Topic 9 - Plant Biology Flashcards
cuticle
- waxy outermost layer of a leaf
- protects it against water loss and insect invasion
palisade mesophyll
- located in upper portion of leaf (where light is most available)
- chloroplast-rich to allow max photosynthesis
vascular bundles
- the “veins” of a plant
- contains the xylem and phloem tubes
- distributed throughout the leaf to transport raw materials and products of photosynthesis
- occur roughly in the middle of the leaf so they’re near all the leaf cells
spongy mesophyll
- contains many air spaces for gas exchange
- located just superior to the stomata
- to allow continuous channels for gas exchange
stomatal pores
- on the bottom surface of the leaf
- receives less light so the temp is lower than on the upper surface
- lower temp minimizes water loss from the pores and the plant
- so the lower epidermis usually has a thinner cuticle than the upper epidermis
- positioning of the epidermis allows the remaining structures of the leaf to be protected and supported
xylem
- supports the plant
- also specialized water-conducting tissue of terrestrial plants
- composed of multiple cell types
- their primary walls include pits or pores to allow water to move laterally
types of xylem cells
- tracheids
- vessel elements
vessel elements
- most important xylem cells
- dead cells with thick, lignified secondary walls
- these secondary walls are interrupted by primary walls that contain pores to allow water movement
- vessel elements are attached end to end to form continuous columns, like the tracheids
- ends of vessel elements have pores to allow water to move freely up the plant
tracheids
- dead cells that taper at the ends
- they connect to one another to form a continuous column
evolution of xylem tube
- ancient plants only had tracheids
- modern flowering plants only have vessel elements
- vessel elements are more effective at their function
why stomatas must open and close
- stomata can only be closed on a short-term basis
- as CO2 must enter the mesophyll region so photosynthesis can occur
- changes in the turgor pressure of the guard cells that surrounds the stomata can affect whether they open and close
how stomatas open and close
- guard cells are cylindrical and their cell wall thickness is uneven
- thickened area of the guard cell wall is oriented
towards the stoma - when guard cells take in water and swell, they bulge more to the outside, opening the stoma
- when the guard cells lose water, they sag towards each other and close the stoma
what affects guard cell activity?
- transport of potassium ions cause guard cells to open
- blue light triggers the activity of ATP-powered proton pumps in the plasma membrane of guard cells
- thus triggering active transport of K+ into cell
- higher solute concentration within the guard cells causes inward water movement by osmosis
- when potassium ions passively leave the cells, water also leaves
- abscisic acid (hormone) can cause guard cells to close
- they cause potassium ions to diffuse rapidly out of the guard cells
- this hormone is produced in the roots during times of water deficiency
- other factors include CO2 levels and even circadian rhythm
cohesion-tension theory
- transpiration occurs bc water moves down a concentration gradient: from leaf air spaces (high) to atmosphere (low)
- water lost via transpiration is replaced by water from vessels due to concentration gradient
- vessel water column is maintained by cohesion (H bonds between H2O molecules) and adhesion (H bonds between H2O and vessel walls)
- continuous tension in column due to continuous transpiration –> replacement cycles
- water is pulled from root cortex into xylem cells and from soil into root due to the tension
adaptations of roots for their function
- the main function of roots is to provide mineral ion and water uptake for the plant
- roots are efficient due to an extensive branching pattern with specialized epidermal structures (root hairs)
- root hairs increase SA so absorption can occur 3x more efficiently
importance of root cap
it protects the apical meristem during primary growth of root through the soil
types of root zones
- zone of cell division (M phase of cell cycle): new undifferentiated cells form
- zone of elongation (G1 phase): cells enlarge in size
- zone of maturation: when the cells become fully functional parts of the plant
movement of water into a plant
- moves from soil to root hairs via osmosis
- because root hairs have a higher solute concentration
- water then moves to the vascular cylinder (which contains xylem and phloem tubes)
movement of mineral ions into a plant
- mineral ions diffuse in
- when there’s a higher conc. of mineral solutes in water outside the root, it may diffuse in
- some may also move via osmosis
bulk flow
AKA mass flow
- passive movement of water and its mineral ion solutes
active transport of minerals into a plant
- occurs if solute conc in plant is higher
- or if the ion can’t cross the lipid bilayer
- results in hypertonic state, also increasing amount of water absorbed
factors affecting plant transpiration
- light
- humidity
- wind: humid air near the stomata is carried away so it increases transpiration
- temperature
- soil water: if soil is water-deficient, turgor loss occurs
- CO2 conc.: causes guard cells to lose turgor, so stomata closes
succulents
plants that store water to survive
xerophytes
plants adapted to live in arid climates
xerophyte adaptations to arid conditions
- Small, thick leaves with decreased SA
- reduced no of stomata
- stomata located in crypts/pits on leaf surface (to cause high humidity around stomata)
- thickened, waxy cuticle (impenetrable barrier to water)
- Hair-like cells on leaf surface trap a layer of water vapour (maintains higher humidity near stomata)
- desert plants shed their leaves and/or become dormant in the driest months
- succulents can store water in their fleshy, watery stems
- xerophytes can use alternative photosynthetic processes
- can close stomata with guard cells
types of photosynthesis
- C3 photosynthetic pathway (most common)
- CAM photosynthesis
- C4 photosynthesis
CAM photosynthesis (brief)
CAM plants close their stomata during the day and store CO2 at night
C4 photosynthesis
C4 plants open their stomata during the day but take in CO2 far quicker than other plants
halophytes
plants adapted to grow in water with high salinity levels
halophyte adaptations to saline conditions
- many are succulents and store water (thus diluting the salt concs)
- some species (e.g. mangrove) secrete salt through salt glands
- some species compartmentalize Na+ and Cl– in cell vacuoles to prevent NaCl toxicity
- sunken stomata (higher humidity around stomata)
- thickened leaves with developed cuticle to minimize water loss
- reduced leaf SA
- can close stomata with guard cells
types of phloem cells
- sieve elements
- companion cells
sieve elements
- connected by sieve plates
- forms sieve tubes
- sieve plates have pores to allow movement of water and dissolved organic molecules throughout the plant
translocation
movement of organic molecules in plants
organic molecules in phloem sap
- sugars
- amino acids
- plant hormones
- small RNA molecules (possibly to aid communication)
pressure-flow hypothesis for movement of phloem sap
- sugar loads into sieve tube at the source (leaf cells) via active transport
- reduces the relative water conc. in the sieve tube members, causing osmosis from the surrounding cells
- water uptake causes hydrostatic pressure, resulting in bulk flow of phloem sap.
- hydrostatic pressure is diminished by removal of sugar (active transport) from the sieve tube at the sink (destination)
- the sugars are changed at the sink (storage cells in roots) to starch
- as starch is insoluble, it exerts no osmotic effect
- xylem then recycles the relatively pure water by carrying it from the sink back to the source
basic types of plant tissue
- dermal tissue
- ground tissue
- vascular tissue
dermal tissue
- outer covering
- protects against physical agents and pathogenic organisms
- prevents water loss
- may have specialized structures for various purposes
ground tissue
- thin-walled cells
- storage
- plays a role in photosynthesis
- helps support the plant
- secretes substances
vascular tissue
- made up of xylem and phloem
- carry out long-distance conduction of water, minerals, and nutrients within the plant
- provide support.
meristematic tissue
- the base of all 3 types of plant tissue
- aggregates of small cells
- basically stem cells but for plants
- when dividing, one cell remains meristematic while the other joins the plant body
- thus the population of meristematic cells is continually renewed
initials
meristematic cells that remain meristematic after division
derivatives
meristematic cells that differentiate after division
determinate growth
growth ceases after a certain size
e.g. animals exhibit determinate growth
indeterminate growth
continuous growth throughout the organism’s lifespan
e.g. plants exhibit indeterminate growth
apical meristem
AKA primary meristem, shoot apex
- occur at tips of roots and stems
- produces new tissue and causes primary growth via mitosis and cell division
- results in herbaceous, non-woody stems and roots
lateral meristem
AKA secondary meristem
- allow growth in thickness
there are 2 types of lateral meristems:
- vascular cambium
- cork cambium
vascular cambium
- type of lateral meristem
- produces secondary vascular tissue (i.e. secondary xylem and secondary phloem)
- lies between xylem and phloem in vascular bundle
cork cambium
- type of lateral meristem
- occurs within the bark of a plant and produces the cork cells of the outer bark
target cell
- cells on which a hormone has an effect
- they have specific receptors in their plasma membrane/cytoplasm/nucleus
tropism
- growth or movement to directional external stimuli
- can be positive (towards stimuli) or negative
phototropism in plants
- plant stems exhibit positive phototropism
- plant roots exhibit negative phototropism
importance of phototropism
- plants need sunlight to carry out light-dependent photosynthetic reactions
- if an area is crowded or dark, it’s important for seedlings to grow towards the sunlight
auxin
- plant hormone causing positive phototropism
- found in seed embryos, apical meristems, and young leaves
- works by redistributing itself away from light stimuli (NOT from increased production on one side), in order to stimulate growth on the side away from the light source
- their effect: increasing the flexibility of plant cell walls, enabling cell elongation
- also affects cell growth by changing the pattern of gene
expression, usually by interacting with a repressor of a particular gene
Example of auxin: indoleactic acid (IAA)
how does auxin work?
- auxin efflux pumps move auxins using ATP into nuclei of cells away from light
- auxin and receptor in nuclei activates a proton pump
- proton pump moves H+ into the spaces of the cell wall
- H+ ions cause drop in pH
- pH change breaks hydrogen bonds between cellulose fibres of cell wall
- result: greater elongation of cells on the stem side away from the light and, therefore, curvature towards the light source
auxin influx
- movement of auxin into a cell
- the auxin efflux pumps don’t directly move auxins into the nuclei
- rather, the pumping action creates high conc of auxin in space between cells (on the side away from light source!!)
- this results in high conc of auxin in intercellular space
- thus relatively low concs in adjacent cells
- auxins diffuse down conc gradient into cell nuclei
other functions of auxin
- stimulation of cell division in meristematic tissue
- differentiation of xylem and phloem
- development of lateral roots
- suppression of lateral bud growth (if present in apical bud)
- stimulation of growth of flower parts
- induction of food production without pollination
angiospermophyte
a flowering plant
types of angiospermophytes
- monocotyledonous plants
- dicotyledonous plants
monocots vs dicots
MONOCOTS vs DICOTS
- leaves: parallel venation vs netlike venation
- petals: multiples of 3 vs multiples of 4/5
- seeds: 1 seed leaf (cotyledon) vs 2 seed leaves
- vascular bundles: arranged throughout stem vs arranged as a ring in stem
- root system: fibrous vs taproot
- pollen grain: 1 opening vs 3 openings
flower parts
- sepals
- petals
- stamen (anther + filament)
- carpel (stigma + style + ovary)
sepal
protects developing flower bud
petals
colorful to attract pollinators
anther
- part of stamen
- produces male sex cells
filament
- stalk of stamen
- holds up anther
stigma
- sticky top of the carpel
- pollen lands here
style
- support structure of carpel
- supports stigma
ovary
- base of carpel
- develops female sex cells
types of flowers
- complete flower - have all 4 basics
- incomplete flower - lack 1 or more of the basics
- staminate flower - only stamens, no carpels
- carpellate flowers - only carpels, no stamens
generations in a plant life cycle
- gametophyte generation (haploid)
- sporophyte generation (diploid)
plants alternate between these two over their lifetime
gametophyte generation
- produces plant gametes by mitosis
- flowering period of plant life cycle
- sexual reproduction
sporophyte generation
- produces spores by meiosis
- asexual reproduction
pollination
process in which male sex cells (pollen) are placed on the female stigma
adaptations of flowers to attract insect/animal vectors
- red flowers: more conspicuous to birds
- yellow/orange flowers: more conspicuous to bees
- heavily scented flowers: can be easily located at night
process of fertilization
- stigma is covered by a sticky, sugary substance
- when the pollen grain adheres to the stigma it germinates to grow a pollen tube
- pollen tube grows down the style of the carpel.
- within the growing pollen tube is the nucleus that will produce the sperm
- pollen tube enters an opening at the bottom of the ovary
- sperm moves from the tube to combine with the egg of the ovule to form a zygote
- when zygote is formed it develops with the surrounding tissue into the seed
- as the seed develops, the ovary around the ovule matures into a fruit
- the fruit encloses and helps to protect the seed
parts of seeds
- testa
- cotyledons
- micropyle
- embryo root
- embryo shoot
testa
tough, protective outer coat
cotyledon
- seed leaves
- nutrient storage structures
micropyle
- scar at the opening
- where the pollen tube enters the ovule
embryo root/shoot
becomes the new plant when germination occurs
factors needed for seed germination
- water: to rehydrate seed
- oxygen: for cell respiration
- warmth: for enzyme activity
plant types (according to flowering)
- long-day plants
- short-day plants
- day-neutral plants
long-day plants
- flowers in midsummer
- when days are longest and nights are shortest
short-day plants
- flowers in spring, late summer, and autumn
- when days are shorter
day-neutral plants
- flowers without regard to day length
phytochrome
- special blue-green pigment in plants
- controls flowering by activating (to Pfr) upon being exposed to red light (660 nm)
- Pfr converts back to Pr (inactive form) upon being exposed to far-red light (730 nm)
- Pfr can rapidly convert back in daylight, but conversion takes longer in darkness
- this slow conversion allows the plant to time the dark period and therefore control flowering
- effect of activation depends on the plant
how do short-day plants regulate flowering?
Pfr inhibits flowering
how do long-day plants regulate flowering?
Pfr promotes flowering
how does phytochrome stimulate flowering?
- generally the promoting form of P stimulates flowering by activating specific genes of shoot apex cells
- activation results in changes in DNA transcription (gene expression), thus allowing the production of flowers
adaptations of phloem cells
- companion cells have a lot of mitochondria to fulfill the energy requirements of both the companion cell and the sieve tube elements
- infolding to increase SA, allowing more phloem loading of substances from the source
- larger plasmodesmata (channel between two cell walls allowing them to communicate)
- rigid cell wall to maintain turgor
- sieve between sieve tube cells to prevent continuous flow if the phloem tube is ruptured at some point (e.g. due to an animal)
