Lecture 23: Glucose as a fuel molecule 1 Flashcards
How many carbon atoms does a glucose molecule have?
6 Carbon molecule (a Hexose)
What two main polysaccharides are sources of glucose in the human diet?
Starch from plants (polymers of up to 1 million glucose units joined by glycosidic bonds)
Glycogen from meat (liver, muscle) and storage
What is the name for two sugars joined together?
Disaccharides
Examples include:
- sucrose (glucose + fructose)
- lactose (glucose + galactose)
- maltose (glucose + glucose).
Carbohydrate digestion involves the hydrolysis of ______________ bonds, catalysed by the enzyme ________ which digests starch into ________.
glycosidic bonds, catalysed by the enzyme amylase which digests starch into disaccharides (maltose)
Why are monosaccharides like glucose unable to cross the cell membrane unaided?
- Sugars are highly polar (water soluble) and cannot simply diffuse across cell membranes
So.. - Require specific transporter proteins anchored in the membrane
What is meant by ‘active transport’ and ‘facilitative transport’ of molecules across a membrane? What are some examples for glucose?
active transport:
moves molecules against a concentration gradient, requires energy input
- for glucose: SGLT (sodium-glucose linked transporter; SGLT1 in absorption from the GI)
facilitative transport:
molecules move down a concentration gradient, does not require energy input
- for glucose: GLUT (Glucose transporter: GLUT2 in absorption from the GI)
What is meant by ‘symport’ transport of glucose by the SGLT 1 transporter?
Symport transport means to move two different substances across the membrane simultaneously.
Symport of glucose and Na+ into the epithelial cell
- glucose up concentration gradient
- Na+ down concentration gradient
Why is having a Na+/K+ ATPase present in epithelial cells important?
To maintain a concentration gradient for sodium within the cell so the uptake of glucose can continue
Na+/K+ ATPase:
Na+ pumped against concentration gradient into interstitial fluid (removal from the cell)
Requires ATP hydrolysis (energy as it is active transport)
K+ moved into epithelial cells during this process
K+ is then returned to the interstitial fluid down
concentration gradient through a different channel
What organisms can use glucose as a fuel molecule?
Oxidized in glycolysis in all organisms (animals, fungi,
plants, bacteria)
Where in the cell does glycolysis occur in mammals?
Glycolysis is usually takes place in the cytoplasm in eukaryotes
In which cell of the human body is glucose ‘essential’ or favoured as a fuel? Why?
Glucose is essential as fuel for red blood cells
Red blood cells do not have mitochondria, so do not have
the other pathways (therefore rely on glucose)
What is the preferred fuel molecule of the eye and why?
Glucose is the favoured fuel in the eye
Why?
Blood vessels (bringing oxygen) and mitochondria (using
oxygen) would refract light in the optical path (lens, cornea) to retina. So it utilises glucose
What is the preferred fuel molecule for the brain and why?
Glucose is the preferred fuel in the brain
- High energy requirement: human brain requires around 120 g of glucose per day
- Brain cells have mitochondria
- Glucose easily crosses the blood-brain barrier, but fats do not.
- Using glucose provides a quick source of ATP without risk of damage
Can also utilise ketone bodies when starving
What does white muscle use as fuel?
White muscle cells tend to use glucose (short bursts of energy) (anaerobic)
What fuel does red muscle often use as fuel?
red muscle cells tend to use fats as the preferred fuel. Endurance based under aerobic conditions and can be sustained for much longer
