IB Biology Plant Biology (9.1-
9.4) Comprehensive Questions
and Solutions Graded A+
9.1.1 Outline that transpiration is the inevitable consequence of gas exchange in the leaf. -
Answer: Transpiration is the loss of water vapor from the stems and leaves of plants
Stomata are pores on the underside of the leaf which facilitate gas exchange (needed for
photosynthesis)
As photosynthetic gas exchange requires stomata to be open, transpiration will be affected by
the level of photosynthesis
Hence, transpiration is an inevitable consequence of gas exchange in the leaf
9.1.2 Explain how plants transport water from the roots to the leaves to replace losses from
transpiration. - Answer: -Light energy converts water in the leaves to vapor, which evaporates
from the leaf via stomata
-New water is absorbed from the soil by the roots, creating a difference in pressure between the
leaves (low) and roots (high)
-Water will flow, via the xylem, along the pressure gradient to replace the water lost from leaves
(transpiration stream)
,9.1.3 Outline the use of models of water transport in xylem by using simple apparatus including
blotting or filter paper, porous pots and capillary tubing. - Answer: The movement of water up
the length of the xylem can be modeled using a number of simple apparatus
These include capillary tubing, filter or blotting paper, and porous pots
Capillary Tubing:
Water has the capacity to flow along narrow spaces in opposition to external forces like gravity
(capillary action)
This is due to a combination of surface tension (cohesive forces) and adhesion with the walls of
the tube surface
The thinner the tube or the less dense the fluid, the higher the liquid will rise (xylem vessels are
thin: 20 - 200 µm)
Filter Paper:
Filter paper (or blotting paper) will absorb water due to both adhesive and cohesive properties
When placed perpendicular to a water source, the water will hence rise up along the length of
the paper
This is comparable to the movement of water up a xylem (the paper and the xylem wall are
both composed of cellulose)
Porous Pots:
Porous pots are semi-permeable containers that allow for the free passage of certain small
materials through pores
The loss of water from the pot is similar to the evaporative water loss that occurs in the leaves
of plants
, If the porous pot is attached by an airtight seal to a tube, the water loss creates a negative
pressure that draws more liquid
9.1.4 Draw the structure of primary xylem vessels in sections of stems based on microscope
images. - Answer:
9.1.5 Explain how the cohesive property of water and the structure of the xylem vessels allow
transport under tension. - Answer: The flow of water through the xylem from the roots to the
leaf, against gravity, is called the transpiration stream
Water rises through xylem vessels due to two key properties of water - cohesion and adhesion
Cohesion:
Cohesion is the force of attraction between two particles of the same substance (e.g. between
two water molecules)
Water molecules are polar and can form a type of intermolecular association called a hydrogen
bond
This cohesive property causes water molecules to be dragged up the xylem towards the leaves
in a continuous stream
9.1.6 Explain how the adhesive property of water and evaporation generates tension forces in
leaf cell walls. - Answer: Adhesion:
Adhesion is the force of attraction between two particles of different substances (e.g. water
molecule and xylem wall)
The xylem wall is also polar and hence can form intermolecular associations with water
molecules
As water molecules move up the xylem via capillary action, they pull inward on the xylem walls
to generate further tension
9.4) Comprehensive Questions
and Solutions Graded A+
9.1.1 Outline that transpiration is the inevitable consequence of gas exchange in the leaf. -
Answer: Transpiration is the loss of water vapor from the stems and leaves of plants
Stomata are pores on the underside of the leaf which facilitate gas exchange (needed for
photosynthesis)
As photosynthetic gas exchange requires stomata to be open, transpiration will be affected by
the level of photosynthesis
Hence, transpiration is an inevitable consequence of gas exchange in the leaf
9.1.2 Explain how plants transport water from the roots to the leaves to replace losses from
transpiration. - Answer: -Light energy converts water in the leaves to vapor, which evaporates
from the leaf via stomata
-New water is absorbed from the soil by the roots, creating a difference in pressure between the
leaves (low) and roots (high)
-Water will flow, via the xylem, along the pressure gradient to replace the water lost from leaves
(transpiration stream)
,9.1.3 Outline the use of models of water transport in xylem by using simple apparatus including
blotting or filter paper, porous pots and capillary tubing. - Answer: The movement of water up
the length of the xylem can be modeled using a number of simple apparatus
These include capillary tubing, filter or blotting paper, and porous pots
Capillary Tubing:
Water has the capacity to flow along narrow spaces in opposition to external forces like gravity
(capillary action)
This is due to a combination of surface tension (cohesive forces) and adhesion with the walls of
the tube surface
The thinner the tube or the less dense the fluid, the higher the liquid will rise (xylem vessels are
thin: 20 - 200 µm)
Filter Paper:
Filter paper (or blotting paper) will absorb water due to both adhesive and cohesive properties
When placed perpendicular to a water source, the water will hence rise up along the length of
the paper
This is comparable to the movement of water up a xylem (the paper and the xylem wall are
both composed of cellulose)
Porous Pots:
Porous pots are semi-permeable containers that allow for the free passage of certain small
materials through pores
The loss of water from the pot is similar to the evaporative water loss that occurs in the leaves
of plants
, If the porous pot is attached by an airtight seal to a tube, the water loss creates a negative
pressure that draws more liquid
9.1.4 Draw the structure of primary xylem vessels in sections of stems based on microscope
images. - Answer:
9.1.5 Explain how the cohesive property of water and the structure of the xylem vessels allow
transport under tension. - Answer: The flow of water through the xylem from the roots to the
leaf, against gravity, is called the transpiration stream
Water rises through xylem vessels due to two key properties of water - cohesion and adhesion
Cohesion:
Cohesion is the force of attraction between two particles of the same substance (e.g. between
two water molecules)
Water molecules are polar and can form a type of intermolecular association called a hydrogen
bond
This cohesive property causes water molecules to be dragged up the xylem towards the leaves
in a continuous stream
9.1.6 Explain how the adhesive property of water and evaporation generates tension forces in
leaf cell walls. - Answer: Adhesion:
Adhesion is the force of attraction between two particles of different substances (e.g. water
molecule and xylem wall)
The xylem wall is also polar and hence can form intermolecular associations with water
molecules
As water molecules move up the xylem via capillary action, they pull inward on the xylem walls
to generate further tension
