STPM Biology Structure of Cell and Organelle (Part 12)

Chloroplast of higher plants

Distribution:

• Generally found in all photosynthesizing cells of higher plants.
• Abundant in mesophyll cells, especially palisade mesophyll cells.

Structure:

• Chloroplast exists in various shapes. The common shape is lense shape.
• General size: 5μm (width); 8-10μm (length).
• The fluid matrix of chloroplast is called stroma. The main structure embedded in the stroma are grana (singular = granum) and intergranal lamellae.
• Granum is a stack of thylakoids. A typical chloroplast contains about 40 to 60 grana.
• Neighboring grana are linked by intergranal lamellae.
• Chlorophyll molecules are embedded in thylakoid membranes in groups called photosystems. A photosystem usually consists of about 300 chlorophyll molecules.
• Also embedded in the thylakoid are ATPase enzymes that catalyze ATP synthesis during photosynthesis.
• Stroma also contains starch granules (temporary storage only), DNA, lipid globules and ribosome (70S) and enzymes catalyzing dark reactions.

  

  Structure of Chloroplast

Functions:

• Carry out photosynthesis.
• Temporary storage site for photosynthesis products such as starch.