Cell Organelles

Cell organelles are cellular components present inside the cells. It is a subcellular (present within the cell) structure that performs one or more specific jobs in the cell.

Cell organelles on the basis of membrane

The cell organelles includes membranous as well as non- membranous bound organelles. There are different types of organelles present inside the cell, these may be classified into three categories on the basis of the presence or absence of a membrane.

  1. Cell organelles without membrane: These are cell walls, ribosomes, and cytoplasm. These organelles are present in both prokaryotic and eukaryotic cells.
  2. Cell organelles with single membrane-bound: These include lysosome, vacuole, Golgi bodies, and Endoplasmic reticulum. These organelles are present only in eukaryotic cells.
  3. Double membranous cell organelles: These include mitochondria, nucleus, and chloroplast. present only in the eukaryotic cells.

Each cell possesses three common features plasma membrane, nucleus, and cytoplasm, which help in its internal activities as well as in its interaction with the external environment. 

eukaryotic cell

Eukaryotic cell

Plasma membrane

Each cell, whether a plant cell or an animal cell, is covered by a thin membrane called a plasma membrane or cell membrane.

The plasma membrane is made up of proteins and lipids. It is a living membrane, the outermost covering in animal cells but inner to the cell wall in the plant cell. It is a living, delicate, elastic, and selectively permeable membrane.

Functions of plasma membrane:

As we know it is selectively permeable and helps in the movements of certain substances, it also helps in providing the shape of some cells such as RBCs, nerve cells, etc.

The plasma membrane isolates cell contents from the external medium. It helps in maintaining the internal composition of the cell which is necessary to sustain life. 

Movement of Substances Across the Cell Membrane:

The movement of substances through the plasma membrane or cell membrane takes place by the following method:

  1. Active Transport
  2. Passive Transport: Movement of molecules towards the concentration gradient without using any energy, e.g. diffusion, osmosis. 

  • Diffusion: molecules of substances move from the region of higher concentration to the lower concentration. For this energy is not required, Ex: absorption of glucose in a cell.
    • The difference between solute molecules is called the concentration gradient.
  • Osmosis: During osmosis, water molecules move from the region of their higher concentration to the region’s lower concentration through a semipermeable membrane, and energy is not required for this. Example:
    • Opening and closing of stomata.
    • Absorption of water from the soil by root hair
    • Conduction of water in the plant body
    • Cell drinking by Amoeba and Paramecium.

  • Active Transport– in the active transport direction of movement of a certain molecule is reverse to diffusion i.e. from the region of their lower concentration to the region of their higher concentration, it requires energy. In this case, energy is provided by adenosine triphosphate i.e. ATP.

Several models of Plasma membranes were proposed regarding the arrangement of proteins and lipids by many biologists but The fluid mosaic model is widely accepted. In 1972, Singer and Nicolson explained the Ultrastructure of the plasma membrane by the fluid mosaic model.

Fluid Mosaic Model

(i) The plasma membrane is composed of a lipid bilayer of phospholipid molecules into which various globular proteins are embedded.
(ii) Each phospholipid molecule has two ends, an outer head hydrophilic means water-attracting, and an inner tail pointing centrally hydrophobic, i.e. having a nature of water-repelling
(iii) The protein molecules are shown in two different ways as-
(a) Peripheral proteins or extrinsic proteins: these are proteins present on the outer and inner surfaces of the lipid bilayer.
(b) Integral proteins or intrinsic proteins: These are proteins that penetrate the lipid bilayer partially or completely.

Fluid mosaic model of plasma membrane

fluid mosaic model of plasma membrane

Nucleus: The Largest Cell Organelle

The nucleus is the largest cell organelle in a eukaryotic cell. It is double membranous and found in all eukaryotic cells.

  • The nucleus is the control center of cellular activities and it is functioning as the storehouse of the DNA of the cell. It is structurally dark and round, surrounded by a nuclear membrane.
  • The nucleolus is found inside the nucleus. It carries chromosomes.

The nucleus consists of the following parts:

Nuclear Envelope: The nucleus is surrounded by a double-layered covering called a nuclear membrane or nuclear envelope. It acts as a barrier and separates the cytoplasm of a cell from the nuclear contents. It contains small pores, called nucleopores, which allow the transfer of selective material from inside the nucleus to the outside cytoplasm.

Nucleoplasm: The nucleus is filled with a transparent, granular, liquid matrix called nucleoplasm. The nuclear components such as nucleolus and chromatin material are embedded in it.

Nucleolus: The small spherical body present at one side of the nucleus. These are not bounded by any membrane and may be one or more in number. It is rich in proteins and ribonucleic acid. It is the site of ribosome synthesis. 

Chromatin Material: The thin, thread-like intertwined mass of filaments present in the nucleoplasm comprises chromatin material. 


The cytoplasm is the fluid portion of the cell present between the plasma membrane and the nuclear envelope and contains many cell organelles some help in the release of energy and some in synthesis and movement. eg. mitochondria, Golgi bodies, etc.

Functions of Cytoplasm:

  • It gives the shape of the cell.
  • The cytoplasm contains dissolved nutrients and helps in dissolving water substances.
  • It helps the materials to move around the cell by cytoplasmic streaming.
  • The cytoplasm is the site of most cellular activities.
  • It consists of Cytosol, Cytoskeleton, and Cytoplasmic Inclusions.

Mitochondria– (Singular = mitochondrion)

Mitochondria are found in both animal and plant cells. Mitochondria contain their own DNA and RNA so they can self-duplicate to produce their own kind without the help of the nucleus. Some important facts about mitochondria are:

  • They release energy
  • In size approximately 0.5 to 1.00 μm (micrometer) having a double membranous wall
  • The inner membrane is folded inside and forms projections termed ‘cristae’ which project into the inner side compartment known as the ‘matrix’.
  • They Oxidise pyruvic acid which breakdown the product of glucose and takes place to release energy stored in the form of ATP for ready use.
  • This process of ATP formation in mitochondria is called cellular respiration. Mitochondria are also called the ‘powerhouse’ of a cell as it releases energy.

Mitochondria in a cell



Plastids are found in plant cells only and may be colored or colorless. These are characteristic features of plant cells and a few protists. These are absent in animal cells. 

  • Like mitochondria, plastids also consist of numerous membrane layers which are embedded in a matrix called the stroma. 
  • They possess their own DNA and ribosomes and also can synthesize their own protein.

Plastids are of three types as-

  • Leucoplast -These plastids are of white color or colorless
  • Chromoplast –The chromoplasts may be of the color blue, red, yellow, etc.
  • Chloroplast – The chloroplasts are green-colored organelles. 

Functions of Plastids: 

Photosynthesis: Chloroplast performs photosynthesis with the help of chlorophyll. It converts solar energy into chemical energy and synthesizes carbohydrates using CO2 and H2O.

Pigments: Chomoplasts consist of different kinds of pigment which provide distinguishing colours to the fruits, flowers, and other parts of the plant that store food.

Synthesis of fatty acids: Chloroplasts contain certain enzymes which are essential for the synthesis of fatty acids.

Storage Organs: Leucoplasts store certain materials such as starch, protein, and oils.


The chloroplast is found in the cytoplasm of green plant cells. It may be disc-shaped or laminate as in most plants cell. In some ribbon-shaped for eg. in an alga Spirogyra or cup-shaped as in alga Chlamydomonas.

Some important facts about chloroplast:

  • wall of the chloroplast is a double membrane
  • A fluid medium called stroma is filled in the inner side of the chloroplast.
  • Chloroplast contains their own DNA and RNA so they can self-duplicate to produce their own kind without the help of the nucleus.
  • Chloroplasts are the site of photosynthesis as plant synthesize their food by the process (with the help of carbon dioxide and water in the presence of sunlight sugar is synthesized by plants).

Endoplasmic reticulum

The Endoplasmic reticulum (ER) is a single membrane-bound structure having a thickness between 50 – 60A°. It is found scattered in the cytoplasm. Its membranes form a continuous sheet that encloses a fluid-filled space called ER lumen.

There are two types of ER found in a cell–

  1. Rough Endoplasmic Reticulum (RER)
    • Rough endoplasmic reticulum (RER) i.e. when ribosomes are attached to ER, and Smooth endoplasmic reticulum (SER) when no ribosomes are attached to ER
    • The ribosomes are the site of protein synthesis and thus are present in all active cells.
    • The synthesized protein is then transported by ER to the different parts of the cell based on their requirements.
  2. Smooth Endoplasmic Reticulum (SER)
    • These look smooth under the microscope because of the absence of any ribosomes.
    • SER is a site for steroid synthesis and stores carbohydrates.
    • SER is found in those cells which are engaged in the secretion, synthesis, or metabolism of fats.

The functions of the Endoplasmic Reticulum:

  • The network of the endoplasmic reticulum provides mechanical support to the cytoplasm.
  • ER serves as a channel for the transport of materials between different parts of the cytoplasm or between the nucleus and the cytoplasm.
  • RER helps in protein synthesis, while SER helps in lipid synthesis.
  • The proteins and lipids synthesized by the ER membranes help in the synthesis of cellular membranes. This process is called biogenesis.
  • The membranes of SER consist of numerous enzymes which catalyze various biochemical reactions.
  • The SER brings about the detoxification of many drugs and toxic substances in the liver cells of vertebrates.
  • The membranes of ER are the site of ATP synthesis and act as a source of energy for the metabolism and transport of materials within the cells.

    Golgi bodies

    The Golgi bodies are single membrane-bound structures. They vary in shape and size. Camillo Golgi (1898) first observed dense stained reticular structures near the nucleus, those are named after him i.e. Golgi bodies.

    Some important facts about Golgi bodies:

    • In animal cells, Golgi bodies are present around the nucleus and are 3 to 7 in number.
    • In-plant cells are many in number and present scattered throughout the cell termed dictyosomes.
    • Golgi bodies consist of many flat, disc-shaped sacs called cisternae which play a significant role as it modifies and packs proteins and polysaccharides
    • As Golgi bodies package proteins into membrane-bound vesicles inside the cell then the vesicles are transported to their recognized sites.
    • The Golgi apparatus is the most important site for the formation of glycoproteins and glycolipids


    The Ribosomes are spherical in shape and do not have membranes. Ribosomes are involved in the synthesis of proteins in the cell.

    Some important Facts about ribosomes:

    • The ribosomes are spherical in shape and about 150 – 250 Å in diameter,
    • Ribosomes are made up of large molecules of RNA and proteins called ribonucleoproteins
    • Ribosomes are present as free particles in the cytoplasm or attached to the ER.
    • Also found stored in the nucleolus inside the nucleus. 80S types are found in eukaryotes
    • The 70S is found in prokaryotes
    • The ribosomes are the site for protein synthesis
    Lysosome-(lysis = breaking down)

    Lysosomes are found in almost all animal cells and some non-green plant cells. They consist of many enzymes capable to digest carbohydrates, proteins, lipids, and nucleic acids, by nature these are phagocytic and they digest their own damaged cell so these are also called “suicidal bags”.


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    Rajeev Ranjan · August 20, 2020 at 2:04 pm


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