CC28: Photosynthesis - Where Did the Energy in Your Pizza Come

Which process provides the link between Earth’s physical environment and the energy needs of an entire ecosystem?

• cellular respiration

• decomposition

• photosynthesis

Answer explanation

Photosynthesis is the process of converting light energy (typically from sunlight) into chemical energy (stored in organic molecules). This process provides energy for the rest of life on Earth.

Organisms that can harness energy from sunlight through photosynthesis are called producers. Producers provide energy for all other organisms on Earth. Which term best describes these other organisms?

• consumers

• decomposers

• photosynthesizers

• predators

Answer explanation

Producers, also called autotrophs, are organisms that make their own food, usually through photosynthesis (though some use another process called chemosynthesis). Organisms that can’t make their own food are called consumers (a) or heterotrophs. They get energy by consuming other organisms

This diagram depicts the organelle in a plant cell that carries out photosynthesis. The organelle and some of its internal structures are labeled with letters.

Match the parts to the letters on the diagram.​

A ​​ (a)  

B ​ (b)  

C ​ (c)  

• Chloroplast
• Stroma
• Thylakoid

Photosynthesis can be broken down into two main reactions. What are these reactions called?

• calvin cycle and cellular respiration

• light-dependent and light-independent

• thylakoids and stroma

Answer explanation

The two main processes of photosynthesis are the light-dependent reactions, which take place in the thylakoids, and light-independent reactions (the Calvin cycle), which occur in the stroma (the fluid-filled space surrounding the thylakoids).

Photosynthesis starts when sunlight hits chlorophyll, green pigment molecules found in plants and algae. Sunlight energizes the chlorophyll molecules and causes them to:

• continue to the next part of photosynthesis, the Calvin cycle

• lose their electrons, which go to the electron transport chain

• release signaling molecules, which begin the cellular growth cycle

Answer explanation

When chlorophyll molecules are energized by sunlight, they lose their electrons. The electrons are transferred through the electron transport chain, powering the rest of photosynthesis.

What is the primary function of the light-dependent reactions in photosynthesis?

• to convert light energy into chemical energy

• to produce oxygen, which enables life on Earth

• to use ATP to produce glucose molecules

Answer explanation

With the help of water, the light-dependent reactions convert light energy into chemical energy (a) in the form of ATP and NADPH, which is used to power the light-independent reactions (Calvin cycle). The light-dependent reactions also produce oxygen (b) as a byproduct, but that is not their primary function.

At the beginning of the Calvin cycle, the carbon-containing molecule RuBP combines with which other molecule?

• 3-PGA

• ATP

• CO2

• G3P

Answer explanation

The Calvin cycle begins by combining RuBP with carbon dioxide (CO2) molecules from the air.

What is considered the final product of the Calvin cycle?

• carbon dioxide

• glucose

• G3P

• RuBP

Answer explanation

Through a series of reactions, the Calvin cycle ultimately produces G3P. G3P can be used to make a variety of other molecules, including RuBP, glucose, starch, and cellulose.

Ecosystems often contain organisms and processes that balance each other out. The diagram models the cycling of materials in an ecosystem. What are the materials represented by X and Y in the model?

• X is carbon dioxide and Y is oxygen.

• X is oxygen and Y is carbon dioxide.

• X is glucose and Y is oxygen.

• X is oxygen and Y is glucose

Answer explanation

Carbon dioxide (X) is an output of cellular respiration in humans. It is released into the environment and serves as an input for photosynthesis in plants. Photosynthesis produces oxygen (Y) as an output, which serves as an input for cellular respiration in humans.

What is carbon fixation?

• the complementary processes of photosynthesis and cellular respiration

• the conversion of atmospheric carbon dioxide into organic molecules

• the release of energy stored in glucose and other simple sugars

Answer explanation

Carbon fixation is the conversion of carbon dioxide (CO2) into organic molecules by producers