Scope, Sequence, and Coordination

A Framework for High School Science Education

Based on the National Science Education Standards


Plants, Chloroplasts and Photosynthesis

Cell Chemistry: Metabolism, Catalysts, and Photosynthesis
Most cell functions involve chemical reactions. Food molecules taken into cells react to provide the chemical constituents needed to synthesize other molecules. Both breakdown and synthesis are made possible by a large set of protein catalysts, called enzymes. The breakdown of some of the food molecules enables the cell to store energy in specific chemicals that are used to carry out the many functions of the cell.

Plant cells contain chloroplasts, the site of photosynthesis. Plants and many microorganisms use solar energy to combine molecules of carbon dioxide and water into complex, energy rich organic compounds and release oxygen to the environment. This process of photosynthesis provides a vital connection between the sun and the energy needs of living systems.


Further Description:

Metabolism, which is defined as the sum of all chemical reactions occurring in a cell, involves the activities of enzymes. Enzymes are large protein molecules that serve as catalysts to lower the energy required for reactions to take place in cells. Enzymes as catalysts also function in speeding up rates of reactions. Because of enzymes, cell metabolism is carried out with greater speed and at lower temperatures than would otherwise be required. Therefore, the breakdown and synthesis of materials in a cell is more efficient.

Cells can manufacture thousands of enzymes. However, cells will only produce enzymes needed for specific reactions. Therefore, not all cells carry all the same enzymes.

Because of the catalytic activities of enzymes, complex chemical processes like the conversion of sugar to release energy can take place. This energy is stored in the bonds of a chemical molecule called ATP. ATP is used for all chemical reactions requiring energy. In cells some of the most important reactions requiring energy are active transport, protein synthesis, DNA synthesis, and cell division.

Considered to be the most important chemical reaction on this planet, photosynthesis is a process that converts the physical energy of light into the chemical energy of ATP and sugar in the form of glucose. Not only do plants undergo photosynthesis, but also some protists (e.g., algae) and some bacteria (e.g., purple sulfur bacteria). Over the history of Earth, the photosynthesis of first bacteria and then algae is largely responsible for Earth’s level of oxygen concentration, which occurred over a period of 1.5 billion years before green plants began to appear.

Glucose, a sugar, is the fundamental energy material of the food chain. In plant and animal cell respiration, mitochondria use glucose to make ATP. Photosynthesis as a process is restricted to plants and a few specialized bacteria and protists. Chloroplasts are the "factories" of carbohydrate production in plants. Chloroplasts are organelles that contain the necessary pigments, named chlorophyll, that will capture light energy. Along with chlorophyll, the chloroplast contains other important chemical compounds, such as enzymes, that are necessary for the synthesis of glucose.

The overall process of photosynthesis requires light, chlorophyll, CO2, and water as basic raw materials to produce glucose. Through a series of complex chemical pathways involving light and dark reactions and two photosystems, I and II, ATP and carbohydrates are synthesized.


Concepts Needed:

Grade 9

Energy, matter, chemical reaction, synthesis, metabolism, energy bonds, enzymes, pigments, carbohydrates

Grade 10

Energy of activation, enzyme action

Grade 11

Anabolism, catabolism, catalyst cofactors in enzyme, allosteric effect, competitive inhibition, feedback mechanisms, ATP synthesis

Grade 12

Grana, stoma, ATP action spectrum, chlorophyll, light, wavelengths


Empirical Laws or Observed Relationships:

The process of photosynthesis, the light and dark reactions, photolysis of water, Calvin-Benson C3 and C4 pathways


Theories or Models:

Enzyme substrate complex, enzyme specificity, induced-fit hypothesis, photosystems I and II, CO2 fixation, bond energy, cyclic photophosphorylation, chemiosmosis, photolysis, clock reactions


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Micro-Unit Description:

Plants, Chloroplasts and Photosynthesis
Students should observe chloroplasts of plant cells, the site of photosynthesis. They should investigate how cells in green plants trap sunlight and convert light energy into the chemical-bond energy of sugar, giving off oxygen as a by-product. The energy in sugar then become available for use by plants and all other living organisms.


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