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7.4B: Chloroplasts

Skills to Develop

  1. Briefly describe chloroplasts and state their function.
  2. State where in the chloroplasts the pigments and the electron transport chains needed to convert light energy into ATP are located.

Chloroplasts (see Fig. 41) are disk-shaped structures ranging from 5 to 10 micrometers in length. Like mitochondria, chloroplasts are surrounded by an inner and an outer membrane. The inner membrane encloses a fluid-filled region called the stroma that contains enzymes for the light-independent reactions of photosynthesis. Infolding of this inner membrane forms interconnected stacks of disk-like sacs called thylakoids, often arranged in stacks called grana. The thylakoid membrane, that encloses a fluid-filled thylakoid interior space, contains chlorophyll and other photosynthetic pigments as well as electron transport chains. The light-dependent reactions of photosynthesis occur in the thylakoids. The outer membrane of the chloroplast encloses the intermembrane space between the inner and outer chloroplast membranes (see Fig. 41).

The thylakoid membranes contain several pigments capable of absorbing visible light. Chlorophyll is the primary pigment of photosynthesis. Chlorophyll absorbs light in the blue and red region of the visible light spectrum and reflects green light. There are two major types of chlorophyll, chlorophyll a that initiates the light-dependent reactions of photosynthesis, and chlorophyll b, an accessory pigment that also participates in photosynthesis. The thylakoid membranes also contain other accessory pigments. Carotenoids are pigments that absorb blue and green light and reflect yellow, orange, or red. Phycocyanins absorb green and yellow light and reflect blue or purple. These accessory pigments absorb light energy and transfer it to chlorophyll.

They are found in plant cells and algae. Like Mitochondria, chloroplasts are surrounded by two membranes. The outer membrane forms the exterior of the organelle while the inner membrane folds to form a system of interconnected disclike sacs called thylakoids. The thylakoids are arranged in stacks called grana. The space enclosed by the inner chloroplast membrane is called the stroma. Chloroplasts replicate giving rise to new chloroplasts as they grow and divide. They also have their own DNA and ribosomes.

The thylakoid membranes contain the pigments chlorophyll and carotenoids, as well as enzymes and the electron transport chains used in photosynthesis (def), a process that converts light energy into the chemical bond energy of carbohydrates. Energy trapped from sunlight by chlorophyll is used to excite electrons in order to produce ATP by photophosphorylation. The light-dependent reactions that trap light energy and produce the ATP and NADPH needed for photosynthesis occur in the thylakoids. The light-independent reactions of photosynthesis use this ATP and NADPH to produce carbohydrates from carbon dioxide and water, a series of reactions that occur in the stroma of the chloroplast.

 

Concept map for Eukaryotic Cell Structure

Summary

  1. Chloroplasts are disk-shaped structures ranging from 5 to 10 micrometers in length. Like mitochondria, chloroplasts are surrounded by an inner and an outer membrane.
  2. Chloroplasts carry out photosynthesis, the process of converting light energy to chemical energy stored in the bonds of sugar.
  3. Chloroplasts replicate giving rise to new chloroplasts as they grow and divide. They also have their own DNA and ribosomes.

Questions

Study the material in this section and then write out the answers to these questions. Do not just click on the answers and write them out. This will not test your understanding of this tutorial.

  1. Briefly describe chloroplasts and state their function. (ans)
  2. State where in the chloroplasts the pigments and the electron transport chains needed to convert light energy into ATP are located. (ans)

Contributors

  • Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)