Topic 9: Plant Biology Flashcards
Explain the functions of the different tissues of a leaf. 8 marks
Function must be given to award mark.
cuticle (produced by epidermis) prevents water loss
epidermis protects cells inside the leaf
stomata (in epidermis) for gas exchange
palisade parenchyma / mesophyll / layer for photosynthesis
spongy parenchyma / mesophyll / layer for photosynthesis
air spaces for diffusion of O2 / CO2 / gases
spongy mesophyll for gas exchange / absorption of CO2
xylem transports water / mineral salts / ions to the leaves
phloem transports products of photosynthesis / sugars (to flowers / new leaves / stem / roots / fruit)
stomata allow transpiration (which helps transport of mineral nutrients)
guard cells open and close stomata
guard cells close stomata to reduce transpiration
Explain the role of auxin in phototropism. 8 marks
Accept clearly annotated diagrams for phototropism marking points.
auxin is a plant hormone;
produced by the tip of the stem/shoot tip;
causes transport of hydrogen ions from cytoplasm to cell wall;
decrease in pH / H+ pumping breaks bonds between cell wall fibres;
makes cell walls flexible/extensible/plastic/softens cell walls;
auxin makes cells enlarge/grow;
gene expression also altered by auxin to promote cell growth;
(positive) phototropism is growth towards light;
shoot tip senses direction of (brightest) light;
auxin moved to side of stem with least light/darker side
causes cells on dark side to elongate/cells on dark side grow faster;
Outline the adaptations of plant roots for absorption of mineral ions from the soil. 5 marks
mineral ions are absorbed by active transport;
large surface area;
branching (increases surface area);
root hairs;
root hair cells have carrier protein/ion pumps (in their plasma membrane);
(many) mitochondria in root (hair) cells;
to provide ATP for active transport;
connections with fungi in the soil/fungal hyphae;
Describe the process of mineral ion uptake into roots. 5 marks
absorbed by root hairs / through epidermis
root hairs increase the surface area for absorption
uses active transport / uses ATP / uses energy
use of proteins / pumps to move ions across membrane
against concentration gradient / diffusion gradients into cell / root
can enter cell wall space / be drawn through cell walls / apoplastic pathway
selective / only specific ions absorbed
Describe how water is carried by the transpiration stream. 7 marks
transpiration is water loss (from plant) by evaporation;
flow of water through xylem from roots to leaves is the transpiration stream;
evaporation from spongy mesophyll cells;
replaced by osmosis from the xylem;
(diffusion of water vapour) through stomata;
water lost replaced from xylem / clear diagram showing movement of water from xylem through cell(s) (walls) to air space;
water pulled out of xylem creates suction/low pressure/tension;
transpiration pull results;
water molecules stick together/are cohesive;
due to hydrogen bonding/polarity of water molecules;
xylem vessels are thin (hollow) tubes;
adhesion between water and xylem due to polarity of water molecules;
creates continuous column/transpiration stream;
Explain how abiotic factors affect the rate of transpiration in a terrestrial plant. 8 marks
less transpiration as (atmospheric) humidity rises
smaller concentration gradient ( of water vapour)
more transpiration as temperature rises
faster diffusion / more kinetic energy (of water molecules)
faster evaporation (due to more latent heat available)
more transpiration as wind (speed) increases
humid air / water vapour blown away from the leaf
increasing the concentration gradient (of water vapour)
more transpiration in the light
due to light causing stomata to open
wider opening with brighter light hence more transpiration
CAM plants opposite
narrower stomata with high carbon dioxide concentration hence less transpiration
List three abiotic factors which affect the rate of transpiration in a typical mesophytic plant. 3 marks
light
temperature
wind
humidity
Explain how wind affects the rate of transpiration from a leaf. 5 marks
wind blows air / water vapour away from the leaf
water vapour that has diffused out of the stomata is carried away
low humidity maintained near the leaf without wind air becomes saturated
large water concentration gradient between inside and outside the leaf
rapid diffusion of water vapour therefore rapid transpiration
no effect if the air brought by the wind is already saturated
Outline adaptations of xerophytes. 4 marks
xerophytes are plants that live in dry conditions;
reduced leaves/spines to prevent water loss (by transpiration);
rolled leaves to prevent water loss / stomata on the inside / sunken stomata;
thick waxy cuticle/hairs on leaves to prevent water loss (by transpiration);
reduced stomata to prevent water loss (by transpiration) / stomata on one side of leaf;
deep/widespread roots to obtain more water;
special tissue for storing water;
take in carbon dioxide at night / CAM plant to prevent water loss;
Outline the role of the phloem in the active translocation of biochemicals. 5 marks
living tissue
composed of companion cells / sieve tube members
companion cells involved in ATP production
sucrose / amino acids / assimilate / products of photosynthesis transported
bi-directional transport
source / leaves to sink / fruits / roots /storage organs / named storage organ
pressure flow hypothesis / movement of water into phloem causes transport
Explain the conditions needed for seed germination. 4 marks
water needed
water causes swelling which bursts the testa / seed coat /
water softens the testa / seed coat
water mobilises soluble food reserves / enzymes / medium for metabolic processes
water rehydrates cells / tissues
water transports hydrolysed food reserves
water transports growth promoters / hormones
water dilutes / washes out growth inhibitors
oxygen needed
oxygen required for (aerobic) respiration
which provides ATP for metabolic activity
warmth increases enzyme activity (reject enzymes denatured)
fire breaks down inhibitors
chilling breaks down inhibitors
light breaks down inhibitors / stimulates germination in some species
degradation of testa makes it more permeable to water / gases
Explain how flowering is controlled in long-day and short-day plants. 7 marks
flowering affected by light;
phytochrome;
exists in two (interconvertible) forms/Pfr and Pr;
Pr (red absorbing/660 nm) converted to Pfr (far-red/730 nm absorbing) in red or day light;
sunlight contains more red than far red light so Pfr predominates during the day;
gradual reversion of Pfr to Pr occurs in darkness;
Pfr is active form / Pr is inactive form;
in long-day plants, flowering induced by dark periods shorter than a critical length / occurs when day is longer than a critical length;
enough Pfr remains in long-day plants at end of short nights to stimulate flowering;
Pfr acts as promoter of flowering in long-day plants;
short-day plants induced to flower by dark periods longer than a critical length/days shorter than a critical value;
at end of long nights enough Pfr has been converted to Pr to allow flowering to occur;
Pfr acts as inhibitor of flowering in short-day plants;
