Eduqas A-level biology component 3: adaptation for transport: plants

how do plants transport water and mineral ions?

- photosynthetic cells in the leaf need water and mineral ions which are only available from the soil.

- they are transported from the roots to the leaves upwards in hollow tubes formed by dead cells called xylem tissue

how do plants transport products of photosynthesis?

(6CO2+6H20-->C6H12O6+6O2)

- glucose is used to make sucrose and amino acids (photosynthates)

- they must be moves from the leaves to other plant organs

- they are transported up and down (bi-directional movement) in phloem tissue

describe the vascular arrangement of leaf tissue (photo)

1) xylem

2) phloem

(both 1&2 form a vascular bundle arranged as a vein or midrib)

3) guard cells

4) stoma

5) lower epidermis

6) spongy mesophyll

7) palisade mesophyll

8) upper epidermis

9) waxy cuticle

describe the vascular arrangement of stem tissue

1) phloem

2) xylem

3) cambium

(1,2&3 form a vascular bundle arranged around the periphery of the stem)

4) pith

5) epidermis

6) cortex

describe the vascular arrangement of root tissue

1) pericycle

2) endodermis

3) phloem

4) xylem

(there are no vascular bundles but 1,2,3&4 are arranged in the centre of the root and is called the stele (5) which is vascular tissue)

5) stele

6) cortex

7) root hair

8) epidermis

explain the different transport pathways:

1) apoplast

2) symplast

3) vacuolar

1) water is taken up by the root hair cell and moves across the cortex by cohesion via cell walls

2) water moves from the cytoplasm of one cell to the next by osmosis via plasmodesmata

3) water can move via the cytoplasm and vacuoles

describe the uptake of water and mineral ions into and across the root

- mineral ions such as nitrates are actively transported from the soil into root hair cells. this lowers the water potential inside the RHC so water enters by osmosis.

- they move through the apoplast pathway along the cell walls of the cortex via cohesion.

- some water moves through the cortex via the symplast pathway through cytoplasm & plasmodesmata.

describe the uptake of water and mineral ions into the xylem

- when water travels into the epidermis which has a water impermeable casparian strip made of Suberin that blocks the apoplast pathway.

- they are then forced across the membrane and into the symplast pathway.

- active transport moves ions into the cytoplasm of the endodermal cells then diffuse into the pericycle and the xylem creating a low WP at each stage allowing water to follow by osmosis.

why would there be less water supply to the plant if grown in waterlogged conditions?

- the rate of diffusion of CO2 & O2 decreases meaning photosynthesis and respiration is inhibited.

- for ions to be transported into the xylem & lower the WP for water to move in by osmosis, ATP is required for active transport.

- inhibited respiration = a lack of ATP synthesis and so there are a lack of ions in the xylem so a lower WP gradient and less water is moved in by osmosis and so root pressure is reduced.

1)what is root pressure?

2)how does the plant ensure toxic ions cannot enter its cells?

1)when water moves from the epidermal cells of the root and into the xylem by osmosis this generates hydrostatic pressure and forces water a small distance up the xylem.

2) there are no carrier proteins specific to these ions on the membrane so these ions cannot enter

what is transpiration? what are the stages of the transpiration stream?

- transpiration is the evaporation of water from the inside the leaves through the stomata and into the atmosphere.

1) water is absorbed by root hair cells

2) it moves through root tissue, into the xylem(lignin-hydrophillic) and is transported up the xylem in the plant stem to the leaf.

3) water is transported by osmosis from the xylem in the leaf to the cells of the spongy mesophyll where is evaporates from the surface of the cells into air spaces.

4) water vapour then diffuses from the air spaces out of the leaf through the stomata down a water potential gradient.

1)what is cohesion?

2)what is adhesion?

3)what does the cohesion- tension theory state?

1) water molecules are attracted to each other by hydrogen bonds

2) water molecules are attracted to the hydrophilic lining of the lignified xylem vessel walls

3) as water vapour diffuses out of the stomata by transpiration, water molecules are drawn up from behind (across the leaf and up the xylem to replace those lost) because of adhesions and cohesion. this upward movement of water creates tension on the xylem vessel walls.

what are the two other processes that help water move up the xylem a small amount?

1) capillarity;

the forces of adhesion and cohesion allow water molecules to rise up narrow tubes for a short distance which is useful in small plants ( not in large plants because after a small distance it will be opposed by the downward force of gravity)

2) root pressure (again has more effect in small plants)

what are the factors affecting transpiration?

(any factor that increases the water potential gradient between the water vapour in the leaf and the surrounding atmosphere)

1) temperature - rise = more KE of water molecules = increases rate of evaporation and diffusion. WP of the atmosphere is lower in higher temperatures = higher WP gradient

2) wind speed (air movement) - still air means water vapour gathers around stomata (diffusion shell), reducing WP gradient. faster wind blows away the diffusion shell & increases the rate of transpiration.

3) humidity - WP gradient decreases when there is more water vapour in the atmosphere (lower WP gradient)

4) light intensity - light causes the stomata to open to allow gas exchange for photosynthesis so more water loss

what is a potometer experiment? what should you do?

- a potometer measure the rate of uptake of water by the shoot which indicates the rate of transpiration

- it is only an estimate and some water in used for photosynthesis or evaporate off the surface of the leaf

1) cut a leafy shoot under water to prevent air bubbles entering into the xylem (breaks H bonds and affects cohesion)

2) fill apparatus completely with water to avoid air bubbles in the glassware

3) fit leafy shoot and seal all joints with vaseline (air tight)

4) pat leaves dry (water would reduce WP gradient and affect results |)

5) introduce one air bubble into capillary tube (open tap)

6) measure air bubble distance after certain time

repeat 3 times and calculate a mean (improve reliability)

1) what does standard deviation tell you?

2) how do you calculate standard deviation?

1)the measure of the spread of data. low SD = values are close to the mean so more consistent and reliable. high SD = values are further from the mean so less consistent and reliable.

2) calculate the mean, subtract the mean from each value, square each answer, add them all together, divide this number by the number of pieces of data -1, find the square root of that value.

what are the four different types of cell that makes up xylem tissue? (photo)

1) xylem parenchyma - living tissue

2) xylem fibres - no role in transport (support only)

3) xylem vessels - the main cells that conduct water, continuous column of dead cells with dissolved cross walls to form hollow tubes from roots to leaves. walls are thickened with lignin (impermeable to water) to strengthen and support the plant (stop collapsing under pressure).

4) xylem tracheids - also conduct water but less efficient than vessels - more elongated with tapering ends. cells walls are thickened and impregnated with lignin.

- both 4&5 have pits in their side walls to allow movement of water between adjacent vessels. in tracheids, they are involved in movement of water to nearby living tissue.

what are the four different types of cell that makes up phloem tissue? (photo)

1)sieve tubes - transport sucrose and amino acids (organic compounds) up/down plant stem, the thin cellulose wall at the ends are perforated (sieve plates) allowing the cytoplasm to run into adjacent cells.

2) companion cells - connect to sieve tubes via plasmodesmata, they have a nucleus and provide ATP for active transport of sugars in/out of the sieve tube. (this is important because sieve tube cells lose nuclei and organelles as they mature.

3) phloem fibres- for support

4) phloem parenchyma - living tissue

how are sieve tube elements adapted to their function?

- don't contain organelles so the path of sucrose/amino acids is undisrupted and faster with more volume

- no nucleus and very few organelles so less resistance to the flow of sucrose and amino acids in the phloem

- closely associated to companion cells which provide ATP energy and proteins

- sieve plates with pores allow sucrose and amino acids in the cytoplasm to move between cells.

describe translocation

- translocation is the transport of soluble organic materials produced by photosynthesis (sucrose and amino acids) in the phloem.

- movement is bidirectional

- liquid inside the phloem tubes is called the sap

- the source is the region where the products of photosynthesis are produced and exported

- the sink is the region where the products of photosynthesis are stored or used for growth

there are 2 experiments that are used for evident of translocation;

1) describe the ringing experiments

2) describe the aphid experiments

1) removal of a ring of outer bark tissue from a woody stem removes the phloem and is left for a week, organic compounds cannot be transported past that region. a bulge is seen due to accumulated phloem sap that cannot move further down. (shows that its the S & AA are transported by the phloem and that it is positioned on the outside of the stem.

2) aphids are small insects that collect contents of individual phloem sieve tube cells. they have mouthparts (stylets) which penetrate the tubes to feed on the sugary sap inside. if they are anaesthetised with CO2, the stylet can be cut off and left in the stem so pure phloem sap can be collected for analysis. this is more accurate that a syringe/needle as the aphid's enzymes ensure the stylet doesn't get blocked.

how do scientists use radioisotopic labelling?

a plant is supplied with radioactive 14CO2, so the plant fix it during photosynthesis to form radioactive photosynthates at the leaves. after 20 mins, the plant is placed on photographic film. dark areas show radioactivity. this proves bidirectional movement as the radioactive carbon is fixed into the sugar at the source and then translocated to the sink. the radioactivity is seen in the aerial parts of the plant & the roots.

what is the mass flow hypothesis?

sugars flow passively from high conc in the leaf (source) to low conc (sink) down a pressure gradient.

- sucrose made in photosynthesis is loaded by active transport into sieve tubes using ATP

- water enters sieve tubes along a WP gradient by osmosis

- pressure in the sieve tubes increases & sucrose moves down a pressure gradient through the phloem, towards the sink.

- sucrose is unloaded by active transport into cells at the sink

- water moves by osmosis out of the phloem, sucrose is removed & the pressure in the phloem tissue becomes lower at the sink.

what are arguments against the mass flow theory?

- no explanation of sieve plates which seem to act as barriers to flow

- S&AA have been observed moving at different rates & directions

- sieve tubes have high ATP consumption & translocation is slowed or stopped if respiratory inhibitors (cyanide) is added.

- companion cells are found all the way along the sieve tubes (not just source & sink) and contain numerous mitochondria for production of ATP. if they purely load & unload photosynthates from the sieve tubes the would not be needed anywhere but sources and sinks.

- therefore an active process within the sieve tubes may be involved

what are alternative theories to the mass flow theory?

1) streaming in the cytoplasm of sieve tubes could be responsible for bi-directional movement.

2) protein filaments have been observed passing through the sieve pores suggesting different solutes are transported by different filaments.

what is a mesophyte and how are they adapted to their conditions?

- most plants in temperate regions and crop plants.

- they grow best in well drained soils and moderately dry air.

- a number of plant behaviours allow mesophytes to survive at unfavourable times of the year e.g.

- deciduous trees shed leaves in autumn to survive over the winter, new leaves grow in the spring to reserve water & energy from less photosynthesis & transpiration

- bulbs and storage organs are produced by non woody plants to survive underground over winter

- annual plants produce seeds and die in the same year, seeds survive winter frost and germinate the next spring in favourable conditions.

what is a xerophyte and how are they adapted to their conditions?

- low water availability, hot, dry desert conditions, cold regions (frozen soil), or exposed windy locations

- marram grass (dry & windy sand dunes) have;

sunken stomata - water vapour is trapped in pits decreasing WP gradient so less water lost by transpiration.

hairs on leaf surface - trapped water vapour between hairs decreases WP gradient so less water is lost by transpiration.

thick waxy cuticle - reduced water loss from the epidermis

rolled leaves - less exposed stomata & trapped water vapour so WP gradient decreases so less water loss via transpiration.

reduced leaf size (spine) - reduces SA from which transpiration can occur

(some have shallow/deep roots to maximise absorption)

what is a hydrophyte and how are they adapted to their conditions?

- water plants, submerged or partially submerged in water.

- stomata one the upper epidermis to allow gas exchange with the air above

- large air spaces to provide buoyancy for the leaves and act as a reservoir of O2 & CO2

- thin or no waxy cuticle as there is no need to reduce water loss as they live in or on water

- poorly developed xylem tissue as there is no need to transport large quantities of water as plant is aquatic

- little lignin as water is a supportive medium and so little lignin id required to support the xylem tissue