"Cellular Metabolism" is all of the hundreds of coordinated chemical reactions that a cell uses to convert "raw materials" into the energy and complex organic molecules that cells need.
Cellular metabolism is a huge production. In every moment all of the trillions of cells in your body are carrying out thousands of chemical reactions (1), using the food you eat, the water you drink and the air you breathe to power all your bustling about and to build and maintain your body.
In metabolism, your cells use enzymes to precisely control the complicated multi-step chemical reactions that keep you alive.
There are two main parts of metabolism - catabolism and anabolism.
In catabolism, larger "food" molecules are broken into smaller ones and the energy released is immediately used to make "energy-transfer molecules".
These "energy-transfer molecules" can then be used to drive whatever
chemical reactions the cell needs, including the chemical reactions used (in anabolism) to build new molecules - specifically
the proteins, fats,
and etc. that cells are made out of.
Without any encouragement, the chemical reactions cells need to stay alive would not happen fast enough. You can generally speed up the rate of chemical reactions by turning the temperature up but too much heat destroys cells.
Luckily, we've got enzymes. "Enzymes" are protein molecules (or once in a while RNA molecules) that fit very precisely to very specific molecules, creating a new temporary molecule that is much more likely to react in the desired way than the original molecule was.
The end result is that without adding any extra energy, enzymes cause chemical reactions to happen much more quickly than they would have on their own (2). This speeding-up of chemical reactions is called "catalyzing reactions".
After the enzymes have done their job, they are released and are free to catalyze another reaction.
Catalyzing specific reactions in specific sequences is a big part of how our genetic code (which determines what enzymes are made) controls the destiny of our cells and bodies (see definition of life).
How much more quickly? It varies but some of the enzymes in our body increase the rate of their targeted chemical reactions by 1016 (that's 10,000,000,000,000,000) times! (3)
Why bother with catabolism (breaking larger molecules into smaller ones)? Well, first of all, catabolism:
(1) frees-up "foundational molecules" - particularly important are essential amino acids and essential fatty acids (these acids are "essential" because human cells can't make them) (amino acids are part of proteins and fatty acids part of fats and other lipids) (4).
These "foundational molecules" can be used in anabolism as building-blocks for the more complex organic molecules that cells are made out of.
Secondly, catabolism:
(2) frees-up vitamins and minerals. These are substances that cells require in small amounts but cannot make themselves.
Vitamins are organic molecules (example: Vitamin C - C6H8O6). Minerals are inorganic ions
(example calcium ions - Ca2+). (4)
Thirdly, during
catabolism,
Energy is released and can be used to immediately make:
(4) "energy-transfer molecules" - high-energy molecules that are useful because the
energy stored in their molecular
bonds can be released and used to drive chemical reactions that would otherwise never
happen - including the molecule-building task of anabolism.
It is important to keep in mind that "energy-transfer molecules" aren't used for long-term energy storage. They are used only for moment-to-moment energy-transfers in the cell. (5) [Fats and carbohydrates are used for long-term energy storage and they are made during anabolism].
The two most important of the energy-transfer molecules are ATP and NADPH. Both are made (in catabolism) with the energy released from oxidation-reduction reactions.
ATP is used to supply energy for all sorts of things, including the molecule-building work of anabolism. NADPH is used as a "reducing agent" in the oxidation-reduction reactions of anabolism.
As we'll see on ATP, every day the average adult makes and consumes an amount of ATP equal to his or her body weight. And you thought you were just hanging around doing nothing!
To see how the energy released by the molecule-busting catabolism is used to create ATP and NADPH and how these molecules are used to power other cellular activities including the molecule-building work of anabolism, go to the next page on cellular metabolism, ADP and NADPH.
For a definition of "oxidation-reduction" reactions, see oxidation-reduction.
Perhaps it would be appropriate to end this page with a poem. It might've been more appropriate to end definition of life with this poem but when I was writing that page, I was really smitten by the joke I ended up using to conclude it.
Gratitude to the Unknown Instructors
1 There are two facts here: (1) the number of chemical reactions carried out each instant by the cells in your body and
(2) the number of cells in your body.
(1) "in every moment all ... the cells in your body are carrying out thousands of chemical reactions" - this fact comes from the beginning
of Chapter 14 of the second edition of the textbook Biochemistry by Garrett and Grisham, currently (6/20/2011) available online at
Heidi.
(2) There are trillions of cells in your body - this came from the press release for "The Nobel Prize in Physiology or Medicine 2001" that
went to Leland H. Hartwell, Tim Hunt, Sir Paul Nurse "for their discoveries of 'key regulators of the cell cycle'"; In this press release,
it was stated: "An adult human being has approximately 100 000 billion cells..." So, an average adult has about 100 trillion cells. Younger people
have less but still trillions since - if you are old enough to be reading about cell metabolism - it is unlikely that you weigh 1/100th as much
as the average adult. In fact, now that I think about it, it is unlikely that if you are out of the womb, you are that slight. The
press release is currently (6/20/2011) online at laureate press release.
2 Information about the working of enzymes can be found in "14.2 · Introduction to Enzyme Kinetics" of the textbook (Biochemitry by Garrett and Grisham) discussed and linked-to in the first part of footnote #1. Particularly relevant to our discussion here is the subsection "Decreasing DG‡ Increases Reaction Rate" in which we find, "Catalysts work by lowering the energy of activation rather than by raising the average energy of the reactants. Catalysts accomplish this remarkable feat by combining transiently with the reactants in a way that promotes their entry into the reactive, transition-state condition." An explanation of transition-states is found in the subsection "Free Energy of Activation and the Action of Catalysts" - "In a first-order chemical reaction, the conversion of A to P occurs because, at any given instant, a fraction of the A molecules has the energy necessary to achieve a reactive condition known as the transition state. In this state, the probability is very high that the particular rearrangement accompanying the A -> P transition will occur."
3 This fact also comes from Biochemistry discusssed and linked-to in the first part of footnote #1. Specifically, it is at the beginning of section "14.1 · Enzymes—Catalytic Power, Specificity, and Regulation".
4 Cholesterol is an example of another
lipid made out of fatty acids.
The information about essential fatty acids, essential amino acids, vitamins and minerals
comes from an online biology textbook "Kimball's Biology Pages", written by retired biology professor John W. Kimball.
The chapter referenced in this footnote is "Nutrition" and it is within the
"Physiology" section. The book is online at http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/TOC.html
5 "Energy-transfer molecules" are discussed throughout the textbook Biochemistry (see footnote #1 for
details about this book) where they go by the name "high-energy biomolecules".
In section "3.6 · The High-Energy Biomolecules", these molecules are introduced and we learn,
"A small family of universal biomolecules mediates the flow of energy from exergonic reactions to the energy-requiring
processes of life. These molecules are the reduced coenzymes and the high-energy phosphate compounds." In that section,
they also point out that these two molecules are not used for long-term storage, but rather,
"They are transient forms of stored energy, meant to carry energy from point to point,
from one enzyme system to another, in the minute-to-minute existence of the cell."
