Plant Structure, Cell Types, and Primary Growth Notes

Plant Cell Walls

  • Mostly composed of cellulose; also contain hemicellulose, pectin, and, in some cases, lignin.

  • Cellulose is a polysaccharide made of glucose units, providing structural support to the cell wall.

  • Cellulose molecules form microfibrils, which are made of adjacent, parallel cellulose molecules held together by hydrogen bonds, giving tensile strength.

  • Plant cell walls are extracellular, meaning they are located outside of the cell membrane, protecting and shaping the cell.

  • Two types of cell walls:

    • Primary wall: thin and present in all cells, allowing for cell growth.

    • Secondary wall: thicker and found in some cells, providing strength and rigidity; may contain lignin.

Functions of Plant Cell Wall

  • Determines and maintains cell shape by providing a rigid framework.

  • Provides support and mechanical strength, enabling plants to grow tall and hold out leaves for light capture; prevents bending and breaking.

  • Prevents the cell membrane from bursting by resisting water pressure through turgor pressure regulation.

  • Controls the rate and direction of cell growth and regulates cell volume through the deposition of new cell wall material.

  • Responsible for the architectural design of the plant by dictating cell arrangement and tissue organization.

  • Acts as a physical barrier to:

    • Pathogens, preventing infection.

    • Water in suberized (waxy) cells, controlling water movement.

  • The wall is porous and allows the free passage of small molecules, facilitating nutrient and signaling molecule transport.

  • Functions in carbohydrate storage; wall components can be reused in other metabolic processes, especially in seeds, providing energy for germination.

Polysaccharides in Plant Cell Walls

  1. Cellulose: A polymer (chain) of up to 25,000 glucose molecules.

    • Approximately 36 chains bond to form a microfibril, providing high tensile strength.

  2. Cross-linking glycans (hemicellulose): Bonds with cellulose, providing structural support and flexibility.

  3. Pectin: Jellylike substance that acts as glue, holding cells together and providing flexibility.

Cells Sticking Together

  • Middle lamella: material between cells made of pectic substances, cementing adjacent cells together.

  • Plasmodesmata: pit pairs with plasmodesmata (vac.), channels through cell walls that connect the cytoplasm of adjacent cells, allowing for communication and transport.

The 3 Tissue Systems in Vascular Plants

  1. Dermal tissue: Single layer that secretes a waxy cuticle, protecting the plant from water loss and pathogens.

  2. Vascular tissue: Xylem and phloem; provide support and supply water, nutrients, and sugars throughout the plant.

  3. Ground tissue: Tissue other than dermal or vascular.

    • The bulk of the young plant.

    • Fills the space between dermal and vascular tissues.

    • Mostly parenchyma cells.

    • Functions in storage, photosynthesis, and support.

Plant Cell Types

  • Dermal tissue:

    • Epidermis: Outermost layer of cells.

  • Ground tissue:

    • Parenchyma: Versatile cells with various functions.

    • Collenchyma: Provides flexible support.

    • Sclerenchyma: Provides rigid support and protection.

  • Vascular tissue:

    • Tracheids and vessel elements: Water-conducting cells in xylem.

    • Sieve elements: Sugar-conducting cells in phloem.

Epidermal Cells

  • Outermost cells.

  • Single layer covering leaves, stems, and roots of non-woody plants or plant parts.

  • Functions:

    • Waterproofing: Preventing water loss.

    • Protection from pathogens (viruses, bacteria, fungi): Acting as a barrier.

    • Other specialized roles:

      • Hairs (trichomes): Reducing water loss and providing defense.

      • Nectary: Secreting nectar to attract pollinators.

      • Guard cells for gas exchange: Regulating stomata opening and closing.

Cuticle

  • Formed by epidermal cells; outermost part is wax.

  • Protects against:

    • Water loss: Preventing desiccation.

    • Pathogens: Acting as a barrier to infection.

    • Ultraviolet (UV) radiation: Reducing damage from sunlight.

Secretory Trichomes

  • Examples include stinging nettle trichomes (one cell), mint trichomes (>1 cell), and oregano (mint) trichomes (8 cells).

  • Cannabis trichomes (stalked gland) are found on the female plant and produce cannabinoids.

  • Trichomes on marjoram (mint family) secrete oils for defense and hinder crawling insects for defense.

Floral Nectary

  • For nectar secretion, attracting pollinators.

Parenchyma Cells

  • Functions:

    • Many, often specialized.

    • Most metabolic processes (photosynthesis, storage, secretion, food storage).

  • Features:

    • A type of ground tissue (when in stems & roots).

    • Usually lack a secondary wall.

    • Can often divide & differentiate at maturity.

  • Examples:

    • Fruit flesh: Storage and nutrition.

    • Endosperm: Nourishment for developing embryos.

    • Pith & cortex of stems & roots: Storage and support.

    • Chloroplast-laden cells inside leaves: Photosynthesis.

Collenchyma Cells

  • Functions:

    • Support, especially of young & growing organs.

  • Features:

    • A type of ground tissue.

    • Thick, uneven primary wall.

    • No secondary wall.

    • Always occur just below the epidermis.

    • Often occur in strands.

    • Alive at maturity; can elongate.

  • Examples:

    • Celery leaf midrib & petiole: Flexible support.

Sclerenchyma Cells

  • Functions:

    • Support, protection (of tissues no longer elongating).

  • Features:

    • A type of ground tissue.

    • Thick secondary wall with lignin (a rigid polymer).

    • Cannot elongate at maturity (may be dead).

    • Two forms: fibers & sclereids.

  • Examples:

    • Fibers (e.g. Burlap (jute fibers)): Providing tensile strength.

    • Sclereids (e.g. Pear): Giving a gritty texture.

Water-Conducting Cells of the Xylem: Tracheids & Vessel Elements

  • Functions: Support & supply of water and minerals

  • Features: Dead at maturity

    • Secondary wall lignified, often spiral

    • Two kinds: Tracheids & vessel elements

    • A type of vascular tissue

Programmed Cell Death

  1. Cytoplasmic streaming

  2. Secondary wall formed

  3. Death (vacuole implodes)

Lignin

  • Class of complex organic polymers (NOT a polysaccharide).

  • Second-most abundant natural polymer, after cellulose.

  • Deposited in cell walls; fills spaces and binds cellulose, hemicellulose & pectin.

  • Gives strength to wood & bark.

  • Can occur in cell walls of non-woody plants (palm trees, bamboo, wheat straw).

  • Human uses:

    • Wood: 20–33% lignin by dry weight.

    • Rope, clothing, rugs (sclerenchyma fibers).

    • Paper—lignin removed to leave cellulose.

    • Artificial vanillin.

Sugar-Conducting Cells of the Phloem: Sieve-Tube Elements

  • Functions: Transport sugars (esp. sucrose), other organic compounds, some minerals

  • Features: A type of vascular tissue

    • Alive at maturity

    • With companion cell (parenchyma)

Plants Grow from Meristems

  • Meristems: Ultimate source of all parts of the mature sporophyte

    • Undifferentiated

    • Retain “forever” the ability to divide

Types & Location of Meristems

  • Apical meristems (AM): Root AM and shoot AM

    • Primary growth

  • Lateral meristems: Vascular cambium & cork cambium

    • Secondary growth

The 3 Tissue Systems (revisited)

  1. Dermal tissue: Single layer; secretes cuticle (waxy)

  2. Vascular tissue: Xylem & phloem; support & supply

  3. Ground tissue: Bulk of young plant; fills space between dermal & vascular tissues; mostly parenchyma; storage, photosynthesis, support

  • These tissue systems arise from 3 new meristems made by the apical meristem

Apical Meristem

  • Source of primary growth of both root & shoot

  • Makes 3 other meristems:

    • Protoderm: Dermal tissue (epidermis)

    • Procambium: Vascular tissue

    • Ground meristem: Ground tissue

Shoot Apical Meristem & Differentiation

  • Youngest tissues at shoot tip

  • Cell differentiation, cell elongation, and cell division occur following the apical meristem.

Shoot Structure & Development

  • Cross-section of young shoot: Eudicot vs. Monocot

Root Apical Meristem

  • Root apical meristem & development

Root Primary Growth: 3 Zones

  1. Zone of cell division

  2. Zone of elongation

  3. Zone of maturation

  • Root cap protects the apical meristem.

Root Growth

  • Root hairs increase surface area.

    • 4-month-old rye plant has 14 billion root hairs.

    • Surface area: 400 m2m^2

Root Structure & Development

  • Cross-section of young root: Eudicot vs. Monocot

Young Roots

  • Endodermis is innermost layer of cortex.

  • Stele or Vascular Cylinder is all cells inside endodermis; arise from procambium.

Lateral Root

  • Originates in pericycle (outermost layer of vascular cylinder

  • Grows out through cortex & epidermis

Structure of Primary (non-woody) Roots—Summary

  • Root cap: protects.

  • Root hairs: epidermal cells, increase absorption area.

  • Endodermis: innermost layer of