Plants - Cells, tissues, and organs

September 10, 2002

1. One of the 'design principles' of multicellular organisms is division of labor between different parts.

• In plants, the shoot system contains the photosynthetic machinery, and needs access to sunlight and CO₂. The predominant site of photosynthesis is the leaves.
• The root system is responsible for acquiring water and minerals, and so must be embedded in soil or other moist material.
• In higher plants, an essential link between the root and shoot systems is the vascular tissue.
  • Xylem is the vascular tissue that transports water and minerals (called xylem sap) upward from root to shoot.
  • Phloem is a second vascular tissue that transports phloem sap rich in organic chemicals (e.g. maple syrup). The direction of phloem sap movement can be changed depending on circumstances - e.g. sugar is synthesized in the leaves, and the phloem may carry sugar to maturing fruit during the summer months but to the roots, for winter storage, during the fall.

2. Plant cells have certain features that are not found in animal cells.

• Some plant cells have a cytoplasmic organelle called the chloroplast. The chloroplast contains chlorophyll, and is the site of photosynthesis.
• Every plant cell contains a large, water-filled vacuole surrounded by an intracellular membrane called the tonoplast. [Plant cells can grow rapidly by simply taking water up into their vacuoles, unlike animal cells that must synthesize cytoplasmic proteins to increase their size.]
• Like animal cells, plant cells secrete an extracellular matrix (ECM) composed of molecules that precipitate in the extracellular fluid. But in plants this ECM forms a thick cell wall that glues adjoining cells together and entraps the living cell (called the protoplast).
  • The primary component of cell walls is a glucose polymer called cellulose [see Campbell, Fig. 5.8]. Cotton fiber is virtually pure cellulose; dry wood is about 50% cellulose.
  • Other compounds beside cellulose give cell walls their unique properties. For instance, lignin is found in the hard, woody cell walls of tree trunks and nut shells.
• Plant cells have a kind of intercellular junction called. Plasmodesmata pass through holes in the cell wall, and have a continuous channel of cytoplasm surrounded by a tube cell membrane. Macromolecules and organelles use these channels to pass between neighboring cells.

3. A tissue is a collection of cells that share certain anatomical and physiological properties in common. Plant cells are organized into three distinct kinds of tissue.

• The outermost tissue is called epidermis or dermis. The epidermis is a nearly continuous layer of cells covering the external surface of the plant. The epidermis serves as a barrier, keeping out pathogens and dissuading foraging herbivores.
• The shoot epidermis is exposed to the atmosphere, and therefore must minimize water loss through evaporation. It does this by secreting a waxy cuticle into the outer cell walls.
• In contrast, roots need to absorb water from the soil through their epidermis. Thus, root epidermis has no cuticle, and many root epidermal cells grow root hairs that greatly increase their surface area for water absorption.

4. Vascular tissue lies at the center of the plant, and is specialized to transport fluids. This transport is non-selective, i.e. chemicals dissolved in the water are carried along with the flowing liquid.

• Xylem is an array of tubular channels composed of empty cell walls.
• An immature xylem cell takes on an elongated morphology, then dies and degenerates leaving behind only its cell wall as a water-filled tube.
• Cell walls slow the rate of water flow. To accelerate the rate of water flow through the xylem, the cell walls have perforations that easily pass water from one 'cell' to the next.
• Phloem is composed of two closely associated cell types called sieve-tube members and companion cells. Unlike xylem cells, both of these phloem cells still have a living protoplast when they reach functional maturity.
  • The sieve-tubes actually transport phloem sap. This sap is an intracellular liquid (in essence, cytoplasm) that passes from one sieve-tube member to the next via plasmodesmata.
  • To facilitate the movement of the phloem sap, the sieve-tube members discard their nuclei and cytoplasmic organelles as they mature.
  • Without a nucleus, the mature sieve-tube member can not synthesize its own RNAs, proteins, or ATP. To compensate, the companion cells are biochemical 'factories' which synthesize macromolecules and transport them to the sieve-tube via plasmodesmata.
• In addition to their role in transport, vascular tissues provide the plant with mechanical support. Part of this support is hydrostatic pressure [what happens to a flower when it wilts?], but many vascular tissues also have hardening factors in their cell walls (e.g. lignin in wood).

5. Ground tissues fill the spaces between epidermis and vascular tissue, and vary in structure and type. Ground tissues can provide metabolic activities or mechanical support.

• In the shoot, ground tissues account for nearly all of the plant's photosynthesis.
• Ground tissues are also used to store nutrients. Plants do not produce fat, and store energy in the form of starch, a glucose polymer distinct from cellulose [see Campbell's Fig. 5.8]. Starch is sometimes stored in tubers (e.g. potatoes) or bulbs (e.g. onions), or in green fruit - where it will be converted back to simple sugars at ripening.

6. Multiple tissues come together to form organs. One of the most distinctive plant organs is the leaf, whose shape and tissue organization are specialized for photosynthesis.

• The outer surface of the leaf is covered by a nearly continuous epidermis. Inside is ground tissue called mesophyll, and veins of vascular tissue that deliver H₂O from the root system.
• Photosynthesis is carried out by the mesophyll. The flattened shape of a leaf insures that all mesophyll cells are close to the upper surface, and therefore well exposed to sunlight.
• During photosynthesis, mesophyll cells absorb CO₂ from the atmosphere and release the waste product O₂. [Plant cells use O₂ for oxidative metabolism. But photosynthesis generates an excess of O₂, which is released back into the atmosphere.]
  • To increase the area of contact between the mesophyll cells and the air, the inside of the leaf is honeycombed by a labyrinth of airspaces [see Campbell, Fig. 35.19].
  • The loose mesophyll surrounding these airspaces is called spongy parenchyma. The solid-packed mesophyll in the upper part of the leaf is called palisade parenchyma.
  • The mesophyll airspaces are connected to the atmosphere by small holes (~ 20 micron dia.) in the epidermis. These holes are called stomata (singular: 'stoma').
• Unlike the epidermis, the mesophyll does not have a cuticle (which would retard the exchange of O₂ for CO₂). Thus, an unavoidable byproduct of photosynthesis is that the leaf can not help but lose some H₂O to the atmosphere by evaporation. Leaves minimizes this loss in two ways:
  • Nearly all stomata are on the underside of the leaf. Some water vapor is lost out of the stomata by diffusion, but the hot water vapor inside the leaf tends to rise and has no openings to get out through the upper side.
  • Each stoma is encircled by a pair guard cells which can open or close the hole. When photosynthesis is not possible (e.g. at night), the guard cells close the stomata tightly to prevent any unnecessary water loss.

Learning Goals

1. Learn the basic organization of plants: root vs. shoot; direction of transport in xylem vs. phloem.

2. Learn the four anatomical features that distinguish plant cells from animal cells.

3. Be able to name the three general types of plant tissue, and describe their general functions and features.

4. Learn the anatomy of a leaf. Where does photosynthesis occur? Where are CO₂ and O₂ exchanged with the atmosphere?