plant exam qs

Question 1

Describe how the apparatus should be set up to ensure that valid measurements can be made. (8)

Answer 1

1 cut shoot under water (to stop air entering xylem vessels);
2 cut shoot at a slant (to increase surface area);
3 check apparatus is full of water / is air bubble free / no air locks;
4 insert shoot into apparatus under water
5 remove potometer from water and ensure airtight / watertight,
joints around shoot;
6 dry leaves;
7 keep conditions constant;
8 allow time for shoot to acclimatise;
9 measure per unit time;

Question 2

explain how hairs around the stomata affect rate of transpiration (3)

Answer 2

trap, moisture / water vapour;
reduces the water potential gradient as air surrounding leaf is more humid;
so transpiration rate is reduced;

Question 3

why do large plants need a transport system? (3)

Answer 3

surface area relative to volume too small/AW;
diffusion too slow/AW; idea of speed needed
distance too great/some cells deep in body/not all cells in contact with
environment/AW; R large if unqualified
insufficient/AW, oxygen/(named) nutrient, supplied/(named) waste removed;
idea of linking (named) areas; look for ‘from…’ ‘to…’ with an
implication of organs, not just ‘all over body’
(may be,) more (metabolically) active/AW/, homoiothermic;

Question 4

transpiration

Answer 4

transpiration is the loss of water, vapour/by evaporation;

Question 5

why does a photometer not give you a direct value for transpiration? (3)

Answer 5

transpiration is the loss of water, vapour/by evaporation;
(apparatus) measures water uptake;
to replace loss;
assumes all uptake is lost/AW; ora some may be used
explanation of how some uptake may be used e.g. used to regain
turgor/used in photosynthesis;
uptake by detached shoot may not be same as whole plant/AW;

Question 6

how does temperature increase rate of transpiration

Answer 6

temperature increased;
more KE/energy;
more evaporation/faster diffusion;

Question 7

how does light intensity increase rate of transpiration

Answer 7

light (intensity) increased;
stomata opened (wider);
allowed more water vapour out/AW;
must be linked to stomatal point above
temp increase linked to light;

Question 8

how does humidity increase rate of transpiration

Answer 8

humidity dropped/air less saturated;
internal spaces c. 100% saturated;
steeper water potential gradient;

Question 9

how does wind speed increase rate of transpiration

Answer 9

wind (increased);
removed, saturated air/diffusion shells;
steeper water potential gradient;

Question 10

adaptations of root hair cells for uptake of water

Answer 10

large surface area (to volume);
low water potential; 
thin wall / short diffusion path;
uncutinised / permeable / unlignified / AW;
rapid, growth / replacement;
large no;
long;
many mitochondria;

Question 11

how does water move from soil to xylem? (6)

Answer 11

1 osmosis in correct context;
look for across membrane, or, into / out of, cell / root
2 moves down a water potential gradient;
3 most negative / lowest, in the xylem;
4 (uptake of) ions / minerals / solutes, into xylem / root hair;
5 tension in xylem / transpiration pull / cohesion-tension;

relate to pathway in root
6 (moves) via the cell walls;
7 (moves) via, cytoplasm / vacuoles;
8 passage via the plasmodesmata; look for linking cytoplasm /through wall
9 Casparian strip / suberin / waxy / fatty, blocks,
cell wall route / apoplast; A waterproof
10 water, crosses membrane / enters, cytoplasm / vacuole / symplast;
11 AVP; e.g. pits in xylem / passage cells /aquaporins /
protein channels / capillarity in cell wall (spaces)

Question 12

name the adaptations of xylem vessels

Answer 12

lignified walls

no cytoplasm

no end walls

pits

large lumen

Question 13

what does lignin help with ?

Answer 13

(allows) adhesion / waterproof / stops collapse (under tension);
rings / spirals / thickening;
prevents collapse (under tension);

Question 14

what does the lack of cytoplasm in xylem help with?

Answer 14

less resistance to flow / ease of flow / AW / more space (linked to
lack of contents);

Question 15

what does the lack of end walls in xylem help with?

Answer 15

continuous tube;

less resistance to flow / ease of flow;

Question 16

what do pits in xylem allow?

Answer 16

lateral movement / get round air bubbles / supplies(water) to cells or
tissues / water in or out;

Question 17

role of sieve tube elements and companion cells in the transport of carbohydrates (3)

Answer 17

1 sieve elements end to end or sieve plates perforated;
2 companion cells metabolically active / have many mitochondria / produce ATP / release energy;
3 (active) loading into, companion cell;
4 ref to proton pump;
5 ref to co-transporter;
6 role of plasmodesmata;
7 sieve element has few organelles for ease of flow
8 ref to, unloading mechanism / (hydrostatic) pressure gradient;

Question 18

Explain how, at different times, the same plant root may be a source or a sink. (2)

Answer 18

source when root converts starch into sugars;

sink when root stores starch or uses carbohydrate for respiration growth;

high hydrostatic pressure makes it a source and low hydrostatic pressure a sink;

when loading it is a source and when unloading a sink;

Question 19

explain how mass flow of materials between the source and the sink would be brought about

Answer 19

water will enter source;
by osmosis;
down a water potential gradient;
increase in (hydrostatic) pressure;
as source / sink cannot expand;
force solution along (tube to sink);

Question 20

State one piece of evidence for the involvement of an active process in transport of sugars

Answer 20

ATP involved / respiration involved / many mitochondria in companion
cells / reduced by metabolic inhibitors / oxygen dependent / temperature
dependent / loading against a concentration gradient

Question 21

Describe an active mechanism which could possibly be involved in the transport of sugars from sources to sinks.

Answer 21

loading, into companion cell / from transfer cell / into sieve tube /
into phloem – implied;
H ions / protons, pumped out of, companion cell / sieve tube / phloem;
diffuse back in with sucrose;
protein carrier / co-transporter;
possible active unloading by reverse mechanism;

Question 22

explain how transpiration results in water moving up a plant stem

Answer 22

loss of water from mesophyll;
cell walls;
more drawn from, cytoplasm / cell;
cohesion of water molecules;
hydrogen / H, bonds;
water under tension / ref to hydrostatic pressure gradient implied;
via, symplast / apoplast / vacuoles;
(water from) xylem / xylem vessels;
ref to water potential gradient;