Plant Cell Structure and Function 6

Plant Cell: Comprehensive Notes

Mini Recap: Fundamental Cell Components

• Cytoplasm:

  • Consists of cytosol (the jelly-like substance in the cell),

  • Organelles,

  • Membranous systems,

  • Non-membranous entities.

• Nucleus:

  • Contains the nuclear genome,

  • Enclosed by the nuclear envelope,

  • Contains chromatin,

  • Nucleolus: Responsible for synthesizing ribosomes.

• Ribosomes: Sites of protein synthesis, found on the ER and the nucleus.

• Plastids: A diverse group of organelles including:

  • Chloroplasts,

  • Chromoplasts,

  • Leucoplasts,

  • Proplastids: Undifferentiated plastids that can develop into other plastid types.

• Mitochondria: Key roles in cellular respiration and apoptosis.

• Peroxisomes: Involved in various metabolic processes.

• Vacuoles: Primary function is storage of both primary and secondary metabolites.

  • Primary metabolites: Essential for basic plant functions (e.g., food storage).

  • Secondary metabolites: Often involved in defense mechanisms; can be toxic to the plant's own cells but serve as toxins against herbivores or pathogens when released (e.g., when the cell is broken).

• Glyoxysomes: Found in seeds, they play a crucial role in converting stored fats into carbohydrates for seedling growth. They release nucleases, proteases, and cytochromes (though cytochromes are more typically associated with mitochondria).

Endoplasmic Reticulum (ER)

• A three-dimensional membrane system with an interior lumen (cisternal space).

• Varies significantly in size and form depending on the specific cell type.

• Divided into two main types:

  • Rough ER (RER):

    • Characterized by flattened sacs called cisternae with many polysomes (ribosomes).

    • Prevalent in protein-rich cells due to its role in protein synthesis and modification.

  • Smooth ER (SER):

    • Consists of tubular structures lacking ribosomes.

    • Extensive in lipid-rich cells, involved in lipid synthesis, detoxification, and calcium storage.

• The ER is in constant flux, changing its shape.

Cortical Endoplasmic Reticulum

• An interconnection of cisternae and tubules located just inside the plasma membrane, forming a large surface area.

• Functions:

  • Regulates the levels of Ca^{2+} in the cytosol.

  • May anchor cytoskeletal structures.

  • Can indicate the metabolic and developmental state of a cell.

  • Instrumental in cell-to-cell communication through plasmodesmata, allowing calcium transport and other signals between cells.

Golgi Apparatus

• Composed of a collection of Golgi bodies, each consisting of 5-8 flattened, disc-shaped cisternae that branch into tubules at their margins.

• Distinction from mammalian cells: In plant cells, the Golgi sacs are separate and remain active even during mitosis and cytokinesis.

• Poles of the Golgi stack:

  • Forming Face (Cis Face): Receives membrane-bound substances (e.g., from the ER via transition vesicles).

  • Maturing Face (Trans Face): Processes and sorts materials, moving them from the Golgi to their final destinations (e.g., plasma membrane).

Golgi Transportation and Synthesis

• Cell Wall Component Synthesis: The Golgi apparatus synthesizes and secretes hemicellulose and pectin for incorporation into the cell wall.

  • Each cisternae within the Golgi stack may be responsible for a different part of the synthesis process.

• Glycoprotein Processing: Receives glycoproteins from the rough ER via transition vesicles.

  • These glycoproteins are then moved to the maturing face via shuttle vesicles.

  • Sorted in the trans-Golgi network (TGN) before being transported to the vacuole or plasma membrane.

• Vesicle Packaging:

  • Vacuolar proteins: Packaged in coated vesicles surrounded by a clathrin cage. Clathrin is a protein that forms a lattice-like structure around vesicles.

  • Secreted glycoproteins: Released from the cell without being coated in clathrin, typically via exocytosis to the plasma membrane.

Endomembrane System

• A network of membranes that connect different organelles within the cell.

• Components: Includes the endoplasmic reticulum, Golgi apparatus, vacuoles, and plasma membrane.

• Exclusions: Does NOT include plastids, mitochondria, or peroxisomes.

• Interconnectedness: All other membranes form an interconnected system.

• ER as membrane source: The endoplasmic reticulum is the primary source of new membranes.

  • Membranes are sent to the Golgi apparatus and then distributed to other parts of the cell, including the plasma membrane.

  • The trans-Golgi network also supplies membranes to the tonoplast (the membrane surrounding the vacuole).

Cytoskeleton

• A dynamic network of protein filaments extending throughout the cytosol.

• Plays vital roles in cell division, growth, differentiation, and organelle movement.

• Three main types of protein filaments in plant cells:

  • Microtubules

  • Actin filaments

  • Intermediate filaments (very little is known about plant intermediate filaments, unlike their well-characterized counterparts in animal cells).

Microtubules

• Structure: Cylindrical in shape, with widely varying lengths and in constant flux (dynamic instability, meaning they can rapidly assemble and disassemble).

• Assembly: Occurs at microtubule organizing centers, such as the nuclear envelope and cortical cytoplasm.

• Composition: Made up of tubulin subunits arranged helically to form a 13-row protofilament.

  • \alpha-tubulin and \beta-tubulin subunits form dimers, which then polymerize to form the microtubule.

• Roles:

  • Cell wall growth, expansion, and direction of cell growth.

  • Guide Golgi vesicles to their destinations.

  • Chromosome movement during cell division (forming spindle fibers for replication and elongation).

Actin Filaments

• Composition: Composed of actin molecules, also in constant flux.

• Forms: Can exist as single filaments or bundle together to form thicker structures.

• Roles:

  • Cell wall deposition.

  • Tip growth of the pollen tube.

  • Movement of the nucleus during cell division.

  • Vesicle secretion.

  • Cytoplasmic streaming (cyclosis).

The Plant Cell Wall

• Often considered the characteristic feature that distinguishes plant cells from animal cells.

• Used to identify the type of cell within a plant.

• Essential Functions:

  • Prevents rupture of the plasma membrane due to protoplast expansion (turgor pressure).

  • Contains enzymes that play important roles in absorption, transport, and secretion.

  • Plays an active role in defense against pathogens; surface receptors can trigger phytoalexin production (antimicrobial compounds).

Composition of the Cell Wall

• Cellulose:

  • Principal structural component.

  • Composed of repeating monomers of \beta-glucose.

  • Cellulose molecules are bundled into microfibrils.

  • Microfibrils can become crystalline due to their orderly arrangement in micelles.

  • Cellulose is remarkably strong, even stronger than steel, due to the coiling of microfibrils to form macrofibrils.

  • Cellulose is interlocked within a matrix of other non-cellulosic molecules.

• Hemicellulose:

  • Its composition differs depending on the plant taxonomic group and among cell types.

  • Forms hydrogen bonds to cellulose microfibrils and acts as tethers, holding microfibrils together.

  • Plays a significant role in regulating cell enlargement by allowing some movement while maintaining structural integrity (stretchy).

• Pectins:

  • Hydrophilic polysaccharides, giving the plant cell wall a somewhat plastic nature.

  • Draw water into the cell wall, which initially decreases intracellular Ca^{2+} levels.

  • As calcium is restored, crosslinks form, preventing further stretching and helping the cell wall maintain its limits before bursting.

• Callose:

  • A widely spread cell wall polysaccharide, formed of wound chains of glucose.

  • Most well known for its roles in phloem tissues.

  • Rapidly deposited in response to wounding or stress to plug plasmodesmata between cells, preventing the spread of pathogens or damage to neighboring cells (acts like gum that dries and hardens).

More Cell Wall Components

• Glycoproteins: Structural proteins and enzymes.

  • Extensins: Originally thought to aid in cell extension, but now shown to strengthen the cell wall and prevent excessive extension.

  • Other enzymes include peroxidases, phosphatases, cellulases, and pectinases.

• Lignin: Adds rigidity and compressive strength to the cell wall, particularly in woody tissues.

• Water Retention Compounds:

  • Cutin

  • Suberin

  • Waxes

The Secondary Cell Wall

• Primary Cell Wall: The first cellulosic layers formed, linking to adjacent cells via the middle lamella.

  • Middle Lamella: Primarily composed of pectins, acts as an adhesive layer between cells.

  • Primary Pit-fields: Thinner regions within the primary cell wall containing many plasmodesmata, allowing passage of substances.

• Secondary Cell Wall Deposition: A secondary cell wall may be laid down on the inner surface of the primary cell wall by the protoplast.

  • This occurs only after the cell has stopped increasing in size.

  • Its primary function is to strengthen the cell.

  • Many cells die after the secondary cell wall is deposited, forming structural elements (e.g., xylem vessels for transport).

Secondary Cell Wall Structure

• Pits: The secondary cell wall is not deposited over primary pit-fields, creating pits, which are openings in the secondary wall.

• Pit Membrane: Composed of the two primary walls and the middle lamella where pits are located.

• Pit-pair: Refers to two opposite pits and the intervening pit membrane.

• Pit Varieties:

  • Simple pits

  • Bordered pits

Plasmodesmata

• Microscopic channels that connect the protoplasts (living parts) of adjacent plant cells, forming a communication network.

• Desmotubule: A modified tubule of the endoplasmic reticulum that traverses through the center of the plasmodesmata, allowing for direct transport of substances and signaling molecules between cells.