Uptake and Use of Water, Minerals and Light

Chapter 8


Why do plants need water?

Photosynthesis. Water used in photosynthesis is a tiny fraction of the water used by plants.

Transport materials (food, nutrients). Water acts as a solvent, materials dissolved within are solutes.

Fill vacuoles. Water is about 90% of cell volume.

Keep turgor pressure.

Keep cell membranes wet, necessary to pass carbon dioxide through.

Most chemical reactions in the cell must occur in solution.

Evaporation from leaves (called transpiration) on hot days keeps leaf temperature moderate.

Molecular Movement of Water

Diffusion = Movement of molecules or ions from a region of higher concentration to a region of lower concentration

Diffusion gradient concentration difference in one area relative to another; molecules naturally move with the concentration gradient, from high to low concentration

State of Equilibrium concentration same across an area


Diffusion of water through a selectively permeable membrane from a region of higher concentration to a region of lower concentration

Osmosis requires no energy

Active transport uses energy to move substances across cell membranes against a diffusion or osmotic gradient




Turgor pressure

the pressure that is exerted against the cell wall as a result of water entering the cell

turgid swollen and firm due to internal water pressure

flaccid soft and flexible, non-turgid, wilted



loss of water from a cell accompanied by the shrinkage of the protoplasm

cell walls retain structure, cytoplasm pulls away from cell wall

usually irreversible


Root Pressure and Guttation

Osmotic pressure from water entering roots through epidermis, then through endodermis

Results in water under higher pressure in roots relative to rest of plant

Water or sap oozes from wounds in stem because of root pressure

Water drops exude from hydathodes in tips of leaves during guttation

How does water reach top of trees?

Early theories

Mechanical pumps (never found)

Root pressure (can only move water up a few feet)

Capillarity (only reaches a few inches)


The Cohesion-Tension Theory

Polar water molecules stick together by hydrogen bonds

called "cohesion"

Water also sticks to walls of xylem tracheids and vessels

called "adhesion"

Water evaporates from mesophyll cells creating a pull or tension on the water column



loss of water vapor from the plant, mostly through stomates in leaves

Regulation of Transpiration

Stomates open by active transport of potassium

(Active transport uses energy to move substances across cell membranes against a diffusion gradient)

When photosynthesis begins in chloroplasts of stomates, the energy produced is used to move potassium (K) ions across cell membrane into guard cells

Water concentration is therefore depressed in guard cells when K concentration increases

Water enters guard cells by osmosis

Guard cells expand, opening pore

Regulation of Transpiration

Stomates close by diffusion of potassium

When photosynthesis in guard cell ceases, active transport of K ions stops

K ions leave guard cells passively as concentration outside stomate is lower than inside guard cell

Water leaves guard cells by osmosis

Stomates close


Effect of Abscisic Acid on Stomates

Released when water stress is high

Plant starts producing ABA

Causes membranes to leak

Loss of potassium occurs

Stomates close as a result


Other Ways Plants Regulate Water Loss

Stomates open at night

desert plants with CAM photosynthesis

Stomates sunken below leaf surface

pine trees, desert plants


Transport of Food in Solution

The Pressure-Flow Hypothesis

Food substances in solution into phloem tissue from a "source" (a place where food is produced or stored) by active transport

With increase in solutes, water is taken up by osmosis creating turgor pressure

Food substances actively removed from the phloem at a "sink" (a place where food is utilized)

Food substances flow from the region of higher pressure at the "source" to the lower pressure of the "sink"



Soils and Nutrients

Mineral Requirements for Growth

Plant nutrients = minerals = elements

Macronutrients and Micronutrients

Macronutrients used by plants in greater amounts

nitrogen, potassium, calcium, phosphorus, magnesium, sulfur

Micronutrients needed by plant is very small amounts

iron, manganese, boron, sodium, cobalt, silicon, selenium




Soil Profile

O = Organic

Partially decomposed plant material, including litter, fermentation and humus layers

A = Topsoil

Mostly organic mixed with mineral

B = Subsoil

Mostly mineral mixed with organic

C = Parent material

Partially to unweathered bedrock


Soil Development

Five factors influence soil development

Parent material


Biological activity



Living Organisms and Organic Composition

Bacteria and fungi

primary biological decomposers

break down complex organic molecules into smaller components

Ants, earthworms, plant roots

burrowing organisms

change soil structure

add air spaces

Typical composition of soil

Minerals, 48%

Water, 25 %

Air, 25 %

Organic matter, 2%

Soil Texture

Soil texture refers to the sizes of the individual soil particles

Sand 0.05 2 mm

Silt 0.002 0.05 mm

Clay < 0.002 mm



Soil Properties

Soil structure is the arrangement of soil particles

Mixture of particle sizes is most common

Pore spaces between particles determines water-holding capacity

Larger particles have larger spaces between

Water in the Soil

Hygroscopic water

physically bound to soil particles, unavailable to plants

Gravitational water

drains out of the pore spaces after a rain

Capillary water

held against the force of gravity, form used by plants

Field capacity

Amount of water remaining in soil after drainage by gravity

mostly capillary water

governed by soil texture

Permanent wilting point

when field capacity cannot support plant growth

Available water

soil water between field capacity and the permanent wilting point

pH scale

pH of soil

Alkaline soil

Minerals become less available

Addition of sulfur makes soil more acid

Acid soil

Inhibits growth of nitrogen-fixing bacteria

Common in areas of high precipitation

Adding calcium or magnesium counteracts soil acidity


The Essence of Photosynthesis

Sunlight energy is converted into chemical bond energy in the form of carbohydrates

Summary equation for photosynthesis

Principle ingredients

Carbon dioxide (CO2)

from atmosphere, enters through stomates


from soil, enters through roots

less than 1% of water entering plant used in photosynthsis

water (not CO2) is the source of O2 released


has properties of both waves (of different lengths, in nanometers, nm) and particles (photons)



Light Wavelengths

violet-blue and red-orange wavelengths used extensively for photosynthesis

green mostly reflected

accessory pigments absorb wavelengths not used by chlorophyll a

Light Intensity

plants vary in how much light then need for photosynthesis

intensity=number of photons

measured in μmol units

some reach their maximum with very little light, others can take advantage of extra light given enough CO2



Light and temperature

too much light often increases temperature which can reduce photosynthesis


Several types of chlorophyll and other pigments are used in photosynthesis

All magnesium containing molecules, similar to hemoglobin



Long lipid tail anchors molecule in the lipid layer of thylakoid membranes of chloroplast


Light harvesting complex called a photosynthetic unit

250-400 pigment molecules


The Major Steps of Photosynthesis

The Light-dependent reactions

Reactions take place in thylakoid membranes of chloroplast

Light required

Water split, producing hydrogen ions and electrons, O2 gas released

ATP created

NADPH + H+ created

The Major Steps of Photosynthesis

The Light-independent reactions (Calvin cycle)

Reactions take place in stroma of chloroplast

used to be called "dark" reactions

now called light-independent reactions because light is indirectly necessary to activate enzymes

Calvin cycle

Carbon dioxide from air combined with 5-carbon sugar

Glucose formed through several reaction steps


The Major Steps of Photosynthesis

Types of photosynthesis

This summary is of C3 photosynthesis

most common

most efficient (stores greatest proportion of available solar energy)

Other forms of photosynthesis are less efficient but better for conserving water

C4 plants like grasses

CAM plants like cactus in deserts