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Ok here goes - please be patient! I will try to go through it in a way that makes sense.
Living organisms are made up of cells - some are single cells while others are multicellular like us. Cells are made up of molecules that are themselves made up of atoms. Atoms associate with one-another by sharing energy in a chemical bond. Therefore molecules will bind to one another via their atomic chemical bonds.
To stay alive means that the body/plant/single cell has to maintain these chemical bonds. Without a continuous source of energy to do this, the bonds will undergo entropy where the energy dissipates into the universe, the atoms drift apart and decay occurs. This can be imagined like the relationship between a mother and son. She supplies the energy to keep his room tidy. She leaves the house for a few hours (lack of energy input) and when she returns the clothes are hung neatly on the floor - energy is lost, entropy is occurring and decay is imminent!
Ok so this is why we need energy. We get energy in the form of food - plants and animals that have eaten plants. This means that all energy comes from plants. They get their energy from the sun. The sun therefore powers all life on the planet. It does this because it shines. Light is composed of photons which are used by plant biochemical reactions - they make sugars and oxygen out of water and carbon dioxide.
We can eat the plants (and gain complex sugars, some proteins and some fats), we breathe in the oxygen the plants produce. We can eat the animals (proteins, fats and sugars) that ate the plants etc.
Now we can get to cellular respiration:
the chemical and molecular processes within cells by which food is converted into energy (packaged into a molecule known as ATP - adenosine triphosphate) to enable the synthesis of new molecules and maintain the chemical bonds of the organism as a whole.
What happens? We eat food - it is broken down in the gut and constituent molecules such as glucose (which is simply sugar that you would sprinkle on your cornflakes) which is then absorbed into the bloodstream from the intestine. Once in the blood it will be transported around the body, exiting in tissues where it will enter individual cells.
At the same time we are breathing - respiring. Oxygen is breathed in, transferred from the lungs into red blood cells which also move around in the blood system and when they enter capillaries (tiny vessels surrounding individual cells) the oxygen they carry is released and absorbed by individual cells.
The cells now have supplies of glucose and oxygen.
How does the body get energy from sugar? Glucose has this chemical formula - C6 H12 O6 - meaning 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms. Molecular oxygen comes in pairs - 2 oxygen atoms joined together.
If you put sugar into a test-tube and heat it up with a flame it will explode - lots of energy tied up in all the bonds that hold these atoms together.
The body needs to get this energy without explosions for obvious reasons, so it uses chemical reactions to break the molecule down in steps, releasing small packets of energy bit by bit. The process uses oxygen to pull the energy out so the chemical equation would look like this:
C6H12O6 (aq) + 6 O2 (g) ? 6 CO2 (g) + 6 H2O
The glucose + oxygen makes carbon dioxide and water.
GLYCOLYSIS
The first set of reactions is called glycolysis - lysis meaning "splitting". It takes place in the cytoplasm of cells. It converts one molecule of glucose into two molecules of pyruvate.
These pyruvates are converted into two molecules called Acetyl Co-A which enter another reaction called the Citric Acid Cycle. Glycolysis is powered by two molecules of ATP (but later on a total of 4 molecules ATP are made, so that the two needed to kick start it are repaid, and making an energy profit of two ATP).
So:
Glucose converted to Glucose-6-Phosphate using ATP and enzyme Hexokinase.
Glucose-6-Phosphate into Fructose-6-Phosphate using enzyme Phosphoglucoisomerase.
Fructose-6-Phosphate into Fructose 1,6-bisphosphate using ATP and enzyme phosphofructokinase.
Fructose-1,6-bisphosphate into dihydroxyacetone which is then converted into Glyceraldehyde 3-phosphate using triose phosphate isomerase (which also converts G3P back to F1,6BP).
G3P into 1,3-bisphosphoglycerate using enzyme G3PDehydrogenase (energy provided by NAD and phosphate) - effectively two molecules made
two 1,3-bisphosphoglycerate into two 3-phosphoglycerate using phosphoglycerate kinase. This step MAKES two ATP.
two 3-phosphoglycerate into two 2-phosphoglycerate using phosphoglycerate mutase.
two 2-phosphoglycerate into two phosphoenolpyruvate using enolase and RELEASING two water molecules ( 2 H20).
two Phosphoenolpyruvate into two Pyruvate using pyruvate kinase and MAKES two ATP.
two Pyruvate into two Acetyl-CoA.
Acetyl-CoA now enters citric acid cycle which oxidises to produce carbon dioxide and water and ATP (the reverse of plants which use carbon dioxide and water to make glucose and oxygen)
Acetyle CoA combines with the last molecule in the 8 step citric acid cycle (oxaloacetate) into citrate
Citrate into Isocitrate
Isocitrate into alpha-ketoglutarate PRODUCES NADH
Alpha-ketoglutarate into succinyl-CoA PRODUCING NADH and carbon dioxide
Succinyl CoA into Succinate making GTP and CoA
Succinate into Fumarate PRODUCING FADH2
Fumarate into Malate using water to help
Malate into oxaloacetate PRODUCING NADH
Oxaloacetate + Acetyl-CoA coming infrom glycolysis to start cycle over again at citrate.
The three NADH produced per turn of this cycle makes 3 ATPs and the single FADH makes 2 ATPs. One GP is used up (succinyle CoA to succinate) so that in total the cycle produces 12 ATP from one glucose - remember GD3 ends up making two of everything downstream.
Its a bloomin' nightmare! Took me forever to figure it out - a lot of it is just learning it off by heart. If u need more or easier explained info send me a message.
Best of luck!
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