CELL STRUCTURE
Author: Dr. R. Menaka
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Cell is the smallest unit of living organisms. The cell consists of nucleus and cytoplasm. Both are suspended in cytosol or cytoplasmic matrix. The cytosol is made up of microtubules, microfilaments and intermediate filaments. These structures are collectively known as cytoskeleton.
Cell membrane:
The membrane separating the cytoplasm of the cell from surrounding structures is called the cell membrane. The cell membranes are made up of predominantly of lipids. That is the trilaminar structure of membranes which forms the basic frame work of the cell membrane. In addition to molecules of lipids, the cell membrane contains several proteins and carbohydrates. The protein molecules are arranged an each side of the phospholipids molecules i.e. forming a protein -phospholipids sandwich. The proteins of the membrane plays a major role in the structure of the cell membrane. for example: Membrane proteins help to maintain the structure integrity of the cell by giving attachment to cytoskeletal filaments. Cell membranes may show a high degree of specialization in some cells. E.g. The membranes of rod and cone cells bear proteins that are sensitive to light.
Cytoplasm:
The cytoplasm of a typical cell contains various structures that are referred as cell organelles. These are permanent features of cytoplasmic differentiation which vary with the different type of cell.
They include,
1. Mitochondria
2. Ribosomes
3. Endoplasmic reticulum
4. Golgi apparatus
5. Various type of vesicles
6. Cytoskeleton - viz microfilaments, microtubules and intermediate filaments.
Mitochondria
size: Mitochondria vary in size. 0.5 to 2 micrometer in length. The number of mitochondria varies from cell to cell based on their high metabolic rate of the cells. When living cells are stained with supravital dyes (Janns Green B) the mitochondria become visible as small rods or spheres approximately 0.5 to 0.2 micrometer in diameter and up to 12 micrometer in length. Mitochondria are the chief source of ATP (energy) for the cell. At the fine structural level, the mitochondria comprise an outer and inner membrane, an inter membranous space and a matrix space.
Ribosomes
Ribosomes play a major role in protein synthesis. Ribosomes are small electron dense and not visible with the light microscope. The ribosome is made up of two sub units. The subunits involves the coding of protein chain.
Endoplasmic Reticulum
There are two types
a. Rough endoplasmic reticulum which is studded with ribosomes. The rough ER represents the site at which proteins are synthesized.
b. Smooth endoplasmic reticulum which is a prominent feature of cells processing lipids. Smooth ER are the ribosome free terminal portion of rough ER.
Golgi Apparatus
After their synthesis in the rough ER, proteins are transferred to the golgi complex, in which they are modified and packed. Materials synthesized in rough ER travels through the ER lumen into smooth ER. Vesicles budding off from smooth ER transport this material to the Cis face of the golgi complex.
Cytoskeleton
The cytoskeleton favours the cell mobility. e.g by forming cilia.
Nucleus
Shape: Nuclei are spherical to ovoid but they may also be spindle shaped (smooth muscle fiber), bean -kidney shaped (monocytes), mutilobed - in the neutrophils
Number: Usually cells contain only one nucleus but certain cell types contain more than one nucleus. e.g. skeletal muscle cells, oseoclasts.
Location: Location of nucleus within the cell varies and more influenced by functional stage, circadian rhythm and type of tissue.
The nucleus contains inherited information which is necessary for directing the activities of the cell.
E/M - Nucleus is seen to be surrounded by a double layered nuclear membrane or nuclear envelop.
Nucleoli: Nucleoli are larger and more distinct in cells that are metabolically active.
Nuclear Matrix is the insoluble material which is mainly consists of proteins that bind to specific DNA sequences and may participate in DNA transcription and replication.
Nucleolus: The nucleolus is a conspicuous spherical basophilic organelle.
Cellular Inclusions
These don't take part in the vital processes as permanent constituents of cytoplasm but merely occur in it as inert particles or products of metabolism.
example: fat, protein, yolk granules, glycogen, pigment granules and secretory granules.
Nuclear Sap - (Karyolymph) is a proteinaceous substance of gelatinous consistency and homogenous appearance in vivo.
Inter cellular junction:
Most cells in the mammalian organism are attached to other cells to from tissues and organs i.e. also referred as cellular communication. There are 3 major types,
Intercellular - Cell to Cell Junctions:
a. Tight junctions - occluding junctions which forms a barrier between lumen and intercellular space.
b. Gap or communicating junctions- Which mediates movement of molecules between cells.
c. Adhering junctions: These are 3 types, i. Zonula adherens, ii. Desmosome and iii. Hemidesmosome.