Plants and Animals for the DAT
/Learn key DAT concepts related to plant and animal biology, including seed germination, primary and secondary growth, nitrogen fixation and animal characteristics, plus practice questions and answers
Everything you need to know about plants and animals for the DAT
Table of Contents
Part 1: Introduction to plants and animals
Part 2: Plants
a) The seed and germination
b) Cell division and alternation of generations
c) Development and primary vs secondary growth
d) Plant tissues
d) Structures of the root, stem, and leaf
f) Transport of water and food
g) Stomata Regulation
h) Major Divisions
i) Flower Structures
j) Plant hormones
k) Nitrogen fixation
Part 3: Animals
a) Characteristics
b) Phyla
Part 4: High-yield terms
Part 5: Questions and answers
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Part 1: Introduction to plants and animals
Studying plant and animal biology is important in understanding life sciences on the DAT. By grasping these basics, you will learn how living things grow and survive. This guide offers detailed insight into plants and animals, covering their structures, how they work, and how they’re connected. Pay attention to high-yield bolded terms, and test your understanding with practice questions and answers at the end.
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Part 2: Plants
a) The seed and germination
Seeds, fundamental to plant propagation, contain components essential for germination and growth. These components include the seed coat, serving as a sturdy outer layer that shields and safeguards the seed, and the endosperm, which acts as a nutrient storehouse and provides essential nutrients to the embryo for germination. The embryo itself is comprised of distinct parts crucial for plant development. These include the radicle, from which the initial root emerges to anchor the plant in the soil, the hypocotyl, constituting the lower part of the young shoot, the plumule, which develops into foliage as embryonic leaves, and the epicotyl, forming the shoot tip in the upper region. The germination process itself marks the reactivation of a dormant seed into a sprouting seedling, and is triggered by specific environmental cues. Water absorption is a key initiator, prompting the breaking of the seed coat and setting off growth.
FIGURE 1: SEED ANATOMY
During the process of seed germination, several key stages unfold to initiate growth and development. First, imbibition occurs as the seed absorbs water, causing swelling and ultimately leading to the rupture of the seed coat. This kick-starts the growth process. Following imbibition, root emergence takes place as the radicle emerges from the seed, extending to form roots that establish the plant's foundation in the soil. Concurrently, shoot elongation begins as the hypocotyl and culminates in the formation of the shoot. As growth progresses, active growth takes place at the root and shoot tips, where apical meristems are located. This process is known as primary growth, and is responsible for primary tissue formation as well as elongation.
b) Cell division and alternation of generations
The life cycle of plants, known as alternation of generations, involves distinct phases of meiosis, spore production, and gamete fusion, perpetuating the cycle of reproduction and growth. It begins with meiosis and spore production, where sporangia undergo meiosis to generate haploid spores (n), the initial stage of the reproductive process. These spores then undergo spore maturation, transitioning into multicellular gametophytes through mitosis, maintaining the haploid state (n). Following this, gamete fusion occurs, as haploid gametes fuse to form a diploid zygote (2n). The diploid zygote then matures into a sporophyte through mitosis, marking the transition to the next phase of the cycle. The sporophyte's cycle then ensues, as cells within sporophytic sporangia undergo meiosis once more, generating haploid spores and restarting the life cycle anew. This intricate process ensures the continuity of plant species and the perpetuation of genetic diversity within plant populations.
FIGURE 2: ALTERNATION OF GENERATIONS
c) Development and primary vs secondary growth
Plant growth is orchestrated through two distinct mechanisms, primary and secondary growth.
Primary growth is responsible for the initial vertical expansion of plants. It originates from mitotic activities occurring at apical meristems situated at both root and shoot tips. These meristems play a crucial role in driving growth by continuously dividing. As roots extend into the soil, their growth is safeguarded by the presence of a root cap, which has three pivotal zones.
Firstly, the Zone of Division houses actively dividing cells that originate from the apical meristem. Secondly, the Zone of Elongation, positioned above the apical meristem, facilitates water absorption and elongation of cells. This zone contributes to root lengthening. Finally, in the Zone of Maturation, cells undergo specialization and develop into distinct plant tissues essential for the plant's overall structure and function.
FIGURE 3: PRIMARY GROWTH ZONES IN PLANT ROOTS
Secondary growth, characterized by lateral expansion, is a phenomenon unique to woody plants. It is orchestrated by lateral meristems, namely the vascular cambium and cork cambium. The vascular cambium is positioned between the primary xylem and phloem, and it plays a central role in this process. It produces secondary xylem inward, contributing to the formation of wood, while simultaneously generating secondary phloem outward. This activity results in the annual growth rings observed in tree trunks. Meanwhile, the cork cambium—situated outside the phloem—is responsible for generating cork, which forms the protective outer layer known as the periderm in woody plants. Together, these lateral meristems enable woody plants to undergo significant expansion and structural reinforcement as they mature.
d) Plant tissues
Plants have distinct tissue types and transport mechanisms promoting structural support, nutrient movement, and protection.
FIGURE 4: PLANT TISSUES
Ground tissue constitutes a significant portion of the plant's mass and provides structural reinforcement. There are three types of ground tissue:
Parenchyma: Forms filler tissue and constitutes the plant's bulk, characterized by thin cell walls.
Collenchyma: Offers additional support, especially in actively growing areas, displaying irregular cell walls.
Sclerenchyma: Provides primary structural support due to thick cell walls.
Vascular tissue facilitates the transport of materials through the plant from sources (where materials are produced) to sinks (where materials are utilized). The two types of vascular tissue in plants are phloem and xylem.
Phloem: Transports sugars from sources (like leaves) to sinks (roots and other plant parts). Consists of sieve cells forming fluid-conducting columns, accompanied by companion cells for metabolic support.
Xylem: Transports water from roots to leaves, also serving as a structural framework. Comprises tracheids and vessel elements, facilitating water movement through pits or perforations in cell walls.
Dermal tissue makes up the plant's outer layer and offers protection. It also helps regulate various functions.
Epidermis: Covered by a cuticle—a waxy layer that reduces water loss.
Root hairs: Extensions that enhance root surface area, augmenting nutrient and water absorption.
Roots take up water via the symplastic (within the cell cytoplasm) or apoplastic (outside the cell through cell walls) pathways. The Casparian strip, composed of fat and wax, forms an impermeable barrier in root cell walls, directing water into the cell cytoplasm for filtration before dispersion throughout the plant. Understanding these diverse tissue types and transport systems elucidates the intricate workings of plant growth, resource distribution, and defense mechanisms.
e) Structures of the root, stem, and leaf
Now that we’ve covered the primary tissues making up plants, let's dive deeper. The epidermis, which forms the outermost layer, promotes water absorption largely starting with root hairs in the zone of maturation. As the zone matures, older root hairs wither away while new ones emerge from the zone of elongation. Additionally, the aged epidermis serves as a protective shield for the root. Moving inward, the cortex constitutes the bulk of the root, acting as a reservoir for starch storage and hosting intercellular spaces vital for cellular respiration. The cortex also facilitates aeration and metabolic activities.
Positioned at the innermost boundary of the cortex is the endodermis, characterized by a ring of tightly packed cells and the presence of the Casparian strip—a hydrophobic barrier composed of suberin. This strip regulates the movement of water into the root's core, compelling it to pass through endodermal cells and preventing backflow to the cortex. Finally, the vascular cylinder, or stele, encompasses the tissues within the endodermis and includes essential elements such as phloem, xylem, and the pericycle, from which lateral roots emerge. Monocot roots are characterized by scattered vascular bundles. Conversely, vascular bundles in dicot roots are arranged in a circle.
FIGURE 5: CROSS-SECTIONS OF MONOCOT AND DICOT ROOTS
Exploring the primary structure of stems reveals the architectural marvels that sustain a plant's functionality and growth. Beginning with the epidermis, the outermost layer fortified by a waxy, fatty substance called cutin, it forms a protective shield known as the cuticle. This armor shields the stem against various environmental stresses, fortifying it against dehydration and external threats. Moving inward, the cortex, nestled between the epidermis and the vascular cylinder, hosts a diversity of ground tissue types. Many of these cells, adorned with chloroplasts, play a vital role in photosynthesis, harnessing the sun's energy for the plant's sustenance.
The vascular cylinder, comprising xylem, phloem, and pith, serves as the central hub for nutrient transport and structural support. In dicots and conifers, xylem and phloem bundles encircle a central pith area. Between these bundles, a single layer of undifferentiated cells persists, poised to evolve into the vascular cambium—a zone responsible for stem growth and secondary tissue formation.
The structure of leaves is designed to facilitate gas exchange and photosynthesis. The structure also regulates transpiration, which is the movement and evaporation of water within a plant.
The epidermis is the outer, protective layer of the leaf. It is coated with a waxy cuticle that minimizes water loss by evaporation. Trichomes, such as hair, scales, or glands, may protrude from this layer. Moving inward, the palisade mesophyll, located closer to the upper epidermis, consists of densely packed parenchyma cells adorned with chloroplasts. It boasts a significant surface area and specializes in photosynthesis. Below the palisade mesophyll sits the spongy mesophyll, where loosely arranged parenchyma cells create ample intercellular spaces. These spaces facilitate the transfer of CO2 to photosynthesizing cells and O2 to respiring cells.
FIGURE 6: LEAF CELLULAR STRUCTURE
Guard cells are specialized epidermal cells that orchestrate the opening and closing of stomata (more on stomata later). The opening and closing of stomata regulates gas exchange between the leaf and the environment. Vascular bundles, containing xylem and phloem, facilitate water supply for photosynthesis. They also transport sugars and other photosynthetic bi-products to various plant parts. Bundle sheath cells encapsulate the vascular bundle, ensuring an anaerobic environment for CO2 fixation in C4 plants and guarding against air bubble interference in water movement. For a review of photosynthesis, see our guide on metabolism.
f) Transport of water and food
Water transportation in plants encompasses several mechanisms that facilitate the movement of water from the roots to the leaves. Initially, water enters the roots through root hairs via osmosis, following two primary pathways: the apoplast and the symplast. In the apoplast, water traverses through cell walls and spaces between cells, while in the symplast, it moves through the cytoplasm of living cells via plasmodesmata.
Once within the endodermis, water enters the stele (aka vascular cylinder) through the symplast and primarily travels via the apoplast through the xylem. The xylem serves as the primary pathway for water conduction and is made of tracheids and vessels. Osmosis plays a significant role in this process, as water moves from the soil into the root and eventually into the xylem due to a gradient between the root and the soil. This movement creates root pressure occasionally seen as guttation—water being expelled—on leaf tips. Additionally, capillary action aids water ascent up the xylem vessels, where liquids rise in narrow tubes due to adhesive forces between water and xylem. As a result, a continuous water column is formed.
Ultimately, the cohesion-tension theory drives water movement, where transpiration, the evaporation of water from leaves, creates tension within the xylem, resulting in negative pressure. This negative pressure, combined with the cohesive nature of water molecules, facilitates bulk flow from roots to leaves.
Water transportation in plants involves several mechanisms facilitating the movement of water from the roots to the leaves.
FIGURE 7: WATER TRANSPORTATION IN PLANTS
The transportation of food in plants primarily occurs through the phloem in a process known as translocation. This hypothesis proposes that the movement of sugars and other organic solutes within the phloem is driven by a pressure gradient established by the active loading of sugars into the phloem at source tissues and their subsequent unloading at sink tissues. Initially, soluble carbohydrates are actively transported into phloem sieve-tube members, creating a higher solute concentration at the source than at the sink (such as the root). To balance the lower water concentration resulting from sugar loading, water diffuses into the source by osmosis. Pressure builds up in the sieve-tube members due to the influx of water, leading to a bulk flow movement of water and sugars through the sieve tubes towards the sink.
FIGURE 8: MOVEMENT OF SUGAR AND WATER IN SIEVES
Rigid cell walls prevent expansion and facilitate the movement of sugars and water. Sugars are moved by bulk flow through the sieve plates between the sieve-tube members. At the sink, where sugars are utilized by nearby cells, pressure decreases in the sieve-tube members. Sugar removal via active transport at the sink increases water concentration, causing water to diffuse out of the cells, relieving pressure. Plants store energy as insoluble starch, allowing any cell to function as either a source or sink for transported sugars and water. For instance, plant roots break down starch at night when photosynthesis is minimal, acting as a source of sugars. Additionally, root hairs facilitate mineral exchange with the soil.
g) Stomata regulation
Stomata are the tiny pores found on plant leaves. They play a pivotal role in regulating gas exchange, transpiration, and the photosynthesis process. Their opening and closure are finely orchestrated to balance the need for CO2 uptake and the risk of excessive water loss.
Guard cells, specialized cells that surround the stomatal pores, control their opening and closing. When water diffuses into the guard cells, they swell, creating an opening called a stoma. Conversely, when water exits the guard cells, they become less congested, causing the stoma to close.
Several factors influence this opening and closure. High temperatures often trigger stomatal closure to prevent excessive water loss, while low concentrations of CO2 within leaves prompt stomatal opening, enabling CO2 uptake necessary for photosynthesis.
Additionally, the day-night cycle influences stomatal behavior, with stomata typically closing at night and opening during the day, possibly in response to variations in CO2 levels caused by respiration and photosynthesis.
Stomatal opening involves ion movement, or the diffusion of potassium ions (K+) into guard cells, establishing an osmotic gradient that draws in more water. As potassium ions enter guard cells, creating an imbalance in charge, chloride ions (Cl-) may enter or hydrogen ions (H+) may be pumped out, contributing to the regulation of osmotic pressure. Guard cells possess a blue light receptor on their plasma membrane, making them sensitive to light. Exposure to blue light triggers water intake, leading to stomatal opening, a mechanism beneficial for efficient photosynthesis.
FIGURE 9: GUARD CELLS OPEN ON THE LEFT AND CLOSE ON THE RIGHT
h) Major divisions of plants
a. Bryophytes
Bryophytes encompass mosses, liverworts, and hornworts. They represent nonvascular plants that lack specialized conducting tissues like roots, stems, or leaves.
Gametes are produced within protective structures called gametangia, with the gametophyte serving as the dominant haploid stage that houses these reproductive structures. Antheridia (male gametangia) generate flagellated sperm, while archegonia (female) produce eggs. Upon fertilization, the zygote develops into a diploid structure while still attached to the gametophyte. Bryophytes utilize rhizoids, root-like absorptive structures, for water absorption and minimal anchorage. Their life cycle is predominantly spent in the gametophyte stage, with a comparatively reduced sporophyte stage that relies on and remains attached to the gametophyte. By staying close to moist habitats and utilizing specialized structures like rhizoids, bryophytes manage to thrive despite their small stature and lack of complex vascular systems.
b. Tracheophytes
Tracheophytes, distinguished by their vascular systems enabling vertical growth and robust stature, exhibit prominent root systems for stability. The majority of their life cycle is spent in the sporophyte stage. Tracheophytes can be seedless or seed-bearing.
Seedless tracheophytes:
Lycophytes and Pterophytes: Examples include club mosses, quillworts, ferns, and horsetails. Predominantly heterosporous, these plants typically produce flagellated sperm.
Seed-bearing tracheophytes:
Gymnosperms: Gymnosperms bear seeds without protective coverings. Examples include conifers like firs, spruces, pines, and redwoods. Their non-flagellated sperm is dispersed within seeds by the wind.
Angiosperms: The most abundant plants, angiosperms produce flowers and fruits with protected seeds in ovaries. Their non-flagellated sperm is often disseminated as pollen by wind or animals. Notably, they exhibit double fertilization, where two male sperm fertilize the female gamete.
i) Flower structures and fertilization
Flowering plants consist of a wide array of species like fruits, maples, oaks, and grasses. They represent the dominant form of land plants. The reproductive cycle of these plants centers around the flower, which serves as the key reproductive structure of angiosperms.
FIGURE 10: ANATOMY OF FLOWERS
There are three essential components of flowers that you need to know.
Pistil: The female reproductive structure comprising the ovary (containing eggs), style, and stigma.
Stamen: The male reproductive structure consisting of the anther (producing pollen) and filament.
Petals: Often working alongside sepals, these attract pollinators. Sepals enclose and safeguard flower buds.
Evolutionary advancements in angiosperms:
Attraction of pollinators (including insects and birds).
Protection of the ovule within the ovary, developing into a fruit post-fertilization, aiding in seed dispersal by wind or animals.
Process of fertilization:
The process of fertilization in flowering plants involves several sequential steps: Pollen Deposition begins when a pollen grain lands on the stigma, triggering the growth of a pollen tube containing two sperm cells down the style towards an ovule. Ovule Development occurs within the ovary, where the megaspore mother cell undergoes divisions to form an embryo sac housing various cells like the egg, synergids, antipodal cells, and polar nuclei.
Double fertilization follows, with one sperm cell fertilizing the egg to form a diploid zygote, while the other fuses with polar nuclei to yield a triploid nucleus, resulting in the formation of endosperm, a crucial tissue for seed development. This unique process, known as double fertilization, is a hallmark of angiosperms. It involves the fertilization of both the egg and polar nuclei by separate sperm cells. Overall, fertilization in flowering plants showcases intricate interactions between male and female structures. This leads to the development of seeds enclosed within fruits, providing an evolutionary advantage for gene dispersal.
j) Plant hormones
Plant hormones, also known as phytohormones, play a pivotal role in regulating various growth and developmental processes within plants. Understanding these hormones is fundamental in comprehending the mechanisms behind plant responses to stimuli and environmental cues.
Auxins: These hormones primarily influence cell growth by elongating cells. They achieve this by increasing hydrogen ions in primary cell walls and activating enzymes that loosen cellulose fibers. These changes thereby enhance cell wall plasticity. Auxins are produced at the tips of shoots and roots (apical meristem). They mediate plant responses to light (phototropism) and gravity (geotropism). Additionally, auxins inhibit lateral buds when produced at the terminal bud of a growing tip and exhibit polar transport from shoot to root.
Gibberellins: A group of hormones promoting cell growth, particularly flower and stem elongation. Synthesized in young leaves, roots, and seeds, they are then transported throughout the plant. Gibberellins can collaborate with auxins to stimulate growth and inhibit aging in leaves. They also play crucial roles in promoting fruit development, seed germination, and causing bolting (rapid stem elongation).
Cytokinins: These hormones stimulate cytokinesis (cell division) and influence organogenesis. They promote lateral bud growth, reducing apical dominance (dominance growth of the apical meristem) and delaying leaf senescence (aging). Their effects depend on the target organ and the presence or concentration of auxins.
Ethylene: Ethylene is a gas that accelerates fruit ripening, flower production, and leaf abscission (aging and dropping of leaves). It collaborates with auxins to inhibit the elongation of roots, stems, and leaves. Ethylene stimulates ripening by enzymatically breaking down cell walls and is involved in the growth response to circumvent obstacles.
Abscisic Acid: This growth inhibitor plays a significant role in delaying growth and forming scales in buds. It maintains seed dormancy, which can be broken by an increase in gibberellins or environmental cues like temperature and light.
k) Nitrogen fixation
Plants engage in a symbiotic relationship with specialized nitrogen-fixing bacteria, fostering a crucial partnership for their growth and development. This alliance allows the conversion of atmospheric nitrogen into a usable form that plants can readily absorb and utilize for their essential functions.
The nitrogen cycle consists of five main parts, and you should be familiar with each of them.
Nitrogen fixation by bacteria: Within the root nodules of certain plants like legumes, nitrogen-fixing bacteria perform a remarkable task. They convert atmospheric nitrogen (N2) into more accessible forms, such as ammonia (NH3) and ammonium (NH4+). This transformation of inert nitrogen into usable compounds is the first step in making nitrogen available to plants.
Nitrification process: Following nitrogen fixation, other bacteria step in. Nitrifying bacteria play a role in converting ammonia and ammonium into nitrites (NO2-) and then further oxidize these compounds into nitrates (NO3-). These nitrates, in the form of nitrate ions, become a crucial source of nitrogen for plants.
Plant assimilation: Plants actively absorb these nitrates through their roots. Once inside the plant, nitrogen undergoes assimilation, where it becomes incorporated into essential compounds like amino acids and chlorophyll. These compounds are vital for plant growth and development.
Role of detritus: The soil's nitrogen content is also enriched through the detritus of decomposed organic matter, such as dead plants and animals. As these organisms break down, they release nitrates into the soil, replenishing the available nitrogen for plants.
Denitrification process: Completing the nitrogen cycle, denitrifying bacteria facilitate the conversion of nitrates back into atmospheric nitrogen. This process effectively returns nitrogen to the atmosphere, ensuring the cyclic nature of nitrogen utilization.
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