Carbohydrates
Carbohydrates are a very important biochemical, not just for their central role in metabolism, but also as a structural and functional molecule.
Carbohydrates are carbon-based molecules that are rich in hydroxyl groups, hence the name; hydrated-carbon.
This is quite a nice simple introduction to the topic from the Osmosis team: https://www.youtube.com/watch?v=pCWpvAD1Dzk&t=40s
Carbohydrates are carbon-based molecules that are rich in hydroxyl groups, hence the name; hydrated-carbon.
This is quite a nice simple introduction to the topic from the Osmosis team: https://www.youtube.com/watch?v=pCWpvAD1Dzk&t=40s
Biochemistry
From a structural point of view, carbohydrates, also referred to as saccharides, are fairly straightforward.
They are essentially just a number of building blocks, monosaccharides, that are linked together into longer chains:
They are essentially just a number of building blocks, monosaccharides, that are linked together into longer chains:
- Monosaccharide - the basic ‘unit’, which exists in a few different forms
- Disaccharide - two unit linked together
- Oligosaccharides - a small number of units (3-9) linked together
- Polysaccharides - chains of multiple units
Monosaccharides
These can be thought of as the basic building blocks.
A few different forms exist but all with the similar features of:
Some important ones are:
A few different forms exist but all with the similar features of:
- Carbon ring
- Comprised of carbon, hydrogen and oxygen
- Hydroxyl groups
Some important ones are:
- Glucose
- Fructose
- Galactose
Disaccharides
As noted, these are simply two monosaccharides bonded together.
Some important ones are:
These bonds are glycosidic bonds, formed when two OH groups bind, losing H20 and leaving an O connecting two carbons from the monosaccharides.
Some important ones are:
- Lactose - galactose and glucose
- Sucrose - fructose and glucose
- Maltose - 2 glucose
These bonds are glycosidic bonds, formed when two OH groups bind, losing H20 and leaving an O connecting two carbons from the monosaccharides.
Complex Carbohydrates
These are structures that are larger than disaccharides.
These are generally:
Starches are polysaccharides that can be digested by human enzymes.
Dietary fibers refer to types of polysaccharides that humans are unable to digest, due to the nature of their bonds.
Some gut bacteria may be able to metabolise these molecules.
These are generally:
- Oligosaccharide
- Polysaccharides
Starches are polysaccharides that can be digested by human enzymes.
Dietary fibers refer to types of polysaccharides that humans are unable to digest, due to the nature of their bonds.
Some gut bacteria may be able to metabolise these molecules.
Digestion & Absorption
It is essentially only the monosaccharides that can be absorbed through the gut endothelium
As such, the digestive process, including enzymatic reactions, is undertaken to allow the larger chains to be broken down.
This is a good video of the process: https://www.youtube.com/watch?v=LWfXeCVp7Wk
Dietary sources of carbohydrates comes from 3 main sources:
As such, these all need further processing.
The process starts in the mouth with mastication and mechanical breakdown of food.
The enzymes in saliva (alpha-amylase) from the salivary glands (mainly parotid) begin the breakdown of starch into disaccharides and oligosaccharides.
As the food is only here a short time, this does not account for much digestion.
The activity stops on arrival in the stomach because of the inactivity of amylase in acidic conditions.
Pancreatic amylase is even more effective than the salivary form and continues the process as the food reaches the duodenum and proximal small intestine.
Again, the result of this process is maltose and other oligosaccharides.
Further enzymatic breakdown occurs under the influence of the different enzymes in the intestinal microvilli brush border.
Key enzymes here are:
The resulting monosaccharides (about 80% of which is glucose) are then absorbed from the gut lumen through an active transport process.
The uptake of glucose (and galactose) is through a sodium driven active transport mechanism.
A sodium pump on the basolateral surface of enterocyte cells pumps sodium into the blood, creating a relative low level of sodium in the cell.
Sodium then has a concentration gradient to diffuse down from the gut lumen.
The sodium-glucose linked transporter on the luminal surface requires the presence of both glucose and sodium at its site to allow opening, thus transporting both into the cell (facilitated diffusion).
The GLUT-2 protein then results in subsequent transport of the glucose out of the basolateral side of the cell.
Fructose uses a facilitated diffusion process not coupled to sodium which is subsequently less effective.
As such, the digestive process, including enzymatic reactions, is undertaken to allow the larger chains to be broken down.
This is a good video of the process: https://www.youtube.com/watch?v=LWfXeCVp7Wk
Dietary sources of carbohydrates comes from 3 main sources:
- Starches - found in almost all non-animal foods
- Lactose - in milk
- Sucrose - found in cane sugar
As such, these all need further processing.
The process starts in the mouth with mastication and mechanical breakdown of food.
The enzymes in saliva (alpha-amylase) from the salivary glands (mainly parotid) begin the breakdown of starch into disaccharides and oligosaccharides.
As the food is only here a short time, this does not account for much digestion.
The activity stops on arrival in the stomach because of the inactivity of amylase in acidic conditions.
Pancreatic amylase is even more effective than the salivary form and continues the process as the food reaches the duodenum and proximal small intestine.
Again, the result of this process is maltose and other oligosaccharides.
Further enzymatic breakdown occurs under the influence of the different enzymes in the intestinal microvilli brush border.
Key enzymes here are:
- Maltase
- Lactase
- Sucrase
The resulting monosaccharides (about 80% of which is glucose) are then absorbed from the gut lumen through an active transport process.
The uptake of glucose (and galactose) is through a sodium driven active transport mechanism.
A sodium pump on the basolateral surface of enterocyte cells pumps sodium into the blood, creating a relative low level of sodium in the cell.
Sodium then has a concentration gradient to diffuse down from the gut lumen.
The sodium-glucose linked transporter on the luminal surface requires the presence of both glucose and sodium at its site to allow opening, thus transporting both into the cell (facilitated diffusion).
The GLUT-2 protein then results in subsequent transport of the glucose out of the basolateral side of the cell.
Fructose uses a facilitated diffusion process not coupled to sodium which is subsequently less effective.
Utilisation
The ultimate goal of carbohydrate utilisation in the body is the creation of ATP, the energy currency of the body.
This can be done with saccharides in a few different ways.
Some key parts are:
This can be done with saccharides in a few different ways.
Some key parts are:
- Glycolysis
- Citric acid cycle
- Oxidative phosphorylation
Management
Whilst glucose is readily available after a meal, it is less so afterwards.
The generation of a constant supply of glucose is therefore a key part of human metabolism.
Two key components of this are:
Glycogenesis refers to the creation of glycogen, essentially a macromolecules of multiple glucose monomers stuck together (starting from the glucose-6-phosphate step of glycolysis after a few chemical reactions).
Subsequent hydrolysis of this, glycogenolysis, releases to glucose monomers back for metabolic utilisation, through the enzyme glycogen phosphorylase.
Glycogen is stored in the skeletal muscles and liver, in a ratio of 3:1.
The back and forward of this process is under hormonal control, as will be explored separately.
Gluconeogenesis refers to the synthesis of glucose de novo from other nutrients e.g. proteins.
This is discussed in more detail elsewhere.
The generation of a constant supply of glucose is therefore a key part of human metabolism.
Two key components of this are:
- Glycogenolysis
- Gluconeogenesis
Glycogenesis refers to the creation of glycogen, essentially a macromolecules of multiple glucose monomers stuck together (starting from the glucose-6-phosphate step of glycolysis after a few chemical reactions).
Subsequent hydrolysis of this, glycogenolysis, releases to glucose monomers back for metabolic utilisation, through the enzyme glycogen phosphorylase.
Glycogen is stored in the skeletal muscles and liver, in a ratio of 3:1.
The back and forward of this process is under hormonal control, as will be explored separately.
Gluconeogenesis refers to the synthesis of glucose de novo from other nutrients e.g. proteins.
This is discussed in more detail elsewhere.
Links & References
- Osmosis. Carbohydrates & Sugars. 2018. https://www.youtube.com/watch?v=pCWpvAD1Dzk&t=40s
- Hasudungan, A. https://www.youtube.com/watch?v=WO-YKeJF_zc
- Duff, E. Metabolic pathways. e-LFH. 2012.
- Guyton, A. Hall, J. Textbook of medical physiology (11th ed). 2006. Elsevier Saunders.
- Hasudungan, A. Starch (Carbohydrate) digestion and absorption. Youtube. 2015. https://www.youtube.com/watch?v=LWfXeCVp7Wk
- Hasudungan, A. Human Metabolism map II - gluconeogenesis and glycogenesis. Youtube. 2012. https://www.youtube.com/watch?v=y5CW63kxTsA&feature=youtu.be
