Mitochondria

Mitochondria

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You should already know from your GCSE that mitochondria are the sites of respiration in eukaryote cells.

Textbooks like to describe the mitochondria as "the powerhouses of cells". What they mean is that mitochondria get the energy out of glucose in respiration: they use this energy to make a wonderful chemical called ATP.

Cells need energy for other processes such as the synthesis of proteins from amino-acids and the replication of DNA. This energy usually comes from the breakdown of glucose; though fats and proteins can also be used a sources of energy. Glucose is a stable chemical: it does not just breakdown releasing energy. Since glucose can pass through cell membranes it is used to transport energy from one part of your body to another in your blood.

Glucose cannot be stored by cells. They must convert the glucose into glycogen. Both liver cells and muscle cells can store glycogen. Storing glucose is not possible for two reasons. Firstly it would just get out of the cell. Secondly it would have an enormous osmotic effect. Glycogen is not soluble so it cannot leak out of cells. Instead of storing tens of thousands of glucose molecules, the cell can store a few glycogen molecules which have very little osmotic effect.

ATP is very much less stable than either glucose or glycogen, so it cannot be used to store energy or to transport energy. Cells make ATP when and where they need it. Muscle cells need a lot of ATP so they have lots of mitochondria. Muscle cells convert chemical energy (in ATP) into kinetic energy: striated muscles contain two important proteins, actin and myosin; these can combine to form actinomyosin. Strands of actinomyosin shorten when ATP is put on them. So every time a muscle contracts, chemical energy is converted into kinetic energy.

Energy is also needed for growth and repair. When cells make protein from amino-acids, they require energy from ATP. It is also necessary to use ATP to link glucose molecules together to form starch or glycogen.

Mitochondria:

have a double membrane

	the inner membrane
	the outer membrane

have their own DNA

this carries the information to make the enzymes

have their own ribosomes

	more like the ribosomes of prokaryotes than eukaryotes
	are used to synthesise proteins (the enzymes) from amino-acids

make their own enzymes

required for the Kreb's Cycle

require glucose

require oxygen

produce ATP from ADP and phosphate ions

The inner membrane of a mitochondrion

	is folded into cristae
	has a large surface area

You should already know from your GCSE studies that mitochondria are the sites of ATP production, but for "A" level you must be able to explain where all the chemical reactions of tissue respiration take place. Glycolysis, the breakdown of glucose to form acetyl coenzyme A, takes place in the cytoplasm outside mitochondria. The Kreb's cycle, breaking down acetyl coenzyme A into carbon dioxide and NADPH, takes place in the matrix of the mitochondrion. Some ATP is generated directly during the Kreb's cycle, but most of the ATP produced in tissue respiration is generated by the electron transfer chain which takes place across the membranes of the mitochondrion. Stalked particles on the inner surface of cristae contain the enzymes required to make ATP from ADP and phosphate.

Both photosynthesis and tissue respiration require a large number of enzymes. Therefore both chloroplasts and mitochondria contain DNA and ribosomes.

Some of the ATP produced by mitochondria is generated directly during glycolysis and the Kreb's Cycle, which is also called the Tri-Carboxylic Acid (TCA) Cycle. More ATP is generated from a chemical called NADPH. This is the starting point of the electron transfer chain (also called Hydrogen transfer). The final Hydrogen acceptor is Oxygen. NADPH is produced during the Kreb's Cycle. One molecule of NADPH can be used to generate three molecules of ATP.

Glycolysis, the breakdown of glucose into pyruvic acid, takes place outside mitochondria. This produces about 15% of the energy of aerobic respiration.

The Kreb's Cycle, also called the Tri-Carboxylic Acid (TCA) Cycle and the Citric Acid Cycle, takes place inside mitochondria. The most important chemical produced during the Kreb's Cycle is NADPH.

The energy in NADPH is used to generate ATP energy in the inner membranes of mitochondria in a process called the Hydrogen Transfer Chain.

Coming soon: pages on glycolysis, the Kreb's cycle and the electron transfer chain.

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Last Revised: 10 November 2006

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