BIOLOGY- CELLULAR SYSTEMS

The framework of life

Life is maintained by a network of chemical reactions. These reactions have reactants and products. The reactants have higher chemical potential and drives the formation of the products. Enzymes are also a kind of driver of the reactions. Chemical potential is determined by chemical thermodynamics. Chemical reactions can be considered as a form of power relationship between chemical compounds. Reactants have the power to drive the reaction. The living organism is a network of these power relationships. The overall network is metabolism and it has both anabolic and catabolic pathways.

     The cells in addition communicate between these pathways by chemical signals. These signals modify the rate or choose alternate pathways to adapt to availability of nutrients or toxins. The metabolic pathways and chemical signals constitute a power framework. Organisms evolve by changing this framework by mutations in the genetic molecules,a component of the pathway itself.The entire pathway of metabolism and life is encoded in the genetic code. It is the code which determines how the the framework is organised. In metazoans cells form organs. The function of these organs is to make oxygen available through respiration or circulation,make food available through ingestion ,digestion and circulation and to remove waste products outside the body. These organs create a physiological framework of interdepedance of  chemicals between them. They are also subject to change and evolution by mutations in developmental chemical networks

Major metabolic systems : 

The major functions of a cell is organised into systems. The system consists of metabolic pathways in a certain senetwork. These networks arise by genetic evolution and holds the key to life and its further evolutionary characteristics. The origin and evolution of these systems is therefore the key to life.

Cell replication system

There are a number of cellular systems which are based on interdependent reactions and coordinated by signals. The first is the cell replication system. The cell undergoes four phases in cell division ,the S phase of DNA replication and M phase of chromosome separation and cytoplssmic division. In between are G1 phase before S phase and G2 phase before M phase. Transitions between stages are promoted by cycle specific protien kinase enzymes and proteins called cyclins. On comming  together kinases phosphorolyse a number of enzymes required for DNA synthesis and make them active.The cyclin for S phase is cycles D In this system other proteins which play a role are in amplification in S phase are RB1 and E2F which phosphorylase cyclin E.Cyclins for M phase is cyclin A and B.The cell cycle is under control by a number of different systems ,such as WNT in development,P13/AKT system,RAS system, MYC system.

DNA repair system

DNA frequently gets damaged by extrinsic radiation,free radicals and this requires either repair of DNA or apoptosis. The signals of DNA damage are detected by a protien ATM along with BRCA1 which phosphorylase a protien p53. P53 .Other detection proteins are CHEK1 and CHEK 2(checkpoint proteins). Once activated p53 it causes cell cycle arrest by inhibiting cycling’s,causes apoptosis and stimulates DNA repair proteins. The level of P53 is brought down by protiens MDM2 and PPM1D.Apart from p53 there are other cyclin dependent kinase inhibitors such as p18 and p14.

Apoptosis and cell death

Apoptosis or programmed cell death plays a role in embryonic development as well as elimination of damaged cells.  Extrinsic stimuli cinitiate apoptosis by TNF and Fas whereas intrinsic signals arise from defective DNA by a protien BCL2. BCL2 is directly responsible for increasing mitochondrial wall permeability and cell death. During this process intra cellular enzymes called caps ashes are activated which lyse cell components.

Tissue homeostasis

Tissue homeostasis means controll of cell number by replacing cell death or initiating cell proliferation where required for growth. The proteins in this group have membrane receptors like TGF ‘beta, and subsequent phosphatases SMAD-4 which initiate nuclear transcription for cell proliferation. Another protien which increases cell biomass by nucleotide biosynthesis, ribosome biogenesis,RNA processing and DNAsynthesis is MYC.

Control of cell energetics

The cells need to switch their anabolic and catabolic metabolism in response to availability of nutrients and oxygen. The protien mTOR regulates protin synthesis. It is phosphorylase by AMPK which phosphorylase’s when nutrients are low.

The immune system

The immune system is a system of interdependent cells communicating with each other to protect the body against foreign pathogens. It has evolved to protect the body against extra cellular pathogens as well as intra cellular viruses and bacteria. The receptor for cytokines involved in immune reaction occur on cell surface. This phosphorylase’s the JAK protiens which in turn phosphorylase’s STAT proteins. The receptor complex so formed enters the nucleus and stimulates proteins for immune reaction.

Morphogenetic system

A large number of receptor pathway are involved in communicating between cells during embryonic development. . Thus the WNT /APC protien signals another protien Beta catenin which is responsible for nuclear synthesis and stem cell proliferation.Beta catenin also causes cell adhesion by protien Cadherin.There is the RAS family of transmembrane receptors. These phosphorylase RAS and RAF proteins. These are important in intra cellular coordination of synthesis of membranes and micro tubules.Another receptor family is receptor tyrosine kinases and cytoplasmic tyrosine kinases which control cell proliferation and differentiation.The notch and hedgehog proteins are also involved in morphogenesis.

Neuron system

The neurotransmitters involved in neural communication include surface neurotransmitter receptors and intra cellular protien kninases which determine the bias of these receptors . Also protien kinases determine synthesis of neural proteins like actin and control adaption of memory. Drug’s used in psychiatry stimulate proteins; kinases and change bias to transmitters. Thus in depression anti depressants cause increased

 protien synthesis and response to the low level of mono amines in synaptic cleft.

Ack: principles of cancer genetics