8. Respiration

What you need to know...
  • The chemical energy stored in glucose must be released by all cells through a series of enzyme-controlled reactions called respiration
    • The energy released from the breakdown of glucose is used to generate ATP from ADP and phosphate
    • The chemical energy stored in ATP can be released by breaking it down to ADP and phosphate. 
    • This energy can be used for cellular activities including muscle cell contraction, cell division, protein synthesis and transmission of nerve impulses. 
    • ATP can be regenerated during respiration. 
    • The breakdown of each glucose molecule via pyruvate to carbon dioxide and water in the presence of oxygen yields 38 molecules of ATP
    • The breakdown of each glucose molecule via the fermentation pathway yields 2 molecules of ATP when oxygen is not present. 
    • Breakdown of glucose to lactic acid via pyruvate in animal cells. 
    • Breakdown of glucose to alcohol/ethanol and carbon dioxide via pyruvate in plant and yeast cells. 
  • Fermentation occurs in the cytoplasm. Aerobic respiration starts in the cytoplasm and is completed in the mitochondria.

Source: SQA

In the final topic of our unit about cells, we're going to be learning lots more about a process which is crucial to all cells: respiration. Once again, you'll already know at least something about respiration. You'll probably know it's the way we release energy from our food, and you may even remember the word equation:

Just like in the previous topic, we're going to be delving into the world of respiration in more depth here. But first, a little bit more about ATP.


We introduced the molecule ATP in the previous topic. Remember it is the battery of the cell. The high energy ATP molecule is formed by combining ADP and a phosphate molecule (Pi). Breaking up ATP releases energy and forming ATP requires the input of energy - this happens as a result of respiration. The energy released from breaking up ATP into ADP and phosphate can be used for many cellular activities including active transport, muscle cell contraction, cell division, protein synthesis and transmission of nerve impulses.

Aerobic Respiration 

So, the purpose of respiration is to release energy from the sugar glucose via a series of enzyme controlled reactions in order to convert ADP and phosphate molecules into ATP. But, how and where does this happen and how much ATP is produced? These are the sorts of questions we'll answer in the remainder of this topic.

The most efficient form of respiration occurs when oxygen is present. We call this from of respiration aerobic respiration. The presence of oxygen allows our cells to completely break down a glucose molecule all the way to carbon dioxide. This releases the maximum amount of energy possible. One molecule of glucose broken down to carbon dioxide can yield 38 molecules of ATP for the cell. 

Aerobic respiration involves a series of enzyme catalysed steps. It begins in the cytoplasm when glucose is broken down into the smaller molecule pyruvate. These pyruvate molecules can then enter the mitochondria to be broken down into carbon dioxide and water in the presence of oxygen. The majority of the 38 molecules of ATP are produced during this stage. These stages are summarised in the following diagram:


Oxygen is not always available cells for various reasons. Despite this, cells must still respire to release energy - we can describe this form of respiration without oxygen as fermentation. Fermentation is much less efficient than aerobic respiration. Without oxygen the final stage of respiration shown in the diagram above cannot occur. Fermentation therefore is restricted to the cytoplasm - mitochondria are not involved. Fermentation only yields 2 molecules of ATP for the cell from each glucose molecule. 

You have already experienced fermentation. Any time you have sprinted so fast your legs have got sore, or you have held your arms up in the air until they've got sore you've experienced fermentation. In animal cells, if there is insufficient oxygen available for enough aerobic respiration to occur, fermentation can take place. This involves the glucose being broken down to pyruvate as above, but then the pyruvate is converted into lactic acid. It's this molecule which makes your muscles ache after sprinting. 

Plant and yeast cells can also use fermentation to release energy from glucose, but they do not convert their pyruvate into lactic acid. Plant and yeast cells convert the pyruvate molecules into the alcohol ethanol and carbon dioxide. That's why yeast is used to produce alcoholic drinks and bread - the carbon dioxide gas causing bread to rise. Fermentation is summarised in the following diagram:

Again, Crash Course has a brilliant video on ATP and Respiration but it does go into more depth than you need.