Eukaryotic animal cells are enclosed by an outer cell membrane unlike the plant cells that have an external cell wall. This plasma membrane is semi-fluid and selectively permeable in nature. It consists of a phospholipids’ bi-layer with integral proteins embedded on the outer or inner and trans-membrane proteins spanning the entire breadth of the membrane. Important functions of this structural component of the cell include secretion of chemical signaling molecules between two cells, selective entry of chemical messengers, water molecules, minerals and ions into the cytoplasm and protection of the cell as a whole from invading pathogens and foreign particles entering the human body
The plasma membrane also encloses within itself the cytoplasm that contains the cellular organelles. The most important of these is the nucleus. This is a significant organelle occupying the central position in a cell. The nucleus appears as a double walled envelope enclosing thread-like structures within it. Two membranes form the outer covering or nuclear envelope with intermittent pores for excluding signaling molecules into cytoplasm. The inner nucleoplasm contains the chromatin: thread-like fibres of DNA wound around histone proteins. Chromatin threads are extremely coiled and when stretched can reach up to two metres long.
The DNA present inside the nucleus forms the hereditary unit of the cell. It performs the function of life by converting itself into messenger RNA strands based on the inherent genetic code. This mRNA travels outside the nucleus and fixes itself into the ribosomes in Rough Endoplasmic Reticulum for translation. This is how the production of proteins happens inside a cell based on the amino acid sequence indicated by the mRNA template. Ultimately the central dogma of biology is applied. DNA produces mRNA and mRNA in turn synthesizes proteins to carry out the entire functions of a living animal cell.
Closely associated with the nuclear membranes is the Endoplasmic Reticulum (ER).There are two types of endoplasmic reticulum: Rough; ribosomes attached onto their membranes and Smooth; do not have any ribosomes. Likewise the function differs in both types. While Rough ER is responsible for the synthesis of proteins or translation in the cell, Smooth ER is involved in the production and transport of lipid molecules. Though the presence of ribosomes is seen in Rough ER, these assembled structures can also be found suspended freely in the cytoplasm. Ribosomes are the critical requirements for protein synthesis involving mRNA and tRNA.
Apart from ribosomes, the endoplasmic reticulum can have a special feature. It resembles the Golgi apparatus too. Golgi apparatus is the aggregation of convex shaped organelles having the capacity to pinch off vesicles from either of their ends during protein transport or modification. The most important function of Golgi bodies is to aid in the transport and modification of protein molecules synthesized by the endoplasmic reticulum.
Next in hierarchy is mitochondria. Commonly known as the powerhouse of the cell, it generates energy in the form of ATP (Adenosine Triphospate) molecules by the process of respiration. This cellular component has two membranes; outer oval-shaped membrane and an inner convoluted one showing ridges or cristae. While the electron transport chain occurs on the inner membranes of the mitochondria, the Kreb’s or Tricarboxylic acid cycle proceeds within the mitochondrial matrix.
Besides the synthesis, modification and transport of macromolecules, there is a cellular component responsible for detoxification. Throughout the various cellular processes, waste or toxic by-products can be produced by chemical reaction. To control these unwanted chemicals,lysosomes are present. These are spherical bodies containing digestive enzymes to breakdown the organic wastes or excretions of the cytoplasm. These are also called ‘suicidal bags’ or ‘cellular scavengers’.
