IB Bio: Plant Biology

Xylem

Xylem

Tracheids and vessel elements that transport water and minerals from roots to leaves, in vascular bundles

Transpiration

Loss of water vapour from stems/leaves, light energy turns water into vapour which exits through the stoma, a consequence of gas exchange in the leaf

Transpiration stream

Movement of water from roots to leaves, created by transpirational pull, results from cohesion and adhesion

Factors affecting transpiration

Heat ( +temp = +transpiration), Light (limits photosynthesis), wind (drying of atmosphere affects concentration gradient)

water evaporates in _______ of the leaf

spongy parenchyma (when light energy transformed into heat energy)

Transpiration pull

• water diffusing out of stomata creates a negative pressure gradient

• new water absorbed at roods creates a pressure differential (high in roots low in leaves)

Gaurd cells function

guard cells become turgid or flaccid to open or close the stomata

abscisic acid hormone

triggers potassium efflux from guard cells, lessens water pressure, traps water in leaf, released by dehydrated mesophyll cells

ABA released by…

dehydrated mesophyll cells

describe xylem cells

dead cells with no protoplasm, therefore, tube-like. Cell walls stiffened with lignin (necessary to withstand negative pressure; usually lower pressure in xylem than atmosphere) and thickened cellulose

Pits

pores in xylem/vessel elements for water transfer between cells

roots need a large surface area because…

they uptake water and minerals from the soil

Fibrous roots

highly branched therefore large surface area for absorption

Tap roots

one larger root with lateral roots, penetrates deep into the soil to access reservoirs

Root hairs

small extensions on epidermis of roots, increase SA:V

Describe diffusion of water into the root

water diffuses into the root hair by osmosis (follows the active transport of mineral ions from soil into root), across the cortex, then moves across the Casparian strip of the endodermis through plasmodesmata

Casparian strip

in the endodermis, impermeable to the passive flow of water and ions

Cuticle (leaf tissue)

Traps moisture inside leaf

Epidermis (leaf tissue)

Provides protection for the leaf

Palisade mesophyll

Tightly packed cells which increases SA, contains many chloroplasts, a major site of photosynthesis

Spongy mesophyll

loosely packed tissue, water vapour binds to it, major site of gas exchange

Vascular bundles

transport tissue containing xylem, phloem, and vascular cambium (undifferentiated)

Stomata

Pores through which water, CO2 and O2 pass

Monocots vs Dicots comparison

single cotyledon vs double cotyledon

long narrow leaf vs broad leaf

parallel veins vs network of veins

scattered vascular bundles vs ring of vascular bundles

floral parts x3s vs floral parts x4s or 5s

Angiosperms vs Gymnosperms comparison

flower/fruit producing vs no flower/fruit

seeds encased in fruit vs unisexual naked seeds (cones) on leaf surface

Meristem

undifferentiated plant tissue capable of indeterminate growth

apical meristem

perform primary/apical growth at roots and shoot tip (can become new flowers and leaves)

lateral meristem

perform secondary growth at cambium

apical growth

primary growth at roots and shoots tips due to cell enlargement (mitosis) and cell division (cytokinesis)

nodes

sections of apical growth in nodes

inactive auxilary buds

formed from apical meristem in stems not used for nodes, creates new shoots, including leaves and flowers

shoot apex

apical meristem and surrounding tissue

lateral growth

increases width at cambium, most active in woody or shrub plants, growth rates are not constant

vascular cambium

gives rise to secondary xylem and phloem

cork cambium

contributes to bark

auxins

hormones that promote apical growth (promote cell elongation, prevents growth in axillary bulbs) and tropic responses

abscisic acid

promotes leaf death and stomatal closures

hormone travel in plants

directly cell to cell or through phloem

tropism

growth in response to directional stimuli

auxins produced in…

apical meristem

phototropism

growth in response to movement of light

plant stem exhibits ______ phototropism while roots exhibit ______ phototropism

positive, negative

auxin efflux pumps

• membrane proteins that create concentration gradients of auxin in tissues by pumping auxin out of cell, into intracellular space, to diffuse into neighbour cell

• can change location due to the fluidity of the cell membrane

auxin influx

movement of auxin into a cell

indoleacetic acid

the type of auxin involved in phototropism

explain how auxin increases flexibility of shoot cell walls

• auxins activate a proton pump (membrane protein) which pumps H+ ions into cell wall

• lowers pH and breaks bonds in cellulose , loosens cell wall

• auxins also upregulate expression of expansins which increase cell wall elasticity

• an influx of H2O, stored in vacuole, can then increase cell size

angiosperms

flowering plants, either monocots or dicots

methods of asexual reproduction in plants

propogation

methods of sexual reproduction in plants

spore formation, pollen transfer

three phases of plant reproduction

pollination, fertilization, seed dispersal

pollination

transfer of pollen grains from stamen to stigma

mutualistic pollinators

• a relationship in which both the pollinator and plant benefit

• pollinators gain nutrients and facilitate plant reproduction

cross-pollination

sexual reproduction between two plants, preferable to self-pollination to increase genetic diversity

monoecious plants

capable of self-pollination, have both stamen and stigma

flowers are colourful and fragrant to…

attract pollinators

fertilization

• nuclei of male gamete (in pollen grains) joins with nuclei of female gamete in ovule

• pollen attaches to sticky stigma and forms a pollen tube to deliver sperm to egg in ovule

Cotyledon

embryonic leaves capable of photosynthesis, stores nutrients, makes up the plant seed

Testa

a strong and protective seed coat that usually must be broken for water intake before germination occurs.

Epicotyl

aka plumule, embryonic shoot protected in the seed

radicle

embryonic root protected in the seed

Micropyle

small pore in the outer layer of the seed, allows for passage of water, also where pollen tube enters the seed

the ovule develops a seed once it is ….

fertilised, sometimes develops a fruit around it

seed dormancy period

a time of low metabolism and no growth/development, seed dehydrates to water content 10-15% total weight

Germination

process by which a seed emerges from the dormancy period and begins to sprout

essential conditions for germination

• water - rehydrates plant tissues, triggers gibberellin

• O2 - needed for resperation to create ATP

• temperature - must be viable for enzyme activity

• pH levels - must be viable for enzyme activity

specialised conditions for germination

• fire - must be exposed to intense heat

• freezing - must experience and extreme cold period

• washing - must be cleaned before germinating

• digestion - to erode seed coat

• scarification - seed coat weakened by physical damage

flowering triggered by…

changes in gene expression

Gibberellins

plant hormones that trigger stem elongation and germination of dormant seeds

gene activation for flowering triggered by…

abiotic factors, linked to seasons when pollinators are most active, and day/night length

Stamen composed of…

• anther - pollen-producing organ

• filament - stalk holding up anther so accessible to pollinators

Pistil

female sex organ in plants

Stamen

male sex organ in plant

Pistil composed of…

• stigma - sticky tip that catches pollen

• style - elevates stigma and connects it to the ovule

• ovule - contains female gametes, where seed develops

Support structures/organs in plants

• petals - attract pollinators

• sepal - strong leaves that protect flower when in bud

• peduncle - stalk

photoperiodism

response of plant to lengths of light and darkness

phytochromes

pigment in plants used to detect light and dark periods

long-day plants

plants that flower around summer, require dark periods to be less than an uninterrupted critical night length to flower, because Pfr promotes flowering

short-day plants

plants that flower in winter/fall, require dark periods to exceed an uninterrupted night length, because Pfr inhibits flowering

Pr

the inactive form of phytochrome, this form is present when absorbing far red light (725 nm)

Pfr

the active form of phytochrome, present when absorbing red light (660 nm)

Pfr converted to Pr when it…

absorbs far red light or in sustained absence of light (darkness reversion)

Pfr is most active during the day because…

there is an abundance of red light (660 nm) in sunlight

Translocation

• the movement of organic material in the phloem (sucrose, amino acids, hormones) from source to sink

• Uses active transport

Sugar is transported as ______ in plants because…

sucrose, soluble but metabolically inert

Turgor pressure

water pressure in plant cells, water stored in vacuole

Sieve tube cells

• aka sieve tube members, long narrow cells connected to form a sieve tube

• present in phloem

• have few organelles and no nuclei to maximize space

• have thick and rigid cell walls to withstand hydrostatic pressure

• Have perforated end plates where cells fused, with holes

sieve plate

connects sieve element cells together, porous to enable bidirectional flow, maintain pressure, and prevent congestion

companion cells

• present in phloem along with sieve tube members

• provide metabolic support for sieve element cells and facilitate loading and unloading of materials at source and sink

• possess an infolding plasma membrane, increase SA for material exchange

• have many mitochondria to fuel active transport

• have many transport membrane proteins to move materials

plasmodesmata

small channels that connect the cytoplasms of sieve elements and companion cells and facilitate symplastic movement of materials

Initial processes of seed germination

• Water absorption

• gibberellin is released which causes the production of amylase which hydrolyses starch → maltose → glucose

• Glucose is used for cell resp.

• Glucose can be converted into cellulose to produce cell walls

Effects of light on transpiration

speeds up transpiration bo warming the leaf and opening stomata

Effects of humidity on transpiration

decreased humidity increases transpiration bc concentration gradient of water in air vs leaf

Effects of wind on transpiration

increases transpiration by moving humid air away from the leaf

Effects of temperature on transpiration

high temperatures increase transpiration because more water evaporates

Effects of soil water on transpiration

if water intake at roots is slow, turgor loss occurs and stomata close, decreasing rate of transpiration

Effects of CO2 on transpiration

high CO2 levels in the air around the plant often cause the guard cells to lose turgor and the stomata close.

Vessel elements

dead cells with no cytoplasm, cellulose walls reinforced by lignin

only present in angiosperms, tracheids are present in all vascular plants

tracheids

tapered cells, exchange H₂O by pits only, cellulose walls with lignin

Negative pressure gradient in xylem

• water is pulled from the xylem under tension (low pressure)

• due to the adhesive attraction between H₂O and leaf cell walls

Adaptations of xerophytes

• CAM physiology; stoma open at night, closed in the day, reducing loss of H₂O by evaporation

• reduced and rolled leaves, lower SA for evaporation and less exposure of stomata to air

• thick cuticle to prevent water loss

• low growth - shaded and less wind