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Biology - AS Level > Plant transport > Flashcards

Plant transport Flashcards

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1
Q

how does water enter the root hair cell from the soil

A

enters by osmosis
high water potential to lower water potential
down water potential gradient

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2
Q

how are root hair cells adapted for the absorption of water

A

increased SA for absorption
thin surface layer ( cell wall + membrane) - so reduced diffusion pathway

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3
Q

how do plants use water

A

-lost in transpiration
-provide water for photosynthesis turgidity and other metabolic processes

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4
Q

in which direction does water move from the root hair cell

A

move across parenchyma cells (cortex)
the to the xylem

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5
Q

what two methods can water move across the root to the xylem

A

-symplast pathway
-apoplast pathway

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6
Q

what is the symplast pathway

A

-water travels as a result of osmosis
-passes through the cytoplasm of the cells
water enters the symplast pathway by osmosis from high WP in the soil to low WP in the cell through partially permeable mem
passes into cytoplasm and continues from one cell to the next via plasmodesmata - connect the cells of cell wall
water pass along cytoplasm cells by osmosis until the xylem is reached

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7
Q

what is the apoplast pathway

A

movement of water through the cell wall
water is drawn along in a continuous column due to cohesion
this creates a tension which draws water along

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8
Q

how does water enter into the xylem vessels

A

salts from endodermis actively pumped in xylem
lowers WP which draws water in from high WP from endodermis and pericycle - high pressure in xylem - causes root pressure increases and water moves up - more water then can move in - water molecules stick together ( cohesion )

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9
Q

what is the evidence for root pressure

A

-root pressure increases with a rise in temp and decreases with decrease in temp
- metabolic inhibitors cause root pressure to cease - these prevent energy release by respiration
- decrease in availability of oxygen and respiratory substrates causes a reduction in root pressure

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10
Q

how does water move up the xylem

A

Cohesion - stick to wall
Adhesion - xylem narrows
Root pressure - push up
Tension - pull up

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11
Q

how does tension work

A

-water lost in transpiration pulls water up xylem
-sucked up and also pulled down by gravity
-this causes tension
- water moves down pressure gradient from high to low - by mass flow
- pull creates continuous stream of water

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12
Q

how is tension caused

A

when water evaporates water in xylem pulled up as force is generated - can be called negative pressure
- as passes from atmospheric pressure in roots to lower atmos pressure in leaves

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13
Q

how does transpiration affect flow of water

A

greater the water loss the faster water is pulled up - this force causes a decrease in stem diameter and trunk diameter

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14
Q

how does weather affect tension

A

humid - reduced
dry/hot/windy days - maximum

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15
Q

how does cohesion work

A

water molecules stick together - force of attraction created by formation of H bonds

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16
Q

what is cohesion-tension theory

A

force combined of transpiration and cohesion - principle mechanism responsible for water movement

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17
Q

how does adhesion work

A

narrowness of xylem means there is a large SA:V and lots of water is in contact with xylem
-attraction of water to another material = adhesion
-the narrower the tube the more water that can stick

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18
Q

what does capillarity mean

A

the narrower the tube the more water that can stick

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19
Q

what is root pressure

A

when salts pumped in actively and lower WP which draws water in by osmosis - pressure pushes water up to the xylem causing root pressure

20
Q

what is transpiration

A

Transpiration refers to the loss of water vapour via the stomata by diffusion
Note that this is different to the transpiration stream which is the movement of water from the roots to the leaves

21
Q

how is transpiration important for the plant

A

It provides a means of cooling the plant via evaporative cooling
The transpiration stream is helpful in the uptake of mineral ions
The turgor pressure of the cells (due to the presence of water as it moves up the plant) provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants

22
Q

what are some external factors that effect transpiration

A

humidity - difference in moisture inside and out of the leaf is reduced - reduces rate of diffusion - low humidity leads to an increase rate of transpiration as max diffusion

temp - high temp increase rate - water molecule shave more kinetic energy so move out leaves

dry soil - stomata close as plants unavible to replace water that is lost in transpiration

light - more light = more photosynthis so stomata open - when less light stomata close so less transpiration

23
Q

internal factors that affect transp

A

leaf SA - increase SA increase rae sd more surface with stomata is exposed to air

distrubution of stomata - most stomata on lower side as does not face sun or wind

waxy cuticle - waterproof layer to reduce water loss

leaf structure - sunken stomata, curled leaves and hairs which trap layer of air in front of stomata and reduce conc gradient

24
Q

what are xerophytes

A

plants adapted to live in dry conditions

which have special features

-specialised leaves - reduce water loss
-extensive root system - to increase water uptake
-swollen stem - to store water
- thick waxy cuticle
-small leaf surface - less area for evaporation
- low density - reduced outlets for evap
-sunken stomata - reduced conc gradient
-hairs - trap moist air
-folded or rolled up grass leaves - maintains humid air around stomata

eg marram grass - sand dunes/ costal

25
Q

what are the xylem vessels

A

-long system of dead cells that have no living contents and connected to form tubes
-transport water and provided structural support
-have lignin in cell walls - again structural support
-narrow to allow adheson and are waterproof - so no water leaves

26
Q

what are the phloem tubes

A

mixed tissue that contains - made of live cells
-sieve tube elements
-companion cells
-phloem fibres
-parenchyma cells

27
Q

evidence for cohesion tension theory

A

change in diameter of the ree trunk according to the rate of transpiration
-transpiration increases = more -ve pressure = xylem walls move inwards = trunk shrinks
-transpiration decreases = less negative = xylem out = trunk increases

xylem vessel broken = air in = no water enters tree ( continuous column of water so molecules don’t stick)
also water does not leak out = instead air drawn in as constantly under tension

28
Q

what is translocation

A

movement of sugars in a plant

29
Q

how is the phloem structured

A

-sieve tube elements (cells)
-companion cells

30
Q

what are the sieve tube elements

A

-cells that are elongated with pores at each end which make up the sieve plate
-have cytoplasm with a few organelles pushed to the side
-no nucleus

31
Q

What are companion cells

A

cells found next to the sieve tube cells
contain cytoplasm, a nucleus and more organelles including mitochondria
linked to sieve tubes by plasmodesmata

32
Q

what are sources

A

the sites of sugar production

33
Q

what are sinks

A

places where sugars will be used directly or stored for future use

34
Q

what is the mass flow hypothesis for translocation

A

explains the movement of sugars and amino acids in solution from high pressure region to a low pressure region through phloem sieve tubes

35
Q

state and explain the first stage of translocation

A

transfer of sucrose into sieve elements
-leaves make sugar in photosynthesis - they are the source of the sugar
-glucose is converted to sucrose - sucrose is higher energy store - so better
-however sucrose is too large to simply diffuse across membranes
-so companion cells actively load sucrose into the phloem against a conc gradient
-H ions pumped out of the companion cell using ATP creating H+ gradient
-sucrose along with H+ diffuses into the companion cell by facilitated diffusion
-sucrose enters the sieve tube element by facilitated diffusion - using carrier protein-sugar enters phloem sieve tube and lower WP
-water then enter sieve tubes by osmosis making them swollen and turgid - causes high hydrostatic pressure in the phloem near the leaves

36
Q

state and explain the 2nd part of translocation

A

mass flow of sucrose through the sieve tube elements
-at respiring cells (sinks) sucrose is used or converted to starch - ie sugar is removed
-sucrose is too large to simply diffuse out so carries proteins are involved
-sucrose is actively transported into these cells from the sieve tubes - lowers the cells WP
-water follows by osmosis
-this lowers the hydrostatic pressure of the sieve tubes

37
Q

how does hydrostatic pressure differ bwt the sources and sinks

A

high at source
low at sink
- move down pressure gradient

38
Q

is translocation active or passive

A

mostly passive but only occurs due to the active transport of sugar - so affected by temp and metabolic poisons

39
Q

who came up with the mass flow hypothesis

A

The mass flow hypothesis was modelled by Ernst Münch in 1930. His simple model consisted of:
Two partially permeable membranes containing solutions with different concentrations of ions (one dilute the other concentrated)
These two membranes were placed into two chambers containing water and were connected via a passageway
The two membranes were joined via a tube
As the membranes were surrounded by water, the water moved by osmosis across the membrane containing the more concentrated solution which forced the solution towards the membrane containing the more dilute solution (where water was being forced out of due to hydrostatic pressure)

40
Q

evidence for mass flow of translocation

A

-When the phloem sieve tube is punctured phloem sap oozes out
This suggests that it is under pressure
-Phloem sap taken from near a source has a higher sucrose concentration than sap taken from near a sink
This suggests that different water potentials would result in osmosis into/out of the sieve tubes at those two locations
-companion cells possess many mitochondria and readily produce ATP for movement

41
Q

evidence against mass flow translocation

A

-function of sieve plates are unclear- seem to hinder mass flow
-It has been suggested that some sieve tubes translocate at different times
The mass flow hypothesis states that nearly all sieve tubes should be involved in translocation at the same time as they are all connected to the same leave
-The rate of translocation of different organic substances was measured and the results showed that amino acids appeared to travel more slowly than sucrose
The mass flow hypothesis states should be flowing at the same rate

42
Q

what methods are used to investigate translocation

A

tracer and ringing - the removal of a ring of surface tissues from the stem of the plant while leaving the stem core intact

43
Q

what is the process of the tracer ad ringing experiments

A

As the phloem is located towards the outside of the stem and the xylem towards the centre, the ring removes the phloem only with the xylem remaining intact
The site of ringing is often shown on diagrams as a small black bar dividing the stem - remove bark and phloem
After the ringing has been done the plant can then be exposed to a radioactive tracer so that the direction and rate of translocation can be investigated
14C is commonly used for these experiments as it is readily absorbed by the leaves and used in photosynthesis to produce sucrose
The sucrose formed will be radioactive so its subsequent movement around the plant via translocation can be traced
The amounts of radioactive carbon present in different parts of the plant can be detected
If the mass flow hypothesis is correct then the bulk flow of phloem sap should be in one direction (from source to sink) and occur at the same rate in any sieve tube at the same time
As leaves are the site of photosynthesis they are the source tissue, sink tissues can be above or below the leaves
If the xylem is damaged during the ringing process the plant will not have an adequate supply of water and will wilt

44
Q

what other methods can be used to track transport in plants

A

using aphids - aphids mouthparts inserted into phloem then cut off the body leaving mouthparts in place
-mouthparts then exude the sap which can be collected and analysed for radioactive sugar

45
Q
A

Biology - AS Level (17 decks)

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