3.4 Mass transport plants MSA

Describe the mass flow hypothesis for the mechanism of translocation in plants (4 marks)

In source/leaf sugars actively transported into phloem

By companion cells

Lowers water potential of sieve cell/tube and water enters by osmosis

Increase in pressure causes mass movement (towards sink/root)

Sugars used/converted in root for respiration for storage

Name the process that produced the 14CO2 released from the trunk (1 mark)

Respiration

The pieces of leaf tissue examined were very thin (optical microscope) explain why (2 marks)

Single/few layers of cells

So light can pass through

Other than distribution of stomata, suggest and explain two xerophytic features (2 marks)

Hairs so trap water vapour and water potential gradient decreased

Stomata in pits/grooves so trap water vapour and WP gradient decreased

Thick cuticle/waxy layer so increases diffusion distance

Waxy layer/cuticle so reduces evaporation/transpiration

Rolled/folded/curled leaves so trap water vapour and WP gradient decreased

Spines/needles so reduces SA:V

Suggest and explain two reasons why the rate of water uptake by a plant might not be the same as the rate of transpiration (2 marks)

Water used for support/turgidity

Water used in photosynthesis

Water used in hydrolysis

Water produced during respiration

Describe the processes involved in the transport of sugars in plant stems (5 marks)

At source, sucrose is actively transported into the phloem/sieve element/tube

By companion/transfer cells

Lowers WP in phloem/sieve element/tube and water enters by osmosis

Produces high hydrostatic pressure

Mass flow/transport towards sink/roots/storage tissue

At sink/roots sugars are removed/unloaded

Define translocation (1 mark)

Organic compounds synthesised in the leaves of a plant are transported to the plants roots

One theory of translocation states that organic substances are pushed from a high pressure in the leaves to a lower pressure in the roots. Describe how a high pressure is produced in the leaves (3 marks)

WP becomes lower/more negative as sugar enters phloem

Water enters phloem by osmosis

Increased volume of water causes increased pressure

The student assumed that water uptake was equivalent to the rate of transpiration. Give two reasons why this might not be a valid assumption (2 marks)

Used to provide turgidity/support/description of

Used in photosynthesis/produced in respiration

Apparatus not sealed/leaks

A potometer measures the rate of water uptake rather than the rate of transpiration. Give two reason why it does not truly measure rate of transpiration (2 marks)

Water used for support/turgidity

Water used in photosynthesis/produced in respiration

Apparatus not sealed/leaks

Explain how water enters xylem from the endodermis in the root and is then transported to the leaves (6 marks)

In the root;

Casparian strip blocks apoplast pathway/only allows symplast pathway

Active transport by endodermis

Of ions/salts into xylem

Lower WP in xylem/water enters xylem by osmosis/down a WP gradient

Xylem to leaf;

Evaporation/transpiration from leaves

Creates cohesion/tension/H-bonding between water molecules/negative pressure

Adhesion/water molecules bind to xylem

Creates continuous water column

Describe the mass flow hypothesis for the mechanism of translocation in plants (4 marks)

In source/leaf sugars actively transported into phloem

By companion cells

Lowers water potential of sieve cell/tube and water enters by osmosis

Increase in pressure causes mass movement (towards sink/root)

Sugars used/converted in root for respiration for storage

Explain the relationship between stomatal opening and photosynthesis (2 marks)

Stomata allow uptake of CO2

CO2 used in/required for photosynthesis

Most water uptake in a root takes place in the region just behind the root tip. Explain why (2 marks)

Presence of root hair creates a large surface area

This area is permeable to water

In a root, water passes through the endodermis and enters the xylem. Explain how (4 marks)

Suberin/waterproof material in cell walls of endodermis

Blocks apoplast pathway/only symplast pathway

Ions moved by active transport into xylem

This lowers the WP in the xylem and water enters by osmosis into xylem

The leaves of plants that grow in dry areas usually have a low number of stomata per mm. Suggest 3 other adaptations the leaves might have (3 marks)

Thick cuticle

Small leaves/low SA

Hairy leaves

Sunken stomata

Rolled leaves

The oxygen dissociation curve of the fetus is to the left of that for its mother. explain the advantage of this for the fetus (2 marks)

Higher affinity/loads more oxygen

At low/same/high partial pressure of oxygen

Oxygen moves from mother/to fetus

After birth, fetal haemoglobin is replaced with adult haemoglobin. Suggest the advantage of this to the baby (2 marks)

Low affinity/oxygen dissociates

Oxygen to respiring tissues/muscles/cells

In which one of the four chambers of the human heart does pressure reach the highest value? (1 mark)

Left ventricle

Explain how the structure of the left ventricle causes high pressure (1 mark)

Thick muscle/thick walls