8.1 Energy and Life

Chemical Energy

• Energy is the ability to do work

  • Objects such as lights, radios, and computers, as well as living things, need energy to work

  • Without the ability to get and use energy, cells would die

  • Light, heat, electricity, and chemical energy are four forms of energy

  • Energy can change from one form to another

• Chemical energy is the energy stored in chemical bonds

• One of the principal chemical compounds that living things use to store and release energy is called adenosine triphosphate (ATP)

  • ATP is made up of adenine, a sugar called ribose, and three phosphate groups

    • The phosphate groups are the key to why ATP can store and release energy

• Adenosine diphosphate (ADP) is similar to ATP but has only two phosphate groups instead of three

  • When a cell has extra energy, it can store small amounts of it by adding a phosphate group to ADP molecules, which turns ADP into ATP

• Cells can give off the energy stored in ATP by breaking the chemical bonds between the second and third phosphate groups

  • A cell can add or subtract these phosphate groups whenever it needs to store or use energy

• Cells use ATP in many ways

  • ATP powers active transport by providing the energy needed to move material into and out of many cells.

  • ATP also helps proteins in muscles to slide closer together, a motion that makes muscles flex

  • Energy from ATP powers many other important events, such as the making of proteins and responses to chemical signals outside of the cell

• Most cells only have a small amount of ATP

  • ATP is great for giving off a small amount of energy very quickly, but it is not good for storing large amounts for a long time

  • Glucose is much better for energy storage, as a single glucose molecule stores more than 90 times the energy of a molecule of ATP

  • Cells can then use the stored glucose to make ATP and ADP as needed

ATP Comes From Food

• ATP comes from the chemical compounds that we call food, though different living things get their food from different sources

• Living things that get food by eating other living things are known as heterotrophs

  • Some heterotrophs get their food by eating plants such as grasses, while others get food by eating other animals; other heterotrophs—mushrooms, for example—get food by breaking down the tissues of dead things

• Living things that make their own food are called autotrophs

  • The process by which autotrophs use the energy of sunlight to produce high-energy carbohydrates—sugars and starches—that can be used as food is known as  photosynthesis

8.2 Photosynthesis: An Overview

Chlorophyll and Chloroplasts

• Living things that carry out photosynthesis use pigments to get energy from sunlight

• Energy from the sun travels to Earth in the form of light

  • Sunlight, which our eyes see as “white” light, is actually a mixture of different wavelengths

• Plants gather the sun’s energy with light-absorbing molecules called pigments

  • The most important pigment in plants is chlorophyll, which absorbs blue-violet light and red light best, though it does not absorb green light very well; leaves look green because they reflect green light

• Photosynthesis takes place inside organelles called chloroplasts

  • These organelles hold many flat, bag-shaped membranes called thylakoids, which are connected to one another and arranged in stacks called grana

  • Chlorophyll and other pigments are found in these thylakoid membranes

  • The liquid-filled space around the thylakoids is known as the stroma

High-Energy Electrons

• When chlorophyll takes in light, much of that energy is moved directly to electrons in the chlorophyll, causing their energy levels to rise

  • A stream of these high-energy electrons is what makes photosynthesis work

  • However, these high-energy electrons cannot keep their energy for long on their own and need a special carrier

• Plant cells use molecules to carry high-energy electrons from chlorophyll to other places

  • One of these carrier molecules is NADP+ (nicotinamide adenine dinucleotide phosphate), and it accepts and holds two high-energy electrons, along with a hydrogen ion (H+), a process that changes NADP+ into NADPH

  • The NADPH carries the electrons to other parts of the cell, where the electrons and their energy can be used to help build useful molecules, such as sugars

A Summary of Photosynthesis

• Photosynthesis uses light energy to change water and carbon dioxide (reactants) into sugars and oxygen (products)

• All of the steps of photosynthesis can be summed up in the following equation:

• There are two stages of photosynthesis: light-dependent reactions and light-independent reactions

  • Light-dependent reactions are reactions of photosynthesis that use ATP, NADPH, and carbon dioxide to make high-energy sugars

    • These reactions take place in the thylakoids and release oxygen

  • Light-independent reactions are reactions of photosynthesis that use light energy to make ATP and NADPH

    • These reactions take place in the stroma

8.3 The Process of Photosynthesis

The Light-Dependent Reactions: Making ATP and NADPH

• The light-dependent reactions take place in the thylakoids of chloroplasts, where they hold groups of chlorophyll and proteins known as photosystems that take in sunlight and use it to add energy to electrons; these electrons are passed to a set of carriers in the thylakoid membrane

  • There are two photosystems, and they are named in order of their discovery—not in the order that they do their work

• The pigments of photosystem II absorb light energy and release high-energy electrons that then get passed down the electron transport chain, a series of proteins in which high-energy electrons are used to change ADP to ATP

• The light-dependent reactions use water and energy from sunlight to make oxygen and change ADP and NADP+ into the energy carriers ATP and NADPH; these compounds are important for the cell because they provide the energy needed to build sugars in the light-independent reactions

• In sum:

  • Photosystem II: Light shining on pigments energizes electrons that come from water

  • Electron Transport: High-energy electrons move down the chain, which pumps H+ ions to the inside of the thylakoid

  • Photosystem I: Electrons are reenergized with more light

  • Electron Transport: The reenergized electrons are transferred to NADP+, to make NADPH

  • ATP Formation: Excess H+ ions spill out through ATP synthase; the protein rotates as each ion passes through, which changes ADP to ATP

Light-Independent Reactions: Making Sugars

• During the light-independent reactions—also called the Calvin cycle—plants build high-energy sugars

  • These sugars are stable, so they can store energy for a long time

  • The Calvin cycle is named after the American scientist Melvin Calvin

• During the light-independent reactions, ATP and NADPH are used to make high-energy sugars

  • The plant uses the sugars to meet its energy needs and to build molecules needed for growth; when other living things eat plants, they, too, get the energy they need

• In sum:

  • Step 1: Carbon dioxide combines with molecules from the cycle that have five carbon atoms; molecules with three carbon atoms result

  • Step 2: Energy from ATP and NADPH energizes the 3-carbon-atom molecules

  • Step 3: Some of these molecules leave the cycle to make sugars and other compounds

  • Step 4: Energy from ATP changes the rest of the 3-carbon-atom molecules back into 5-carbon-atom molecules

  • Step 5: The 5-carbon-atom molecules can go through the cycle again

Factors Affecting Photosynthesis

• Three important factors that affect photosynthesis are temperature, light intensity, and the availability of water

  • Temperature is one factor that can affect the rate of photosynthesis, as photosynthesis enzymes work best between 0°C and 35°C

  • Light intensity is another factor, as very bright light speeds up photosynthesis

  • Water levels also affect the rate of photosynthesis

• Extreme conditions can also affect photosynthesis

  • Plants that close tiny openings in their leaves to keep from drying out in the hot can’t receive as much carbon dioxide; in turn, this slows photosynthesis

• C4 plants have a special chemical pathway that gets carbon into the Calvin cycle even when there is not much carbon dioxide available

  • The pathway uses extra ATP but lets the plants carry out photosynthesis when it is hot

• CAM plants save water by taking air into their leaves only at night

  • In the dark, carbon dioxide is used to make acids

  • During the day, these acids are turned back into carbon dioxide for photosynthesis