Characteristics of Living Things (Try to list 6):

Plant Part

Function & How it helps meet characteristics of life

How its STRUCTURE helps with its FUNCTION

LeafStemRootsFlowerFruitsSeeds

Stem:• Transports water upwards• Transports sugars wherever

needed• Supports/positions leaves

Leaves:• Make sugar through

photosynthesis• Release water through

transpiration

Roots:• Absorb water and nutrients

(N, P, K)• Store sugars• Anchor the plant in the soil

MAJOR PARTS OF A PLANT:

How do roots absorb water & nutrients?• Structural adaptations– High surface area• Branching structure• Root hairs

– Symbioses• Mycorrhizal fungi – increase surface area for water and

mineral absorption• Rhizobium bacteria – fix nitrogen (N2) and convert it

into a form that plants can use

Fungal root symbionts: Mycorrhizae

Mycorrhizae

Root nodules on legumes:formed by Rhizobium bacteria

Example: clover roots

How do roots absorb water & nutrients?• Structural adaptations– High surface area• Branching structure• Root hairs

– Symbioses• Mycorrizal fungi – increase surface area for water and

mineral absorption• Rhizobium bacteria – fix nitrogen (N2) and convert it

into a form that plants can use

Cross section of a root:

How do water and nutrients enter roots?

(First, watch the animation…)

1. Water moves through the root via two routes:• Symplastic route – Through the cytoplasm & plasmodesmata, after

diffusing across the plasma membrane of epidermal cells• Apoplastic route – Along cell walls without entering cells

2. At the endodermis, the Casparian strip (waxy belt around endodermal cells) forces water and solutes into the cell• Endodermal plasma membrane selects which minerals/nutrients

enter• Thereby controlling what enters the xylem

3. Once inside endodermal cells, water & select solutes flow into xylem cells

Water Flow into Roots• Driven by osmosis (sometimes following active

transport of minerals into roots)• Due to differences in ψ (water potential)• Quick review: What’s water potential?– Tendency of water to leave an area and go somewhere else– Determined by• Pressure• Solute Concentration

• If soil is too dry (ψsoil lower than ψroot cells), water won’t enter roots

• Result: wilted plant

How do plants transport water up through the xylem?

• Xylem structure– Long cells (tracheids & vessel elements) are dead

and hollow – just cell walls– Cells connected at ends by holes/perforations for

water flow

High WP

Low WP 1. Transpiration creates low WP in leaf air spaces

2. Water from veins moves into leaves

3. Water in veins is connected by H-bonds all the way down through the xylem. Adhesion & cohesion create a pull on the water below.

4. Water from soil flows in to replace water pulled upwards by TACT

How do plants transport water up through the xylem?

• Transpiration-Adhesion-Cohesion-Tension Mechanism (TACT)– Transpiration pulls water out of leaves– Water molecules in leaves stick to water molecules all the

way down the xylem (cohesion)• Adhesion to cell walls also helps

– This creates negative pressure (tension) on the water column in the xylem

A completely passive way to transport water, driven by solar energy (powers transpiration) and hydrogen bonds!

Lab 9 Procedure – How to measure transpiration rates

1. Take the plant out of the pot with the roots and some soil.

2. Cover the roots/soil with a bag and seal it at the top with a twist-tie.

3. Measure the initial mass.4. Put it in the conditions you’re

testing.5. Later, measure the mass again

(record time between masses).6. Do not calculate % change!7. Divide by leaf surface area.

Procedure – How to standardize based on surface area

Follow the directions on page 6.

Sugar Transport

• Phloem– Cells are alive at functional maturity– Two cell types:• Sieve tube elements

– Contain only cytoplasm (no nuclei or other organelles)– Connected end to end with sieve holes

• Companion cells– Contain the nucleus and organelles to support themselves

AND the sieve tube cells

How do plants transport sugar from source to sink?

• Sugar source – makes or releases sugars– Photosynthesizing leaves– Storage organs that are breaking down/releasing

sugar• Sugar sink – absorbs and uses or stores sugars– Actively growing parts– Storage organs that are storing sugar (roots, fruits)

SucroseH2O

H2O

1. Sucrose pumped into phloem by active transport.Water follows by osmosis.

2. Positive pressure builds up in phloem near sugar source.Forces sugar-water to flow away from source, towards sink.(Pressure-flow mechanism)

3. Sucrose unloaded into sink by active transport.Water diffuses back into xylem to be recycled.Sucrose

H2O

SINK:

SOURCE:

Pulle

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by

TACT

mec

hani

sm

Bulk flow due to positive pressure

Xylem Phloem