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Plant structure and function (parts of chapters 35, 36 and 37). Unlike animals, plants remain in one place and produce food through photosynthesis.
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Plant structure and function(parts of chapters 35, 36 and 37) • Unlike animals, plants remain in one place and produce food through photosynthesis. • In the process of photosynthesis plants (and other photosynthetic organisms such as algae, other protists, and cyanobacteria) trap the energy in sunlight and store it in chemical bonds. • The energy stored in chemical bonds can then be used to fuel metabolic processes.
Plants and photosynthesis • This process is called photosynthesis. • In this class we will not discuss the process of photosynthesis in detail. It is covered in depth in Bio 101.
Photosynthesis • In photosynthesis carbon dioxide (CO2) and water (H20) and the energy provided by light are used to make glucose. • 6 CO2 + 12 H20 + energy C6H12O6 + 6O2 + 6 H20
Chloroplasts • The organelle plants use to carry out photosynthesis is the chloroplast. • In plants chloroplasts are concentrated in the leaves, which generally are thin and flat to allow maximum exposure to light.
Basic structure of plants • To carry out photosynthesis plants must obtain water and minerals from the soil, CO2 from the air, and light from the sun. • The structure of plants reflects their need to carry out these tasks.
Basic structure of plants • Plants have three basic organs: • Roots • Stems • Leaves • These organs are organized into two systems: the largely below-ground root system and the above-ground shoot system (stems and leaves).
Roots • Roots perform several tasks. They • Anchor the plant in place • Absorb minerals and water • Store organic nutrients such as sugars (e.g. carrot, sugar beet, turnip).
Roots • Roots systems may have a central taproot with lateral roots branching off from it (e.g. dandelion). • Alternatively, a root system may have no obvious main root, but instead be a fibrous system with many small roots growing from the stem, each of which has its own lateral roots (e.g. grasses).
Roots • The entire root system anchors a plant in place, but absorption of water and minerals occurs mainly at the root tips. • At the root tips huge numbers of root hairs increase the surface area enormously.
Root hairs • Root hairs are extensions of individual epidermal root cells and are not multicellular structures (as lateral roots are).
Roots • Root hairs are permeable to water and adhere closely to soil particles allowing efficient absorption of water and nutrients. • Most plants forms mutually beneficial relationships with fungi, which facilitate absorption of water and minerals.
Mycorrhizae • The plants and fungi form mycorrhizae: symbiotic associations of plant roots united with fungal hyphae (hyphae are tiny filaments that form the bulk of a fungus). • Most plants form these symbiotic mycorrhizal relationships and they greatly enhance the plants growth. [a symbiotic relationship is a close, mutually beneficial relationship]
Mycorrhizae • The fungal hyphae grow over the root and penetrate into it and may in some cases form a mantle or layer over the root. • The fungus benefits from a steady supply of sugar donated by the host plant.
Mycorrhizae Plant receives numerous benefits: • Fungus greatly increases surface area for absorption (can be as much as 3 meters of hyphae per cm of plant root length). • Fungus selectively absorbs phosphate and other nutrients and supplies them to plant. • Fungus may secrete growth factors that promote root growth. • Fungus may produce antibiotics that protect the plant from pathogenic bacteria and fungi in the soil.
Mycorrhizae • Plant-fungus symbiosis may have been one of the early adaptations that allowed plants to colonize the land, which probably initially was quite nutrient poor. • Fossils of some of the earliest plants show mycorrhizae.
Shoot Systems • Shoot systems consist of stems and leaves. • Stems are elongated structures comprised of nodes and internodes. • Nodes are where leaves are attached and internodes are the sections in between.
Shoot Systems • Stems have a terminal bud at the tip and this is where elongation takes place, enabling the stem to reach upwards towards the light. • If the tip of the stem is eaten or shaded, however, axillary buds (buds on the side) will begin to grow.
Shoot Systems • Gardeners shape plants by pruning them. • By removing terminal buds a bushier plant can be produced or by removing lateral flower buds a single large flower can be produced.
Shoot Systems • Stems have been greatly modified in many plants to perform a variety of functions. • Rhizomes, bulbs, tubers, and stolons are all modified stems although they are often mistaken for roots.
Modified stems • Bulbs: vertical shoots that grow underground. The “flesh” of a bulb (e.g. an onion) consists of leaves modified for food storage. • Stolons and rhizomes: are stems that grow on (stolons) or just under (rhizomes) the soil surface. New plantlets form periodically along the length of these stems (asexual reproduction).
Modified stems • Tubers: are enlarged ends of rhizomes specialized for storing food (e.g. potato). • The “eyes” of a tuber are axillary buds.
Leaves • Leaves are the main photosynthetic organ of plants, although green stems also perform photosynthesis. • Leaves vary in form, but usually have a flat blade and a stalk (petiole) that joins the leaf to the stem.
Leaves • Leaves are generally flat to maximize the area exposed to the sun and minimize the distance gases must be transported to and from photosynthesizing cells. • However, in many cases leaves have been substantially modified by natural selection to perform other functions.
Modified leaves • Tendrils of climbing plants such as clematis are often modified leaves. • Spines of cacti are modified leaves (most photosynthesis being carried out by the fleshy stem. • Some leaves are modifed as storageleaves to store water. • Some leaves called bracts look like petals (e.g. in dogwoods) being brightly colored and enlarged to attract pollinators to the flowers they surround. • Some leaves produce plantlets that drop off the plant and take root in the soil.
Plant vascular system • Plants contain two vascular systems that transport water, minerals, and sugars around the plant. • Xylem transports water and dissolved minerals from the roots into the shoots. • Phloem transports sugars from the leaves to where they are needed in the plant.
Plant vascular system • Xylem cells are dead at functional maturity and form thin elongated tubes that water moves through. • Phloem cells are alive.
Plant secondary growth • Primary growth is stem elongation, secondary growth refers to the thickening of woody plants over time. • Xylem and phloem cells are both produced by a plant tissue called vascular cambium that is located under the bark. • This cambium produces xylem cells on the inside and phloem on its outside.
Plant secondary growth • As the plant grows older inner xylem tissue forms the heartwood of the tree. This tissue no longer transports liquid. • Xylem cells have thick lignified walls (lignin is a complex cross-linked polymer) that provide structural support for the plant. • The outer (more recently produced) xylem is called sapwood and this carries liquid.
Plant secondary growth • On the outside of the cambium layer phloem is produced. Phloem is produced more slowly than xylem and older phloem is sloughed off the tree so it does not accumulate as xylem does.
Plant secondary growth • Because the outer layer of phloem is essential to transportation, a tree that is “ringed” by grazers (i.e., has its outer bark removed around the circumference of the plant) will die. • In contrast, a tree may be hollowed out and still survive.
