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4th Feb 2021

STPM Biology Structure of Cell and Organelle (Part 17)

Vacuole

Distribution:

• Found in plant cells.
• Animal cells do not contain vacuoles.

Structure:

• Varies in shape and sizes
• Each vacuole in bounded by a membrane called tonoplast.
• Vacuole is filled with fluid that consist of amino acids, sugars, minerals, waste products and pigments.

Structure of vacuole

Functions:

• Provides turgor pressure to support cell.
• Temporary storage site for amino acids, sugars and waste products.
• Important role in the growth of plant cells.
• As protection. Vacuoles of some plants contain toxins which deter animals from eating the plants. 

STPM Biology Structure of Cell and Organelle (Part 15)

Centriole

Distribution:

• Found in almost all animal cells.
• Not found in plant cells except for lower plant cells such as fungi, algae and bryophytes.

Structure:

• Shape and size: short cylinder; 500nm long and 200nm in diameter.
• A centriole consists of triplets of microtubules in a ‘9+0’ arrangement.
• Neighboring microtubule triplets are connected by fibrils.

Structure of centrioles

Functions:

• Initiates cell division (mitosis and meiosis). cell division begins when centrioles move towards opposite poles.
• Produces spindle during mitosis and meiosis. Spindles are microtubules.
• Involved in the formation of cilia and flagella.

STPM Biology Structure of Cell and Organelle (Part 14)

Microfilaments

Distribution:

• Randomly distributed in the cytoplasm of cells.

Structure:

• Microfilaments are long, fine protein filaments with a diameter of 6-7nm.
• Microfilament consists of two chains of actin molecules.

Cytoskeleton

Functions:

• Pinocytosis, phagocytosis and exocytosis involve microfilaments.
• Main component of the cytoskeleton. Thus, microfilaments are responsible for maintaining the shape of the cell and providing mechanical support for cell.
• Contraction of microfilaments along the equatorial plane enables cytokinesis to take place in animal cells.
• Actin microfilaments of skeletal muscles enable muscle contraction.

Besides microtubules and microfilaments, yet another filament that forms the cytoskeleton is the intermediate filament. Intermediate filament (8-12nm in diameter) is fibrous protein.

Cytoskeleton in cells have these functions:

1. Provides mechanical support for cells.
2. Maintains the shape of the cell.
3. Site for organelles to adhere.