stpm biology chapter 7

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    THE STOMATA

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    Leaves

    The exchange of oxygen and carbon dioxide in the leaf occurs

    through pores called stomata(singular = stoma).

    Normally stomata open when the light strikes the leaf in the

    morning and close during the night.

    The immediate cause is a change in the turgorof the guard cells.

    The inner wall of each guard cell is thick and elastic. When turgordevelops within the two guard cells flanking each stoma the thin

    outer walls bulge out and force the inner walls into a crescent

    shape. This opens the stoma. When the guard cells lose turgor

    the elastic inner walls regain their original shape and the stoma

    closes.

    The table shows the osmotic pressure measured at different times

    of day in typical guard cells. The osmotic pressure within the other

    cells of the lower epidermis remained constant at !"# lb$in%. When

    the osmotic pressure of the guard cells became greater than thatof the surrounding cells the stomata opened. &n the evening when

    http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/D/Diffusion.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/D/Diffusion.html
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    The immediate cause is a change in the turgorof the guard cells. The inner

    wall of each guard cell is thick and elastic. When turgor develops within the

    two guard cells flanking each stoma the thin outer walls bulge out and force

    the inner walls into a crescent shape. This opens the stoma. When the guardcells lose turgor the elastic inner walls regain their original shape and the

    stoma closes.

    The table shows the osmotic pressure measured at different times of day in

    typical guard cells. The osmotic pressure within the other cells of the lower

    epidermis remained constant at !"# lb$in%. When the osmotic pressure of the

    guard cells became greater than that of the surrounding cells the stomataopened. &n the evening when the osmotic pressure of the guard cells

    dropped to nearly that of the surrounding cells the stomata closed.

    http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/D/Diffusion.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/D/Diffusion.html
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    STOMATA '

    Is a pore on the epidermis of the leaves and

    stems of plants which can open and close

    FUNCTIONS

    !" #na$le gases e%change $etween leave cells

    and atmospheric

    &" 'egulate and control water loss from plants

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    T(# ST'UCTU'# OF STOMATA

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    The stomata)

    Is a tin* openings

    in the epidermis of

    a plant leaf)

    are surrounded

    $* a pairs of guard

    cells"

    The guard cellsopen and close the

    stoma $* changing

    shape

    STOMATA

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    T(# ST'UCTU'# OF STOMATA

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    T *T+,-T,+ / 0 *T10 T02

    ( Stoma are found on epidermal layer of leaves, stems and part of flowers of floweringplant )

    *tomata is bounded by a pair of guard cellswhich are

    kidney shape in dicot and dumbell3shaped in monocot

    leaves. 4uard cells (gc) is a specialised epidermal cellwhich

    contain chloroplasts forphotosynthesis.

    The cellulose cell walls of gc that surround the stoma

    (inner cell wall) are thic+er and less elasticthan those in

    contact with epidermis ( outer cell wall).

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    The different elasticity of the cellulose cell wall is

    contributed by the microfibril cellulose that surround

    the wall of the cell. The two guard cells are fused at their end allowing

    the cell to bend as they expand.

    The diff. causes the outer wall elongates morethan the inner wall when the cell is in turgor

    pressure.

    The presence of chloroplast enables the gc to

    photosynthesise

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    /or gaseous exchange

    35uring the day -%diffuse into the leaves for

    photosinthesis and %produces fromphotusinthesis diffuses out.

    35uring night %diffuse into the leaves for

    respiration and -%diffuse out.

    *tomata regulate the rate of photosynthesis based

    on large number of internal and external signals.

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    0llow transpiration to cooling leaves

    35uring transpiration water vapour diffuse out

    through stomata due to water vapourconcentration gradient.(water vapour

    concentration higher in spaces of leaves and

    lower in atmosphere)

    35iffusion of water vapour create a transpiration pull

    in the xylem so that water in transported up to the

    leaves for photosynthesis.

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    +educe exessive loss of water when pore close

    35one physically as when water is lost from the

    guard cells the cells become flaccid.The stomais automatically closed

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    There are two hypothesis

    !. The starch3sugar hypothesis

    %.The potassium ion accumulation hypothesis

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    STA'C( ,SU-A' (./OT(#SIS

    0t a day time

    the guard cell undergoes photosynthesis due to

    the presence of chloroplast in the gc

    sugarsare formed

    the accumulation of sugar increase the

    osmotic concentrationof gc water potential in gc

    becomes lower than the water potential in

    neighbouring epidemal cells

    5ifference in water potential causes the

    movement of water molecules from epidermal cells

    into guard cells by osmosis"

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    The entrance of water causes the gc to

    become swell and turgid

    Turgor pressure increases the thinnerouter cellulose cell walls elongate more

    than the inner thicker walls

    The gc $end outwardand stoma opens"

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    At night)

    4uard cells do not go through photosynthesis

    There is no sugarproduced in guard cells sugar

    converted to starch reduces osmotic

    concentration in guard cells

    Water potential in guard cells become higher

    than the w.p in epidermis cells

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    Water potential in guard cells become higherthan

    the w.p in epidermis cells

    Water molecule diffuses outof gc into

    neighbouring epidermis cells

    4c becomesflaccid because of no turgor

    pressure.

    -losing the stoma

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    /OTASSIUM IONS 0 123 ACCUMULATION

    (./OT(#SIS

    ( more recent hypothesis)

    6otassium ions from neighbouring epidermis cells

    actively transported to the guard cells

    nergy needed is supplied by 0T6 generates

    during phosphorylation of the light reaction of

    photosynthesis

    0T6 ( from photophosphorylation) is use to drive

    theproton-pumpin the cell membrane of gc

    activate by the light

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    44""/OTASSIUM IONS 0 123 ACCUMULATION (./OT(#SIS

    ( more recent hypothesis)

    0s protons (7) are pumped outof the gc the inner

    gc becomes more negatively charged. This activate

    potassium channels to pumped inpotassium ions 87

    into gc from epidermis cells. -l

    9

    ions then enter tobalance the charge

    The accumulation of 87$ ( malate ions in some

    plants ) causes the water potential of gc more negative

    than that in epidermis cells.

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    The gc becomes turgid)

    Turgor pressure increased thinner outer

    cellulose cell walls expand morethan theinner wall

    4uard cells bend outwardand the stoma

    opens

    Water from epidermis cells enters by osmosis into

    the guard cells

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    6roton pump

    6otassium channel

    7 pumped out of the guard cells

    lowest the electrochemical gradient

    7ion

    (2

    1

    Stoma opens

    87channels activated 87

    ions pumped in

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