The vacuoles are non-cytoplasm areas bounded by a single membrane bilayer present in the cytoplasm. Large vacuoles are characteristic of matured plant cells.
Young or growing plant cell contains many small vacuoles which coalesce to form a large central vacuole. Similarly animal cells have either many very small vacuoles or totally absent. In a mature plant cell the central vacuole may occupy about 90% of the cell volume.
As a result the cytoplasm is pressed against the cell membrane as a thin layer and the nucleus becoming lateral. The, membrane surrounding the vacuole is known as tonoplast and the aqueous solution inside is called cell sap or vacuolar sap.
This sap contains digestive enzymes and ions, metabolites and waste products. The tonoplast is differentially permeable and regulates the movement of ions and metabolites into the vacuole. Like the lysosomes the pH inside the vacuole is slightly lower than surrounding cytoplasm.
This lower pH inside the vacuole is maintained by pumping in of hydrogen ions by hydrogen pump present in tonoplast membrane. High concentration of salts, sugar and many water soluble pigments are present in vacuole.
Pigments like anthocyanin present in the vacuole gives the plant parts its specific color. (Deep purple or red) The vacuole originates from the fusion and enlargement of small vacuoles present in meristematic cells which are believed to originate from endoplasmic reticulum. Functions
1. Storage of reserve food like sucrose.
2. Stores and concentrates minerals.
3. As they contain solute in high concentration water enters the vacuole resulting in an outward turgor pressure on the cytoplasm and the cell wall. This results in turgidity of the cell.
4. Store waste products
5. Contain water-soluble pigments to impart coloration to the plant parts.
6. Some plant vacuoles have hydrolytic enzymes acting at acidic pH. These vacuoles function like lysosomes.
7. Secondary metabolites like Tannin, latex etc. are stored in vacuoles.
8. Contractile vacuoles found in some protists and algal cells take part in osmoregulation and excretion.
9. Gas vacuoles or pseudo vacuoles or air vacuoles found in prokaryotes provide buoyancy and also mechanical strength.
Cytoskeleton is a cellular 'scaffold" of "Skeleton" contained in the cytoplasm of eukaryotic cell . This is a dynamic structure of extensive net-work acting as the skeleton and muscle of the cell, for movement of stability.
They are also involved with distribution and orientation of cell organelles and cellular division. Eukaryotic cells contain three main kinds of cytoskeleton filaments. They are Microfilaments, intermediate filaments and microtubules.
Microfilaments / Actin filaments
Microfilaments are long, narrow cylindrical protein filaments of about 7 nm in diameter. Being the thinnest of the cytoskeleton filaments, they are called as microfilaments.
These filaments are formed by a family of proteins called actin proteins and for this reason these filaments are also known as actin filaments. Monomers of actin protein form long thin chains like 'strings of pearls'. Two chains of actin closely twin around each other to form a filament.
These filaments are mostly concentrated below the plasma membrane, to maintain cellular shape and in some cases form cytoplasm protuberances (like pseudopodia and microvilli). Functions
1. By forming a band below the plasma membrane they maintain the shape of the cell and also provide strength to the cell.
2. Generate locomotion in some cells like W.B.C. and amoeba (Pseudopodia formation)
3. Interact with myosin muscle fibers for muscle contraction.
4. Link Trans membrane proteins (e.g. surface receptors) to cytoplasm proteins.
5. Anchors centrosomes at the opposite poles of cell during cell division.
6. Cytoplsmic streaming movement is cause by the action of microfilaments.
7. Form cleavage furrows at the time of cytokinesis.
These are filaments of 8 - 11 nanometers in diameter, more stable than actin filaments and form heterogeneous constituents of cytoskeleton. These filaments arc constituted by fibrous protein molecules twined together in an overlapping arrangement. There are four types of intermediate filaments:
Vimetins: common structural support of many cells. Provide mechanical strength to muscle and other cells.
Keratin: Found in skin cells, hair and nails, form tonofibrils of desmosomes.
Neurofilaments: Found in axons and dendrons of nerve cells; strengthen the long axons and dendrons of nerve cells; strengthen the long axons of neurons.
Hamin: gives structural support to nuclear envelope.
The nucleus in epithelia cells is held within the cell by a basket-like network of intermediate filaments made up of keratins. Different kinds of epithelial cells use different keratins to build up their intermediate filaments Functions
1. Provide support and strength to cell membrane and nuclear envelope.
2. Form a skeletal network in the cytoplasm.
3. Found as constituents of hair, nail and skin (Keratin).
4. Tonofibrils support the desmosomes.
5. Neurofilaments strengthen the axons.
6. Provide mechanical strength to muscle (vimetins)