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139 Cards in this Set
- Front
- Back
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Plants are made up of several cell types but there are structures common to most plt cells.
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Nucleus, mitochondria, chloroplasts (in many), vacuole, cytoplasmic membrane and cell wall (in living cells)
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Dead plant cells lack all except...
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cell wall
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Where is the cell wall located?
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Outside of the citoplasmic membrane, and therefore was never part of living cell.
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The cell wall is analogous to what?
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Analogous to fur as a protective outer cover for mammals, and is also part of the animal, but not living.
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Several cells types exist
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a) these can be highly specialized cells that do specialized jobs
b) cell structure is based on function and therefore some of these specialized cells will have specialized structures |
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Trichomes
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hairlike projections of the cell wall of some epidermal cells
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Casparian strips
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Gasket-like structures on outside of endodermal cells
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Ground tissue cells
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Make up the body of plant, least specialized
Parenchyma Cells, Collenchyma Cells, Sclerenchyma cells. (2nd of 3 tissue types) a) “background” tissue b) generally less specialized, looks like it just "takes up space" but it generally has a function |
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Parenchyma cells
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(1) least specialized of ground tissue cells
(2) cubical to elongated cube in shape (3) generally thin walled w/o 2˚ walls (4) large central vacuole (5) most metabolic activities present. (6) generally retain totipotency (7) general background/space filling / storage cells or PS cells PS, storage or ground tissue |
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Collenchyma cells
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(1) similar to parenchyma except have thicker, uneven 1° walls
(2) lack 2˚ walls (3) are arranged in strands or fibers (4) Provide strength, especially in young stems (e.g. strings in celery) (5) immature cells (6) often develop into schlerenchyma strength or support in growing tissue |
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Sclerenchyma cells
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(1) 1˚ and thick 2˚ walls w/ lignin.
(2) So stiff that plts can no longer grow in these areas. (3) Many actually are dead when mature. (4) Occur as bundles of fibers or sclereids strength or support in mature tissue |
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Bundles of fibers
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Fibers are very long and thin with pointed ends, generally arranged in bundles of parallel fibers. e.g. hemp or flax fibers (woven into cloth or rope)
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Sclereids
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Short, irreg in shape.
May be highly lignified, e.g. shells of nuts, or in pear flesh. |
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Cells of Vascular tissues
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Xylem elements: Allow transpiration
Phloem elements: Make up sieve tubes |
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Xylem elements
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Tracheids
Vessel elements |
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Tracheids
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(a) 1˚ and thick even 2˚ walls w/ lignin, (in areas of continued elongation 2˚ walls laid down in a spiral to allow stretch (eg palm)
(b) dead and hollow at maturity (c) fiber shaped (d) overlap at ends with communicating pits to allow water conduction. (e) also add strength (f) can have pits on side walls (g) conduct water upwards |
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Vessel elements
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(a) shorter & wider than tracheids
(b) 1˚ and thinner 2˚ walls w/ lignin (c) dead and hollow at maturity (d) perforated ends walls, arranged end to end forming a water conducting pipe (e) also add strength (f) pits on sides (g) conduct water upwards |
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Phloem elements
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Sieve tube members.
Companion cells. |
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Sieve tube members
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(a) alive at maturity but may lack many of the functional organelles normally found (nucleus, mito, chlpsts).
(b) Shape: from a hollow fiber with overlapping ends and communicating plasmodesmata, to vessel element in shape (c) In vessel element shape, cells are lined up end to end and the ends are called sieve plates. (d) sieve plates have plasmodesmata with communicating c.m. between adjacent cells (e) Func: Allow transport of phloem (sap) |
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Companion cells
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-All sieve tube members have 1 or more -companion cells along side.
-Companion cells are small in diameter but long and spindle shaped with many plasmodesmata connecting it to its s.t. member cell. - Comp. cells have a full complement of organelles, full metabolic abilities and is thought to "feed" and care for sieve tube member. -Comp. cells are thought to provide the st member with normal cellular functions and food. - In many plts, companion cells also load sugar into st members, also thought to load phloem sap into s.t. member. |
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Plant tissues
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1) cells of a single type are most often grouped together to form a tissue
2) these tissues have specialized functions 3) the 3 tissue types are based on tissues |
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The 3 general tissue types
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Dermis
Fundamental (ground) tissue Vascular tissue |
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Dermis
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tissues that form the outermost cover of the plant body.
Composed of epidermis and periderm. |
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Epidermis
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1˚ outer cover
(1) usually 1 cell layer thick of parenchyma (2) cell type reflects func. (a) epidermis of leaves: thin to thick, relatively unspecialized cells (except guard cells of stomata), covered on outside by cutin (waxy material) that forms the cuticle layer on the outside surface of epiderm. It waterproofs and airproofs the surface. (b) epidermis of roots: generally thick (protection), unspecialized cells, with trichromes near root tips |
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Periderm
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(1) periderm is a 2˚ outer cover
(2) takes place of 1˚ epidermis in regions of 2˚ growth (3) It is the outer and dry layer of the “bark” (4) true bark is “bark” + all of the tissues outside the vascular cambium |
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Vascular tissue
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a) function: conducts fluids
b) examples: phloem & xylem |
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Distribution of tissues in the Plant body
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FFF: form follows function
a) Plants must grow b) makes food c) conduct water and photosynthate d) reproduce e) protect themselves from desiccation f) don’t fall over 2) Various Plt tissues are distributed where they will do their job best. a) e.g. PS cells and stomata are not found in the roots b) roots and stems have a lot of conductive tissue to conduct and also give strength. |
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Angiosperms
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(flowering plts) are the most widespread and successful plts type at this time in evolution.
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Monocot Characteristics
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1) one cotyledon
(2) leaf veins parallel (3) vasc bundles scattered in stem (4) fibrous root (5) flower parts in 3’s (6) e.g. Grasses like wheat, palm, lily |
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Monocot components
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Roots
Root tissue Cortex Stele Stem |
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Monocot roots
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(1) jobs
(a) absorb and conduct water and minerals (xylem, dermis) (b) anchor plt (xylem, dermis, ground tissue) (c) store food (ground tissue) and conduct food (phloem) (2) generally fibrous in monocots (to increase surface area) to abs water efficiently |
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Monocot root tissue arrangement
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(outside to in) (1) root epidermis
(a) often with trichromes (root hairs for abs, found back from the root tip as they are scraped off with growth) (2) little cuticle near tip, more on mature root shaft (3) some mucin near tip |
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Cortex of monocot root
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Made of ground tissue and endodermis.
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Ground tissue of cortex (monocot root)
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(a) function: strength & storage (can store starch or become lignified to give strength)
(b) loosely packed parenchyma cells. (c) cells connected by plasmodesmata (important in C mobilization) |
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Endodermis of cortex ( monocot root)
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(a separating layer)
Actually is the inner layer of cortex (a) cells different from cortex in that they are tightly packed and have Casparian strips. (physiologically isolates stele from rest of root) |
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Stele (monocot)
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(Central region) Contains vasc. tissue
(1) pericycle (directly inside endodermis): is a lateral meristem & gives rise to lateral roots in monocots (2) bundles of 1˚ xylem: arranged around periphery like #'s on a clock (3) bundles of 1˚ phloem: arranged between the xylem bundles. (4) pith: ground tissue of stele |
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Stem ( monocot)
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Function: support, conduction
Components (Outside in): Epidermis, ground tissue, vascular bundles |
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Epidermis of monocot stem
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has a thick cuticle to prevent water loss
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ground tissue of monocot stem
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(for strength, storage or space
filling): loosely packed parenchyma or more sturdy sclerenchyma |
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vascular bundles of monocot stem
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Function: for strength and conduction.
(a) composed of both xylem (more central) and phloem (more peripheral) (b) bundles are scattered in the ground tissue but are more prevalent near the epidermis (c) bundles may have fiber caps |
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Eudicot characteristics
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(1) 2 cots
(2) leaf veins net like (3) vasc. bundles have ring arrangement in stem (4) tap root (5) flower parts in 4 or 5's e.g. roses, avocados, peas, oaks |
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Eudicot components
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Root
Stem |
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Eudicot roots
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Generally tap roots, to go for deeper water
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Eudicot root epidermis
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(a) often with trichromes (root hairs for abs, found near root tip mostly, as they are scraped off with growth)
(b) little cuticle near tip, more on mature root shaft |
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Eudicot root cortex
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ground tissue (storage or space filling)
(a) thick layer, loosely packed parenchyma cells (b) cells connected by plasmodesmata (c) can store/transport starch (d) this layer can be replaced as the stele enlarges (in 2˚ growth) as seen in most perennial eudicots (e) occasionally lignified |
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Eudicot root endodermis
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(separating layer) Actually inner layer of cortex
(a) different in that tightly packed tissue has Casparian strips (physiologically isolates stele from rest of root) Physiological barrier between cortex and steele |
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Eudicot root stele
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(central): contains vasc. tissue
(a) pericycle (directly inside endodermis): lateral meristem, in eudicots gives rise to lateral roots and 2˚ vasc. tissue (b) bundles of 1˚ xylem: arranged most often in the shape of a cross (c) bundles of 1˚ phloem: arranged between the "spokes" of xylem bundles. (d) pith: little to none in eudicot |
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Eudicot stem epidermis
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single layer with thick cuticle prevents water loss
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Eudicot stem cortex
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(storage, space filling and PS)
(a) thick layer of loosely packed parenchyma cells (b) if PS, then will usually lack endodermis |
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Xylem and phloem (eudicot stems)
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bundles of xylem and phloem combined (relative positions of the X. and P. vary)
(a) bundles arranged around periphery like #'s on a clock OR (b) may be in a central vasc. stele (Tilia spp.) (c) may have fibrous caps |
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Center of eudicot stem
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center: pith of parenchyma
(also found between bundles) OR empty space |
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Plant growth
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1) plts grow in 3D: they grow in length (1˚ growth , both up and down) and they grow in width (2˚ growth, they become thicker) only
2) in general, monocots live only one yr and therefore grow only up and down (i.e. 1˚ growth only). 3) many eudicots grow for more than one yr and therefore need to grow up and down and laterally (i.e. 1˚ and 2˚ growth) 4) in all cases, all growth in plts is derived from meristem tissues only. |
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Meristem
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a. undifferentiated (i.e. embryonic or “stem”) cells that are able to continually divide mitotically.
b. No other tissues are able to add cells to the plt body. Therefore the sites of meristem are the only points at which the plant grows c. Meristems make all new plt tissues |
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Where is meristem found?
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a. 1˚ shoot apex (called apical mertistem)
b. leaf axis (called axial mertistem) c. root tip (called root apical mertistem) (a, b and c produce increases in length and occur in the youngest parts of the plt) d. 2˚vascular cambium: tree "rings“ e. cork cambium: produces bark(d & e produce increases in width, i.e. 2˚ growth, and occur in the older parts of the plt, mostly of perennial eudicots) |
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Primary meristems
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Protoderm : lies around the outside of the stem and develops into the epidermis.
Procambium : lies just inside of the protoderm and develops into primary xylem and primary phloem. It also produces the vascular cambium, a secondary meristem. Ground meristem: develops into the Cortex and the pith. It produces the cork cambium, another secondary meristem. |
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Secondary meristems (lateral meristems)
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Vascular cambium: produces secondary xylem and secondary phloem. This is a process that may continue throughout the life of the plant. This is what gives rise to wood in plants. Such plants are called arborescent. This does not occur in plants that do not go through secondary growth (known as herbaceous plants).
Cork cambium: gives rise to the periderm(cork cambium+ cork), which replaces the epidermis. |
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Meristem primary growth in roots
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a) most apical: root cap
•a thick cover or zone of cells that protects the deeper meristem. •Secretes a polysacc slime (mucin) that lubricates the interface between the root tip and the abrasive soil. b) Growth Zones in roots: 3 overlapping regions of growth •zones are found behind the root cap • zones are w/o sharp boundaries • zones are based on cell activities |
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Growth zones
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Cell division
Elongation Maturation |
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Zone of cell division
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•contains a ball like root apical meristem
•provides dividing cells to the root tip and the three 1˚ meristem tissues in young plts •the three 1˚ meristem tissues rise as 3 cylinders from the root apical meristem. •gives rise to the 3 1° meristems protoderm becomes dermis ground meristem becomes cortex procambium becomes vasc. tissues of stele •region is compact and dense |
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Zone of elongation
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•where cells elongate
•elongation is what pushes the root tip through the soil •is accomplished by the cell increasing in length by more than 10x and by the 1˚ meristems adding cells to the zone. |
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Zone of maturation
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•region where the cells complete their differentiation into adult tissues.
•Root hairs (extensions from the cells, used to increase surface area) can be formed here. |
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Meristem secondary growth in roots
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(a) growth that widens the root.
(b) It adds 2˚ xylem and phloem only. (c) 2˚ growth in root occurs from the vascular cambium •VC is a 2˚ meristem in the shape of a cylinder •VC produces 2˚ xylem to inside and 2˚ phloem to outside. •VC retains meristematic cells (or initials) so that it can continue to divide (c) as 2˚ growth occurs the cortex degrades and is lost. Only 2˚ tissues are left. |
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Meristem primary growth in shoots
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(a) apical meristem
(b) Axillary bud meristems (c) vasc. tissue arises from the procambium • vasc tissue is organized into bundles: 1˚xylem towards the center and 1˚ phloem to the outside. |
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Apical meristem
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•found at tip of shoot,
•increases height only. •Dome shaped mass of initials at the terminal bud •Give rise to the three 1˚ meristems=> Protoderm(-> dermis) Ground meristem (-> ground tissue) Procambium (-> vsc cambium) •meristems elongate downward in cylinders, as in roots. •leaf primordia: enclose the delicate apical meristem |
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Axillary bud meristems
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•flanked by leaf primordia at a node
•give rise to lateral branches. •new axillary bud meristems arise from the three 1˚ meristem tissues left behind by the apical meristem as it grows •as cells elongate and mature between the buds and apex, the internodes are formed. |
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Vasc. tissue arises from the procambium, how is it organized?
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• vasc tissue is organized into bundles: 1˚xylem towards the center and 1˚ phloem to the outside.
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Meristem secondary growth in shoots
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•in general, only perenial woody eudicots do this
•adds width to stems or roots •2˚ tissues often crush and replace 1˚ tissues •most commonly starts in 2nd year of growth |
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Vasc. tissues from 2 ˚ growth of shoots
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Vascular cambium
Cork cambium |
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Vascular cambium
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•VC is a lateral meristem
•produces 2˚ xylem ("wood") to inside •produces 2˚ phloem to outside •2˚ phloem and 2˚ xylem can be found in vasc. bundles or vasc. layers. •VC can also make parenchyma between bundles -> "xylem and phloem rays” •VC is not a thick layer and can be lost to growth of the cork cambium •VC adds Xylem (& Phloem) each yr, giving rise to wood "rings“ •Most of increase in girth is due to this process. |
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Cork cambium
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In trees that have bark, a cork cambium is found
•CC is another lateral meristem •develops outside of 2˚ phloem, in the cortex •CC adds cork to the outside only. •This is the outer most layer in a woody eudicot like an oak tree. •Cork is what you think of as“bark” •Yr by yr, the cortex is destroyed and replaced by cork. •When the cortex is gone, cork cambium arises from phloem initials (meristematic stem cells of the phloem). •The outer layer of cork is constantly sloughed off. •Definition of bark= 2˚phloem+ CC+ cork) |
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Why do plants need water?
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like all org, plts are composed of cells & cells need water to:
a. transport nutriens, wastes and chem signals within and between cells. b. biochem rxns require water c. turgor gives plts physical strength to remain upright (esp. in annuals) d. turgor gives plts physical strength to grow (hydrostatic pressure is used to expand cell walls.) e. provide e-’s for PS |
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Types of fluid transport
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1) Bulk Flow(translocation) in phloem (sap containing PSthate)
2) Transpiration in xylem (H2O and minerals) 3) diffusion over short distances |
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Cell membranes are permeable to...and not permeable to...
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Permeable to gases and alcohols, but not much else. They are not freely permeable to charged or polar molecules, ions, medium and large sized molecules.
But life requires that these types of molecules cross the cm |
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Mechanisms by which large or charged/polar molecules can pass the cell membrane
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a) endocytosis and exocytosis
b) passive diffusion (facilitated diffusion by protein channels for ions) c) active transport |
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Passive diffusion of water through cell membrane is mostly through...
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Passive diff proteins called aquaporins.
Hypertonic Hypotonic |
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Passive diffusion of ions and molecules through cell membrane is mostly through...
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Ion channels/gated ion channels
(1) specific passive protein channels embedded in cm OR (2) specific active transport proteins |
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Active transport of ions or molecules
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a) via carrier proteins/pumps/active transport proteins
b) specific c) able to pump against a gradient d) energy requiring |
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Examples of active transport
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1) Proton pump, pumps protons outside the cell, using up ATP in the process.
2) co-transport protein: 2 different molecules or ions are exchanged across a memb. (a) This is a stoichiometric relationship: In this example, 3Na+for 2K+ (b) ATP required 3) A plt example of co-transport: sucrose/proton pump (a strong proton gradient drives the transport of sucrose) |
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Osmosis
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osmosis: movement of water across a cell memb in response to [solute] gradients. Predicts the movement of water very well in animal cells.
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Osmosis complication factor in plant cells
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Cell wall. Osmosis occurs in plts cells but,
once a plt cell becomes "full" the cell memb exerts pressure on the cell wall and cell wall pushes back and restricts inflow. Therefore osmosis alone can not be used to predict water movement in plts cells. |
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Water potential (ψ or psi)
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Expresses the combo of pressure potential and osmotic potential in a region.
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Unit of measurement of psi
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MegaPascal (MPa)
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How is psi used in plants?
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used to describe the water movement in plants
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A container open to atm and containing pure water is defined as
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ψ = 0 MPa
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high water potential
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relatively low solute &/or high pressure: i.e. “relatively lots of water”
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low water potential
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relatively high solute &/or low pressure: i.e. “relatively little water”
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Water potential of water open to atmosphere with solute...
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Has a lower water potential than zero (i.e. = negative # )
(a) e.g. any 0.1M solution = -0.23MPa (b) lower # = lower water pot, meaning less water or more solute |
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Water with less solute ...
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has higher water pot= a less negative #
(a) e.g. pure water = -.0 MPa (b) higher # = higher water potential, meaning more water or less solute |
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What does pressure do to psi?
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(1) pure water under household pressure in a tap is MPa = +.25Mpa
(2) i.e. more water in smaller area and a higher water pot. |
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Can you achieve a positive psi using only solute?
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No
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Water moves across a memb. from an area of ___ water pot to an area of ___ water pot.
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High to low
( This tendency is governed by the same laws of physics that governs all biological processes. This is the 2nd law of thermodynamics that says: In time, all things go from ordered to random ) |
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How is water pot. used
to access the movement of water in a U shaped Tube (before equilib?)? |
water pot Ψ =
pressure pot + solute pot (Ψ = Ψp + Ψs) |
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The Ψ will be equal in the cell and the environment when...
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a cell is in osmotic equilibrium with its environment
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The transpiration stream
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The abs of water and minerals by roots, transport through out the plt body and evaporation through the leaves.
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Where does the transpiration stream occur?
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in xylem, which forms a
continuous branched system of pipes from the roots to the leaves. Water from xylem evaporates through the stomata of leaves |
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What does transpiration rely on?
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Transpiration relies on the fact that water has the
emergent property of cohesion (Water sticks to itself very tightly) |
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3 major compartments of water in plant cells
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Vacuole
symplast apoplast |
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Vacuole
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main water storage area and is well able to increase and decrease its volume
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Symplast
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(cytoplasm (sol) compartment= symplast)
less able to change its volume due to physiological requirements. |
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Apoplast
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(region outside the cm=
apoplast) in reality outside the cell but not necessarily outside the plt |
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Transpiration stream
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Water is attracted by roots.
Lowered ψ leads to root pressure which pushes water upwards |
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Why is water is attracted to roots?
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Because:
a) plt cell walls are hydrophobic due to a polar carbohydrate structure b) active transport of mineral ions (esp. K+) into root cells, therefore roots have a lower? ψ than the surrounding normal ground water |
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Lowered ψ leads to root pressure which pushes water upwards but...
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amt of root pressure
possible pushes water up only a couple of meters. |
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How do plts transport xylem sap to the tops of trees?
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(1) transpirational loss of water leads to developing water deficits in the leaves.
(i.e. a negative ψp lowers the ψ in leaves as water evaporates through the stomata) |
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There are 2 mech. by which transpiration (xylem transport) is driven, which are they?
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1) root pressure (positive
pressure, therefore high ψp ) pushes water up the roots 2) transpiration pulls water to the top of the tree via water evaporation, cohesion of water and bulk flow |
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notes on transpiration
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1) in some plts
root pressure causes guttation in morning, before transpiration begins 2) most of water is abs through the root hairs (cellular extensions from epidermal cells) or hyphae of mycorrhizal fungi. |
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Once water has been abs by root hairs/hyphae, water travels through...
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The root cortex.
1) Water travels through the cortex apoplast or symplast 2) water then reaches the endodermis. a) Remember the endodermis is a hollow cylinder of tightly packed parenchymal cells. b) endodermal cells have a Casparian Strip |
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Casparian Strip
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A gasket of suberin, like mortar around bricks, not allowing water to enter the stele before passing through a cell wall.
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Endodermis may be thought of as...?
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A brick wall with mortar (suberin) between the bricks
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Before the water passes the endodermis, the water can be ____ or _____
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apoplastic or symplastic
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Apoplastic water
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has not passed through a cm and needing to be “screened”, must pass into the cells of the endodermis before entering the stele.
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Symplastic water
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has passes through a cm, therefore it has been “screened”.
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Water enters cells of endodermis by...?
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Osmosis
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After entering the cells of the endodermis, the water is...?
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Screened (enters symplast)
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After being screened, water can...?
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Water then can continue to travel to the conductive tracheids and vessel elements of xylem of the stele.
Remember that tracheids and vessel elements are dead and are therefore are part of the apoplast. So water once again will pass from the symplast to the apoplast |
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The transport of xylem sap mechanism is known as...
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transpiration-cohesion-tension mechanism
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What is xylem sap?
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(1) The liquid contents of the xylem is known as xylem sap.
(2) Xylem sap is a dilute solution of water and minerals, but it has been screened by the cm of the root cells. |
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The conductive cells of xylem are?
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Tracheids and vessel elements are the conductive cells of the xylem.
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How do tracheids and vessel elements transport sap?
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Tracheids and vessel elements are continuous with each other via pits and perforated end plates. They form the "pipes" of the stele.
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How is water of the xylem sap is held together ?
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By cohesion, making them hard to pull appart.
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What is cohesion?
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cohesion is the sticking together of identical compounds, in the case of xylem sap: water molecules
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What is adhesion?
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something sticking to something different
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Transpiration is driven by ...?
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Water evaporating from the stomata.
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How do transpiration, cohesion and tension all come together to accomplish the movement of xylem sap?
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Water evaporation through the stomata creates a lower ψ in the stomatal space and this will create a draw on water below it in the xylem (this creates a tension on the water, as water sticks together due to cohesion).
Tension (lowered water pot) is generated by the draw of water upward and this will draw more water from the roots. Cohesion and adhesion of water to the cell walls, also helps counteract the forces of gravity. |
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Why is there another conduction pathway beside the xylem?
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xylem moves fluid upwards from root to shoot only
a) good for moving water and minerals up into the plt from the ground b) does not move PSthate c) PSthate is made in the leaves and therefore must move from leaves down to shoots and roots d) Thus, there needs to be a 2nd kind of path in plts to move PSthate |
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What is the path through which PSthat is moved?
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It is called translocation and uses the phloem. It moves PSthate both up and down in a plt
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Phloem contains...?
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Phloem sap.
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Phloem sap contains...?
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a) up to 30% sucrose (derived from PSthate, sucrose is the main form of transported C in plts)
b) symplastic minerals c) hormones d) dissolved organic compounds |
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What are the conductive cells of the phloem?
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Sieve tube members
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How are sieve tube members arranged?
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Sieve-tube members are arranged end to end to make sieve tubes. Sieve plates are at the ends of s.t. members and border with adjacent s.t. member’s sieve plates.
Plasmodesmata: s.t. pores are lined with cm and the cm extend from one cell to the next, therefore there is free communication between cells. Fluid transport between adjacent st members is very efficient. This set up also begins to blur the distinction between one cell and the next. |
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Sieve tube members are alive or dead at maturity?
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Alive, but lack most internal organelles (i.e. they are mostly just cytoplasm on inside)
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Phloem loading and unloading involves...?
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sugar sinks/sources
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Sugar sinks
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Is any organ that is actively using or taking up sugar
(1) therefore a sugar sink draws sugar from the phloem (2) e.g. Meristems of roots and shoots, leaves, fruits |
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Sugar sources
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is any organ that is actively making sugar by PS or is breaking down starch
into sugar (1) therefore a sugar sources moves sugar into the phloem (2) e.g. Leaves, storage roots or stems |
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Describe the process of the loading of phloem from a sugar source.
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a) a source sugar will load sucrose into the phloem (symplastic or combo) mostly through active transport of sucrose into the st members
b) ATPase and a sucrose-H+ co-transport proteins of the companion cells are used c) ATPase in the cm pumps H+ to the apoplast, making the [H+] there high. d) the sucrose pump uses the chemiosmotic potential of the [H+] of the apoplast to co-transport sucrose into the companion cell from the apoplast |
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What moves phloem sap to the sugar sink?
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The bulk flow within the phloem:
a) once the sucrose at the source is loaded in the phloem symplast, the ψ at that point in the phloem is low b) water will move into the area of low ψ from outside the phloem c) that increases the ψp inside the phloem and water will start to flow away from that area towards an area of lower ψ (=bulk flow or pressure flow) d) once the sugar in the phloem sap reaches the sugar sink, it must be unloaded. |
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What happens once the sugar in the phloem sap reaches the sugar sink?
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It must be unloaded:
(a) sucrose is unloaded by the sucrose-H+ transport protein and ATPase to sink cells (b) now ψ is lower outside the phloem (in the sink cells) (c) so there is bulk flow of water through osmosis out of the phloem to xylem and sink (d) once there is increased water in the xylem bulk flow will start there as well and the water will be moved back towards the sugar source via the xylem |
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Whether an organ is a sugar sink or source can change as.... ?
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As demand for C changes.
The demands may vary from season to season and even day to night in some cases. |
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How is C stored and then mobilized?
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1) C is stored as starch (glucose polymer) in granules in the chlpst of leaves or in plastids of parenchyma cells of storage roots or stems.
2) enzymes are utilized to either polymerized glucose into starch when sugar is plentiful OR 3) other enzymes depolymerize starch into glucose and then glucose is loaded into the phloem for transport to wherever the glucose is required |
