Lecture 39 May 5 2003 R.Jones Chapter 6, 5.15-5.16, 32.4

1. We will begin with the oxidative breakdown of glucose one of the most common sugars that is formed in heterotrophic organisms as a result of digestion. A simplified equation for this can be written as:

Glucose is broken down in a three-step series of reactions. Glycolysis occurs in the cytoplasm and breaks glucose into two more simple 3-carbon molecules known as pyruvate. In glycolysis 2ATP are formed when the glucose molecule is split into two pyruvate and as well as the formation of 2 molecules reduced co-enzyme NADH. The formation of ATP in this first step of glucose breakdown is referred to as substrate level phosphorylation.

2. Pyruvate is transported across the outer and inner membranes of the mitochondrion and converted to a 2-carbon molecule known as acetyl-CoA with the loss of 2 x CO2 and the formation of 2 more NADH.

3. Acetyl-CoA enters the Krebs Cycle where it is broken down completely to CO2. When 2 x acetyl COA is broken down 4 x CO2 are released. During the Krebs Cycle 6 NADH, 2 ATP and 2FADH (FADH is a co-enzyme similar to NADH). The net result of the breakdown of glucose is that 4ATP, 10NADH and 2FADH are produced. A simplified summary equation for cellular respiration of glucose can be written as follows to take into account the high energy molecules produced in this process.

4. The last stage in respiration is that the electrons trapped in the co-enzymes FADH and NADH are transferred to O₂ and more ATP is made. All of the oxygen that we take in by breathing is simply used by mitochondria to take electrons and H⁺ from co-enzymes, forming H₂O. This occurs in the mitochondria where an electron transport chain takes NADH and FADH and separates the H+ from the e-. The H⁺ are transported into the space between the inner and outer envelope of the mitochondria causing an accumulation of H⁺ in this inter-membrane space. These H+ travel back into the mitochondrial matrix through an ATP synthase enzyme causing ATP to be formed. The H⁺ and e^- are finally transportd to oxygen where H₂O is produced.

5. For each NADH 3 molecules of ATP are formed and from FADH 2 ATP are formed. So, 10 NADH give 30 ATP, 2 FADH give 4 ATP and 4 ATP were formed by substrate level phosphorylation. A total of 36 ATP are formed during aerobic respiration (2 ATP are used for the transport of NADH into the mitochondrion). The delta G of glucose oxidation is 686 calories and the delta G of ATP hydrolysis is 7.5 calories. Clearly 36 ATP can give 270 calories and this represents an efficieny of respiration of 39%.

6.This is oxidative metabolism, but we know that many organisms such as yeast can function in the absence of O₂, namely anaerobically. How do they do it? They carry out the process of fermentation and in the case of yeast they carry out alcoholic fermentation producing CO₂ and 2 ATP. Yeast use NADH to convert pyruvate to ethanol thus by-passing the need for oxygen to remove electrons form glucose.

7. Humans can carry out limited fermentation in muscles where pyruvate is reduced to lactate with NADH. As in fermentation in yeast 2 ATP can be produced for every glucose molecule oxidized to lactate. Energetically a very inefficient process about a 2% efficiency.

8. Other forms of human food are dealt with in metabolism in a similar way to sugars. Fats for example are broken down into glycerol and fatty acids. Glycerol enters glycolysis and fats are broken down into acetyl CoA and enter the Krebs Cycle. Proteins are broken down into amino acids, the amino group is removed and the resulting acid broken down in the Krebs Cycle.

9. Plants play a central role in the cycling of carbon, nitrogen and phosporus as well as other elements in the biosphere. Carbon cycling is via the action of photosynthesis and plants carefully regulate their uptake of CO₂. The stomates that regulate uptake of CO₂ in plants play an important role in the water economy of plants and for agriculture, it's water that's important. If water is withheld from plants stomates close and plants do not photosynthesize.

10. Stomates are found on the above ground surfaces of plants and are formed from two guard cells. They are generally more abundant on the lower surfaces of leaves. Guard cells in grasses and broad-leafed dicots differ in structure, but functionally they are identical.

11. Specialized thickenings in the cell walls of the guard cells allow the cells to move relative to each other forming a pore or stoma. The movement of the guard cells is brought about by the movement of water into and out of the guard cells by osmosis.