nutrition and transport in plants 1 ch. 26 – plant nutrition & transport essential inorganic...

1Nutrition and TNutrition and Transport in Plaransport in Plantsnts

Ch. 26 – Plant Nutrition & Transport

Essential Inorganic Nutrients About 95% of a plant’s dry weight is carbon, hydrogen, and oxygen

Primary nutrients are carbon dioxide and water

Minerals •Know why plants need the following minerals: N, C, H, O, P, K, Mg • Know the difference between macronutrients & micronutrients

Use book slogan to remember the names of all macro & micronutrients.

“C Hopkins Café Managed by Mine Cousin Clyde Mo”

2Overview of Plant Nutrition

Except for CO2 and O2 all other minerals are

absorbed as minerals by roots


Plant Nutrition & Transport

A Nutrient is an Essential Nutrient: If it has an identifiable roleNo other nutrient can substitute for itA deficiency of this nutrient causes plant to die

Hydroponics •Water culture •Can be used to determine if a nutrient is

essential. •Eliminate mineral from the water given to

plants and see if they can survive.

4Nutrient Deficiencies

Nitrogen

Phosphorus

Calcium


Water and Mineral Uptake

Water moves from soil to roots by osmosis, or by flowing along cell walls.• If water moves by traveling along porous cell

walls it is following the apoplast pathway: Water will move into roots because they have lower

osmotic pressure than the soil solution does. Water moves inward towards center of root & the Casparian strip.

- At this point the water is forced to enter the endodermal cells to get into the xylem.

6Water and Mineral Uptake

Apoplast

Symplast



• If water moves by osmosis from cell to cell it is following the symplast pathway:

Concentration of substances dissolved in cells is higher than in the soil, so water moves in through root hairs.

Water dilutes cell concentration compared to adjacent cells, so water moves into next cell.

This continues to xylem where the Casparian strip is encountered and water must continue through endodermal cells to get into xylem.

8Water and Mineral Uptake

Apoplast

Symplast



• In contrast to water, minerals are actively taken up by plant cells.

The concentration of minerals in roots is as much as 10,000 times greater than in surrounding soil.

Following uptake by root cells, minerals move into xylem & are transported with the water.

Along the way, minerals can exit the xylem & enter cells that require them.


Adaptation of Roots for Mineral Uptake

Important Symbiotic RelationshipsRhizobium bacteria fix atmospheric nitrogen (N2)

Live in root nodules Found in legumes such as soybeans & alfalfa

Turn NH3 into NH4+

Plants can then take up the NH4+ to make organic

compounds The bacteria are supplied with food by the host plant

This is an example of mutualism.

11Root Nodules



Mycorrhizae An association between fungi and plant roots

Fungus increases surface area available for mineral and water uptake & breaks down organic matter

Root furnishes fungus with sugars & amino acids Plants are extremely dependent on mycorrhizae Example: orchid seeds don’t germinate until mycorrhizal fungus has invaded cells

13Mycorrhizae



Epiphytes Some plants have poorly developed roots or no roots at all. Epiphytes are “air plants” They grow on larger plants which give them support But they don’t receive nutrients from their host. Instead they might catch rain and minerals in special pockets at base of their leaves.

Example: Spanish moss

15Spanish Moss


Transport Mechanisms in Xylem

The watery contents of xylem and phloem vessels is called sap.Xylem sap transports water & dissolved minerals from roots to leaves

Phloem sap transports organic nutrients to all parts of the plants

17Plant Transport System

Xylem is continuous from roots through stem to

leaves



Water flows passively from an area of higher water potential to an area of lower water potentialWater potential is the energy of water. It depends on two factors: Water pressure across membrane. Water moves from high pressure to low pressure.

Solute concentration across membrane (osmotic pressure). Water moves from area of low solute concentration to area of high solute concentration.



Increasing water pressure will counter the tendency of water to enter a cell because of solutes. A common situation in plants.

As water enters a plant cell by osmosis, pressure will build up inside due to the strong cell wall.

When will water stop entering the cell? When pressure inside the cell increases to the point that it balances the osmotic pressure from outside.

This leads to turgor pressure.


Water Transport in Xylem

Water entering roots creates a positive pressure (root pressure)Occurs primarily at nightTends to push xylem sap upwardMay be responsible for guttation

Water forced out vein endings along edges of leaves

Root pressure is not likely to be mechanism by which water rises to tops of very tall trees. Can only push water up to about 10m.

21 Guttation


Cohesion-Tension Model of Xylem Transport

Passive mechanism of xylem transportWater molecules tend to cling together (cohesion) - Due to hydrogen bonding between water

molecules. Water forms a continuous column in xylem from roots to leaves.

Polarity of water allows interaction with molecules of vessel walls (adhesion) This gives the water column extra strength & prevents it from slipping back down the xylem.



Water column primarily moves upward passively due to transpiration.Transpiration is the evaporation of water from leaf cells. 90% of water taken up by roots is lost by transpiration eventually Water evaporates from mesophyll cells of leaves into intercellular spaces and then exits the leaf through stomata

H20 molecules lost from mesophyll cells are replaced by H20 molecules from xylem of leaf veins.

This exerts a force, or tension, that draws the water column up in vessels from roots to leaves.



Tension can reach from the leaves to the root only if the water column is continuous. If the water column is broken, as when you cut a stem, the water column “snaps back” making it more difficult for transport to occur. This is why it is best to cut flowers under water. When plants are under water stress, the stomata close.

Now the plant loses little water because of the waxy cuticle. However, plants can’t get CO2 for the Calvin Cycle of photosynthesis.

So plants need abundant water to be healthy.

25 Cohesion-tension Model ofXylem Transport


Opening and Closing of Stomata

Each stoma, or pore, in leaf epidermis is bordered by guard cellsWhen water enters guard cells & turgor pressure increases, the stoma opens

When water exits guard cells & turgor pressure decreases, the stoma closes

Guard cells attached to each other at ends; inner walls thicker than outer walls.

Cellulose microfibrils in walls prevent radial expansion but they can expand lengthwise.

When this occurs the guard cells buckle outward and stoma opens.

27 Opening and Closing of Stomata



What causes water to end or leave guard cells?Potassium ions (K+) accumulate within guard cells when stomata open

Active transport of K+ into guard cells causes water to follow by osmosis & stomata open.

H+ ions accumulate outside guard cells as K+ moves into them.

A proton pump run by the hydrolysis of ATP transports H+ to outside of cell. This creates an electrochemical gradient that allows K+ to enter by way of a channel protein.

29 Opening and Closing of Stomata



What regulates opening & closing of stomata?Blue-light component of sunlight is a signal that can cause stomata to open.

- A flavin pigment may absorb blue light & then sets in motion a response that activates the proton pump.

- Could also be due to a receptor that inactivates pump when CO2 levels rise

- Abscisic acid (ABA) produced by wilting leaves also causes stomata to close



Circadian rhythms are behaviors that occur nearly every 24 hours Plants kept in dark open & close stomata every 24 hours.

- This means there is some sort of internal biological clock that is keeping time.


Transport Mechanisms in Phloem

Phloem transports organic materials to any parts of plants that need them.This could be young leaves, flowers & fruits & roots

Girdling a treeStripping bark from part of tree or leaving a metal band around the circumference for a long time

Girdling of tree below the level of leaves causes bark to swell just above the cut

Sugar will accumulate in the swollen tissue


Organic Nutrient Transport

How to get samples of phloem sap to study?Difficult to get phloem sap from plants without injuring the phloem or having the phloem tubes get clogged up.

Scientists use small insects called aphids. - These have a long mouthpart called a stylet. - They inject their stylet into phloem like a hypodermic needle.

- Can let the aphids attach to a plant & then cut their bodies off & use stylet as a funnel to collect phloem sap.

34Acquiring Phloem Sap

Excess phloem sap (honeydew) passing out of aphid’s body

Cut aphid head off and leave stylus attached to plant to collect the sap


Pressure-Flow Model of Phloem Transport

Positive pressure drives sap in sieve tubes Photosynthesizing leaves produce sugar. They are a source of sugar.

Sucrose is actively transported into sieve tubes of phloem. Sugar is carried across in conjunction with H+ ions.

Water follows passively by osmosis Increase in volume creates flow that moves water and sucrose to a sink

Roots, etc. are sinks for sugar. They are removing sugar from phloem & using it in cell respiration.

Sugar is actively transported OUT of sieve tubes, water exits passively and enters xylem. Water travels back up to the leaves.

36Pressure-flow Model of Phloem Transport

1. Sugar actively transported into sieve tubes

2. Water follows by osmosis

3. Positive pressure causes phloem contents to flow from source to sink

4. At sink, sugar is actively transported out of sieve tubes

5. Some water returns to xylem, where it mixes with more water absorbed from soil

6. Xylem transports water to mesophyll of leaf

7. Most water is transpired, some is used for photosynthesis, and some reenters phloem by osmosis.

nutrition and transport in plants 1 ch. 26 – plant nutrition & transport essential inorganic...

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