Cell: The Unit of Life - Botany

Overview of the Lecture Series

• The lecture series, "The Robust Revision Series," is presented by Dr. Vipin Kumar Sharma on Physics w.
• The series aims to cover biology, specifically botany, from a basic to advanced level.
• The series includes in-depth coverage of topics and elaboration of the past 5 years' PYQs (Previous Year Questions).
• Annotated notes, created by Dr. Sharma, are provided for direct printouts and easy revision.
• The series focuses on explaining concepts clearly and aims to help students excel in both school and NEET exams.
• Effort is put into improving the content each year to ensure depth and appeal to students.
• The chapter "Cell: The Unit of Life" is fundamental to understanding biology.

The Cell: Basic Unit of Life

• The cell is the structural and functional unit of life, an important line from NCERT.
• It is the basic unit of life, capable of independent existence and performing essential functions.
• Unlike living organisms, viruses do not have cells.
• Organisms are unicellular (single-celled) or multicellular (multiple cells).
• Simpler organisms with a single cell exhibit complex metabolic diversity.
• The series aims to cover every point in depth.

Historical Discoveries

• Robert Hook discovered the dead cell in 1665 in cork, observing cell walls.
• Antonie van Leeuwenhoek discovered the living cell.
• Robert Brown discovered the nucleus in 1831.
• Advancements in electron microscopy around 1950 led to the discovery of smaller cell structures.
• The Fluid Mosaic Model, the final model of the cell membrane, was given in 1972 by Singer and Nicolson.

Cell Theory

• Schleiden, a botanist, observed different types of cells in plants forming tissues in 1838.
• Schwann, a British zoologist, reported the presence of a cell membrane in animal cells around 1839.
• Schwann proposed that plants and animals are made of cells or cell products.
• Schleiden and Schwann together proposed the initial cell theory.
• Rudolf Virchow added that new cells arise from pre-existing cells (Omnis cellula e cellula) in 1855.
• The completed cell theory states that all organisms are made of cells and cell products and that cells arise from pre-existing cells.

Variety in Cell Shape and Size

• Cells vary in shape and size, with different cells in the human body.
• Red blood cells (RBCs) are biconcave discs, with a diameter of about 7 micrometers.
• White blood cells (WBCs) are amoeboid and irregular in shape.
• Columnar cells are long and thin with basally located nuclei.
• Nerve cells are branched and long; they are one of the longest.
• Mesophyll cells in plant leaves contain chloroplasts and perform photosynthesis.
• The typical eukaryotic cell ranges from 10 to 20 micrometers in size.
• The typical prokaryotic cell ranges from 1 to 2 micrometers in size.
• Bacteria's size is about 3 to 5 micrometers in length.
• Mycoplasma has a length of .3 micrometers.
• PPLO (Pleuro Pneumonia Like Organisms) have .1 micrometer size for the overall size.
• Viruses are even smaller, ranging from .02 to .2 micrometers.
• Mycoplasma is the smallest living cell.
• An ostrich egg is the largest cell.
• The nerve cell is the longest cell and can be 1 meter long.

Cell Types

• Type cells are used for research; for example, onion peel cells for plants.
• Human cheek cells are used to study animal cells.
• Plant cells have an outer cell wall; animal cells have only a cell membrane.
• Four main differences between plant and animal cells:
  • Cell wall (present in plants, absent in animals)
  • Chloroplast (present in plants, absent in animals)
  • Central vacuole (large in plants, small or absent in animals)
  • Centriole and centrosome (absent in plants, present in animals)

Eukaryotic Cell Structure

• A eukaryotic cell has a cell membrane, cell wall, and a nucleus with a nuclear membrane.
• The cytoplasm is the area between the cell membrane and the nucleus, where metabolic reactions occur.
• It is the main arena of cellular activities.
• The cytoplasm is semi-fluid in eukaryotes and fluid in prokaryotes.
• The cell membrane, cytoplasm, and ribosomes are found in every cell.
• Ribosomes are non-membrane-bound structures made of proteins and ribosomal RNA (rRNA).
• They are found in the cytoplasm, on the endoplasmic reticulum, and within mitochondria and chloroplasts in eukaryotes.
• Ribosomes are called protein factories.
• Ribosomes form a polysome when many attach to a single mRNA strand.
• George Palade discovered ribosomes in 1953.
• Different types of ribosomes exist: 70S in prokaryotes and 80S in eukaryotes.
• 70S ribosomes consist of 50S and 30S subunits.
• 80S ribosomes consist of 60S and 40S subunits.
• The 'S' stands for Svedberg unit, which measures sedimentation coefficient.

Prokaryotic Cells

• Prokaryotic cells lack a well-defined nucleus and membrane-bound organelles.
• Genetic material exists as a double-stranded circular DNA called the nucleoid or genophore
• DNA is associated with polyamine (basic) proteins.
• Plasmids: extra-chromosomal, double-stranded, circular DNA that can replicate independently.
• These plasmids bear genes for specific functions like antibiotic resistance.
• Most prokaryotes have a cell wall made of peptidoglycan
• Cell membrane is made of lipids and proteins.
• The cytoplasm is the site of all metabolic reactions.
• Almost all prokaryotic cells have a cell wall.
• Mycoplasma is a type of prokaryotic cell that lacks a cell wall, hence it can change shape.

Cell Wall and Cell Membrane Extensions

• The cell wall provides shape and structure.
• The cell membrane is flexible.
• Flagella extends from the cell membrane but originates from the cell wall.
• Pili and fimbriae are surface structures that play no role in motility.
• Fimbriae are bristle-like structures that help in attachment.
• Pili are tubular structures involved in conjugation (DNA transfer).
• Mesosomes are infoldings of the cell membrane, functioning in cell wall formation, DNA replication, respiration, and secretion.

Inclusions and Cell Diversity

• Prokaryotes have inclusion bodies that are non-membrane-bound, such as phosphate granules or cyanophycean granules.
• Gas vacuoles provide buoyancy to photosynthetic bacteria like blue-green algae.
• Prokaryotic cells include bacteria, blue-green algae, mycoplasma, and PPLO.
• Mycoplasmas are the smallest.
• Prokaryotic cells divide more rapidly because of simplicity.
• Bacteria come in different shapes: coccus (spherical), bacillus (rod-shaped), spirillum (spiral), and vibrio (comma-shaped).

The Cell Envelope

• The cell envelope found in most prokaryotic cells is composed of the cell membrane, cell wall, and glycocalyx.
• Cell membrane: interacts with the outside.
• Cell wall: provides protection and shape and stops cells from bursting.
• Glycocalyx: the outermost layer that provides protection.
  • This can be the loose slime layer or a tough capsule.
• Gram-positive bacteria retain stain; Gram-negative bacteria do not.
• Gram-positive and Gram-negative are divided by cell envelope

Eukaryotic Cells: A Comprehensive Overview

• Eukaryotic cells are divided with clear compartments and complex locomotory structures.
• Eukaryotic flagella contain tubulin protein.
• The cytoskeleton maintains cells.

The Cytoskeleton

• The cytoskeleton consists of proteins for structural support, shape, and motility.
  • Components:
    • Microtubules
    • Microfilaments (actin filaments)
    • Intermediate filaments

The Cell Membrane: Composition and Structure

• The cell membrane structure was studied in RBCs.
• It is mainly composed of lipids and proteins and arranged in a bilayer
• Lipids: Amphipathic; consists of - hydrophilic heads (phospholipids) facing outward, and hydrophobic tails facing inward
• Proteins: are the integral and peripheral proteins depending on ease of extraction
• Protein-to-lipid ratio varies; for example: RBCs: Protein: 52%, Lipid: 40%

Fluid Mosaic Model (1972)

• Lipids enable cell growth, division, and endocytosis processes.
• Proteins act as a mosaic to serve different purposes.
• Cholesterol adds strength.

Passive Transport

• Facilitated diffusion involves moving a high concentration to low concentrations through trans-membrane protein.
• Simple Diffusion moves high concentration to low concentration without help.

Active Transport

• Active Transport needs a pump to transport from a low concentration to a high concentration.

Eukaryotic Cell Wall Anatomy

• Cell walls are rigid and provide protection, with plants having walls of cellulose and calcium carbonate in algae.
• Plant cell walls consist of cellulose, hemicellulose, pectins, and proteins.
• Middle Lamella is a structure that connects plant cells made of calcium pectate.
• Plasmodesmata: Strands that connect the cytoplasm.

Endomembrane System

• Endoplasmic Reticulum (ER), the Golgi complex, lysosomes, and vacuoles.

Endoplasmic Reticulum (ER)

• Consists of the network of tubules.
• Divided by:
  • Luminal: Inside area of the ER
  • Extra-luminal: Outside area of the ER
• Two types:
  • Rough (RER): is for protein synthesis
  • Smooth (SER): is for Lipid/Steroid synthesis

Golgi Apparatus

• Consists of flattened discs which is Cisternae diameter =0.5-1.0µm
• Cis- face, convex or the forming face to receive
• Trans- face, concave or the maturing face to ship
• Functions to do Glycosylation for protein

Lysosomes

• Hydrolytic enzymes such as proteases, lipases, and nucleases
• Active at the high PH (around 5%5 )

Vacuoles

• Membrane bond; single membrane; store the wastes like ions, other waster
• Osmoregulation uses contractile vacuoles
• Food vacuoles are to engulf food

Microbodies

• Single-membrane bound body
• Has the important enzymes like the Peroxisomes with the Peroxidase.

Mitochondria

• Double membrane-bound
• Outer membrane: forms
• Cristae increases the membranous inside
• Matrix: It is filled with material

Plastids

• Chloroplasts: Contain the green pigment
• Chromoplasts: Contain the other colors besides the green color
• Leucoplasts: Has no pigments and stores the protein.

Chloroplasts

• Has DNA, RNNA, and ribosomes
• Double membrane-bound that is for photosynthesis
  • outer
  • inner
• Thylakoids are the stack-like coins, called grana
• Dark Respiration occurs in Sroma

Cilia and Flagella

• They help function to help move either the cell or the surrounding fluids

Core of Cilia and Flagella

• Core: Axoneme has 9+2 arrangement
  • Nine pair on the outside (Doublets) and two in side (singlets) connected through Inter-doublet bridges and Inter-Singlet
  • Inter-doublet bridges uses Nexin
• Inside they are connected to Radial Spokes

Centrioles

• Are in Pairs and are in the Angles of 90
• The pericentriolar material has 9x3x0 structure called Cartwheel

The Nucleus

• Brain of the cells
• Chromatin: it is made of DNA, RNA and it is acidic
• Nuclear pore-allows transport in both directions
• Nucleolus: it has only ribosomal RNA
  • DNA is not divided and is lose

Chromosomes

• Chromatid has two arm. P arm small and q arm large
• Primary construction-centromere-is primary and present
• Secondary constrictions-only some-has that. Called Sat Chromosomes

The type of chromatids

• metacentric (Middle)
• sub-metacentric ( a but off center)
• acrocentric ( off by far)
• telocentric (end position)