Chapter 21: Plant Structure & Function

Chapter 21: Plant Structure & Function

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

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

Chapter 21: Plant Structure & Function Need for transport systems in larger organisms: 1. Smaller surface area/volume ratio 2. 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.

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

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)

Adaptations for Life on Land Two groups evolved: Nonvascular plants: : Mosses (Bryophytes( 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.

Nonvascular Plants

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.

Vascular Plants

Vascular Tissue

Adaptations for Life on Land Other challenges to life on land for plants ( (See Fig. 7.1, p. 186): Light and CO 2 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 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.

Adaptations for Life on Land

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.

Adaptations for Life on Land

Adaptations for Life on Land

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

Transport Systems in Plants

Water Transport

Water Transport Corn stem

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.

Xylem

Xylem

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 t have pumping systems for transporting water from roots to aerial system of shoots and leaves.

Xylem

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.

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

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 replaces water that leaves the mesophyll cells via the stomates. Less polar liquids would not be able to do this, as they are less cohesive.

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.

Phloem (Plasmodesmata)

Phloem

Nutrient Transport

Nutrient Transport Pressure-flow hypothesis: : water and dissolved sugars move from areas of high pressure (sources( sources) ) to areas of low pressure (sinks( 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 Cotyledon: : single (monocots( monocots) ) or double (dicots( dicots) ) seed leaf of a flowering plant embryo (angiosperms( angiosperms). Endosperm: : nutrient rich structure formed by the union of a sperm cell and a large cell having two nuclei; double fertilization ion is complete when a second sperm cell fertilizes the egg; the endosperm provides nourishment to the developing embryo in seeds of angiosperms.

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 Growing leaf buds Root tips Flowers Fruits Seeds

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.

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.

Nutrient Transport in Phloem

Pressure Flow Hypothesis