Plant Cell Biology Overview

Cell Characteristics

• Size: Medium-sized, between prokaryotic and eukaryotic cells
• Cell wall: Absent, but has a pellicle or theca
• Nucleus: Well-organized, surrounded by a membrane
• Membrane-bound organelles: Present, including mitochondria and plastids
• Ribosomes: 80s type
• Histone protein: Absent
• Chromosome: Present, formed with acidic proteins
• Cell division: Amitosis
• Flagella: Present, consisting of filaments

Plant Cell Wall Structure

• Middle Lamella: Outermost layer, contains pectins for cell adhesion
• Primary Cell Wall: Middle layer, formed during cell growth, composed of cellulose microfibrils, hemicellulose fibers, and pectin polysaccharides
• Secondary Cell Wall: Innermost layer, formed in some plant cells, composed of cellulose, hemicellulose, and lignin

Plasmodesmata

• Structure:
  • Plasma membrane lining (plasmalemma)
  • Cytoplasmic sleeve
  • Desmotubule
• Functions:
  • Transport of ions and molecules
  • Cell-to-cell communication
  • Regulation of growth and development
• Types:
  • Primary plasmodesmata: Formed during cell division
  • Secondary plasmodesmata: Formed de novo, aided by cytokinin

Bacterial Cell Wall Structure

• Peptidoglycan: Unique molecule found in bacterial cell walls, composed of sugars and amino acids

Gram Staining

• Step 1: Apply crystal violet to a heat-fixed smear
• Step 2: Add Gram's iodine
• Step 3: Rapidly decolorize with alcohol, acetone, or a mixture of both
• Step 4: Counterstain with safranin

Plasma Membrane Structure and Function

• Selectively permeable membrane: Regulates the entry and exit of materials in and out of the cell.
• Components:
  • Phospholipids: Provide structure and fluidity
  • Cholesterol: Regulates fluidity and stability
  • Integral proteins: Embedded within phospholipid layers, regulate transport and signaling
  • Peripheral proteins: Located on inner or outer surface of the phospholipid bilayer, regulate transport and signaling
  • Carbohydrates: Attached to proteins/lipids on the outside of the membrane, provide cell-cell recognition and adhesion

Fluid Mosaic Model

• Proposed by S.J. Singer and Garth L. Nicholson in 1972
• Describes the plasma membrane as a mosaic of components that give the membrane a fluid character.

Timeline of Plasma Membrane Models

• 1890: Overton - Lipid Nature
• 1905: Langmuir - Lipid Monolayer
• 1926: E.Gorter & F.Grendel - Lipid Bilayer
• 1960: Robertson - Unit Membrane
• 1972: Singer and Nicholson - Fluid Mosaic Model

Lipids in the Plasma Membrane

• Phospholipids: Important for cell growth and division (e.g., Phosphatidylcholine, Phosphatidylserine, Phosphatidylethanolamine)
• Glycolipids: Involved in cell recognition and adhesion (e.g., Cerebroside, Gangliosides)
• Sterols: Important for cell growth and viability, regulate membrane fluidity and permeability (e.g., Cholesterol, Stigmasterol)

Lipid Rafts

• Membrane microdomains enriched with cholesterol and glycosphingolipids, along with proteins.
• Involved in signal transduction, cholesterol trafficking, and endocytosis.
• Types:
  • Caveolar: Flask-shaped invagination with caveolin protein, found in various cells including brain cells and endothelial cells.
  • Non-caveolar: Planar lipid rafts containing flotillin proteins, found in neurons.

Lipid Aggregates

• Interact with water in three forms:
  • Micellar structure: Small spherical structure formed by fatty acid side chains (< 20 nm)
  • Lipid sheets: Two lipid monolayers forming sheets
  • Liposomes: Closed, solvent-filled vesicles

Function of Lipids in the Plasma Membrane

• Used for energy storage
• Primarily function as anhydrous reservoirs for caloric reserves

Plasma Membrane Proteins

• Responsible for most of the dynamic processes in the plasma membrane.
• Types:
  • Peripheral proteins: Bound by electrostatic or hydrogen bonds to the outer layer of the membrane, without interaction with the hydrophobic core of the lipid bilayer.
  • Integral proteins: Embedded within phospholipid layers, interacting with the hydrophobic core of the lipid bilayer.
• Non-Brownian Motion of Cytoplasmic Components: Constituents of cytoplasm move as a separate entity, theorized to be channeled by motor proteins.

Cytoplasm

• Structure and Composition:
  • Cytosol: Liquid-like portion of the cytoplasm, making up about 70% of its composition
  • Cytoplasmic Organelles: Specialized structures within cells that carry out specific tasks
  • Inclusion Bodies: Small, insoluble particles suspended in the cytosol

Cytosol

• Functions:
  • Site of many chemical reactions
  • Involved in osmoregulation and cell signaling
  • Generates action potentials in cells like nerve and muscle cells

Cell Organelles

• Types:
  • Organelles without membrane: Ribosomes, Cytoskeleton
  • Single membrane-bound organelles: Vacuole, Lysosome, Golgi Apparatus, Endoplasmic Reticulum
  • Double membrane-bound organelles: Nucleus, Mitochondria, Chloroplast

Cytoplasmic Inclusions

• Examples:
  • Lipid droplets: Store lipids like fatty acids
  • Cytoplasmic inclusions in plant cells: Store excess glucose during photosynthesis

Overall Functions of Cytoplasm

• Support and Structure: Aids with cell structure and turbidity, keeping organelles in place
• Protection: Protects the cell and its components from damage
• Storage: Contains materials such as storage units and enzymes essential for metabolic activities
• Transport: Assists with the transport of organelles and cytoplasmic inclusions through cytoplasmic streaming

Cytoplasmic Streaming

• Movement of cytoplasm within the cell membrane, acting as a transportation system to move necessary components.

Cell Organelles: Definition and Functions

• Cellular components enclosed in structures/compartments, coordinating and functioning efficiently for cell function.

Nucleus

• Definition: Double membrane-bound organelle located centrally in a eukaryotic cell, enclosing the DNA, the genetic material.
• Structure and Characteristics:
  • Largest and most prominent organelle in the cell
  • Accounts for almost 10% of the entire cell volume
  • Average diameter of approximately 6 μm
  • Shape is usually spherical or oblong

Parts and Function of Nucleus

• Nuclear Envelope and Nuclear Pores: Surrounding the nucleus, made of a phospholipid bilayer, contains tiny openings called nuclear pores.
• Perinuclear Space: Fluid-filled region between the inner and outer nuclear membranes.
• Functions:
  • Controls and coordinates the functioning of the entire cell
  • Contains genetic material and regulates gene expression

Nuclear Envelope

• Selectively permeable membrane that separates the nuclear content from the cytoplasm.
• Regulates the flow of molecules into and out of the nucleus through nuclear pores.