Chloroplasts

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Chloroplasts

Cross-section of a Chloroplast
Cross-section of a Chloroplast. Encyclopaedia Britannica/UIG/Getty Images

What Are Chloroplasts?

Photosynthesis occurs in eukaryotic cell structures called chloroplasts. A chloroplast is a type of plant cell organelle known as a plastid. Plastids assist in storing and harvesting needed substances for energy production. A chloroplast contains a green pigment called chlorophyll, which absorbs light energy for photosynthesis. Hence, the name chloroplast indicates that these structures are chlorophyll-containing plastids. Like mitochondria, chloroplasts have their own DNA, are responsible for energy production, and reproduce independently from the rest of the cell through a division process similar to bacterial binary fission. Chloroplasts are also responsible for producing amino acids and lipid components needed for chloroplast membrane production. Chloroplasts can also be found in other photosynthetic organisms such as algae.

Chloroplast: Structure

Plant chloroplasts are commonly found in guard cells located in plant leaves. Guard cells surround tiny pores called stomata, opening and closing them to allow for gas exchange required for photosynthesis. Chloroplasts and other plastids develop from cells called proplastids. Proplastids are immature, undifferentiated cells that develop into different types of plastids. A proplastid that develops into a chloroplast, only does so in the presence of light. Chloroplasts contain several different structures, each having specialized functions. Chloroplast structures include:

  • Membrane Envelope - contains inner and outer lipid bilayer membranes that act as protective coverings and keep chloroplast structures enclosed.​ The inner membrane separates the stroma from the intermembrane space and regulates the passage of molecules into and out of the chloroplast.
  • Intermembrane Space - space between the outer membrane and inner membrane.
  • Thylakoid System - internal membrane system consisting of flattened sac-like membrane structures called thylakoids that serve as the sites of conversion of light energy to chemical energy.​
  • Thylakoid Lumen - compartment within each thylakoid.
  • Grana (singular granum) - dense layered stacks of thylakoid sacs (10 to 20) that serve as the sites of conversion of light energy to chemical energy.​
  • Stroma - dense fluid within the chloroplast that lies inside the envelope but outside the thylakoid membrane. This is the site of conversion of carbon dioxide to carbohydrates (sugar).​
  • Chlorophyll - a green photosynthetic pigment within the chloroplast grana that absorbs light energy.

Chloroplast: Photosynthesis

In photosynthesis, the sun's solar energy is converted to chemical energy. The chemical energy is stored in the form of glucose (sugar). Carbon dioxide, water, and sunlight are used to produce glucose, oxygen, and water. Photosynthesis occurs in two stages. These stages are known as the light reaction stage and the dark reaction stage. The light reaction stage takes place in the presence of light and occurs within the chloroplast grana. The primary pigment used to convert light energy into chemical energy is chlorophyll a. Other pigments involved in light absorption include chlorophyll b, xanthophyll, and carotene. In the light reaction stage, sunlight is converted to chemical energy in the form of ATP (free energy containing molecule) and NADPH (high energy electron carrying molecule). Both ATP and NADPH are used in the dark reaction stage to produce sugar. The dark reaction stage is also known as the carbon fixation stage or the Calvin cycle. Dark reactions occur in the stroma. The stroma contains enzymes which facilitate a series of reactions that use ATP, NADPH, and carbon dioxide to produce sugar. The sugar can be stored in the form of starch, used during respiration, or used in the production of cellulose.

Plant Cell: Structures and Organelles

To learn more about organelles that can be found in typical plant cells, see:

  • Cell (Plasma) Membrane - a thin, semi-permeable membrane that surrounds the cytoplasm of a cell, enclosing its contents.​
  • Cell Wall - outer covering of the cell that protects the plant cell and gives it shape.​
  • Centrioles - organize the assembly of microtubules during cell division.​
  • Chloroplasts - the sites of photosynthesis in a plant cell.​
  • Cytoplasm - gel-like substance within the cell membrane composed of water, enzymes, salts, organelles, and various organic molecules.​
  • Cytoskeleton - a network of fibers throughout the cytoplasm that helps the cell maintain its shape and gives support to the cell.​
  • Endoplasmic Reticulum - extensive network of membranes composed of both regions with ribosomes (rough ER) and regions without ribosomes (smooth ER).​
  • Golgi Complex - responsible for manufacturing, storing and shipping certain cellular products.​
  • Lysosomes - sacs of enzymes that digest cellular macromolecules such as nucleic acids. They are rare in plant cells.​
  • Microtubules - hollow rods that function primarily to help support and shape the cell.​
  • Mitochondria - generate energy for the cell through respiration.​
  • Nucleus - membrane bound structure that contains the cell's hereditary information.​
  • Nucleolus - structure within the nucleus that helps in the synthesis of ribosomes.​
  • Nucleopore - tiny hole within the nuclear membrane that allows nucleic acids and proteins to move into and out of the nucleus.​
  • Peroxisomes - tiny structures bound by a single membrane that contain enzymes that produce hydrogen peroxide as a by-product.​
  • Plasmodesmata - pores or channels between plant cell walls that allow molecules and communication signals to pass between individual plant cells.​
  • Ribosomes - consisting of RNA and proteins, ribosomes are responsible for protein assembly.
  • Vacuole - typically large structure in a plant cell that provides support and participates in a variety of cellular functions including storage, detoxification, protection, and growth.

Source:

  • Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Chloroplasts and Other Plastids. Available from: http://www.ncbi.nlm.nih.gov/books/NBK9905/