NOTES FOR BIOLOGY 1002

SECTIONS 004, 005, 006

Spring 2006

DR. STEVEN POMARICO

CHAPTER 30

PLANT NUTRITION AND TRANSPORT

The broad category of nutrition includes all of the materials and energy needed for an organism to live and reproduce.

The “materials” required for life are nutrients.

Nutrition in all organisms involves four main steps:

          -Acquisition of nutrients

          -Digestion, if required

          -Distribution of nutrients throughout the body.

          -Synthesis of molecules for the organism's body.

Plants and animals get their nutrition differently

Plants acquire all their nutrients from soil (minerals), water (hydrogen and oxygen), or air (CO₂)

Some of the nutrients are needed in large quantities (>1% of the plant’s dry weight), these are the macronutrients

          C, H, O, P, K, N, S, Ca, Fe, Mg

How to remember the macronutrients

          C HOPKiNS CaFe Mighty good

Another group of nutrients is only needed in small amounts (<1% of the plant’s dry weight), these are the micronutrients

          Cl, Mn, Mo, Cu, B, Z

Only two of these macro/micronutrient come from the air the remainder are obtained from the soil.

Usually nitrogen and phosphorus are “limiting”. This doesn’t mean the plants don’t grow, it means that growth is limited by the supply of these nutrients. Most typical fertilizers supply N, P, and K.

ACQUISITION OF MINERALS

Only minerals dissolved in soil water are accessible to roots.

Four step process of mineral absorption by roots

          1) Active transport into root hairs.

          2) Diffusion through root hair cytoplasm to endodermis cells via pores called                                 plasmodesmata

          3) Active transport from endodermis cytoplasm into the extracellular space                                 of the vascular cylinder.

          4) Diffusion into the xylem.

Using this process the plant may concentrate minerals inside their tissues.

Role of Casparian strip

The endodermis cell layer is coated with a waxy coating called the Casparian strip (See fig 30.4).

This waxy coating seals the spaces between endodermal cells so that any material that moves between the vascular tissue and the cortex must pass through (instead of between) the endodermal cells.

Casparian strip "leakproofs" the vascular cylinder, retaining the concentrated mineral solution within the extracellular space of the vascular cylinder.

The acquisition of minerals by plant is aided by symbiotic relationships.

Nitrogen fixation

Since most of the atmosphere is N₂ gas (79% ) why should nitrogen be a “limiting” mineral?

Plants make all of their organic molecules from scratch.

They need nitrogen (N) for amino acids (building blocks of proteins), nucleic acids (building blocks of RNA and DNA), and chlorophyll

Plants can only absorb N through their roots, usually in the form of ammonium (NH₄) or nitrate (NO₃) ions.

Nitrogen-fixing bacteria have the enzymes necessary to convert N₂ into NH₄ and NO₃ but it is an energetically costly process.

Nitrogen-fixing bacteria live in root nodules (See fig 30.6) of legumes (peas, alfalfa, soybeans) .

The plant supplies the bacteria with sugar for energy.

The bacteria produce excess NH₄ or NO₃ for the plant.

Mycorrhizae

The fungus converts insoluble soil nutrients into simpler water-soluble compounds that root hairs can absorb and transport.

The fungus obtains sugars and amino acids from the plant.

ACQUISITION OF WATER

Water moves straight in from outside of root hairs into xylem by osmosis

Osmosis - diffusion of water across along a concentration gradient of free water molecules; water moves from solution with higher free water conc (lower dissolved materials) into lower free water concentration (higher dissolved materials).

So by moving minerals into the root, the plant has created conditions to drag in water as well.

How this works:

          1) Higher water concentration in soil relative to the root cells.

          2) The waterproof Casparian strip blocks movement of water between                             cells at the endodermis.

          3) The cell membranes of the endodermis act as a gate separating the                 outer low mineral (high water) solutions from the inner

high mineral (low water) solutions.

          4) Therefore, water moves from the cells and the extracellular space                    outside the Casparian strip, through the endodermal cells, and

into the extracellular space inside the vascular

cylinder by osmosis.

          5) Water moves from the extracellular space of the vascular cylinder into              tracheids and vessel elements of xylem through the cell wall pits.

          6) Water is then pulled up the xylem, powered by evaporation of water                            from the leaves.

Only this last step is not osmosis.

TRANSPORT OF MINERALS AND WATER

Water and dissolved minerals move from roots to stems and leaves by a process known as bulk flow where water and mineral move together.

Not a big deal when you think of a small seedling, but what about a huge oak, or a giant sequoia over a football field high.

The cells at the top of the plant are the most metabolically active (that's where growing is taking place), need water and minerals.

Minerals are dissolved in water, so the question becomes:

How does water move up a plant against gravity?

Water is pulled up the xylem powered by the force of evaporation of water from the leaves.

This process is transpiration.

Transpiration requires the "Cohesion-tension theory" to work.

Two fundamental parts of the cohesion-tension theory:

          1) cohesion - water within xylem tubes sticks together (by hydrogen                    bonds). Water molecules in the xylem tube resist being pulled apart

          2) tension - water is pulled up by negative pressure.

 Because water molecules bind to each other, they can pull water up like a chain.

Transpiration - How it works (See fig 30.9)

          1) Water evaporation occurs thru stomata of leaves.

          2) Water leaving leaf makes it “dry”, so it pulls in more water from xylem.

          3) The water molecule leaving the xylem is “stuck” to other water molecules,

so it pulls up on those molecules, thereby pulling the "chain"

of water molecules up the tree.

Evaporation in the leaves controls the movement of water and minerals up from the roots.

The flow of water in xylem is unidirectional because only the shoot has openings in the epidermis where water can escape.

The rate of transpiration is controlled by opening and closing the stomata which is in turn controlled by the guard cells which surround the stomata (See fig 30.11).

How do guard cells open and close the stomata?

Stomata open when guard cells take up water and swell, they close when guard cells lose water and shrink.

This may seem backwards because you might expect the cells to get fatter as they swell. BUT, something stops them from getting fatter: they have "belts" of cellulose fibers surrounding them.

As the stomata increase in volume, they can't get fatter, so they have to get longer. This makes them bow out apart from each other, thereby opening the stoma.

How does the plant control the swelling of guard cells?

By osmosis.

-Plants can move potassium into (active transport) and out of (diffusion) guard cells.

-When the potassium (K⁺) concentration is high, water moves in, and the stoma

          opens.

-When potassium leaves the guard cells, the stomata close.

Three important triggers to open or close stoma:

1) Light reception (opens) - Plants need CO₂ during photosynthesis.

When light strikes certain pigments in guard cells they trigger a pumping of potassium into guard cells (opening stomata). At night, potassium diffuse back out, closing the stomata.

2) Low carbon dioxide concentration (opens) - Plants use CO₂ (in photosynthesis) and produce CO₂ (in cellular respiration).

During the daytime the use of CO₂ is much faster than its production. This leads to low CO₂ and triggers potassium pumping into the guard cells, opening the stoma.

3) Water loss (closes) - If water vapor leaves the stomata faster than it can be replaced from xylem, the leaf will wilt.

This causes the mesophyll cells to release a hormone (abscisic acid - ABA).

This hormone stops potassium from being pumped into the guard cells, which eventually makes them close until water levels return to normal.

TRANSPORT OF SUGARS

The fluid in the phloem is under positive pressure (it’s pushed through the sieve tubes).

This fluid contains a high sugar concentration because of sucrose moving from a source to a sink.

Sources, Sinks and movement of sucrose.

A. Phloem flow is directed by sugar production and use.

B. Any structure that makes sugar will be a source of phloem flow.

C. Any structure that uses up sugar will be a "sink" towards which phloem will flow.

D. Fruits, roots, flowers, and new leaves are all "sinks."

Thus phloem can move nutrients either up or down the plant.

"Pressure-flow theory" for sugar movement in phloem governs sucrose movement from leaf into either a developing fruit or into storage in the roots (See fig 30.16).

1) Sugar from photosynthesis enters leaf companion cells by active transport.

2) Sucrose then diffuses into adjacent sieve tube elements.

3) Water enters the sieve tubes by osmosis from nearby xylem cells.

4) The developing fruit is a "sucrose sink." Sucrose is actively transported out of the

          sieve-tube elements into the cells of a fruit (or root), lowering the

sugar level in that end of the system.

5) Water leaves these sieve tubes due to osmosis, and follows the sugar into the

           fruit/root.

6) Bulk flow, driven by hydrostatic pressure gradient (like in a garden hose).