Transport in plants

Question 1

Describe the functions of the transport systems in plants.

Answer 1

1. Metabolic demands- places that don’t photosynthesise need glucose and oxygen transported to them and waste products of cell metabolism removed.
2. Metabolic demands-Hormones made in one part of the plant need to be transported to the area where they have an affect.
3. Metabolic demands- mineral ions absorbed by the roots need to be transported to all cells to make proteins to make enzymes and the structure of the cell

Question 2

Explain 2 reasons why multicellular plants have to have transport systems.

Answer 2

1. Size- Some are very large so need effective transport system to move substances up and down from the tip of the roots to the stems and leaves
2. SA- Overall have relatively small SA:Vol ratio, despite large leaf SA- once trunks and stems and roots are taken into account- so can’t rely on diffusion alone.

Question 3

Define the term “herbaceous”

Answer 3

Refers to a plant that has a non-woody stem and which dies back at the end of the growing season.

Question 4

Define the term dicotyledonous plants (dicots)

Answer 4

Plants that produce seeds containing two cotyledons, which act as food stores for the developing embryo and form the first leaves when the seed germinates

Question 5

Define the term vascular system

Answer 5

A system of transport vessels in animals or plants

Question 6

Define the term vascular bundles

Answer 6

The vascular system of herbaceous dicots made up of a xylem and phloem tissue

Question 7

Name the two types of transport vessels in vascular bundles.

Answer 7

Xylem and Phloem

Question 8

Draw, label and annotate diagrams of transverse sections (cross-sections) through a typical stem, root and leaf of an herbaceous dicotyledonous plant.

Answer 8

1. Stem- outer layer is epidermis, middle layer is the cortex, middle area is the parenchyma and little beans half in parenchyma and half in cortex are vascular bundles- half closest to middle is xylem, half pointing out is phloem
2. Root- Root hairs pointing out, exodermis is outer layer, then epidermis, then cortex, then a endodermis surrounds the middle circle- where the xylem makes and x shape and the phloem are 4 circles in between the branches of the x
3. Leaf- upside down hill shape- circle in the hill- lower half of circle is the phloem and upper is the xylem,

Question 9

State the function of the xylem

Answer 9

1. Transport of water and mineral ions

2. Support

Question 10

State the function of the phloem

Answer 10

1. Transports food in the form of organic solutes around the plant from the leaves where they are made in photosynthesis
2. It provides cells with the sugars and amino acids it needed for cellular respiration and synthesis of all other useful molecules.

Question 11

Describe the structure of xylem and explain how it is adapted for its function.

Answer 11

1. Largely non-living tissue
2. Flow is from roots to shoots and leaves
3. Xylem vessels are long hollow structures made by several columns of tubes fusing together end to end- contain no cytoplasm or end cell wals- improves flow of water
4. Also contain xylem parenchyma and lignin
5. Water and mineral ions move into and out of the vessels through small pits in the walls where there is no lignin

Question 12

Describe the patterns of lignification in xylem and state its function.

Answer 12

1. Cells walls are thickened with lignin which helps to support the walls and stops them collapsing inwards
2. It can be in spirals or as distinct rings
3. These patterns allow flexibility and prevents the stem form breaking
4. As the cell gets older the amount of lignin increases

Question 13

Describe the function of xylem parenchyma

Answer 13

1. Packs around the xylem vessels storing food and containing tanin deposits- bitter chemicals that protects plant tissue from attack by herbivores

Question 14

Identify xylem and phloem from longitudinal cross section of stem

Answer 14

1. Xylem is relatively thin on the inside

2. Layer between and phloem is further to the outside

Question 15

Define the term sieve tube element

Answer 15

The mains cells of the phloem that have a greatly reduced living content and sieve plates between the cells

Question 16

Define the term sieve plates

Answer 16

Areas between the cells of the phloem where the walls become perforated giving many gaps and a sieve-like appearance that allows the phloem contents to flow through.

Question 17

Define the term companion cell

Answer 17

The active cells found next to sieve tube elements that supply the phloem vessels with all of their metabolic needs

Question 18

Describe the structure of phloem and explain how it is adapted for its function.

Answer 18

1. Sieve tube elements which are made up of many cells joined end to end to form a long hollow structure
2. Unlike xylem tissue the phloem tubes are not lignified.
3. Between the cells there are sieve plates
4. The tonoplast, nucleus and other organelles breakdown- becomes a tube filled with phloem sap - mature phloem cells have no nucleus
5. Companion cells (maintain their nucleus and organelles) are linked to the sieve tube elements by plasmodesmata.- they are very active cells and act as life support system for sieve tube cells
6. Phloem tissue also contains supporting tissues including fibres and sclereids- cells with extremely thick cell walls

Question 19

Describe how to produce stained sections (both transverse sections and longitudinal sections) of plant stems for viewing under a light microscope.

Answer 19

1. Use a scalpel to cut a cross-section of the stem (transverse or longitudinal)- very thin
2. Place the cut sections in water using tweezers, until you use them- stops drying out
3. Add drop of water to microscope slide add the plant section and add one or two drops of a stain - leave for 1 min
4. put cover slip on

Question 20

Define transpiration

Answer 20

The loss of water vapour from the stems and leaves of a plant as a result of evaporation from the cell surfaces inside the leaf and diffusion down a concentration gradient out through the stomata

Question 21

Define transpiration stream

Answer 21

The movement of water through a plant from the roots until it is lost by evaporation from the leaves

Question 22

Define transpiration pull

Answer 22

The force which aids in drawing the water upward from roots to leaves.

Question 23

Explain why water loss is inevitable for plants.

Answer 23

Because the stoma must open to exchange oxygen and carbon dioxide, and water vapour is also lost by diffusion

Question 24

Outline the route water takes through a plant.

Answer 24

1. Drawn from the root hair cells by osmosis down the water potential gradient
2. Up the plant to the leaves down the water potential gradient- soil around the roots have higher water potential than leaves

Question 25

Define the term stomata

Answer 25

Pores in the surface of a leaf or stem that may be opened and closed by guard cells

Question 26

Define the term guard cell

Answer 26

Cells that can open and close the stomatal pores controlling gaseous exchange and water loss in plants

Question 27

Define the term adhesion

Answer 27

The force of attraction between unlike molecules

Question 28

Define the term cohesion

Answer 28

Cohesion refers to the sticking together of alike molecules, such as water molecule being attracted to another water molecule.

Question 29

Explain how transpiration results in water moving through the plant (the cohesion-tension theory), and state whether it is an active or passive process.

Answer 29

Passive process- Cohesion-tension theory

1. Water moves into the xylem down the water potential gradient
2. Root pressure/ high hydrostatic pressure at the bottom of the xylem
3. Water vapour is lost by transpiration form the top of the leaves
4. This creates a low hydrostatic pressure at the top of the xylem
5. Water under tension is pulled up in a continuous column
6. There is cohesion between the water molecules and adhesion between the water molecules and the xylem (hydrogen bonds with carbohydrates in the cell wall).
7. This results in capillary action
8. Water moves up the xylem by mass flow down the hydrostatic gradient- to replace water lost by evaporation-transpiration pull

Question 30

Describe 3 sources of evidence for the cohesion-tension theory.

Answer 30

1. changes in the diameter of trees- when transpiration is at it’s height during the day the tension in the xylem vessels is at it’s highest too- tree shrinks in diameter. Opposite happens during the night
2. When a xylem is broken- when you cut a flower stem - air is drawn in rather than water leaking out
3. If the above happened, the plant can no longer move water up the stem as the continuous stream of water molecules held together by cohesion forces has been broken

Question 31

Explain how guard cells can open and close stomata.

Answer 31

1. When turgor is low the asymmetric configuration of the guard cell walls closes the pores
2. When turgor is high- the cell swells and because the inner wall is less flexible than outer wall the cells become bean-shaped and open the pore.
3. When water becomes scarce hormonal signals from the roots can trigger turgor loss from the guard cell, which closes the stomatal pore

Question 32

State 5 environmental factors that can affect the rate of transpiration and for each explain how they have their effect.

Answer 32

1. Light intensity- Lighter the faster- stomata open when it gets light (photosynthesis), increasing the rate of water vapour diffusing out and therefore increasing the evaporation from the surfaces of the leaves
2. Temperature- higher the faster- increases the kinetic energy of the water molecules ad therefore increases the rate of evaporation. It also increases the concentration of water vapour that the external air can hold before becomes saturated- decrease water potential
3. Humidity- lower humidity, faster transpiration- if air around plant is dry, it increases the water potential gradient between the leaf ad air
4. Wind- Windier the faster- blows water molecules away from the around the stomata, increasing the water potential gradient
5. Soil-water availability- if dry soil the plant will be under water stress and rate will decreases

Question 33

Draw, label and annotate a diagram of a potometer to show how it can measure transpiration rate.

Answer 33

1. A capillary tube with a measuring device attached- ruler etc
2. has a tap and fills the potometer with water- pushes the air bubble to the start
3. Cut a leafy shoot under water and transferred to the apparatus to avoid introducing air bubbles to the stem
4. Measure the rate of water uptake by measuring the distance the air bubble moves in a given time
5. You can vary conditions to see their affect on the rate of transpiration

Question 34

Describe 5 functions of water in plants.

Answer 34

1. Turgor pressure- provides a hydrostatic skeleton to support the stems and leaves
2. Turgor- also drives cell expansion- force that enables plant roots to force their way though tarmac and concrete
3. Loss of water through evaporation keeps the plant cool
4. Mineral ions and the products of photosynthesis are transported as aqueous solutions
5. Water is the raw material for photosynthesis

Question 35

Describe 4 ways that the root hairs of root hair cells are adapted as exchange surfaces.

Answer 35

1. Microscopic size- can easily penetrate between soil particles
2. Each microscopic hair has large SA;Vol ratio - thousands on each tip
3. Each hair has a thin surface layer- diffusion and osmosis can take place quickly
4. Concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell

Question 36

Explain why water move from the soil into root hair cells.

Answer 36

1. Soil water has a very low concentration of dissolved minerals so has a very high water potential
2. The cytoplasm and vacuolar sap of the rot hair cell contain many different solvents including sugars, mineral ions and amino acids so the water potential in the cell is lower
3. So water moves into the root hair cells by osmosis

Question 37

Name the 2 pathways by which water travels across the root to the xylem.

Answer 37

1. Appolast pathway

2. Symplast pathway

Question 38

Describe the symplast pathway of water movement.

Answer 38

1. Water moves through the symplast- continuous cytoplasm of the living plant cells that is connected through the plasmodesmata- by osmosis.
2. Root hair cell has higher water potential than the next ell along- water diffuses from the soil making the cytoplasm more dilute- so water moves by osmosis to the next cell
3. This continues until the xylem is reached

Question 39

Describe the apoplast pathway of water movement.

Answer 39

1. Movement of water through the apoplast- cell walls and intercellular spaces
2. Water fills spaces between the loose, open network of fibres in the cellulose cell wall- by diffusion
3. As water molecules move into the xylem, ore water molecules are pulled through the apoplast behind hem due to cohesive forces
4. This pull from the water into the xylem and up the plant along with cohesive forces causes a tension- meaning there is a continuous flow of water through he open structure of the cellulose cell wall- little or no resistance

Question 40

Define the term endodermis

Answer 40

The layer of cells surrounding the vascular tissue of the roots,

Question 41

Define the term casparian strip

Answer 41

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

Question 42

Define root pressure

Answer 42

The active pumping of minerals into the xylem by root cells that produces a movement of water into the xylem by osmosis

Question 43

Describe structure, function and location of the Casparian strip.

Answer 43

1. When water in the apoplast pathway gets to the endodermis cells in the root, it’s path is blocked by the casparian strip in the cell walls
2. Now the water is forced to take the symplast pathway
3. This means that water has to go through a plasma membrane- partially permeable so can control whether or not substances in the water get through
4. Once past this barrier the water moves into the xylem

Question 44

Explain the role of active transport by endodermal cells for the movement of water.

Answer 44

1. The solute concentration in the cytoplasm of the endodermal cells is relatively dilute compared to the cells in the xylem
2. The endodermal cells appear to move mineral ions into the xylem by active transport
3. As a result the water potential of the xylem cells is much lower than the water potential of the endodermal cells
4. This increases the rate of water moving into the xylem by osmosis

Question 45

Describe the evidence for the role of active transport in moving water from root endodermis into the xylem.

Answer 45

1. Some poisons - cyanide- affect the mitochondria and prevent the production of ATP - if cyanide is applied to root cells so there is no energy supply, root pressure disappears
2. Root pressure increases with a rise in temperature and falls with a fall in temperature suggesting chemical reactions are involved
3. If levels of oxygen or respiratory substrates fall, root pressure falls
4. Xylem sap may exude from the cut end of stems at certain times

Question 46

Define the term xerophyte

Answer 46

Plants with adaptations that enable them to survive in dry habitats or habitats where water is in short supply

Question 47

Define hydrophyte

Answer 47

Plants with adaptations that enable them to survive in very wet habitats or submerged or at the surface of water

Question 48

State 3 examples of xerophytic plants

Answer 48

1. conifers and marram grass found on sand dunes and coastal areas
2. Many plants in very cold or icy conditions- water is frozen
3. Cacti- deserts etc

Question 49

State 3 examples of hydrophytic plants.

Answer 49

1. Water lilies and water cress which grow at the surface
2. Duckweeds- submerged or free-floating
3. Bulrushes and yellow irises- grow at edge of water

Question 50

State 3 ways most plants conserve water or gain better access to water.

Answer 50

1. Waxy cuticle- reduce transpiration from the leaf surfaces
2. Stomata are mainly found on the underside of the leaf and can be closed to prevent water loos
3. Roots grow down to the water in the soil

Question 51

Describe the environmental conditions where water loss, or water access, may become a real problem for plant species.

Answer 51

1. Hot, dry, breezy conditions- water will evaporated very rapidly
2. Cold or icy climate- water is frozen

Question 52

Draw a table to show 10 adaptations xerophytes have for conserving, storing or accessing water and for each adaptation explain how it benefits the plant.

Answer 52

1. Thick waxy cuticle- minimise water loss
2. Sunken stomata- stomata located in pits, which reduce air movement, producing a micro-climate of humid air that reduces the water potential gradient
3. Reduced number of stomata- reduce water loss by transpiration but also reduce gas exchange capabilities
4. Reduced leaves- greatly reduced SA:Vol ratio minimising water loss by transpiration
5. Hairy leaves- like spines on cacti create a micro-climate of still, humid air, reducing the water potential gradient- some have micro-hairs in the sunken stomatal pits
6. Curled leaves- confines all the stomata within a micro-environment of still, humid air to reduce diffusion of water vapour form the stomata
7. Succulents- store water in specialised parenchyma tissue in their stems and roots- water is stored for droughts
8. Leaf loss- Some plants lose their leaves when water isn’t available- palo verde loses all it’s leaves in long, dry seasons and it’s trunk and branches turn green and photosynthesise with minimal water loss
9. Root adaptations- long roots that can grow deep into the ground or a mass of widespread shallow roots that can absorb any available water before it evaporates.
10. Avoiding the problems- many lose their leaves and become dormant or die completely leaving seeds behind to germinate.

Question 53

Describe two problems faced by hydrophytes.

Answer 53

1. Leaves need to float so they are near the surface of the water to get the light needed for photosynthesis
2. Water-logging is a major problem- air spaces of plants need to be full of air, not water

Question 54

Draw a table to show 8 adaptations of hydrophytes and for each adaptation explain how it benefits the plant.

Answer 54

1. Very thin or no waxy cuticle- don;t need to conserve water
2. Many always-open stomata on the upper surfaces- maximising the number of stomata maximises gaseous exchange- no risk of loss of turgor- stomata open all the time and guard cells are inactive. Need to be on upper surfaces so are in contact with the air
3. Reduced structure to the plant- water supports the leaves so no need for the strong supporting structures
4. Wide, flat leaves- spread across the surface of water to capture as much light as possible
5. Small roots- water an diffuse directly into stem and leaf tissue- less need for uptake by roots
6. Large SA of stems and roots under water- maximises area for photosynthesis ad for oxygen to diffuse into submerged plants.
7. Air sacs- enable the leaves or flowers to float to the surface of the water
8. Aerenchyma- specialised parenchyma tissue forms in the leaves,stems and roots - many large air spaces- making leaves and stems more buoyant and forming low-resistance internal pathway form the movement of substances such as oxygen to tissues below the water.

Question 55

State the form in which carbohydrates are transported in plants.

Answer 55

Sucrose

Question 56

Define the term translocation

Answer 56

The transport of organic solutes around a plant in the phloem

Question 57

Define the term source

Answer 57

Regions of a plant that produce assimilates (e.g glucose) by photosynthesis or form storage materials e.g leaves, storage organs

Question 58

Define the term sink

Answer 58

Regions of a plant that require assimilates to supply their metabolic needs e.g. roots, fruits

Question 59

Define the term assimilates

Answer 59

The products of photosynthesis that are transported around a plant e.g sucrose

Question 60

State 3 examples of sources

Answer 60

1. Green leaves and stems
2. Storage organs such as tubers and tap roots
3. Food stores in seeds when they germinate

Question 61

State 3 examples of sinks.

Answer 61

1. Roots that are growing and or actively absorbing mineral ions
2. Meristems that are actively dividing
3. Any part of the plant that are laying down food stores e.g developing seeds, fruits or storage organs

Question 62

Define the term “phloem loading” and state the two main ways this occurs.

Answer 62

1. When the soluble produts of photosynthesis are moved into the phloem form the sources by an active process
2. Two main ways plants load assimilates into their phloem for transport- symplast and apoplast route

Question 63

Describe the symplast route for phloem loading and explain how it occurs.

Answer 63

1. Sucrose moves through the cytoplasm of mesophyll cells and on into the sieve tubes by diffusion through the plasmodesmata
2. This route is largely passive,
3. The sucrose ends up in the sieve elements and water follows by osmosis
4. This creates a pressure of water that moves the sucrose through the phloem by mass flow

Question 64

Describe the apoplast route for phloem loading and explain how it occurs.

Answer 64

1. Sucrose travels through the cell walls and inter-cell spaces to the companion cells and sieve elements by diffusion down the concentration gradient, maintained by the removal of sucrose into the phloem vessels.
2. Companion cells use ATP to pump H+ ions out of the cell by active transport
3. This creates a concentration gradient
4. Facilitated diffusion of H+ ions in down the concentration gradient.
5. Sucrose moves in with the hydrogen ions by co-transport
6. This also results in water moving into the companions cell and sieve tube element by osmosis- high hydrostatic and turgor pressure in companion cells
7. Sucrose move into the sieve tube elements by diffusion through the plasmodesmata

Question 65

Describe how the water and assimilates in phloem move from source to sink.

Answer 65

1. Sucrose in the companion cell diffuses into the sieve tube element through plasodesmata down the concentration gradient
2. The water potential is lowered, so water follows passively by osmosis and increases the hydrostatic pressure at the source.
3. Water and dissolved sucrose moves down the phloem form higher to lower hydrostatic region (at the sink)- Mass flow
4. At the sink- sucrose is removed from the sieve tube into surrounding cells by diffusion or active transport.
5. This increases the water potential of the sieve tube so water moves out of the sieve tube by osmosis to lower the pressure.

Question 66

Describe how companion cells are adapted for their function.

Answer 66

1. Have many infoldings in their cell membrane to give an increased surface area for active transport of sucrose into the cell cytoplasm.
2. They also have many mitochondria to supply ATP needed transport pumps

Question 67

Give two examples of what sucrose can be converted into, and what purpose each serves.

Answer 67

Glucose for respiration

Starch for storage

Question 68

Describe and explain 4 sources of evidence for the processes involved in translocation.

Answer 68

1, Microscopy allow us to see the adaptations of the companion cells for active transport

2. If mitochondria of companion cells are poisoned translocation stops
3. The flow of sugars in the phloem is 10000 times faster than it would be by diffusion alone suggesting an active process is driving the mass flow