Plants PLANT STRUCTURE AND GROWTH

The angiosperms/flowering plants are the most diverse group of plants.

Divided into 2 classes: 1. Monocots. Ex: grass, corn, grains, rice,

lilies, orchids 2. Dicots. Ex: maple trees, beans, roses,

oak The morphology of a plant body has a

subterranean root system and an aerial shoots system.

2 types of vascular tissue/stele: 1. Xylem: conducts water upward from

the roots 2. Phloem: transports food made in the

leaves throughout the plant. 3 types of ground plant cells:

1. Parenchyma: a. Most common plant cells b. Have thin flexible primary cell walls c. Their functions are for photosynthesis in the chloroplasts of the leaves

of parenchyma cells d. Some of these cells are involved in storage (starch) e. Carry out most of the plant’s metabolic needs f. Ex: potatoes, spinach leaves

2. Collenchyma: a. Have thicker primary cell walls b. Are grouped into cylinder like the “strings” of a celery stalk c. Their function is support to parts of the plant still growing

3. Sclerenchyma: a. Thickest cell walls of all three cell types b. At maturity, these cells are dead and cannot grow in length like the

other cells c. Their function is support which is reinforced by a secondary cell wall

hardened with lignin d. Are found in things like tree bark, shrubs, nutshell, pears, etc.

Ground Tissue Parenchyma Collenchyma Sclerenchyma Tissue Tissue Tissue Function -Photosynthesis (leaf) -support in young -rigid support - Food storage (root) stems, roots, and -protection -Healing and tissue petioles -tracheids and vessels are Regeneration components in some plants Cell Types Parenchyma cells Collenchyma cells Sclereid cells & fiber cells In This Tissue

Xylem: Its function is to conduct water and minerals and

provide support (it is dead) Have both a primary and secondary cell wall for added

strength 2 types of xylem cells:

1. Tracheids: tall and thin 2. Vessel elements: wider and short

Both tracheids and vessel elements are arranged in tubes laid end-to-end where water flows through the openings at their ends and through the holes in the cells called pits

The tubes are hollow and the cells are dead

Phloem: Its function is to conduct sugars Made up of cells laid end-to-end called sieve-tube members Have no secondary cell walls Are alive at maturity Pores on the end of

these sieve-tube members form sieve plate—where sugars and ions move between the food-conducting cells

Sieve plate allows the cytoplasm of one sieve-tube member to make contact with the next cell

Sieve tubes have parenchyma cells next to each cell that are called companion cells—connected to the sieve-tube member by many plasmodesmata.

Plasmodesmata: similar to gap junctions in animal cells Companion cells: make proteins for the sieve-tube member cells that lack nuclei and ribosomes; some help load sugar

made in the leaf into the sieve-tube member cells. Other plant cell types include the epidermis/epidermal cells that cover the outside of the plant. This is generally a single

layer of tightly packed cells that protect the plant and form dermal tissue. Some epidermis cells like those found on root hairs are involved in absorption Other epidermis cells like those on leaves secrete a waxy coating called the cuticles that helps the plant retain water. Seed forms when an ovule and pollen come together in fertilization. The seed is the embryo, a seed coat, and storage materials like endosperm, and cotyledons. After a seed reaches maturity, it remains dormant until environmental cues cause it to germinate. Cues include water,

temperature, light, opening of the seed coat. Water is the most important cue to activate various enzymes inside the seed which starts cellular respiration. Then radical (young root) breaks out of the seed coat and anchors the seedling. Apical meristem: in the young seedling, growth occurs at the tips of the roots and shoots. This is very active growth/cell divisions and called primary growth. Secondary growth: occurs only in woody plants that thicken their roots and shoots. This leads to plants like conifers and

woody dicots. The roots of dicots are taproots that consist of one large vertical root and many smaller roots.

Taproot: this is a firm anchor and helps plants “tap” water from far below ground.

The roots of monocots are fibrous thread-like mats that spread out in the surface layers of soil helps to hold soil in place and prevent erosion.

Root hairs on root tips increase the surface area of the root tremendously.

Mycorrihizae: fungi living on plant roots and in many plants and also absorb water and minerals as well.

In some plants, the roots rise above ground to gain access to water and help support stems, these roots are adventitious.

The root tip is protected by a thimble-like root cap that protects the cells from the abrasive soil; the cap also secretes a polysaccharide slime to help push through the soil.

Zone of cell division: up from the root cap; these rapidly dividing cells absorb water and blend into the next region the zone of elongation.

Zone of elongation: cells elongate to more than 10x their original length.

Zone of maturation: cells being to differentiate into specialized functions such as mature into xylem or parenchyma cells. Root hair form extensions here.

There are 3 tissue systems to the roots, stems and leaves: 1. Dermal 2. Ground/cortex tissue system in between 3. Vascular tissue/stele with the xylem and phloem

The ground tissue system has the cortex which stores starch and has lots of space between the cells. Ground tissue system has the endodermis in the innermost part of the cortex. Endodermis: a semi-permeable barrier that determines what can enter into and out of the vascular tissue; it especially

control ion movement. Vascular cylinder/stele: includes the xylem and phloem. The steles of monocots have a central core of parenchyma cells called the pith.

Primary tissue in the stem is similar to primary tissue in the roots. The epidermis, however, has a waxy cuticle and the cortex contains chloroplast. The vascular cylinders are different. In dicots, the stem’s vascular bundles are in a circle around the outside of the stem. In monocots, the vascular bundles are random throughout the ground tissue.

Epidermis of leaves is covered by a waxy cuticle reduces water loss or transpiration.

Transpiration: evaporation of water from plants. The palisade mesophyll cells have many

chloroplasts where photosynthesis primarily occurs.

The spongy mesophyll cells are loosely packed and thus allow for gas exchange (O2 and CO2) to respiring cells.

The guard cells on the epidermis control the opening and closing of stomata.

The vascular bundles consist of the normal xylem and phloem.

Some plants like C4 plants also have bundle sheath cells to provide an anaerobic environment for the Calvin cycle to only fix CO2.

TRANSPORT IN PLANTS Water and minerals/ions enter plants through roots by osmosis. The water must first pass the epidermis then go through the cortex, and then to the selectively permeable endodermis

before entering into the xylem. 2 possible routes to getting to the xylem:

1. Intracellular path: first crosses the cell walls of the root hair and then the cell membrane. The water and minerals then move across the cortex via plasmodesmata (gaps from cell to cell) before arriving at the semi-permeable plasma membrane of the endodermis and xylem.

2. Extracellular path: moves water from the root hair to the xylem, but the water remains within the cell wall of the root. And once the water arrives at the endodermis it encounters a barrier called the Casparian strip—waxy belt prevents water and ion from entering the xylem without first be selectively allowed in by the cell membrane of the endodermis.

The intracellular pathway just described moves water/ions through the “living” cytoplasm/plasmodesmata route and is called symplastic.

The extracellular route for water and ions through the “nonliving” cell walls is called the apoplastic. The endodermal plasma membrane is selective not so much to water (water enters via osmosis), but to the ions/minerals in

the water. Plants need K+ ions, but

not Na+ ions. So the endodermis only allows K+ to enter through specific channels.

These ions enter rather easily when the cells are negative charged.

This negative charge is established because plant cells have proton pump—actively pump H+ out of the cells, generating a membrane potential.

3 ways that water and dissolved ions move from the roots to the xylem and eventually out of the plant via stomata on the undersides of leaves (transpiration): 1. Osmosis: passively transports water into root hairs because the water direction goes from hypotonic hypertonic or

from a high water potential to a low water potential. So the mineral concentration inside the stele is always higher than that of the soil to drive into the plant’s roots. Osmotic force generates a small root pressure and is what causes dew/sap (water and minerals) on the grass in the early morning. But osmosis isn’t a major force in moving water up a plant’s xylem.

2. Capillary action: the rise of liquid in narrow tubes contributes to water moving up the xylem. Cohesion—water molecules like to stick to other water molecules through hydrogen bonding. Adhesion—water is attracted to unlike substances also (cellulose in xylem cells). These 2 attractions help water to be pulled continuously up the xylem, but these are not the major contributor to water being pulled.

3. Cohesion-tension mechanism: most water movement in plants is due to this. It is based on the idea that as water evaporates from plant’s stomata during transpiration, a negative pressure or tension (pulling) develops within the xylem. Couple this to the cohesion between water molecules forming a column of water from the roots to the leaves. This leads to bulk flow of water via the xylem’s tracheids and vessel element cells and out the leaves.

The ultimate driving force of the cohesion-tension mechanism is the heat from the sun.

When stomata open, CO2 can enter and O2 and H2O can exit. When stomata closed, photosynthesis cannot occur because no CO2

is entering the Calvin cycle and no water is being pulled up by transpiration.

When Stomata are open, and both CO2 and H2O are made available the plant is at risk of desiccation or drying out.

The opening and closing of stomata must be controlled by 2 cells that surround the stomata openings called the guard cells.

^ Cell wall that borders the stomata is thicker than the rest of the cell. This is important because when water enters the guard cell by osmosis, the guard cells become turgid and swell then buckle outward. This increases the space between the two cells or the opening of the stomata. Then when the cells lose water and become flaccid, the space is closed.

The changes in the turgor pressure that open and close the stomata occur due to the loss and uptake of K+ by the guard cells. Stomata open when guard cells actively take up K+ from neighboring cells. This makes the guard cells hypertonic and then

H2O enters by osmosis.

In C3 plants, stomata close when temperatures are high to reduce desiccation, but his turns off photosynthesis. Stomata close at night and open during the day. This allows CO2 to enter during the day and keep photosynthesis cannot occur. Stomata also can open when CO2 levels are low inside the leaves.

Phloem sap moves throughout the plant. Phloem sap is a sugary solution (sugar + water) and may also contain things like hormone. Sugar source: a location in a plant where sugar is made. Sugar can be produced either by photosynthesis in the leaves mesophyll, in stems, and in bulb plants. Phloem usually moves sugar out of a sugar source, such as leaves or stems, to the nonphotosynthetic parts of the plant. Sugar sink: a location in a plant where sugar is then stored or consumed. Ex: growing roots, shoots, and fruits. Storage structures like the taproot of a beet, the tubers of a potato plant, and the bulbs of a tulip are sugar sinks during the

summer. In spring the next year, the beet roots, tubers, and bulbs are no longer sugar sinks, but sugar sources. The season determines source and sink. Translocation: movement of sugars from source to sink. Pressure-flow mechanism: it causes phloem sap to always flow from source to sink. In beet plants:

a. At the sugar source, the leaves, sugar is made by photosynthesis then actively loaded into the phloem increases concentration of sugar in the phloem drives in water from the xylem because of osmosis.

b. This inward flux of water is necessary to move the otherwise dry glucose/sucrose and other simple sugars and increase the water pressure at the source end.

c. At the sink end, the beet root, sugar 1st exit the phloem to be stored in beet root cells, so water automatically leaves as well as osmosis.

d. The exit of water decreases the hydrostatic pressure in the phloem. So the building of water pressure at the source and the reduction of water pressure at the sink causes water to flow down a gradient of hydrostatic pressure.

e. Since the sugar is dissolved in the water and the sieve plate allow free movement of sugar and water, the sugar gets carried at the same rate as the water.

f. The xylem then transports water back from the sink to the source.

PLANT HORMONES Plants grow toward the light. Positive phototropism: growth of a shoot towards light. The cells on the dark side of seedling are longer than those on the bright side of plant. These different growth rates allowed the shoot’s apical meristem (actively dividing tips of roots and shoots) to bend towards

light.

When the tips of a grass shoot were removed, the shoot grew straight and wouldn’t bend toward the light. The shoot grew straight if the tip was covered by an opaque cap. Proved that the tip was responding to light. Experiments 6 and 7 showed the same with permeable gelatin and impermeable mica.

Involved cutting the tips of grass seedlings and putting them on a block of agar.

Then the chemical messenger or hormone diffused from the tip into the agar. The agar was then put as a substitute onto the different “tipless” plants below and

grown in the dark to test only the chemical and not the sunlight This showed that when shoots grow towards light they do so because of a high

concentration of the growth promoting chemical on the dark side of the shoot which is the hormone auxin.

Another tropism—an “irreversible” (long-lasting change) growth response to an environmental stimulus is gravitropism. Gravitropism: if you place a seedling on its side, it will grow so the shoot bends upwards and the root curves downwards. This response to gravity ensures that roots go in the soil and shoots reach sunlight regardless of how the seed landed when

it was planted. Botanists are still not sure what causes this, but one hypothesis is by the settling of statoliths. Statoliths: organelles containing heavy/dense starch. An uneven distribution of organelles may in turn signal the cell to

redistribute auxin.

Thigmotropism: growth movement in response to touch. Ex: when ivy and other climbing plants contact an object and wrap around it. This is considered to be a long lasting change. Ex: mimosa/sensitive plant.

Hormone: made in one part of the organism and influences cells at another part. They can pass through cell walls and alter plant physiology.

5 main hormones in plants: 1. Auxin: Like IAA (indoleacetic acid) promote growth by cell elongation primarily

in the stem/shoots of plants (the apical meristem). But IAA promotes growth only when at a certain concentration. When IAA gets too high, this causes the plant to make another hormone

called ethylene that inhibits stem elongation. Auxins make plants cells elongate by increase the H+ inside primary cell

walls. These H+ then activate enzymes that break bonds of the cellulose in the

cell walls. The cell then swells with water and elongates because the weakened cell

wall no longer resists the cell’s tendency to take up water osmotically. The cell then stays bigger by synthesizing more cell wall material and

cytoplasm.

2. Cytokininis: Promote cell divisional also

called cytokinesis Are found in actively growing

tissues, especially in the roots, which then may be transported throughout the plant to other target tissues

Also retard senescence the aging of flowers and fruits so are used to keep flowers fresh

Influence organ development called organogenesis.

For example, the amount of cytokinin and auxin together can influence either a plant is tall or wide.

The tall plant (plant B) has its terminal bud intact so auxin traveled down the stem and allowed the stem to elongate inhibits the side auxiliary branches.

The short plant (plant A) had its terminal bud removed early on and meant no auxin was there to both elongate the stem and inhibit the side branches allowed the cytokinins transported from the roots to activate the side auxiliary buds and the plant grew bushy.

The length/fullness of a plant is controlled by the interplay of auxins and cytokinins. 3. Gibberellins (GA): Are made at the tips of roots and shoots Produce a wide variety of effects One main effect is the elongation of stems and leaves Bulting: when high concentration of GA causes the rapid elongation of stems; it enhances the effects of the auxins. In combination with auxins, GA influence fruit development. Spraying GA on some fruits makes them develop without fertilization (no pollen) giving us such fruits as seedless

grapes. GA are also important in seed germinate and can cause a seed to sprout when sprayed onto seeds.

4. Abscisic acid (ABA): Produced in the buds signals the buds

to form scales that will protect them from harsh conditions

It’s a growth inhibitor Causes seed to remain dormant This is important to annual plants in

desert because germination without enough water would quickly kill the plants

Seeds will not germinate until a downpour of rain washes ABA out of the seeds, getting rid of this inhibitor

Ratio of ABA to GA (promotes germination) determines if a seed sprouts or remains dormant

Acts as stress hormone in growing plants

Helps them cope with adverse conditions

Ex: if a plant is dehydrated, ABA stimulates in the leaves causing the stomata to close

ABA is named because it was once believed to cause abscission or breaking off of leaves from trees in the fall, but there’s no connection between them; this has never been proven.

5. Ethylene: It’s a gaseous by-product It triggers fruit ripening Fruits ripen by the breakdown of cell wall that cause the characteristic color changes (green to yellow or red) Growers try and retard the action of ethylene Growers do this by flushing stored apples with CO2 that inhibits the action of ethylene It also plays a role in the fall color changes in leaves and drying and promotes abscission, the loss of leaves

Plants display rhythmic behavior like the opening and closing of stomata and sleep movements. Circadian rhythm: a biological cycle of about 24 hours These rhythms persist even when an organism is removed from environmental/external cues—like in constant dark or

constant light. Biological clock: internal timekeepers control circadian rhythms; it continues to mark time in the absence of environmental

cues; but to remain tuned to a period of exactly 24 hours, it requires daily signals from the environment. Ex: if an organism is keep in a constant environment such as constant darkness, its sleep movements change slightly to a cycle of about 26 hours.

In humans, the biological clock is a cluster of nerve cells in the pineal gland of the brain.

In plants, we don’t know what they are or where they are located.

Unlike most metabolic processes, biological clocks and the circadian rhythms they control are not affected by temperature shifts.

This is important when you consider if a clock speed up or slowed down based on outside temperature, this would be a very inaccurate timepiece.

A biological clock not only times a plant’s daily activities, it also influences seasonal events. Flowering, seed germination and the onset and ending of dormancy are all examples of stages in plant development that

occur at specific times of the year. Photoperiod: the environmental stimulus plants most often use to detect the time of year; it’s the relative lengths of night

and day. Night is most important. Plants whose flowering is triggered by photoperiod fall into 2 groups:

1. Short-day/long-night plants: its plants flower in the late summer, fall, or winter when there is little sunlight or long nights. Ex: Poinsettias

2. Long-day/short-night plants: its plants flower in the late spring or early summer. Ex: garden plants (spinach, lettuce), iris, and many cereal grains. Spinach only flowers when exposed to 10 consecutive hours of darkness. If this is interrupted by even a single flash of light, spinach will not flower.

Flowering and other responses are controlled by night length.

There is no absolute night length for when plants flower.

The actual length of the critical period varies from species to species.

Plant measure photoperiod or night length with aide of pigment called phytochrome.

Phytochrome: a colored protein that absorbs light.

Phytochrome pigment molecule alternates between 2 forms: 1. One absorbs red light (Pr) 2. Other form absorbs far-red light (Pfr)

The conversion from one form to the other happens quickly. But plants only make Pr So if a plant is kept in the dark, the pigment remains as Pr. Normally, each day at sunrise, the Pr form is converted to Pfr and this sets a plants biological clock Then each day at sunset, the Pfr form is converted to Pr. The biological clock measure the time between the conversion of these 2 forms, or the time from sunset to sunrise. This pigment tells time of day and the season and Pfr especially triggers responses such as seed germination, flowering , and

stomata opening.

NOTES

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