Plant Physiology

To understand how plants work, you need to combine knowledge of plant structure with understanding of plant physiology. This lecture covers some aspects of physiology, emphasizing transport and photosynthesis.

Transport

You’ve already seen the structure of xylem and phloem. How does transport in these systems work? How is a redwood tree able to move water from the soil to leaves 100m above the soil?

Successful movement is based on the chemical nature of water. Water molecules are bonded to each other by hydrogen bonds. That makes the water in roots, xylem, and leaves a continuous network. How does water move?

There are 5 major forces that move water from place to place:

1. diffusion – the net flow of molecules from regions of higher to regions of lower concentration. This is the major force moving water in gaseous (vapor) phase.

2. osmosis – the diffusion of liquid water molecules from a dilute solution (more water, less solute) across a selectively permeable membrane into a more concentrated solution (less water, more solute). Osmosis is important in moving water from the solution bathing cells (the apoplast) into the cytoplasm. This flow will continue until the hydrostatic pressure (turgor pressure) inside the cell balances the osmotic pressure.

In intercellular spaces

Within cellular cytoplasm

3. capillary forces – not only is water cohesive (tends to stick together), it is also adhesive, sticking to hydrophilic surfaces. That includes carbohydrates (cellulose) of the xylem tubes’ walls. They are very narrow in bore, and water is pulled to cover the surface of the inside of the tube. The force pulling is capillary force. How large can it be? 1,000 atmospheres, or 15,000 lbs. Eventually the force of gravity balances the upward pull, in theory. That balance is not reached in plants, and capillary force moves water upward to replace evaporative loss.

4. hydrostatic (turgor) pressure – this has already come up.

5. gravity – has also already been mentioned.

How much pressure is involved?

To move water to the top of a 33m elm tree (species doesn’t matter) requires a pressure of 6.7 atmospheres (for those into proper SI units, this is equivalent to 0.67 megapascals).

To move water to the top of a 100m redwood requires a pressure of 20 atmospheres or 2 MPa.

Ecologists can measure the force exerted in a plant stem using a tool called a Schollander Bomb.

Since the water column is continuous from the roots to the leaves, water loss from transpiration affects the entire column.

Water is ‘pulled’ upward from the roots from the cohesion of water in the entire system. How much water is transpired (and replaced by absorption from soil into roots)?

One corn plant in the central U.S. uses (transpires) 50 – 100 gallons of water (text: 196 l)

A tomato plant ~ 120 l

An apple tree ~ 8000 l

date palm (warmer, wetter, more tropical habitat) ~ 140,000 l

Plant biologists determine how water will flow by combining these forces into a measure called water potential. Water flows from a region of high water potential to one having lower potential.

There is generally a higher potential in roots and shoots than in leaves. Transpiration involves water evaporating from the humid interior of leaves and diffusing through stomates. That loss generates strong forces pulling water up through the plant, first from stems into leaves, then upward through the xylem, from roots into xylem, and from soil into root tissues.

Let’s begin at the leaves and (briefly) follow the process and forces…

Air spaces within the leaves are generally in equilibrium with the liquid water in cellulose fibrils of cell walls, i.e. at 100% relative humidity. Air outside the leaves is almost always at a lower relative humidity.

That difference drives diffusion, as long as there is an available pathway. Given hydrophobic cuticle, the pathway is through stomates when they are open.

How fast water diffuses out is in part determined by the thickness of the boundary layer around the leaf, a region of almost unstirred air. Thicker boundary layers slow diffusive loss.

What can make for a thicker boundary layer? A dense layer of trichomes does the job. So does putting stomatal openings below the surface of the leaf, in stomatal crypts.

Here’s a digrammatic representation of a cross section of a yucca leaf:

Loss of water from intercellular spaces within the leaf causes water to evaporate from the surfaces of cellulose cell walls. That produces capillary forces attracting water from adjacent areas of the leaf.

Much of that water comes from inside plant cells, moving across the plasma membrane (osmosis). Turgor pressure decreases. Cell walls ‘relax’, and, if sufficient water is lost without replacement, the leaf wilts.

If the plant is well watered, then replacement is available from the xylem. This water flows out of tracheids through the pits in their secondary cell walls and into fibrous cell walls of the mesophyll cells.

What follows on the next slide is a diagrammatic representation of these movements…

Water flowing out of a tracheid pulls on the rest of the water in the tracheid and on the walls of the tracheid. That force is transferred by hydrogen bonding of water through the system.

The walls of the tracheid are strong and rigid, so the force effectively acts only on the water column, producing a hydrostatic tension.

Water may move among neighboring tracheids under this pressure, and may move up through the column of tracheids. However, tracheids are connected only by pits, which makes this path high resistance. Vessels are uninterrupted, have larger diameters, and are, therefore, a low resistance path.

Tracheids are too small for bubbles to form, blocking flow and there are many of them. Blocking one tracheid has little impact. Not so in vessels.

What causes bubbles? Very high tension and freezing mostly.

Angiosperms have xylem with both tracheids and vessel elements. Conifers (dominant in boreal forests with cold climates) have only tracheids. This may explain, in part, their dominance there.

Finally, there is flow into the xylem in roots. The xylem pulls water from the intercellular space (the apoplast) of the stele (the core of the root). The water flowing from apoplast into xylem is replaced by water flowing into the stele from root cortex. In turn, that cortical water is replaced by water drawn into the root from the soil.

The movement from cortex to stele involves both apoplast and symplast (the interconnected cytoplasms of adjacent cells). The pathways work in parallel as shown below:

There is a limit to water movement through the apoplast. There is a layer of cells around the stele called endodermis, and these cells have something called a Casparian strip on their walls made of suberin (and sometimes lignin) that prevents intercellular water movement into the stele. Water is transported to the stele at this point by the symplastic path only.

Now let’s move to transport of sugars in phloem. It is commonly sucrose (common table sugar) that is exported from leaves. Export is by means of the sieve tubes of phloem.

The rate of movement in sieve tubes is ~ 1-2 cm/min. This is faster than diffusion or cell-to-cell transport, but slower than the movement of water in vessel elements of xylem.

The current belief is that the sucrose is carried along with a bulk flow of solution. The flow is directed by a gradient in hydrostatic pressure, and is powered by an osmotic pump.

Sucrose is the solute that is osmotically active. It is pumped from photosynthetically active cells into sieve tubes of small, minor veins. Accumulation of sucrose in sieve tube cells pulls water into the cells by osmosis. That increases hydrostatic pressure at a source of sucrose. That initiates flow.

Flow directs the water and sucrose to areas where sucrose is in low concentration (a sink). At the sink, sucrose is removed from sieve tubes (and water, as well) by companion cells. That decreases hydrostatic pressure in the sieve tube at the sink, so that a difference in pressure is maintained, and flow continues from the source to the sink.

The same tissue can be a sink at one time and a source at another, e.g.

• a young, growing leaf starts out as a sink; once mature its sucrose is exported and it is a source

• carrots are biennial plants – they complete their life cycle over two years. In year 1 the root is a sink, a storage organ for starch and sugar in its parenchyma cells. In year 2, when the shoot starts to bolt and flower, the root becomes a source.

This diagram does not incorporate the importance of companion cells in sieve tube unloading.

Photosynthesis

Photosynthesis involves two sets of reactions: the light reactions and ‘dark’ reactions (that are otherwise called the Calvin cycle).

The Light Reactions

These reactions occur on the thylakoid membranes of chloroplasts. There are two photosystems involved, named, logically enough, Photosystem I and Photosystem II.

Each of these photosystems contains proteins complexed with cholorophyll pigments, and photosystem II also contains carotenoids.

The chlorophyll and carotenoids are organized into light harvesting complexes. They trap photons.

The energy of the trapped photon excites a chlorophyll a molecule and, through what is called resonance, that energy is transferred to a Photosystem reaction centre.

Either directly (if the photon excited Photoystem I) or indirectly via electron transport (if the photon excited Photosystem II), light energy is converted into electron energy that is used in electron transport to NADP to reduce it to NADPH, splitting water and releasing an electron and oxygen.

Showing both photosystems I and II…in addition to reducing NADP to NADPH (photosystem I), ATP is produced ‘directly’when light excites photosystem II.

Fd – ferredoxin Pq – plastoquinone Pc - plastocyanin

The diagram on the previous slide showed the excitation of Photosystem II caused P680 to become oxidized (lose an electron). That electron is replaced by one from water (as part of splitting water into hydrogen and oxygen). That electron is passed through a series of carriers, losing some energy in each step. The labeled sequence of acceptors are Pq (plastiquinone), a cytochrome complex and plastocyanin. That energy is captured (partly) in the formation of an ATP molecule from ADP (called photophosphorylation).

The electron is eventually passed to an oxidized P700 of photosystem I. When P700 was itself excited by light, it was oxidized, and passed an electron through a series of acceptors, with the energy used to reduce NADP to NADPH.

This whole process is non-cyclic photophosphorylation.

Just to make it all more complicated, photosystem I can function independent of photosystem II, in cyclic photophosphorylation. This sequence of electron transfer produces ATP directly, rather than forming NADPH.

The energy captured in ATP and NADPH is used to drive the chemical reactions of the Calvin-Benson cycle.

Melvin Calvin, from the University of California, won a Nobel Prize for the ‘discovery’ and description of the dark reactions (meaning not light requiring) of photosynthesis.

Here’s one diagram of the process.

The molecules involved: RuBP – ribulose biphosphate PGA – phosphoglyceric acid PGAL – phosphoglyceraldehyde rubisco – ribulose biphosphate

carboxylase

The steps of the Calvin cycle are:

1. Fixation of CO2 by enzymatically adding a carbon to ribulose 1,5 biphosphate. The enzyme is rubisco (ribulose biphosphate carboxylase. Rubisco is a common protein in photosynthetic plants, representing from 1/8 to 1/4 of total leaf protein.

2. The 6-carbon molecule formed is unstable, and very rapidly splits into two 3-carbon molecules of phosphoglyceric acid (PGA).

3. PGA is modified enzymatically (with the energy input from one NADPH and one ATP from the light reactions) into two molecules of glyceraldehyde phosphate (PGAL). Most of the PGAL (10 out of every 12) is used to regenerate RuBP. That makes the series of reactions cyclic.

4. The other two PGAL are re-combined enzymatically to form a 6-carbon sugar, fructose 1,6 biphosphate. That sugar molecule is converted rapidly to glucose, which is, in turn, converted into sucrose or starch.

The Calvin-Benson cycle is universal in photosynthetic plants. However, as you already know, there are alternatives in carbon fixation.

In C4 photosynthesis, the initial carbon fixation step uses PEP carboxylase to attach a carbon from CO2 to phosphoenol- pyruvate, a 3-carbon molecule, to form a 4-carbon molecule, oxaloacetate. There are then a cycle of reactions during which a CO2 is passed to the Calvin cycle. Note the location of these steps within the leaf. This mode of carbon fixation is called the Hatch-Slack pathway.

CAM carbon fixation occurs as in the Hatch-Slack pathway, fixing carbon into 4-carbon acids. The difference is in timing and location. CAM plants fix carbon at night in the same mesophyll cells that undergo light reactions during day.

This is the C4 pathway.Carbon fixation in the mesophyll, but Calvin cycle reactions in the bundle sheath.

In CAM, both light and dark reactions occur in mesophyll, but carbon fixation is limited to nighttime hours.

Photorespiration

Some of the CO2 fixed in photosynthesis is lost to photorespiration.

The actions of rubisco depend on the relative concentrations of CO2 and O2 in the leaf. When CO2 is high, rubisco acts to catalyze the addition of CO2 to RuBP. However, when O2 is high and CO2 low, rubisco catalyzes the addition of O2 to RuBP. Eventually, CO2 is formed, but without formation of ATP or NADPH.

This occurs in C3 plants, but not in C4 plants, and, on a hot day, may cost a C3 plant as much as 50% of fixed carbon, at a high energy cost. On cooler days (or in cooler climates) when photorespiration is low or unlikely to occur, C3 plants are more efficient (expend less energy) to fix CO2.

Photorespiration is, therefore, a key factor in explaining the distributions of C3 and C4 species.

Cellular Respiration

All living organisms use energy, and form ATP to use in the many enzymatic reactions involved in molecular synthesis. The basic steps are glycolysis and the reactions of the Krebs cycle.

Glycolysis means the splitting (lysis) of sugars. The usual steps are to split a glucose molecule into two three-carbon glyceraldehyde phosphate, then convert those molecules into pyruvate molecules. The net energy-yielding result is 2 ATP and 2 NADH being formed.

The pyruvate is transferred into mitochondria, where the reactions of the Krebs cycle occur. The details (which you were probably forced to learn in high school biology) are not critical. What is important is the energy result of the cycle of reactions.

The two molecules of pyruvate that are produced from one molecule of glucose yield 2 ATP, 8 NADH and 2 FADH2 in being carried through the Krebs cycle.

These energy-rich molecules are passed to the electron transport system of the mitochondria, where more ATP is produced (24 ATP from 8 NADH). The process is called oxidative phosphorylation.

The Krebs cycle is diagrammed on the next slide.

And here is a diagram of the electron transport chain. These proteins are all located on the internal membrane (the crista) of the mitochondrion.

There are many more important components of plant physiology, some of which may arise later in the semester (e.g. plant hormones). This has been a “bare bones” treatment of a few basics.