Structure and Function of Plants
Plants are as intricate and complicated as animals. But you wouldn’t know that from looking at the SAT II Biology. Though the test covers almost all aspects of animal organismal biology, it covers plants in much less detail. This is not to say that you can ignore plants completely while studying for the test, just that you need to study the right things.
To begin with, you need to know what plants are, how plants differ from animals on both the cellular and organismal level, and the different categories of plants that exist. You should also know about the structure and function of the three most important parts of vascular plants: leaves, stems, and roots. In addition, it’s a good idea to have a basic knowledge of how plants grow. Plants also have various and unique means of reproducing themselves that the SAT II may test. And finally, you should have some sense of plant “behaviors,” which are called tropisms.
The General Plant Cell
We covered plant cells back in the chapter on Cell Processes, but we’ll provide a summary here.
- Plants have all the organelles animal cells have. Nucleus, ribosomes, mitochondria, endoplasmic reticulum, Golgi apparatus, etc.
- Plants have chloroplasts. Chloroplasts are special organelles that contain chlorophyll and allow plants to carry out photosynthesis.
- Plant cells can sometimes have big vacuoles for storage.
- Plants are surrounded by a rigid cell wall made of cellulose, in addition to the cell membrane that surrounds animal cells. These walls provide support.
Types of Plants
Just as there are many different kinds of animals, there are also many different kinds of plants. The earliest plants had no special vascular tissues devoted to transport, meaning they could not grow to great heights because they couldn’t transport necessary liquids and minerals over long distances. These plants are called nonvascular or nontracheophyte. Tracheophytes, also called vascular plants, do have special vascular tissues for transport.
Vascular Tissues
Vascular tissues are composed of specialized cells that create “tubes” through which materials can flow throughout the plant body. These vessels are continuous throughout the plant, allowing for the efficient and controlled distribution of water and nutrients. In addition to this transport function, vascular tissues also support the plant. The two types of vascular tissue are xylem and phloem.
XYLEM
Xylem consists of a “pipeline” of dead cells arranged end to end for water and mineral transport. When the cells that form xylem die at maturity, the nucleus and cytoplasm disintegrate, leaving a hollow tunnel through which fluids can move. The xylem carries water and dissolved minerals upward from the roots through the stem and leaves of the plant. In larger seed plants, xylem cells are specialized into tracheids and vessel elements. Vessels are wider and better at conducting water than the tracheids.
In addition to distributing nutrients, xylem provides structural support. In fact, the material commonly known as “wood” is actually xylem. After a time, the xylem at the center of older trees (woody dicots) ceases to function in transport and takes on a purely supportive role.
PHLOEM
Unlike xylem, the cells that make up phloem are living at maturity and can carry materials both up and down the plant body. Phloem consists of sieve elements, which are arranged end to end to form passageways, and companion cells, which are closely associated with the sieve elements, even though their exact function is unknown. Mature companion cells have both a nucleus and cytoplasm, while the mature sieve elements contain only cytoplasm; for this reason it is thought that the nuclei of companion cells may control the activities of neighboring sieve elements. Phloem is responsible for distributing the products of photosynthesis, such as amino acids and carbohydrates, from the leaves to the rest of the plant.
Types of Tracheophytes
There are also distinctions among the tracheophytes. Ferns were the first tracheophytes to evolve. Ferns are notable because they reproduce through the use of spores and do not use seeds. Otherwise, in SAT II Biology world, you don’t really have to worry about them. The two tracheophytes that are more important are gymnosperms and angiosperms.
GYMNOSPERMS
Gymnosperms were the first tracheophytes to use seeds for reproduction. The seeds develop in protective structures called cones. A gymnosperm will contain some cones that are female and some that are male. Female cones produce spores that, after fertilization, become eggs enclosed in seeds that fall to the ground. Male cones produce pollen, which is taken by the wind and fertilizes female eggs by that means. Coniferous trees such as pines and firs are good examples of gymnosperms.
ANGIOSPERMS
Angiosperms, the flowering plants, are the most highly evolved plants and the most dominant in present times. Angiosperms are also the type of plant that the SAT II mainly focuses on. In fact, you don’t really need to know anything about nontracheophytes, ferns, and gymnosperms beyond what we’ve already told you. The further discussions of plant biology in this chapter will focus on angiosperms (though what is true for angiosperms is also often true for the other types of plants).
As for angiosperms, there are actually two kinds: monocots and dicots. Monocots include grasses, grains, and other narrow-leaved angiosperms. Monocots are named for the presence of a single cotyledon (seed leaf) during embryonic development. In general, the veins of monocot leaves are parallel, the flower parts occur in multiples of three, and a fibrous root system is present. Bundles of vascular tissue are scattered throughout the stem instead of appearing in a single ring. Dicots, such as maples, oaks, elms, sunflowers, and roses, originate from embryos with two cotyledons. They are further distinguished from monocots by the branched network of veins in their leaves, the occurrence of their flower parts in groups of four or five, and the presence of a taproot, which is a single main root with tributaries off it. The vascular bundles of dicots are arranged in a tubular pattern in the stem.
Leaves
Leaves are the sites of photosynthesis in plants. The leaves’ broad, flattened surfaces gather energy from sunlight, while apertures on their undersides bring in carbon dioxide and release oxygen. On its two exteriors, the leaf has layers of epidermal cells that secrete a waxy, nearly impermeable cuticle to protect against water loss and fungal or bacterial attack. The only way gases can diffuse in or out of the leaf is though stomata, small openings on the underside of the leaf. The opening or closing of the stomata occurs through the swelling or relaxing of guard cells. If the plant wants to limit the diffusion of gases and the transpiration of water, the guard cells swell together and close the stomata.
The tissues between the epidermal cells are called mesophyll. The mesophyll can be further broken down into two layers, the palisade layer and the spongy layer. Both layers are packed with chloroplasts, the factories of photosynthesis. In the palisade layer, chloroplasts are lined in columns just below the epidermal cells to facilitate the capture of light. In the spongy layer, cells are less ordered and more diffuse, leaving large intracellular spaces that facilitate the exchange of carbon dioxide and oxygen.
Overall, it is to the plant’s advantage to maximize the gas exchange and sunlight trapping surface area while keeping leaf thickness to a minimum so that once gases enter the leaf through the stomata, they can diffuse easily throughout the leaf cells.
The Chloroplast
Like mitochondria, chloroplasts consist of a double membrane layer with an intermembrane space between. The inner membrane is folded into multiple stacks of flattened, disk-shaped compartments. Each such compartment is called a thylakoid, and a stack of thylakoids is called a granum. The thylakoid membrane separates the chloroplast interior into two very different compartments: the thylakoid space inside the thykaloids where the photosynthetic chlorophyll resides and the stroma, a fluid that lies outside the stacked disks and takes up the rest of the organelle.
Photosynthesis
In photosynthesis, plants (and other photosynthetic autotrophs) use the energy from sunlight to create the carbohydrates necessary for cell respiration. More specifically, plants take water and carbon dioxide and transform them into glucose and oxygen:
6CO2 + 6H2O + light energyC6H12O6 + 6O2
This general equation of photosynthesis represents the combined effects of two different stages. The first stage is called the light reaction, since it is dependent on light. The dark reaction, the second stage, does not need light.
THE LIGHT REACTION
The light reaction, also called the light-dependent reaction, takes place within the thylakoid spaces of the grana in the chloroplast. When sunlight strikes the chlorophyll contained within the thylakoid spaces, electrons become excited and infused with energy, and they are transferred down an electron transport chain similar to the one found in aerobic respiration. The energy in the electrons is used to set up a proton gradient across the membrane of the thylakoid spaces. Protons flowing back across the thylakoid membrane according to the concentration gradient are harnessed to produce ATP and NADPH (the reduced form of NADP). As a by-product of this process, molecules of water are split into molecular hydrogen and oxygen. The plant needs the hydrogen to produce ATP. The oxygen you’re breathing right now is some of that waste product.
The purpose of the light reaction is to make the usable energy necessary to run the dark reaction.
THE DARK REACTION
The dark reaction is also referred to as the light-independent reaction, the Calvin cycle, or carbon fixation. The reaction takes place in the stroma of the chloroplast.
In the dark reaction, the carbon from carbon dioxide is added to the five-carbon sugar ribulose bisphosphate (RuBP) to produce a six-carbon compound. This six-carbon sugar is immediately split into two three-carbon molecules, which in a chain reaction using the ATP and NADPH from the light reaction are modified to form glyceraldehyde 3-phosphate. The glyceraldehyde 3-phosphate can be synthesized into carbohydrates such as glucose, and it can also be synthesized back into ribulose biphosphate. One of the glyceraldehyde 3-phosphate molecules is made into carbohydrates, while the other molecules remain in the Calvin cycle to serve as raw materials for the next round of production.
Roots
The roots of a plant draw water and minerals from the soil and pass them upward through xylem and phloem to the stem and leaves. Roots are also responsible for storing the plant’s organic nutrients, which are passed downward from the leaves through the phloem. Radiating from the roots is a system of root hairs, which vastly increase the absorptive surface area of the roots. Roots also anchor the plant in the soil.
Growth in Vascular Plants
Vascular plants undergo two kinds of growth, primary growth and secondary growth. Primary growth occurs in the apical meristems, located at the tip of both root and shoot, and is mainly a growth of vertical length. The meristems are regions of rapid mitotic division, churning out cells like a factory. When a cell divides, one of its offspring moves down into the plant body, where it elongates, and the other remains in the meristem to divide again.
Secondary growth is a growth of thickness. Secondary growth is a product of two different, though related, tissues, which both fall under the umbrella-term lateral meristems. Vascular cambium exists between xylem and phloem: on its inside the cambium produces what is known as secondary xylem; on its outside it forms secondary phloem. The primary xylem and phloem are pushed farther inward and outward. The vascular cambium is more productive during the growing seasons. During the rest of the year it creates little growth. This phenomenon creates distinct rings of growth, each ring representing a single growing season. By studying these rings, it is possible to calculate the age of a plant, and it’s even possible to determine the specific conditions of a particular growing season. The second lateral meristem is called cork cambium and is responsible for the formation of cork (bark), which replaces the epidermis to form the protective covering of shoot and root.
Controlling Growth: Plant Hormones
Plant growth is controlled by plant hormones, which influence cell differentiation, elongation, and division. Some plant hormones also affect the timing of reproduction and germination.
- Auxins. The primary function of the auxin hormones is to elongate plant cells in the stem. Auxins are also responsible for root development, secondary growth in the vascular cambium, inhibition of lateral branching, and fruit development.
- Kinins promote cell division and tissue growth in leaf, stem, and root. Kinins are also involved in the development of chloroplasts, fruits, and flowers. In addition, they have been shown to delay senescence (aging), especially in leaves, which is one reason that florists use cytokinins on freshly cut flowers.
- Gibberellins stimulate growth, especially elongation of the stem, and can also end the dormancy period of seeds and buds by encouraging germination. Additionally, gibberellins play a role in root growth and differentiation.
- Ethylene controls the ripening of fruits. It also contributes to the senescence of plants by promoting leaf loss and other changes. Ethylene can bring buds and seeds out of dormancy, initiate flower development, and promote radial (horizontal) growth in roots and stems.
- Inhibitors restrain growth and maintain the period of dormancy in seeds and buds.
Plant Behavior: Tropisms
When people think about plants growing, they generally think of them growing straight up or growing wider. But plants also display other types of growth in response to the stimuli within their environment. These responses to stimuli are called tropisms and are controlled by plant hormones. There are three main tropisms:
- Phototropism is the tendency of a plant to move toward light. Phototropism results from the rapid elongation of cells on the dark side of the plant, which causes the plant to bend in the opposite direction.
- Gravitropism refers to a plant’s tendency to grow toward or against gravity. A plant that displays positive gravitropism will grow downward, toward the earth. A plant that displays negative gravitropism will grow upward, away from the earth. Most plants are negatively gravitropic. Gravitropism is also controlled by auxin. In a horizontal root or stem, auxin is concentrated in the lower half, pulled by gravity. In a positively gravitropic plant, this auxin concentration will inhibit cell growth on that lower side, causing the stem to bend downward. In a negatively gravitropic plant, this auxin concentration will inspire cell growth on that lower side, causing the stem to bend upward.
- Thigmotropism, a reaction to touch, causes parts of the plant to thicken or coil as they touch or are touched by environmental entities. Tree trunks, for instance, grow thicker when exposed to strong winds and vines tend to grow straight until they encounter a substrate to wrap around.
Photoperiodism
Plants have a wide variety of flowering strategies involving what time of year they will flower and, consequently, reproduce. In many plants, flowering is dependent on the duration of day and night; this is called photoperiodism.
All flowering plants have been placed in one of three categories with respect to photoperiodism: short-day plants, long-day plants, and day-neutral plants. Despite their names, however, scientists have discovered that it is the uninterrupted length of night rather than length of day that is the most important factor in determining when and whether plants will bloom. Short-day plants begin to bloom when the hours of darkness in a 24-hour period rise above a critical level, as when days shorten in the autumn. These plants include poinsettias, chrysanthemums, goldenrod, and asters. Long-day plants begin to flower when the duration of night decreases past a critical point, as when days lengthen in the spring and summer. Spinach, lettuce, and most grains are long-day plants. Finally, many plants are day neutral, which means that the onset of flowering is not controlled by photoperiod at all. These plants, which are independent both of night length and day length, include tomatoes, sunflowers, dandelions, rice, and corn.
Plant Reproduction
Plants can reproduce both asexually and sexually. Each type of reproduction has its benefits and disadvantages.
Asexual Reproduction
Through asexual reproduction, many plants can produce genetically identical offshoots (clones) of themselves, which then develop into independent plants. This process is also called vegetative propagation. The many modes of vegetative propagation include the production of specialized structures such as tubers, runners, and bulbs. Grafting is an artificial form of vegetative propagation. The advantages to this kind of asexual reproduction, which can occur either naturally or artificially, stem from the fact that it can occur more rapidly than seed propagation and can allow a genetically superior plant to produce unlimited copies of itself without variation.
TUBERS:
As seen in potatoes, tubers are fleshy underground storage structures composed of enlarged parts of the stem. A tuber functions in asexual propagation as a result of the tiny scale leaves equipped with buds that grow on its surface. Each of these buds can form a new plant that is genetically identical to the parent.
RUNNERS:
Runners are slender, horizontal stems that spread outward from the main plant, such as those found on strawberry plants. Entirely new plants can develop from nodes located at intervals on the runners; each node can give rise to new roots and shoots.
BULBS:
Bulbs such as onions and tulips are roughly spherical underground buds with fleshy leaves extending from their short stems. Each bulb contains several other buds that can give rise to new plants.
GRAFTING:
In grafting, two young plants are joined, first by artificial means and then by tissue regeneration. Typically, a twig or bud is cut from one plant and joined to a rooted plant of a related species or variety. The twig or bud is called the scion, and the plant onto which is it grafted (and that provides the roots) is called the stock. The scion eventually develops into an entire shoot system. Grafting often allows horticulturalists to combine the best features of two different plants into one plant. Sometimes the stock and scion retain independent characteristics, and sometimes the stock alters the characteristics of the scion in some desirable way.
Sexual Reproduction in Plants
All plants undergo a life cycle that takes them through both haploid and diploid generations. The multicellular diploid plant structure is called the sporophyte, which produces spores through meiotic division. The multicellular haploid plant structure is called the gametophyte, which is formed from the spore and gives rise to the haploid gametes. The fluctuation between these diploid and haploid stages that occurs in plants is called the alternation of generations. The way in which the alternation of generations occurs in plants depends on the type of plant. In nonvascular plants, the dominant generation is haploid, so that the gametophyte constitutes what we think of as the main plant. The opposite is true for tracheophytes, in which the diploid generation is dominant and the sporophyte constitutes the main plant. The SAT II Biology only deals with the specifics of the tracheophyte alternation of generations, though nonvascular plants have a similar life cycle.
The dominant phase in the tracheophyte life cycle is the diploid (sporophyte) stage. The gametophytes are very small and cannot exist independent of the parent plant. The reproductive structures of the sporophyte (cones in gymnosperms and flowers in angiosperms) produce male and female haploid spores: microspores (male) and megaspores (female). These spores give rise to similarly sexually differentiated gametophytes, which in turn produce gametes. Fertilization occurs when a male and female gamete join to form a zygote. The resulting embryo, encased in a seed coating, will eventually become a new sporophyte.
REPRODUCTION IN FLOWERING PLANTS
Angiosperms are special because they have developed special reproductive systems, which are none other than the flowers you should always take time to stop and smell. To understand angiosperm reproduction, then, the first thing you have to do is know the structure of the flower.
THE FLOWER
Flowers, the reproductive structures of angiosperms, are adaptations designed to attract insects and other pollen-bearing animals to the plant to aid in pollen dispersal. For this reason, flowers are most often colorful and showy; not surprisingly, plants that rely on wind (instead of insects) for pollen dispersal have flowers that are more likely to be small and drab.
The flower is composed of four organs: the sepal, petal, stamen, and pistil. Sepals and petals are not directly involved in reproduction, while the stamen and pistil are the male and female reproductive organs. The stamen holds pollen, which is the male gamete, and the stigma is the site where a pollen grain must land in order to fertilize the female ovules that are located in the ovary at the base of the pistil. This ovary, an exclusive feature of angiosperms, encloses the ovules and develops into a fruit after fertilization.
ANGIOSPERM FERTILIZATION
The female reproductive organ of angiosperms is the pistil, located in the middle of the flower. As in gymnosperms, the male gametophyte is the pollen grain. In order for fertilization to occur in most flowering plants, insects or other animals must transport the pollen to the pistil. A major distinguishing feature of angiosperms is the process of double fertilization, in which an angiosperm ovule contains an egg cell and a diploid fusion nucleus, which is created through the joining of two polar nuclei within the ovule.
When a pollen grain comes into contact with the stigma, or top of the pistil, it sends a pollen tube down into the ovary at the pistil’s base. As the pollen tube penetrates the ovule, it releases two sperm cells. One fuses with the egg to create a diploid zygote, while the other joins with the fusion nucleus to form a triploid nucleus. This triploid nucleus turns into an endosperm, which nourishes the developing embryo (filling the role of gametophyte tissue in the gymnosperm seed). As in gymnosperms, the ovule becomes a seed, encasing the embryo and endosperm in a seed coat. But unlike gymnosperms, in angiosperms, the ovary containing the ovules develops into a fruit after fertilization. The fruit gives the embryos the double benefit of added protection against desiccation and increased dispersal, since it is eaten by far-ranging animals that then excrete the seeds.
Angiosperms either self-pollinate, in which a particular plant fertilizes itself, or cross-pollinate, in which one plant is fertilized by another of the same species. Cross-pollination generally produces far more vigorous plants and is encouraged through differential development of the male and female gametophytes on a flower or through the positioning of these gametophytes so that self-pollination is difficult.
