Tag: cell: the unit of life

Sphaerosomes are single half unit membrane covered small spherical organelles which synthesize and store fats. The cell wall of bacteria is made up of glycocalyx (carbohydrate and amino acids). Chloroplasts are eukaryotic plant cell organelles which have an internal thylakoid membrane system. The thylakoid membranes carry out the light reactions part of photosynthesis. The chromosomes of a eukaryotic cells are called its karyotype. The diagrammatic representation of a cells karyotype is called an idiogram.

Plants are autotrophic organisms which manufacture their own food. Plant cells have chlorophyll containing chloroplasts which are involved in the process of photosynthesis. A plant cell is surrounded by cellulosic cell wall which is dead at maturity. Chloroplasts and cellulosic cell wall are characteristics of plant cells only.

Most of the energy is produced in the mitochondria through the process of oxidative phosphorylation during aerobic respiration. Hence, mitochondria is called powerhouse of cell. Energy is also produced through glycolysis and anaerobic fermentation in cytoplasm but major energy production occurs in mitochondria through aerobic respiration. Chloroplasts have green pigment chlorophyll and carry out the process of photosynthesis which results in production of sugars. This sugar forms food for plant themselves and all the heterotrophic organism. Hence, chloroplast are called 'Kitchen of cell'.

Mitochondria and chloroplasts are unique cell organelles in a eukaryotic cell. Both are double membraned, contain DNA, have their own ribosomes and protein synthesizing machinery. They are semi-autonomous entities. The ribosomes of these organelles resemble prokaryotic ribosomes. These characteristics are seen as an evidences supporting the endosymbiotic theory. The theory of endosymbiosis  postulates that chloroplasts and mitochondria are the result of evolution initiated by the endocytosis of bacteria and blue - green algae. According to this theory, blue green algae and bacteria were not digested; they became symbiotic instead. This concept has been expressed by the phrase, 'A Cell within a cell'. 

In a eukaryotic cell, energy transformation reactions majorly occur in cytoplasm, mitochondria and chloroplast. In cytoplasm, the energy is captured and used to produce ATP by substrate level phosphorylation during glycolysis. In mitochondria, the energy is captured and used to produce ATP by oxidative phosphorylation. In chloroplasts, the energy is captured and used to produce ATP by photophosphorylation. Thus, two cell organelles involved in energy transformation reactions are chloroplast and mitochondria.

Both chloroplast and mitochondria were once independent free living organisms and later on became lodged in eukaryotic cells as endosymbionts. This hypothesis is supported by many evidences example both of these organelles are surrounded by double membranes, have their own DNA which is circular like prokaryotic chromosome, have their own ribosomes which are 70S type like prokaryotic ribosomes and are not dependent on nucleus for their multiplication but can increase in number by fission like bacteria.

Chloroplasts have internal membranes called thylakoids suspended in ground substance called stroma. The thylakoids are of two types : appressed and non-appressed. The appressed membranes are stacked like a pile of coins called grana. The thylakoid membranes enclose small chambers. This thylakoid membrane system is absent in mitochondria. In mitochondria, the inner mitochondrial membrane is folded into cristae.

Internally, the chloroplast has two main components; the thylakoid lamellae and stroma. The stroma contains enzymes of Calvin cycle and carries out the dark reaction. The light reaction is carried out by thylakoid lamellae arranged as grana. Carotenes and chlorophyll pigments are present in the grana. These pigments are involved in capturing light energy for the light reaction. The inner space of mitochondria is filled with mitochondrial matrix.

A eukaryotic cell has DNA in nucleus as well as cell organelles like mitochondria and chloroplasts. The DNA in cell organelles resembles prokaryotic DNA. The organelle DNA is present as a circular molecule while the DNA in nucleus is organised as linear structures.

DNA is a long polymer made from repeating units called as nucleotides. DNA is found in mitochondria, plastid & nucleolus. Eukaryotic organisms like animals, plants, fungi, and protists store most of their DNA inside the cell nucleus and some of their DNA in organelles, such as mitochondria or chloroplasts. In contrast, prokaryotes like bacteria and archaea store their DNA only in the cytoplasm.