First up: GLYCOLYSIS, BITCHES! The energy investment phase
You ate some potatoes. You ate some bread. You ate an entire goddamn bag of 7-11 brand sour cream and cheddar chips (because there's a recession on and you're thrifty). Or you ate some turnip-braised seitan with sesame-crusted yeast powder, because you're some kind of jerkwad health nut who thinks he's better than everyone. Here's your crown, your majesty. It doesn't matter what stupid thing you shoved down your gullet, meathead, 'cause it's all going to the same place: the cytosol.
No, that's not some fancy new brand of vodka endorsed by Cee-lo, that's the liquidy goo inside all of your cells. Don't confuse the cytosol with the cytoplasm. That is exactly the sort of behavior that will get you laughed out of a biochemistry conference.
All that crap you're not supposed to be eating gets broken down into glucose by your digestive system and enters your cells. Don't ask me how it gets broken down or how it gets in the cell because that is another topic. But like some sort of teenaged loser trying to date your daughter, that glucose is unsuitable for entry into your mitochondria.
ATP, is of course, adenosine triphosphate, the famed energy-storing molecule that is comprised of adenosine (yes, the same adenosine that shows up in the nucleic acids; adeonsine is the ribose sugar + the nitrogenous base adenine) and three phosphate groups.
Phosphate groups are just a bunch of loser oxygens hanging around a phosphorous atom, so theu'r epretty negatively charged. This has all sort of important biochemical implications, including effects on a given molecule's polarity and solubility in water, the orientation of DNA's sugar-phosphate backbone towards the outside, and blah blaha blah dork stuff. You know those bitchy people that you hang out with for no reason that you can actually identify? They just smoke cigarettes and complain about how there isn't anything to do? Well, those are your phosphate groups. They will leave you at the drop of a hat, which in biological terms means in the presence of the phosphorylating enzyme HEXOKINASE. I know, I know. It sounds like a shitty German metal band, I know. It isn't one.
Your pal HEXOKINASE phosphorylates the useless hippie glucose molecule. Are you scared of the big scary word phosphorylation? Huh? Are you scawed? Widdle weader weawy scawed??! Sack up and learn!
Phosphorylation is basically a fancy term for "transfers a phosphate group from ____ to _____", eg an ATP to a glucose molecule. It's kinda like a postal worker! Just think of the USPS as the United States Phosphorylation Service, and you'll never fail a test or be bothered with pesky social invitations again!
Anyway: now that you've got two o' dem phosphates attached to your commie pinko layabout glucose, you can split it into two 3-carbon sugars: glyceraldehyde-3-phosphate (rap name: G3P) and its structural isomer, the Gunnar to its Matthew Nelson, the Cathy to its Patty Lane, the obese motorcycle twin to its other obese motorcycle twin.... some freakin' thing i don't feel like typing out.
Now we have reached the end of the energy investment phase! Why is called that? Well, if you've been paying attention, you'll have noticed that it cost us two ATP to break apart this glucose molecule. Hey wait a minute! I thought this cellular respiration dealie was supposed to CREATE energy, not spend it! What is this unfair tax n' spendocrat system?!? Is OBAMA somehow to blame?
Slow your roll, Chr-istrionic Duh-lumbus! Cellular respiration is all about net gain, maaaan! You gotta give a little to get a little! Ya gotta expend energy in the form of ATP to drive the necessary reactions to phosphorylate ADP, resulting in ATP! Ya gotta spend money to make money! THIS GUY knows!