Chapter 6: How Cell Acquire Energy - Study Guide

Required Reading: Introduction, 6.1, 6.2, 6.3, 6.4, and “6.5” (see below)

Learning Objectives for Chapter 6:

 

I. Introduction

Definitions:  Autotroph, Photoautotroph, Heterotroph, Photosynthesis

Key Concepts: Plants (and algae) are the primary producers of food for our planet. In other words, all food energy consumed by other organisms comes directly or indirectly from plants (and algae), which can make glucose (a sugar) from carbon dioxide and sunlight using a process known as photosynthesis. Because plants can make glucose from non-living sources, they are called autotrophs ("auto" = self, "troph" = feeder). More specifically, they are called photoautotrophs, since they use light energy ("photo" = light) to make glucose. Photosynthesis is the main pathway through which carbon enters the life cycle for later use in other organic compounds. Heterotrophs (“hetero” = other, “troph” = feeder) eat other living organisms as their food energy and carbon source, both of which entered the web of life via autotrophs.

II. Section 6.1

Definitions: Chloroplast, Stroma, Thylakoids, Light-dependent step, Light-independent step.

Key Concepts: Plants are able to take sunlight and carbon dioxide gas and create glucose from them. The complete reaction requires a few steps, and each step is carried out in a separate area of the plant cell’s chloroplast. Chloroplasts are organelles found in plants and algae that are able to carry out the process of photosynthesis. The interior structure of the chloroplast is related to this function – the main features being the light-collecting thylakoids (stacked together to absorb as much light as possible) and the gel-like stroma (which is similar to cytoplasm) in between the thylakoids and the outer chloroplast membrane (see Fig. 6.3, p. 94). The first step of photosynthesis – the light-dependent step – is a light-absorbing step, which occurs in the membranes of the thylakoids. The end result of light absorption is the formation of a short-term, energy-storing ATP bond. ATP bond energy is later used in the light-independent step – which occurs in the stroma –  to bond carbon atoms together, forming glucose. The glucose formed is then used to make other plant molecules, like starch cellulose or sucrose.

 

Note: Not all plant cells contain chloroplasts, but it is easy to spot those that do – they appear green to the human eye. A close-up look at a plant cells reveals that most of the cell interior is colorless, but the chloroplasts inside the cells make the plant look green to the human eye (see Fig. 6.3 b and c, p. 94.).

III. Section 6.2

Definitions: Photon, Pigments, Chlorophyll, Absorption of light.

Key concepts: Chloroplasts in plants and algae absorb light from the visible-light region of the electromagnetic spectrum (see Fig. 6.5, p. 96). In other words, plants are able to absorb light energy that we see as colored light. The reason they are able to absorb this light is because they contain pigments – molecules that absorb colored light and therefore appear colored themselves. The primary pigment used by chloroplasts is chlorophyll. There is more than one type of chlorophyll, but we won’t concern ourselves with this detail. However, it is important to know that pigments absorb only specific colors of light and don't absorb other colors. If a pigment absorbs both red and blue light (see Fig. 6.6 a, p. 97), the other colors of light will not be absorbed, causing the pigment to look green to our eyes. Since plants appear to be green, they are actually absorbing red and blue colored light.

IV. Section 6.3

Definitions: Electron transport system, Reaction center, Photolysis, Concentration gradient, ATP synthase.

Key concepts: This section focuses on the light-dependent reactions that occur in the chloroplast – the first step in photosynthesis. When chlorophyll absorbs light energy, it transfers that energy to an electron. The light energy "kicks" or "zaps" the electron up to a higher energy level.. Once the electron is in the high-energy state, it acts somewhat like water at the top of a water wheel. The water flows over the water wheel, and as it falls, it pushes the water wheel, which then spins and turns its motion into usable energy (like electricity, for example).

 

In photosynthesis, as the high-energy electron falls back down to a lower level, it gives off its energy to an “ATP wheel” – the electron energy is used to form energy-containing bonds ATP. The electrons used in photosynthesis come from water molecules that are split by light energy (called photolysis, "photo" = light, "lysis" = break) in the interior of a thylakoid and release their electrons (see Fig. 6.11 a, p. 99). The light-“zapped” electrons pull hydrogen ions into the interior of the thylakoid. As hydrogen ions collect inside the thylakoid, the concentration of hydrogen ions on the outside is depleted. This cause a concentration gradient – there are many more H+ ions on one side of the membrane than the other. The hydrogen ions want to go from the area of high concentration (inside the thylakoid membrane) to an area of low concentration (outside the thylakoid) but are only given one escape route – which is to move through a transmembrane protein called ATP synthase. The hydrogen ions, which are positively charged ions, repel one another – especially in such high concentration - and therefore are full of energy, which is released when they escape. ATP synthase allows the hydrogen ions to pass through, but only if the H+ ions give their energy to ATP synthase for use in production of ATP bonds. All of these steps together – from the electron being zapped to the formation of ATP bonds – constitute the light-dependent reactions of photosynthesis.

V. Section 6.4

Key concepts: This section Summarizes the light-dependent reactions described in section 6.3. Light splits water, releasing an electron, hydrogen ions, and oxygen inside the thylakoid membrane. The light-“zapped” electron travels through the membrane, dragging more H+ ions into the membrane. As H+ ions collect, they repel each other more and more, and want more and more to escape the thylakoid and move into the stroma. They can do this only if they pass through ATP synthase, which takes their energy and puts the energy to ATP bonds. Understand these key steps and be able to draw and explain them on a test.

V. A summary of section 6.5 written in something closer to English!

Key concepts: Since section 6.5 is nearly written in a foreign language, you need not read it. But do look at Fig. 6.14, p. 101 and read this summary of what happens in the light-independent reactions of photosynthesis. The “light-independent” or “dark” reactions occur once ATP has been formed. The ATP formed in the light-dependent reactions collects up in the stroma of the chloroplast. In the stroma, the ATP is then combined with carbon dioxide molecules, and a series of reactions result, leading to the formation of glucose. Essentially, the ATP bond energy formed in the light-dependent reactions is used to “stitch” carbon atoms (from carbon dioxide) together to form glucose molecules. The high-energy ATP bond gets broken, and the energy released is put into carbon-carbon bonds to make glucose.

Suggested Additional Study Resources:

From Textbook:

Review Questions: 1, 2, 6

Self quiz: 1, 2, 3, 4, 5, 6

Critical Thinking: 1

From Interactive concepts in Biology CD-ROM:

Chapter 6 Quiz Questions (Under “Learning Tools”): 1, 7, 9, 10

Unit 1 (From Main Menu), Chapter 6:

                                    Methods of Energy Acquisition: Reading, Activity1

                                    Photosynthesis - An Overview: Reading, Activity 2

Light Trapping Pigments: Reading, Activities 1 & 2 (remember things look the colors that they DON’T absorb)

Light Dependent Reactions: Movies 2, 3 & 6, and Activity 5

Light-Independent Reactions: Movie 2