Cell - The Unit of Life Test 15

A plant cell has rigid wall on the outside. It has plastids and a centrally located large vacuole. All of these are absent in an animal cell.

Golgi complex consists of a stack of cisternae which are curved to give definite polarity to the Golgi apparatus. The convex side (forming/cis face) receives vesicles from endoplasmic reticulum. The concave side (maturing /transface) buds off vesicles as secretion.

Ribosomes may-occur singly as monosomes or in rosettes and helical groups called polyribosome. The different ribosomes are connected with a 10-20A thick strand of mRNA. They are formed during periods of active protein synthesis when a number of copies of the same polypeotide are required.

Golgi apparatus was discovered by Camillo Golgi in nerve cells of barn owl and cat.

Cell wall is the outer rigid protective and supportive covering of plant cells,fungi and some protists. Cell membrane is covering of protoplasm which occurs inside the cell wall. 

Ribosomes are naked ribonucleoprotein protoplasmic particles (RNP) which function as a site for protein synthesis.

According to Robertson's concept of unit membrane, the cell membrane is trilaminar. It has a thickness of about 75Å with a central lipid layer of 35Å thick and two peripheral protein layers of 20Å each.

Proteins have very specific shapes which make them ideal as receptor molecules for chemical signalling between cells. Branching side chain glycolipids on the outer surface of cell membranes are also involved in cell-cell recognition.

In tracellular membrane is involved in establishing compartments in the cells and provides organization of enzymes and pigments. It provides a channel for nutrient distribution. Rigidity to the cell is provided by cell wall.

Plasmodesmata are cytoplasmic bridges between adjacent plant cells which develop in minute pores of their walls. They form a protoplasmic continuum called symplast.
It provides channels for controlled passage of small sized particles between adjacent cells as well as transfer of some specific signals.

The lipid molecules are amphipathic, i.e., they possess both polar hydrophilic (water loving) and nonpolar hydrophobic (water repelling) ends. The hydrophilic region is in the form of a head occurring at outer surface while the hydrophobic part contains two tails of fatty acids. Hydrophobic tails usually occur towards the centre of the membrane. It results in the formation of a lipid bilayer. Most common lipid in the bilayer is phospholipid.

Lysosomes are single membrane bound vesicles which contain hydrolytic enzymes. They are believed to be formed by the joint activity of ER and Golgi complex. Precursors of hydrolytic enzymes are synthesized at RER and are transferred to the forming face of Golgi complex. These precursors are then changed to enzymes. Enzymes are packed in larger vesicles and are pinched off at maturing face. These vesicles combine with endosomes to produce lusosomes.

The SER provides surface for the synthesis of  including phospholipids, cholesterol, steroid hormones and hormones, adrenal corticoid hormones), ascorbic acid are visual pigments.

Succinate dehydrogenase, anenzyme complex involume in TCA cycle, is located in inner mitochondrial membrare.

ER functions as cytoskeleton or skeletal framework by providing mechanical support to colloidal cytoplastic matrix. It conducts information from cell to cell. RER helps in protein synthesis. SER takes part in detoxification of tox chemicals in liver.

Glyoxysomes are microbodies which contain enzyres for β-oxidation of fatty acids and glyoxylate pathway. They are considered to be special peroxisomes. They occurs abundantly in germinating endosperm cells of oil seeds.

The lysosomes may be called "suicide bags’ of the cell in view of their autolytic role, or "disposal units’ of the cell because they digest the incoming food materials and remove the foreign bodies, toxic molecules, and debris, or "recyling centres" as they break down worn out cells cell organelles to component molecules for building organelles and cells.

Nucleolus is a dense, rounded, dark-staining, granular structure without a limiting membrane. It consists largely of RNAs and proteins. Nucleolus syntesizes and stores RNA. It also receives ribosomal proteins from the cytoplasm for storage. It forms ribosomal subunits by wrapping the RNAs with nitosomal proteins. The ribosomal subunits later have the nucleus through the nuclear pores.

Sphaerosome, lysosome and glyoxysome are bounded by single membrane.

A covering membrane is absent around ribosomes, centrioles and nucleolus.

Chloroplasts and mitochondria are semi-autonomous cell-organelles possessing their own DNA and proteins. In animals an individual gets mitochondria and mitochondrial. genes from its mother because the middle piece of a sperm that contains mltochondria does not enter the egg during fetilization. This inheritance pattern of mitochondrial genes is called uniparental inheritance or extra nuclear inheritance or maternal inheritance.

In many cells proteins released from the endoplasmic reticulum are combined with carbohydrate to produce complexes like glycoproteins. This occurs in both Golgi complex and rough endoplasmic reticulum as they contain enzymes called glycosyl-transferases.

Autolysis, caused by the lysosomal enzymes, plays a role in the disappearance of tail during the metam orphosis of frog's tadpole, and in softening of gum tissue to allow eruption of teeth in vertebrates. For autolysis, enzymes of primaryly so somes are  released from the cell. Autolysis occur in ageing, dead and diseased cells.

Dictyosome is a cup-shaped structure of flattened membranous vesicles found in plant cell. They modify proteins from the ER, and may also polymerise sugars to  polysaccharides. They then package these materials for delivery to destinations within the cell, for secretion, or for storage.

Ribosomes are granular structures which were discovered by Robinson and Brown (1953) in plant cell and by Palade (1955) in animal cell. Palade (1955) also coined the term ribosomes hence, they are also called Palade particles. Ribosomes occur in both prokaryotic and eukaryotic cells.

Acentriole possesses a whorl of nine peripheral fibrils. Fibrils are absent in the centre. The arrangement is, therefore, called 9 + 0. Flagella contains an axoneme of a peripheral doublet fibrils and 2 central singlet fibrils. This arrangement is called 9 + 2 or 11 stranded.

Chemically, ribosomes consist of two parts, proteins and rRNA. Proteins are both structural and enzymatic.

Peroxisomes are found in photosynthetic cells and perform photorespiration. They also take part in lipid metabolism. Lysosomes are the cells' garbage disposal system.

Camillo Golgi discovered the Golgi apparatus as reticular structure. Typically it is 0.5 - 1.0 pm in diameter but its shape and size depend upon the physiological state of the cell.

RBCs lack nucleus and may other cell organelles thus they are best suited to study cell membrane without any complication.