5. plant transport

Question 1

why do plants need a transport system?

Answer 1

A plant transport system ensures all areas of the plant receives a sufficient amount of nutrients to survive.

Question 2

why do larger plants require specialised transports systems?

Answer 2

• increasing transport distance
• surface area:volume ratio
• a higher metabolic rate meaning a higher metabolic demand

Question 3

list some structural differences between a monocot and a dicot plant

Answer 3

Question 4

what are dicot plants?

Answer 4

plants that make seeds that contain 2 cotyledons

Question 5

what are the two main groups of dicot plants?

Answer 5

1) herbaceous dicots (non woody stem) e.g. daises
2) woody dicots e.g. oak

Question 6

what is the vascular system?

Answer 6

A plant has a series of transport vessels running through the roots, stems, and leaves

Question 7

what are the two vascular systems?

Answer 7

xylem phloem

Question 8

what are the two different types of plant transport systems?

Answer 8

• Transpiration System
• Translocation System

Question 9

what is the transpiration system?

Answer 9

• The movement of water molecules and dissolved minerals
  ions
• Xylem vessels
• Passive process

Question 10

what is the translocation system?

Answer 10

• The movement of sugars (Sucrose) & amino acids
• Phloem vessel – sieve & companion cells
• Active process

Question 11

What is a vascular bundle?

Answer 11

• Xylem and Phloem are arranged in vascular bundles in the roots, stems
  and leaves. The arrangement of xylem and phloem is different in different
  organs.
• There is a layer of cambium in between xylem and phloem, that is
  meristem cells which are involved in production of new xylem and phloem
  tissue.

Question 12

Answer 12

Question 13

Vascular Bundles in roots

Answer 13

• This provides a ‘drill’ like
  structure
• This enables the plant to push
  down into the root
• Xylem tissues is the strongest
  so is in the centre – X structure
• Phloem in four separate
  sections

Question 14

Vascular Bundles in leaf

Answer 14

• Xylem is located on top of the phloem
• This only applies to dicotyledonous plants, other plants
  types have a different structure – you don’t need to know
  these

Question 15

Vascular Bundles in Stem

Answer 15

• Xylem is located on the
  inside – in non-wooded
  plants
• This provides additional
  support to the stem
• The cambium layer
  contains meristem cells

Question 16

Question 17

Structure of Xylem

Answer 17

Structure:
*A dead tissue - there is no cytoplasm and no
nuclei in xylem tissue.
* Hollow tubes
*Cell wall contains spiralised lignin that gives
the tissue high strength.
*Pits in wall (non-lignified areas)
Function:
*Transports water and dissolved minerals
upwards from the root hair cells to the
leaves.
*This is called the transpiration stream.

Question 18

Adaptations of Xylem tissue

Answer 18

Answer 19

Question 20

Structure of phloem tissue

Answer 20

Structure:
*A living tissue.
* Composed of tubes of elongated cells called phloem sieve
tubes.
* Transports assimilates (sucrose, amino acids etc) from the
leaves (source) to other parts of the plant (sinks).
*Process is called translocation.
* Assimilates move from one phloem cell to the next
through pores in the end walls called sieve plates.
* Each sieve tube has an associated companion cell (with a
nucleus, organelles, enzymes.
Function:
*Transports food (in the form of sucrose) upwards and
downwards, depending on where food is needed –
bidirectional transport.
*This is called Translocation

Question 21

Phloem tissue adaptations

Answer 21

Question 22

Comparison of plant tissues

Answer 22

Question 23

Need for water
in plants

Answer 23

• Mineral ions and sugars
  are transported in
  aqueous solution
• Water is a raw material
  of photosynthesis
• Cooling effect (by
  transpiration)
• Turgor pressure –
  hydrostatic skeleton

Question 24

Adaptations of root hair cells

Answer 24

• Very thin cellulose walls, that are readily permeable
  to water and dissolved mineral ions.
• Microscopic in size
• Large SA:V ratio
• Concentration of solutes in the cytoplasm of root
  hair cells maintains a water potential gradient
  between the soil water and the cell.

Question 25

Structure of a root
can you label

Answer 25

Question 26

Water movement pathways

Answer 26

• Symplast Pathway (through cytoplasm)
• Vacuolar Pathway (through vacuoles)
• Apoplast Pathway (through cell walls)

Question 27

What is transpiration

Answer 27

Transpiration is the loss of water vapour (by evaporation and diffusion) from the
surface of leaves and stems of a plant.

Question 28

How is the rate of transpiration controlled?

Answer 28

• 1) Waxy cuticle (=waterproof layer)
• 2) Guard cells can open or close stomata
• 3) Very few stomata on upper surface of leaf

Question 29

Transpiration stream

Answer 29

the flow of water (in continuous
columns), up the xylem vessels from roots to leaves.

Question 30

Adhesion

Answer 30

• 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.

Question 31

Cohesion

Answer 31

• 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.

Question 32

Cohesion-tension theory

Answer 32

• Water vapour diffuses/evaporates out of the leaf via the
  stomata (transpiration) from an area of high ψ to an area
  of low ψ.
• This loss of water vapour creates a low hydrostatic
  pressure at the top of the xylem (i.e. in the leaf).
• Water is drawn into the xylem in the root (higher
  hydrostatic pressure). Pressure gradient is created.
• This creates a tension (suction) in the xylem which pulls
  up water in a continuous column.
• Within the xylem vessels the columns of water are held
  together by cohesion (the molecules are hydrogen
  bonded to each other) and by adhesion (the attraction
  between a water molecule and the walls of the xylem
  vessels).
• Column (of water) is pulled up by tension.

Question 33

Stomata

Answer 33

Guard cells control the opening and closing of stomata.

Guard cells are turgid – Stomata Open
* Water moves into the vacuoles by osmosis.
* Outer wall is more flexible than the inner wall,
so to cell bends and opens the stoma.
Guard cells are flaccid – Stomata closed
* Water moves out of the vacuoles by osmosis.
* Outer wall is more flexible than the inner wall,
so to cell bends back and closes the stoma.

Question 34

Conditions for stomatal opening

Answer 34

• Stomata open during the day and close during the night.
• High water potential outside the stomata
• Low concentrations of CO2 cause stomata to open. High CO2
  causes stomata to close.
• Light causes stomata to open.

Question 35

How are guard cells adapted?

Answer 35

• Unevenly thickened (cell) wall – wall beside the pore is thicker.
• Able to change shape/bend
• Transport proteins/ion channels in the plasma membrane. Absorption
  of K+
  ions by the guard cells.
• K+
  ions decrease the water potential hence water enters by osmosis and
  guard cells can become turgid.
• Presence of chloroplasts & mitochondria to provide ATP energy.

Question 36

Mesophytes

Answer 36

able to take up sufficient water
to replace transpiration (most plants).

Question 37

Hydrophytes

Answer 37

live either partially or
completely submerged in water – problems
with O2 uptake.

Question 38

Xerophytes

Answer 38

live in areas where water lost via
transpiration is greater than taken up by roots.

Question 39

Translocation

Answer 39

• Translocation is the movement of assimilates within phloem sieve
  tubes (e.g. sucrose/amino acids, hormones etc) from where they are
  made (source) to where they are required (sink). It is an active
  process.
• Translocation occurs in phloem vessels.
• Requires ATP energy to create a pressure difference.
• Movement is bidirectional (from source to sink).
• Liquid being transported is called ‘phloem sap’
• Glucose is transported as sucrose in the phloem sap (20-30%)

Question 40

Sources

Answer 40

This is the site where
sucrose/assimilates are made and
loaded into the phloem. (high
concentration).
E.g.
* Green leaves and green stem
(photosynthesis produces glucose
which is transported as sucrose, as
sucrose has less of an osmotic effect
than glucose).
* Storage organs eg. tubers and tap
roots (unloading their stored
substances at the beginning of a
growth period).
* Food stores in seeds (which are
germinating).

Question 41

Sinks

Answer 41

• The site where sucrose /assimilates are
  unloaded from the phloem for use or
  storage
  E.g.
• Meristems (apical or lateral) that are
  actively dividing.
• Roots that are growing and / or actively
  absorbing mineral ions.
• Any part of the plant where the
  assimilates are being stored (eg.
  developing seeds, fruits or storage
  organs).

Question 42

Process of translocation

Answer 42

There are 3 stages to the movement of sucrose and
assimilates from source to sink.
1) Active loading at the source into the phloem sieve
tube.
2) Mass flow of sucrose through the sieve tube
elements (involves water from xylem).
3) Active unloading of sucrose at the sink

Question 43

Loading of assimilates – pathways

Answer 43

• The pathway that sucrose molecules use to travel to the sieve
  tubes is not fully understood yet.
• The molecules may move by the:
  Symplastic pathway (through the cytoplasm and
  plasmodesmata) which is a passive process as the sucrose
  molecules move by diffusion.
  Apoplastic pathway (through the cell walls) which is an active
  process.

Question 44

1. Active loading at the source

Answer 44

Question 45

Adaptations for active loading

Answer 45

• Companion cells have infoldings in their cell surface membrane to increase the available
  surface area for the active transport of solutes and many mitochondria to provide the
  energy for the proton pump.
• This mechanism permits some plants to build up the sucrose in the phloem to up to three
  times the concentration of that in the mesophyll.

Question 46

2. Mass flow of sucrose through phloem sieve tubes.

Answer 46

• Sugars/sucrose/assimilates enter the sieve tube element (at the
  source), this lowers the water potential in the sieve tube.
• Water enters the sieve tube by osmosis from the xylem.
• This raises the hydrostatic pressure at the source.
• When assimilates leave the sieve tube at the sink, this
  increases the water potential inside the sieve tube.
• Water leaves the sieve tube by osmosis, down a water potential
  gradient and lowers the hydrostatic pressure (at the sink).
• Water moves down the hydrostatic pressure gradient (from
  high to low) towards the sink, also moving sucrose (and other
  assimilates) along the phloem.
• This is called mass flow.

Question 47

3. Active unloading of sucrose at the sink

Answer 47

• Sucrose needs to be actively unloaded into the sink where it is needed.
• Unloading of sucrose is thought to be similar to the loading of sucrose.
• Sucrose is actively transported out of the companion cells and then moves
  out of the phloem sieve tubes into the sinks via the apoplastic or symplastic
  pathways.
• In the sink sucrose is converted into other molecules e.g. starch. This helps
  to maintain a concentration gradient.
• When sucrose diffuses out of the sieve tubes, this increases the water
  potential of the tube.
• Water therefore moves out of the sieve tube (back into the xylem
  vessels) by osmosis.
• This creates a low hydrostatic pressure at the sink, compared to the
  higher hydrostatic pressure at the source.