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6.9.2.5.1. Chloroplasts

Chloroplasts

The chloroplasts are a type of cromoplastos that are located in the cell plant photosynthesis and the algae green.

They can be moved by amoeboid and contractile movements, and are placed in the part of the cell that receives the greatest amount of light. Photosynthetic algae usually have one large chloroplast, but plant cells have a few dozen.

The internal environment of the chloroplast is called the stroma, and it is composed of a solution of carbohydrates, lipids, protides, nucleic acids, pigments, nucleotides, salts and elements.

Its main function is to capture light energy and use it in the synthesis of organic matter.

Structure of chloroplasts

Chloroplasts are made up of a double membrane (external and internal), an intermembrane space and an interior space or stroma, where the thylakoids are found, in the shape of flattened saccules. In the chloroplast it is distinguished:

  • External and internal membrane. Its structure is similar to that of the rest of the membranes (60% are lipids and 40% are proteins). The outer membrane contains porins, making it highly permeable, and the inner membrane, which is less permeable, has specific transport proteins that regulate the passage of substances between the hyaloplasm and the stroma. They lack chlorophyll and, as in mitochondria, these membranes also have no cholesterol .
  • Intermembrane space. Its composition is very similar to the cytosol, due to the permeability of the outer membrane.
  • Thylakoids and scarlet. They are isolated or interconnected flattened saccules, similar to a pile of coins forming a membranous internal network. Each of these stacks is called grana, with a variable number of bags. Thylakoid membranes contain everything necessary for photosynthesis .

38% are lipids, 50% proteins and 12% pigments (carotenoids and chlorophylls). Proteins can be classified into three groups:

    • Proteins associated with pigments : they form large complexes, integrated into the membrane.
    • Electron transport proteins: as in mitochondria, although they transport electrons from waterto NADP, which is reduced.
    • ATP-synthetase, similar to that of the inner mitochondrial membrane .
  • Stroma It is the central space of the chloroplast. Contains inside:
    • double-strandedcircular DNA molecule, encoding chloroplast protein synthesis.
    • Ribosomes, ( plastoribosomes) of 70S, such as those of mitochondria and bacteria.
    • Enzymes, of two types:

Chloroplast functions

The main functions of chloroplasts are:

Two phases of photosynthesis are distinguished:

    • Luminous (obtaining energy). It occurs in lamelas and sprinkles. They depend directly on the light. As a product, you get ATP (chemical energy), and NADPH (a powerful reducer). Solar energy activates an electron in chlorophyll, which moves along a transport chain. The energy obtained is used to pump protons through the thylakoid membrane. The electrochemical gradient that is created is used to synthesize ATP. Chlorophyll recovers the lost electrons, by breaking the water molecule, which oxidizes, leaving oxygen free. The last electron acceptor in the chain is NADP, which is reduced to NADPH.
    • Dark (fixation of CO2). It occurs in the stroma. These reactions can be carried out in the dark, since they do not depend directly on light. ATP and NADPH, obtained in the light phaseare used as an energy source and as a reducing agent, respectively, transforming inorganic compounds (carbon dioxide) into organic compounds (carbohydrates).
  • Fatty acid biosynthesis. They use carbohydrates, NADPH and ATP synthesized in the light phase of photosynthesis.
  • Reduction of nitrates to nitrites. Nitrites are reduced to ammonia, which is the nitrogen source for the synthesis of amino acids and nucleotides.

Chloroplasts in all plant cells?

Although chloroplasts are unique to eukaryotic plant cells and it is one of the main differentiating characteristics of animal cells, not all plant cells have chloroplasts. For example, plant cells in the root will not have chloroplasts or photosynthesize.

Similarities and differences between mitochondria and chloroplasts

  • Similarities between mitochondria and chloroplasts:
    • At a structural level:
      • Mitochondria and chloroplasts have a double membrane (outer membrane, inner membrane) and an intermembrane space that separates them.
      • Mitochondria and chloroplasts present internal membranous folds intended to increase the membrane surface (mitochondrial ridges and thylakoid grains).
      • Mitochondria and chloroplasts have an internal space (mitochondrial matrix and stroma) that contains:
        • Circular double-stranded DNA, similar to bacterial.
        • 70 S ribosomes, similar to bacterial ones. These ribosomes synthesize the necessary proteins specific to the mitochondria and chloroplast respectively.
      • Mitochondria and chloroplasts have multi-enzyme complexes that form electron transport chains with which ATP is produced.
    • At a functional level:
      • Mitochondria and chloroplasts are the energy transducing organelles, producing ATP from ADP and Pi.
      • Mitochondrial DNA replicates in the mitochondrial matrix and chloroplast DNA in the stroma.
      • Both reproduce by bipartition, have electron transport chains, synthesize ATP, their own proteins, etc.
  • Differences between mitochondria and chloroplasts
    • At a structural level:
      • Mitochondria have an inner mitochondrial membrane with folds called mitochondrial ridges, while chloroplasts have stacked thylakoids within the stroma, with a much larger surface area.
      • Chloroplasts contain the pigment chlorophyll that gives them their green color. In contrast, the mitochondria, no.
      • Chloroplasts have photosystems I and II. In contrast, the mitochondria, no.
    • At a functional level:
      • Mitochondria intervene in catabolism (aerobic respiration) transforming the chemical energy of organic molecules into ATP usable by cells. They produce energy from the oxidation of nutrients through catabolic processes. They perform the Krebs cycle.
      • Chloroplasts are involved in autotrophic anabolism, transforming energy from the sun into chemical energy from organic biomolecules. They synthesize organic molecules through anabolic processes. They do the Calvin cycle.
      • Mitochondria need oxygen to produce energy. Chloroplasts produce oxygen as a waste substance by making organic molecules.
      • Mitochondria are in eukaryotic cells (animal and plant), chloroplasts only in plant cells.

Basic ideas about chloroplasts

The chloroplasts are organelles unique to plants eukaryotic cells photosynthetic and the  algae  green.

The chloroplasts are composed of: