Chapter 8 Photosynthesis

8-1 Energy and Life Energy is the ability to do work. Living things depend on energy.

Autotrophs and Heterotrophs The energy that living things need comes from food. The energy in most food comes from the sun. Plants and some other types of organisms are able to use light energy from the sun to produce food.

Autotrophs – organisms that make their own food (plants) Heterotrophs – get energy from the foods they consume (animals)

Chemical Energy and ATP Energy comes in many forms, including light, heat, and electricity. Energy can also be stored in chemical compounds.

ATP and ADP The activities of the cell are powered by chemical fuels.

Adenosine triphosphate (ATP) – one of the principal chemical compounds that living things use to store energy. An ATP molecule consists of a nitrogen-containing compound called adenine, a 5-carbon sugar called ribose, and three phosphate groups.

Adenosine Diphosphate (ADP)- similar structure to ATP, but ADP has two phosphate groups instead of three. When a cell has energy available, it can store small amounts of energy by adding a phosphate group to ADP molecules, producing ATP molecules.

Releasing Energy From ATP The energy stored in ATP is released when ATP is converted into ADP and a phosphate group. Because a cell can add and subtract a third phosphate group, it has a way of storing and releasing energy as needed. The characteristics of ATP make it a very useful molecule that is used by all types of cells as their basic energy source.

Using Biochemical Energy Cells use the energy provided by ATP in many ways. -active transport – many cell membranes contain a sodium-potassium pump that moves sodium ions out of the cell and potassium ions into it. ATP molecules provide the energy to do this. -movement within the cell- cell organelles are moved along microtubules by motor proteins that use the energy of ATP to generate force.

ATP and Glucose Cells can regenerate ATP and ADP as needed by using the energy in carbohydrates like glucose.

8-2 Photosynthesis: An Overview

Photosynthesis – plants use the energy of sunlight to convert water and carbon dioxide into oxygen and high-energy carbohydrates (sugars and starches)

Investigating Photosynthesis Van Helmont’s Experiment In the 1600s, a Belgian physician named Jan van Helmont wanted to see if plants grew by taking material out of the soil. He planted a seedling and after 5 years it grew into a small tree. He saw that the mass of the soil had remained the same, so he concluded that most of the mass the plant gained had come from water, because that was the only thing that he had added to the plant.

Van Helmont didn’t realize it, but carbon dioxide in the air made a major contribution to the mass of the tree.

Priestley’s Experiment More than 100 years later, an English minister named Joseph Priestley placed a glass jar over a lit candle and watched the flame gradually die out. He concluded that something in the air was necessary to keep a candle flame burning. This was oxygen. He then put a plant under the glass jar and saw that the candle would stay lit for a while. This showed that the plant produced the substance required for burning – it released oxygen.

Jan Ingenhousz Dutch scientist Jan Ingenhousz showed that the effect that Priestley observed only occurred when the plant was exposed to light. The results of these experiments showed that light is necessary for plants to produce oxygen.

These experiments reveal that when there is light, plants transform carbon dioxide and water into carbohydrates and release oxygen.

The Photosynthesis Equation Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into oxygen and high energy sugars.

In addition to water and carbon dioxide, photosynthesis requires light and chlorophyll, which is a molecule in chloroplasts.

Plants gather the sun’s energy with light- absorbing molecules called pigments. The plants’ main pigment is chlorophyll. There are two main types: chlorophyll a and chlorophyll b.

8-3 The Reactions of Photosynthesis

Inside a Chloroplast Chloroplasts contain saclike photosynthetic membranes called thylakoids. They are arranged in stacks called grana. Thylakoids contain clusters of chlorophyll and other pigments and protein known as photosystems that are able to capture the energy of sunlight.

NADPH When sunlight excites electrons in chlorophyll, the electrons gain energy. These high-energy electrons require a special carrier.

One of these carrier molecules is a compound known as NADP+ (nicotinamide adenine dinucleotide phosphate)

NADP+ holds 2 high-energy electrons along with a hydrogen ion. This converts the NADP+ into NADPH. This conversion is one way in which some of the energy of sunlight can be trapped in chemical form.

Light-Dependent Reactions Light-dependent reactions need light. Light- dependent reactions produce oxygen gas and convert ADP and NADP+ into the energy carriers ATP and NADPH

The Calvin Cycle During the Calvin Cycle, plants use the energy that ATP and NADPH contain to build high- energy compounds that can be stored for a long time. The Calvin Cycle uses ATP and NADPH from the light-dependent reactions to produce high-energy sugars.

-Six carbon dioxide molecules enter the cycle from the atmosphere. The carbon dioxide molecules combine with six 5-carbon molecules. The result is twelve 3-carbon molecules -The twelve 3-carbon molecules are then converted into higher-energy forms. The energy for this conversion comes from ATP and high-energy electrons from NADPH

-Two of the twelve 3-carbon molecules are converted into two similar 3-carbon molecules. These 3-carbon molecules are used to form various 6-carbon sugars and other compounds. -The remaining ten 3-carbon molecules are converted back into six 5-carbon molecules. These molecules combine with six new carbon dioxide molecules to begin the next cycle.

As photosynthesis proceeds, the Calvin cycle works steadily, turning out energy rich sugars and removing carbon dioxide from the atmosphere. The plant uses the sugars for energy and to build more complex carbohydrates such as starches and cellulose, which it needs for growth and development.