Cilia and flagella function in animal cell
1.3.11 Cilia and Flagella
The cell membrane and elongated appendages. They are present on the Cilia (L. cilium, eyelash) and Flagella (L. flagella means whip) are hair-like, and also contain cilia (such as epithelial lining in the respiratory system).
The stationary surface of some cells. They help in the movement of the cell. In some stationary cells, cilia help in the movement of materials over the surface of the cell.
Flagella are five to twenty times longer than cilia.
However, both cilia and flagella have the same internal structure. They are membrane-bounded cylinders. This membrane encloses a matrix. The matrix contains axonemes or axial filaments.
The axonemes consist of nine pairs (doublets) of microtubules, which are arranged in a circle around two central tubules. This arrangement is called the 9+2 pattern of microtubules. Microtubules slide over each other during the movement of cilia and flagella.
Each microtubule has two structures, the dynein arms_ which project towards the neighboring doublets, and spokes which extend towards the center. Dynein can hydrolyze ATP and release energy for ciliary or flagellar movements.
The flagella and cilia originate from the basal body (also called Kinetosome) which is a modified form of the centriole.
The basal body controls the growth of cilia and flagella. Microtubules in the basal body form a 9+0 (9 triplets) pattern. The basal body exhibits a cartwheel structure.
Mechanism of movement:
The movement of these structures is due to the sliding of double fibrils into groups one after the other. (suggested by Bradford, 1955).
Effective stroke:
During effective stroke five out of nine double fibrils contra cilium bends.
Recovery stroke:
As a result During recovery stroke four out of nine double fibrils contract and make the cilium straight.
1.3.12 Nucleus
The nucleus is a double membrane-bound cell organelle of eukaryotic cells. It was discovered by Robert Brown in 1831 in the orchid cell. The nucleus controls all cellular metabolism and contains the genetic information of the cell. The nucleus is considered the controller of the heart or brain of the cell.
It is self self-replicating organelle, that arises from the division of a pre-existing nucleus. Generally, each cell contains one nucleus but sometimes there may be two too many, dikaryotes in Paramecium and many in Opalina.
It is absent in some eukaryotic cells, such as in mature phloem sieve tube elements in plants and mature RBCs of most mammals.
In animal cells, it generally occupies the central space while in the case of plant cells, it is pushed toward the periphery due to the presence of a large central vacuole.
It may be spherical, oval, elongated, or irregular in shape. It is only visible when the cell is in the non-dividing stage. In dividing cells it disappears and chromatin material is replaced by chromosomes.
Structure:
The nucleus of the nondividing cell (inter-phase) consists of a nuclear membrane, nucleoplasm, chromatin net, and nucleolus.
Each nucleus is covered by two parallel membranes with a space between (10-50 nm) called the perinuclear space. It is composed of a protein and lipid bilayer, like a plasma membrane. The outer nuclear membrane is at places continuous with the endoplasmic reticulum while the inner nuclear membrane encloses the nuclear contents.
The ribosomes are also attached to the outer surface of the nuclear membrane. At certain points nuclear membrane is provided by nuclear pores, around the margins of these nuclear pores both membranes are fused with one another.
These pores are also guarded by permeases in the form of a pore complex which regulates RNA, and ionic exchange (i.e., nucleocytoplasmic traffic) between nucleoplasm and cytoplasm. The nuclear membrane is also known as the nuclear envelope.
Nucleoplasm:
Nucleoplasm is the ground substance of the nucleus, which is also known as a nuclear matrix or karyoplasm.
Chemical composition of nucleoplasm:
It is a transparent complex colloidal form of solution or fluid containing water, protein, and enzymes like ATPase, DNA and RNA polymerases, and endonucleases. It also contains nucleotides and mineral ions.
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