Cell – The Unit of Life Part -II (EUKARYOTIC CELLS)

As we have read definition of cells and 1st type of cell (Prokaryotic). in this post we gonna read about 2nd type of human or animal cells (eukaryotic cells).
If you haven’t read its first part CLICK HERE.



– They have well organized membrane bound nucleus and organelles (endoplasmic reticulum (ER), Golgi complex,lysosomes, mitochondria, microbodies etc).

– There is a clear compartmentalization of cytoplasm due to the membrane bound organelles.

– They have complex locomotory & cytoskeletal structures.

– Their genetic material is organized into chromosomes.

– Eukaryotes include protists, plants, animals and fungi.

Cell organelles in eukaryotic cells

1. Cell Membrane

Chemical studies on the human RBCs showed that the cell membrane is composed of a lipid bilayer.

The lipids are arranged within the membrane with the outer polar head and the inner hydrophobic tails. This ensures that the non-polar tail of saturated hydrocarbons is protected from the aqueous environment.

– The lipid component of the membrane mainly consists of phosphoglycerides.

Cell membranes also possess protein and carbohydrate.

Ratio of protein and lipid varies in different cells. E.g. In human RBC, membrane has 52% protein and 40% lipids.

Depending on the ease of extraction, membrane l proteins are 2 types:

  • Integral proteins : Partially or totally buried in membrane.
  • Peripheral proteins : Lie on the surface of membrane.

    Fluid mosaic model of cell membrane : Proposed by Singer & Nicolson (1972). According to this, the quasifluid nature of lipid enables lateral movement of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity.


  • Transport of the molecules. The membrane is selectively.
  • Due to the fluid nature, the plasma membrane can help in cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc.

    Types of Transport

    1. Passive transport: It is the movement of molecules across the membrane along the concentration gradient (i.e., from higher concentration to the lower) without the expenditure of energy.

    It is 2 types:

    A. Simple diffusion: It is the movement of neutral solutes across the membrane.

    B. Osmosis: It is the movement of water by diffusion across the membrane.

    As the polar molecules cannot pass through the non-polar lipid bilayer, they require a carrier protein of the
    membrane to facilitate their transport.

    2. Active transport: It is the movement of molecules across the membrane against the concentration gradient (i.e. from lower to the higher concentration) with the expenditure of energy (ATP is utilized). E.g. Na+/K+ pump.

    2. Cell Wall

    It is a non-living rigid structure found outer to the plasma membrane of fungi and plants.

    Cell wall of Algae is made of cellulose, galactans, mannans and minerals like CaCO3. In other plants, it consists of cellulose, hemicellulose, pectins and proteins.

    Cell wall of a young plant cell (primary wall) is capable of growth. It gradually diminishes as the cell matures and the secondary wall is formed on the inner side (towards membrane) of the cell.

    The middle lamella is a layer mainly of calcium pectate which glues the different neighbouring cells together.
    The cell wall and middle lamellae may be traversed by plasmodesmata which connect the cytoplasm of neighbouring cells.


    A. It gives shape to the cell.

    B. It protects the cell from mechanical damage & infection.

    C. It helps in cell-to-cell interaction.

    D. It acts as barrier to undesirable macromolecules. permeable to some molecules present on either side of it.

    3. Endomembrane System

    It is a group of membranous organelles having coordinated functions.

    They include endoplasmic reticulum (ER), Golgi complex, lysosomes and vacuoles.


    • These are a network of tiny tubular structures scattered in the cytoplasm.
    • ER divides the intracellular space into 2 compartments: luminal (inside ER) & extra luminal (cytoplasm).

    Endoplasmic reticulum is 2 types:

    A. Rough endoplasmic reticulum (RER): Bear ribosomes on their surface. RER is frequently observed in the cells actively involved in protein synthesis and secretion. They are extensive and continuous with the outer membrane of the nucleus.

    B. Smooth endoplasmic reticulum (SER): Ribosomes are absent. SER is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesized in SER.


    Densely stained reticular structures near the nucleus.

    First observed by Camillo Golgi (1898).

    They consist of flat, disc-shaped sacs (cisternae) of 0.5 – 1.0 micrometer diameter. These are stacked parallel to each other.

    Cisternae are concentrically arranged with convex cis (forming) face and concave trans (maturing) face. Cis &
    trans faces are totally different, but interconnected.

    Function of Golgi apparatus: –

    • Packaging materials, to be delivered either to the intracellular targets or secreted outside the cell. Materials to be packaged in the form of vesicles from the ER fuse with the cis face and move towards the maturing face. This is why the Golgi apparatus remains in close association with the endoplasmic reticulum.
    • Proteins synthesized by ribosomes on the ER are modified in the cisternae of the Golgi apparatus before they are released from its trans face.
    • Golgi apparatus is the important site
      of formation of glycoproteins and


    – These are membrane bound vesicular structures formed by the process of packaging in the Golgi apparatus.

    – Lysosomal vesicles contain almost all type’s of hydrolytic enzymes (hydrolases– lipases, proteases, carbohydrases). They are active at the acidic pH. These enzymes can digest carbohydrates, proteins, lipids and nucleic acids.


    – These are the membrane-bound space found in the cytoplasm. It contains water, sap, excretory product and other materials not useful for the cell.

    – Vacuole is bound by a single membrane called tonoplast.

    – In plant cells, the vacuoles can occupy up to 90% of the volume of the cell.

    – In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole; hence their concentration is significantly higher in the vacuole than in the cytoplasm.

    – In Amoeba, the contractile vacuole is important for excretion. In many cells, as in protists, food vacuoles are formed by engulfing the food particles.


    – Mitochondria are clearly visible only when stained.

    – Number, shape and size of mitochondria per cell are variable depending on the physiological activity.

    – It is sausage-shaped or cylindrical having a diameter of 0.2 -1.0 micrometer (average 0.5 micrometer ) and length 1.0 – 4.1 micrometer.

    – A mitochondrion is a double membrane-bound structure with the outer membrane and the inner membrane. It divides lumen into 2 aqueous compartments, i.e., the outer compartment and the inner compartment (matrix).

    – Inner membrane forms a number of infoldings (cristae) towards the matrix. They increase the surface area.

    – The two membranes have their own specific enzymes associated with the mitochondrial function.

    – Matrix possesses a circular DNA, a few RNA molecules, ribosomes (70S) and components for protein synthesis.

    – The mitochondria divide by fission.


    Mitochondria are the sites of aerobic respiration. They produce energy in the form of ATP. So they are called ‘power houses’ of the cell.


    – Plastids are found in all plant cells and in euglenoides.

    – Large sized. Easily observable under the microscope.

    – They contain some pigments.

    – Based on the type of pigments, plastids are 3 types:

    A. Chloroplasts: Contain chlorophyll and carotenoid pigments. They trap light energy for photosynthesis.

    B. Chromoplasts: Contain fat soluble carotenoid pigments like carotene, xanthophylls etc. This gives a yellow, orange or red colour.

    C. Leucoplasts: These are colourless plastids of varied shapes and sizes with stored nutrients.

    They include:

  • Amyloplasts: Store starch. E.g. potato.
  • Elaioplasts: Store oils and fats.
  • Aleuroplasts: Store proteins.

    – These are double membrane bound organelles mainly found in the mesophyll cells of the leaves.

    – These are lens-shaped, oval, spherical, discoid or ribbon like organelles.

    – Length: 5-10 mm. Width: 2-4 mm.

    – Their number varies from 1 (e.g. Chlamydomonas, a green alga) to 20-40 per cell in the mesophyll.

    – Inner membrane of chloroplast is less permeable.

    – The space limited by the inner membrane of the chloroplast is called the stroma. It contains a number of organized flattened membranous sacs called thylakoids.

    – Membrane of thylakoids encloses a space called lumen.

    – Chlorophyll pigments are present in the thylakoids.

    – Thylakoids are arranged in stacks called grana or the intergranal thylakoids.

    – There are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana.

    – The stroma contains small, double-stranded circular DNA molecules, ribosomes and enzymes for the synthesis of carbohydrates and proteins.

    – The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).

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    – These are the non-membranous granular structures composed of ribonucleic acid (RNA) and proteins

    – It is first observed by George Palade (1953).

    – Eukaryotic ribosome has 2 subunits- 60S (large subunit) and 40S (small subunit). They together form 80S.


    – It is a network of filamentous proteinaceous structures present in the cytoplasm.

    – It provides mechanical support, motility, maintenance of the shape of the cell etc.


    – They are hair-like outgrowths of the cell membrane.

    – Cilia are small structures which work like oars, causing the movement of either the cell or the surrounding fluid.

    – Flagella are comparatively longer and responsible for cell movement.

    – Flagella of prokaryotic bacteria and eukaryotes are structurally different.

    – Cilium and flagellum are covered with plasma membrane. Their core (axoneme) possesses a number of microtubules running parallel to the long axis.

    – The axoneme usually has 9 pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. This is known as 9+2 array.

    – The central tubules are connected by bridges and are also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublet by a radial spoke. Thus, there are nine radial spokes. The peripheral doublets are also interconnected by linkers.

    – Both the cilium and flagellum emerge from centriole-like structure called the basal bodies.


    – Centrosome is an organelle usually containing two non- membrane bound cylindrical structures called centrioles.

    – They are surrounded by pericentriolar materials.

    – The centrioles lie perpendicular to each other. They are made up of 9 evenly spaced peripheral fibrils of tubulin.
    Each of the peripheral fibril is a triplet. The adjacent triplets are also linked.

    – The central part of the centriole is also proteinaceous and called the hub, which is connected with tubules of the.

    – The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.


    – Nucleus was first described by Robert Brown (1831).

    – Later the material of the nucleus stained by the basic dyes was given the name chromatin by Flemming.

    – Normally, there is only one nucleus per cell. Variations in the number of nuclei are also observed. Some mature cells lack nucleus. E.g. mammalian erythrocytes and sieve tube cells of vascular plants.

    – The interphase nucleus contains

  • Nuclear envelope: Double layered membrane with a space between (10 – 50 nm) called the perinuclear space. It forms a barrier between the materials present inside the nucleus and that of the cytoplasm. The outer membrane usually remains continuous with the ER and also bears ribosomes on it.
    The nuclear envelope has minute pores, which are formed by the fusion of its two membranes. These are the passages for the movement of RNA and protein between the nucleus and the cytoplasm.

  • Nuclear matrix (nucleoplasm)
  • Chromatin: A network of nucleoprotein fibres. It contains DNA and basic proteins (histones), some non- histone proteins and RNA. During cell division, chromatins condense to form chromosomes.
  • Nucleolus: One or more non-membranous spherical bodies. It is continuous with the nucleoplasm. It is a site for ribosomal RNA synthesis.


    – A single human cell has about 2 m long thread of DNA distributed among its 46 (23 pairs) chromosomes.

    – Every chromosome has a primary constriction (centromere) on the sides of which disc shaped structures called kinetochores are present.

    – Based on the position of the centromere, chromosomes are 4 types.

  • Metacentric chromosome: It has middle centromere forming two equal arms of the chromosome.
  • Sub-metacentric chromosome: It has centromere nearer to one end of the chromosome resulting into one shorter arm and one longer arm.
  • Acrocentric chromosome: Centromere is situated close to its end forming one extremely short and one very long arm.
  • Telocentric chromosome: It has a terminal centromere. Some chromosomes have non-staining secondary constrictions at a constant location. It is called satellite.


    – These are many membrane bound
    minute vesicles that contain various

    – Present in both plant and animal cells.

    So that was all about cells and its types. i you haven’t read its first part.. Check that.. thanks for visiting..

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