Transport in Plants

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  • Created by: LBCW0502
  • Created on: 24-12-16 14:39
Why do plants need a transport system?
Metabolic demands (need to transport glucose/oxygen/metabolic waste), size (need to move substances up/down the plant), SA:Vol (small SA:Vol ratio so they cannot rely on diffusion alone)
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What are dicotyledonous plants?
Plants which make seeds that contain two cotyledons (organs that act as food stores for developing embryo plant and form first leaves when the seed germinates)
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What are herbaceous dicots?
Plants with soft tissues and a relatively short life cycle
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What are woody dicots?
Plants which have hard lignified tissues and a long life cycle
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What is a vascular system?
A series of transport vessels running through the stem, roots and leaves
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What are the two main types of transport vessels?
Xylem and phloem (transport tissues arranged together in a vascular bundle)
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Where are the vascular bundles located in the stem?
Around the edge (to give strength and support)
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Where are the vascular bundles located in the roots?
In the middle (to help the plant withstand the tugging strains that result as the stem and leaves are blown in the wind)
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Describe the structure of a dicot leaf
The midrib of a dicot leaf is the main vein carrying the vascular tissue through the organ. It also helps to support the structure of the lead. Many small, branching veins through the leaf functioning both in transport and support
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Describe the structure of xylem
Largely non-living tissue, made up of several types of cells, most of which are dead when they are functioning in the plant. Xylem vessels are the main structures. They are long, hollow structures made by columns of cells fusing together end to end
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Describe the function of xylem
Transports water and mineral ions and provides support. The flow of materials is up from the roots to the shoots/leaves.
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Where is xylem parenchyma located?
Thick-walled xylem parenchyma packs around xylem vessels (storing food and containing tannin deposits)
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What is tannin?
A bitter, astringent-tasting chemical that protects plant tissues from attack by herbivores
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Which structure provides extra mechanical strength for the xylem?
Lignin (xylem fibres are long cells with lignified secondary walls - they do not transport water). Lignin can form rings, spirals or solid tubes
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What are bordered pits?
Relatively solid tubes with lots of small unlignified areas (this is where water leaves the xylem and moves into other cells of the plant)
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Describe the function of the phloem
Transports food in the form of organic solutes around the plant from the leaves. They supply cells with sugars/amino acids needed for cellular respiration/synthesis of useful molecules. Flow of materials is up/down plant
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What are sieve tube elements?
The main transporting vessels of the phloem. They are made up of many cells joined end to end to form a long, hollow structure
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Are phloem tubes lignified?
No
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What are sieve plates?
In areas between the cells, the walls become perforated to form sieve plates (look like sieves and let phloem contents through)
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What is the vacuole membrane called?
Tonoplast
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What is the phloem tube filled with?
Phloem sap
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Do mature cells have a nucleus?
No
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What are companion cells?
Cells which are linked to the sieve tube elements by many plasmodesmata. They maintain their nucleus/all organelles. Companion cells are very active and function as 'life support system' for sieve tube cells, which have lost most normal cell function
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What is plasmodesmata?
Microscopic channels through the cellulose cell walls linking the cytoplasm of adjacent cells
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Phloem tissue contains which supporting tissue?
Sclereids, fibres and cells with extremely thick cell walls
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Why is water transport important in plants?
Turgor pressure (hydrostatic skeleton to support stems/leaves), turgor drives cell expansion (enables plant roots to force their way through tarmac/concrete), loss of water via evaporation keeps plants cool and to transport substances
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What is a root hair?
A long thin extension from a root hair cell, a specialised epidermal cell found near the growing root tip
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What are the features of the root hairs as an exchange surface?
Microscopic size (penetrate between soil particles), large SA:Vol ratio, thin surface layer for diffusion/osmosis, high solute concentration to maintain water potential gradient between soil water and cell
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Describe the symplast pathway
Water moves through continuous cytoplasm of living plant cells (connected through plasmodesmata) by osmosis. Water moves from cell to cell until the xylem is reached
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How is the water potential gradient maintained through the symplast pathway?
As water leaves the root hair cell by osmosis, the water potential of the cytoplasm falls again which maintains the steep water potential gradient to ensure that as much water as possible continues to move into the cell from the soil
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Describe the apoplast pathway
Water moves through cell walls and intercellular spaces. Water fills spaces between loose open network of fibres in the cellulose cell wall. As water moves into xylem, water is pulled through apoplast due to cohesive forces (tension)
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What does the tension in the apoplast pathway lead do?
A continuous flow of water through the open structure of the cellulose cell wall, which offers little or no resistance
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How does water move into the xylem? (1)
Water moves across the root in the apoplast/symplast pathways until it reaches the endodermis (layer of cells surrounding vascular tissue). At the Casparian *****, water in the apoplast pathway moves into cytoplasm/symplast pathway
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How does water move into the xylem? (2)
Solute concentration in endodermal cells is dilute compared to cells in xylem. Ions are actively transported into xylem to increase rate of movement of water. In the vascular bundle, water returns to apoplast pathway to enter xylem
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What is root pressure?
The active pumping of minerals into xylem to produce movement of water by osmosis results in root pressure (independent of any effects of transpiration). Gives water a push up xylem (not a major factor in the movement of water)
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What is the Casparian *****?
A band of waxy material (suberin) that runs around each of the endodermal cells forming a waterproof layer
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What is the significance of the diversion of water from the apoplast pathway to the symplast pathway?
Water must pass though the selectively permeable cell surface membranes. This excludes any potentially-toxic solutes in the soil water from reaching living tissues, as the membranes would have no carrier proteins to admit them
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How do poisons provide evidence for the role of active transport in root pressure?
Poisons (e.g. cyanide) affect mitochondria (prevents production of ATP). If cyanide is applied to root cells, there is no energy supply and the root pressure disappears
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How does temperature provide evidence for the role of active transport in root pressure?
Root pressure increases with a rise in temperature and falls with a fall in temperature, suggesting chemical reactions are involved
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How does the level of oxygen provide evidence for the role of active transport in root pressure?
If levels of oxygen or respiratory substrates fall, root pressure falls
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What is guttation?
Xylem sap may exude from the cut end of stems at certain times. In the natural world, xylem sap is forced out of special pores at the ends of leaves in some conditions e.g. overnight when transpiration is low
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Why do leaves have a very large surface area?
To capture more sunlight in order to carry out photosynthesis
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Why are the surfaces of leaves covered with a waxy cuticle?
To make then waterproof (prevents leaf cells losing water rapidly and constantly by evaporation from their surfaces)
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Describe the gases involved in gaseous exchange
Carbon dioxide moves from the air into the leaf and oxygen moves out of the lead by diffusion down a concentration gradient through microscopic pores in the lead (o the underside of the leaf)
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Stomata can be opened by which type of cells?
Guard cells (which surround the stomatal opening)
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What is transpiration?
Evaporation of water from the leaves and stems of plants (it is an inevitable consequence of gaseous exchange)
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Why is the lost of water vapour from leaves and stems inevitable?
During the day, the plant needs carbon dioxide for photosynthesis and at night oxygen is needed for respiration. The stomata needs to open to allow this gaseous exchange (some stomata need to be open all the time). Water vapour leaves via stomata
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Describe the transpiration stream
Water enters the roots, moves across membranes, diffuses into the apoplast pathway from xylem through leaf cells. It evaporates from freely permeable cellulose cell walls of mesophyll cells and into air spaces. Water vapour leaves
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Describe the cohesion-tension theory (1)
Water molecules evaporate from the surface of mesophyll cells into the air spaces in the lead and move out of the stomata into the surrounding air by diffusion down a concentration gradient
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Describe the cohesion-tension theory (2)
The loss of water by evaporation from a mesophyll cell lowers the water potential of the cell, so water moves into the cell from an adjacent cells by osmosis, along the apoplast and symplast pathways
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Describe the cohesion-tension theory (3)
This is repeated across the leaf to the xylem. Water moves out of the xylem by osmosis into the cells of the leaf
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Describe the cohesion-tension theory (4)
Water molecules form hydrogen bonds with the carbohydrates in the walls of the narrow xylem vessels (adhesion). Water molecules also form hydrogen bonds with each other and so they tend to stick together (cohesion). This leads to capillary action
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What is capillary action?
Water rises up a narrow tube against the force of gravity
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What is the transpiration pull?
Water is drawn up the xylem in a continuous stream to replace the water lost by evaporation
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What does the transpiration pull lead to?
Tension in the xylem. This in turn helps to move water across the roots from the soil
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Describe evidence for cohesion-tension theory
Changes in the diameter of trees, broken xylem vessels and air pulled into a xylem vessel
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Name the apparatus used to measure transpiration
Potometer (measures water uptake from plants - assumes all water is lost via transpiration and not stored in the stem)
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Describe the structure of stomata and function in controlling the rate of respiration
Stoma control the rate of transpiration (turgor driven process). At low turgor, the asymmetric configuration of the guard cell walls closes the pore.
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What happens to guard cells when environmental conditions are favourable?
Guard cells pump in solutes by active transport, increasing their turgor. Cellulose hoops prevent cells from swelling in width so they extend lengthways. The inner wall in less flexible in the outer wall (bean-shaped - opens pore)
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What happens to guard cells when water becomes scarce?
Hormonal signals from the roots can trigger turgor loss from the guard cells, which close the stomatal pore to conserve water
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What are the factors that affect the rate of respiration? (5)
Light, humidity, temperature, air movement and soil-water availability
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What is translocation?
The process of transporting organic compounds in the phloem from sources to sinks
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What are assimilates?
Products of photosynthesis which are transported
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What is the main assimilate?
Sucrose (less likely to be metabolised whilst being transported)
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Give examples of sources of assimilates (3)
Green leaves/stems, storage organs (tubers, tap roots) and food stores (in seeds when they germinate)
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Give examples of sinks (3)
Growing roots, meristems and any part of the plant that are laying down food stores (e.g. fruits, developing seeds, storage organs)
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What is phloem loading?
Soluble products of photosynthesis are moved into the phloem from the sources by an active process
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Describe phloem loading via the symplast route
Sucrose moves from source, to cytoplasm of mesophyll cells and into sieve tubes (via diffusion through plasmodesmata). Water follows via osmosis to create pressure to move sucrose through phloem by mass sflow
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Describe phloem loading via the apoplast route
Sucrose travels from source through cell walls/inter-cell spaces to companion cells and sieve tube elements by diffusion along a concentration gradient
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How is the concentration gradient maintained?
By the removal of sucrose into the phloem vessels
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How is sucrose transported into the cytoplasm in companion cells?
Active process. H+ ions are actively pumped out of the companion cell into surrounding tissue using ATP. H+ ions return to companion cell down a concentration gradient via co-transport protein. Sucrose is co-transported
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In which areas will there is a high sucrose concentration?
In companion cells and in the sieve elements through the many plasmodesmata between the two linked cells
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Describe the structure of companion cells
Have many infoldings in cell membranes to give increased SA for active transport of sucrose into the cell cytoplasm. Many mitochondria to supply the ATP needed for transport pumps
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What happens as a result of build up of sucrose in the companion cell and sieve tube element?
Water moves in by osmosis. This leads to a build up in turgor pressure due to rigid cell walls. Water carrying assimilates moves into sieve tube elements, reducing the pressure in companion cells and moves up/down plant by mass flow to sinks
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Describe phloem unloading
Sucrose is unloaded from phloem at any point into cells that need it. Sucrose moves into cells via diffusion and is converted into another substance (concentration gradient of maintained between contents of phloem and surrounding cells)
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What happens in the phloem when there is a lost of solutes?
Leads to a rise in water potential in phloem. Water moves out into the surrounding cells via osmosis. Some of the water that carried the solute to the sink is drawn into the transpiration stream in the xylem
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Describe the evidence for translocation (4)
Microscopy, if mitochondria of companion cells are destroyed translocated stops, flow of sugars in phloem is 10,000 times faster than that of diffusion (active process) and aphids
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What are xerophytes?
Plants in dry habitats evolved to enable them to live and reproduce in places where water availability is very low (found in dry salty conditions or very cold icy conditions)
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Describe the ways of conserving water (9)
Thick waxy cuticle, sunken stomata, reduced number of stomata, reduced leaves, hairy leaves, curled leaves, succulents, leaf loss and wide/deep roots
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What are hydrophytes?
Plants that live in water
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What are the adaptations of hydrophytes? (8)
Very thin/no waxy cuticle, many always open stomata on upper surfaces, reduced structure to the plant, wide/flat leaves, small roots, large SA of stems/roots underwater, air sacs and aerenchyma
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Card 2

Front

What are dicotyledonous plants?

Back

Plants which make seeds that contain two cotyledons (organs that act as food stores for developing embryo plant and form first leaves when the seed germinates)

Card 3

Front

What are herbaceous dicots?

Back

Preview of the front of card 3

Card 4

Front

What are woody dicots?

Back

Preview of the front of card 4

Card 5

Front

What is a vascular system?

Back

Preview of the front of card 5
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