Structure and Functions of Cell Organelles

Cell organelles are the specialised structures that share the work of a eukaryotic cell. This page covers the endomembrane system (endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles), then mitochondria, plastids, ribosomes, the cytoskeleton, cilia and flagella, centrosome, nucleus, chromosomes and microbodies. It follows the NCERT Class 11 chapter Cell: The Unit of Life. NEET asks cell organelles mostly as match-the-list and statement questions on structure, function and numbers such as 70S, 80S and the 9+2 array.

1. The Endomembrane System

• Each membranous organelle is distinct in its structure and function.
• Many of them are still studied together as the endomembrane system, because their functions are coordinated.
• ★ Exam imp Members: endoplasmic reticulum (ER), Golgi complex, lysosomes and vacuoles.
• Not included: mitochondria, chloroplasts and peroxisomes. Their functions are not coordinated with the members above.

2. Endoplasmic Reticulum and Golgi Apparatus

2.1 Endoplasmic reticulum (ER)

• Electron microscopy shows a network, or reticulum, of tiny tubular structures scattered in the cytoplasm: the endoplasmic reticulum (ER).
• The ER divides the space inside the cell into two compartments: luminal (inside the ER) and extra-luminal (the cytoplasm).
• Ribosomes are often attached to the outer surface of the ER.
• ★ Exam imp Rough endoplasmic reticulum (RER): ER bearing ribosomes on its surface. Smooth endoplasmic reticulum (SER): ER without ribosomes, so it looks smooth.
• The ER is made of tubules or cisternae.
• The ER helps in the transport of substances and in the synthesis of proteins, lipoproteins and glycogen.

2.2 Golgi apparatus

• Camillo Golgi (1898) first saw densely stained reticular structures near the nucleus. They were later named Golgi bodies after him.
• They consist of many flat, disc-shaped sacs, the cisternae, of 0.5 $\mu$m to 1.0 $\mu$m diameter.
• The cisternae are stacked parallel to each other. The number of cisternae in a Golgi complex varies.
• The cisternae are arranged concentrically near the nucleus.
• The Golgi has two faces. They are entirely different but interconnected.

How the Golgi packages materials:

1. Ribosomes on the ER synthesise proteins.
2. Materials to be packaged leave the ER as vesicles.
3. The vesicles fuse with the cis face of the Golgi apparatus.
4. The materials move towards the maturing (trans) face, and proteins are modified in the cisternae.
5. They are released from the trans face, to targets inside the cell or for secretion outside it.

• ★ Exam imp The Golgi apparatus mainly packages materials, for delivery to targets inside the cell or for secretion outside it.
• Material arrives from the ER, which explains why the Golgi stays closely associated with the ER.
• The Golgi apparatus is an important site of formation of glycoproteins and glycolipids.

3. Lysosomes and Vacuoles

3.1 Lysosomes

• Lysosomes: membrane-bound vesicular structures formed by packaging in the Golgi apparatus.
• ★ Exam imp Isolated lysosomal vesicles are very rich in almost all types of hydrolytic enzymes (hydrolases): lipases, proteases and carbohydrases.
• These enzymes work best (optimally) at acidic pH.
• They can digest carbohydrates, proteins, lipids and nucleic acids.

3.2 Vacuoles

• Vacuole: a membrane-bound space in the cytoplasm.
• It contains water, sap, excretory products and other materials not useful for the cell.
• ★ Exam imp The vacuole is bounded by a single membrane called the tonoplast.
• In plant cells, vacuoles can occupy up to 90 per cent of the cell volume.
• In plants, the tonoplast moves ions and other materials into the vacuole against their concentration gradients.
• So these materials are far more concentrated in the vacuole than in the cytoplasm.

4. Mitochondria

• Mitochondria (singular: mitochondrion) are not easily seen under the microscope unless specially stained.
• The number of mitochondria per cell varies with the physiological activity of the cell.
• Their shape and size also vary considerably.
• Typically, a mitochondrion is sausage-shaped or cylindrical, with a diameter of 0.2-1.0 $\mu$m (average 0.5 $\mu$m).
• Its length is 1.0-4.1 $\mu$m.

4.1 Structure

• ★ Exam imp Each mitochondrion is double membrane-bound: an outer membrane and an inner membrane.
• The two membranes divide its lumen into two aqueous compartments: the outer compartment and the inner compartment.
• The inner compartment is filled with a dense, homogeneous substance, the matrix.
• The outer membrane is smooth and forms the continuous limiting boundary of the organelle.
• The inner membrane forms many infoldings, the cristae (singular: crista), towards the matrix.
• The cristae increase the surface area.
• Each of the two membranes has its own specific enzymes for mitochondrial function.

4.2 Functions

• ★ Exam imp Mitochondria are the sites of aerobic respiration.
• They help in oxidative phosphorylation and produce cellular energy as ATP, so they are called the 'power houses' of the cell.
• The matrix has a single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components needed for protein synthesis.
• Mitochondria divide by fission.

5. Plastids

• ★ Exam imp Plastids are found in all plant cells and in euglenoids.
• They are large, so they are easily seen under the microscope.
• They bear specific pigments, which give plant parts specific colours.
• By the type of pigment, plastids are chloroplasts, chromoplasts or leucoplasts.

Kinds of leucoplasts

5.1 Chloroplasts

• Most chloroplasts of green plants are in the mesophyll cells of leaves.
• They are lens-shaped, oval, spherical, discoid or even ribbon-like.
• Length 5-10 $\mu$m; width 2-4 $\mu$m.
• Number: from one per cell in Chlamydomonas, a green alga, to 20-40 per cell in the mesophyll.
• ★ Exam imp Like mitochondria, chloroplasts are double membrane-bound. The inner membrane is relatively less permeable.
• Stroma: the space enclosed by the inner membrane.
• Thylakoids: organised, flattened membranous sacs in the stroma.
• Thylakoids are arranged in stacks, like piles of coins, called grana (singular: granum) or the intergranal thylakoids.
• Stroma lamellae: flat membranous tubules that connect the thylakoids of different grana.
• The thylakoid membrane encloses a space called the lumen.
• The stroma contains enzymes for the synthesis of carbohydrates and proteins.
• The stroma also has small, double-stranded circular DNA molecules and ribosomes.
• Chlorophyll pigments are present in the thylakoids.
• Chloroplast ribosomes are 70S, smaller than the 80S ribosomes of the cytoplasm.
• The grana are the site of the light reactions; the stroma is the site of the dark reactions.

6. Ribosomes and Cytoskeleton

6.1 Ribosomes

• George Palade (1953) first saw ribosomes under the electron microscope as dense, granular particles.
• Ribosomes are made of ribonucleic acid (RNA) and proteins.
• ★ Exam imp Ribosomes are not surrounded by any membrane.
• Eukaryotic ribosomes are 80S; prokaryotic ribosomes are 70S.
• Each ribosome has two subunits, a larger one and a smaller one.
• 'S' stands for the Svedberg unit, the sedimentation coefficient. It indirectly measures density and size.

6.2 Cytoskeleton

• Cytoskeleton: an elaborate network of filamentous protein structures in the cytoplasm.
• It consists of microtubules, microfilaments and intermediate filaments.
• Functions: mechanical support, motility and maintenance of the shape of the cell.

7. Cilia, Flagella and Centrosome

7.1 Cilia and flagella

• Cilia (singular: cilium) and flagella (singular: flagellum) are hair-like outgrowths of the cell membrane.
• Prokaryotic bacteria also have flagella, but these are structurally different from eukaryotic flagella.

Structure seen under the electron microscope:

• A cilium or flagellum is covered by the plasma membrane.
• Its core, the axoneme, has many microtubules running parallel to the long axis.
• ★ Exam imp The axoneme usually has nine doublets of radially arranged peripheral microtubules and a pair of central microtubules: the 9+2 array.
• The two central tubules are connected by bridges and enclosed by a central sheath.
• The central sheath joins one tubule of each peripheral doublet by a radial spoke, so there are nine radial spokes.
• The peripheral doublets are interconnected by linkers.
• Both the cilium and the flagellum emerge from centriole-like structures called basal bodies.

7.2 Centrosome and centrioles

• Centrosome: an organelle usually containing two cylindrical structures, the centrioles.
• The centrioles are surrounded by amorphous pericentriolar materials.
• The two centrioles of a centrosome lie perpendicular to each other.
• ★ Exam imp Each centriole has a cartwheel organisation of nine evenly spaced peripheral fibrils of tubulin protein.
• Each peripheral fibril is a triplet, and adjacent triplets are linked.
• The central part of the proximal region is also protein; it is called the hub.
• The hub is joined to the tubules of the peripheral triplets by radial spokes made of protein.
• Centrioles form the basal body of cilia or flagella.
• In animal cells, they form the spindle fibres that give rise to the spindle apparatus during cell division.

8. Nucleus and Chromosomes

8.1 Discovery and parts

• ★ Exam imp Robert Brown first described the nucleus as a cell organelle, as early as 1831.
• Flemming later named the nuclear material that stains with basic dyes chromatin.
• Interphase nucleus: the nucleus of a cell that is not dividing.
• It has highly extended, elaborate nucleoprotein fibres (chromatin), the nuclear matrix and one or more spherical nucleoli (singular: nucleolus).

8.2 Nuclear envelope and pores

• Electron microscopy shows that the nuclear envelope has two parallel membranes.
• The space between them, 10 to 50 nm wide, is the perinuclear space.
• The envelope is a barrier between materials inside the nucleus and those in the cytoplasm.
• The inner membrane encloses the nucleoplasm and the chromatin material.
• The outer membrane usually stays continuous with the endoplasmic reticulum and also bears ribosomes.
• At many places, the envelope is interrupted by minute nuclear pores, formed by fusion of its two membranes.
• ★ Exam imp Nuclear pores are the passages for movement of RNA and protein molecules in both directions between the nucleus and the cytoplasm.

8.3 Number of nuclei

• Normally there is only one nucleus per cell, but variations in number are often seen.
• Some mature cells even lack a nucleus: erythrocytes of many mammals and sieve tube cells of vascular plants.

8.4 Nucleoplasm and nucleolus

• The nuclear matrix, or nucleoplasm, contains the nucleolus and chromatin.
• Nucleoli are spherical structures in the nucleoplasm.
• A nucleolus is not membrane-bound, so its contents are continuous with the rest of the nucleoplasm.
• ★ Exam imp The nucleolus is a site of active ribosomal RNA (rRNA) synthesis.
• Cells that actively synthesise proteins have larger and more numerous nucleoli.
• The nucleus controls the activities of the organelles and plays a major role in heredity.

8.5 Chromatin and chromosomes

• In the interphase nucleus, chromatin is a loose, indistinct network of nucleoprotein fibres.
• During cell division, structured chromosomes are seen in place of the nucleus.
• Chromatin contains DNA, basic proteins called histones, some non-histone proteins and RNA.
• A single human cell has about two metres of DNA thread, spread over its 46 chromosomes (23 pairs).
• Chromosomes are visible only in dividing cells.
• ★ Exam imp Every chromosome has a primary constriction, the centromere; disc-shaped kinetochores lie on its sides.
• The centromere holds the two chromatids of a chromosome.

By the position of the centromere, chromosomes are of four types:

• Some chromosomes have non-staining secondary constrictions at a constant location.
• This gives the appearance of a small fragment called the satellite.

9. Microbodies

• Microbodies: many minute, membrane-bound vesicles that contain various enzymes.
• They are present in both plant and animal cells.

10. Exam Essentials

Pairs to Match

Numbers to Remember

• Golgi cisternae: 0.5-1.0 $\mu$m diameter; Camillo Golgi, 1898.
• Vacuole: up to 90% of a plant cell's volume.
• Mitochondrion: diameter 0.2-1.0 $\mu$m (average 0.5 $\mu$m); length 1.0-4.1 $\mu$m.
• Chloroplast: length 5-10 $\mu$m, width 2-4 $\mu$m.
• Chloroplasts per cell: 1 in Chlamydomonas; 20-40 in a mesophyll cell.
• Ribosomes: 80S (60S + 40S) and 70S (50S + 30S); George Palade, 1953.
• Axoneme: 9 doublets + 2 central microtubules; 9 radial spokes. Centriole: 9 triplets.
• Nucleus: Robert Brown, 1831; perinuclear space 10 to 50 nm.
• Human cell: about 2 m of DNA in 46 chromosomes (23 pairs).
• 4 types of chromosomes by centromere position.

Examples to Remember

11. Quick Revision

• Endomembrane system: ER, Golgi, lysosomes, vacuoles; their functions are coordinated.
• RER: ribosomes, protein synthesis and secretion; SER: lipids and steroidal hormones.
• ER helps in transport of substances and in synthesis of proteins, lipoproteins and glycogen.
• Golgi: cisternae; cis face receives, trans face releases; glycoproteins and glycolipids.
• Lysosomes: from the Golgi; acid hydrolases digest all types of macromolecules.
• Vacuole: tonoplast; up to 90% of a plant cell; contractile vacuole in Amoeba.
• Mitochondria: smooth outer membrane, cristae, matrix; oxidative phosphorylation and ATP; divide by fission.
• Plastids: chloroplasts, chromoplasts, leucoplasts; found in plant cells and euglenoids.
• Chloroplast: grana for light reactions, stroma for dark reactions; 70S ribosomes.
• Ribosomes: RNA and protein, no membrane; free in the cytoplasm or on the ER.
• Cytoskeleton: microtubules, microfilaments, intermediate filaments.
• Cilia and flagella: 9+2 axoneme from basal bodies; centrioles: nine triplets in a cartwheel.
• Nucleus: double envelope with pores; nucleolus makes rRNA; chromatin forms chromosomes.
• Chromosomes: metacentric, sub-metacentric, acrocentric, telocentric; satellite beyond a secondary constriction.
• The cell is the structural and functional unit of life.

12. Solved Examples

13. Practice Questions

Common Mistakes to Avoid