Download BL 1021 – Unit 2-3 Plants III

BL 1021 – Unit 2.3-2.5
Plant Life Cycles and Growth
Genetics Terminology
• When one cell divides, creating two new cells, each
with a complete and identical set of DNA, it is
called mitosis. This creates two daughter cells that
are identical to the mother. This occurs in growing
parts of an organism and in asexual reproduction.
• When a cell divides twice in a row to create four
cells, each with half of the DNA of the original, this is
meiosis. In this process, each of the four gamete
cells created are unique. These cells fuse at
fertilization to create a new organism with a
complete set of DNA.
2.3 Life Phases
• When a cell contains only one copy of each
chromosome, it is a haploid cell (n). Cells with two
copies of each chromosome are diploid (2n).
• Plants spend a phase of their lives as haploid and a
phase diploid. Some plants will feature a portion of
them going through a triploid (3n) phase.
Alternation of Generations
• In many species of plants, both the haploid and
diploid phases of plants create complex structures,
grow and exist for a long period of time.
• This does not happen in humans. Haploid cells in
humans (sperm/egg) exist only for a brief period
and quickly fertilize into a diploid organism.
• In these species, one generation of a plant will exist
as haploid and then another diploid, plants that
follow this life cycle are said to have alternation of
generations.
Alternation of Generations
• The Alternating plant Life Cycle:
• Begins with a fertilized diploid zygote. This cell will
divide over and over by mitosis to create a
complex diploid plant known as a sporophyte.
• Eventually certain cells within the sporophyte will
undergo meiosis to create haploid cells known as
spores.
Alternation of Generations
• These spores will go on to divide by mitosis to create
a complex multicellular plant known as a
gametophyte.
• Eventually the gametophyte will create gamete
cells by mitosis. This haploid gametes will find a
partner and fuse together to form a diploid zygote,
completing the cycle.
Alternation of Generations
2.4 Life Cycle of Flowering Plants
• Flowering plants have a life cycle where a diploid
organism creates both male and female gametes
which then combine to create a new plant.
• As the sperm and egg do not mature into full
multicellular plants prior to fertilization, it can be
argued there is not true alternation of generations in
angiosperms.
• The basic live cycle involves a sperm and egg fusing
into a zygote in a seed, which will mature into a
plant that will produce more sperm spores and
house eggs in an ovary.
2.4 Life Cycle of Flowering Plants
• On the stamen (male reproductive organ) of a
flower, spores are created by cells undergoing
meiosis. These spore cells will undergo mitosis to
create multiple cells in the spore. Unlike with human
sperm which can accomplish fertilization
individually, there is need for multiple supporting
cells.
• When a pollen grain attaches to the entry to the
female reproductive structure, the carpel, cells
within it begin dividing and extends a pollen tube
into the carpel to deliver the sperm to the ovary.
2.4 Life Cycle of Flowering Plants
• Two sperm end up being introduced to the ovary.
One fertilizes the female gamete, the egg, creating
a new diploid zygote.
• The other sperm contributes to the formation of a
triploid (3n) cell that will develop to assist the
zygote.
• As two sperm are used, this process is known as
double fertilization.
2.4 Life Cycle of Flowering Plants
2.4.3 Formation of a Seed
• The ovary contains several ovules, each with their
own egg cell. Each ovule is double fertilized and so
will have a 2n zygote and a 3n helper cell. After
fertilization, each ovule of the ovary will mature into
a seed.
• The zygote will divide by mitosis to create an
embryo.
• The 3n cell will develop into a multicellular structure
known as the endosperm, a mass of cells around
the embryo that offer food and protection.
2.4.3 Formation of a Seed
• The embryo develops the cotyledon(s), which
begin to absorb nutrition from the endosperm.
• As well, the hard outer coating of the shell is
formed, housing both the embryo and endosperm
inside.
• After a certain point, the embryo will stop
development and wait until it arrives in a fertile soil
environment to grow in.
2.4.4 The Fruit
• As the fertilized seeds mature, the ovary of a flower
develops into a fruit. This structure works to protect
the seeds as well as to help disperse them.
• Many fruits are designed to be eaten. The flesh of
the fruit attracts animals. The seeds survive digestion
and can begin life in a pile of fertilizer.
• Some fruit are not designed to be eaten. Dandelion
fruit are designed to be transported by winds.
2.5.1 Types of Growth
• Most animals have a limit to how large they can
grow regardless of food supply or time. This is
determinate growth – the maximum size of the
creature is determined.
• Most plants, however, do not operate like this and
will grow as large as resources allow. This type of
growth is known as indeterminate growth. This allows
a plant to continuously grow to take in more
sunlight, water, and minerals.
2.5.1 Types of Growth
• Even with unlimited growth, most plants have a
limited life span.
• Many plants will grow, reproduce and then die with
the seasons of one year. These are annuals. Some
will last for two years, typically with reproduction
occurring in the second year. These are biennials.
• Some plants, however, will live for many, many
years. These are perennials.
2.5.2 Types of Growth
• Plants can grow in two ways – either by expanding
their reach, or expanding their bulk.
• Primary growth in plants involves the lengthening of
the plant.
• Secondary growth in plants involves the thickening
and widening of roots and stems.
• At any given time, both may occur at once.
2.53 Plant Metabolism
• Plants are autotrophs which means they can build
any organic compound they need from basic
carbon dioxide.
• Plants make their sugar fuel source from water and
carbon dioxide.
• Other compounds are made through a
combination of carbon dioxide, water and/or trace
minerals absorbed through the roots. The atoms of
carbon dioxide and water for these compounds
may be used directly or derived from sugar.
2.5.4 Plant Nutrients
• Macronutrients are those substances that a plant
needs in large amounts to survive. These substances
are used to build vital parts of the cells – proteins,
DNA, cell walls, etc. Carbon, oxygen, and hydrogen
are examples of macronutrients.
• Materials needed in much lower numbers are
called micronutrients. Many of these are simple
metals, ions and inorganic compounds. Examples
include iron, chlorine and copper.
2.5.5 Plant Hormones
• Plants use hormones to control, regulate and direct
growth and development.
• Plants produce hormones in very small amounts, but
their effects are still significant.
• Two major hormones:
• Abscisic acid – deactivating hormone. Keeps seeds
dormant. Keeps plants dormant in the winter. Closes
up stomata.
• Gibberellins – activating hormones. Promotes
growth, fruit development, returning from winter
dormancy.
2.5.5 Plant Hormones
2.5.6 Directed Growth
• Plant growth is dependent on the environment it is
placed in. Plants need to have their leaves in the
sunlight as directly as possible and their roots
securely cemented into the ground. If not, the plant
will die.
• Plants have phototropism –they grow in the
direction of sunlight. Feedback from cells receiving
less or more sunlight will direct the plant in one
direction or another.
2.5.6 Directed Growth
• Plants also are directed by gravity. Stems grow
against the pull of gravity and thus have negative
gravitropism. Roots, on the other hand, show
positive gravitropism, growing towards the pull of
gravity. Certain hormones in the plant will trigger
these responses by themselves being pulled down
by gravity.
• Plants also respond to the touch of the surface.
Thigmotropism appears in many plants to direct
their growth. Plants with tendrils will often “feel out”
a suitable target and wrap around them for
support.