1. Module 5 Intro

1.6. Page 4

Lesson 1

Module 5—Photosynthesis and Cellular Respiration

Read

 

REDOX

 

oxidation: a loss of electrons

 

reduction: a gain of electrons

Oxidation and reduction, oxidized and reduced. These are words that you will encounter as you learn more about photosynthesis and cellular respiration. Read page 167 in your textbook.

 

The words reduction or reduced can sometimes create confusion. Normally one would associate the idea of less with these words; but instead, there is a gain. How does this work?

 

There is a simple mnemonic device to remember the definition of the terms oxidation and reduction. All you have to do is remember that LEO the lion goes GERRR! (That is a growl.)  Spelled out, it looks like this:

 

Lose Electrons → Oxidation,     Gain Electrons → Reduction

 

redox reaction: a reaction involving the transfer of electrons

When electrons transfer in a chemical equation, this is called a redox reaction. Here is a chemical equation to illustrate.

 

Xe- + Y → X + Ye-

 

Reduction: Carbon dioxide gains electrons and hydrogen to become glucose. It is reduced.
Oxidation: Water loses electrons and loses hydrogens to become oxygen. It is oxidized.

 

It turns out that the overall reaction of photosynthesis is an oxidation-reduction (redox) reaction. In the first step, the oxygen in water is oxidized by the light energy. This means that the water molecule has lost electrons (LEO). You will learn more about this process when you study light-dependent reactions.

 

H2O + light energy→1/2 O2 + 2 [H.]+ 2 e-

 

proton: a hydrogen ion containing one electron

Here, [H.], or a proton, represents a reducing agent. Next, the [H.] reduces the carbon in CO2. This is not as simple as is shown. However, the idea is that at some point carbon dioxide is reduced (GER). The result is a charboydrate molecule. You will learn about this step during your study of light-independent reactions.

 

2[H.] + CO2 → (CH2O){carbohydrate molecule}

 

Note: In order for a glucose molecule to be created, more than one water molecule and one carbon dioxide molecule are involved. This situation is indicated by the balanced chemical equation

 

6 CO2(g) + 6 H20(l) + energy → C6H12O6(s) + O2(g)

 

Conversely, in cellular respiration, glucose is oxidized and oxygen is reduced.

 

Self-Check

 

SC 7.  A molecule such as NADP+ can be reduced to NADPH. NADPH is said to have reducing power. What does this mean?

 

SC 8.  Hydrogen ions are also called ______________.

 

SC 9. True or False: [H+] has one electron.

 

Check your work.
Self-Check Answers

 

SC 7. NADPH has more available stored energy because it has gained electrons. It has more power.

 

SC 8. Hydrogen ions are also called protons.

 

SC 9. True. Protons carry one electron and can reduce a compound because a proton will add an electron (GER).

 

Watch and Listen

 

What is an electron transport chain?

 

You will be introduced to an electron transport chain in the following animation. In an electron transport chain, electrons pass from carrier to carrier through a series of oxidation-reduction reactions. During each transfer, some energy is released.



chemiosmosis: the process by which ATP is generated through the movement of protons down a concentration gradient

 

The protons move across the inner membranes to the mitochondrion and thylakoid membrane of the chloroplast and combine with ADP and phosphate molecules to form ATP.

Electron transport chains are essential to photosynthesis and cellular respiration. In an electron transport system, energy from electron transfer during oxidation-reduction reactions enables certain carriers to pump protons (H+) across a membrane. As the H+ concentration increases on one side of the membrane, an electrochemical gradient develops and chemiosmosis occurs.

 

Watch the following animation to get an idea of what chemiosmosis looks like.