CHAPTER 15
REVIEW QUESTIONS
15.1
The arrangement of the leaf – which has a large surface area – on the stem and petiole exposes
much of the leaf to sunlight and air and maximises light absorption. It is thin, with many air
spaces, allowing for rapid diffusion of carbon dioxide and water to cells and penetration by
light. The epidermis, the protective layer of cells which secretes a waterproof cuticle, has no
chloroplasts allowing sunlight to penetrate the mesophyll cells which are packed with
chloroplasts which are moved within the cell by cytoplasmic streaming to ensure maximum
exposure to sunlight. The arrangement of the spongy and palisade mesophyll is such that light
is captured by the chloroplasts and there is easy movement of materials within the leaf from
stomata on the lower surface and xylem and to the phloem.
15.2
The leaves. Photosynthesis occurs in both the palisade mesophyll, a layer of vertically oriented
cells directly below the upper epidermis, and spongy mesophyll cells between the palisade
cells and the lower epidermis. These latter cells capture any radiant energy which passes
through the palisade layer, and are widely spaced, allowing movement of water vapour and
gases to move efficiently from either the stomata or the xylem to the mesophyll cells.
15.3
Plants need energy to convert the carbohydrates formed during photosynthesis to other organic
compounds and in performing their life functions. This energy is attained by the respiration of
glucose.
15.4
Glucose + oxygen  carbon dioxide + water + energy (as ATP)
15.5
Compensation point is the light intensity at which the rate of photosynthesis and the rate of
respiration are the same. There is no net gain or loss of carbon dioxide, oxygen or glucose.
15.6
15.7
Because of their cellular structure, plants do not locomote and thus do not require as much
energy as animals. In addition they produce oxygen as a by-product of photosynthesis and so
do not need specialised organs for gas exchange.
15.8
Stomata reduce water loss but provide areas for gas exchange.
Moist air spaces between spongy mesophyll cells allows carbon dioxide to dissolve and pass
rapidly into the mesophyll cells; water carrying carbon dioxide can pass from cell to cell via
three routes, mostly via the cellulose cell walls.
15.9
Lenticels are areas of loosely packed cork cells on the surface of woody stems and some roots.
The air can diffuse through the air spaces between these cells to underlying living cells.
15.10
The mangrove habitat is composed of shifting, anoxic, water-logged soil. Thus the oxygen
required for respiration in root cells is not available. Some roots grow above the soil (and
water level), e.g. prop roots and looping pneumatophores, and obtain their oxygen supplies
through the lenticels.
15.11
Digestion is the break down of large, complex, insoluble molecules into small, simple, soluble
molecules that can pass through cell membranes.
15.12
Storage molecules need to be transported from sites of production or storage to nonphotosynthetic parts of the plant to perform their life functions.
15.13
Parasitic plants send hair-like structures (hautoria) into the xylem and phloem of the host
species. Insectivorous plants have specialised structures in which they trap insects.
Extracellular enzymes are released in these structures to digest the insect, so that simple
soluble molecules can be absorbed by the cells and transported to the rest of the plant.
15.14
Intracellular digestion occurs within the cell (usually in a membrane bound vacuole to prevent
digestion of the cell contents by the enzymes). In extracellular digestion, enzymes are released
to the outside of the cell where digestion takes place. The digested molecules are then taken up
by the cells (diffusion, active transport, etc.).
15.15
If the soils in which the plants are normally found do not contain adequate nitrate and
phosphate minerals, some plants may have had a genetic mutation which allowed them to
supplement their nutrient requirements using heterotrophic nutrition. Since these plants would
have had a greater survival advantage than members of their species without this ability, over
time they became the dominant type of the species.
15.16
They are either retained within a membranous vacuole or broken down to their constituent
amino acids which will be used in later synthesis of macromolecules. This is essential since
the enzymes would start to digest the plant if they were loose in the cytoplasm.
15.17
Water is required in the process of photosynthesis. It provides the hydrogen needed in the
formation of carbohydrate. Water is drawn through the plant from the soil in the transpiration
stream, the energy being provided by solar radiation in the evaporation of water from the leaf
in transpiration. If soil water levels are low, the stomata close to decrease water loss by the
plant. This then limits the rate of photosynthesis to the rate of respiration since there can be no
gas exchange with the atmosphere.
15.18
The stoma is a small pore in the lower epidermis of the leaf. It is surrounded by two guard cells
which contain chlorophyll. The walls of the guard cell are thicker next to the pore than
adjacent to the surrounding epidermal cells. If the guard cells are turgid, there is uneven
pressure in the cells, causing the thin walls to stretch more than the thick ones, thus the two
cells curve away from the pore which opens. It is thought that photosynthesis in these cells
accumulates glucose which is osmotically active. Water is thereby drawn into the cells making
them turgid and thus opening the stoma.
15.19
Although the stoma only take up a small proportion of the leaf’s surface, diffusion through
them is extremely efficient.
15.20
Transpiration rate is affected by light intensity, temperature, humidity, wind and soil water
levels.
15.21
Increase in water uptake (extensive shallow roots or deep ‘water-seeking’ tap roots; leaves
which channel any condensation down towards the roots system); presence of water storage
tissues; leaves with low SA : Volume ratio, reduction of stomata or time they remain open,
sunken stomata, hairy surface or rolling or folding to increase humidity.
15.22
Physiological drought means that although water is present in the habitat it is not available for
plant use, e.g. it is saline or in the form of ice. Plants in saltmarshes have root cells which exert
a higher than normal osmotic pressure allowing them to take in water, or they have water
storage tissues.
15.23
Osmoregulation is the control, within set limits, of the salt and water balance of the organism.
Excretion is the removal of metabolic waste products.
15.24
Excretory gases are removed by diffusion. Plants do not need large amounts of protein since
they do not locomote, and can regulate the synthesis of proteins according to their needs, so
they do not normally excrete nitrogenous wastes. Other waste substances are generally
converted to a relatively non-toxic form and stored either inside cells or intercellular spaces.
15.25
Intracellular transport is the movement of matter within the cell, due to diffusion and
cytoplasmic streaming. Extracellular transport is the movement of materials outside the cells
usually from place to place within the organism in a tubular system of vessels.
15.26
Diffusion and cytoplasmic streaming.
15.27
a.
b.
15.28
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
o.
p.
15.29
a.
b.
c.
d.
e.
Simple organisms are usually aquatic or live in moist terrestrial habitats, and each cell is
in close proximity to the external environment. Thus transport of matter is achieved by
diffusion, osmosis and active transport (carrier molecules, pinocytosis and phagocytosis)
between the cell and the environment.
Any increase in size, or distance of cells from the external environment, results in
decreased efficiency of transport. Thus there is a limitation to size and specialisation of
tissues for specific functions.
Adhesion: force of attraction between different types of molecules.
Capillarity: movement of fluid in a narrow tube due to forces of adhesion between the
fluid molecules and the walls of the tube.
Cohesion: force of attraction between molecules of the same type.
Casparian strip: a thickened strip of waxy suberin found around one diameter of the wall
of endodermis cells in the root.
Cytoplasmic streaming: the flow of cytoplasm around the cell.
Diffusion: the movement of a substance from an area of its high concentration to an area
of its low concentration.
Drip tip: specialised, downwardly pointed tip of the leaf where excess water can be
removed
Guttation: ability of some plant leaves to actively secrete water from special glands at the
end of veins.
Humidity: measure of the amount of water vapour in the air.
Mass flow: movement of dissolved solutes in plants resulting from a high turgor pressure
difference.
Plasmodesmata: cytoplasmic bridges between plant cells.
Phloem: a complex tissue composed of living and dead cells, the functional unit of which
is composed of a sieve element (through which translocation occurs) and companion cell
(which are believed to provide the energy for translocation).
Root pressure: pressure exerted by the continued uptake of water by the roots from the
soil and which forces the water a distance up the plant stem.
Translocation: movement of organic matter through the phloem from one part of the plant
to another.
Transpiration: evaporative water loss from the leaves of plants.
Xylem: complex tissue composed of living and dead cells; the tracheids and vessels are
the non-living components through which water and dissolved mineral ions move from
the roots to the leaves.
Osmosis: occurs along a diffusion gradient from the leaves down to the roots and soil.
Root pressure: causes water to move a limited distance up the stem until counteracted by
the force of gravity.
Cohesion: occurs between the water molecules forming thin water columns in the xylem.
Adhesion: of the water molecules to the walls of the vessels and tracheids and the spongy
mesophyll cells aiding the movement of water upwards.
Transpiration: causing water loss from the stomata and thus maintaining the flow of water
from the roots to the leaves.
15.30
Place a plant in a beaker of water which has been coloured with a dye. After a period of time
cut sections of the plant stem and determine which tissues were stained by the dye.
15.31
From one cell vacuole to the next by osmosis through the cell walls and all cell contents.
Through the plasmodesmata from one cell to the next by osmosis.
Moving along and through intercellular spaces and cellulose walls from one cell to the next.
15.32
The Casparian strip is thought to force the water to move through the cytoplasm of the
endodermis cells rather than along the cell walls, regulating the movement of water into the
xylem and possibly regulating salt concentration as well as preventing the entry of toxins or
pathogens.
15.33
As soluble monomers.
15.34
Phloem.
15.35
a.
b.
c.
d.
Cytoplasmic streaming:
For: explains opposite movements of solutes in one tube.
Against: no evidence that cytoplasmic streaming occurs in mature sieve elements;
velocity of cytoplasmic streaming in other cells is too low to account for the speed of
translocation.
Activated diffusion:
For: accounts for the rate of flow.
Against: not all solutes travel along a concentration gradient.
Mass flow:
For: accounts for rate of flow.
Against: does not explain the need for companion cells or why factors which affect
metabolic rate also affect speed of translocation; bidirectional flow within a sieve
element; when turgor pressure is very low translocation also occurs.
Electro-osmotic mass flow:
For: explains need for companion cells and the presence of phosphate ions in sieve tubes.
Against: does not account for bidirectional flow.