Plants What is a Plant?

Plant Biology Basic Leaf Structure Water is lost in the form of a gas through openings called stomata (singular stoma) Transpiration – The term given to the loss of water vapor from leaves and other aerial parts of the plant

Plant Biology Transpiration can be affected by: Temperature Humidity Wind Speed

Plant Biology

Plant Biology Transpired water has to be replaced by the intake of water at the roots There is a continuous stream of water from the roots to the upper part of the plant The stream of water provides the plants with minerals and water needed for photosynthesis Epidermis of most plants secrete wax to form a water proof coating to the leaf (waxy cuticle)

Plant Biology Xylem Involved in supporting the plant as well as being the specialized water-conducting tissue of terrestrial plants

Plant Biology Xylem A tissue that carries water upward from the roots to every part of the plant Xylem is actually a complex tissue composed of many cell types. The 2 types of cells largely involved in water transport are tracheids and vessel elements.

Plant Biology Xylem Tracheids are dead cells that taper at the ends and connect to one another to form a continuous column specialized type of water conducting cell Vessel elements are the most important xylem cells involved in water transport Also dead cells Have thick lignified secondary walls Lignified walls are interrupted by areas called primary walls

Plant Biology Xylem Primary walls include pits and pores that allow water to move laterally Lignin - cellulose that thickens the walls of xylem to prevent inward collapse in times of low pressure. Thickening cell walls with lignin make walls hard and “woody” See page 384

Plant Biology Must be able to draw

Stomata and Guard Cells Plant Biology Stomata and Guard Cells Plants take in carbon dioxide (CO₂) and give off oxygen (O₂) Stomata – openings of the leaves that allow for gas exchange

Plant Biology Plants maintain homeostasis by keeping their stomata open just enough to allow photosynthesis to take place but not so much that the lose an excess amount of water Guard Cells – highly specialized cells that surround the stomata and control their openings and closing The stomata open and close because of changes in the turgor pressure of the guard cells that surround them When the cells take in water they swell, they bulge more to the outside – open stomata

Plant Biology Guard Cells opposite occurs when H₂O is scarce When the guard cells lose water they sag towards each other – close the stomata

Plant Biology Guard Cells Regulate the movement of gases Especially water vapor and CO₂ in and out of the leaf When H₂O is abundant, it raises the H₂O pressure and opens the stomata CO₂ can enter the through the stoma and H₂O is lost by transpiration The gain or loss of water in the guard cells is largely because of the transport of potassium ions.

Plant Biology Generally; stomata are open during the daytime (active photosynthesis) and closed at night Can close also during bright sunlight Guard cells also respond to wind and temperature

Plant Biology-Cohesion-Tension Theory Process Explanation Water moves down concentration gradients The spaces within a leaf have a high concentration of water vapor. Water moves from this location to the atmosphere, which has a lower water concentration Water lost by transpiration is replaced by water from the vessels Replacing water from the vessels maintains a high water vapor concentration in the air spaces of the leaf The vessel water column is maintained by cohesion and adhesion Cohesion involves the hydrogen bonds that form between water molecules. Adhesion involves the hydrogen bonds that form between water molecules and the sides of the vessels; adhesion counteracts gravity

Plant Biology-Cohesion-Tension Theory Process Explanation Tension occurs in the columns of water in the xylem This is because of the loss of water in the leaves and the replacement of that lost water by xylem water. The water columns remain continuous because of cohesion and adhesion Water is pulled from the root cortex into xylem cells Cohesion and adhesion maintain the columns under the tension created by transpiration Water is pulled from the soil into the roots This happens because of the tension created by transpiration and the maintenance of a continuous column of water

Plant Biology Water and Osmosis Water is a raw material for photosynthesis As water is absorbed minerals (nutrients) are absorbed from the soil needed for plant growth Water moves into the root hairs from the soil because the root hairs have a higher solute concentration and a lower water concentration from the surrounding soil Water moves through the plasma membrane into the root hair cells Most of the water entering a plant comes in through the root hairs by osmosis

Plant Biology Phloem – transports solutions of nutrients and carbohydrates (organic material) Made up of living cells (xylem are not) Xylem moves water and minerals upward from the roots Phloem cells transport their contents in various direction

Plant Biology

Plant Biology Sink – a plant organ that uses or stores sugar

How cells pull water upward… Cohesion – a property of water; water molecules are attached to one another Cohesion is formed by Hydrogen bonds

How cells pull water upward… Adhesion – water’s hydrogen bonds, bond to other substances or attraction between unlike molecules Capillary Action – the tendency of water to rise in a thin tube It is a combination of both cohesion and adhesion Both are also properties of water

How cells pull water upward… Capillary Action adhesion

Plant Biology Pressure – Flow Hypothesis Remember that in the phloem sieve tube cells are alive Active transport moves sugar into the sieve tube from surrounding tissues The uptake of water at the source causes a positive pressure called hydrostatic pressure, in the sieve tube This results in bulk flow of the phloem sap Water then follows by osmosis, creating pressure in the tube at the source of the sugars Hydrostatic pressure is controlled by the removal of sugar from the sieve tube at the sink Sugars are turned to starch at the sink (starch in insoluble to water) If another region of the plant needs sugars they are actively pumped out of the tube into surrounding tissues

Plant Biology Osmosis causes water to leave the tube reducing the pressure in the tube in these areas Results: pressure driven flow of nutrient rich sap from the sources of sugars to the places where sugars are stored or used Xylem recycles relatively pure water by carrying it back to the source

Plant Biology Growth in Plants Meristematic tissue is composed of aggregates of small cells that have the same function as stem cells in animals. When these cells divide, one cell remains meristematic while other is free to differentiate and become part of the plant body.

Plant Biology Growth in Plants Apical Meristems – AKA Primary Meristems, occur at the tips and stems Shoot Apex – a typical meristems and the surrounding developing tissue Primary growth allows the root to extend throughout the soil Also allows the stem to grow to collect CO2 and light

Plant Biology Lateral Meristems – allow growth in thickness of plants Secondary growth Trees (woody plants) have active lateral meristems

Plant Biology Auxins & Phototropism Tropisms – growth or movement to directional external stimuli such as light, gravity, chemical, and touch Positive – towards stimuli Negative – away from stimuli Phototropism – the tendency of a plant to grow towards a light source Plants usually exhibit a positive phototropism and plant roots exhibit a negative phototropism

Plant Biology Auxins & Phototropism Auxins are plant hormones that cause the positive phototropism of plant shoots and seedlings Found in: embryo of seeds, the meristems of apical buds and young leaves Auxins appear to increase the flexibility of plant cell walls in young developing shoots. This enables cell elongation on the side of the shoot necessary to cause growth towards the light

Plant Biology Auxins & Phototropism Auxin Efflux Pumps – specialized membrane proteins, move auxin out of the cells closer to the light using ATP This pumping action creates a high concentration of auxin in the intercellular space The result is high concentration gradient from the intercellular space from the adjacent cell The result is a greater elongation of cells on the stem side away from the light and then curvature towards the light

Plant Biology Whenever auxin affects cell growth it does so by changing the pattern of gene expression

Plant Biology Angiosperms Flowering plants Bear their seeds in flowers inside a layer of tissue that protects the seed. Flowering involves a change in gene expression in the shoot apex The switch to flowering is a response to the length of light and dark periods in many plants

Plant Biology Flowers and Fruits Ovaries – contained in flower, they surround and protect the seeds Thus the name Angiosperm which means “enclosed seed”

Reproduction in Flowering Plants

Plant Biology Flower Part Function Sepals Petals Protect the developing flower while it is inside the bud Petals Often colorful to attract pollinators Anther Part of the stamen that produces the male sex cells (pollen) Filament Stalk of the stamen that holds up the anther Stigma Sticky top of the carpel, on which pollen lands Style Structure of the carpel that supports the stigma Ovary Base of the carpel in which the female sex cells develop

Plant Biology Pollination – the transfer of pollen from the male reproductive structure to the female reproductive structure Success in plant reproduction depends on pollination fertilization, and seed dispersal Most flowering plants use mutualistic relationships with pollinators in sexual reproduction

Plant Biology Seed – a plant embryo and a food supply, encased in a protective covering Seed Coat Cotyledons Shoot Apex (embryonic shoot) Micropyle Hilum Root Apex (embryonic root)

Factors Needed for Seed Germination Plant Biology Factors Needed for Seed Germination Water must be available Oxygen Warmth (enzyme activity)