Download ch 38 - ltcconline.net

Principles of Biology - Biology 102
Spring Quarter
Written by: Sue Kloss
Lake Tahoe Community College
Instructor: Ralph Sinibaldi
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Ch. 38 Angiosperm Reproduction
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Intro- Rafflesia - parasitic plant of Indonesia - Cabbage sized floral bud that produces a tire-sized flower with
an odor of carrion. It attracts carrion flies that eventually pollinate it. A few days after opening, it shrivels and dies,
producing 4 million seeds. Many plants are propagated or reproduce asexually as well.
A Life Cycle of a flowering plant. Plant Reproduction - fitness = ability to replace yourself with fertile offspring
1. flower - reproductive structure of plants (Fig. 38.2)
2. Parts of a flowera. petals - colorful to attract dispersers or pollinators
b. reproductive organs
1. stamens- male organs
a. anther - sac at tip where meiosis occurs and pollen grains
develop here. Pollen grains house cells that develop into sperm
2. carpel - female organ of flower.
a. Tip of carpel = stigma; receives grains of pollen
b. base of carpel = ovary, hoses reproductive structures called
ovules (contain developing eggs and cells that support it)
3. life cycle of generalized angiosperm
a. fertilization in ovule, which matures into a seed with embryo
b. ovary develops into fruit, protects seed and aids dispersal
c. seed germinates, embryo develops into a seedling, then into mature plant
B. development of pollen and ovules culminates in fertilization
1. haploid and diploid alternation of generations- review (Fig. 38.2)
a. most reproductive structures of all plants are diploid, plant body is sporophyte
c. ovules and anthers in angiosperms have cells that undergo meiosis
d. haploid spores result from meiosis
e. spores divide (mitosis) and become multicellular gametophyte (haploid
generation)
f. gametophyte produces gametes by mitosis
g. fertilization = union of male and female gametophyte (diploid zygote)
h. zygote divides by mitosis and develops into sporophyte (Fig. 38.3)
2. without microscope, we can see only sporophyte and occasionally pollen (male
gametophytes). Let’s look at gametophytes.
a. In anthers, there are chambers called pollen sacs that have cells that develop into pollen
(Fig. 38.4)
1. Each cell, called a microspore mother cell or microsporocyte, undergoes
meiosis, forming 4 haploid spores, each of which can give rise to a haploid male
gametophyte
2. spores divide mitotically, forming 2 haploid cells, called tube cell and
generative cell. At this point it is an immature male gametophyte- the key is
whether the structure is haploid and single celled (spore) or haploid and multicelled
(gametophyte).
3. a thick wall forms around these 2 cells and resulting pollen grain is
ready for release
a. During maturation of male gametophyte, generative cell passes into
tube cell so that the tube cell has a completely free standing generative cell
inside of it
b. usually, the generative cell will divide again so that it produces 2 sperm
cells
c. the tube cell will grow after pollination thru the style and into the
micropyle, depositing the sperm in the vicinity of the embryo sac
b. most plants have ovary with several ovules
1. ovule contains central cell (megasporocyte or megaspore mother cell)
surrounded by protective covering of smaller cells (sporangium)
2. central cell enlarges, undergoes meiosis producing 4 haploid spores
3. 3 of the spores degenerate, the 4th enlarges and divides mitotically
to produce a multicelled structure called the embryo sac
4. embryo sac is female gametophyte. It contains a large central cell
with 2 haploid nuclei. One of its other cells is the egg.
a. the ovule now consists of the embryo sac and the surrounding
integument
b. synergids flank the egg and function to attraction and guidance of the
pollen tube to the egg
c. at pollination, pollen grain lands on the stigma of the carpel. most
angiosperms need an animal to deliver pollen; gymnos and most grasses use
wind. some plants use water for pollination
1. pollen grain germinates on the stigma.
2. tube cell grows downward as the pollen tube into micropyle and the ovary
3. generative cell divides mitotically, forming 2 sperm as tube grows
4. When pollen tube reaches base of ovary, it enters ovule and deposits
2 sperms into embryo sac
5. Double fertilization occurs:
C. Mechanisms that prevent self fertilization
1. sex increases genetic variation
2. self fertilization is often called selfing; some plants use this to ensure that seeds develop
3. most angios have barriers to prevent selfing
a. dioecious plants - have different reproductive structures on different flowers
b. timing of development of floral parts provides self incompatibility
c. structural arrangement makes it unlikely that animal pollinator will transfer pollen to a
stigma of the same flower (Fig. 38.5)
d. self incompatibility- ability of a plant to reject its own pollen, or pollen of closely related
individual
II. Ovule develops into a seed and ovaries into fruits
A. Double fertilization (Fig. 38.6)
1. one sperm fertilizes egg
2. other sperm contributes nucleus to large central cell of embryo sac.
a. embryo develops from zygote
b. endosperm
3. this occurs only in plants, mainly angiosperms.
4. after fertilization, ovule contains triploid central cell and zygote.
a. ovule begins developing into seed
b. triploid cell divides; develops into endosperm- nutritious, multicellular milky mass
c. endosperm nourishes seedling till it becomes a self supporting seedling
1. in some plants, endosperm provides seed w/ nourishment
2. in other plants, nutrients transferred to cotyledons, so mature seed lacks
endosperm
d. embryonic development- zygote divides by mitosis into 2 cells. (Fig. 38.7)
1. one cell (still called zygote) divides repeatedly, becomes a ball called
of cells called embryo
2. the other cell divides to form a thread of cells that pushes the embryo
into the endosperm called the suspensor
3. bulges on embryo in figure on cotyledons, monocot would have only 1
4. mature seed forms; ovule’s coat has lost most water, formed
resistant seed coat. Encloses and protects embryo and food supply, the
endosperm.
5. Embryo stops developing at this point. Seeds become dormant.
6. Further development occurs at germination.
7. Seed dormancy - growth and development are temporarily suspended
important adaptation - allows time for plant to disperse seeds and for
good germination conditions (temperature and moisture) to be present
8. Dormant embryo- miniature root and shoot, each equipped with
apical meristem; also, 3 tissue cylinders that form epidermis, the cortex
and primary vascular tissues.
9. Fig 38.8- seed structure of dicot and monocot
a. bean is dicot - no endosperm- absorbed by cotyledons
seed coat, epicotyl, hypocotyl, radicle, cotyledons
b. corn - monocot; a kernel is a fruit containing one seed.
contains a large endosperm and single cotyledon.
c. cotyledon absorbs endosperm’s nutrients during germination.
B. Ovary develops into a fruit . Fruits develop at same time seeds do, but from outer parts of ovary. A fruit
is a matured ovary (Fig. 38.9)
1. protects seeds,
2. helps disperse seeds (corn kernels, peach, orange, tomato, cherry, pea pod.
a. After pollination, flower drops petals
b. hormonal changes cause ovary to begin to grow.
c. it expands tremendously, walls thicken
d. wall forms pod or fruit (Fig 38.9- pea plant seeds)
e. after full fruit development, pod would harden and eventually
split open, releasing seeds
f. in some cases, the true fruit is harbored inside a fleshy part of the
flower (apples; the ovary is actually inside the apple core)
3. Simple fruits develop from a flower with a single ovary and carpel.
4. Aggregate fruits develop from a flower with many carpels. Each part of the
berry fruit develops from a single ovary.
5. Multiple fruits develop from a group of separate flowers tightly clustered
together. When the walls of the ovaries thicken, they fuse together and form one
fruit (Pineapple)
C. Seed germination is the resuming of growth of a plant that is already formed in the seed, the embryo (it
has an embryonic root and shoot) - end of the dormancy period
1. imbibation - taking up water - seed expands and ruptures seed coat
a. (Fig. 38.10) germination begins when seed takes up water.
b. expansion of seed ruptures coat
c. triggers metabolic change in the embryo to make it begin growing again
d. enzymes in endosperm or cotyledons begin digesting stored nutrients
e. nutrients are transported to growing region of the embryo
f. roots generally emerge first in a germinating seed
g. shoots of dicots emerge as a hook so the tender shoot tip is pulled thru
soil rather than pushed
h. shoots of a monocot are protected by a sheath, and pushed straight up thru
soil. The corn cotyledon remains in the soil and decomposes (38.10)
E. Asexual Reproduction - vegetative reproduction - produces clones
1. Fragmentation - separation of parts from parent and subsequent regeneration of
rest of plant - (cuttings)
2. A garlic bulb is actually an underground stem that functions in storage
a. bulb fragments into cloves
b. each clove can give rise to a plant
c. white sheaths are leaves attached to stem (bulb)
3. A redwood tree will sprout new copies of itself from undifferentiated tissue called
burls near the base of the tree.
4. ring of creosote bushes in desert are root sprouts from an original that probably
came from a seed 12000 ya.
5. Many grasses can sprout new stems and roots from runners, or aspens can sprout clones from
roots (Fig. 38.11)
6. Since meristems remain active throughout the life of a plant, they retain the ability to
sustain growth or vegetative reproduction indefinitely
7. vegetatively reproduced plants are mature copies of the original, not seedlings; so
developmentally and ecologically they may avoid some of the problems of seedlings.
8. Sexual and asexual reproduction have both played an important role in adaptation of
plants to their environments
Questions for homework/ Lesson Objectives for Chapter 38
1. In general terms, explain how the basic plant life cycle with alternation of generations is modified in angiosperms.
2. List four floral parts in order from outside to inside a flower.
3. From a diagram of an idealized flower, correctly label the following structures and describe the function of each
structure:
a. sepals
b. petals
c. stamen (filament and anther)
d. carpel (style, ovary, ovule, and stigma)
4. Distinguish between:
a. complete and incomplete flowers
b. bisexual and unisexual flowers
c. monoecious and dioecious plant species
5. Explain by which generation, structure, and process spores are produced.
6. Explain by which generation, structure, and process gametes are produced.
7. Name the structures that represent the male and female gametophytes of flowering plants.
8. Describe the development of an embryo sac and explain the fate of each of its cells.
9. Explain how pollen can be transferred between flowers.
10. Distinguish between pollination and fertilization.
11. Describe mechanisms that prevent self-pollination.
12. Outline the process of double fertilization. Explain the adaptive advantage of double fertilization in angiosperms.
13. Explain how fertilization in animals is similar to that in plants.
14. Describe the fate of the ovule and ovary after double fertilization. Note where major nutrients are stored as the embryo
develops.
15. Describe the development and function of the endosperm. Distinguish between liquid endosperm and solid
endosperm.
16. Describe the development of a plant embryo from the first mitotic division to the embryonic plant with rudimentary
organs.
17. From a diagram, identify the following structures of a seed and state a function for each:
a. seed coat
b. proembryo
c. suspensor
d. hypocotyls
e. radicle
f. epicotyl
g. plumule
h. endosperm
i. cotyledons
j. shoot apex
18. Explain how a monocot and dicot seed differ.
19. Explain how fruit forms and ripens.
20. Distinguish among simple, aggregate, and multiple fruit. Give an example of each type of fruit.
21. Explain how selective breeding by humans has changed fruits.
22. Explain how seed dormancy can be advantageous to a plant. Describe some conditions for breaking dormancy.
23. Describe the natural mechanisms of vegetative reproduction in plants.
24. Explain the advantages and disadvantages of reproducing sexually and asexually.
Ch. 39 Control Systems in Plants
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Intro: Plants are constantly tuned in to and responding to their environments. Being rooted to the ground requires plants
to respond to everything that comes their way. When plants are exposed to light, cells respond to quantity, color,
direction of light, and cells begin biochemical changes that influence the chemical signals in a plant. Ultimately, these
cause the plant to bend toward the light. Animals use movement to respond to enviro cues, plants use a change in growth
and development. For this reason, there’s much more variability in plant form within a species than in animal form. (eg.
lions vs oaks - # of limbs, size, etc)
I Signal transduction pathways - link signal reception in plants to their responses.
A. signals elicit a response from an organism via receptors in cells - eg. UV light has no receptor in our eye
cells, but bees are guided to nectar
1. eyes of a potato are axillary buds- they sprout pale, white shoots in the dark - etiolation
a. all energy goes toward elongation to find light source eventually and break ground and Ps
2. when shoot reaches sunlight, plant undergoes de-etiolation
a. slow growth, put on leaves, roots grow, shoot produces chlorophyll
B. General model of cell - signal processing
1. reception, transduction and response (fig. 39.3)
C. Reception
1. signals first detected by receptors - proteins that undergo conformational changes in response to
specific stimulus
2. phytochrome is a receptor that occurs in the plasma of a plant cell
3. when phytochrome interacts with light, process of de-etiolation begins
D. Transduction
1. receptors are sensitive to very weak environmental and chemical signals
a. eg. moonlight is enough in some plants to cause a slowing of growth in some oat seedlings
2. How is info from weak signals amplified, and how is response generated?
a. second messengers - small internally produced chemicals that transfer and amplify signal
from receptor to toher proteins that cause response
1. phytochrome - each molecule activated in turn activates hundreds of second
messenger molecules
2. each second messenger may activate hundreds of molecules of a specific enzyme
3. result is rapid amplification of signals in plant
b. second messengers in de-etiolation - (Fig. 39.4) - phytochrome causes activity along 2
different biochemical pathways
1. one pathway ramps up production of cGMP, which changes properties of ion
channels and increases Ca2+
2. second pathway uses cGMP as a second messenger to activate a specific protein
kinase - these work by phosphorylating proteins at specific site to activate them
c. both pathways influence activity of specific ion channels and protein kinases which will
influence plant enzyme activity and other properties
E. Response
1. signal transduction pathways lead to regulation of one or more cellular activities
2. two main mechanisms for activating enzymes via signal pathways
a. stimulation of RNA transcription (mRNA) to produce particular enzymes
b. activating existing enzymes
3. transcriptional regulation - transcription factors bind to specific regions of DNA and control
transcription of specific genes
a. activation of some of these transcription factors requires cGMP, other transcriptional factors
require Ca2+
4. posttranslational modification of proteins - works on existing proteins
a. most modified by phosphorylation
b. many signal pathways use phosphorylation cascades (one phosphorylated protein in turn
phosphorylates another) to regulate synthesis of new proteins, usually by turning genes on/off
F. De-etiolation proteins
1. some function in Ps, some supply chemical precursors of chlorophyll; others affect plant hormones
that regulate growth
II. Plant Hormones - chemical signals that coordinate different parts of organisms - only minute amts. needed to induce
substantial change
A. Phototropism - plants turn toward light
1. cells on darker side elongate faster, pushing plant toward light (Fig. 39.5)
2. if light is uniform, plant grows straight
3. hormones regulate plant growth; discovered in series of experiments
a. 1880 - Darwin and son, Francis- grass seedlings would bend only if shoot tips were
exposed to light, growth response occurs below shoot tip
b. Boysen-Jensen - 1913 1) tested chemical signal idea block of gelatin between tip and rest of shoot: no
cell contact, chemicals can diffuse; bending occurred
2) mica block -impermeable- no bending
c. both support signal as mobile chemical
d. Fritz Went - isolated chemical messenger - 1926
1) removed tips, placed them on agar. messenger should diffuse in.
2) centered agar blocks on shoots with tips removed- faster growth than no tips
3) offset agar blocks produced bending
4) untreated agar blocks = no effect
5) Went reasoned that there is a higher conc. of chemical he called auxin on dark
side of plant
e. later research found that auxin produced equally on all sides, but travels to dark side to
increase concentration
B. 6 major types of plant hormones and classes
1. produced in very small amts., but just a few molecules may have profound effect
a. redirect metabolism and development of cells
b. turn genes on or off, activate or inactivate enzymes, change membranes
c. all 6 types influence growth, most influence development
1) they can cause increase division or elongation
2) inhibit growth by decreasing division or elongation
3) effects depend on species of plant, stage of development, concentration of
hormone
4) mostly hormones balanced together to control growth and development
2. Auxins - class of chemicals- promote elongation of developing shoots
a. indoleacetic acid = IAA (Fig. 39.8) causes pathway for cell expansion; above a certain
range, IAA makes plants synthesize ethylene, another hormone which counters IAA
b. same hormone may have different effects on same target cell depending on conc.
c. different target cells may respond differently to same concentration
d. auxins also may trigger devel. of vascular tissue and induce cell division in vascular
cambium to promote stem diameter growth
e. IAA in developing seeds may promote growth of fruit
f. if you spray developing plants with auxins, you don’t need fertilization to produce fruits
result is seedless fruits - tomatoes, cucumbers, eggplants
3. Cytokinins are growth regulators- they promote cell division (cytokinesis)
a. natural and synthetic
b. produced in actively growing tissues - roots, embryos and fruits
c. made in the roots, move up through xylem sap
d. enhance division, growth and development of cultured plant cells
e. retard aging of fruit and flowers - florists use as spray on cut flowers to last longer
f. most plants have complex growth patterns, if apical meristem removed, auxin won’t
exert influence on axillary buds, so cytokinins from roots can cause bushiness (Fig. 39.9)
g. even if meristem is intact, these 2 hormones balance each other- perhaps how the
plant regulates shoot vs. root growth
4. Giberellins affect stem elongation (Fig 39.11)
a. synthesized in root tip and stem tip
b. along with auxins, stimulate stem and leaf elongation
c. along with auxins, influence fruit development
d. important in seed development in many plants; giberellins probably the cue for
environmentally controlled metabolic processes e.g. if H2O available, giberellins released
allowing germination by mobilizing nutrients stored in the seed.
5. Brassinosteroids - steroids that are chemically similar to cholesterol and sex hormones; they
act very similar to auxins - induce cell elongation and divison in stems and seedlings, and retard leaf
abscission and promote xylem differentiation
6. Abcisic acid (ABA = Abcisic acid) - induces dormancy, suspending primary and secondary
growth. What are such times?
a. in many plants, ABA acts as growth inhibitor. When downpour washes ABA out of
seeds, they germinate (conditions are good)
(Fig. 39.12)
b. ABA signals buds to form scales protecting them in harsh conditions
c. in many plants, ratio of ABA to gibberellins is important in germination
d. acts as “stress hormone”
6. Ethylene triggers fruit ripening and other aging processes
a. ripening = breakdown of fruit cell walls, and changes in color; sometimes drying
b. produced by all cells of a plant
c. fruits produce more of it as they age
d. diffuses through plant in spaces btn cells
e. diffuses through air from plant to plant
f. one bad apple spoils the bunch
g. tomatoes picked green, piped in ethylene ripens them, plastic bags increase effect
h. sometimes growers will retard action of ethylene w/CO2
I. leaf color change - new colors made in autumn, also loss of chlorophyll that masks
other colors; ethylene probably plays a role
j. leaf abcission - base of leaf stalk separates from the stem (abcission layer) protective
layer helps tree avoid infection; weight of leaf and wind split abcission layer; leaf falls.
C. Growth Responses and Biological Rhythms
1. tropisms = directed growth response, growth toward or away from a stimulus
a. phototropism - growth toward light
1) auxin migrates to dark side
b. gravitropism - growth response to gravity
c. thigmotropism - response to touch- tendril = modified leaf grows slower in contact area
causing curling around object
D. Internal clocks - 24 hr innate biological clock = circadian rhythm. occurs even if organism is sheltered
from environmental cues
1. gene that controls clock- when concentration is high enough, protein production stops. When
concentration of protein falls, transcription starts- roughly a 24 hour cycle
2. adapt for temperature; if not, our clocks would be a mess
E. Plants mark season by measuring photoperiod
1. flowering, seed production, germination and dormancy occur at specific times each year.
2. Plants use photoperiod - relative lengths of day vs night
3. flowering and other responses actually controlled by night length, not day length
a. short day plants (long night) - will not flower if night length interrupted by a flash needs 10 continuous hours of dark
b. long day plants (short night) will only flower if night length is shorter than some
critical number (say, 10 hours) flowering can be induced by flashing long nights
F. How do plants measure photoperiod? Pigments called phytochromes
1. phytochromes - proteins with lightabsorbing component.
a. light absorbed at reddish end of spectrum
b. molecules appear bluish
c. light detectors
d. 2 forms of phytochromes 1) Pr absorbs red light (660 nm in length) and is converted to Pfr.
2) Pfr absorbs far red (730 nm in length) and is converted to Pr
3) Occurs slowly in natural light,
G. Plant defense
1. herbivory a. thorns,
b. plants recruit predators to defend them from certain herbivores (Fig. 33.14a)
c. chemicals (distasteful or toxic compounds) such as canavanine - similar in
structure to arginine, an amino acid. It takes place chemically, but not functionally abnormal functions, so insect dies.
d. morphology - ground cover
2. pathogens- bacteria, viruses, fungi
a. epidermis = like skin (can be overcome by wounds or nat’l openings (stomates)
b. chemicals to kill infection and to signal other cells
c. chemical changes in cell wall, toughening the walls
d. inherited ability to recognizecertain pathogens
1) pathogen gains access but doesn't kill plant
2) plant said to be resistant
Study Questions:
Ch. 39
1. Compare the growth of a plant in darkness (etiolation) to the characteristics of greening (deetiolation).
2. Describe the signal pathways associated with de-etiolation.
3. Describe the role of second messengers in the process of de-etiolation.
4. Describe the two main mechanisms by which a signaling pathway can activate an enzyme.
5. Explain, using several examples, what researchers have learned about the activity of plant
hormones by study of mutant plants
6. For the following scientists, describe their hypothesis, experiments, and conclusions about the
mechanism of phototropism:
a. Charles and Francis Darwin
b. Peter Boysen-Jensen
c. Frits Went
7. List six classes of plant hormones, describe their major functions, and note where they are
produced in the plant.
8. Describe how abscisic acid (ABA) helps prepare a plant for winter.
9. Describe the effects of ABA on seed dormancy and drought stress.