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Topic 9 Plant Biology
Introduction
• Plants are extremely important
• They produce oxygen
• They remove carbon dioxide from the
atmosphere.
• They produce food (carbohydrates)
• We will study transportation (xylem &
phloem), growth, and reproduction.
9.1 Xylem
• Water is lost as a vapor from leaf openings
called stomata, called transpiration.
• This lost water must be replaced by
absorption.
• Leaves vary in form but generally consist of
the flat blade, and a stalk called the petiole
that attaches the blade to the stem.
Structure of a leaf
Leaf Structure
• Outer layer for many is a waxy cuticle for
protection and to prevent water loss.
• Next layer is epidermis which also is for
protection.
• Vascular tissue, xylem brings water to the leaf
while phloem carries products of
photosynthesis to the plant.
• Both are found in bundles called veins.
• Under the epidermis on the top of the leaf is the
palisade mesophyll where most of the
chloroplasts are located.
• Above the epidermis on the bottom of the leaf is
the spongy mesophyll, loosely packed cells with
lots of room for gas exchange.
• Stomata are on the bottom of the leaf and are
the site of gas exchange. Water can be lost here.
Protected by guard cells which control the size of
the openings.
Water and Mineral Movement
• Water lost from leaves (transpiration) is
replaced by water from the roots.
• This is a constant flow which also carries
minerals with it.
• Transpiration helps to cool leaves and stems.
• Xylem is involved in this upward movement of
water as well as support of the plant.
Xylem
• Two main types of cells, tracheid and vessel
elements.
• Tracheids are dead cells that taper at the ends
and connect to form a tube.
• Vessel elements are also dead cells that have
thick walls supported by lignin and ave pits to
allow lateral movement.
• Tracheids evolved first, vessel elements came
later.
• Water movement within the xylem is cohesiontension mechanism.
Stomata and Guard Cells
• Stomata openings are controlled by the two
guard cells around them.
• When full of water, high turgor pressure opens
the guard cells, when low on water, low turgor
pressure causes the stomata to close.
• Water movement is controlled by potassium ions.
• Blue light causes protein pumps to pump
potassium into the guard cells, which causes
water to move into the cells.
Abscisic Acid
• Abscisic acid is a hormone that causes
potassium ions to rapidly diffuse out of the
guard cells, causing closure.
• Produced in the roots when water levels are
low.
• Prevents water loss from transpiration, but
there’s a problem with this. What is it???
Cohesion-Tension Theory
• Water leaves the plant due to more water in leaf than
in atmosphere. (transpiration)
• Lost water is replaced by water from vessels (xylem)
• Water in the vessels in maintained by cohesion and
adhesion.
• Tension occurs on the column of water as water leaves
the leaf.
• Water is pulled from the root cortex into the xylem.
• Water is pulled from the soil into the roots.
Roots and fluid movement
• Main job of roots is uptake of water and
minerals inn the form of ions.
• Root hairs increase surface area 3X.
• Root cap protects apical meristem during
growth.
• Root has 3 zones: cell division zone
(mitosis),elongation zone where cells grow
(G1), and maturation zone where cells
become functional. ( root hairs etc.)
Mineral ions
• Mineral ions get into roots 3 ways:
Diffusion
Fungal hyphae (symbiotic relationship) called
mycorrhiza
Active transport
Adaptations for water conservation
• Light speeds up transpiration by warming and
opening stomata.
• Humidity decreases transpiration
• Wind increases transpiration
• Temperature increases transpiration
• Soil water decrease causes transpiration
decrease
• Carbon dioxide increase causes transpiration to
decrease.
Desert and salt plants
• Xerophytes are adapted to arid climates by
limiting transpiration loss
Small thick leaves decrease surface area
Fewer stomata
Stomata found in recessed pits
Thick waxy cuticle
Hair like cells on surface help to trap moisture
Lose leaves during the driest times
• Succulents store water in their thick stems and
leaves
• Xerophytes use CAM and C4 processes
• CAM plants close stomata during the day and
make sugar at night.
• C4 plants open during the day but can bring in
carbon dioxide quicker than usual.
Halophytes
• Grow in salt
• Mangroves secrete salt through glands
9.2 Transport in the phloem
Movement of organic molecules
• Organic molecules move via the phloem.
• Phloem is made of living cells.
• Made from sieve tube members and their
companion cells.
• Sieve tube members connected to each other
by plates to form sieve tubes.
• The plates have pores for movement.
• Companion cells connect by plasmodesmata
• Movement of organic molecules in plants is
called translocation.
• Fluid is called sap and is mostly sucrose, with
some amino acids, hormones and RNA
Pressure-Flow hypothesis
• The movement of sugar into and out of the
phloem causes water movement into and out
of the phloem from the xylem.
• This causes pressure differences and aids in
movement.
• Used aphids to learn about this.
9.3 Growth in Plants
• Most plants have 3 basic types of tissue
• Dermal tissue, outer covering for protection
and water loss.
• Ground tissue, thin walled cells that do
storage, photosynthesis support, secretion.
• Vascular tissue, xylem and phloem, move
water, minerals and nutrients around plant
and offer some support.
• All are derived from meristematic tissue, small
undifferentiated cells sort of like stem cells.
• When they divide, one cell remains an
undifferentiated meristem, the other
differentiates to become some part of the
plant.
• The one that stays a meristem is called an
initials, the ones that differentiate are celled
derivitives.
• Unlike animals, plants grow all during their
lives, called indeterminate growth.
• Determinant growth ceases after a certain
period of time.
• Plants do die however. Some are annuals,
living only one year, some are biennials, living
two years, perennials live for many years and
usually die from an infection or other
environmental factor.
Apical Meristems
• Apical meristems, sometimes called primary
meristems, are at the tips of roots and stems.
• This area is referred to as a shoot apex.
• New growth is called primary growth and is
mitosis.
• This causes non-woody stems and roots
Lateral meristems
• Causes growth in width or thickness of plants.
• Most trees and shrubs have two kinds of
lateral meristems:
• Vascular cambium, which is between the
xylem and phloem. Makes secondary xylem
that forms wood, and secondary phloem
• Cork cambium, in the bark of the plant, forms
outer bark.
Plant Hormones
• Factors that affect plant development:
Environmental such as day length and water
availability
Receptors which allow plant to detect
environmental factors.
Genetic makeup of the plant
Hormones , chemical messengers
• Receptors in certain cells receive stimuli causing
the cell to release a hormone.
• Hormones have a particular cell they effect called
the target cell.
• Unlike animal hormones, a plant hormone can
have a different effect on cells in different parts
of a plant.
• In animals there is usually one gland that makes a
specific hormone, in plants, hormones are made
throughout the plant.
Auxins and phototropism
• Tropisms are movements based of an external
stimulus.
• Positive tropism is toward the stimulus,
negative tropism away from the stimuli.
• Common stimuli for plants are chemicals,
gravity, touch, and light.
• If the stimulus is light, its called phototropism.
• Plant stems show positive phototropism while
plant roots show negative.
• Allows plants to move toward the light for
survival.
• Auxins are hormones that cause positive
phototropism.
• Found in the embryo of seeds, apical
meristems, and young leaves.
• Auxin only works on cells that have auxin
receptors.
• Auxin causes elongation of cells by making
their cell walls more flexible.
• Auxin is produced by all cells in the area but is
goes to the side of the plant that is away from
the light.
• Active transport pumps auxin out of cells on
the light side. Indoleacetic acid
• Auxins interact with the nucleus of the cell
changing gene expression, which activatse a
proton pump that pumps protons into the cell
wall and changes the pH.
• This causes hydrogen bonding within the
cellulose to break down.
• Auxins also help with cell division,
differentiation between xylem and phloem,
lateral roots, growth of flowers.
9.4 Reproduction in plants