Plants Cross Section of a Leaf Waxy cuticle - protects leaf from water loss/insect invasion Epidermis - protects leaf Palisade mesophyll - site of photosynthesis (many chloroplasts) Spongy mesophyll - allow for gas exchange, some photosynthesis Air space - surfaces for gas exchange (around spongy mesophyll) Vascular Bundles - vascular tissue containing xylem + phloem in veins (mid-leaf) - Xylem - water transport to leaf - Phloem - nutrient transport (sugars from photosynthesis) Guard Cells - controls opening/closing of stomata Stomata - pores in bottom of leaf for water loss (transpiration) + gas exchange Cohesion-Tension Theory 1. Water moves down concentration gradients a. High water [c] in air spaces, evaporates to atmosphere = negative pressure 2. Water pulled from soil into roots, then from root cortex into xylem vessels a. High ion [c] in root hair cells promotes osmosis Transpiration stream from tension created by transpiration, lignan keeps structure under hydrostatic pressure 3. Vessel water column maintained by cohesion and adhesion = tension a. Continuous cohesion b/n water (h-bonds), adhesion b/n water and vessel sides (counteracts gravity) = capillary action 4. Lost water from transpiration is replaced by water from vessels a. Maintain high water concentration in air spaces Xylem Structure and Function Tracheids = dead cells, more primitive, no lignan, not as effective transpiration Vessel elements = dead cells, more evolved, lignan (secondary wall, protect against pathogens/water), pores in primary wall for lateral water exchange, end-to-end Transpiration = loss of water from leaves/stems of plants 1. Xylem vessels transport water through plant 2. Water heated by sunlight in mesophyll + vapourizes 3. Vapour transpires out of stomata 4. Water loss creates negative pressure + transpiration pull of molecules into the xylem (transpiration stream) 5. More water drawn into the leaf - measured with a potometer (water uptake) - when stomata open for gas exchange, water vapour can escape (evaporates from cell walls into air spaces and diffuses out of leaf) Stomata and Guard Cells - Turgor is the pressure in a cell that liquid exerts on membrane Open = sunlight (photosynthesis), reduced CO2 [c], guard cells turgid + high pressure in cytoplasm Closed = water shortage (abscisic acid from roots removes K+ ions so water into epidermal cells and guard cells close), darkness, guard cells flaccid + low pressure in cytoplasm - blue wavelength triggers protein pump so increases K+ ions in guard cells → water follows ions to dilute (osmosis) → guard cells plump (stoma opens) → water leaves when K+ leaves (abscisic acid) Factors Affecting Transpiration Wind Speed - blows water on leaves away - increased [c] gradient + rate - high can cause stomata close - no effect if wind saturated Humidity - water diffuses out at high [c] - low transpiration as atmospheric humidity rises (low gradient) Temperature - faster particle movement - increased rate (evaporation) - stomata may close at very high temperatures Light Intensity - stomata open (photosynthesis) - water out + increased rate Mineral Movement 1) Minerals absorbed by root hairs (increased SA for absorption) 2) Apoplastic pathway through cell walls (active transport) 3) Selective (only specific ions absorbed) - taken in through passive (simple diffusion in rich soil), bulk transport (with water), active (in poor soil), and mutual relationship with fungi (swap nutrients, hyphae = mushroom roots) - adaptations = large SA, branching, root hairs, many mitochondria for ATP, connect with fungal hyphae, carrier proteins (pumps) in root hair cell membranes - active transport of minerals → increased solute [c] in root cells → higher [c] than soil → osmosis Plant Adaptations Xerophytes = adaptations for dry habitats = vertical stems (prevent sunlight absorption at midday), thick waxy cuticles, less stomata + in pits + hairs on leaves (less wind exposure), deep roots + extensive surface roots (water, night humidity), small/needle shaped leaves (spines reduce SA), fleshy leaves (more vacuoles), leaves roll up (trap humidity) Halophytes = adaptations for saline soil = high K+ or sugar [c] in cytoplasm to prevent osmosis into saline environment, maintain high Na+/Cl- [c] in vacuoles (no metabolism there) Phloem Structure and Function Sieve element cells = alive, long/narrow, rigid, connected to form sieve tube, sieve plates link at transverse ends (porous for flow), no nuclei, reduced organelles for max. translocation space Companion cells = metabolic support for sieve element cells, many mitochondria, facilitate loading and unloading of materials with appropriate transport proteins in membrane, infolding membrane to increase SA:Vol ratio Schlerenchymal + parenchymal cells = ground tissue for support Apoplastic loading = materials pumping across with membrane proteins 1) H+ ions actively transported out of phloem cells by protein pumps 2) Creates pressure gradient with high H+ ions outside cell 3) H+ ions passively diffuse back into cell with cotransport protein with requires sucrose movement = mutualistic 4) Build up of sucrose in sieve tube for transport from the source Plasmodesmata = connect cytoplasm b/n sieve element cells and companion cells → mediate the symplastic exchange of materials Phloem properties - bidirectional transport of water/nutrients - sieve element cells connect in tube with sieve plate + supported by companion cells - sap movement mediated by hydrostatic pressure from xylem Translocation = the movement of organic compounds from sources to sinks (synthesized → delivered) in plant sap in phloem - sugars - sucrose (disaccharide, soluble + metabolically inert) - rate affected by photosynthesis, cell resp, + transpiration rate, sieve tube diameter (pressure) Symplastic loading = materials through plasmodesmata into sieve tube (sneaks in) Water uptake by osmosis = sap hypertonic b/c high solute [c] from active transport → osmosis from xylem to decrease solute [c] - hydrostatic pressure increases due to incompressibility of water = mass flow to lower pressure (towards sinks) - solutes unloaded by companion cells into sinks where hypotonic (lower solute), water back to xylem, keeps hydrostatic pressure lower at sink Xylem and Phloem Identification Stems (phloem on outside) monocot = scattered; dicot = circle around pith (centre), cambium b/n xylem and phloem SMS SDC Roots (phloem on outside) monocot = stele is large + vessels in circle around pith; dicot = stele is small, xylem in cross in centre RMC RDX Monocotyledon = one cotyledon, branched roots, long leaf veins, 3 flowers, scattered vascular Dicotyledon = two cotyledons, tap roots, ringed vascular, net-like leaf veins, 4 or 5 flowers Measuring Phloem Transport Rates Using Aphids - protruding mouthpiece called stylet pierces phloem tube to extract sap + digestive enzymes help - plant grown in lab with leaves sealed in glass with radioactively labelled 14-CO2 → leaves convert to sugars → used along plant’s length to collect sap → severed stylet = sap still flows due to hydrostatic pressure in sieve tube → measure phloem transport rates from time radioisotope took to be detected Plant Growth - undifferentiated cells in meristems allow indeterminate growth (can become any cell type) - apical meristems at shoot/root tips (primary growth = new leaves/flowers) - lateral meristems at cambium (secondary growth = bark) - cell enlargement + repeated cell divisions - growth from nodes in stem and in axillary (lateral) buds - plant hormones control growth in shoot apex (like glands) Auxin and Phototropism - auxin produced in shoot apical meristem = a plant hormone that changes the pattern of gene expression in a plant’s cells (activates diff. gene pathways in shoots/roots) - apical dominance = prevents growth in lateral (axillary) buds (use energy to grow up), can grow bigger with increased distance b/n terminal and axillary bud → removing terminal bud with selective pruning allows bushier/dominant lateral buds Auxin efflux pumps = set up [c] gradient of auxin + make cell walls flexible In roots = inhibits cell elongation; in shoots = stimulates cell elongation Shoots - auxin increases cell flexibility + promotes growth with elongation 1) Activates protein pump which secretes H+ ions in cell wall 2) Decreased pH causes cellulose fibres in cell wall to loosen 3) Increased expression of expansins causes lengthening b/n cellulose 4) Water influx to be stored in vacuole (cell wall more flexible, can fit more) Tropisms = movement of a plant in response to a directional external stimulus - phototropism light, geotropism gravity, hydrotropism water gradient, thigmotropism tactile stimulus - positive phototropism = in shoots, dark side elongates + growth towards light - negative phototropism = in roots, dark side becomes shorter + growth away from light Micropropagation 1) Pick perfect plant (stock plant + sterilised) 2) Explant grown on a sterile nutrient agar gel 3) Growth hormones to stimulate development 4) Multiplication for new samples 5) Transfer to soil - asexual reproduction cultured in lab (in vitro) - tissues of shoot apex, nutrient agar gels, growth hormones to produce clones from stock plant - rapid bulking - conserve characteristics, GMO - virus-free strains - non-infected meristems - propagation of rare species - hard to breed, commercially in demand (orchids) Plant Reproduction - vegetative propagation, spore formations, pollen transfer Sexual Reproduction 1) Pollination - transfer of pollen from anther to stigma (some can self-pollinate) 2) Fertilisation - fusion of male and female gametes to form a zygote 3) Seed Dispersal - reduce competition for resources - cross pollination = pollen from one plant to ovule of another, pollinators in a mutualistic relationship with flowing plant (both benefit → nutrition for animals, means of sexual reproduction for plant) - flowering = when pollinator most abundant (seasons), triggered by day/night length, gene expression changes trigger growth of shoot apical meristem + influenced by abiotic factors - factors affecting germination (O2, H2O, range of suitable temps + pH) Flower and Seed Structure Phytochromes
