Green Algae and Land Plants: Structure, Diversity, and Adaptations

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Green Algae and Land Plants

Introduction

Green algae and land plants are essential groups within the domain Eukarya, playing a critical role as primary producers in terrestrial and aquatic ecosystems. This section explores their key features, evolutionary adaptations, and life cycles, focusing on the transition from aquatic to terrestrial environments.

Key Terms and Definitions

Green algae: Photosynthetic eukaryotes, primarily aquatic, considered the closest relatives of land plants.
Land plants: Multicellular, primarily terrestrial organisms that evolved from green algal ancestors.
Artificial selection: The process by which humans selectively breed plants for desirable traits.
Primary producers: Organisms that synthesize organic compounds from inorganic sources, forming the base of food webs.
Vascular tissue: Specialized tissue (xylem and phloem) for transporting water, minerals, and sugars throughout the plant.
Cuticle: A waxy, protective layer covering the aerial parts of plants, reducing water loss.
Stoma / stomata: Pores on the plant surface that regulate gas exchange and water loss.
Guard cells: Specialized cells that control the opening and closing of stomata.
Lignin: A complex polymer that strengthens cell walls, especially in vascular tissue.
Vessel elements: Short, wide cells in angiosperms that form part of the xylem, facilitating efficient water transport.
Gametangia: Multicellular organs that produce gametes (sperm or eggs) in plants.
Embryophytes: Land plants that protect their developing embryos within parental tissues.
Alternation of generations: A life cycle alternating between multicellular haploid (gametophyte) and diploid (sporophyte) stages.
Gametophyte: The haploid, gamete-producing phase of the plant life cycle.
Sporophyte: The diploid, spore-producing phase of the plant life cycle.
Heterospory: The production of two distinct types of spores: microspores (male) and megaspores (female).
Pollen: Male gametophyte enclosed within a protective coat, enabling fertilization without water.
Ovule: Structure in seed plants that develops into a seed after fertilization.
Flower: The reproductive structure of angiosperms, containing organs for sexual reproduction.
Stamen: The male reproductive organ of a flower, producing pollen.
Carpel: The female reproductive organ of a flower, containing the ovary.
Fruit: A mature ovary that contains seeds, aiding in their dispersal.
Monocot: Angiosperms with one seed leaf (cotyledon).
Dicot: Angiosperms with two seed leaves (cotyledons).
Rhizoid: Hair-like structures in non-vascular plants that anchor the plant and absorb water.
Root: Organ in vascular plants that anchors the plant and absorbs water and nutrients.

Diversity and Evolution of Land Plants

Are all plants multicellular? Are all plants eukaryotic?

All land plants are multicellular and eukaryotic.
Some green algae are unicellular, but all land plants are multicellular.

Plant Cell Walls and Storage Molecules

Cell walls are primarily composed of cellulose, a polysaccharide.
Plants store metabolic products as starch, a polymer of glucose.

Monophyly and Ancestry of Plants

Plants are monophyletic, meaning they share a single common ancestor.
Their most recent common ancestor is believed to be a group of green algae (specifically, charophytes).

Major Taxa of Land Plants

Non-vascular plants (e.g., mosses, liverworts, hornworts)
Seedless vascular plants (e.g., ferns, club mosses)
Seed plants (gymnosperms and angiosperms)

Adaptations for Life on Land

Key Adaptations

Cuticle: Prevents desiccation by reducing water loss from plant surfaces.
Stomata: Allow regulated gas exchange while minimizing water loss.
Water-conducting cells: Tracheids and vessel elements transport water efficiently, supporting larger plant bodies.
Gametangia: Protect gametes from drying out and mechanical damage.
Embryophyte condition: Retention and protection of the developing embryo within parental tissues.

Tracheids vs. Vessel Elements

Feature	Tracheids	Vessel Elements
Structure	Long, tapered cells with pits	Short, wide cells with perforations
Occurrence	All vascular plants	Mainly angiosperms
Efficiency	Less efficient water transport	More efficient water transport

Antheridia vs. Archegonia

Structure	Antheridia	Archegonia
Function	Produces sperm	Produces eggs
Location	Male gametophyte	Female gametophyte

Alternation of Generations

Overview

Land plants exhibit alternation of generations, a life cycle that alternates between multicellular haploid (gametophyte) and diploid (sporophyte) stages.

Life Cycle Steps

The sporophyte (diploid) produces haploid spores by meiosis.
Each spore grows into a gametophyte (haploid) by mitosis.
The gametophyte produces haploid gametes (sperm and eggs) by mitosis.
Fusion of gametes (fertilization) forms a diploid zygote, which develops into a new sporophyte.

Key Features

Gametophyte is haploid (n) and produces gametes by mitosis.
Gametes fuse to form a diploid zygote, which becomes the sporophyte.
Sporophyte is diploid (2n) and produces spores by meiosis.
Each spore grows into a new gametophyte.

Summary Table: Alternation of Generations

Stage	Ploidy	Produces	Process
Gametophyte	Haploid (n)	Gametes	Mitosis
Sporophyte	Diploid (2n)	Spores	Meiosis

Heterospory

Heterospory refers to the production of two types of spores: microspores (male) and megaspores (female).
This adaptation is seen in seed plants and some seedless vascular plants.

Additional Information

Pollen and ovules are key innovations in seed plants, allowing reproduction without water.
Flowers and fruits are unique to angiosperms, enhancing reproductive success and seed dispersal.
Monocots and dicots are two major groups of angiosperms, distinguished by the number of seed leaves (cotyledons).
Rhizoids are found in non-vascular plants, while roots are present in vascular plants.

Example: Moss Life Cycle

The dominant stage is the gametophyte (haploid).
Sporophyte (diploid) grows from the gametophyte and produces spores by meiosis.
Water is required for sperm to swim to the egg in the archegonium.

Example: Fern Life Cycle

The dominant stage is the sporophyte (diploid).
Gametophyte is small and independent, producing both antheridia and archegonia.
Spores are dispersed by wind and grow into new gametophytes.

Equations

Meiosis in sporophyte:
Mitosis in gametophyte: