Ch. 4 Plant Physiology

Net flow in whole plants

Some Key Concepts

• Diffusion: movement of molecules from high to low concentration.

• Osmosis: diffusion across a semi-permeable membrane.

• Mass or bulk flow: movement of fluid due to pressure or gravity differences.

Long-distance movement of water

• Plants mostly obtain water & minerals from soil.

• Water moves up the xylem by bulk flow.

• Movement of water depends on transpiration pull, cohesion & adhesion of water molecules, capillary forces, and strong cell walls.

Fig. 39.12a

Fig. 39.12b

•transpirational pull•flow from greater to lower water concentration

•relies on cohesion & adhesion of water

–cavitation breaks chain of water molecules

Ascent of xylem sap

Figure 35.11  Water-conducting cells of xylem

Other mechanisms of water transport not important.

• Diffusion (note mosses, etc.)

• Capillary forces

• Osmotic pressure (guttation)

Fig. 39.11

The availability of soil water and minerals

Long-distance transport of water from roots to leaves

Mineral Uptake Key Points• Mineral movement to root by diffusion or bulk

flow or root growth.• Uptake controlled at root endodermis.• Uptake by either simple diffusion (no protein),

facilitated diffusion (protein channel), or active uptake (requires energy and a protein carrier).

• Organisms concentrate minerals and most other substances.

• Usually biggest energy expenditure of roots, cause nutrients are being concentrated.

Movement of sugars

• Sugars (etc.) move from the source– Photosynthetic leaves– Storage organ

• To the sink – Growing organs– Developing storage tissue

• Through mass flow in phloem• Pressure Flow Hypothesis

Phloem transport

• pressure flow1 high sugar concentration at

“source”2 sugar diluted with water

from xylem creating pressure for flow

3 sugar unloaded at “sink” where it is metabolized or converted to starch

4 excess water flows to xylem back to “source”

• translocation: movement of food from “source” to “sink(s)”

Pressure flow in a sieve tube

Transport Movies

Some “hot” areas in plant water and nutrient research

• Improving plant water-use efficiency

• Improving salt tolerance

• Improving nutritional value of plants (e.g., golden rice, increasing Fe content)

• Phytoremediation

Life on Earth depends on flow of energy from sun

Life processes are driven by energy

• Plants are dynamic metabolic systems– 1000s of reactions occur every second

• Processes can be – energy consuming (endergonic) or – energy releasing (exergonic) and– catabolic (breakdown) or– anabolic (synthesis)

Fig. 8.6

Fig. 8.7

The most common and important forms of cellular energy.

• Chemical bonds (e.g., ATP, CH2O)

• Electrons (redox reactions)

• Electrochemical gradients

Fig. 8.14

Cellular respiration

• Chemical-bond energy in sugars is converted to energy-rich compound ATP which can then be used for other metabolic reactions

Fig. 9.19

Energy yield depends on oxygen

• Aerobic (with oxygen)– 36 ATP molecules per glucose molecule

• Anaerobic (without oxygen)– 2 ATP molecules per glucose molecule