Cellular Respiration and Protein Synthesis

Cells have lots of functions for humble little things that we can only see under a microscope. The many chemical reactions that take place within them are known as metabolism; those reactions that break down larger molecules into smaller ones are known as catabolism, while those that build up larger molecules from smaller ones are known as anabolism. Catabolism releases energy, while anabolism requires it. Hence balance here is key in keeping the cell functioning.

Cellular respiration is a metabolic process in which the organic molecules from the food are broken down to release energy for the cells. Glucose formed from the breakdown of carbohydrates, amino acids formed from the breakdown of proteins, and fatty acids and glycerol from the breakdown of lipids can all be broken down to release energy in this way. The main food material used, however, is glucose, so that's what I'm going to focus on in this post.

There are a lot of reactions that occur during respiration, each of which produces a small amount of energy, thus controlling the amount of energy released rather than releasing it all at once. Here's an equation for the overall reaction:

C₆H₁₂O₆ + 6O₂ à 6CO₂ + 6H₂O + energy

(Text equation: Glucose + oxygen --> carbon dioxide + water + energy)

60% of the energy produced is released as heat, which is used for keeping body temperature constant. The remaining energy is used to join an inorganic phosphate group to a molecule of adenosine diphosphate (ADP), which then forms adenosine triphosphate (ATP). This bond between ADP and the phosphate group is relatively easy to break, and breaking this bond releases energy. Thus ATP can transfer energy by moving around to where energy is needed before breaking down into ADP and a phosphate group (well, this is the mental image I'm getting in my head anyway). The resulting ADP can be recycled to store more energy later on.

There are two main kinds of cellular respiration: aerobic and anaerobic. Let's take a look at them in greater detail:

Aerobic Respiration

Aerobic respiration requires oxygen, but it's more effective. In the first phase, known as glycolysis, the glucose molecule is broken down in the cytoplasm over ten steps (nope, respiration isn't as simple as you might want it to be) to two molecules of pyruvic acid, simultaneously releasing two molecules of ATP.

The pyruvic acid molecules then enter a mitochondrion, where two more series of reactions occur. The first is known as the Krebs cycle (or citric acid cycle), which forms two more ATP molecules per two molecules of pyruvic acid (i.e. two more ATP molecules per original glucose molecule). The next series of reactions is called the electron transport system and can produce up to 34 molecules of ATP per original molecule of glucose.

The maximum yield from aerobic respiration of one mole of glucose is therefore 38 molecules of ATP (2 + 2 + 34). However, this is a maximum only- often the actual yield is lower than this.

Anaerobic Respiration

Anaerobic respiration is respiration without oxygen. Once glycolysis is complete, if oxygen is not present or is only present in insufficient amounts for the amount of energy required (during exercise, for instance), the pyruvic acid molecules will be converted into lactic acid, allowing cells to release some energy without oxygen. Lactic acid is what causes muscle pain and fatigue during exercise.

Lactic acid is then transported to the liver via the blood, where it recombines with oxygen to form glucose and then glycogen. I've spoken a little bit about glycogen in my second post about the digestive system. It's a molecule that can convert back and forth into glucose, and is thus a form of energy reserve for the body.

If you've been reading closely you might have noticed that, although anaerobic respiration is meant to be respiration without oxygen, the process converting lactic acid to glucose and glycogen does require oxygen! This is partly why you have to breathe heavily after exercising- the body needs lots of oxygen to repay this "oxygen debt." This extra oxygen is also known as recovery oxygen.

Uses of Energy

Why do our cells need so much energy? I've touched on the need for cells to break molecules down and build them up, but why are our cells doing this, exactly? Here's a quick list of functions that our cells need energy for:

• Building complex molecules (e.g. proteins) by combining smaller ones (synthesis)
• Cell division and growth
• Movement of cell organelles
• Movement of the whole cell
• Maintaining cell organisation
• Active transport
• Transmission of nerve impulses

Let's have a look at the first one in a little more detail.

Protein Synthesis

As mentioned in one of my earlier posts, proteins are made up of amino acids which are linked together via condensation reactions (the amino acids combine by losing molecules of water). The proteins formed are determined by genes, which are parts of the DNA. Each amino acid corresponds to a sequence of three bases on the DNA. For example, the sequence cytosine-adenine-guanine on the DNA corresponds to the amino acid valine.

Although the DNA is located in the nucleus, the proteins are assembled in the ribosomes in the cytoplasm. One type of RNA (ribonucleic acid), known as messenger RNA, is used to transfer the genetic code from the DNA to the ribosomes. RNA, as opposed to DNA, has only one strand of sugars, phosphates and nucleotide bases. Also, instead of thymine, it has a different base called uracil (which, like thymine, pairs up with alanine).

To transfer the message across, parts of the DNA molecule break apart, allowing the messenger RNA to form by matching up bases. The messenger RNA then leaves the nucleus and attaches to a ribosome.

In the cytoplasm, a different kind of RNA, transfer RNA, brings the amino acids to the ribosomes. Each transfer RNA molecule has three bases that attach to the messenger RNA molecule according to the rules of complementary bases and all that.

Now, the funky part is that there's a lot of opposites going on. Firstly, the messenger RNA is made up of bases that complement (i.e. are pretty much opposite to) those on the selected strand on DNA. Not to worry, though- the messenger RNA then matches up with transfer RNA containing "opposites" of the messenger RNA- rather, the same as what was originally on the DNA!

Confused? Here's an example...

Original DNA strand: CAGTTCCGA

Messenger RNA created: GUCAAGGCU

Transfer RNA required: CAG TTC CGA (i.e. the same as the original DNA strand. Not sure whether it should be uracil instead of thymine- the text of my book says that RNA contains uracil, not thymine, but the diagrams have thymine instead of uracil...)

CAG corresponds to valine, TTC corresponds to lysine and CGA corresponds to alanine. Hence this original DNA strand corresponds to a protein chain consisting of valine, lysine and alanine, in that order.

That's pretty much all from me on this topic. If you're still awake, though, read on for some info that might not be useful now but might be later if you decide to undertake further study:

Radioactive Tracers

By now you might be wondering how scientists know so much about the inner life of cells. This is all done through scientific experimentation (obviously), some of which involves the use of tracers. Tracers are substances that can be identified and followed. Many are made up of radioactive isotopes (forms of an element that are unstable and break down, emitting radiation). (If you want to know more about isotopes, check out one of my earlier Chemistry posts on Atomic Structure and the Periodic Table.)

Now for an example of how radioactive tracers can be used to help us learn about the body! In one experiment, amino acids were labelled with radioactive tracers and injected into the blood of rats. A few minutes later radioactive protein was found in the ribosomes of the pancreas of the rat. This helped scientists discover that proteins are made from amino acids at the ribosomes. Some other cellular processes have been discovered through this use of radioactive isotopes.