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How Does Electron Transport Work

Background Information

The cell is accepted as the smallest unit of life. Living cells, as well as more complex organisms, depend on many chemical reactions in order to carry out the processes needed for life. (Growth, reproduction..) It takes energy to carry out all of these chemical reactions. Energy can be obtained from food or from light, but in each case it has to be converted into a useable form of energy for living cells. ATP is the useful form of energy used by living cells. It is a complex chemical and the energy is stored in the 3 phosphate bonds.
Cells break down food during respiration or convert light energy into ATP during the light reaction of photosynthesis. The cells then use the ATP energy to move, grow reproduce or to make food ( during the dark reaction of photosynthesis). In all cases, electron transport systems are the major sites of ATP synthesis.

What is the ETC?

The electron transport chain is a group of chemicals that can accept and pass on electrons from other molecules. ETC's are organized in membranes which also serve to insulate charges as they move.
Electrons move from one chmeical to another in a specific order and it happens very FAST. Atoms are composed of protons(+), neutrons (no charge) and electrons(-). Molecules are made up of atoms held together by covalent bonds. If we can pretend for a momenet that molecules have personalities, some molecules are "happy" with the number of electrons they have. Other molecules have more than they want, and tend to donate electrons to other molecules. Still other molecules ared "electron hogs" and take electrons from other molecules. Electrons tend to move from a strong donor to and acceptor which then donates to a stronger acceptor which then donates to a stronger acceptor, etc. until it reaches an electron hog which will keep it. The electron strat out at a high energy and loses some energy The particular chemicals involved and the order in which they pass on the electron depends upon the type of cell and the location within the cell.

Where, How and What

In eukaryotic cells, electron transport can happen in mitochondria during a process called respiration and in chloroplasts during photosynthesis.

As electrons move through the electron transport chain, the enrgy they lose is used to add a phosphate to ADP to make ATP. This is done by moving protons (H+, as hydrogen ions) from one side of the membrane causing a build up (high concentration gradient) on the other side. As the protons move back across, they have to go through a special spot called ATP synthetase where ATP gets made.

In respiration, ETC is controlled by whether or not food and oxygen are present. In photosynthesis light and water are required.

Chloroplast ETC

This is tricky because there is a cyclic and noncyclic electron flow that can happen and the different products that result. Light energy is used to excite electrons in particular reaction center chlorophyll a molecules (PHOTOSYSTEMS 1 and 2). These excited chlorophyll molecules are strong electron DONORS and will pass on electrons to other molecules. ATP is produced as a result.

In non-cyclic electron flow, electrons move from water to NADP+ to make NADPH ( which is used to help make sugar from carbon dioxide) and ATP is also made.

In cyclic electron flow the electrons are recycled and only ATP is made.

ACTING OUT NONCYCLIC electron flow

1. Ask students to be photosystem 1 chlorophyll
acceptor molecules A, B and C (if enough students)
photosystem 2 chlorophyll
acceptor molecule Fd
Light (1 -2 students)
NADP+ (1 -2 students)
any remaining students can be water
Have a box or basket labeled sugar making!
Water molecules and the chlorophyll start out with electrons (candy)

2. Organize students into chloroplast membrane.

3. Light shoves the chlorophyll from PHOTOSYSTEM 2 which is then energized and it runs and gives electrons to the ETC -molecules A, B and C. chlorophyll then runs back and grabs electrons from available water. Water is now oxygen and should leave. Light should then shove the chlorophyll again.

While this is happening.....
Light shoves the chlorophyll of PHOOSYSTEM 1 which is then energized and passes its electrons to Fd. Fd then passes the electrons to NADP+. NADP+ is now NADPH and it leaves and puts its electrons in the sugar making box/basket.. The chlorphyll of PHOTOSYSTEM 1 goes to molecule C and takes its electrons. Now POTO 1 awaits a shove from light to start again.
Note Water is the major componenet of living cells and there is no shortage of that (it is not limiting). Therefore as long as there is light and as long as NADPH is being used to make sugar the process can just keep going on.

Students should not end up with the goodies, they should end up in the box/basket.... Students should be rewarded at the end of the activity only!
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