3.7 Plant Nutrition and Transport

Water potential

potential energy of water

How do solutes reduce water potential

consuming potential energy in water

How water moves in root cells

difference in water potential

Transpiration

• water evaporates from leaves

Why does water evaporate from leaves

• cool the leaf

• occurs through stomata

• main driver of water movement in xylem

• creates negative pressure at the leaf surface

Cohesion

pulls nearby water molecules closer to stomata

Cohesion tension theory

• transpiration creates negative water potential gradient

• water pulled to roots

• cohesion and adhesion of water to the cell walls transports water through the xylem up the tree

Turgor Pressure

• force of water pushing against cell wall

Turgor Pressure- if water potential is lower outside cells

• water moves out of the cells, plant wilts

Turgor Pressure- if water potential is higher outside

• water moves into the cells keeping plant errect

Role of Phloem

• creates positive pressure pushing sap from source to a sink

source

produces sugars (leaves)

Sink

receives sugars (roots and fruits0

Pressure Flow Theory

1. sugars actively transported by companion cells from photosynthetic cells into sieve tube cells

2. water moves via osmosis from xylem to sieve tube, increasing sieve tube pressure

3. pressure pushes sugars toward the sink

4. at the sink, transport proteins move sugars out of sieve tube

5. because solute conc. in phloem decreased, water leaves sieve tube via osmosis

Translocation

1. Active transport of sugars from source into phloem

2. high amount of solute in phloem moves water into phloem

3. pressure pushes sugars to the sink

4. water moves back into xylem

Response to light- Chromophores

• light absorbing pigments

Photoperiodism

uses light to track time (seasonal, germination, or growth response

Phototropism

• reaction to light toward or away from light

positive phototropism

• toward light

negative phototropism

• away from light

Gravitropism

• response to gravity

• negative: shoots grow up

• positive: roots grow down

Thigmotropism

• response to touch

thigmomorphogensis

• developmental change in plant change due to mechanical stress

• ex: thickened tree trunk on windy coast

Difference between plant hormones and human

plant hormones can be produced in any cell

animal hormones produced in specific glands

Plant defenses

• bark/waxy cuticle=barrier

• thorns: modified branches and spines: modified leaves

• volatile odors

• repellent tastes

• toxins

Role of cellulose

structure component of cell wall

Parasitism- holoparasites

lack chlorophyll

Parasitism- hemiparasites

have chlorophyll

Insectivorous Plants

• uses specialized leaves to attract and digest insects

• in acidic environments

Commensalism- epiphyte

• grows on other plants, not dependent on other plant for nutrition