Botany – Part 2
• Plant Diversity
Tropism
• Tropisms is growth that occurs in response
to an environmental stimulus
– Three major categories:
• Gravitropism: Response to gravitational force; auxin
and gibberellins create an uneven growth pattern
when a plant is placed on its side
• Phototropism: response to light; auxin elongates one
side of a plant more than the other so it “stretches”
toward a light source
• Thigmotropism: response to contact; vines growing
on the side of a building
Cellular Mechanisms: Growth
• Growth: the irreversible increase in size resulting from
cell division and cell enlargement
• The zygote divides using mitosis to produce a multicellular embryo in the seed which is dormant until
conditions are met
• Mitosis resumes in the root and shoot apical meristems
after germination
• Enlargement of the newly produced cells results in most
of the actual size increase
• Tropisms contribute to the overall shape and growth of the
plant
Cellular Mechanisms:
Morphogenesis
• Morphogenesis: the development of body shape
and organization
• Begins in the early divisions of the embryo to
produce the cotyldons (nutrient structure) and
rudimentary roots and shoots
• Continues to shape the root and shoot systems
– The meristem stays embryonic (early development) for
the life of the plant and continues morphogenesis as
needed for growth
Cellular Mechanisms: Cellular
Differentiation
• Cellular differentiation: the divergence in a
structure and function of cells as they become
specialized during development
• Gives rise to various unique organs in physiology
• Each cell of each organ is fixed in a certain
location and performs a specific function
– Ex: guard cells, xylem, phloem, etc.
Metabolic Processes
• As plants grow, they need to take in
nutrients to allow metabolism
• Chloroplasts: Photosynthesis; produces
G3P, a sugar precursor used to make sugars
and starches
• Mitochondrion: Cellular respiration;
produces ATP to power cellular processes
and Mitosis
Obtaining Nutrients - Photosynthesis
• Stomata in the mesophyll open to allow CO2 to
enter
– C4 plants have photosynthesis occur in mesophyll cells
(light dependent rxn) and bundle sheath cells (light
independent rxn)
– CAM plants open stomata at night to limit transpiration
• Sunlight is captured by chlorophyll and converted
to energy in light dependent reactions using
NADPH
• Water is transported through the Xylem into the
cells to donate the electrons for light dependent
reactions
Stomata Open and Close
• Stomata open at dawn:
– 1. Light induces guard cells to take in K+
– 2. Decrease of CO2 in leaf air spaces due to
photosynthesis
– 3. Internal clocks induce the opening of stomata
• Guard cells close if:
– There is a water deficiency
– Production of abscisic acid (a hormone) signals closing
– High temperatures increase CO2 in leaf air spaces
Water through Osmosis
• Water is absorbed through the root system
and transferred into the xylem for transport
• Water in the xylem uses a concentration
gradient and capillary action to fight gravity
as it travels up
– This means more water is in the roots vs the top
of the plant
– Transpiration (loss of water when stomata
open) helps to maintain this gradient
Three Levels of Transport
• 1- uptake and loss of
water and solutes by
individual cells (root cells)
• 2- short-distance transport
from cell to cell (sugar
loading from leaves to
phloem)
• 3- long-distance transport
of sap within xylem and
phloem in whole plant
Cellular Transport
• Water transport
√ Osmosis: movement of H2O
– hyperosmotic - gain water
– hypoosmotic – loose water
– isotonic – approximately the same
• Cell wall creates physical
pressure: water potential
– solutes decrease; pressure increase
• Water moves from high to low
water potential
– The goal is equilibrium between
the interior of the cell and the
exterior environment
Cellular Transport
• Flaccid (limp, iostonic);
– Will gain/loose water
depending on surrounding
solution
• Plasmolysis (cell loses
water in a hypertonic
environment; plasma
membrane pulls away);
• Turgor pressure (influx of
water due to osmosis;
hypotonic environment)
Transport of Xylem Sap
• Transpiration: loss of water
vapor from leaves pulls water
from roots (transpirational
pull); cohesion and adhesion
of water
• Root pressure: at night (low
transpiration), roots cells
continue to pump minerals
into xylem; this generates
pressure, pushing sap
upwards
Translocation of Phloem Sap
• Translocation: food/phloem transport
– Products of photosynthesis
• Sugar source: sugar production organ
(mature leaves)
– Where sugar is produced through
photosynthesis or breakdown of starch
• Sugar sink: sugar storage organ
(growing roots, tips, stems, fruit)
– Consumes the sugar produced
• Source and sink depends on the season
• Minerals can be transported to the sink
• Direction of flow depends on the
locations of the source and sink
Nutrients
• Essential- required for the plant life cycle
• Macro- (large amounts) carbon, oxygen, hydrogen, nitrogen, sulfur,
phosphorus, potassium, calcium, magnesium
• Micro- (small amounts; cofactors of enzyme action) chlorine, iron,
boron, manganese, zinc, copper, molybdenum, nickel
Sexual Reproduction
• Alternation of generations:
haploid (n) and diploid (2n)
generations take turns
producing each other
• Sporophyte (2n): produces
haploid spores by meiosis; these
spores divide by mitosis giving
rise to male and female haploid
plants called….
• Gametophytes (n): develop and
produce gametes
Floral variations
• Floral organs: sepals, petals,
stamens (male), carpels (female)
• complete: all 4 floral organs
• incomplete: lacking 1 or more
floral organs
• perfect: both stamens and carpels
on 1 flower (capable of self
pollination and reproduction)
• imperfect: lacking either a
stamen or carpel (must have
other flowers to reproduce)
Gametophyte development
• Male gametophyte:
microsporocyte (in pollen sacs
of anther) divides by meiosis
into 4 haploid microspores;
mitosis produces a generative
cell (sperm) and a tube cell
(pollen tube)= a pollen grain
– Extremely durable due to
tough coat
– Pollen grain is an immature
male gametophyte that
develops within
Gametophyte development
• Female
gametophyte:
megasporocyte (in ovule)
divides by meiosis to 4 cells,
only 1 survives to a haploid
megaspore; 3 mitotic divisions
forms the embryo sac; includes:
1 egg cell (female gamete) and
2 polar nuclei
Pollination
• Plants use several methods to distribute the pollen, or
microsporocytes
– Wind: the wind distributes the pollen to other plants within the area
– Animal: several animals are used to spread pollen within plants, the most
common are birds and bees
– Cross pollination is possible for some plants, but most must cross pollinate
• Pollen must land on the stigma at the tip of the carpel
• These methods allow for distribution of organisms over a larger
are of land and randomly, ensuring greater survival and less
competition for resources
– Clumped dispersion: many in a small area
– Uniform distribution: evenly spaced across an area
– Ransom distribution: randomly spaced