Chloroplasts

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Chloroplasts

• by
• Chris Boyce

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Chloroplasts

• Chloroplasts are large, specialised organelles
  found in the photosynthetic tissue of plants.
• They occur in large numbers in the palisade cells
  of the leaves of flowering plants.
• Chloroplasts belong to a group of organelles
  known as plastids, which often contain pigments.
• They contain the pigment chlorophyll which is
  responsible for the green colour of plants.

Plant cell with visible chloroplasts
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Structure

• Chloroplasts are disc-shaped structures, which
  range from 2 10 micrometres in diameter and are
  around 1 micrometre thick.
• They are surrounded by a double membrane, made up
  of the inner and outer membrane, known as the
  chloroplast envelope.
• This chloroplast envelope encloses a gel-like
  matrix known as the stroma, which contains all of
  the important structures needed for chloroplasts
  to function.

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Inside the Chloroplast

• Situated within the stroma is a system of
  flattened membrane sacs called thylakoids.
• These thylakoids appear in stacks called grana.
• Ribosomes, DNA and photosynthetic enzymes are
  also contained in the stroma, along with the
  thylakoids.
• Starch grains and lipid droplets are also stored
  within the stroma.

Diagram showing chloroplast structure
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Function

• Chloroplasts are the organelles that conduct
  photosynthesis.
• The light dependant reactions of photosynthesis
  take place in the membrane system formed by the
  thylakoids.
• The chlorophyll molecules, which have the
  function of trapping light energy, are situated
  on the thylakoids.
• The photosynthetic enzymes in the stroma are
  necessary for the light-independent reactions in
  which carbon dioxide is converted into
  carbohydrates.

Diagram showing photosynthesis
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Summary

• Chloroplasts are large organelles, about 2 - 10
  µm in diameter, that are found in large
  quantities in the palisade cells of plants.
• They have a double membrane called the
  chloroplast envelope, which encloses a gel-like
  matrix called the stroma.
• Thylakoids, ribosomes, DNA, photosynthetic
  enzymes, starch grains and lipid droplets are
  found in the stroma.
• Chlorophyll, located on the thylakoids, traps the
  light energy needed for photosynthesis.
• The photosynthetic enzymes are needed for the
  light-independent reactions that convert carbon
  dioxide into carbohydrates.