5 Steps to a 5: AP Biology 2017 (2016)

STEP 4

Review the Knowledge You Need to Score High

CHAPTER 14

Plants

IN THIS CHAPTER

Summary: This chapter discusses the anatomy of plants, the mechanisms of root and shoot growth, plant hormones, tropisms, and the mechanism of water and nutrient movement from roots to shoots and back.

Key Ideas

Roots are the portions of the plant that are below ground; shoots are the portions of the plant that are above ground.

There are three plant tissue systems to know: ground, vascular, and dermal.

Two important plant vascular structures: xylem and phloem.

Regions of plant growth: root cap, zone of cell division, zone of elongation, and zone of maturation.

Five important plant hormones: abscisic acid, auxin, cytokinins, ethylene, and gibberellins.

Three important tropisms: gravitropism, phototropism, and thigmotropism.

Introduction

This chapter begins with a quick tour of the anatomy of plants, starting with the roots and moving to the shoots. In these two sections, the mechanisms of root and shoot growth will be examined and the important players will be identified. From there we will turn our focus to plant hormones and tropisms. A discussion on photoperiodism follows, and the chapter concludes with a look at the mechanism by which water and nutrients travel through plants from roots to shoots and back.

Anatomy of Plants

The anatomy of a plant in its most simplistic form can be divided into the roots and the shoots. Roots are the portions of the plant that are below the ground, while shoots are the portions of the plant that are above the ground. The roots wind their way through the terrain, working as an anchor to keep the plants in place. In addition, the roots work as gatherers, absorbing the water and nutrients vital to a plant’s survival.

Tissue Systems

There are three plant tissue systems to know: ground, vascular, and dermal.

BIG IDEA 2.D.2

Homeostatic mechanisms (e.g., plants obtaining CO₂ and eliminating O₂ ) reflect common ancestry and divergence due to adaptation to different environments .

Ground Tissue

The ground tissue, which makes up most of the body of the plant, is found between the dermal and vascular systems and is subdivided into three cell types: collenchyma cells, live cells that provide flexible and mechanical support—often found in stems and leaves; parenchyma cells, the most prominent of the three types, with many functions—parenchyma cells found in leaves are called mesophyll cells, and allow CO₂ and O₂ to diffuse through intercellular spaces (owing to the presence of large vacuoles, these cells play a role in storage and secretion for plants); and sclerenchyma cells, which protect seeds and support the plant.

Vascular Tissue

Plant vascular tissue comes up often on the AP Biology exam. The two characters you need to be familiar with are the xylem and the phloem.

Joscelyn (12th grader): “Know these two and what their driving forces are.”

Xylem . This structure has multiple functions. It is a support structure that strengthens the plant and functions as a passageway for the transport of water and minerals from the soil. One interesting (and sad) note about xylem cells is that most of them are dead and are simply there as cell walls that contain the minerals and water being passed along the plant. Xylem cells can be divided into two categories: vessel elements and tracheid cells. They both function in the passage of water, but vessel elements move water more efficiently because of structural differences that are not pertinent to this exam.

Phloem . This structure also functions as a “highway” for plants, assisting in the movement of sugars from one place to another. Unlike the xylem, the functionally mature cells of the phloem, sieve-tube elements, are alive and well.

Dermal Tissue

Dermal tissue provides the protective outer coating for plants. It is the skin, or epidermis. This coating attempts to keep the bad guys (infectious agents) out, and the good guys (water and nutrients) in. Within the epidermis are cells called guard cells, which control the opening and closing of gaps called stomata that are vital to the process of photosynthesis as was discussed back in Chapter 8 , Photosynthesis.

Roots

Root Systems

How do plants get their nutrients? Through the hard work of roots, whose tips absorb nourishment for the plant (minerals and water) via root hairs. Most of the water and minerals are absorbed by plants at the root tips, which have root hairs extending from their surface. These hairs create a larger surface area for absorption in much the same way as the brush border does in the human intestines—improving the efficiency of nutrient and water acquisition.

BIG IDEA 2.A.3

The high surface area of root hairs helps plants exchange matter with the environment .

A root is not just a root, for not all root structures are the same. In Chapter 13 , Taxonomy and Classification, two types of angiosperm plants were mentioned: dicots and monocots. Dicots are known for having a taproot system, while monocots are associated with fibrous roots. The taproot (e.g., carrot) system branches in a way similar to the human lungs; the roots start as one thick root on entrance into the ground, and then divide into smaller and smaller branches called lateral roots underneath the surface, which serve to hold the plant in place. Fibrous roots provide plants with a very strong anchor in the ground without going very deep into the soil. The root system can be summarized as follows:

Dicots → taproot → thick entry root → division into smaller branches

Monocots → fibrous root → shallow entry into ground → strong anchor effect

Root Structure

Let’s take a look at the structure of a root moving from outside to inside. The root is lined by the epidermis, whose cells give rise to the root hairs that plants must thank for their ability to absorb water and nutrients. Moving farther in, we come to the cortex, the majority of the root that functions as a starch storage receptacle. The innermost layer of the cortex is composed of a cylinder of cells known as the endodermis. These cells are important to the plant because the walls between these cells create an obstacle known as the casparian strip, which blocks water from passing. This is one of the mechanisms by which plants control the flow of water. Moving in through the endodermis, we come to the vascular cylinder, which is composed of a collection of cells known as the pericycle . The lateral roots of the plant are made from the pericycle, and hold the vascular tissue of the root—our friends from earlier, the xylem and phloem.

Root Growth

Plants grow as long as they are alive as a result of the presence of meristemic cells. Early on in the life of a plant, after a seed matures, it sits and waits until the time is right for germination. At this point, water is absorbed by the embryo, which begins to grow again. When large enough, it busts through the seed coat, beginning its journey to planthood. At the start of this journey, the growth is concentrated in the actively dividing cells of the apical meristem.Growth in this region leads to an increase in the length of a plant: primary plant growth. Later on growth occurs in cells known as the lateral meristems, which extend all the way through the plant. This growth leads to an increase in the width of a plant and is known as secondary plant growth.

Regions of Growth

Root cap: protective structure that keeps roots from being damaged during push through soil.

Zone of cell division: section of root where cells are actively dividing.

Zone of elongation: next section up along the root, where cells absorb H₂ O and increase in length to make the plant taller.

Zone of maturation: section of root past the zone of elongation where the cells differentiate to their finalized form (phloem, xylem, parenchyma, epidermal, etc).

The Shoot System

Now that we have discussed roots—the part of the plant that is in the ground—let’s take a look at shoots (leaves and stems), the parts of the plant that are out of the ground.

Structure of a Leaf

Leaves are protected by the waxy cuticle of the epidermis, which functions to decrease the transpiration rate. Inside the epidermis lies the ground tissue of the leaf, the mesophyll, which is involved in the ever-so-important process of photosynthesis. There are two important layers to the mesophyll: the palisade mesophyll and the spongy mesophyll. Most of the photosynthesis of the leaf occurs in the palisade mesophyll, where there are many chloroplasts. Inside a bit farther is the spongy mesophyll whose cells provide CO₂ to the cells performing photosynthesis. Important structures to successful photosynthesis are stomata, which are controlled by the guard cells that line the walls of the epidermis. Extending a bit farther inside the leaf, we find the xylem, the supplier of water to photosynthesizing cells, and the phloem, which carries away the products of photosynthesis. In C₄ plants, a second type of cell called a bundle sheath cell surrounds the vascular tissue to make the use of CO₂ more efficient and allow the stomata to remain closed during the hot daytime hours. These cells prevent excessive transpiration.

Structure of Stems

Again, let’s travel from the outside in and discuss the basic structure. The epidermis for the stem provides protection and is covered by cutin, a waxy protective coat. The cortex of a stem contains the parenchyma, collenchyma, and schlerenchyma cells mentioned earlier in this chapter. You’ll notice that there is no endodermis in the stem because this portion of the plant is not involved in the absorption of water. As a result, the next structure we see as we move inward is the vascular cylinder and our friends the xylem and phloem.

A term to know is the vascular cambium, which extends along the entire length of the plant and gives rise to secondary xylem and phloem. Over time, the stem of a plant will increase in width because of the secondary xylem produced each year.

Another term to know is the cork cambium, which produces a thick cover for stems and roots. This covering replaces the epidermis when it dries up and falls off the stem during secondary growth, forming a protective barrier against infection and physical damage.

The growth of plants is not a continuous process in seasonal environments. There are periods of dormancy in between phases of growth. Have you ever seen the rings of a tree after it has been cut down? These rings produced each year are a window into the past, and give insight into the amount of rain a tree has encountered in a given year. The wider the ring, the more water it saw.

Plant Hormones

Hormones perform the same general function for plants that they do for humans—they are signals that can travel long distances to affect the actions of another cell. There are five main plant hormones you should study for this exam.

1. Abscisic acid . This is the “babysitter” hormone. It makes sure that seeds do not germinate too early, inhibits cell growth, and stimulates the closing of the stomata to make sure the plant maintains enough water.

2. Auxin . This is a popular AP Biology exam plant hormone selection. Auxin is a hormone that performs several functions—it leads to the elongation of stems, and plays a role in phototropism and gravitropism, which we will discuss a bit later.

3. Cytokinins . Hormones that promote cell division and leaf enlargement. They also seem to have an element of the “fountain of youth” in them, as they seem to slow down the aging of leaves. Supermarkets use synthetic cytokinins to keep their veggies fresh.

4. Ethylene . This hormone initiates fruit ripening and causes flowers and leaves to drop from trees (associated with aging).

5. Gibberellins . Another hormone group that assists in stem elongation. When you think gibberellins, think “grow.” It is thought to induce the growth of dormant seeds, buds, and flowers.

BIG IDEA 3.B.2

The ethylene signal causes changes in production of different enzymes .

Plant Tropisms

A tropism is growth that occurs in response to an environmental stimulus such as sunlight or gravity. The three tropisms you should familiarize yourself with are gravitropism, phototropism, and thigmotropism.

1. Gravitropism . This is a plant’s growth response to gravitational force. Two of the hormones mentioned earlier play a role in this movement: auxin and gibberellins. A plant placed on its side will show gravitropic growth in which the cells on the upward-facing side will not grow as much as those on the downward side. It is believed that the relative concentrations of these hormones in the various areas of the plant are responsible for this imbalanced growth of the plant.

BIG IDEA 2.E.2

In plants, physiological events involve interactions between environment (light) and internal molecular signals .

2. Phototropism . This is a plant’s growth response to light. Auxin is the hormone in charge here. Auxin works its magic in the zone of elongation. While the mechanics of the phototropism process may not be vital to this exam, it is still quite interesting to know. When a plant receives light on all sides, auxin is distributed equally around the zone of elongation and growth is even. When one half of a plant is in the sun, and the other is in the shade, auxin (almost as if it feels bad for the shady portion) focuses on the darker side. This leads to unequal growth of the stem with the side receiving less light growing faster—causing the movement of the plant toward the light source.

3. Thigmotropism . This is a plant’s growth response to contact. One example involves vines, which wind around objects with which they make contact as they grow.

How in the world did we figure out that auxin played such a large role in phototropism? A series of experiments performed by two scientists proved vital to the understanding of this process. Grass seedlings are surrounded by a protective structure known as the coleoptile. Peter Boysen-Jensen performed an experiment in which a gelatin block permeable to chemical signals was placed in between this coleoptile structure and the body of a grass seedling. When the piece of grass was exposed to light on one side, it grew toward the light. When a barrier impermeable to chemical signals was placed in between the two structures instead, this growth toward light did not occur. Another scientist, F. W. Went, came onto the scene and took Jensen’s experiment a step further. Went wanted to show that it was indeed a chemical and not the coleoptile tip itself that was responsible for the phototropic response. He cut off the tip and exposed it to light while the tip was resting on an agar block that would collect any chemicals that diffused out. The block was then placed on the body of a tipless grass seedling sitting in a dark room. Even in the absence of light, a block placed more toward the right side of a seedling caused the seedling to bend to the left. A block placed more toward the left side of a seedling caused the seedling to bend to the right. Because there was no further light stimulation causing the growth, the agar block must indeed have contained a chemical that induced a phototropic response. This chemical was given the name auxin .

Photoperiodism

BIG IDEA 2.E.1

Timing of specific events is important in the development of an organism .

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 florigen is thought to assist in the blooming of flowers. An important pigment to the process of flowering is phytochrome, which is involved in the production 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 plants and long-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 H₂ 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 across selectively permeable membranes. It flows from a region with a high water concentration to a region with a low water concentration. There is a higher mineral concentration inside the vascular cylinder, which drives water into the xylem contained in this cylinder by a force known as root pressure.

Capillary Action

Capillary action is the force of adhesion between water and a passageway that pulls water up along the sides. Along with osmosis, this mechanism is a minor contributor to the movement of water up the xylem due to the counteracting force of gravity.

Cohesion-Tension Theory and Transpiration

This process is the major mover of water in the xylem. Transpiration creates a negative pressure in the leaves and xylem tissue due to the evaporative loss of water. Water molecules display molecular attraction (cohesion) for other water molecules, in effect creating a single united water molecule that runs the length of the plant. Imagine that you tie a bunch of soda cans to a rope. If you are standing in a tree, and pull up on the cans at the top of the rope, the cans at the bottom will follow—not really because they are loyal to the other cans, but because they are connected to them, they are bonded. This is similar to the movement of water through the xylem. When water evaporates off the surface of the leaf, the water is pulled up through the xylem toward the leaves—transpiration is the force pulling water through the plant.

The Changing of the Guard: Regulating Stomata Activity

The stomata are structures vital to the daily workings of a plant. When closed, photosynthesis is halted because water and carbon dioxide are inaccessible. When open, mesophyll cells have access to water and carbon dioxide. But with every reward, there is always a risk. When the stomata are open, the plant could dry out as a result of excessive transpiration. This process of opening and closing the stomata must therefore be very carefully controlled. Guard cells are the ones for the job. They surround and tightly regulate the actions of the stomata. When water flows into neighboring guard cells (leading to an increase in turgor pressure), a structural change occurs that causes the opening of the stomata. When the water flows out of the guard cells (a decrease in turgor pressure), the stomata will close. It is by this mechanism that guard cells control the opening and closing of the stomata.

“Move Over, Sugar”: Carbohydrate Transport Through Phloem

The transport of carbohydrates through the phloem is called translocation. After their production, carbohydrates, the all-important product of photosynthesis, are dumped into the phloem (the sugar superhighway) near the site of their creation, to be distributed throughout the plant. The movement of the sugar into the phloem creates a driving force because it establishes a concentration gradient. This gradient leads to the passive diffusion of water into the phloem, causing an increase in the pressure of these cells. This pressure drives the movement of sugars and water through the phloem. As the sugars arrive at various destination sites, the sugar is consumed by plant cells, causing a reversal in the driving force for water that pushes water out of the phloem. As water exits the phloem, the increased pressure disappears and all is good once again.

1 . Which of the following is not a time when most stomata tend to be open?

A. When CO₂ concentrations are low inside the leaf

B. When temperatures are low

C. When the concentration of water inside the plant is low

D. During the day

E. On a cold, rainy day

For questions 2–5, please use the following answer choices:

A. Abscisic acid

B. Auxin

C. Cytokinins

D. Ethylene

E. Gibberellins

2 . This hormone is used by supermarkets for its “fountain of youth” effect.

3 . This hormone initiates fruit ripening and works hard during the autumn months.

4 . This hormone prevents seeds from germinating prematurely.

5 . This hormone is known to induce growth in dormant seeds, buds, and flowers.

6 . A vine is observed to wrap around a tree as it grows in the forest. This is an example of

A. gravitropism.

B. phototropism.

C. thigmotropism.

D. photoperiodism.

E. phototaxis.

7 . This portion of the root of a plant is responsible for the visual perception of growth:

A. Zone of cell division

B. Vascular cylinder

C. Zone of elongation

D. Endodermis

E. Zone of maturation

For questions 8–10, please use the following answers:

A. Sieve-tube elements

B. Vessel elements

C. Tracheids

D. Guard cells

E. Collenchyma cells

8 . These cells are responsible for controlling the opening and closing of the stomata.

9 . These cells are the more efficient of the two types of xylem cells.

10 . These cells are live cells that function as structural support for a plant.

11 . The unequal growth of the stem of a plant in which the side in the shade grows faster than the side in the sun is an example of

A. gravitropism.

B. phototropism.

C. thigmotropism.

D. photoperiodism.

E. phototaxis.

1 . C —When the concentration of water inside the plant is low, the stomata close in an effort to minimize transpiration.

2 . C

3 . D

4 . A

5 . E

6 . C —Thigmotropism is a plant’s growth in response to touch. Phototropism is growth in response to light, and gravitropism is growth in response to gravitational force. Photoperiodism is the response by a plant to the change in the length of days, and phototaxis is the sad phenomenon whereby moths fly kamikaze-style into burning hot lights at night.

7 . C

8 . D

9 . B

10 . E

11 . B —Phototropism, a plant’s growth response to light, is controlled by auxin. This hormone is produced in the apical meristem and sent to the zone of elongation to initiate growth toward the sun.

The following terms and topics are important in this chapter:

Anatomy of plants: tissue systems are divided into ground, vascular, and dermal .

Ground tissue: the body of the plant is divided into three cell types:

• Collenchyma cells: provide flexible and mechanical support; found in stems and leaves.

• Parenchyma cells: play a role in storage, secretion, and photosynthesis in cells.

• Sclerenchyma cells: protect seeds and support the plant.

Vascular tissue: xylem (transports water and minerals) and phloem (transports sugar).

Dermal tissue: protective outer coating for plants: epidermis .

Roots

Types: taproot system (dicots)—system that divides into lateral roots that anchor the plant; fibrous root system (monocots)—anchoring system that does not go deep down into soil.

Structure: epidermis → endodermis (casparian strip) → vascular cylinder → xylem/phloem.

Growth: occurs for lifetime of the plant thanks to meristem cells:

• Primary growth: increased length of a plant (occurs in region of apical meristems).

• Secondary growth: increased width of a plant (occurs in region of lateral meristems, limited in monocots).

• Three main growth regions: zone of cell division (cells divide), zone of elongation (cells elongate), zone of maturation (cells mature to specialized form).

Stems (Shoots)

Structure: epidermis (cutin) → cortex (ground tissue) → vascular cylinder → xylem/phloem.

Vascular cambium: gives rise to secondary xylem/phloem; runs entire length of plant.

Cork cambium: produces protective covering that replaces epidermis during secondary growth.

Leaves (Shoots)

Structure: epidermis (cuticle) → mesophyll (photosynthesis) → vascular bundles → xylem/phloem.

C₄ plants: leaves contain another cell type, bundle sheath cells, which assist in respiration in hot and dry regions.

Stomata: structure, controlled by guard cells, that when open allows CO₂ in, and H₂ O and O₂ out.

Plant hormones: abscisic acid (inhibits cell growth, helps close stomata), auxin (stem elongation, gravitropism, phototropism), cytokinins (promotes cell division, leaf enlargement, slows aging of leaves), ethylene (ripens fruit and causes leaves to fall), gibberellins (stem elongation, induces growth in dormant seeds, buds, flowers).

Plant tropisms: gravitropism (a plant’s growth in response to gravity—auxin, gibberellins), phototropism (plant’s growth in response to light—auxin), thigmotropism (plant’s growth in response to touch).

Photoperiodism: response of a plant to the change in length of days; remember florigen and phytochrome.

Driving force for H₂ O movement in plants: transpiration is the major driving force that draws H₂ O up the xylem because of the cohesive nature of water molecules. Osmosis and capillary action are minor contributors.

Driving force for sugar movement in plants: sugar, when created, is dumped into the phloem, creating a concentration gradient that draws water in, increasing the pressure that drives the sugar through the phloem.

CHAPTER 14

Plants

1 . Which region represents the area of the plant where cells differentiate to their finalized form (phloem, xylem, parenchyma, etc.)?

(A) Root cap

(B) Zone of cell division

(C) Zone of elongation

(D) Zone of maturation

2 . The growth of the tendrils of a plant around a hard substance, such as a pole or post, represents an example of

(A) gravitropism.

(B) phototropism.

(C) thigmotropism.

(D) coleotropism.

3 . Which of the following are the xylem cells in charge of water transport in gymnosperms?

(A) Sieve-tube elements

(B) Vessel elements

(C) Tracheid cells

(D) Collenchyma cells

4 . The response of a plant to the change in hours of sunlight per day is known as

(A) photoperiodism.

(B) photophosphorylation.

(C) photorespiration.

(D) phototropism.

Answers and Explanations

1 . D —This choice is logical if you really think about what the question is asking. If cells are differentiating into their finalized form, they are maturing.

2 . C —OK, coleotropism was made up, but the other three are real. Gravitropism is a plant’s growth response to gravitational force. Phototropism is a plant’s growth response to light. Thigmotropism is a plant’s growth response to contact, which is the winner here.

3 . C

4 . A