topic 9 Flashcards
what is transpiration an inevitable consequence of and why?
gas exchange in the leaf; if the stomata allow co2 to be absorbed, they must let water vapour escape
define transpiration
the loss of water vapour from the leaves and stems of plants
give the function of guard cells
- to minimise water loss
- guard cells control the aperture of the stoma
describe xylem vessels
- long continuous tubes
- walls thickened and impregnated with lignin (polymer)
- formed from files of non-living cells, arranged end-to-end
how does lignin help the xylem
lignin strengthens the xylem walls, so that they can withstand very low pressures without collapsing
the pressure inside xylem vessels is usually much ——- than atmospheric pressure but…
lower; the rigid structure prevents the vessels from collapsing
where would the xylem be in a vascular bundle
on the inside, next to the pith
why are the cohesive and adhesive properties of water important in transpiration?
- O atom in one molecule attracts H atom in other (due to polarity)= cohesion
- H2O attracted to hydrophilic part of cell walls of the xylem=adhesion
-> water can be pulled up from the xylem in a continuous stream
what maintains the transpiration stream?
tension in leaf cell walls
what generates tension forces in leaf cell walls?
the adhesive property of water and evaporation
describe how tension acts in transpiration
water evaporates:
- adhesion causes water to be drawn through the cell wall from the xylem
- this reduces xylem pressure, which generates a pulling force
adaptations of xerophytes (6)
- reduced/rolled leaves
- thicker wait cuticle
- stomata in pits with hairs (traps water vapour, making surroundings more humid)
- utilise CAM physiology
- lower growth to ground so less exposed to environmental conditions
- shallow roots
Really tired tudents may use loads of energy sources
CAM physiology
- stomata only open at night
- CO2 taken in at night is stored as magic acid, and this is done via the C4 pathway
what are halophytes
plants that live in saline soils
adaptations of halophytes (7)
- leaves reduced to small scaly structures/spines
- leaves shed when water scarce (stem can photosynthesise)
- water storage structures develop in leaves
- thick cuticle + multiple layer epidermis
- sunken stomata
- long roots
- structures for removing salt buildup
Little little water tickles spraying little sickles
how is water absorbed into the root cells?
by osmosis
why does water move into root cells via osmosis?
solute concentration inside root cells is greater than in the soil
describe how mineral ions move into the root cells
by active transport, using protein pumps in the plasma membranes of root cells.
symplastic pathway
through cytoplasm
apoplastic pathway
through cell wall
draw the xylem
pg 403
describe the structure of the phloem (4)
- porous sieve tube plate
- companion cells (folded); undertake metabolic and genetic functions for the sieve cell; many mitochondria
- sieve tube member cell; specialised cell (no organelles)
- plasmodesmata; gaps in cell wall for apoplast pathway
why are companion cells folded?
to give a large SA for the transport of sucrose
describe movement in the phloem
both ways, contrasts with the xylem
state the function of the phloem
to move around products of photosynthesis from sources to sinks
give 4 examples of sources
photosynthetic tissues;
- mature green leaves
- green stems
storage organs unloading their stores:
- storage tissues in germinating seeds
- tap roots of tubers at the start of the growth season
give 2 examples of sinks
roots growing/absorbing mineral ions
parts of the plant with developing food stores (developing fruits, seeds, leaves, tap roots/tubers)§
why is sucrose transported rather than glucose
- it is a non-reducing sugar so will have no immediate reactions with other molecules; glucose is quite reactive so would be taken up by cells on route
- osmotic effect
- sucrose is more of an efficient energy story (disaccharide more ee than monosaccharide)
describe translocation
- Sucrose can be loaded into the phloem at sources such as leaves via the symplastic or apoplastic pathway.
- In the apoplastic pathway, sucrose is co-transported into companion cells along with hydrogen ions. The concentration of sucrose increases in the companion cells so it diffuses into sieve tubes from the companion cells, through plasmodesmata.
- In the symplastic pathway, sucrose diffuses from cytoplasm to cytoplasm of neighbouring cells.
- The high solute concentration causes water to move, by osmosis, into the sieve tubes from xylem vessels.
- The flow of water into the sieve tubes increases the hydrostatic pressure which causes water to flow to areas of lower pressure, such as sinks. Sinks are areas of the plant which don’t synthesise organic molecules, such as the roots.
- At sinks, sucrose is unloaded from the sieve tubes, which decreases the solute concentration.
- This leads to water moving back into the xylem vessels by osmosis and as a result, the hydrostatic pressure in the sieve tubes decreases.
describe how sucrose is moved into/out of different cells
sucrose is moved from cells to companion cells and sieve tube elements by diffusion along a concentration gradient, but is moved into companion cells and sieve tube elements by an active process
how does phloem loading occur
via the apoplastic pathway
state the evidence that backs up the ideas of translocation
- companion cells have membrane folding for a large SA for transport of sucrose
- also have many mitochondria to release the ATP needed for active transport
- if these mitochondria are poisoned, translocation in the phloem stops
- the pH of the companion cells is higher than the surrounding cells, supporting the idea of a hydrogen pump
- the flow of sugars in the phloem is about 10000 times faster than it would be by diffusion alone, suggesting an active process is driving the mass flow
Why are sieve elements are unable to sustain independent metabolic activity without the support of a companion cell?
This is because the sieve element cells have no nuclei and fewer organelles (to maximise flow rate)
State how the structure of phloem sieve elements and companion cells is adapted to their relationship
Plasmodesmata exist between sieve elements and companion cells in relatively large numbers
These connect the cytoplasm of the two cells and mediate the symplastic exchange of metabolites
look at xylem and phloem images of stem and root
describe the type of growth that plants undergo
indeterminate growth
what enables the indeterminate growth of plants?
undifferentiated cells in the meristems of plants
what phenomenon sets plant cells apart from most animals?
many plant cells, including some fully differentiated types, have the capacity to generate whole plants (ie the cells are totipotent)
describe plant meristems
primary meristems are found at the tips of stems and roots (apical meristems)
many dicotyledonous plants also develop lateral meristems
what is the role of mitosis in plant growth?
mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves
describe the method by which the shoot apical meristem aids growth
With each division one cell remains in the meristem while the other increases in size and differentiates as it is pushed away from the meristem region
each apical meristem can give rise to additional meristems including
protoderm (gives rise to epidermis), procambium (gives rise to vascular tissue), ground meristem (gives rise to pith)
what is the function of plant hormones?
they control growth in the shoot apex
what are auxins?
plant hormones that initiate growth of roots, influence development of fruits and regulate leaf development
what is the most abundant auxin? and what is its role?
indole-3-acetic acid (IAA); control of growth in the shoot apex.
When produced by the shoot apical meristem, it promotes growth in the shoot apex via cell elongation and division
prevents/inhibits growth in lateral (axillary) buds - apical dominance
describe the sites of production and use of IAA
synthesised in the apical meristem of the shoot and transported down the stem to stimulate growth
give 2 additional examples of plant growth promoters
Gibberellins and Cytokinins
describe apical dominance
- as the shoot apical meristem grows and forms leaves, regions of meristem are left behind at the node
- growth here is inhibited by auxin produced by the shoot apical meristem
- the further distant a node is from the apical meristem, the lower the concentration of auxin and the less likely that growth in the auxiliary bud will be inhibited by auxin
what is the role of cytokinins?
produced in the root and promote auxiliary bud growth. the relative ratio of cytokinins:auxins determine whether the auxiliary bud will develop
how do plants respond to their environment?
by tropisms (directional growth responses to directional external stimuli_
define phototropism
growth towards light
define gravitropism
growth in response to gravitational force
how does light influence patterns of gene expression?
- when light falls on a plant, light energy is absorbed by phototropins (photoreceptor proteins), causing a change in their 3D shape
- the altered shape of the phototropins allows them to bind to receptors inside the cell, affecting the expression of certain genes
- the affected genes could code for glycoproteins called PIN3 proteins, thought to be involved with the transport of auxin from cell to cell
describe phototropism
- phototropins in the tip detect a greater intensity of light on one side of the stem than the other
- IAA is transported laterally from the light to the shaded side due to changed expression of PIN3 proteins
- higher concentrations of IAA on the shaded side cause greater growth on this side, so the stem grows in a curve towards the source of the brighter light
in the root, auxin —— shoot elongation
inhibits
describe gravitropism
- when a root is placed on its side, auxin will accumulate on the lower side of the plant in response to the force of gravity
- higher concentrations of auxin inhibit root cell elongation so the top cells elongate at a higher rate than the bottom cells, causing the root to bend downward
auxin efflux pumps can
set up concentration gradients of auxin in plant tissue
how does gravity cause accumulation of auxins at the bottom?
- cellular organelles called statoliths accumulate on the lower side of cells
- this leads to distribution of PIN3 transporter proteins to the bottom of the cell
describe how micropropagation is carried out
- Tissues from the stock plant are sterilized and cut into pieces called explants (for most applications, the least differentiated tissue serves as the source tissue such as a meristem).
- The explant is placed into sterilized growth media that includes plant hormones.
- Inclusion of equal proportions of auxin and cytokinin into the media leads to the formation of an undifferentiated mass called a callus. If the growth media contains a ratio of auxin that is greater than ten times the amount of cytokinin, then this is called rooting media and roots develop. If the ratio of auxin to cytokinin is less than 10:1, then this is called shoot media and shoots develop.
- Once roots and shoots are developed, the cloned plant can be transferred to soil.
define micropropagation
When plant tissues are cultured in the laboratory (in vitro) in order to reproduce asexually
state and explain 3 uses of micropropagation
- rapid bulking up of new varieties
- production of virus-free strains of existing varieties
- propagation of orchids and other rare species with desired characteristics
multiplication phase of micropropagation
The growing shoots can be continuously divided and separated to form new samples
why is the apical meristem free of viruses
viruses are transported within a plant from cell to cell through vascular tissue and via plasmodesmata
describe the relationship between flowering and gene expression
flowering involves a change in gene expression in the shoot apex
what is the change involved in flowering?
a trigger causes the shift from vegetative phase (young plant) -> reproductive phase, when meristems in the shoot start to produce parts of flowers instead of leaves
the switch to flowering in many plants is a response to
the length of light and dark periods in many plants (where length of darkness is what matters)
long day plants
flower in summer when the nights have become short enough
short day plants
flower in the autumn when the nights have become long enough
what is used to measure the length of dark periods in plants? and how?
phytochrome (pigment) that can switch between two forms, Pr and Pfr
- when Pr absorbs red light of wavelength 660nm it is converted into Pfr
- when Pfr absorbs far-red light of wavelength 730nm it is converted into Pr (this is not of great importance as sunlight contains more light of wavelength 660nm so in normal sunlight phytochrome is rapidly converted to Pfr)
- Pr is more stable so in darkness Pfr very gradually changes into Pr
which is the active form of phytochrome and why?
Pfr; receptor proteins are present in the cytoplasm to which Pfr but not Pr binds
in long day plants
large amounts of Pfr remain at the end of short nights and bind to the receptor, which then promotes transcription of genes needed for flowering
in short day plants
Pfr inhibits transcription of the flowering time gene (FT gene), preventing flowering; so flowering requires low levels of Pfr (resulting from long nights)
methods used to induce short-day plants to flower out of season
providing additional light in the middle of the night
draw and label a diagram of an animal pollinated flower
give the functions of
- nectar-secreting glands
- petals
- sepals
- anthers
- filaments
- carpel (stigma, style, ovary)
- nectar-secreting glands attract insects, especially bees
- petals are large and colourful, helping insects to find the flower.
- sepals protect the flower bud during its development and at night when buds close.
- anthers produce pollen, containing the male gametes.
- filaments hold the anthers in a position where they are likely to brush pollen onto visiting insects
- female part of the flower is called a carpel. It consists of a stigma, style and ovary. The stigma is sticky and will capture pollen from the visiting insect. The stigma is held up by the style. The ovary is located inside a small rounded structure called an ovule.
why do most flowering plants use mutualistic relationships with pollinators in sexual reproduction? what does this involve?
sexual reproduction in flowering plants depends on the transfer of pollen from the stamen to a stigma of another plant
pollinators gain food in the form of nectar; plant gains a means to transfer pollen to another plant.
describe fertilisation in plants
- from each pollen grain on the stigma a tube grows down the style to the ovary
- the pollen tube carries male gametes to fertilise the ovary, which is located inside a small rounded structure called an ovule
- the fertilised ovule develops into a seed and the ovary develops into a fruit
give 2 advantages of seed dispersal
- reduces competition between offspring and parent
- helps to spread the species
draw and label the internal and external structures of seeds
explain the conditions needed for germination
Oxygen – for aerobic respiration (the seed requires large amounts of ATP in order to develop)
Water – to metabolically activate the seed (triggers the synthesis of gibberellin, which stimulates mitosis and cell division)
Temperature and pH – for optimal function of enzymes
adaptations of phloem (sieve tube)
- sieve plate with sieve pores - allows for continuous movement
- cellulose cell wall - strengthens wall to withstand hydrostatic pressure
- no nucleus, vacuole or ribosome - maximise space for translocation
- thin cytoplasm - reduces friction to facilitate movement
adaptations of phloem (companion cells)
- nucleus and other organelles present - provides metabolic and genetic support to sieve tube elements, helping w loading and unloading
- transport proteins in plasma membrane - moves assimilates into and out of sieve tube elements
- large numbers of mitochondria (provide ATP for active transport)
- plasmodesmata - allows organic compounds to move from companion cells to sieve tube elements
explain what the Rf values represent in chromatography
equation
- depends on relative solubility of the pigment
- each pigment has a specific Rf value
- can be used to identify different pigments
give 3 examples of photosynthetic pigments
chlorophyll, carotene, xanthophyll
explain how aphid stylets can be used to study the movement of solutes in plant tissues
a. aphids tap into phloem with their stylets «to use sap as a food source»
b. plants grown in radioactive CO2/14CO2 incorporate it into carbohydrate
c. phloem contents/sap/fluid flows through the stylet
d. aphid body severed/cut from stylet «after stylet inserted into phloem»
e. analyze «sap/fluid exuded from stylet» for solutes/carbohydrates
OR
radioactive-labelled carbon can be detected «in the phloem sap»
f. stylets at different parts of the plant can show sequence/rate of movement
explain the effects of auxin on plant cells
a. increases cell elongation/growth/enlargement OR has effect on rate of mitosis
b. changes the pattern of gene expression
promotes transcription of some genes
c. changes the pH of the extracellular environment/cell wall and
increases activity of proton pumps
d. breaks cross links/connections between cellulose fibres in cell wall
e. increases cell wall plasticity
f. «varying» auxin concentrations have different effects in different parts of
the plant
