A LEVEL BIOLOGY 7402 PAPER 3 2026 ACTUAL
SCRIPT COMPLETE QUESTIONS AND
ANSWERS 100% CORRECT
◉ Mass transport Answer: Is the movement of molecules over large
distances due to pressure differences
◉ Transpiration Answer: When the stomata are open water vapour
diffuses out of the stomata from the air spaces in leaf down a water
potential gradient. This loss of water is called transpiration. To
replace this , water evaporates from the walls of mesophyll cells into
the air spaces forming water vapour. The water in mesophyll cells is
replaced by continuous columns of water from the xylem vessels in
the leaf.
◉ Features of xylem vessels Answer: Their cell walls contain lignin
which strengthens xylem wall and makes them water proof causing
the cells to die. This leaves a hollow lumen with no cytoplasm which
offers little resistance to the mass flow of water and minerals. Walls
contain small holes called pits which can divert water laterally if
xylem vessel is blocked. There are no end walls so water can travel
in continuous columns up the xylem vessel.
◉ Cohesion tension theory Answer: As transpiration occurs through
open stomata , water evaporates from the cell walls of mesophyll
,cells of the leaf and is replaced by water from the xylem due to
cohesive forces between water molecules. This creates negative
pressure at the top of the xylem in the stem and creates tension
which pulls water up towards the leaves. Continuous columns of
water are maintained due to cohesion between water molecules and
adhesion between water molecules and the walls of the xylem
vessel. Water enters the stem through the roots.
◉ Four main factors effecting transpiration rate Answer: Light -
greater light intensity faster transpiration rate due to stomata
opening when it is light to let in CO2 for photosynthesis
Temperature - higher temperature faster transpiration rate due to
water molecules having more energy so evaporate from mesophyll
cell walls faster, which increases water potential gradient allowing
water to diffuse out of the leaf faster
Humidity - lower humidity faster transpiration rate as if the air is
dry the water potential gradient increases which increases
transpiration
Wind - the windier the faster transpiration rate as air blows away
water molecules from the stomata which increases water potential
gradient
◉ Evidence for cohesion tension theory Answer: Tension has been
measured in xylem as plants transpire.
If a column of water in xylem is broken air bubbles can form and
stop any further upward movement of water.
,Respiratory inhibitors , cyanide or lack of oxygen, do not inhibit this
process.
Diameter of trees decrease during transpiration as tension pulls
xylem walls in and can be measured by a dendrometer
◉ Potometer Answer: Estimates transpiration rates by measuring
water uptake by a plant. Cut a shoot underwater and at a slant to
prevent air from entering the xylem and to increase surface area for
water uptake. Assemble potometer in water and insert the shoot
underwater so no air can enter. Remove apparatus from water but
keep the end of the capillary tube submerged as its water and air
tight. Dry leaves and allow time for shoot to acclimatise and the shut
tap. Remove capillary end tube till one air bubble forms and put tube
back in water. Record starting positions of air bubble. Start a stop
watch and record distance moves by bubble per unit time. The rate
of air bubble movement is transpiration rate. Only change one
variable at a time and all other conditions must remain constant. Use
reservoir to return bubble to start for repeats.
◉ Why is the trunk diameter lowest at noon ? Answer: Stomata are
open so transpiration rates are high, increased tension so water
column is pulled up xylem faster , this pulls walls of xylem in and
xylem vessels desecrate in diameter.
◉ Xerophytic adaptations Answer: Thick waxy cuticle to reduce
evaporation/ water loss from upper epidermis , greater thickness
increases length of diffusion pathway for water.
, Stomata sunken in pits to build humidity but trapping moist air
which reduced water potential gradient so reduces water loss
Hairs on lower epidermis of lead traps water vapour between the
hairs which reduces water potential gradient
Curled leaves with stomata inside protect from wind and traps
humid air with high water potential which reduces water potential
gradient
Smaller surface area to volume ratio slows rate of diffusion
Stomata confined to underside of leaf where the temperature is
cooler so less heat energy to evaporate water
Close stomata during the day to reduced transpiration
◉ Features of phloem tissue Answer: Contains sieve tube elements
which don't contain a nucleus and few organelles but are living cells
due to plasmodesmata connections with the companion cell which
contains a nucleus , mitochondria and other organelles to carry out
functions for sieve cells. Each sieve tube is connected by a sieve plate
which is perforated with pores.
◉ Translocation Answer: Is the movement of sucrose and organic
solutes in the phloem tissue from sources to sinks. A source is where
the organic solutes are produced and at a high concentration and are
usually the mesophyll cells of leaves by condensation fo fructose and
glucose. A sink is where the organic solutes is used up and are at a
low concentration and are other parts of the plant, especially
growing parts where the sucrose is hydrolysed into glucose and
SCRIPT COMPLETE QUESTIONS AND
ANSWERS 100% CORRECT
◉ Mass transport Answer: Is the movement of molecules over large
distances due to pressure differences
◉ Transpiration Answer: When the stomata are open water vapour
diffuses out of the stomata from the air spaces in leaf down a water
potential gradient. This loss of water is called transpiration. To
replace this , water evaporates from the walls of mesophyll cells into
the air spaces forming water vapour. The water in mesophyll cells is
replaced by continuous columns of water from the xylem vessels in
the leaf.
◉ Features of xylem vessels Answer: Their cell walls contain lignin
which strengthens xylem wall and makes them water proof causing
the cells to die. This leaves a hollow lumen with no cytoplasm which
offers little resistance to the mass flow of water and minerals. Walls
contain small holes called pits which can divert water laterally if
xylem vessel is blocked. There are no end walls so water can travel
in continuous columns up the xylem vessel.
◉ Cohesion tension theory Answer: As transpiration occurs through
open stomata , water evaporates from the cell walls of mesophyll
,cells of the leaf and is replaced by water from the xylem due to
cohesive forces between water molecules. This creates negative
pressure at the top of the xylem in the stem and creates tension
which pulls water up towards the leaves. Continuous columns of
water are maintained due to cohesion between water molecules and
adhesion between water molecules and the walls of the xylem
vessel. Water enters the stem through the roots.
◉ Four main factors effecting transpiration rate Answer: Light -
greater light intensity faster transpiration rate due to stomata
opening when it is light to let in CO2 for photosynthesis
Temperature - higher temperature faster transpiration rate due to
water molecules having more energy so evaporate from mesophyll
cell walls faster, which increases water potential gradient allowing
water to diffuse out of the leaf faster
Humidity - lower humidity faster transpiration rate as if the air is
dry the water potential gradient increases which increases
transpiration
Wind - the windier the faster transpiration rate as air blows away
water molecules from the stomata which increases water potential
gradient
◉ Evidence for cohesion tension theory Answer: Tension has been
measured in xylem as plants transpire.
If a column of water in xylem is broken air bubbles can form and
stop any further upward movement of water.
,Respiratory inhibitors , cyanide or lack of oxygen, do not inhibit this
process.
Diameter of trees decrease during transpiration as tension pulls
xylem walls in and can be measured by a dendrometer
◉ Potometer Answer: Estimates transpiration rates by measuring
water uptake by a plant. Cut a shoot underwater and at a slant to
prevent air from entering the xylem and to increase surface area for
water uptake. Assemble potometer in water and insert the shoot
underwater so no air can enter. Remove apparatus from water but
keep the end of the capillary tube submerged as its water and air
tight. Dry leaves and allow time for shoot to acclimatise and the shut
tap. Remove capillary end tube till one air bubble forms and put tube
back in water. Record starting positions of air bubble. Start a stop
watch and record distance moves by bubble per unit time. The rate
of air bubble movement is transpiration rate. Only change one
variable at a time and all other conditions must remain constant. Use
reservoir to return bubble to start for repeats.
◉ Why is the trunk diameter lowest at noon ? Answer: Stomata are
open so transpiration rates are high, increased tension so water
column is pulled up xylem faster , this pulls walls of xylem in and
xylem vessels desecrate in diameter.
◉ Xerophytic adaptations Answer: Thick waxy cuticle to reduce
evaporation/ water loss from upper epidermis , greater thickness
increases length of diffusion pathway for water.
, Stomata sunken in pits to build humidity but trapping moist air
which reduced water potential gradient so reduces water loss
Hairs on lower epidermis of lead traps water vapour between the
hairs which reduces water potential gradient
Curled leaves with stomata inside protect from wind and traps
humid air with high water potential which reduces water potential
gradient
Smaller surface area to volume ratio slows rate of diffusion
Stomata confined to underside of leaf where the temperature is
cooler so less heat energy to evaporate water
Close stomata during the day to reduced transpiration
◉ Features of phloem tissue Answer: Contains sieve tube elements
which don't contain a nucleus and few organelles but are living cells
due to plasmodesmata connections with the companion cell which
contains a nucleus , mitochondria and other organelles to carry out
functions for sieve cells. Each sieve tube is connected by a sieve plate
which is perforated with pores.
◉ Translocation Answer: Is the movement of sucrose and organic
solutes in the phloem tissue from sources to sinks. A source is where
the organic solutes are produced and at a high concentration and are
usually the mesophyll cells of leaves by condensation fo fructose and
glucose. A sink is where the organic solutes is used up and are at a
low concentration and are other parts of the plant, especially
growing parts where the sucrose is hydrolysed into glucose and
