Cell Theory & Cell Overview

• The cell is the fundamental structural and functional unit of all living organisms.
• All living organisms are composed of cells and their products.

Cell Theory

• Proposed by Matthias Schleiden and Theodor Schwann in 1839.
• States that all plants and animals are composed of cells.
• Rudolf Virchow expanded the cell theory, stating that new cells arise from pre-existing cells (Omnis cellula e cellula).
• The cell theory states: all living organisms are composed of cells and products of cells; all cells arise from pre-existing cells.

Overview of the Cell

• Cells vary significantly in shape, size, and activities.
• Mycoplasmas are the smallest cells (0.3 μm in length).
• Human red blood cells are about 7.0 μm in diameter.
• Bacteria range from 3 to 5 μm in size.
• The largest single cell is the egg of an ostrich.
• Nerve cells are among the longest cells.
• Cells can have diverse shapes like disc-like, polygonal, columnar, cuboid, thread-like, or irregular.

Prokaryotic Cells

• Lack a membrane-bound nucleus and other membrane-bound organelles.
• Generally smaller and simpler than eukaryotic cells.
• Represented by bacteria, archaea, mycoplasmas, and PPLO (Pleuro Pneumonia Like Organisms).
• Genetic material is essentially naked, not enveloped by a nuclear membrane.
• Have a cell wall surrounding the cell membrane, except in mycoplasmas.
• Plasmids: Circular DNA outside the genomic DNA, conferring unique phenotypic characters like antibiotic resistance.
• Mesosomes: Infoldings of the cell membrane, aiding in cell wall formation, DNA replication, and respiration.
• Flagella: Thin filamentous extensions from the cell wall, used for motility.
• Pili and Fimbriae: Surface structures that attach bacteria to rocks in streams and the host's tissues.

Eukaryotic Cells

• Have membrane-bound organelles, including a nucleus.
• Found in protists, plants, animals, and fungi.
• Possess an organized nucleus with a nuclear envelope.
• Have a variety of complex locomotory and cytoskeletal structures.

Cell Membrane

• Composed of a lipid bilayer with proteins.
• Lipids are arranged with the polar head towards the outer sides and the hydrophobic tails towards the inner part.
• Integral proteins are partially or totally buried in the membrane.
• Peripheral proteins lie on the surface of the membrane.
• Fluid Mosaic Model: Proposed by Singer and Nicolson in 1972, describes the membrane as a fluid structure with proteins dispersed in the lipid bilayer.
• Membrane functions include transport of molecules across it.
• Passive transport: Movement of substances across the membrane without energy, via diffusion.
• Osmosis: Movement of water across the membrane from an area of high water concentration to low water concentration.
• Active transport: Movement of substances across the membrane using energy (ATP).
• Sodium-potassium pump: An example of active transport.

Cell Wall

• A non-living rigid structure that provides shape and protection to the cell.
• Found in plant cells and fungi.
• Algae have cell walls made of cellulose, galactans, mannans, and minerals like calcium carbonate.
• Plant cell walls consist of cellulose, hemicellulose, pectins, and proteins.
• Middle lamella: A layer mainly of calcium pectate which glues the cell walls of adjacent cells together.
• Plasmodesmata: Cytoplasmic connections between neighboring cells, facilitating transport and communication.

Endomembrane System

• A group of membrane-bound organelles that coordinate their functions.
• Includes the endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles.

Endoplasmic Reticulum (ER)

• A network of tubules and sacs (cisternae) that extends throughout the cytoplasm.
• Rough ER (RER): Has ribosomes attached to its surface, involved in protein synthesis and secretion.
• Smooth ER (SER): Lacks ribosomes, involved in lipid synthesis, steroid hormone synthesis, and detoxification.

Golgi Apparatus

• Consists of flattened, disc-shaped sacs or cisternae.
• Has two faces: cis (forming) face and trans (maturing) face.
• Functions include packaging materials, modifying proteins and lipids, and forming glycoproteins and glycolipids.

Lysosomes

• Membrane-bound vesicular structures containing hydrolytic enzymes.
• Enzymes are optimally active at acidic pH.
• Functions include digesting cellular waste and foreign materials.

Vacuoles

• Membrane-bound space found in the cytoplasm.
• Contains water, sap, excretory products, and other materials.
• In plant cells, vacuoles can occupy up to 90% of the cell volume, maintaining turgor pressure.
• In Amoeba, contractile vacuole helps in excretion.
• Food vacuoles are formed by engulfing food particles.

Mitochondria

• Double-membrane-bound organelles, considered the "powerhouses" of the cell.
• Outer membrane is smooth, inner membrane is folded into cristae.
• Cristae increase the surface area for ATP generation.
• Matrix contains enzymes for the Krebs cycle, DNA, RNA, and ribosomes.
• Carry out cellular respiration to produce ATP.

Plastids

• Found in plant cells and euglenoids.
• Chloroplasts: Contain chlorophyll, involved in photosynthesis.
• Chromoplasts: Contain carotenoid pigments, responsible for color in flowers and fruits.
• Leucoplasts: Colorless plastids, store nutrients like starch (amyloplast), oils (elaioplast), and proteins (aleuroplast).
• Chloroplast structure includes grana (stacks of thylakoids) and stroma.
• Thylakoids contain chlorophyll.
• Stroma contains enzymes for the Calvin cycle, DNA, RNA, and ribosomes.

Ribosomes

• Granular structures composed of RNA and proteins.
• Not membrane-bound.
• Eukaryotic ribosomes are 80S (60S and 40S subunits).
• Prokaryotic ribosomes are 70S (50S and 30S subunits).
• Site of protein synthesis.
• Polyribosomes or polysomes: Several ribosomes attached to a single mRNA.

Cytoskeleton

• A network of protein filaments in the cytoplasm.
• Provides mechanical support, motility, and maintains the shape of the cell.
• Includes microtubules, microfilaments, and intermediate filaments.

Cilia and Flagella

• Hair-like outgrowths from the cell membrane.
• Cilia are small and numerous, flagella are longer and fewer.
• Structure: Axoneme with nine pairs of peripheral microtubules and two centrally located microtubules (9+2 arrangement).

Centrosome and Centrioles

• Centrosome contains two centrioles.
• Centrioles are cylindrical structures made of microtubules.
• Involved in cell division, forming the spindle apparatus.

Nucleus

• Contains the cell's genetic material (DNA).
• Nuclear envelope: Double-membrane structure enclosing the nucleus.
• Nuclear pores: Openings in the nuclear envelope, allowing transport of molecules.
• Nucleolus: Site of ribosome synthesis.
• Chromatin: DNA and protein fibers that condense to form chromosomes during cell division.

Chromosomes

• Condensed chromatin fibers.
• Composed of DNA and histones.
• Each chromosome has a primary constriction called the centromere.
• Kinetochores: Disc-shaped structures on the centromere, attachment sites for spindle fibers.
• Types of chromosomes: Metacentric, submetacentric, acrocentric, and telocentric, classified based on the position of the centromere.
• Telomere: The tip of the chromosome.

Microbodies

• Membrane-bound vesicles containing various enzymes.
• Examples include peroxisomes (involved in photorespiration).