Name: ______Date: ______Period: ____
Unit 6: Energy in Living Things
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Section 2: Photosynthesis
Photosynthesis is the process in which green plants use the energy of sunlight to convert water and carbon dioxide into high-energy carbohydrates and oxygen.
What did early experiments on photosynthesis uncover about how plants get their energy?
1. Mass a plant gains does not come from soil.
2. Plants produce oxygen.
3. Light is needed for plants to produce oxygen.
What do these chemical abbreviations stand for?
CO2 = Carbon Dioxide H2O = Water
O2 = Oxygen C6H12O6 = Glucose
Process of photosynthesis can be chemically expressed like this:
sunlight
6 CO2 + 6 H2O à C6H12O6 + 6 O2
Photosynthesis takes place in the chloroplasts which
are located in the leaves of plants and are bright
green. It is important to know the structure
of a chloroplast when learning about photosynthesis.
On the following page, label the parts of the chloroplast.
1. Chloroplasts contain thylakoids, which are saclike photosynthetic membranes. Thylakoids are arranged in stacks known as grana. A single stack is called a granum. They look like a stack of green pancakes!
2. Just like mitochondria, chloroplasts have a double membrane. They are simply called the inner and outer membrane.
3. The jelly-like filling inside the chloroplast is called the stroma.
The Stages of Photosynthesis:
1. Absorption of Light Energy
2. Electron Transport Chain
3. The Calvin Cycle
Stage 1: Energy is captured from sunlight
Plants gather the sun’s energy with light-absorbing molecules called pigments.
The main pigment in plants is called chlorophyll. There are two types chlorophyll “a” and “b” but both look green/yellow.
Sunlight is actually made up of all the colors of the spectrum but looks white. You only see the colors that are not absorbed by objects.
Look at the graph below, the two lines represent where on the white light spectrum and how much energy that chlorophyll absorbs. The higher the line, the more energy absorbed. Where on the spectrum does chlorophyll absorb the least amount of light energy? Green and Yellow
Plants also include other pigments, in smaller amounts, they are called carentenoids
These pigments can be seen during the autumn when the leaves are changing colors or in flowers. These pigments are also called accessory pigments because they are extra just like sunglasses or jewelry are considered extra to your clothes. Most carotenoids absorb green and yellow.
When chlorophyll and carotenoids get hit with sunlight, their molecules get excited and the electrons in the molecules begin to move around. Remember that electrons are negatively charged particles that rotate around the nucleus of atoms. Once this happens, we move to the second phase of photosynthesis.
Stage 2: Electron Transport Chain
The electron transport chain is located in the thylakoid membranes. The reactions that must use the energy of sunlight are called light-dependant reactions.
The reason why plants need water in this step is because water molecules have loosely held electrons which are easy to remove when hit with sunlight. The electrons gain a great deal of energy from sunlight. There is an enzyme whose sole job is to split the water molecule and is simply called the Water Splitting Enzyme.
These freed electrons are then passed down a chain of proteins within the thylakoid membranes, creating ATP and * NADPH as it goes. It is like a game of hot potato!
* NADPH is another form of energy carrier just like ATP and ADP, when not energized its name is: NADP+
Stage 3: The Calvin Cycle
So far we have talked about why a plant needs water (to provide electrons) and sunlight (to excite electrons), but what about carbon dioxide? Why does a plant need that?
Remember that plants make organic compounds from inorganic substances. This property makes them autotrophs. Carbon dioxide is used to make these organic substances. This process is called carbon dioxide fixation.
The Calvin Cycle takes place in the stroma and is known as the light-independent reaction or “dark reactions” because it does not require sunlight to function.
The Calvin Cycle can be summed up by saying that the plant uses carbon dioxide, NADPH, and ATP to create a type of sugar known as glucose. This sugar is how plants store energy.
Section 2 Vocabulary
Chlorophyll – principal pigment of plants and other photosynthetic organisms; captures light energy.
Accessory Pigments/Carotenoids – compound other than chlorophyll that absorbs light at different wavelengths than chlorophyll.
Chloroplast – organelle found in cells of plants and some other organisms that uses energy from sunlight to make energy-rich food molecules by photosynthesis.
Electron Transport Chain – a series of proteins in which the high-energy electrons from the Krebs cycle are used to convert ADP into ATP.
Calvin Cycle – reactions of photosynthesis in which energy from ATP and NADPH is used to build high-energy compounds such as sugars.
Thylakoid – saclike body in chloroplasts made of photosynthetic membranes that contain photosystems.
Carbon Dioxide Fixation – the process of using carbon dioxide to create organic substances.
Section 2 Questions
1. What are the 3 stages of photosynthesis?
a. ______
b. ______
c. ______
2. What role does sunlight actually play in photosynthesis?
3. What is the highest and lowest energy of light on the visible spectrum?
a. Highest energy - ______
b. Lowest energy - ______
4. What are the reactants and products of photosynthesis? In other words, what does photosynthesis need to happen and what does it produce?
a. Reactants - ______
b. Products - ______
5. Where does Stage 1 and Stage 2 take place in the chloroplast?
a. Stage 1 Location - ______
b. Stage 2 Location - ______
6. Do plants use photosynthesis at night? If not, what can they do at night?
7. What factors do you think affect the rate of photosynthesis?
a. ______
b. ______
c. ______
Unit 6, Section 2, Page 7