Chapter C3: Multicellular Organisms – Plants

Multicellular OrganismsMulticellular organisms have specialized cells of many different types that allow them to grow to a larger size than single-celled organisms.

Redwood Tree

3000 year old cedar tree on Vancouver Island

• These specialized cells form tissues and organs that carry out specific functions.

• Many multicellular organisms have internal transport systems for efficient exchange of materials.

Levels of Organization in Multicellular Organisms

Organism (e.g. human)

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Organ System (e.g. circulatory system)

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Organ (e.g. heart)

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Tissue (e.g. cardiac tissue)

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Cell (e.g. cardiac cell)

Plant StructurePlants have two main organ systems:

1. Shoot system – everything that is above the ground. Includes the stem, leaves, buds, flowers, fruit, and tubers.

2. Root system – everything that is below the ground except tubers. Includes roots as well as aerial roots above ground.

Plants have three main tissue types:

1. Dermal tissue, or the epidermis, is the outer layer of cells that covers all herbaceous (non-woody) plants.

• The dermal tissue of the shoot system is responsible for the exchange of oxygen and carbon dioxide.

The dermal cells of the leaves and stems secrete a waxy substance that forms the cuticle – protects the plants from microorganisms and from water loss.

• The dermal tissue of the root system is responsible for the uptake of water and minerals from the soil; root hairs increase surface area.

2. Ground tissue makes up the majority of the plant and is found as a layer beneath the epidermis.

• In the stem, ground tissue provides strength and support to the plant.

• In the roots, it is involved in food and water storage.

• In the leaves, it is the location where photosynthesis occurs.

• Ground tissue cells are loosely packed together to allow gases to diffuse rapidly.

3. Vascular tissue – xylem and phloem

• Xylem tissue moves water and minerals from the roots to the shoots of the plant.

• Xylem is composed of hollow, dead cells that fuse together with perforations in the walls between the cells.

• Phloem tissue transports dissolved sugars from the shoots to the roots of the plant.

• Phloem vessels are formed from individual sieve tube cells with perforated end walls.

• Sieve tube cells are alive but they lose their nuclei at maturity, so they are connected to small, nucleated companion cells that direct their activity.

• Sugars transported by the plant are used for various cellular processes, converted to cellulose for plant structure, or stored as starch.

*NOTE - Increased size in plants results from rapid cell division in the meristems (meristematic tissue).

The Leaf and Photosynthesis

The chloroplast is the organelle where photosynthesis reactions take place; contains the pigment chlorophyll that absorbs light energy from the Sun.

Photosynthesis is the process by which plants make sugars; oxygen is a waste product of this reaction.

6H2O (l) + 6CO2 (g) → C6H12O6 (aq) + 6O2 (g)

Cellular respiration is the process by which glucose is used to produce ATP that can be used as energy for various cellular processes.

C6H12O6 (aq) + 6O2 (g) → 6H2O (l) + 6CO2 (g) + ATP

During the day the plants take in CO2 and release O2 but since there is no light energy at night, the CO2 produced from cellular respiration is released.

Structure and Function of the Leaf

Dermal tissue:

• The epidermis of the leaf is clear and usually one or two cells thick to allow light to enter the leaf.

• Stomata are tiny openings in the leaf (mostly on the lower epidermis) that allow for the diffusion of CO2 and O2 as well as transpiration.

• Guard cells regulate the opening and closing of the stomata.

• Light striking the leaf causes the guard cells to accumulate potassium ions by active transport, resulting in water entering the guard cells by osmosis – this increases turgor pressure.

• When turgor pressure is high the guard cells change shape and the stomata open, allowing for gas exchange and transpiration.

• When turgor pressure is low the guard cells change shape and the stomata close to prevent water loss.

Turgor Pressure

•Osmosis is what keeps plant cells plump and full of water so that the plants do not look wilted.

•Water pressure inside a plant cell pushes against the rigid cell wall causing turgor pressure.

•The walls of plant cells are able to withstand a great deal of turgor pressure.

•This is not the case with animal cells. If an animal cell takes in too much water by osmosis, it will burst (lysis).

• In woody plants the epidermis is replaced by bark or cork, so gas exchange and transpiration occur in the lenticels (pores) along the stems.

Ground tissue

• The palisade tissue cells are tightly packed together and contain many chloroplasts for photosynthesis.

Ground tissue

• The spongy mesophyll tissue is loosely packed to allow space for gases to diffuse through the leaf; also contain some chloroplasts.

Vascular tissue

• Xylem and phloem are arranged in vascular bundles that extend throughout the plant and branch into finer veins within the spongy mesophyll.

• Xylem carries water and dissolved minerals absorbed by the roots to the rest of the plant.

• Phloem carries the dissolved sugars produced by photosynthesis from the leaves to the rest of the plant.

Water Transport in PlantsCohesion: the tendency for water molecules to stick together because they are polar.

• Helps create an upward pull of water from the roots to the tips of leaves.

•Water is polar: one end is negative, the other is positive

•This results in a strong attraction between water molecules

Water Transport in Plants

Adhesion: the tendency of water molecules to stick, or adhere, to certain surfaces.

• The adhesion of xylem sap to xylem walls helps prevent the sap from falling back down to the roots.

Root pressure pushes –root cells actively transport bring minerals into the xylem, resulting in water following by osmosis

root pressure is the turgor pressure inside the xylem that pushes fluid upward.

Transpiration pulls – transpiration from the leaves generates tension that pulls the water upward in an unbroken column.

As water evaporates from the leaves, more water is drawn upward to take its place.

The Effect of Tonicity in Plant Cells

If a plant cell is placed in a hypertonic solution water will leave the vacuole by osmosis resulting in plasmolysis –the cell membrane and its contents will begin to pull away from the cell wall causing the plant to wilt.

If a plant cell is placed in a hypotonic environment water will enter the cell by osmosis and the plant will become turgid and stand upright.

Sugar Transport in PlantsPhloem sap is composed of sugars from photosynthesis, minerals, other nutrients, and water.

Pressure-flow theory can be used to describe the movement of materials through phloem.

• Sugars from photosynthesis, minerals, and other nutrients are actively transported into the leaf phloem, resulting in water entering by osmosis.

• The nutrients from the phloem are taken up by growing shoots and roots, as well as by fruits and other storage organs, resulting in water leaving by osmosis.

• This causes the pressure in the roots to be lower than in the leaves, resulting in the consistent downward flow of phloem from source to sink.

Plant Control Systems/Tropisms

Gravitropism – growth in response to the settling of starch within a plant due to gravity.

Phototropism – growth in response to light; cells facing away from the light source elongate.

• Positive phototropism – growth of a plant toward a light source.

Charles and Frances Darwin

• Concluded that the tip of the stem was the area responsible for the detection of light stimulus and was somehow communicating with the cells in the area of the bending.

Peter Boysen-Jensen

• Snipped off the grass seedlings, covered the stump with gelatin and replaced the tip – positive phototropism continued normally.

• Repeated the procedure using a thin slice of the mineral mica instead of gelatin – phototropism was not observed.

• Concluded that positive phototropism is the result of a chemical moving from the tip to the area of elongation.

• Auxin moves to the darker side of the plant, causing the cells there to grow larger than cells on the lighter side

Went’sExperiment