Next Lesson - Energy Reactions in Cells
Core
Metabolism
Defined as the chemical processes that occur within a living thing to keep that thing alive. There are four main pathways of metabolism:
- Oxidative - the conversion of food to energy.
- Detoxification - the removal of toxins from the body.
- Biosynthetic - the production of the basic building blocks needed to keep cells alive.
- Fuel storage and mobilisation - the ability to store fuel when it is in surplus (such as after eating) and to mobilise these stores later when fuel is in shortage (such as in the middle of the night).
There are also two classifications of metabolic processes:
- Catabolic processes start with large molecules, breaking them down into smaller parts and thus releasing energy in the form of reducing power. E.g. breaking down glucose and producing NADH from NAD+. (I remember this as cats like to break stuff).
- Anabolic processes start with small molecules and energy, and build up larger molecules. E.g. taking amino acids and building them together into a gigantic protein. (I remember this as anabolic steroids build up muscles).
Energy is the capacity to do work. In humans, this energy is stored in chemical bonds.
All living things require energy for:
- Biosynthetic work - the synthesis of cellular components. Such as the production of the lipid cell membrane.
- Transport work - the movement of ions and nutrients across membranes. Such as using the Na+/K+ ATPase pump to create the Na+ gradient in the gut that is used to absorb glucose (co-transport).
- Mechanical work - e.g. muscle contraction where ATP is used to form cross bridges between actin and myosin.
- Electrical work - e.g. nervous conduction where Na+/K+ ATPase is used to regulate membrane potential.
- Osmotic work - e.g. in the kidneys. The kidneys use the Na+/K+ ATPase to regulate water balance, because water will follow Na+ wherever it goes due to osmolarity being higher where there is a lot of Na+.
As you can see, many processes use the Na+/K+ ATPase pump as a regulator. The Na+/K+ ATPase pump uses ATP to pump 3Na+ out of the cell for every 2K+ it pumps into the cell, both against their gradient (hence the need for ATP as an energy source).
Diagram - The ATP-ADP cycle. Here you can see how energy production and usage are linked
SimpleMed original by Dr. Maddie Swannack
The kilojoule (kJ) is the standard unit of food energy, but the general public use the term ‘calorie’. This can get confusing, since ‘one calorie’ is a term used in physics, whereas the calories written on food containers are really referring to kilocalories - which are just referred to as calories for short.
So 1kcal (referred to as a calorie by the public and food packaging) = the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius.
1 kcal = 4.184 kJ of energy (often rounded to 4.2 kJ).
Digestion converts larger carbohydrates (sugar polymers called polysaccharides) into monosaccharides (one sugar, known as a monomer). Examples of major carbohydrates are:
- Starch - the carbohydrate storage molecule in plants, a polymer of glucose.
- Glycogen - the carbohydrate storage molecule in animals, a highly branched polymer of glucose.
- Sucrose - table sugar, a glucose-fructose disaccharide.
- Lactose - milk sugar, galactose-glucose disaccharide.
- Maltose - glucose-glucose disaccharide.
Proteins are made up of amino acids in a chain, joined by peptide bonds. Digestion breaks these peptide bonds, to allow for the amino acids to be absorbed into the blood. There are 20 amino acids, some of which we can make through chemical reactions within the body, others of which we can’t. The ones we cannot produce are called essential amino acids, and can be found in the table below, along with a helpful mnemonic for learning them.
Table - Helpful mnemonic for learning the essential amino acids in humans
SimpleMed original by Dr. Maddie Swannack
The kicker is that some amino acids are ‘Conditionally Essential’. This means that although the body can synthesise these amino acids, it cannot do so in high enough quantities in the body, so they become essential in the diet. During periods of growth, illness or physiological stress, some amino acids may become conditionally essential. In children and pregnancy, requirements for amino acids such as arginine, tyrosine and cysteine may exceed the body’s capacity for synthesis.
Electrolytes are ions in body fluids, derived from salts, acids or bases, which help establish ion gradients across membranes and maintain water balance. They have many important functions, such as:
- Ca2+ used in bone structure and signalling
- Fe2+ used in haemoglobin
- Mg2+, Cu2+ and Zn2+ for enzyme co-factors
Table - Examples of electrolytes
SimpleMed original by Dr. Maddie Swannack
Vitamins are dietary substances which are essential for life, so any deficiencies can lead to disease. They are divided into fat soluble and water soluble.
Table - A list of vitamins and the conditions associated if they are deficient
SimpleMed original by Dr. Maddie Swannack
Fibre is found in plant material that forms cereal type foods (bread, beans), and includes non-starch polysaccharides such as cellulose, which is a polymer of glucose. However, unlike starch and glycogen which feature α1-4 and α1-6 glycosidic bonds, cellulose contains β1-4 bonds, which humans cannot break down. This means we do not absorb the glucose from cellulose, but it is still needed for a healthy GI tract. It helps prevent:
- Constipation - by providing something solid for the bowel to push against.
- Bowel cancer - by helping to move the toxic waste through the bowel quicker.
- High cholesterol - prevents reabsorption of bile acids so the liver must make more.
- Reduces risk of diabetes - slows the absorption of sugar.
- 10392
