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Comparison of Animal cell with Precellular Forms of life

Functional systems of the cellThe following are the important functions of cell that make it a living thing.

1-Ingestion by the cell-Endocytosis

Includes:- 1-Pinocytosis 2-Phagocytosis2-Digestion by lysosomes

3-Excretion (Exocytosis)4-Synthesis: a)RER (protein synthesis) b)SER (Lipid synthesis) c)Golgi apparatus (Galactose, Hyaluronic acid)

5-Energy Extraction by Mitochondria

6-Bactericidal action by lysosomes and peroxisomes

7-Transport

EndocytosisThis is a process by which different substances are engulfed by the cell.

It is the reverse of exocytosis.Types:- 1-Pinocytosis:- Pinocytosis is the ingestion of extremely small vesicles containing extracellular fluid.

This is the process by which cell ingest ECF and its contents.

It is involved in the formation of minute invaginations of the cell membrane.

It occurs continuously at the cell membrane.This process occurs rapidly by the macrophages.

Pinocytic vesicles are very small, only 100-200 nm in diameter.

By this process some macromolecules, proteins can enter the cell.

On the surface of CM receptors are present which are specific for the types of protein that are to be ingested.

Once the protein molecules are bound with the receptor, the surface properties change.

The CM invaginates inwards and contractile proteins cause its border to close over the attached protein along with ECF, forming pinocytic vesicles.

It can cross the cell in 2-3 minutes.

2-PhagocytosisThis is a process by which there is

ingestion of large particles such as bacteria and proteins of damaged tissues.

It occurs in the same way as pinocytosis.Only certain cells have a capability of

phagocytosis, most important cells are tissue macrophages and neutrophils.

Pre-requisits are:-1-The proteins or large poly saccharides on the surface of particles that are phagocytosed.

2-The surface of particles should be rough like dead tissues, foreign particles or bacteria.

Phagocytosis occurs in following steps:-

1-The CM receptors attach to the surface of particle.

2-The edges of the membrane around the points of attachment evaginate outwards within a fraction of a second to surround the particle.

After it more membrane receptors are attached to the particles.

Digestion by LysosomesCell can digest different substances with the help of enzymes.

Lysosomes contain hydrolytic enzymes.

ExocytosisRemoval of substances by the cells is called exocytosis.

It is the reverse of endocytosis .This occurs when residual material in an endocytic vesicle is discharged from the cell.

It is involved in cellular secretion of gland cells.

The substances that are secreted by the cell move from ER to golgi apparatus.

They are then extruded into secretary granules .

The granules move toward the CM, which fuses and leaves the contents of granules or vesicles outside the cell.

This process requires Ca and energy.

Synthesis Specific functions of ER:-1-Proteins formation by RER- Protein molecules are synthesized inside the ribosomes.It extrude some protein into cytosol but mostly into the endoplasmic matrix.

Lipid formation by SER:- SER mostly forms phospholipids

and cholestrol.These go inside the membrane of ER, causing it to grow more extensive.

Occurs mainly in SER.Also small ER vesicles or transport vesicles continuously breaks off from it and go to GA.

Other functions of ERProvides enzymes that control

glycogen breakdown when glucose is used for energy.

Provides vast number of enzymes which detoxify substances such as drugs

It achieves detoxification by coagulation, oxidation, hydrolysis, conjugation with glycuronic acid etc

Specific functions of GAProcessing of already formed substances.

Capable of forming carbohydrates that cannot be formed by ER e.g Hyaluronic acid, chondroitin sulfate.

Functions of HA and CS1-Major component of proteoglycans secreted in mucus and other glandular secretions.

2-Major portion of ground substance, outside cell in interstitial spaces, acting as filler b/w collagen fibers and cells

3-Principal component of organic matrix in cartilage and bone.

Processing by GAThe transport vesicles fuse with GA and empty their secretions here.

More carbohydrates are added to secretions.

It compact the ER secretions into highly concentrated packets.

Release in the cell in the form of lysosomes and secretary vesicles.

SV are abundant in secretary cells, which attaches to CM and release its secretions outside by exocytosis.

This exocytosis is stimulated by Ca ions.

The membranous system of ER and GA represents a highly metabolic organ capable of forming new intracellular structures as well as secretory substances to be extruded from the cell.