3.1: From Algae to Terrestrial Plants
pg. 90
Key Terms: alga, plant, embryo, sporic reproduction, gametophyte,
and sporophyte.
Algae are photosynthetic members of the kingdom of Protista. These
organisms can be either unicellular or multi-cellular.
Multi-cellular algae are also know as seaweed, and are classified into
three phyla based on colour; brown, red, or green.
Classifying an organism as either algae or plant is very difficult.
Plants have evolved from algae, and the green algae is the
evolutionary link between the two kingdoms.
The Multi-cellular Algae
All the unicellular plant-like protists are called algae. There are six
phyla of algae, three unicellular algae; dino-flagellates, diatoms, and
euglenoids, and three multi-cellular Seaweeds; brown, red, and green.
Figure 3.2 Unicellular Algae, including dinoflagellates, diatoms, and euglenoids, were
discussed in Chapter 2. The multi-cellular algae, or seaweeds, include brown, red, and green
algae.
Brown Algae (Phylum Phaeophyta)
Brown algae is the largest and most complex of the protests and are
also key components of marine and tidal environments. Some species
of kelp can grow up to 60 m. in height, creating underwater forests.
These areas form habitats for many species of fish and other
organisms, supplying food and shelter.
Brown algae do not have true roots and leaves, but do have
specialized tissues. Holdfast is a structure responsible for anchoring
algae to rocks or shells. Stipe acts like a stem-like structure. Blades
are flat-like coming from the stipe that act like leaves, collecting
sunlight energy for photosynthesis.
Figure 3.3 (A) Kelp are giant brown algae found in cool oceans, such as along British
Columbia’s Pacific coast. They form underwater forests. (B) Rockweed grows abundantly in
the tidal zones of rocky Atlantic coastlines. (C) The basic structure of a brown alga consists
of a holdfast, a stipe
Red Algae (phylum Rhodophyta)
Appears to be the first multi-cellular organism on Earth, between 1.5
to 1.2 billion years ago.
There are approximately 6000 living species of red algae. May grow
up to 1 metre in length, and most abundant in warm tropical coastal
waters.
Red algae as a photosynthetic species, have green chlorophyll and
additional pigments, phycoerythrin which can use light rays that reach
deeper depths. This allows Red algae to live at depths up to 100
metres.
Red algae is important economically, use in the food industry to wrap
sushi and in dairy products like ice cream.
Green algae (Phylum Chlorophyta)
Green algae is an aquatic organism commonly found in fresh water,
but can live in salt water environments.
Green algae maybe found in other ecosystems, such as; sea ice,
attached to surface of trees, and in the fur of sloths of the tropical rain
forests.
Green algae, Chlamydomonas are single celled organisms which use
flagella to move about. These cells can live in colonies. Other green
algae, Ulva (sea lettuce), can grow up to a metre in length, yet only 2
cells thick. These are the most plant-like of the algae. They contain
chlorophyll and are able to photosynthesize, and have cell walls to
store food in the form of starch.
Figure 3.5: The ancestor of plants is thought to be an organism very similar to today’s green
algae, like the examples shown here. Some green algae are unicellular, some are colonial, and
still others, including Spirogyra, are multi-cellular.
The Shift to Land
Scientific hypothesis states that the green algae are the closest
evolutionary relatives of land plants, and is based on several structures
within the cell.
- Plants have chloroplasts that contain green chlorophyll a and b.
- Both algae and plants have cell walls made up of cellulose
- Both stored food energy in the form of starch. (other organisms
store food as glycogen)
- The DNA of both organisms show similar sequences
Figure 3.6: Green algae are the organisms most closely related to plants.
Adaptations to Life on Land
Adaptations were required for aquatic organisms to land and become
terrestrial.
- The first adaptation required the organism to develop a method
to protect it from drying out. (roots and cuticle)
- Another adaptation was a development of a system to transport
water and dissolved substances from the outer environment to
the cells of the organism. (roots and vascular tissue)
- The development of support system to left the organism up
towards the light and air, instead of lying flat on the ground.
(stem)
These processes did not occur at once, but over long periods of time.
The first plants were small and grew in moist areas, which allowed
them to rely on osmosis and diffusion to obtain nutrients and water.
Plant Embryos
The transition from aquatic to terrestrial environments occurred about
460 millions years ago.
Plants, (different from algae), reproduce using embryos. Embryos are
small, simple, multi-cellular plants that are dependent on the parent
plant for a time.
Other evolutionary developments included vascular tissue fro
transporting materials, seeds for protecting the embryo, and flowers
for gamete exchange.
Figure 3.7 Significant developments in the evolution of land plants include the production of
embryos; the development of vascular tissue; and the production of seeds, and the flowers
(mya means millions of years ago)
Vascular Tissue, Leaves, and Roots
Vascular plants have two types of vascular tissue, xylem and phloem.
Xylem transports water ad minerals from the roots to the rest of the
plant.
Phloem transport larger molecules like glucose (sugars).
Vascular tissue allowed for the evolution of roots, providing a strong
anchoring ability, and the specializing of cells to absorb and transport
water and minerals.
Leaves evolved as specialized structures to maximize the capture of
light energy. They increased surface area of the plant above ground to
allow for a better exchange of gases (CO2 and O2) and the capture of
sunlight for photosynthesis.
Alternation of Generations
Sporic Reproduction has two multi-cellular stages in the
life cycle, a haploid version and a diploid version.
Gametophyte is a haploid version of the organism.
Sporophyte is a diploid version of the organism after two
gametophytes have fused together.
The sporophyte produces spores, by meiosis, which
develop into haploid gametophyte.
Figure 3.8: In sporic sexual reproduction, generations alternate between haploid stage and a
diploid stage.
Learning Check: Questions 1 – 6
page 93
Section 3.1: Review Questions 1- 12
page 95
UNIT 1 Chapter 3: Multicellular Diversity
Section 3.1 Review
Section 3.1