin regulated)

Question 23

What is glycolysis?

Answer 23

When glucose is broken down into two pyruvate molecules.

Question 24

What is phase 1 of glycolysis?

Answer 24

An investment phase where 2 ATP are used to form intermediates.

Question 25

What is phase 2 of glycolysis?

Answer 25

The payback phase. Here 2 NADH+H+ are formed in total and 4 ATP are formed as well in total.

Question 26

What are the net products of glycolysis?

Answer 26

2 NADH+H+
2 ATP
2 pyruvate

Question 27

What are some important features of glycolysis?

Answer 27

It can operate anaerobically
Occurs in all tissues
Exergonic
An irreversible pathway

Question 28

What are some key enzymes of glycolysis? Why are they?

Answer 28

Hexokinase (glucokinase in the liver)
Phosphofructokinase-1
Pyruvate kinase
Because they are all responsible of the irreversible steps of glycolysis

Question 29

Why is phosphofructokinase-1 especially important?

Answer 29

Because it is the committing step (step 3). This is the first step that commits glucose to metabolise via glycolysis. Here there is no turning back.

Question 30

Which other pathway in catabolism can operate anaerobically?

Answer 30

LDH pathway.

Question 31

Why are there so many steps of glycolysis?

Answer 31

Because chemistry is easier in small steps
Efficient energy conservation
Gives versatility for the pathways
Can be controlled

Question 32

How can glycolysis be used in a clinical setting?

Answer 32

Imaging with PET scan where a contrast is used. A radioactive labelled hexokinase substrate which is trapped in the glycolysis. It can’t be broken down completely. So high metabolising cells take up more. These are cancer cells.