Cell is a fundamental unit of life notes

Cell is a fundamental unit of life notes (part 2)

Vacuoles

Vacuoles are the fluid filled cavity in cytoplasm. These are covered by membrane called Tonoplast. Its fluid is called cell sap. Vacuoles can occupy up to 90 per cent of the volume of the cell. It has both organic and inorganic substances. In plants, the Tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole; hence their concentration is significantly higher in the vacuole than in the cytoplasm.

In Amoeba the contractile vacuole is important for excretion. In many cells, as in protists, food vacuoles are formed by engulfing the food particles. They includes amino acid, protein etc. its function is to keep the turgidity of cell, Tonoplast act as semi permeable membrane. Vacuoles in amoeba help in excretion.

Plastid

The term plastid was used by the A.F.W. Schimper in 1885. He classified the plastid into three types that are:

Leucoplast—these are colourless plastids that are found in the embryonic & germ cells. They are also found in that region which not receive light. They store food materials as carbohydrate (Amyloplast) protein (Proteinoplast) &fat (Eleioplasts).

Chromoplast-these are coloured plastid containing carotenoids & other pigments. They impart colour to certain portion of plants such as petals, fruits etc.

Chloroplast—the chloroplast is the most abundant pigment molecules that provide green colour. It is the site where process of photosynthesis occur that results formation of food for plant body.

Microtubules

These are very fine microscopic fibre that made up of protein. Microtubules provide strength, it brings about movement inside the cell, and they help in formation of spindle fibres during cell division.

Centrosome

It is found in the animal cells and present near the nucleus. It concerned with the cell division and formation of basal bodies of cilia and flagella.

Micro bodies

Many membrane bound minute vesicles called micro bodies that contain various enzymes, are present in both plant and animal cells.

Nucleus

Nucleus is a cell organelle that was first described by Robert Brown in 1831.The nucleus is bounded by double membrane, porous, selectively permeable nuclear envelope.

• It is filled with a jelly like substance called nucleoplasm.
• A network of thread like structure in it is called chromatin that occurs around the nucleolus.
• Electron microscopy has revealed that the nuclear envelope, which consists of two parallel membranes with a space between (10 to 50 nm), called the peri-nuclear space, forms a barrier between the materials present inside the nucleus and that of the cytoplasm.
• The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it. Nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm.
• Normally, there is only one nucleus per cell, variations in the number of nuclei are also frequently observed. The nuclear matrix or the nucleoplasm contains nucleolus and chromatin.

Functions: nucleus controls the functioning of cell, it helps in the transmission of characters from parents to the offspring and it also regulates the cell cycle.

CHROMOSOME

During different stages of cell division, cells show different structured chromosomes in place of the nucleus. Chromatin contains DNA and some basic proteins called histones, some non-histone proteins and also RNA. A single human cell has approximately two metre long thread of DNA distributed among its forty six (twenty three pairs) chromosomes.

Based on the position of the centromere, the chromosomes can be classified into four types. The metacentric chromosome has middle centromere forming two equal arms of the chromosome. The sub-metacentric chromosome has centromere nearer to one end of the chromosome resulting into one shorter arm and one longer arm. In case of acrocentric chromosome the centromere is situated close to its end forming one extremely short and one very long arm, whereas the telocentric chromosome has a terminal Centromere.

Types of cells

prokaryotes

The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia like Organisms). They are generally smaller and multiply more rapidly than the eukaryotic cells. They may vary greatly in shape and size.

• The four basic shapes of bacteria that are bacillus (rod like), coccus (spherical), vibrio (comma shaped) and spirillum (spiral).
• Organisation of the prokaryotic cell is fundamentally similar even though prokaryotes exhibit a wide variety of shapes and functions.
• All prokaryotes have a cell wall surrounding the cell membrane. The fluid matrix filling the cell is the cytoplasm.
• There is no well-defined nucleus. The genetic material is basically naked, not enveloped by a nuclear membrane.
• In addition to the genomic DNA (the single chromosome/circular DNA), many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids. The plasmid DNA confers certain unique phenotypic characters to such bacteria that is resistance to antibiotics. Plasmid DNA is used to monitor bacterial transformation with foreign DNA.
• Nuclear membrane is found in eukaryotes. No organelles, like the ones in eukaryotes, are found in prokaryotic cells except for ribosome. Prokaryotes have something unique in the form of inclusions. A specialised differentiated form of cell membrane called mesosome is the characteristic of prokaryotes. They are essentially infolding of cell membrane.

Eukaryotes

The eukaryotes include all the protists, plants, animals and fungi. In Eukaryotic cells there is an extensive compartmentalisation of cytoplasm through the presence of membrane bound organelles.

• Eukaryotic cells possess an organised nucleus with a nuclear envelope. In addition, Eukaryotic cells have a variety of complex locomotory and cytoskeletal structures.
• Their genetic material is organised into chromosomes. All eukaryotic cells are not identical.
• Plant and animal cells are different as the former possess cell walls, plastids and a large central vacuole which are absent in animal cells. On the other hand, animal cells have centrioles which are absent in almost all plant cells.

Difference b/w Plant and Animal cell

PLANT CELL

ANIMAL CELL