3.9 - Transport in plants

Question 1

Transport systems in dicotyledonous plants

Answer 1

have a series of transport vessels running through the stem, roots and leaves known as a vascular system
- made up of two types of transport vessel, xylem and phloem
- arranged in vascular bundles

Question 2

Vascular bundles in the stem

Answer 2

Around the edge to give strength and support
- phloem on the outside and xylem on the inside

Question 3

Vascular bundles in the root

Answer 3

In the middle to help plants withstand tugging strains as the stems and leaves are blown in the wind
- The xylem is a cross in the middle and the phloem are arranged around the xylem in the endodermis

Question 4

Vascular bundles in leaves

Answer 4

In the midrib of the leaf (main vein carrying vascular tissue)
- the Xylem is above the phloem, nearer the top of the leaf (where the palisade mesophylls are)

Question 5

Why do multicellular plants need transport systems

Answer 5

• metabolic demands (oxygen, glucose, hormones and mineral ions need to be transported around the plant)
• size (plants continue to grow throughout their lives, so some are very large e.g. trees)
• surface area to volume ratio (size and complexity of multicellular plants means that SA:V is relatively low)

Question 6

Structure and function of the xylem

Answer 6

• the transport of water and mineral ions up from the roots to the shoots and leaves
• provides structural support
• made up of mostly non-living tissue
• The main structures are xylem vessels, long hollow structures made of dead cells with no end cell walls fused together
• spirals of lignin run around the lumen of the xylem, helping reinforce vessels and providing waterproofing
• xylem vessels have lots of small unlignified areas called bordered pits, where water leaves the xylem and moves into other cells
• thick-walled xylem parenchyma are packed around the xylem vessels, storing food and containing tannin deposits
• xylem fibres are long cells with lignified secondary walls that provide extra mechanical strength but do not transport water

Question 7

Adaptations of plants to increase SA:V ratio

Answer 7

• Plants have a branching body shape
• Leaves are flat and thin
• Roots have root hairs

Question 8

Structure and function of the phloem

Answer 8

• a living tissue that transports food in the form of organic solutes around the plant from the leaves where they are made by photosynthesis
• supplies cells with sugars and amino acids needed for cellular respiration and synthesis of molecules
• main transporting vessels are sieve tube elements, unlignified cells joined end to end to form a long hollow tube
• in the areas between the cells, the walls become perforated to form sieve plates, letting the phloem contents move through
• companion ells linked to the sieve tube elements by many plasmodesmata maintain nucleus and all their organelles, so act as a ‘life support system’ for the sieve tube system, which have no organelles
• companion cells actively transport sugar into sieve cells, and water through osmosis
• contains supporting tissues including fibres and sclereids with very thick cell walls

Question 9

How is water pulled up through the xylem through the transpiration stream (cohesion-tension theory)

Answer 9

1. water vapour evaporates from the mesophyll layers into the air spaces in the leaf and move out of the stomata into the surrounding air by diffusion, meaning there is a higher concentration of solutes at the leaf end of the plant
2. water enters the leaf from the xylem tissue (osmosis)
3. water molecules stick together (cohesion - weak H bonds)
4. water molecules pull up further molecules as they leave the xylem, a column of water is pulled up the xylem
5. the water is under tension as evaporation is pulling the water column upwards, and gravity is pulling it downwards
6. adhesion of water molecules to the sides of the xylem stops the column from breaking

Question 10

What are dicotyledonous (dicot) plants

Answer 10

Plants that make seeds that contain 2 cotyledons (organs that act as a food store for the developing embryo plant and forms the first leaves when it germinates), so 2 leaves grow from the seed

Question 11

The two pathways that water moves across the root into the xylem

Answer 11

• the symplast pathway (through cytoplasm)
• the apoplast pathway (along cell walls)

Question 12

The symplast pathway

Answer 12

• water moves through the symplast (the continuous cytoplasm of the living plant cells that are connected through the plasmodesmata) by osmosis
• the root hair cell has the highest water potential as a result of water moving in from the soil, so water moves away from the root hair cell to the next cell along through osmosis
• this continues from cell to cell until the xylem is reached
• the water leaving the root hair cell maintains a steep water potential gradient

Question 13

The apoplast pathway

Answer 13

• the movement of water through the apoplast (the cell walls and the intercellular spaces- spaces between cells)
• water fills the spaces between the loose, open network of fibres in the cellulose cell wall
• as water molecules move into the xylem, more molecules are pulled through the apoplast behind them due to cohesion

Question 14

What happens when the apoplast pathway reach the endodermis of the vascular bundle

Answer 14

• the epidermis contains the Casparian strip, a waterproof layer. This means that water cannot move through the epidermis in intercellular spaces, so water in the apoplast pathway then diffuse through a selectively permeable membrane by osmosis like in the symplast pathway.

Question 15

What is the difference between the apoplast and symplast pathway

Answer 15

• The apoplast route is the fully permeable route in which the water movement occurs in passive diffusion
• the symplast is a selectively permeable route in which the water movement occurs by osmosis, so any solutes are regulated
• water moving in the xylem through the apoplast, is regulated since it must enter the symplast in the endodermis.

Question 16

What is the casparian strip

Answer 16

a band of waxy material called suberin that runs around each of the endodermal cells, forming a waterproof layer

Question 17

Why do solutes need to be regulated before water enters the xylem

Answer 17

• the water in the symplast and apoplast systems travelling through partially permeable membranes on the way to the xylem excludes any potentially toxic solutes in the soil water from reaching living tissues, as the membranes would have no carrier proteins to admit them

Question 18

Transpiration

Answer 18

The loss of water vapour through stomata by diffusion. It is an inevitable consequence of gas exchange

Question 19

Transpiration stream

Answer 19

The continuous movement of water from the root to the atmosphere

Question 20

The process of transpiration

Answer 20

• oxygen diffuses out of the leaf by diffusion down a concentration gradient through the stomata (usually on the underside of the leaf)
• the stomata can be open and closed by guard cells
• when the stomata are open to allow exchange of carbon dioxide and oxygen, water vapour also moves out by diffusion

Question 21

Evidence for the cohesion-tension theory

Answer 21

• changes in the diameter of trees, when transpiration is at its highest in the day, the xylem vessels have the most tension, so the tree shrinks in diameter
• when a xylem vessel is broken (stem is cut), air is drawn into the xylem rather than water leaking out. This also means the plant can no longer move water up the stem as the continuous stream of water has been broken

Question 22

Factors affecting the rate of transpiration

Answer 22

• temperature
  Increases the kinetic energy of the water molecules and therefore increases the rate of evaporation from the spongy mesophyll cells into the air spaces of the leaf. Increases the concentration of water vapour that the external air can hold before it can become saturated
• humidity
  A high relative humidity will lower the rate of transpiration because of the reduced water potential gradient between the inside of the leaf and the outside air. Very dry air increases the rate of transpiration
• wind speed
  increases transpiration as wind maintains the concentration gradient between the inside and outside of the leaf
• stomata density
  more stomata = more diffusion pathways = more transpiration
• number of leaves on plant
  more stomata = more transpiration
• light intensity
  increased amount of photosynthesis opens stomata for gas exchange, so increased rate of transpiration

Question 23

Measuring transpiration using a potometer

Answer 23

A plant cutting is placed in a water filled tube with an air bubble in. The rate of transpiration is measured by measuring the movement of the air bubble over time

Question 24

Translocation

Answer 24

The movement of assimilates from source to sink
- main sources are green leaves and stems, storage organs such as tubers or tap roots that are unloading their stores, food stores in seeds where they germinate
- main sinks are roots that are actively growing or absorbing mineral ions, meristems that are actively dividing, an parts of the plant that are laying down food stores

Question 25

Active loading

Answer 25

The process of sucrose entering the phloem.
- hydrogen ions are pumped out (proton pump) of companion cells into the surrounding tissue by active transport, creating a concentration gradient between the companion cells and the surrounding tissue
- H+ acts as a co-transporter, so it brings sucrose into the companion cells by facilitated diffusion back down the concentration gradient
- the concentration of sucrose in the companion cells increases, so the sucrose diffuses out of the
companion cells down the concentration gradient into the sieve tube elements through plasmodesmata
- as a result of the build up of sucrose in the sieve tube elements, water moves in through osmosis, building up hydrostatic pressure
- the water and sucrose moves up or down the plant by mass flow down a hydrostatic pressure gradient to the sinks (areas of low hydrostatic pressure)

Question 26

plasmodesmata

Answer 26

a narrow thread of cytoplasm that passes through the cell walls of adjacent plant cells and allows communication between them.

Question 27

Evidence of translocation/mass flow

Answer 27

• advances in microscopy allows us to see the adaptations of the companion cells for active transport (lots of mitochondria)
• if the mitochondria of the companion cells are poisoned, translocation stops
• the flow of sugars in the phloem is a lot faster than it would be by diffusion alone
• using evidence from aphid studies, there is a positive pressure in the phloem the sap out through a hole in the phloem

Question 28

Xerophyte

Answer 28

A plant adapted to be able to survive in an environment with little available water or moisture

Question 29

Adaptations of plants to conserve water

Answer 29

• thick waxy cuticle to minimise water loss
• sunken stomata so there is less of a concentration gradient
• reduced numbers of stomata
• reduced leaves
• hairy leaves to trap water and reduce water vapour potential gradient
• curled leaves confining the stomata within a microclimate of humid air
• storing water in specialised parenchyma (succulents)
• leaf loss when water is not available
• root adaptation such as long tap roots or widespread shallow roots
• avoiding low levels of water through dormancy or surviving as storage organs

Question 30

Hydrophytes

Answer 30

plants that live either partially or completely submerged in water

Question 31

Adaptations of hydrophytes

Answer 31

• stomata on top of leaf to allow for gas exchange
• very thin waxy cuticle, only to prevent damage
• surface areas of leaves increased as water reduces light reaching plant
• smaller or absent roots, or roots to anchor in flowing water
• Aerenchyma (air sacs) allowing leaves to float on surface for increases gas exchange