Cell Structure and Function

  • Cell is the fundamental and structural unit of life.
  • All living organisms are composed of cells.

  • Amoeba, chlamydomonas, paramoecium and bacteria are unicellular organism.
  • In multicellular organisms, many cell group together to form various body parts and perform different functions.
  • Cells divide to produce cells of their own kind.
  • Amoeba can change their shape.

Structural organisation of Cell

  • Each cell has specific components within it called cell organelles.
  • Cell organelles perform specific functions inside a cell.
  • All cells are found to have the same organelles.
  • Interaction of cell with its environment is possible due to cell organelles.
  • Plasma membrane or cell membrane is the outermost covering of the cell separating the contents of the cell from external environment.


  • Plasma membrane is selectively permeable.
  • Carbon dioxide and oxygen can move across the cell membrane by diffusion.
  • Diffusion is the process by which substance moves from a region of its high concentration to a region of low concentration
  • Osmosis is the process by which water moves across a selectively permeable membrane from a region of low concentration to a region of low concentration
  • Osmosis is a special type of diffusion.
  • If the medium surrounding the cell has more water concentration then water will move inside the cell and the medium is said to be hypotonic solution.
  • If the medium surrounding the cell and the cell has equal water concentration then water will move inside as well as outside the cell at same rate and thus there will be no net movement of water, such type of a medium is called isotonic solution.

  • If the medium surrounding the cell has less water concentration then the water will move outside the cell and the medium is called hypertonic solution.
  • Cell when placed in a hypertonic solution will swell
  • Cell placed in a isotonic solution will not change its size.
  • Cell placed in a hypertonic solution will shrink.
  • Plant roots absorb water by diffusion.
  • The plasma membrane is flexible and is made up of proteins and lipids.
  • Endocytosis is a process by which amoeba engulfs its food and other material from external environment due to flexibility of plasma membrane.

Animal Cell
Animal Cell


  • Along with plasma membrane or cell membrane plant cells have another outer rigid covering called the cell wall.
  • The plant cell wall is mainly composed of cellulose.
  • Plasmolysis is the phenomena in which the contents of the cell contract and move away from the cell wall when plant cell looses water due to shrinkage.
  • Cell walls allow cells of plants, fungii and bacteria to withstand very dilute external media without bursting, i.e cell walls can allow cells to withstand much greater changes as compared to the animal cells
Plant cell
Plant cell


  • When cells are stained by iodine solution or safranin solution, different parts of the cells get differently coloured according to their chemical composition
  • Nucleus has a double layered covering called nucleus membrane.
  • Transfer of materials from inside of the nucleus to its outside takes place through pores present on nucleus membrane.
  • Nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
  • Chromosome have DNA ( Deoxyribo Nucleic Acid) molecules which have information of inheritance.
  • DNA is present as a part of chromatin material in a cell which is not dividing.
  • Nucleus plays an important role in cell reproduction and determining the way in which the cell will develop and the form it will exhibit at maturity by directing chemical activities.
  • An poorly defined nuclear region containing only nucleic acid is called nucleoid.
  • Prokaryotes are organisms lacking nucleus membrane.
  • Eukaryotes are organisms having nucleus membrane.
  • Prokaryotic cells also lack other cytoplasmic organelles present in eukaryotic cells.


  • Cytoplasm is the fluid content inside the plasma membrane.
  • Cytoplasm has many specialised cell organelles that perform special functions.
  • Cell organelles also have membranes.
  • Prokaryotes also lack membrane bound cell organelles.
  • Viruses lack any membranes and thus do not show characteristics of life until they enter a living body.


  • A cell organelle is a specialized sub-unit within a cell that has a specific function.
  • Organelles are either separately enclosed and may called as membrane-bound organelles or are spatially distinct functional units without a membrane.

Endoplasmic reticulum

  • The endoplasmic reticulum is a large network of membrane-bound tubes and sheets.
  • Endoplasmic reticulum is similar in structure to plasma membrane.
  • There are two types of Endoplasmic reticulum – Rough Endoplasmic Reticulum(RER) and Smooth Endoplasmic Reticulum(SER)
  • Ribosomes attached to the surface of Rough endoplasmic reticulum makes it look rough under a microscope.
  • Ribosomes are site for manufacture of protein.
  • The Smooth Endoplasmic Reticulum helps in manufacture of fat molecules or lipids.
  • Membrane biogenesis is the process by which cell membrane is manufactured using some proteins and lipids.
  • Other proteins and lipids function as enzymes.
  • Endoplasmic reticulum serves as channels for transfer of materials between various regions of the cytoplasm and between cytoplasm and the nucleus.
  • Endoplasmic reticulum also functions as cytoplasmic framework, providing a surface for biochemical activities.
  • Smooth endoplasmic reticulum detoxifies poisons and drugs.

Golgi apparatus

Golgi apparatus
Golgi apparatus
  • Golgi apparatus consists of a system of membrane-bound vesicles arranged approximately parallel to each other in stacks called cristens.
  • The material synthesised at Endoplasmic reticulum is packaged and dispatched to various targets inside and outside the cell through golgi apparatus.
  • Golgi apparatus makes complex sugars from simple sugar.
  • Golgi apparatus is also involved in formation of lysosomes.


  • Lysosome keeps the cell clean by digesting any foreign material as well as worn out cell organelles.
  • Lysosomes contain powering digesting enzymes capable of breaking down all organic material.
  • When the cell gets damaged the lysosomes may burst and the enzymes digest their own cells
  • They are structurally membrane bound sacs filled with digestive enzymes
  • Lysosomes are known as the suicide bag of the cell.
  • The digestive enzyme present in Lysosome is made by Rough Endoplasmic Reticulum.


  • Mitochondria is the power house of the cell as it produces the energy required for various chemical activities needed for life.
  • Energy released in mitochondria is released in the form of ATP(Adenosine Triphosphate)
  • ATP is the energy currency of the cell.
  • Mitochondria has two membranes, the outer membrane is porous while the inner membrane is deeply folded providing surface for generation of ATP.
  • Mitochondria has its own DNA and ribosome and make their own protein.


  • Plastids are present only in plant cells.
  • Leucoplasts are colourless plastids.
  • Chromoplasts are coloured plastids.
  • Plastids containg the pigment chlorophyll is called chloroplasts.
  • Chloroplasts also contains orange and yellow pigment.
  • Leucoplasts are primary organelles in which starch, protein and oil is stored.
  • Plastids contain various membrane layers embedded in a material called storma.
  • Plastids also have their own DNA and ribosome.

Test on Class 9th Biology on Fundamental Unit of Life:CELL


  • Vacuoles are storage sacs for solid and liquid contents.
  • Vacuoles present in animal cells are smaller as compared to vacuoles present in plant cells.
  • In plant cells vacuoles are full of sap and provide rigidity and turbidity to the cell.
  • Vacuoles store amino acids, sugars and proteins.
  • In some unicellular organisms, specialized vacuoles play important role in expelling excess water and some waste from the cell.

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