bio 112 final exam unit 2 plants

🌿 Unit 2: Plant Form and Function

Origins of Plants

• Evolution from Green Algae (Charophytes):
  Plants evolved from green algae (Charophytes) around 500 million years ago. They transitioned from aquatic to terrestrial environments, developing key features such as a cuticle to prevent water loss and a vascular system to transport water and nutrients.
  Cuticle: A waxy coating that prevents water loss, helping plants survive on land.

Nonvascular Plants (Bryophytes)

• Characteristics:

  • Lack vascular tissues (xylem and phloem).

  • Depend on diffusion for water and nutrient transport.

  • Small size due to lack of vascular system.

  • Require moist environments for reproduction because they need water for sperm to swim to the egg.

• Reproduction:
  Nonvascular plants reproduce via spores (no seeds).

• Examples:
  Mosses, liverworts, hornworts.

Seedless Vascular Plants

• Characteristics:

  • Have vascular tissues (xylem and phloem), allowing them to transport water and nutrients.

  • Reproduce via spores, not seeds.

  • Require water for fertilization (sperm swims to egg).

• Examples:
  Ferns, club mosses, horsetails.

Gymnosperms (Naked Seed Plants)

• Characteristics:

  • Seed-producing plants with seeds that are not enclosed in an ovary (seeds are "naked").

  • Vascular system with xylem (tracheids) and phloem (sieve tube members).

  • Reproduce with seeds but do not have flowers.

• Examples:
  Pines, cycads, ginkgos.

Angiosperms (Flowering Plants)

• Characteristics:

  • Produce seeds enclosed within a fruit (ovary).

  • Vascular system with xylem (vessel elements) and phloem (sieve tube members).

  • Flowers and undergo double fertilization (1 sperm fertilizes the egg, and another fuses with two polar nuclei to form endosperm).

Eudicots vs. Monocots

• Eudicots:

  • Two cotyledons (seed leaves).

  • Net-like (reticulate) leaf venation.

  • Flower parts in multiples of 4 or 5.

  • Vascular bundles arranged in a ring in the stem.

• Monocots:

  • One cotyledon.

  • Parallel leaf venation.

  • Flower parts in multiples of 3.

  • Vascular bundles scattered throughout the stem.

Root System

• Fibrous Roots:

  • A network of smaller roots that spread out (common in monocots like grasses).

• Taproot:

  • A single, large root that grows deep (common in eudicots like carrots).

• Modified Roots:

  • Prop roots: Provide support (e.g., corn).

  • Pneumatophores: Allow gas exchange in waterlogged soils (e.g., mangroves).

  • Storage roots: Store nutrients (e.g., beets).

  • Adventitious roots: Arise from non-root tissues, providing support or helping with propagation (e.g., aerial roots).

Shoot System

• Components:
  Includes stems, leaves, and flowers.

• Leaf Types:

  • Simple Leaf: Single, undivided leaf blade.

  • Compound Leaf: Multiple leaflets attached to a single petiole.

  • Doubly Compound Leaf: Leaflets are also divided into smaller leaflets.

• Leaf Arrangements:

  • Alternate: One leaf per node.

  • Opposite: Two leaves per node.

  • Whorled: Three or more leaves per node.

  • Rosette: A circular arrangement of leaves at the base of the plant.

• Modified Leaves:

  • Spines: Reduce water loss and deter herbivores (e.g., cacti).

  • Bulbs: Store nutrients (e.g., onions).

  • Succulents: Store water (e.g., aloe).

  • Tendrils: Aid in climbing (e.g., peas).

  • Traps: Capture prey (e.g., Venus flytrap).

  • Floral Mimics: Attract pollinators (e.g., some orchids).

• Stem Functions:

  • Support for leaves and flowers.

  • Transport of water, nutrients, and sugars.

• Modified Stems:

  • Rhizomes: Underground stems for vegetative propagation.

  • Tubers: Storage organs (e.g., potatoes).

  • Stolons: Horizontal stems aiding in vegetative reproduction (e.g., strawberries).

Plant Tissues

• Dermal Tissue System:

  • Epidermis: Outer layer of cells, often covered by a cuticle (waxy coating) to reduce water loss.

  • Stomata: Openings for gas exchange (CO₂ in, O₂ and H₂O out).

  • Guard Cells: Control the opening and closing of stomata.

  • Trichomes: Hair-like structures that provide protection against herbivores or help in water conservation.

  • Cuticle: A waxy layer secreted by the epidermal cells. It serves as a protective coating to prevent excessive water loss and reduce the risk of water entry from pathogens.

• Ground Tissue System:

  • Parenchyma: Thin-walled cells involved in photosynthesis, storage, and healing.

  • Collenchyma: Cells with thickened walls, providing flexible support.

  • Sclerenchyma: Cells with very thick walls, often dead at maturity, providing structural support.

    • Fibers: Long, strong cells that provide rigidity.

Plant Tissues

Vascular Tissue System:

• Vascular Bundles:
  Groups of xylem and phloem tissues found in the stems of plants. These bundles are responsible for transporting water, nutrients, and sugars.

• Arrangement in Monocots:
  In monocots, vascular bundles are scattered throughout the stem, not in a ring.

• Arrangement in Eudicots:
  In eudicots, vascular bundles are typically arranged in a circle or ring in the stem.

• Xylem:
  Tissue responsible for transporting water and minerals from the roots to other parts of the plant.

  • Tracheids: Long, tapering cells with pits that allow water to pass between them.

  • Vessel Elements: Shorter, wider cells that form vessels, allowing more efficient water flow.

• Phloem:
  Tissue responsible for transporting sugars and other organic compounds.

  • Sieve Tube Members: Cells that form tubes to transport sugars and nutrients throughout the plant.

  • Companion Cells: Support sieve tube members by helping with metabolic functions like loading and unloading sugars.

Plant Growth

Primary Growth (Lengthening):

• Apical Meristems:
  Located at the tips of roots and shoots, these regions of active cell division are responsible for primary growth, increasing the length of the plant.

• Roots:

  • Root Cap: A protective covering at the tip of the root that protects the apical meristem as the root grows through the soil.

  • Zone of Elongation: The area just behind the root tip where cells elongate, causing the root to lengthen.

  • Zone of Maturation: The area where cells differentiate into their final forms, such as root hairs.

  • Root Hairs: Tiny projections that increase the surface area for water and nutrient absorption.

  • Stele: The central part of the root, composed of vascular tissues (xylem and phloem), which transport water and nutrients.

• Shoots:

  • Terminal Buds: The buds at the tips of the stems that allow the plant to grow taller or longer.

  • Axillary Buds: Buds found in the angles between the stem and the leaves, which can develop into branches or flowers.

• Leaves:

  • Primordia: The earliest stage of leaf development.

  • Mature: Fully developed leaves that are responsible for photosynthesis.

Secondary Growth (Thickening):

• Lateral Meristems:
  Meristems that are responsible for the thickening of the plant, specifically in stems and roots.

  • Vascular Cambium: Produces secondary xylem (wood) and secondary phloem (inner bark), contributing to the plant's girth.

  • Cork Cambium: Produces cork, a protective layer for the plant, found on the outer layers of stems and roots.

  • Cork: A protective, waterproof tissue produced by the cork cambium that helps protect the plant from water loss and injury.

• Secondary Xylem:
  Forms wood and is responsible for transporting water and nutrients in mature plants.

• Secondary Phloem:
  The inner bark, which is responsible for transporting sugars and other organic compounds.

Plant Transport

Water and Nutrients:

• Xylem Function:
  Transports water and dissolved minerals from the roots to the rest of the plant. This is crucial for maintaining turgor pressure and supporting photosynthesis.

• Total Water Potential (Ψ):
  The overall potential energy of water in a plant system. It determines the direction of water movement.

  • Solute Potential (Ψs): The effect of solutes on the water potential. More solutes (like salts or sugars) lower the solute potential, making it more negative.

  • Pressure Potential (Ψp): The physical pressure on water in the system. It can be positive (as in turgor pressure) or negative (in xylem vessels under tension).

  Formula:
  Total Water Potential (Ψ) = Solute Potential (Ψs) + Pressure Potential (Ψp)

  You should be able to solve water potential calculations by using this formula, considering both solute and pressure potentials.

Evapotranspiration in Leaves:

• Evapotranspiration is the loss of water through both evaporation from the soil and transpiration from the leaves. It helps create a negative pressure in the xylem, driving the movement of water from the roots to the leaves.

Role of Guard Cells:

• Guard cells control the opening and closing of stomata. When guard cells take up water, they become turgid, opening the stomata. When they lose water, they become flaccid, closing the stomata. This helps regulate gas exchange (CO₂ in, O₂ and H₂O out) and water loss.

Role of K+ (Potassium):

• Potassium ions (K+) play a crucial role in regulating the opening and closing of stomata by affecting the turgidity of guard cells. When K+ enters the guard cells, it causes them to take up water and swell, opening the stomata.

Apoplastic, Symplastic, and Transmembrane Flow in Roots:

• Apoplastic Flow:
  Water and nutrients move through the cell walls and intercellular spaces, bypassing the cytoplasm of cells.

• Symplastic Flow:
  Water and nutrients move through the cytoplasm of cells via plasmodesmata (small channels that connect plant cells).

• Transmembrane Flow:
  Water and nutrients move across cell membranes from one cell to another.

Role of Casparian Strip in Root:

• The Casparian strip is a band of waterproof material (suberin) that surrounds the endodermal cells in the root. It prevents the passive flow of water and solutes from the apoplast into the vascular tissue, ensuring that all water and nutrients pass through the selective membrane of the endodermal cells before entering the xylem.

Phloem Transport

Photosynthate:

• Photosynthate refers to the sugars and other organic compounds produced during photosynthesis, primarily glucose, that are transported through the phloem to various parts of the plant.

Phloem Function:

• The phloem transports the products of photosynthesis (like sugars) from the source (leaves) to sinks (growing tissues, storage organs).

Pressure Flow Model:

• This model explains how phloem sap moves through the plant. It suggests that sugars (mainly sucrose) are actively loaded into the phloem at the source, creating high pressure. This pressure pushes the sap towards areas of low pressure (the sink), where sugars are unloaded.

• Source:
  The part of the plant (like leaves) where sugars are produced or stored and loaded into the phloem.

• Sink:
  Any part of the plant (like roots, fruits, or growing tips) that requires sugars for growth or storage.

Role of Xylem:

• Although primarily for water transport, the xylem also helps with the movement of some nutrients to the phloem and plays a role in plant structure and growth.

Plant Nutrition

Macronutrients:

• Plants need large amounts of these elements for growth and development:
  Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sulfur (S)

Nitrogen Limitation:

• Nitrogen is critical for amino acids, proteins, and DNA. However, nitrogen is often limited in soil, which can affect plant growth. To compensate, plants often rely on nitrogen-fixing bacteria.

Nitrogen Cycle:

• Nitrogen-Fixing Bacteria:
  These bacteria (e.g., Rhizobium) convert nitrogen gas from the air into ammonia or other forms usable by plants.

• Nitrifying Bacteria:
  Convert ammonia into nitrates, which plants can take up.

• Ammonifying Bacteria:
  Break down organic matter into ammonia.

• Denitrifying Bacteria:
  Convert nitrates back into nitrogen gas, completing the nitrogen cycle.

Specialization of Legumes with Rhizobium:

• Legumes (like peas) have a mutualistic relationship with Rhizobium bacteria. The bacteria fix nitrogen in root nodules, providing the plant with nitrogen, while the plant provides the bacteria with sugars.

Carnivory as an Adaptation to Low Nitrogen:

• Some plants, like Venus flytraps and pitcher plants, have evolved carnivorous behavior to capture and digest insects to supplement nitrogen, especially in nutrient-poor soils.

Micronutrients:

• Essential elements that plants need in smaller amounts:
  Iron (Fe), Manganese (Mn), Boron (B), Copper (Cu), Zinc (Zn), Molybdenum (Mo), Chlorine (Cl)

Adaptations for Retrieving Cations from Soil:

• Plants use proton pumps in root hairs to actively pump hydrogen ions (H⁺) out of the roots, which helps displace cations (like calcium or potassium) from soil particles, making them available for absorption.

Mycorrhizae:

• Symbiotic relationships between fungi and plant roots. Fungi increase the surface area for nutrient absorption, particularly phosphorus, while the plant provides sugars to the fungi.

Plant Sensory Systems

Phototropism:

• The plant’s growth response to light. Plants grow toward light (positive phototropism) or away from it (negative phototropism).

• Role of Auxin:
  Auxins are plant hormones that promote elongation in cells on the shaded side of the plant, causing it to bend toward the light source.

Gravitropism (or Geotropism):

• The plant's growth response to gravity. Roots grow downward (positive gravitropism), while shoots grow upward (negative gravitropism).

• Statoliths:
  Specialized starch-filled organelles in plant cells that help sense gravity and guide root and shoot growth.

• Role of Auxin:
  In gravitropism, auxin is distributed unevenly in response to gravity. In roots, high concentrations of auxin inhibit growth on the lower side, causing the root to bend downward.

Apical Dominance:

• The phenomenon where the main central stem of the plant suppresses the growth of lateral (side) buds. This helps the plant focus on upward growth.

• Role of Auxin:
  Auxin produced in the apical bud inhibits the growth of axillary buds.

Role of Cytokinins:

• Cytokinins are plant hormones that promote cell division, particularly in the roots and shoots. They work in conjunction with auxins to regulate growth patterns.

Seed Dormancy:

• Dormancy prevents premature germination. It ensures that seeds germinate only under optimal environmental conditions.

• Role of ABA (Abscisic Acid):
  ABA is a hormone that promotes seed dormancy and inhibits seed germination.

Seed Germination:

• The process by which a seed begins to grow. It typically begins with water absorption, followed by the breakdown of stored food in the seed.

• Role of Gibberellins:
  Gibberellins are hormones that trigger the breakdown of stored food in seeds, allowing for the growth of the embryo.

Fruit Ripening:

• Ripening is the final stage of fruit development, during which fruits change color, soften, and become more palatable.

• Role of Ethylene:
  Ethylene is a gaseous plant hormone that promotes fruit ripening by regulating the expression of genes involved in color change, softening, and sugar accumulation

Angiosperm Reproduction

Alternation of Generations Lifecycle of All Plants:

• Alternation of generations refers to the plant lifecycle that alternates between two multicellular stages: the haploid gametophyte (which produces gametes) and the diploid sporophyte (which produces spores).

• The sporophyte generation is typically the dominant, multicellular form in most plants (like the tree or flowering plant).

• The gametophyte is usually smaller and lives inside the sporophyte in angiosperms, where it produces gametes (eggs and sperm).

Function of the Gametophyte:

• The gametophyte is responsible for producing gametes (sperm and egg cells) by mitosis.

• In angiosperms, the male gametophyte is the pollen grain, and the female gametophyte is the embryo sac inside the ovule.

Gamete Making by Mitosis:

• Gametes (sperm and egg cells) are produced by mitosis. This is because gametes need to be haploid (having only one set of chromosomes) to maintain the correct chromosome number after fertilization.

Function of the Sporophyte:

• The sporophyte is the diploid, multicellular stage of the plant, and its function is to produce spores through meiosis.

• In angiosperms, the sporophyte is the whole plant (flower, stem, leaves, roots), and it houses the reproductive organs (flower structures) that facilitate fertilization and seed production.

Spore Making by Meiosis:

• The sporophyte produces spores through meiosis. These spores are haploid (having one set of chromosomes) and will grow into the gametophyte generation.

Flower Form and Function

Female Structures:

1. Carpel (Pistil):
   The female reproductive organ of a flower. It consists of three parts:

   • Stigma: The sticky tip that catches pollen.

   • Style: The long stalk that connects the stigma to the ovary.

   • Ovary: The swollen base that contains the ovules (which become seeds after fertilization).

2. Ovule:
   The structure inside the ovary that contains the female gametophyte (embryo sac) and the egg cell.

3. Megaspore:
   The haploid spore that develops into the female gametophyte (embryo sac) after meiosis. Typically, only one of the four megaspores survives and matures.

4. Embryo Sac:
   The female gametophyte inside the ovule. It typically consists of 7 cells, including the egg cell and 2 polar nuclei.

5. Egg:
   The female gamete (haploid cell) inside the embryo sac. It will fuse with the male sperm to form the zygote.

6. Two Polar Nuclei:
   Two nuclei located in the center of the embryo sac. These will fuse with a sperm to form the endosperm (a nutrient-rich tissue that supports the developing embryo).

Male Structures:

1. Stamen:
   The male reproductive organ of the flower, consisting of two parts:

   • Anther: The part of the stamen that produces and releases pollen (male gametophytes).

   • Filament: The stalk that supports the anther and holds it in place.

2. Microspore:
   The haploid spore that develops into the male gametophyte (pollen grain) after meiosis.

3. Pollen Grain:
   The male gametophyte that contains sperm cells. The pollen grain is carried by wind or pollinators to the female stigma.

4. Pollination:
   The transfer of pollen from the anther to the stigma, which enables fertilization. It can occur via wind, insects, birds, or other methods.

Fertilization and Seed Production

Double Fertilization:

• In angiosperms, fertilization involves double fertilization, which means two sperm cells are involved:

  1. Egg + Sperm: One sperm fuses with the egg cell to form a zygote (the fertilized embryo), which will develop into the new plant.

  2. Sperm + Two Polar Nuclei: The second sperm fuses with the two polar nuclei to form the endosperm, a tissue that provides nourishment to the developing embryo.

Role of the Pollen Tube:

• The pollen tube forms after pollen lands on the stigma. The tube grows down the style to reach the ovary, where it releases the sperm cells. This is crucial for the sperm to travel and fertilize the egg and polar nuclei.

Fusion of Nuclei:

• During fertilization, the two sperm nuclei from the pollen grain fuse with the respective nuclei in the ovule:

  1. Egg + Sperm: To form the zygote (fertilized egg).

  2. Two Polar Nuclei + Sperm: To form the endosperm, which will help nourish the developing embryo.

Seed Production:

• After fertilization, the ovule develops into a seed containing the embryo, endosperm, and a seed coat. The seed is a mature ovule.

Endosperm:

• The endosperm is the tissue that forms from the fusion of the sperm and the two polar nuclei. It acts as a food reserve for the developing embryo.

Embryogenesis and Fruit Production

Embryogenesis:

• Embryogenesis is the process by which the fertilized egg (zygote) develops into an embryo. This involves cell division and differentiation to form the structures of the young plant (e.g., cotyledons, stem, root).

Fruit Production:

• After fertilization, the ovary of the flower develops into a fruit. The fruit's primary role is to protect the developing seeds and assist in their dispersal. The fruit is derived from the ovary, and its form varies (e.g., berries, nuts, capsules).

Role in Seed Dispersal:

• The fruit aids in seed dispersal by attracting animals (which eat the fruit and later excrete the seeds), or through mechanisms like wind or water. This ensures that seeds are spread over a wide area, helping the plant species to colonize new locations.