These notes for CBSE Class 11 Biology, Chapter 8, focus on 'Cell - The Unit of Life'. They define the cell as the fundamental structural and functional unit of life, introducing the Cell Theory formulated by Schleiden and Schwann, and modified by Virchow. The notes differentiate between prokaryotic cells, lacking a defined nucleus and membrane-bound organelles, and eukaryotic cells, which possess these structures. Key components like the cell envelope (glycocalyx, cell wall, plasma membrane), mesosomes, ribosomes, and flagella in prokaryotes are detailed. For eukaryotes, the notes cover the cell membrane with its fluid mosaic model, the protective and structural cell wall, the endoplasmic reticulum (RER and SER), Golgi apparatus, lysosomes, vacuoles, mitochondria (powerhouses of the cell), plastids (chloroplasts, chromoplasts, leucoplasts), ribosomes, cilia, flagella, centrosomes, centrioles, and the nucleus with its chromatin and chromosomes. These comprehensive notes are ideal for quick revision and understanding of cell structure and function for exams.
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Gram positive bacteria : Bacteria that take up gram stain.
Gram negative bacteria : Bacteria that do not take up gram stain.
Prokaryotic cells : Cells which lack a well defined nucleus and membrane bound
cell organelles. e.g., bacteria, cyanobacteria, mycoplasma.
Eukaryotic cells : Cells which have a well defined nucleus and membrane bound
cell organelles. e.g., all protists, plants, animals and fungi cells.
Passive transport : Transport of molecules across a membrane along the con-
centration gradient, i.e., from higher to lower concentration without the consumption
of energy.
Active transport : Transport of molecules against concentrataion gradient, i.e.,
from lower to higher concentration with the consumption of energy (ATP).
Polyribosome/polysome : A chain like structure formed when several ribosome
are attached to a single mRNA.
PPLO : Pleuro Pneumonia Like
Organisms.
Cell : Cell is the structural and functional unit of life. Cell Theory was formu-
lated by Scheleiden and Schwann and was modified by
Rudolf virchow states :
(a) All living organisms are composed of cells and products of cells.
(b) All cells arise from pre-existing cells.
Prokaryotic cells
Genetic material is not enveloped by nuclear envelope.
Many bacteria contain
extra chromosomal DNA ?
plasmids.
Cell Envelope
Prokaryotic cells have a chemically complex cell envelope which consists
of a tightly bound 3 layered structure i.e., outermost glycocalyx followed by cell wall and then plasma membrane.
A specialised structure ? mesosome is formed by the extension of plasma membrane into the cell. Mesosomes help in cell wall formation, DNA replication
and distribution to daughter cells, respiration, secretion process, to increase
surface
area of plasma-membrane and enzymatic content.
Bacterial cells may be motile or non-motile. Motile bacterias have flagella composed of three parts ? filament, hook and basal body. Pili and fimbriae are
surface structures which do not play any role in motality. These structures help
the bacteria to attach with rocks and the host tissues.
70S ribosomes are associated with plasma membrane and is made of two
subunits ? 50S and 30S. Ribosomes are site of protein synthesis.
Possess an organized nucleus with nuclear envelope and have a variety of complex locomotory and cytoskeletal structures.
Singer and Nicolson (1972) gave ‘Fluid mosaic model’. According to this
the quasi-fluid nature of lipid enables lateral movement of proteins within the
overall bilayer
Functions : Selectively permeable.
Cell Wall is a non-living rigid structure which gives shape to the cell and
protects cell from mechanical damage and infection, helps in cell-to-cell interaction
and provides barrier to undesirable macromolecules.
Cell wall of algae is made of cellulose, galactans, mannans and minerals
like calcium carbonate. Plant cell wall consists of cellulose, hemicellulose, pectins
and proteins.
Middle lamella is made of calcium pectate which holds neighbouring cells
together.
Plasmodesmata connect the cytoplasm of neighbouring cells.
Endoplasmic Recticulum (ER)
Consists of network of tiny tubular structures. ER divides the intracellular
space into two distinct compartments ? luminal (inside ER) and extra luminal
(cytoplasm).
(i) Rough Endoplasmic Reticulum (RER) :
• Ribosomes attached to outer surface.
• Involved in protein synthesis and secretion.
(ii) Smooth Endoplasmic Reticulum (SER) :
• Lack ribosomes.
• Site for synthesis of lipid.
Golgi apparatus : Consists of cisternae stacked parallel to each other. Two faces of the organelle
are convex cis or forming face and concave trans or maturing face.
Functions : Performs packaging of materials, to be delivered either to the
intra-cellar targets or secreted outside the cell. Important site of formation
of glycoproteins and glycolipids.
Lysosomes : Membrane bound vesicular structures formed by the process of packaging
in the golgi apparatus. Contain hydrolysing enzymes (lipases, proteases,
carbohyrases) which are active in acidic pH. Also called ‘Suicidal Bag’.
Function : Intracellular digestion.
Vacuoles :
Membrane bound space found in the cytoplasm. Contain water, sap, excretory
product, etc.
Function : In plants tonoplast (single membrane of vacuole) faciliates transport
of ions and other substances.
Contractile vacuole for excretion in Amoeba and food vacuoles formed in
protistis for digestion of food.
Mitochondria :
Double membrane structure. Outer membrane smooth and inner membrane
forms a number of infoldings called cristae.
Function : Sites of aerobic respiration. Called ‘power houses’ of cell as
produce cellular energy in the form of ATP. Matrix possesses single circular
DNA molecule, a few RNA molecules, ribosomes (70S). It divides by
fission.
Plastids : Found in plant cells and in euglenoides. Chloroplasts, chromoplasts and
leucoplasts are 3 types of plastids depending on pigments contained.Chloroplasts are double membraned structure. Space limited by inner membrane
is called stroma.
Flattened
membranous sacs called thylakoids
in stroma.
Chlorophyll pigments are present in thylakoids.
Function : Site of photosynthesis.
Ribosomes
Compased of RNA and proteins; without membrane. Eucaryotic ribosomes
are 80S.
Function : Site of protein synthesis.
Cilia and Flagella
Cilia are small structures which work like oar, which help in movement.
Flagella are longer and responsible for cell movement. They are covered
with plasma membrane. Core is called axoneme which has 9 + 2 arrangement.
Centrosome and Centrioles
Centrosome contains two cylindrical structures called centrioles. Surrounded
by amorphous pericentriolar material. Has 9 + 0 arrangement. Centrioles form
the basal body of cilia or flagella and spindle fibres for cell division in animal
cells.
Nucleus : With double membrane; nuclear pores; has chromatin, nuclear
matrix and nucleoli (site for rRNA synthesis).
Chromatin : DNA + non histone proteins.
Chromosomes (on basis of position of centromere) :
Metacentric : Middle centromere.
Sub-metacentric : Centromere nearer to one end of chromosome.
Acrocentric : Centromere situated close to its end.
Telocentric : Has terminal centromere.
Satellite : Some chromosomes have non-staining secondary constrictions
at a constant location, which gives the appearance of small fragment called satellite.
Cell Theory states that all living organisms are composed of cells and products of cells, and all cells arise from pre-existing cells.
Prokaryotic cells lack a well-defined nucleus and membrane-bound organelles, while eukaryotic cells possess both.
The Fluid Mosaic Model, proposed by Singer and Nicolson, describes the cell membrane's quasi-fluid nature allowing lateral movement of proteins within the lipid bilayer.
RER is involved in protein synthesis and secretion due to attached ribosomes, while SER is the site for lipid synthesis and lacks ribosomes.
Mitochondria are called powerhouses because they produce cellular energy in the form of ATP through aerobic respiration.
The three types of plastids are chloroplasts (for photosynthesis), chromoplasts (for pigments), and leucoplasts (for storage).
Cilia and flagella have a core called the axoneme with a 9+2 arrangement of microtubules, covered by a plasma membrane.
The Golgi apparatus packages materials for delivery to intra-cellular targets or secretion, and is important for forming glycoproteins and glycolipids.
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