what is cell in biology ? | structure and function





what is cell its structure and function ?


The cell: structure and functions -



Cells are the body's smallest functional units.

They are grouped to form tissue, each of which has a specialised function,

For example - blood,muscle,bone.


Different tissues are grouped to form an organ, e.g, the heart, stomach and brain. 


The human body develops from a single cell called the zygote, which results from the fusion of the ovum and the spermatozoa.







A cell consists of a plasma membrane enclosing several organelles, literally 'small organ'.


They include; the nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes and the cytoskeleton.



1- Plasma membrane - the plasma membrane consists of two layers 

of phospholipids with protein and sugars embedded in them. 


Lipid cholesterol is also present. The phospholipid molecules have a head, which is electrically charged and hydrophilic and a tail which has no charge and is hydrophobic.


 




what is cell organelles ?


 2- Organelles - 


a) - Nucleus- All body cells have a nucleus, except mature erythrocytes fibre and some other cells, contains several nuclei. the nucleus contains the body's genetic material in the form of DNA, this directs all its metabolic activities. In a non- dividing cell DNA is present as a fine network of threads called chromatin forms a distinct structure called chromosomes.


b) - Mitochondria - mitochondria are membranous, sausage-shaped 


structure in the cytoplasm, sometimes described as the 'Powerhouse ' of the cell. they are central to aerobic respiration, the processes by which chemical energy is made available in the cell.this is in the form of ATP which release energy when the cell breaks it down.


c) - Ribosomes- these are tiny granules composed of RNA and protein. They synthesis proteins form amino acids, using RNA as the template .when in the cytoplasm, 


the ribosomes units or in small clusters in the cytoplasm, the ribosomes make proteins for use within the cell. these include enzymes required for metabolism.


d) - Endoplasmic reticulum - Endoplasmic reticulum is an extensive series of interconnecting membranous canals in the cytoplasm. 


There are two types - 


 1. Smooth Endoplasmic Reticulum (SER)


  2. Rough Endoplasmic Reticulum (RER)


SER synthesis lipids and steroid hormones and are also associated with the detoxification of some drugs. 


RER is studded with ribosomes. These are the site of synthesis of proteins, some of which are 'exported' from cells. 


For example - enzymes and hormones that leave the parent cell by exocytosis to be used by cells elsewhere.


e) -Golgi apparatus - the Golgi apparatus consists of stacks of closely folded flattened membranous sacs. it is present in all cells but is larger in those that synthesise and export proteins.


f) -Lysosomes - lysosomes are small membranous vesicles pinched off from the Golgi apparatus. They contain a variety of enzymes involved in breaking down fragments of organelles and large molecules.


g) - Cytoskeleton- this consists of an extensive network of tiny protein fibres-


For example - microfilaments, microtubules, centrosomes and cell extensive. 


1) Microfilaments - these are the smallest fibres. They provide structural support, maintain the characteristic shape of the cell and permit contraction, e.g actin in muscle cells.


2) Microtubules- these are larger contractile protein fibres that are involved in the movement of: 


  • Organelles within the cell
  • Chromosomes during cell division
  • Cell extensions


3) - Centrosomes - this direct organisation of microtubules within the cell. It consists of a pair of centrioles and plays an important role in cell division.


4) - Cell extensions - these projects from the plasma membrane in some types of cell and their main components are microtubules which allow movement. They include 

  • Microvilli
  • Cilia
  • Flagella 


a) Microvilli- tiny projections that contain microfilaments.


b) Cilia- microscope hair-like projections containing microtubules that lie along the free borders of some cells.


c) Flagella - single, long whip-like projections, containing microtubules.






 Q what is cell theory ?


  Answer: schleiden and Schwann formulated the cell theory which states that new cells are formed from a pre-existing one