Plant Responses to the Environment, Transpiration, Photosynthesis, Respiration

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Lecture Outlines by Gregory Ahearn, University of North Florida

Adapted from

45.1 What Are Some Major Plant Hormones?

Plants respond to their environment in sophisticated ways – React to stimuli like touch, gravity, moisture,

light, and day length – They use hormones—chemicals that are

secreted by cells and transported to other cells, where they exert specific effects

Auxins promote or inhibit elongation in different target cells – Controls positive phototropism (growth toward light) in

shoots and gavitropism (growth toward or away from gravity) in shoots and roots

Radish Seedlings Show Positive Phototropism

Fig. 45-3

Auxin is transported to the lower side of theshoot, where it stimulates cell elongationand causes the stem to bend upward

Auxin is transported to the lower side of theroot, where it inhibits cell elongation andcauses the root to bend downward

(b) The shoot and root are orientedhorizontally

Positive Gravitropism

Fig. 45-2b

Gibberellins are primarily active in plant shoots – Promote stem elongation

by increasing both cell division and cell elongation

Ethylene is a gas – Recognized as a plant “stress hormone” – Causes certain fruits to ripen

Ripe Fruit Become Attractive to Animal Seed Dispersers

Fig. 45-11

Abscisic acid helps plants withstand unfavorable environmental conditions – Causes stomata to close when water is scarce – Promotes root growth and inhibits stem growth under dry

conditions – Maintains dormancy in seeds

Florigens control the timing of flowering in response to environmental cues

45.2 How Do Hormones Regulate Plant Life Cycles?

Some plants display thigmotropism, a directional movement or change in growth in response to touch

A Rapid Response to Touch

Fig. 45-14

A Sundew with Its Insect Prey

Fig. 45-14

Photosynthesis Review

Occurs in two steps: Light reaction and dark reaction (Calvin Cycle)

Light Reaction

Requires light Converts light energy

into chemical energy (NADPH and ATP)

Dark Reaction

AKA Calvin cycle Does not require light Makes sugar from CO2

and hydrogen ions carried by NADPH

Energy to do this comes from ATP produced in light reaction

C4 and CAM plants separate light and dark reactions

Cellular respiration in plants?

Yes! Sugar must be converted into ATP for cells to use

Sugar + O2 CO2 + H2O + ATP In mitochondria

Transpiration

Plants breathe! Gas exchange occurs

through stomata Evaporation from

leaves (transpiration) draws water up the plant – like a straw!

Relies on cohesion and adhesion

Plant Responses to the Environment, Transpiration, Photosynthesis, Respiration

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