Biology For Dummies

Part V It’s Not Easy Being Green: Plant Structure and Function

Chapter 21

Probing into Plant Physiology

In This Chapter

Moving water and other nutrients through plants

Getting sugars where they need to go

Triggering plant responses with hormones

Photosynthesis (described in Chapter 5) isn’t the only life-sustaining process carried out by plants. Below the surface, plants are busy collecting and transporting water and other nutrients upward, as well as moving sugars upward and downward to the cells that need them. Their hormones, meanwhile, are sending messages to promote growth toward sunlight and blossoming, among other things.

Consider this chapter your introduction to the physiology of plants (physiology is the study of the function of organisms and their parts). Here, you discover the processes plants use to transport nutrients, fluids, and sugars throughout their bodies. You also get to know the plant hormones that regulate growth and development.

How Nutrients, Fluids, and Sugars Move through Plants

Just like you have a circulatory system that moves food and oxygen throughout your body, plants have a system to move nutrients, fluids, and sugars throughout their bodies. (Even though plants make food in their leaves by photosynthesis, the entire plant needs some of that food and the nutrients it provides.) The following sections fill you in on the different nutrients plants must absorb to stay healthy as well as how they move sugars from their leaves and water from their roots (without losing too much of it).

Taking an inventory of the nutrients plants need to survive

All plants require carbohydrates, proteins, fats, and nucleic acids to function — the same as you do. They also need mineral elements to build their molecules and make sure their enzymes are working properly. Fortunately, plants can obtain all the nutrients they need to survive from their environment.

 Plants get carbon, hydrogen, and oxygen by taking carbon dioxide from the atmosphere and water from the soil. With energy from the Sun, plants combine these molecules to form carbohydrates during the process of photosynthesis.

Plants obtain their necessary mineral elements from the soil as well. The mineral nutrients found in soil dissolve in water, so when plants absorb water through their roots, they obtain both macronutrients and micronutrients. Macronutrients help with molecule construction, andmicronutrients act as partners for enzymes and other proteins to help them function. Plants generally require large amounts of macronutrients and smaller amounts of micronutrients. Table 21-1 lists the specific macro- and micronutrients plants absorb from soil.

Table 21-1 The Essential Nutrients Plants Pull from Soil



Calcium (Ca)

Boron (B)

Magnesium (Mg)

Chloride (Cl)

Nitrogen (N)

Copper (Cu)

Phosphorous (P)

Iron (Fe)

Potassium (K)

Manganese (Mn)

Sulfur (S)

Molybdenum (Mb)


Zinc (Zn)

 You can remember the most important elements for plants with the phrase “C. Hopkins Café, Mighty Good.” This phrase stands for CHOPKNS CaFe Mg — in other words, carbon, hydrogen, oxygen, phosphorous, potassium, nitrogen, sulfur, calcium, iron, and magnesium. All of these elements are macronutrients for plants, with the exception of iron, which is considered a micronutrient.

If plants don’t get enough of one of these important elements, they can’t function correctly. Without carbon, hydrogen, and oxygen (from carbon dioxide and water), plants can’t grow at all. And even though plants need smaller amounts of minerals, even one missing mineral can cause a specific problem. We list these problems and the mineral deficiencies they’re associated with in Table 21-2.

Table 21-2 The Effects of Mineral Deficiencies on Plants

Mineral That’s Missing

Effect of the Deficiency


Leaves on the ends of the plant die and fall off early; plant growth is stunted; flowers and seeds usually aren’t produced


Leaves roll and curl; roots are poorly developed and may look gelatinous


Terminal shoots wilt and die; leaves appear faded in color


White marks show in the veins; leaves look bleached; tips of leaves look scorched


Veins look green, but leaf tissue looks white or yellow and brittle; leaves may wilt, fall off, or die


Same as magnesium but stems are yellowish-green in color and often hard to the touch


Leaves are light yellow and may not grow


Stunted growth; leaves turn light green, then yellow, and then dry out and fall off


Stunted growth; leaves sometimes look purplish; stems are thin


Leaves have pale green or streaked yellow color and look wrinkled between the veins


Leaves appear light green to yellow in color; stems are thin


Leaves die; white streaks show between the veins in older leaves

Transporting water and other nutrients from the ground up

Several processes work together to transport water (as well as other nutrients) from where a plant absorbs it (the roots) upward through the rest of its body. To understand how these processes work, you first need to know one key feature of water: Water molecules tend to stick together, literally. Water molecules are attracted to each other by weak electrical attractions called hydrogen bonds. The stickiness of water helps keep the water molecules together when you drink water through a straw — a process that’s very similar to one of the methods plants use to move water through their bodies.

Water moves from the soil, into a plant’s roots, and then throughout the plant thanks to a combination of three processes:

Osmosis: The method plants use to draw water from the soil into the xylem cells in their roots is called osmosis. Root cells have a higher concentration of minerals than the soil they’re in, so during osmosis, water flows toward the higher concentration of dissolved substances found in the root cells. This intake of water increases pressure in the root cells and pushes water into the plant’s xylem (see Chapter 20 for the full scoop on plant structure).

Capillary action: This causes liquids to rise up through the tubes in the xylem of plants. This action results from adhesion (when two things stick together), which is caused by the attraction between water molecules and the walls of the narrow tube. The adhesion forces water to be pulled up the column of vessel elements in the xylem and in the tubules in the cell wall.

Transpiration and cohesion: Transpiration is the technical term for the evaporation of water from plants. As water evaporates through the stomates in the leaves (or any part of the plant exposed to air), it creates a negative pressure (also called tension or suction) in the leaves and tissues of the xylem. The negative pressure in the leaves and xylem exerts a pulling force on the water in the plant’s xylem and draws the water upward. When water molecules stick to each other through cohesion (where like — as opposed to different — substances stick together), they fill the column in the xylem and act as a huge single molecule of water. As water evaporates from the plant through transpiration, the rest of the water gets pulled up, causing the need for more water to be pulled into the plant.

 The back and forth of transpiration and cohesion is known as the cohesion-tension theory. It’s similar to what happens when you suck on a straw. The suction you apply to the straw is like the evaporation from the leaves of the plant. Just like you can pull up a column of liquid through your straw, a plant can pull up a column of liquid through its xylem.

Water plus sap equals . . . a dewdrop?

Those droplets of water that you see on a plant’s leaves in the morning, what you think of as a bunch of dewdrops, aren’t just water. They’re a mixture of water and sap, a sugar solution from the phloem (Chapter 20 describes this and other elements of a plant’s structure). These sap droplets are proof that water and minerals get pulled up from the soil and transported throughout the entire plant. (We describe how this occurs in the nearby “Transporting water and other nutrients from the ground up” section.)

Translocating sugars upward and downward through the phloem

Phloem moves sap, a sticky solution containing sugars, water, minerals, amino acids, and plant hormones throughout the plant via translocation, the transport of dissolved materials in a plant. Unlike xylem, which can only carry water upward, phloem carries sap upward and downward from sugar sources to sugar sinks.

Sugar sources are plant organs such as leaves that produce sugars.

Sugar sinks are plant organs, such as roots, tubers, or bulbs, that consume or store sugars.

The specific way translocation works in a plant’s phloem is explained by the pressure-flow theory, which we outline step by step in the following list:

1. First, sugars are loaded into phloem cells called sieve tube elements within sugar sources, creating a high concentration of sugar at the source.

The concentration of sugars in sink organs is much lower.

2. Water enters the sieve tube elements by osmosis.

During osmosis, water moves into the areas with the highest concentration of solutes (in this case, sugars).

3. The inflow of water increases pressure at the source, causing the movement of water and carbohydrates toward the sieve tube elements at a sugar sink.

 You can think of this like turning on a water faucet that’s connected to a garden hose. As water flows from the tank into the hose, it pushes the water in front of it down the hose.

4. Sugars are removed from cells at the sugar sink, keeping the concentration of sugars low.

As a sugar sink receives water and carbohydrates, pressure builds. But before the sugar sink can turn into a sugar source, carbohydrates in a sink are actively transported out of the sink and into needy plant cells. As the carbohydrates are removed, the water then follows the solutes and diffuses out of the cell, relieving the pressure.

 Sugar sinks that store carbohydrates can become sugar sources for plants when sugars are needed. Starch, a complex carbohydrate, is insoluble in water, so it acts as a carbohydrate storage molecule. Whenever a plant needs sugar, like at night or in the winter when photosynthesis doesn’t occur as well, the plant can break down its starches into simple sugars, allowing a tissue that would normally be a sugar sink to become a sugar source.

Because plant cells can act as both sinks and sources, and because phloem transport goes both upward and downward, plants are pretty good at spreading the wealth of carbohydrates and fluid to where they’re needed. As long as a plant has a continuous incoming source of minerals, water, carbon dioxide, and light, it can fend for itself.

Controlling water loss

Because water is essential to a plant’s functioning, it has built-in mechanisms that help prevent it from losing too much water: a cuticle and guard cells.

The cuticle is a layer of cells found on the top surfaces of a plant’s leaves (see Figure 21-1). It lets light pass into the leaf but protects the leaf from losing water. Many plants have cuticles that contain waxes that resist the movement of water into and out of a leaf, much like wax on your car keeps water off the paint.

Guard cells are found on the bottom of a plant’s leaves, near a stomate, a tiny opening that you can’t see with your naked eye. (An individual opening is called a stomate, or stoma; several openings are called stomates, or stomata.) Plants need to keep their stomates, shown in Figure 21-1, open in order to obtain carbon dioxide for photosynthesis and release oxygen. However, if the stomates are open too long or on a really hot day, the plant can lose too much water. To prevent such water loss from happening, each stoma has two guard cells surrounding it.

 Guard cells can swell and contract in order to open and close the stomates. When the Sun is shining and photosynthesis is occurring, guard cells swell up with water like full balloons, which stretches them outward and opens the stomates. At night, when photosynthesis isn’t occurring, the guard cells release some water and collapse together, closing the stomates.

Aphids suck

Aphids, those tiny little insects that can destroy your houseplants when you’re not looking, live on the sap flowing through the phloem of a plant. They have long, pointy structures called stylets that let them suck sap right out of the plant phloem. Insertion of the stylet doesn’t harm the plant — in fact, aphids can go right into a sieve tube without the plant “feeling” a thing. An aphid can stay attached to a plant for hours, sucking out the sap all the while. It’s the loss of sap — and the cumulative effect of many, many aphids on a plant — that causes damage to a plant. The aphids fill up, leaving the plant starved of its sugar mixture.

In an interesting twist, scientists have found a way to use aphids to study transport in the phloem. They allow aphids to attach to plants and insert their stylets, and then they cut the aphids away but leave the stylets embedded in the plants. Materials flowing through the phloem ooze out through the stylets and can be collected by scientists for further study.

Figure 21-1: A plant’s cuticle and guard cells keep it from losing too much water.

 Some plants that live in very hot, dry environments save water by opening their stomates at night and storing carbon dioxide in their leaves. Then, during the day when it’s hot and dry, they keep their stomates closed to conserve water, performing photosynthesis with the carbon dioxide they stored during the night.

Sending Signals with Plant Hormones

Plant cells communicate with each other via messengers called hormones, chemical signals produced by cells that act on target cells to control their growth or development. Plant hormones control many of the plant behaviors you’re used to seeing, such as the ripening of fruit, the growth of shoots upward and roots downward, the growth of plants toward the light, the dropping of leaves in the fall, and the growth and flowering of plants at particular times of the year.

Five categories of hormones control plant growth and development:

Auxins stimulate the elongation of cells in the plant stem and phototropism (the growth of plants toward light). If a plant receives equal light on all sides, its stem grows straight. If light is uneven, then auxin moves toward the darker side of the plant. This may seem backward, but when the shady side of the stem grows, the stem, in its crookedness, actually bends toward the light. This action keeps the leaves toward the light so photosynthesis can continue.

Gibberellins promote both cell division and cell elongation, causing shoots to elongate so plants can grow taller and leaves can grow bigger. They also signal buds and seeds to begin growing in the spring.

Cytokinins stimulate cell division, promote leaf expansion, and slow down the aging of leaves. Florists actually use them to help make cut flowers last longer.

Abscisic acid inhibits cell growth and can help prevent water loss by triggering stomates to close. Plant nurseries use abscisic acid to keep plants dormant during shipping.

Ethylene stimulates the ripening of fruit and signals deciduous trees to drop their leaves in the fall. Fruit growers use ethylene to partially ripen fruit for sale.

 Some of the flavor-making processes that occur in fruits happen while the fruits are still on the plant. So, even though ethylene can trigger some parts of ripening, like softening after a fruit has been picked, fruit that’s picked unripe doesn’t taste as good as fruit that has ripened on the plant. That’s why you can buy a big, beautiful tomato at the grocery store and take it home only to discover that it doesn’t have much flavor — it was probably picked unripe and then treated with ethylene.