5 Steps to a 5 AP Biology, 2014-2015 Edition

172 ❯ STEP 4. Review the Knowledge You Need to Score High

Photoperiodism

Like all of us, plants have a biological clock that maintains a circadian rhythm—a physiologic

cycle that occurs in time increments that are roughly equivalent to the length of a day.

The month of June has the longest days of the year—the most sunlight. The month of

December has the shortest days of the year—the least sunlight. How is it that plants, which

are so dependent on light, are able to survive through these varying conditions? This is

thanks to photoperiodism,the response by a plant to the change in the length of days. One

commonly discussed example of photoperiodism involves flowering plants (angiosperms).

A hormone known as florigenis thought to assist in the blooming of flowers. An impor-

tant pigment to the process of flowering is phytochrome,which is involved in the produc-

tion of florigen. Because plants differ in the conditions required for flowering to occur,

different amounts of florigen are needed to initiate this process from plant to plant.

One interesting application of photoperiodism involves the distinction between short-

day plantsandlong-day plants,which flower only if certain requirements are met:

Go with the Flow: Osmosis, Capillary Action,

Cohesion-Tension Theory, and Transpiration

Osmosis drives the absorption of water and minerals from the soil by the root tips. Water

then moves deeper into the root until it reaches the endodermis. Once there, because of the

casparian strip, it can only travel through the selective endodermal cells that choose which

nutrients and minerals they let through to the vascular cylinder beyond. The casparian strip

essentially lets only those with a backstage pass through. Potassium has a backstage pass and

can go into the vascular cylinder... sodium does not and gets denied. Once the water gets

to the xylem, it has reached the H 2 O superhighway and is ready to go all over the plant.

There are a few driving forces responsible for the movement of a plant’s water supply.

The three main forces we will cover here are osmosis, capillary action, and cohesion-tension

theory. Of those three, the cohesion-tension theory pulls the most weight.

Osmosis

Osmosis is the driving force that moves water from the soil into xylem cells. How in the

world does the plant keep the concentration gradient such that it promotes the movement

of water in the appropriate direction? There are two contributing factors: (1) the water is

constantly moving away from the root tips creating the space for more water to enter, and

(2) osmosis is defined as the passive diffusion of water down its concentration gradient

FLOWERING

PLANT TYPE EXAMPLE REQUIREMENTS FLOWERS DURING

Short-day plants Poinsettias Exposure to a night End of summer to end

longerthan a certain of winter

number of hours

(e.g., 10 hours)

Long-day plants Spinach Exposure to a night Late spring to early

shorterthan a summer

certain number of

hours (e.g., 8 hours)

BIG IDEA 2.E.1
Timing of specific
events is important
in the development
of an organism.