CHAPTER
7 - LEAVES:
1.
The leaf
is divided into two primary regions, the petiole and the
blade. The
petiole is the stalk that attaches the leaf to the stem, while the blade is the
flattened portion of the leaf, which is adapted for photosynthesis. Some leaves have leaf-like structures
called stipules at the base of the petiole.
2.
The
primary function of the leaf is photosynthesis.
3.
In
alternate leaf arrangement there is only one leaf
displayed per node, while
opposite arrangement has two leaves per node and
whorled arrangement has three or more leaves per
node.
4.
An
axillary bud is located on the stem at the point that
the petiole attaches to the stem. A
simple leaf has a single undivided blade even though the
blade may be deeply lobed in some species.
Compound leaves have a blade that is divided into
individual leaflets that look like individual leaves,
but there is no axillary bud at the point that the leaflet stalk attaches to the
petiole. See illustrations in the
text and online lab. Leaflets of a
pinnately compound leaf are arranged linearly along a
main stalk called the rachis. Leaflets of a palmately
compound leaf all attach to a petiole at the same point like
fingers to the palm. Again
recognize that an axillary or lateral bud is located at the point at which the
petiole attaches to the stem and that there is no axillary bud at the point
where leaflets of a compound leaf join the rachis or petiole.
5.
Veins
in leaves contain vascular tissue, xylem and phloem. Leaves have a specific arrangement of
veins that is characteristic of particular plant types. Leaves with parallel
venation have veins that run the length of the leaf and that are
more or less parallel. Parallel
venation is characteristic of monocots.
Leaves with netted venation have veins that
branch repeatedly forming a net-like arrangement of veins. Netted venation is characteristic of
dicots. Veins of leaves with
pinnately netted venation branch from a single central
vein while leaves with palmately netted veins have
several veins that branch from a single point at the base of the leaf. Some more primitive plants like
the Ginkgo have dichotomously branched veins. Dichotomously branched
veins always branch forming two branches like a fork.
6.
The
epidermis of the leaf is usually only one cell in
thickness and is composed of clear cells with no chloroplast. The outside of the epidermis is covered
with a waxy layer the cuticle. The cuticle reduces the evaporation of
water from the leaf surface.
7.
The
mesophyll is the region of photosynthesis. Some leaves have a mesophyll divided
into an upper region of column like cells (palisade
mesophyll) and a loosely arranges region of irregularly shaped
cells (spongy mesophyll). If present the palisade mesophyll is the
primary region of photosynthesis and the spongy mesophyll although still active
in photosynthesis facilitates gas exchange.
8.
Stomata
are openings in the epidermis that facilitate gas exchange. Guard cells are
found in pair change shape to regulate the size of the stomata.
9.
Leaf
veins contain vascular tissues, sometimes fibers and are surrounded by a layer
of chlorenchymous cells, the bundle sheath. In cross section, xylem tissue is always
located on the upper side and that phloem is always on the lower side of the
bundle. Monocots usually do not
have a palisade layer where dicots do.
10.
Pine
leaves are adapted to arid conditions.
The function of the hypodermis is to reduce the
evaporation of water from the leaf surface.
11.
Sun
leaves are usually smaller, thicker and darker green than
shade leaves that are located toward the inside of the
plant.
Xerophytic leaves are adapted to dry
conditions.
Mesophytic leaves are adapted to normal
terrestrial conditions while hydrophytic leaves are
adapted to very wet conditions such as aquatic plants.
12.
The
following specialized leaves are identified and pictured in your text. Please refer to the text for the
information of the following: leaf tendrils, spines, prickles,
storage leaves, flower pot leaves, window leaves, reproductive leaves, floral
leaves and the leaves of carnivorous plants
like pitcher plants and sundews.
13.
Autumn
color results from a combination of factors including chlorophyll breakdown, the
production of anthocyanins (red to blue pigments) and the appearance of
carotenoids (yellow to orange) pigments that were always present, but mask by
chlorophyll.
14.
Many
trees with broad leaves drop their leaves annually due to seasonal changes. In temperate regions with pronounced
seasons trees drop their leaves in response to changes from summer to winter and
in both temperate and tropical regions with pronounced wet and dry seasons trees
often drop leaves in response to the dry season. The process that facilitates leaf drop
is called leaf abscission. The petiole usually detaches from the
stem along a layer called the abscission zone. In this region the petiole is
structurally weak due to the absence of lignin in the cell walls. As the leaf senesces in the fall pectin
and other materials in the middle lamella of the abscission zone are broken down
by the action of enzymes such as pectinase and cellulase. As the abscission layer softens the
weight of the leaf will cause it to break.
The cells on the stem side of the abscission zone become suberized
forming a corky layer.
1.
Annuals
are plants that complete their life cycle in one growing season and then
die. The next generation arises
from the seed left by previous generations at the beginning of each growing
season. Biennials are plants that complete their life
cycle in two growing seasons. Only
vegetative growth is produced the first season. During the second season the plant
flowers and produces seed.
Perennials are plants that live for many seasons
and have many reproductive efforts over many years.
2.
The
floral parts of monocots are usually displayed in threes or multiples of three,
while dicots are usually in fours or fives or multiples of four or five.
3.
The
flower is a greatly compressed stem with four whorls or sets of
modified leaves that serve as floral parts. The flower is attached to the stem
by a stalk called the peduncle which expands at the base
of the flower to form the receptacle. The floral parts
arise from the receptacle. Starting
at the base and moving upward, the first whorl of floral parts is the
calyx, which is composed of leaf like sepals. The sepals often cover the bud and fold
back as the flower opens. In some
flowers the sepals are brightly colored and petal-like in appearance. In such
cases the sepals are referred to as tepals.
Stamens, the male
organs of the flower, are the next whorl of floral parts. Each stamen is divided into a supporting
filament and an apical capsule-like structure, the
anther. Pollen grains
are produced within the anther.
Each pollen grain is a male gametophyte,
which represents the male plant in the sexual phase in the plant life cycle.
The last whorl of floral parts is
the carpels. The
female gynoecium is composed on one or more carpels, each of which
enclosed at least one ovule. The
female gynoecium may be composed of a single carpel or more than one carpel
fused into a single structure. The
carpel is also referred to as the pistil. The carpel or pistil is composed of a
basal ovary, the style and an apical
stigma. The ovary
contains one or more cavities that contain the ovules. Each ovule contains a female
gametophyte. The style is
the stalk that connects the ovary and the stigma, while the stigma is the
structure to which the pollen grains adhere during
pollination.
4.
If the sepals,
petals and stamens originate from the upper surface of an inferior
ovary due to the floral tube of the sepals, petals and stamens being
fused to the ovary wall.
The sepals, petals and stamens of a superior ovary originate from
the base of the ovary. Ovary
position is used to differentiate between hypogynous,
perigynous and epigynous flowers. The ovary is superior in
hypogynous flowers and there is no floral tube
surrounding the ovary.
Perigynous flowers have a superior ovary but the
ovary is surrounded by a floral tube which is not fused to the ovary wall. Epigynous
flowers have a fully inferior ovary with the floral tube fused to
the ovary wall. he second whorl of floral parts is the corolla, a
structure composed of individual petals, which are brightly colored and
showy in many flowers. The shape
and color of the petals is designed to attract pollinators such as birds and
insects, while plants adapted to wind pollination usually have very small petals
if any at all. The term corolla
means colored wheel.
5.
A
fruit is a ripened of mature ovary. As the ovary wall the fruit provides
protection for the developing seed nad then facilitates dispersal of the seed in
one way or another at maturity.
6.
The
matured ovary wall or fruit is the pericarp, which is divided into three
layers. The outer layer is the
exocarp, the middle the mesocarp
and the inner most layer the endocarp. The layers vary in thickness from one
variety of fruit to another and vary from being soft and fleshy to being dry and
very hard.
7. Fruits are divided into four
basic types: simple fruit, aggregate
fruit, multiple fruit and
accessory fruit. A simple fruit is one that
develops from a single carpel or pistil.
Most fruits are simple fruit and are divided into two major groups,
fleshy and dry.
A berry is a
fruit that is fleshy through out.
The exocarp, mesocarp and endocarp of a true berry is soft
throughout with tomatoes, grapes and bananas as examples. A pepo such as a
watermelon or cucumber has a thick rind, while a hesperidium has a
leathery, oil containing skin.
Hesperidiums are citrus fruit such as oranges,
lemons and grapefruit. Another type
of simple fruit is the drupe. A drupe has a fleshy exocarp and
mesocarp, but a hard, stony endocarp.
Examples of drupes are peaches and apricots.
Dry simple fruit are divided into
those that split along a seam at maturity, dehiscent and those
that do not split, indehiscent. The classification of dehiscent fruit is
based upon whether they split along one, two or many seams. Follicles split along one
seam, legumes two seams and capsules, which consist
of at least two carpels split in a variety of ways. A silique is a type of dry
dehiscent fruit that splits along two seams but the seed are borne on a central
partition.
Dry indehiscent fruit are
grain (caryopsis), nuts, achenes,
samaras and schizocarps. A grain such as corn has a fused
pericarp and seed coat. The
pericarp and seed coat can not be separated. An achene is similar to a
caryopsis except that the pericarp and the seed coat are easily separated. The sunflower seed is an example of an
achene. A nut is similar to an achene except the pericarp is
usually much thicker and harder.
The base of the nut is enclosed in a cup-like structure composed of fused
bracts. An acorn is an example of a
true nut. A maple seed is an example of a samara. The pericarp of the fruit extends out
into a wing like structure that assist in dispersal of the seed. The schizocarp is a double
fruit or twin fruit that separate from each other at maturity. A schizocarp is unique to the carrot
family (Apiaceae).
Aggregate fruit
develop from flowers that have many individual and separate pistils. Each individual pistil develops into a
fruitlet. As the
fruitlets develop they fuse together to produce the aggregate. Examples are blackberries and
raspberries. Multiple fruit develop from many individual flowers
that are borne very close together on an inflorescence. As they develop the individual fruit
fuse together as in pineapples and horse apples.
Much of the fleshy tissue of an
accessory fruit (pomes) develops from tissues other than
the ovary wall. Usually a floral
tube enlarges and becomes fleshy as it grows amount the ovary or true
fruit. Examples are
strawberries, apples and pears.
8. Wind,
animals and water can serve as agents of dispersal for plant seed and
fruit. Many seed have plume like
structures that assist the seed in floating in air and can thus allow the seed
to be carried a considerable distance from the parent plant. Many other plants have hooks or other
kinds of adhesive structures or secretions that allow them to adhere to the
hair, feathers or skin of an animal that can carry the seed a considerable
distance. Still other seed must
pass through the digestive system of an animal before they will germinate or
they are simply transported in the digestive system of an animal. Even when we
consider islands as distance from other land masses as the Hawaiian Islands.
Many species reached the islands by being carried by birds, either on the
feathers or in the gut. A few seed
have adaptations that allow them to be transported by floating in fresh water or
even sea water.
9. The seed, a small self-sufficient reproductive package, has three main parts. These are the outer protective seed coat, which develops from the integument that surrounds the ovule, the embryo and the endosperm. The embryo is composed of the radicle which develops into the root, the plumule which develops into the shoots and the cotyledon(s) which store and transfer food to the developing embryo. The endosperm is a food storage tissue that is present at some point in the development of all seed.
Seed can be divided into two basic types, monocots and dicots. Monocots have a single cotyledon that is not usually used for food storage but transfers food from the endosperm to the developing embryo. Dicot seeds have two cotyledons although there are two types based upon the presence or absence of endosperm. A dicot such as the common bean has no endosperm in the seed at maturity; it has all been absorbed by the cotyledons. In contrast dicots such as caster beans have both endosperm and cotyledons at maturity.
10. Two distinct patterns of
germination can be identified. In monocots and some dicots the plumule pushes
upward through the soil developing into the shoot system while the
radicle grows downward producing the root system. At first roots develop faster
insuring their ability to provide water and minerals for the developing shoot
system.
Some dicots such as beans show a
different pattern of development. In this type of seed the region of the embryo
between the point of cotyledon attachment and the plumule is the hypocotyl. As
the seed germinates the radicle rapidly produces a root system while the
hypocotyl arches thought the soil. As the young plant continues to develop the
hypocotyl arch straightens pulling the cotyledons and plumule out of the
soil.
11. In temperate regions many seed
undergo a period of dormancy before they germinate. This insures that the seed do not
germinate at the end of the growing season. Often seed require a period of chilling
before germination. Plants adapted
to desert conditions or extended periods of drought produce seed that require
extensive flushing with water to break dormancy. In contrast seed produced in wet tropical
areas may germinate while still in the fruit on the tree. When all conditions for germination are
met such as chilling, adequate water, proper temperature or even passing through
the digestive system of an animal, the embryo is able to absorb food from the
endosperm or cotyledons and begins to grow rapidly.
1. DIFFUSION - The random movement of
particles (molecules or ions) from a region of their greater concentration to a
region of their lower concentration.
Results from random molecular motion. Occurs in gases, liquids and
solids. Occurs more rapidly with
substances of lower density and at higher temperatures. WHY?
2. OSMOSIS - The diffusion of a
solvent (usually water in biological systems) across a differentially permeable
membrane.
3. DIALYSIS - The diffusion
of a solute across a differentially permeable membrane.
4. BULK
FLOW - The
massive movement of substances in one direction due to a strong concentration
gradient. Often the organism is
actively involved in the creation of the concentration or pressure
gradient. An example is the flow of
sugars from the roots of plants to the shoots through transport tubes called
phloem. Sugars are actively loaded
into the phloem in the roots causing water to diffuse into the phloem causing a
strong pressure gradient between the roots and the shoots. The loading point is the "source", the
unloading point in the shoot is the "sink".
5. TURGOR -
Internal water pressure in plant cells due to the diffusion of water into the
cell vacuole which pushes outward against the cell wall causing the wall to
stretch and become rigid. Turgor
results from plant roots being in contact with soil water that is hypotonic to
plant cells. As a result water
diffuses into plant cells until the internal physical pressure equals the osmotic tendency
(force) to diffuse into the cell. Turgor is responsible for much of the support
and form of soft plant tissues.
6. Plasmolysis
is the loss of water from the plant cell due to the cell being in a hypertonic
environment.
Imbibition is a process by which a porous
material absorbs a liquid and swells.
This occurs as seed absorb large amounts of water prior to germination
7.
Transpiration
is the evaporation of water from the leaf surface.
8.
Root
pressure develops as a result of positive osmotic pressure in the
xylem. As a result water is pushed
up the xylem column. Root pressure
can only push water a few inches above the surface of the soil.
9.
Active
transport involves the movement of ions or molecules across a
membrane against its concentration gradient. A carrier and energy from ATP is
required to move ions or molecules against the concentration gradient.
10. The
transpiration-cohesion theory involves both the
transpiration of water from the leaf surface and the fact that water molecules
are attracted to each other. This
attraction is cohesion, thus a column of water can be supported against the pull
of gravity to the extent that theoretically a column could be pulled 600 feet
above the surface. As water is transpired from the leaf surface it creates a
negative water pressure in the leaves.
Water is thus pulled up the column of xylem tissue to replace that that
was lost.
11. In most
plants the stoma are open during the day and closed at night. Exposure to light causes an influx of
K+ from
surrounding subsidiary cell. This
produces a negative osmotic pressure in the guard cells causing water to diffuse
into the guard cells.. the result is that the guard cells swell and the stoma
opens. At sundown K+ is transported
out of the guard cells causing them to lose water to surrounding cell. This results in the closing of the
stoma. Overriding all of this is
water stress. Under water stress
the concentration of abscisic acid increases in the guard cells and prevents the
inflow of K+ when leaves are exposed to light, thus the stoma remain closed
reducing the loss of water from the leaf.
CHAPTER
10 – METABOLISM IN PLANTS:
1. The leaf is a specialized stem that
is flattened so that it can collect as much light as possible. Within the leaf, cells are arranged so
that they can collect as much light as possible. Chloroplast located within the cells
serve as the organelles of photosynthesis.
Each chloroplast is surrounded by two smooth outer membranes with a
complex inner network of membranes the thylakoids. Thylakoids are arranged in stacks of
thylakoid disk called grana. Photosynthetic pigments and the
compounds of the electron transport system are located in the thylakoid
membranes. The fluid filled region
of the chloroplast outside the thylakoids, which contains the enzymes involved
with carbon fixation, is the stroma .
2. SUMMARY OF
PHOTOSYNTHESIS:
6CO2 + 12 H2O
---------> C6H12O6 +
6O2 + 6H2O
3.
Visible light makes up a small part of the spectrum of electromagnetic
energy. Different forms of
electromagnetic energy differ from each other in the wavelength of the
energy. Electromagnetic energy
ranges from gamma rays at the short wave end, with a wavelength 10nm, to radio waves, with wavelengths up to
several meters, at the long wave end of the spectrum. Short wave energy has the greatest
amount of energy. Visible light is
found just beyond ultraviolet energy, and has a wavelength that ranges from 400
to 750nm. Light energy is measured
in photons. Photons are absorbed by
plant pigments such as chlorophyll and carotenoid. Light energy is transferred to electrons
of the pigments and is ultimately used in the synthesis of ATP.
4.
Chlorophyll a is the
essential photosynthetic pigment of all green plants. Other pigments such as chlorophyll b, c, d, carotenoids
and xanthrophylls are
accessory pigments that transfer light energy in the form of excited electrons
to chlorophyll a. The absorption spectrum of
chlorophyll identifies the colors of visible light absorbed by chlorophyll. Chlorophyll a and b absorb very little
light in the green range but are very active in the absorption of blue and red
light. The action spectrum of photosynthesis describes the activity of the
various colors of light in photosynthesis.
Accessory pigments collect energy from colors of light not utilized by
chlorophyll a and transfer the energy to chlorophyll a.
5.
The light dependent
phase of photosynthesis is dependent upon light as a source of energy,
while the light independent phase is dependent upon ATP generated in the light
dependent phase as its energy source.
The light dependent phase uses water as a raw material and produces ATP, NADPH and oxygen. The light dependent phase occurs in
association with the thylakoids. The light independent phase occurs in the
stroma and uses H
from NADPH and energy from ATP to reduce carbon dioxide to a
carbohydrate.
6.
Photosystems are
collections of chlorophyll and
accessory pigments that function as light harvesting units in
photosynthesis. The photosystems
are embedded in the thylakoid membrane.
Light energy is transferred to high energy electrons in the
photosystem. This is the conversion
of electromagnetic energy into chemical energy.
7.
Cyclic
photophosphorylation is a process that is not an essential part of
photosynthesis, but is a process through which ATP is produced in the presence
of light. In cyclic
photophosphorylation electrons of photosystem I are excited by
photons and transferred to electron transport pigments which release free
energy. This free energy is used to
pump H+ into the thylakoid space forming an electrochemical gradient
which is used to produce ATP. The
process is cyclic because the same electrons are returned to the photosystem
from which they exited. Noncyclic
photophosphorylation is the light dependent phase of
photosynthesis. It involves both photosystems I and II. It is noncyclic because the electrons
returned to the photosystems are not the same ones that originally exited. The electrons that leave from
photosystem I pass through a series of electron transport pigments eventually
reducing NADP+ to NADP-. NADP- then attracts a
H+ becoming NADPH.
Photosystem II absorbs photons and transfers high energy electrons to an
electron transport system. This
electron transport system releases energy used to establish an electrochemical
gradient across the thylakoid membrane.
This results in the production of ATP. The electrons from photosystem II are
transferred to photosystem I.
Photosystem II is now short
2e-. These are pulled from water in the process of photolysis which produces 2H+ and
O.
8.
In photosynthesis, chemiosmosis occurs by
H+ being pumped from the stroma to the thylakoid space. This establishes an electrochemical gradient across the
thylakoid membrane. As hydrogen ions move back to the stroma
through a channel protein, coupled to ATP synthase, energy is release
and ATP is generated. This is
called photophosphorylation.
9. The light independent phase is also
referred to as the Calvin
Cycle or simply as carbon
fixation. During this phase
carbon dioxide is reduced by H from NADPH producing a carbohydrate. Energy for this highly endergonic
process is derived from ATP produced in the light dependent phase. Carbon fixation is a cyclic process dependent upon the
regeneration of a 5C compound,)
ribulose 1,5 bisphosphate (RuBP). RuBP is the compound that initially
accepts CO2 at the beginning of the Calvin
Cycle.
10.
Photorespiration is a process
in which oxygen competes with carbon dioxide for the binding site on RuBP. The higher the O2
concentration the greater the chance of O2 binding to
RuBP. Bright light and high temperature
increases the chance of photorespiration.
When O2 binds to RuBP it results in the break down of RuBP
thus reducing the rate of photosynthesis by as much as
50%.
11. The
type of photosynthesis discussed so far is C3 photosynthesis. This is because a three carbon compound
is the first identifiable compound produced as carbon dioxide enters the Calvin
Cycle. C3 photosynthesis is
especially sensitive to photorespiration, thus under conditions of bright light
and high temperature, C3 plants suffer greatly from photorespiration. Plants that carry on C4
photosynthesis avoid photorespiration by binding CO2 to
a 3C compound, phosphoenol pyruvate, producing a 4C compound, oxaloacetate, in the mesophyll cells. Oxaloacetate is converted into malate that diffuses into the bundle sheath cells where
CO2 is released, converting malate into 3C pyruvate which diffuses
back to the mesophyll. The release
of CO2 in the bundle sheath cells creates a high concentration of
CO2 in the bundle sheath cell.
As a result the Calvin Cycle is completed in the bundle sheath cells
greatly increasing photosynthetic efficiency under conditions of bright light
and high temperatures. Many
tropical plants adapted to these conditions are C4 plants. These include sugar cane, Bermuda grass
and many others.
12. Crassulacean acid metabolism
(CAM) is a process that occurs in certain plants adapted to very dry
conditions. In this process the
plants close their stomata during the day and open them at night. This allows them to conserve water, but
restricts their CO2
uptake during daylight hours. CAM plants take CO2 from the
atmosphere at night and convert it to organic acids. During the day the acids release
CO2 which is used in photosynthesis. Cacti are common CAM
plants.