8. Transport in plants

Question 1

Xylem

Answer 1

Tissue in a plant that transports water and mineral ions from roots to the leaves and shoots.

Also helps provide support for the plant + keep it upright.

Question 2

Xylem - Structure

Answer 2

• Consists of hollow tubes
• Xylem cells (are dead + hollow with no cell walls between cells in tube) form long continuous tubes from the root to the leaves
• Cellulose cell wall but also strengthened by a substance named lignin that spirals around the xylem supporting it.
• Lumen (cavity of xylem) –> contains no cytoplasm
• No nucleus

Question 3

Phloem

Answer 3

Tissue in a plant that transports amino acids and sucrose.

Question 4

Phloem - Structure

Answer 4

• Consists of living cells arranged end to end
• Phloem vessels contain cytoplasm which go through holes in the sieve plates (cell walls between cells replaced by sieve plates allowing movement of substances through it) from one cell to the next.
• Have companion cells which have mitochondria for energy transfer. This energy is used to move the food through the phloem.

Question 5

Xylem - Position in plant

Answer 5

• Located in vascular bundle, but found in different positions depending on the part of the plant.

Question 6

Position of xylem and phloem in the root of a non-woody dicotylednous plant (dicot).

Answer 6

• Several xylems in the middle fo the root (looking like an x or a +)
• Smaller phloems surrounding the yxlem x

Question 7

Position of xylem and phloem in a stem

Answer 7

• Several vascular bundles located around the edge of the stem.
• The inner half of the vascular bundle is the xylem.
• Whereas the outer half is the phloem.

Question 8

Position of the xylem and phloem in leaves

Answer 8

• Xylem, located in upper half (towards upper mesophyll layer) of the vascular bundle)
• Phloem located in bottom half.

Question 9

Root hair cells - Transport in plants

Answer 9

• Specialised cells found on the surface of plant roots.
• Long hair like structures creating a large surface area for the absorption of water/ mineral ions from the soil.

Nucleus located at base of hair-like protrusion.

Question 10

Pathway of water from the roots to the leaves

Answer 10

Root hair cells on the surface of a root
–>
Root cortex cells inside the root
–>
Xylem vessels from the roots, moves through the stem
–>
Water enters the cells in the leaf
–> Water evaporates into the air spaces in the spongy mesophyll
–>
Water vapour diffuses out of the leaf through the stomata

Question 11

Root cortex cells

Answer 11

Type of cell in a plant roots that surround xylem and phloem.

Question 12

How to show the pathway of water through a plant - Experiment

Answer 12

Water-soluble stains can be used to investigate the pathway of water through the stem of a plant

(Celery often used bc xylem vessels are easily seen both in cross-section and in transverse section.)

Question 13

Transpiration

Answer 13

The loss of water vapour from plant leaves by evaporation at the surface of mesophyll cells, followed by diffusion of water vapour through the stomata.

Question 14

Transpiration pull

Answer 14

A force in xylem that is created by evaporation of water from the leaves.

Question 15

Transpiration - Explained

Answer 15

Spongy mesophyll tissue is loosely arragned with air spaces + large area of cell surfaces –> allows the rapid evaporation of waterfrom the surface of mesophyll cells.

Air spaces int he mesophyll tissue are connected to stomate –> water vapour diffuses through stomata and out of leaf.

Question 16

Increase in temperature –> effect on transpiration

Answer 16

Higher temperature increases evaporation –> increases rate of transpiration

(high evaporation cools plant, bc endothermic reaction)

Question 17

Increase in wind –> Effect on transpiration rate

Answer 17

Wind is moving air –> sweeps awat water vapour from surface of leaves –> speeds up evaporation bc steeper concentration gradient

Question 18

Increasing humidity –> effect on transpiration rate

Answer 18

Increased humidity = decreased transpiration rate bc less steep concentration gradient, slowing down diffusion of water vapour through stomata.

Question 19

Transpiration pull - Explained

Answer 19

• Attractive forces between the molecules in a liquid –> causes the molecules to stick together (why water droplets exist)
  –> This is called cohesion

As one water particle is evaporated from the leaf, it pulls up more water particles in a column behind it.

Question 20

Transpiration pull - Adhesion

Answer 20

Adhesion allows water molecules to stick to xylem walls because of the low force of attraction between them.

When stomata are closed and evaporation isn’t happening, it stops water molecules from travelling down xylem.

(Force of transpiration pull overcomes force of adhesion when water evaporates, thus allowing movement)

Question 21

Effect of stomata number on transpiration rate

Answer 21

The more stomata present on a leaf, the more water is lost through transpiration.

Larger leaves have more stomata due to larger surface area.
—>

Large the leaf surface area, the more stomata, the higher the rate of transpiration.

Question 22

Wilting plants

Answer 22

If plant cells lose more water than they are gaining, they become flaccid.

–> lose their rigidity and the stems/ leaves wilt.

–>

Occurs when the rate of transpiration is greater than the rate of water absorption through the root hair cells.

Question 23

Translocation

Answer 23

The movement of sucrose and amino acids in phloem.

They move from a source (where they are produced) to a sink (where they are stored or used in respiration or growth).

Question 24

Source - Transpiration

Answer 24

• A region of production in a plant

(eg. glucose is being made in the leaves by photosyntehsis and then converted into sucrose for transport around the plant. Leaves = source)

Question 25

Sink - Translocation

Answer 25

• A region of storage (carbohydrates stored as starch would be a sink for sucrose.)
• A region of use in respiration or growth
  (Glucose used in aerobic respiration to release energy or amino acids used to make proteins for growth)

Question 26

Which direction does translocation occur? Is it always the same?

Answer 26

Source –> Sink

However, the same part of tge country can act as a source or a sink but at different times in its life.

Eg.
In root tubers, leaves are the source during the summer. The underground tubers are used as sinks where starch is stored. During winter the roots with stored starch become the source, whereas the leaves are the sink.