Lateral transport:

1. Lateral transport:

2. Bulk flow functions in long-distance transport Diffusion works well over short distances but its too slow to be useful for long distance transport in a plant. Water and solutes move through xylem and phloem by bulk flow, which is the movement of of a fluid driven by pressure. In phloem, hydrostatic pressure is generated at one end of the sieve tube, forcing sap to the opposite end of the tube. Called pressure flow. In xylem negative pressure, or tension, drives long distance transport. Transpiration; the evaporation of water from the leaf, reduces pressure in the xylem.

3. Structure and Function of Xylem and Phloem • Sieve-tube members have no organelles, vessel elements and tracheids are dead at maturity. This leaves the tubes free of obstruction, which facilitates the movement of water and dissolved solutes. • The porous ends of the phloem cells- sieve plates, also enhance transport by bulk flow.

4. Back to absorption of water and minerals by roots Water and minerals enter the roots through the epidermis of the root tips where the root hairs are located. Soil particles adhere to the hairs and the soil solution moves into the hydrophilic walls of the epidermal cells The soil solution moves along the apoplastic route through root hairs, epidermis, cortex to the endodermis. Within the wall of the endodermis is a wall of waxy material called the Casparian strip. This prevents further movement via the apoplastic pathway. To go further water and minerals have to cross the plasma membrane and enter via the symplast pathway

6. Many plant roots have a symbiotic relationship with fungus called mycorrhizae, which help to absorb water and minerals. The hyphae of the fungus absorbs water and minerals and transfers them to the host plant

7. Transport of Xylem Sap • Sap in the xylem flows upward from roots to leaves. Every cell needs to get water and minerals • Water vapor is lost from the leaf through the stomata due to evaporation • Trees can loose more than 200L of water per hour during hot days • Water has to constantly be replaced • How does water get from roots to leaves? Is it pushed or pulled?

8. Root pressure: • Accumulation of solutes inside the root lowers the water pressure compared to the soil and so water moves in • This generates a positive pressure that pushes xylem sap up, this mechanism can only push water up a few meters • Root pressure causes guttation- the exudation of water on the tips of grass or leaf margins. (this is not the same as transpiration, which is loss of water vapor from stomata

9. Since root pressure does not explain movement of water higher than a few meters, we will look at forces that pull sap up the xylem. This theory is called : The Transpiration-Cohesion-Tension Theory • TRANSPIRATION • Air spaces in the spongy mesopyll are saturated with water vapor • In most cases the outside air is drier and so has a lower ψ, so water moves out of the cell- this movement of water vapor by evaporation is called transpiration

10. 2. TENSION • Transpiration is changed into a pulling force by the generation of a negative pressure in the leaf. • Evaporation of water from the outside of the mesophyll cell replaces the water vapor that is lost from the air spaces in transpiration • Water remaining in the cell adheres to the cell wall and water molecules are attracted to each other, creating surface tension • The combination of these 2 forces: adhesion to the wall and surface tension, causes the surface of the water to form a concave shape. • The water film at the surface of the leaf has a negative pressure, which pulls water up

11. A combination of adhesion, cohesion, and surface tension (see below) allow water to climb the walls of small diameter tubes like xylem. This is called capillary action. The U shaped surface formed by water as it climbs the walls of the tube is called a meniscus.

13. Cohesion and Adhesion of water molecules • The transpirational pull created by the negative pressure in the leaf is transmitted all the way down to the root because of a unique property of water. • The atoms in a water molecule are covalent, but they share the electrons unequally, making the molecule have a polarity, difference in charge. • The H ends are slightly +, the O end is slightly negative. • Water molecules are attracted to each other, forming weak bonds (H+ bonds) between the molecules. • So there is one long unbroken chain between the molecules in the column of water in the xylem. • Water molecules also adhere to the walls of the xylem due to H bonds.

14. Hydrogen bonds between water molecules explain cohesive property

16. Outside air -10 to -100 MPa Leaf -1.0 MPa Trunk -0.8 MPa Soil -0.3MPa

17. Review: transpiration, adhesion, cohesion, tension Transpiration involves the pulling of water up through the xylem of a plant utilizing the energy of evaporation and the tensile strength of water. Adhesion is the attractive force between water molecules and other substances. Because both water and cellulose are polar molecules there is a strong attraction for water within the hollow capillaries of the xylem.

18. Cohesion is the attractive force between molecules of the same substance. Water has an unusually high cohesive force due to the 4 hydrogen bonds each water molecule potentially has with any other water molecule. It is estimated that water's cohesive force within xylem give it a tensile strength equivalent to that of a steel wire of similar diameter. Tension a stress placed on an object by a pulling force. This pulling force is created by the surface tension which develops in the leaf tissue.

19. Factors that affect Rate of Transpiration: 1. Humidity- high humidity lowers concentration gradient, so lowers R of T 2. Wind- wind moves the water molecules away from the surface of the leaf, therefore increases the concentration gradient, so increases R of T 3. Temperature: increased Temp, increases movement of particles, and increases the rate of evaporation and so increases R of T

20. 4. Light: • Light increases Temp, therefore increases R of T • Light causes the stomata to open, because light stimulates rate of photosynthesis, PTN needs CO2 • Light also causes increase in ATP from light dependent rctn- ATP is used to power the proton pump (inside becomes more negative), more K+ enters, and so more water enters, the guard cells become turgid and stomata open more

21. Measuring Transpiration

22. Control of Transpiration • When stomata are open water vapor and oxygen leave and CO2 come in. • The plant has to balance the need for CO2 with conservation of water. This is the Photosynthesis-Transpiration Compromise • Transpiration also assists the delivery of minerals from roots to leaves and cools the plant due to water’s high heat of evaporation • Rate of transpiration is controlled by stomata

23. How do stomata open and close? Each stomata has a guard cell on either side, which controls the size of the opening If the guard cells are turgid, the opening is big If the guard cells are flaccid, the opening can be closed The size of the guard cell is controlled by K+ ions, remember mechanism of K+ transport

24. Light triggers the uptake of K+ because of light sensitive pigments in the in the guard cell that stimulate the proton pump. The production of ATP due to photosynthesis also encourages the proton pump. Most plants close their guard cells at night to conserve water.

25. GASEOUS EXCHANGE • Most takes place through guard cells in leaf. • CO2 comes in and O2 goes out due to differences in concentration gradients • Gases can also be exchanged through lenticels in stems and in roots, through root hairs.

26. TRANSLOCATION: transport of organic molecules in the phloem • Phloem sap is made up mostly of sucrose. Other solutes are minerals, amino acids and hormones • Direction of flow is variable but sieve tubes carry sap from a sugar source to a sugar sink. • A sugar source is an organ where sugar is produced by photosynthesis or where starch is broken down into sugar • A sugar sink is an organ that is a net consumer of sugar. Ex: growing roots, shoots, stems, fruit • A storage organ can be a sink or a source depending on the season

27. Phloem loading and unloading • Sugar needs to be loaded into the phloem before it can be translocated • Sugar can move from mesophyll cells to sieve tube members by the symplast pathway (cell to cell through plasmodesmata) • Another route is a combination of symplast and apoplast pathways • Companion cells pass sugar they have accumulated into the sieve tube members through plasmodesmata • Sugar is also moved by active transport by cotransport with H+ ions • At the sink end sugar moves down its concentration gradient into the sink by diffusion and water follows by osmosis

28. Phloem loading

29. Pressure flow: translocation in angiosperms: • Phloem sap flows from source to sink at rates up to 1m/hr • This high rate is achieved by bulk flow, which is driven by pressure • Phloem loading makes a high solute concentration at the source end, which means a low ψ, causing water to move into the phloem • Hydrostatic pressure builds in the source end of the sieve tube • Pressure at the sink end is lowered because water moves out • This causes water to move from source to sink • Water is recycled back via the xylem

30. Pressure Flow

31. Plants store food in: Modified roots, like potatoes- sucrose is converted to starch seeds fruit

32. PLANT REPRODUCTION Sexual Reproduction Life cycles of angiosperms and other plants are characterized by alternation of generations between haploid (n) and diploid (2n). The diploid is called the sporophyte. It produces haploid spores by meiosis. The spores then divide by mitosis giving rise to multicellular male and female plants called gametophytes. Gametophytes undergo growth and differentiation and produce gametes: sperm and egg. Fertilization unites sperm and egg creating a new diploid sporocyte

34. Variations on gametophyte/sporophyte relationships

36. Flower showing reproductive parts Stamen are male, made up of antherand filament. Pollen is produced in the anther Carpels are female, made up of style and stigma. Carpel has an ovary at the base, stigma is sticky,landing place for pollen. Some flowers have more than 1 carpel. Sepals and petals are not reproductive

37. Gametophyte development MALE In the pollen sacs of the anther are numerous microsporocytes- they divide by meiosis into 4 haploid microspores; mitosis produces a generative cell (sperm) and a tube cell (pollen tube) a pollen grain FEMALE Ovules form in the ovary. Megasporocyte divides by meiosis to 4 cells, only 1 survives into a haploid megaspore; 3 mitotic divisions forms the embryo sac; 8 cells:: 1 egg cell (female gamete) and 2 polar nuclei and 2 (synergids, guide sperm), 3 antipodal cells

39. Pollination • Brings male and female gametes together. Pollen can be carried by wind, insects, animals or rain • When the pollen germinates it produces a pollen tube, which grows down through the stigma and into the style. • As it grows the generative nucleus divides and produces 2 sperm nuclei

40. Double Fertilization • One sperm fertilizes the egg to form the diploid zygote, this will develop into the plant embryo • The other sperm combines with the 2 polar bodies to form a triploid nucleus, this develops into the endosperm, tissue that stores food for the seed. • This double fertilization ensures that the endosperm will develop only in fertilized eggs.

42. The ovule develops into a seed containing the embryo and a supply of nutrients in the endosperm. Mature seed is covered by seed coat and has dehydrated. Growth stops until seed germinates

43. Mature seed

44. The ovary develops into a fruit adapted for seed dispersal. • Fruit protects the seed and aids in dispersal • Pollination causes the ovary to transform into fruit, if no pollination, flower dies and falls off

45. Germination • After it matures the seed becomes dormant until conditions are suitable; dormancy increases the chances of survival • Breaking dormancy requires certain conditions: • Water is essential for all seeds • Some need a period of cold, some need intense heat • A few require light • Some need their seed coats be be broken down by chemicals • Seed viability varies from days to decades

46. Seed to seedling Seed imbibes water Causes seed coat to rupture Triggers metabolic activity, gibberellins are released After hydration the enzymes in the seed start to digest the stored food and nutrients are transported to growing regions of the embryo The radicle, embryonic root is the first organ to emerge Then the shoot pushes through the soil surface Light stimulates the growth of the shoot upwards Shoot growth is different in monocots and dicots

47. Hypocotyl emerges, then straightens in response to light. Shoot is pulled up rather than having to push through the soil Coleoptile is a tube that pushes its way through the soil, the shoot emerges from this tube

48. Asexual reproduction • Many plants can clone themselves through a process called vegetative reproduction. • Detached fragments of plants can develop into a new organism called fragmentation • Another type is separate organisms growing from adventitious roots • Apomixis is seed formation without fertilization. Diploid cells in the ovary give rise to a new organism, seeds are dispersed by wind

50. Plants can be propagated in agriculture due to their ability to reproduce asexually: • Cuttings from plants • Grafting of a stem from one plant onto another • Scientist can clone plants from single cells • Genes can be inserted into plants through genetic engineering; genetically modified foods