Cytoplasm

Describe The Structure Of A Generalized Cell

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Structure Of A Generalized Cell

Though cells may differ in shape and size, all of them have the same basic structure, referred to as the generalized cell structure or generalized cell. A cell has both living and non-living parts. The living parts of the cell that have a definite shape, structure, and function are called organelles.

The main parts of a generalized cell are cell membrane, cytoplasm, and nucleus.

Cell membrane: It is the outer covering of a cell. It is a porous membrane through which selected substances can enter or leave the cell. The cell membrane is also called plasma membrane.

Cytoplasm: It is a jelly-like fluid that fills up the part of the cell between the cell membrane and the nucleus. Several organelles are embedded in the cytoplasm.

Nucleus: It is considered to be the brain of the cell. Nucleus is surrounded by a membrane called the nuclear membrane and is filled with a semi-solid substance called the nucleoplasm. Thread-like structures called chromosomes are present in the nucleoplasm. Chromosomes carry genes, which transfer the characteristics of a cell to the new cells that are formed during cell division. Also present inside the nucleus is a round granule called nucleolus, which contains a network of fibrous material called chromatin fibres.

Figure shows a generalized cell. It is important to note that some of the organelles shown in figure may be present only in plant cells and not in animal cells and vice versa.

Structure of a Generalized Cell 1
Structure of a Generalized Cell

Let us learn about the cytoplasmic organelles shown in figure.

Golgi apparatus (or Golgi body): It is made up of tubules (long tube-like structures), vesicles (small vessels), and vacuoles and is responsible for the secretion of chemical substances like enzymes, hormones, and proteins.

Vacuole: It is a fluid-filled space enclosed in a membrane. Vacuoles store excess water, useful minerals, pigments, and many other substances.

Lysosome: Lysosomes contain chemical substances called enzymes that are capable of digesting cells and a variety of intra- and extra-cellular materials [intra, internal (inside); extra, external (outside)]. In times of emergency, lysosomes burst and destroy the cell. Hence, they are also called suicide bags of the cell.

Endoplasmic reticulum :It is a network of tubules and channels and is involved in the synthesis, storage, and transport of cell products.

Ribosomes: These are small granular structures scattered in the cytoplasm that act as sites of protein synthesis.

Centrosome: This structure is present only in animal cells. The main function of centrosomes is to initiate and regulate cell division.

Plastids: These are present only in plant cells. Plastids contain certain pigments that have a specific role to play in the functioning of the plant. Depending on the pigment colour, plastids are of three kinds: chloroplasts, chromoplasts, and leucoplasts.

  • Chloroplasts contain the green pigment chlorophyll, which helps plants in making food.
  • Chromoplasts contain non-green pigments that give colour to flowers and fruits.
  • Leucoplasts are colourless and store food in the form of carbohydrates, fats, and proteins.

Mitochondria (singular mitochondrion): These are tiny spherical or rod-shaped bodies. They act as sites of energy production and are, therefore, called the powerhouses of the cell.

Which Components form the Structure of the Plasma Membrane

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Which Components form the Structure of the Plasma Membrane

Ultra Structure of Cells

Which Components form the Structure of the Plasma Membrane 1
Which Components form the Structure of the Plasma Membrane 2
Plasma Membrane :

Introduction :
Cell surface in all the cells is enclosed by a living membrane which is called cell membrane by
C. Nageli and C. Kramer (1855).
Historical Account :
J.Q. Plower (1931) coined the term Plasmalemma for cell membrane.

Ultrastructure :                

  • Plasma membrane forms outer covering of each cell.
  • It is present in both plant and animal cells.
  • Plasma membrane is a living, thin, delicate elastic, selectively permeable membrane.
  • It separates contents of a cell from the surrounding medium.

Fluid Mosaic Model :

  • In 1972, Singer Nicolson proposed this model. According to this, cell membrane consists-two layers of phospholipid molecules, phospholipid & protein molecules are arranged as a mosaic.
  • Phospholipid molecules have their polar heads  directed outward non polar tail pointing inward.
  • The proteins are of two types ­:
  • Peripheral and integral. Peripheral proteins are located superficially while integral proteins are embeded in the phospholipid matrix. The protein monolayer have elasticity & mechanical support to the lipid matrix.
    Which Components form the Structure of the Plasma Membrane 3

Functions of Plasma Membrane :

  • The main function of plasma membrane is to regulate the movement of molecules inside and outside the cell.
  • It allow the movement of gaseous substance from high concentration to low concentration which known as diffusion.
  • Water also obeys the low of diffusion . The movement of water molecule through a selectively permeable membrane is called osmosis.
  • The flexibility of cell membrane also enables the cell to engulf in food, which is also known as endocytosis. For example – in Amoeba

Cell wall :
In plants, one another rigid is called ‘Cell wall’. It is made up of cellulose which provide structural strength to plant.

Function of cell wall :

  • It maintains the shape of cell.
  • It protect the cells from mechanical injury & prevents their desication.
  • It provide mechanical support against gravity. It is due to rigid cell walls that the aerial part of plant are able to keep erect & expose their leaves to sunlight.  
  • Cell walls permit the cells to with stand very dilute external media without bursting.

Nucleus : It is the most important part of cell which control all the activities of cell.

Structure of Nucleus  : 

  • The nucleus has a double layered covering called nuclear membrane.
  • The nuclear membrane has pores inside the nucleus to its outside, that is, to the cytoplasm.
  • The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
  • Chromosomes are composed of DNA and protein.
  • Functional segments of DNA are called genes.
  • In a cell which is not dividing, this DNA is present as part of chromatin material.

Function of nucleus :

  • It play a important role in cellular reproduction.
  • DNA contain the information necessary for constructing & organizing cells.
  • Nucleoid – In some organisms nuclear region of cell may be poorly defined due to the absence of nuclear membrane. Such an undefined nuclear region called nucleoid.

Note :

  • Prokaryotic cell – Cell which do not have well defined nuclear region . called prokaryotic cells. Pro – Primitive   Karyon –  nucleus
  • Eukaryotic cells – Cells which have well defined nuclear region, called eukaryotic cells.
  • Along with nucleus membrane, prokaryotic cells lack most of cell organells.

Cytoplasm :
The fluid & semifluid matrix of  a cell between the nucleus & the plasma membrane, containing various organelles is called cytoplasm.

Cell organelles :
Small membrane bound structures, which perform a lot of chemical activities to support the function & structure of a cell, called cell organelles –