Chapter 21: Plant Structure & Function

1. Chapter 21: Plant Structure & Function

2. Chapter 21: Plant Structure & Function • All organisms must: • Take in certain materials, e.g. O2, food, drink • Eliminate other materials, e.g. CO2, waste products

3. Chapter 21: Plant Structure & Function • Single-celled organisms can take in/release necessary materials by simple diffusion.

4. Chapter 21: Plant Structure & Function • Need for transport systems in larger organisms: • Smaller surface area/volume ratio • Surface of body is not in contact with liquid • Consider human body (vertebrates): • Our cells are in contact with internal liquid environment. • Specialized systems maintain this environment, provide cells with food and oxygen, remove carbon dioxide and eliminate wastes. • Plants, likewise, possess transport systems. • Transport systems: key to maintaining internal balance necessary for life.

5. Surface-to-volume Ratio • Demo: 1 cm Surface Area = Volume = SA/Volume Ratio = Surface Area = Volume = SA/Volume Ratio = Surface Area = Volume = SA/Volume Ratio =

6. Transport Systems in Plants: Adaptations for Life on Land • First land plants probably evolved from green algae 430 MYA. • Life out of water posed new challenges: e.g. loss of moisture to air. • Early adaptations included: • Protective structure for gametes and embryos • Water-proof covering (waxy cuticle)

7. Adaptations for Life on Land Two groups evolved: • Nonvascular plants: Mosses (Bryophytes) and relatives (liverworts and hornworts) • Do not grow very large. • Restricted to damp environments; require water for fertilization. • Waxy cuticle • No vascular tissue. Water moves through plant by diffusion, capillary action, and cytoplasmic streaming. • No woody tissues for support.

8. Nonvascular Plants

9. Adaptations for Life on Land • Vascular plants: • Evolved specialized vascular tissue (cells joined into tubes) for conducting water and nutrients throughout the body of the plant.

10. Vascular Plants

11. Vascular Tissue

12. Adaptations for Life on Land • Other challenges to life on land for plants (See Fig. 7.1, p. 186): • Light and CO2 must be obtained above ground. • Water and nutrients from soil. • Evolved underground root system for absorbing minerals and water. See Fig. 7.2, p. 187. • Water-absorbing sections of roots generally not covered with cuticle. • Root hairs: fine, long extensions from root cells to maximize absorption surface of roots. • Water acts as transport fluid, carrying nutrients from roots to leaves.

13. Adaptations for Life on Land

14. Adaptations for Life on Land • Also evolved aerial system of stems and leaves for food production. • Lignin: polymer embedded within cellulose matrix that provides rigidity (support) to trees and other vascular plants.

15. Adaptations for Life on Land

16. Adaptations for Life on Land

17. 21.1: Plant Cell Types Parenchyma Cells: • Most common cell type; least specialized • Store starch, oils, water • Thin walls; large, water-filled vacuoles • Photosynthesis • Make up flesh of many fruits • Divide throughout entire life  heal wounds, regenerate plant parts, e.g. cuttings  new plant

18. 21.1: Plant Cell Types Collenchyma Cells: • Provide support • Walls either thick or thin • Found in young tissues of leaves & shoots • Form strands, e.g. celery strings • Flexible, stretchy, can change size/elongate and still provide structure

19. 21.1: Plant Cell Types Sclerenchyma Cells: • Strongest cell type • Possess second cell wall hardened with lignin tough, durable, rigid, do not allow for growth of cells • Found in parts of plants no longer growing, e.g. fruit pits, outer shells of nuts • Within vascular system, sclerenchyma cells die at maturity, cytoplasm disintegrates, leaving rigid cell walls for support • Fibers of sclerenchyma cells are used by humans for linen, rope, etc.

20. 21.1: Three plant tissue systems From outermost to innermost: • Dermal tissue system • Ground tissue system • Vascular tissue system

21. 21.1: Plant Tissue Systems Dermal Tissue System: • Referred to as epidermis • Covers outside of plant  protection • May secrete waxy substance  cuticle • Nonwoody parts of plants, e.g. leaves, stems = composed of live parenchyma cells • Outer bark of woody trees = dermal tissue composed of dead parenchyma cells

22. 21.1: Plant Tissue Systems Ground Tissue System: • Surrounded by dermal tissue • Consists of all three cell types; parenchyma tissue is most common • Makes up much of plant interior • Provides support • In roots and stems: stores materials • In leaves: contains many chloroplasts for photosynthesis • In cacti: parenchyma cells store water; spines of cactus (modified leaves) = rigid sclerenchyma cells

23. 21.1: Plant Tissue Systems Vascular Tissue System: • Surrounded by ground tissue • Main function: transport of water, nutrients, and organic compounds • System of hollow tubes • Two types of vascular tissue: • Xylem: transport of water & nutrients • Phloem: transport of photosynthetic products

24. Adaptations for Life on Land • Pith is a light substance that is found in vascular plants. • Consists of soft, spongy parenchyma cells, and is located in the center of the stem. • Encircled by a ring of xylem (woody tissue), and outside that, a ring of phloem (bark tissue). Pith

25. Transport Systems in Plants

26. Water Transport

27. Water Transport Corn stem

31. Water Transport Xylem:hollow tube-shaped cells that carry water and minerals up from the roots (Fig. 7.4a, p. 189 ). • Consists of two types of water-conducting cells plus strong weight-bearing fibers: • Tracheids: cells with pointed ends and thick walls with pits that connect them to neighboring cells. Water moves through the pits. • Vessel elements: wider, shorter, thinner-walled and less-tapered than tracheids; ends are perforated or missing altogether. Water flows freely through openings.

32. Xylem

33. Xylem

34. Water Transport • Evaporation in plants is great, e.g. a typical red maple may lose 2000 L of water on a humid day. • In trees and tall plants, water must be transported up great distances. • Plants don’t have pumping systems for transporting water from roots to aerial system of shoots and leaves.

35. Xylem

36. Water Transport • Cohesion-tension hypothesis: explanation for water transport in plants; based on molecular properties of water and transpiration. • Roots exert pressure, but insufficient to account for rise of water in taller plants and trees. • Cohesion: tendency of water molecules to stick together because of weak hydrogen bonds. • Adhesion: water molecules are polar (slightly charged); thus, they form weak bonds with other charged molecules, e.g. glass or the walls of xylem tissue cells.

37. Water Transport Capillary action: the process by which water rises in a glass tube; brought about by cohesion and adhesion. • Water adheres to charged walls of glass tube; cohesion causes other water molecules to follow. • Walls of tracheids and vessel elements also have many charged groups which take water up by capillary action. • Process is not very rapid; height to which water can rise is limited by: • Diameter of tube • Gravity

38. Water Transport • Water leaving plant by transpiration tugs on water below it. This tugging is transmitted from one water molecule to another  a long chain of water molecules continually pulled through xylem from root to leaf. • Water molecules in the xylem replace water that leaves the mesophyll cells via the stomates. • Less polar liquids would not be able to do this, as they are less cohesive.

39. Nutrient Transport Fig. 7.4b, p. 189 • Phloem: system of elongated cells arranged into tubes filled with streaming cytoplasm; movement of organic materials is accomplished here by active transport. • Consists of sieve tube members, companion cells and fibers. • Sieve Tubes: elongated cells with perforated ends (sieve plates), resembling strainers (thus, the name “sieve”) through which contents of cells mix. • Sugars and amino acids move through phloem cells from leaves to other parts of the plant. • Rate of movement is greater than could be accounted for by diffusion.

40. Phloem (Plasmodesmata)

41. Phloem

42. Nutrient Transport

43. Nutrient Transport Pressure-flow hypothesis: water and dissolved sugars move from areas of high pressure (sources) to areas of low pressure (sinks). • Sources: areas where sugars are produced or stored; generally, areas where energy is provided; cotyledons and endosperm during germination; leaves during spring and summer; some storage roots during spring • Sinks: (next slide)

44. Nutrient Transport Pressure-flow hypothesis: water and dissolved sugars move from areas of high pressure (sources) to areas of low pressure (sinks). • Sinks: areas where water and sugars are used or food storage areas; where water and sugar must be constantly replenished, e.g. growing leaf buds, root tips, flowers, fruits, seeds.

45. Pressure-flow hypothesis • Sucrose is produced in a leaf by photosynthesis. Then actively transported into sieve tubes from mesophyll cells. • Companion cells produce a protein key in the transport process. • High [sucrose] draws water into the phloem cells, producing higher pressure. • High pressure pushes sucrose toward areas of lower pressure, moving sucrose through sieve tubes, cell to cell, from source to sink.

46. Pressure-flow hypothesis • At sink, active transport removes sucrose from phloem for use or storage. • As this occurs, water leaves phloem cells by osmosis, mostly returning to xylem. • Entire process depends upon uptake of water and sucrose by phloem cells at source areas and active removal of same materials from phloem cells by sink tissues.

47. Nutrient Transport in Phloem

48. Pressure Flow Hypothesis Active transport Osmosis