Comprehensive Study Notes on Cell Structures: Organelles, Cell Walls, and Membranes

Sedimentation and the Svedberg Unit

• Definition of the Svedberg Unit (Symbol S): The Svedberg unit is a measurement of the sedimentation rate of a particle during centrifugation. It indicates the size of the molecule; larger molecules typically possess a larger sedimentation coefficient.

• Influencing Factors: The sedimentation rate is dependent on several physical properties of the particle:

  • Volume.

  • Shape.

  • Molecular mass.

• Comparative Sedimentation: If a particle is heavier and has a more compact shape, its Svedberg value will be greater than that of lighter particles with a less compact shape.

• Historical Context and Eponym: The unit is named after the Swedish chemist Theodor Svedberg ($1884-1971$). He won the $1926$ Nobel Prize in Chemistry for his significant work on colloids and the invention of the ultracentrifuge.

Electron Microscopy Specialized Techniques

• Transmission Electron Microscope (TEM): This instrument uses transmitted electrons to form an image of the specimen.

• Scanning Electron Microscope (SEM): This instrument produces an image by detecting secondary or backscattered electrons from the surface of the specimen.

• Scanning Transmission Electron Microscope (STEM): A hybrid instrument that combines both TEM and SEM functionalities in one device.

• Key Distinguishing Feature: The primary difference between SEM and TEM imaging is the method of electron detection: SEM relies on secondary/backscattered electrons, while TEM utilizes electrons passing through (transmitted) the specimen.

The Cell Wall and Plasma Membrane

• Plasma Membrane: Defined as the outer living boundary of the cell, also known as the cell membrane. It serves as the boundary of the protoplasm and most organelles.

• Cell Wall: A non-living, extracellular component formed exteriorly to the plasma membrane.

• Presence in Organisms: The cell wall is found in:

  • Plant cells (composed of cellulose).

  • Prokaryotes (composed of peptidoglycan).

  • Fungi (composed of chitin).

  • Some protists.

• Absence in Animal Cells: Animal cells do not have a cell wall. This absence is likely attributed to their locomotor mode of life (the self-directed ability of an organism to move its entire body).

• General Functions of the Cell Wall:

  • Protects the plant cell.

  • Maintains the specific shape of the cell.

  • Prevents excessive uptake of water via osmotic regulation.

  • Provides structural support against the force of gravity for the whole plant through specialized cells.

Structural Layers of the Plant Cell Wall

• Permeability: The plant cell wall is a permeable boundary. It is porous and allows the free passage of water and dissolved materials.

• Primary Cell Wall:

  • Recognized as a "true wall."

  • Develops in newly growing cells, specifically during cell division.

  • Location: Found just inner to the middle lamella.

  • Characteristics: Thin and slightly flexible.

  • Composition: Composed of cellulose microfibrils (bundles of cellulose chains) embedded in a matrix of other polysaccharides such as hemicelluloses and pectin.

  • Skeletal Structure: Microfibrils exhibit a crisscross arrangement in layers, providing the cell with great strength.

  • Growth Adaptation: The wall is adapted to growth and stretches plastically, meaning it stretches irreversibly.

• Middle Lamella:

  • The outermost layer located between the primary cell walls of adjacent cells.

  • Function: Acts as the cement that holds adjacent cells together.

  • Composition: Composed of sticky, gel-like pectin and salts of magnesium ($Mg$) and calcium ($Ca$).

• Secondary Cell Wall:

  • Location: Formed between the primary cell wall and the plasma membrane.

  • Specific Distribution: Found only in sclerenchyma cells.

  • Biological State: Cells containing a secondary cell wall are generally dead at maturity and serve to provide mechanical support.

  • Timing of Development: Develops only when the cell has reached its maximum size and completed growth.

  • Physical Properties: Very thick and rigid; its rigidity prevents further growth or expansion.

  • Composition: Primarily consists of cellulose, hemicelluloses, lignin, inorganic salts, and waxes.

  • Structural Integrity: Contains cellulose microfibrils in a crisscross arrangement. Lignin is a key component that cements and anchors these microfibrils together, providing essential rigidity and a definite shape.

Chemical Composition and "Science Tidbits"

• Pectic Acids:

  • Polymers consisting of approximately $100$ galacturonic acid molecules.

  • Highly hydrophilic.

  • Function: Form salts with $Ca$ and $Mg$, resulting in insoluble gels. These are major components of the middle lamella and are also found in primary cell walls.

• Pectin:

  • Polymers consisting of approximately $200$ galacturonic acid molecules.

  • Chemical modification: The majority of its carboxyl groups are methylated ($COOCH_3$).

  • Properties: Less hydrophilic than pectic acid but soluble in hot water. Found in both the middle lamella and primary walls.

The Plasma Membrane (Cell Membrane)

• Distribution: Found in all living prokaryotic and eukaryotic cells.

• Core Function: Regulates and controls the passage of materials entering and exiting the cell.

• Chemical Composition:

  • Proteins: $60-80\%$

  • Lipids: $20-40\%$

  • Carbohydrates: Present in small quantities.

• Svedberg Unit (S): Measures sedimentation rate; larger particles, larger coefficient.

• Influencing Factors: Volume, shape, molecular mass influence sedimentation rate.

• Comparative Sedimentation: Heavier, compact shapes have greater Svedberg values.

• Historical Context: Named after Theodor Svedberg (1884-1971), Nobel Prize in Chemistry 1926.

Electron Microscopy Techniques

• TEM: Uses transmitted electrons to form images.

• SEM: Detects secondary/backscattered electrons for surface images.

• STEM: Combines TEM and SEM functionalities.

• Key Difference: SEM uses backscattered electrons; TEM uses transmitted electrons.

Cell Wall and Plasma Membrane

• Plasma Membrane: Outer living cell boundary, boundary of protoplasm and organelles.

• Cell Wall: Non-living, extracellular to plasma membrane; present in plants, prokaryotes, fungi, some protists.

• Absence in Animals: No cell wall in animal cells; linked to locomotion.

• Functions: Protects cell, shapes it, regulates water uptake, provides structural support.

Plant Cell Wall Structure

• Permeability: Porous, allows water passage.

• Primary Cell Wall: Thin, flexible, composed of cellulose microfibrils and polysaccharides; develops during growth.

• Middle Lamella: Gel-like pectin layer, cements cells together.

• Secondary Cell Wall: Thick, rigid, found in sclerenchyma; supports and prevents growth; contains lignin for rigidity.

Chemical Composition

• Pectic Acids: Polymers from galacturonic acid; hydrophilic, forms salts with $Ca$ and $Mg$.

• Pectin: Similar to pectic acids, fewer gallacturonic acids, less hydrophilic, soluble in hot water.

Plasma Membrane

• Distribution: Found in all prokaryotic/eukaryotic cells.

• Function: Regulates material passage.

• Composition: Proteins (60-80%), lipids (20-40%), trace carbohydrates.