Exam Qs

Question 1

Xylem forms a part of a plant’s transport system
Explain why large multicellular plants need a transport system

Answer 1

• long distance from external structures to inner cells
• small surface area:volume ratio
• diffusion is not sufficient/fast enough
• eg. sucrose needs to be transported

Question 2

Multicellular organisms, such as plants, have evolved internal transport systems.
Explain the benefit to plants of internal transport systems.

Answer 2

• surface area: volume ratio too small
• diffusion from outer surface not sufficient
• (transport system) ensures molecules / nutrients / sugars / water, reach all tissues
• (allows) high metabolic rate

Question 3

This question is about the impact of potentially harmful chemicals and microorganisms.
Salts that a plant needs, such as nitrates and phosphates, are taken into root hair cells by active
transport.
For which macromolecule does a plant need both nitrogen and phosphorus?

Answer 3

DNA / RNA / nucleic acid

Question 4

Sodium chloride in solution dissociates into Na+ and Cl−.
Explain how the Casparian strip prevents these ions from reaching the xylem of the plant by the apoplast pathway.

Answer 4

two from
1 strip is impervious to, water / solutions (1)
2 forces water / solutions, to pass through,
plasma / cell surface, membrane (1)
3 phospholipid (bilayer), repels / AW, ions / charged particles (1)

Question 5

Soluble mineral ions are present in soil.
Explain why water molecules can form hydrogen bonds with nitrate (NO3–) ions.

Answer 5

• water is (a) polar (molecule)
• nitrate (ion) / NO3–, is, charged / negative
• (hydrogen bonds form) between H on water and O on nitrate

Question 6

A group of students wanted to observe the position of xylem vessels in the leaf stalks (petioles)
of celery.
Describe a procedure they could use to do this.

Answer 6

• cut thin transverse section of celery
• add stain

Question 7

Suggest what evidence the scientist might expect to see in companion cells, using an electron microscope.

Answer 7

• many / large, mitochondria ;
  plasmodesmata (between companion cell and sieve tube) / described ;
  many ribosomes / extensive RER ;
  many proteins in the, plasma / cell surface, membrane ;

Question 8

State the function of pits in xylem vessel.

Answer 8

lateral movement of water

Question 9

Suggest why it is beneficial to the plant for the carbohydrate to be transferred throughout the plant in the form of sucrose rather than as an alternative carbohydrate.

Answer 9

sucrose is soluble so can be transported in sap (1)
but metabolically (relatively) inactive so no, used / removed, during transport (1)

Question 10

How is transport in the xylem similar to the phloem?

Answer 10

similar – one of
solutes carried in solution in both (1) both carry mineral salts (1)
both use, mass flow / generated hydrostatic pressure (1)
different – one of
transport in phloem can take place in different directions and transport in xylem only takes place up the plant (1)
phloem carries carbohydrates and xylem does not (1)
phloem transport uses living cells and xylem does not (1)
xylem uses, capillary action / cohesion and adhesion, and phloem does not (1)

Question 11

Explain, with a suitable example, how some parts of the plant can act as both a ‘source’ and a ‘sink’.

Answer 11

certain parts can store and then release carbohydrates when needed (1)
suitable examples include root or leaf, which can act as sink or source at different times of year (1)

Question 12

In the modified plants, the unloading of sucrose is increased in the tubers compared with those that were not modified.
The transport of sucrose to the tubers was also increased in the modified plants.
Using the data and the information given, deduce a possible mechanism to account for the increased unloading and transport of sucrose in the modified plants.

Answer 12

two from
sucrose unloaded at sinks and invertase
converts sucrose into, glucose / monosaccharide (1)
increases sucrose concentration gradient between phloem and sink (1)
causes increased unloading of sucrose from phloem (1)
two from
increases solute gradient between source and sink (1)
removal of water from phloem increases pressure gradient between source and sink (1)
contributes to increased movement in phloem (1)

Question 13

Halophytes are plants that have the ability to live in soils with a very low water potential. In the UK these plants form part of salt marsh communities.
Suggest and explain how the root hairs of halophytes are able to absorb water by osmosis from the soil of the salt marsh.

Answer 13

there is a lower water potential inside root hair (cells)
actively transport / pump, (mineral) ions / salts, into root hair(s) (cells)
or
root hair(s) (cells) store / contain, (mineral) ions / salts / solutes

Question 14

Explain how mass flow of the phloem sap occurs in plants with a vascular system

Answer 14

1 sugar / sucrose / assimilates, in the sieve tube (elements) ✓
2 (assimilates) enter, sieve tube / phloem (at source)
and
lowers water potential (in sieve tube)✓
3 water enters (sieve tube), by osmosis / down water potential gradient / described and
increases hydrostatic pressure ✓
4 (assimilates) leave, sieve tube / phloem (at sink)
and
increases water potential (inside sieve tube) ✓
5 water leaves (sieve tube), by osmosis / down water potential gradient / described and
lowers hydrostatic pressure ✓
6 (assimilates) move, from high to low (hydrostatic) pressure / down pressure gradient ✓

Question 15

What is meant by translocation?

Answer 15

transport / movement / mass flow, of, assimilates / sucrose / amino acids;
from source to sink / description;

Question 16

Explain how increased air movement increases the loss of water vapour from the leaves

Answer 16

water vapour around the, stomata / leaf surface, is blown away;
reduces water (vapour) potential around, stomata;

Question 17

Suggest and explain how the absence of vascular tissue might affect the size to which moss plants can grow

Answer 17

must remain small OR
cannot grow tall / large / big;
no support from vascular tissues / vascular bundles / xylem;
use only diffusion / no mass flow / no rapid transport;
diffusion too slow (to enable substances to move large distances);
idea of:
short diffusion pathway / large surface area to volume ratio;

Question 18

Although a moss plant has no vascular tissue, water still moves through the plant from the root-like structures to the leaves.
Use your knowledge of the mechanisms of water transport to explain the movement of water through the moss plant.

Answer 18

1. idea of water lost by evaporation / transpiration / evapotranspiration;
2. (water moves by) symplast and apoplast pathways;
3. through / along cell walls by, capillary action / adhesion (apoplast pathway);
4. (water loss) reduces the water potential of (leaf) cells;
5. water moves from higher water potential to lower water potential / down water potential gradient (symplast pathway);
6. by osmosis (symplast pathway);
7. through plasmodesmata (symplast pathway);

Question 19

Describe how the guard cells surrounding the leaf pores are adapted to their role.

Answer 19

idea of: unevenly thickened (cell) wall;
able to, change shape / bend;
transport proteins / ion pumps, in plasma membranepresence of) chloroplasts (to provide, ATP / energy);

Question 20

The cohesion-tension theory is often used to explain the mechanism by which water moves up the xylem from the roots to the leaves.
Use this theory to explain how water moves from the roots to the leaves.

Answer 20

evaporation at top of, plant / xylem;
(creates) tension in xylem;
water molecules, stick together / are cohesive / form a chain or column;
(column / chain) pulled up (by tension)

Question 21

Neonicotinoid insecticide molecules are absorbed by the roots and leaves of maize plants.
Describe how, once these molecules have been absorbed, they may be transported to the fruits of the maize plant.

Answer 21

in xylem (by),cohesion-tension / transpiration (stream);
in phloem (by), translocation / mass flow