IB Bio Leaf & Transport System

epidermis

protective covering of one or more cell layers; living tissue; some cells modified in guard cells

cuticle

made of cutin (waxy substance on epidermis); non-living material; reduces transpiration (water loss)

which leaf layers are clear & colorless, allowing light to pass thru?

epidermis & cuticle

guard cells

specialized epidermal cells that control open & close of stomata; make up stomata

palisade mesophyll

consist of parenchyma cells (ground tissue) w/ many chloroplasts; usually in one layer but can be on both sides of leaf in arid climates

stomata

pores in epidermal layer of leaf; found only on bottom of leaves in shade

what do stomata diffuse?

in = co2; out = o2, water vapor

is there cuticle over stomata?

no b/c waxy layer would block diffusion

what happens when stomata lose too much water?

close stomata

where do floating plants have stomata?

on top of leaves

spongy mesophyll

consist of parenchyma cells loosely arranged below palisade mesophyll

why is spongy mesophyll spongy?

has many empty chambers for co2 storage

which leaf layers are green & photosynthetic?

palisade & spongy mesophyll

vascular bundles

consist of xylem & phloem tissue (veins in leaf);

what are vascular bundles surrounded by?

bundle sheath cells (specialized mesophyll cells) --> no exposure of vascular bundles to intercellular spaces

role of bundle sheath cells

prevent air bubbles from entering vascular tissue b/c air bubbles stop flow of water from roots --> leaves & sugar water from leaves --> roots

layers of leaf (top --> bottom)

cuticle, upper epidermis, cuticle, palisade mesophyll and spongy mesophyll w/ vascular bundles partially in between them, cuticle, second lower epidermis, & another cuticle

apoplast pathway

water enters cell walls & moves from one cell to another w/o ever entering cell's other living tissues; blocked by suberin contained in caspian strip

symplast pathway

water moves from cytoplasm of one cell to next cell's cytoplasm thru plasmodesmata; when water reaches endodermis --> can only go into vascular cylinder thru this pathway (endoderm cells let water in but only some ions)

plasmodesmata

tunnel of cytoplasm connecting plant cells

osmosis

diffusion of water; how water moves from soil --> inside root hairs

concentration gradient

maintained by root by keeping certain minerals & removing water

root pressure

concentration gradient forces water even higher up root

cohesion theory

major cause of water theory; due to unequal pressure @ leaves & roots --> water pushed from roots to leaves; movement cont. until pressure is even throughout plant

what occurs during transport into roots?

na+ ions enter (active), water follows (passive)

transpiration

prevents (-) pressure w/ leaves & xylem tissue

cohesion

produces single polymer-like water column from roots --> leaves

how does cohesion occur?

1. bulk flow of water thru xylem cells occurs as water molecules evaporate from leaf surface & pulls up water column behind it
2. H bond btwn H & O of H2O --> water acts as single polymer moving together

pressure level of roots

high pressure b/c water keeps entering by osmosis but doesn't reach equilibrium b/c ions enter by active transport

pressure level of leaves

low pressure b/c water evaporates thru stomata --> less water molecules

stomata goal

minimize water loss, intake lots of CO2

guard cells have...

have uneven cell walls
- thick walls on inside b/c doesn't need to stretch as much
- thin walls on outside b/c needs to stretch more

when water enters thru thin wall of guard cells...

- increase turgor pressure
- cell walls stretch & become kidney-shaped
- stomata open

when water diffuses out of thick wall of guard cells...

- decrease turgor pressure
- cell walls no longer stretch
- stomata close
- prevents water from being released

what affects opening of stomata?

- decreased CO2 concentration
- day of day = night
- increased amounts of K+ ion enter cell

what affects closing of stomata?

- increased temp
- time of day = daytime

how does K+ ion cause stomata to open?

K+ ion enters thru active transport in guard cells --> increases concentration of solution inside of cells --> concentration gradient --> H2O diffuses from surrounding cells into guard cells --> increases turgor pressure inside --> cell walls stretch unevenly --> stomata opens

translocation

movement of carbs thru phloem from source to sink

how to translocation occur?

pressure flow hypothesis
1. glucose enters sieve-tube members by active transport from leaf cells --> high pressure at top of sieve tube --> creates concentration gradient
2. water enters sieve-tube by osmosis (passive)
3. turgor pressure builds up --> water & glucose moves by bulk flow away from source to equalize pressure
4. water leaves sieve-tube by osmosis & glucose leaves by active transport into roots --> tube = area of low pressure again
5. carbs are used by sinks --> solution = hypotonic

if the roots have too high pressure...

eventually pop but water can be transported back up to leaves thru xylem

sinks

where sugar is delivered (roots)

source

where substances are produced or enter plant (leaves)