Transport in plants

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  • Created by: Emily.T
  • Created on: 22-11-17 20:00
Why do multicellular plants need transport systems?
For substances, such as water, minerals and sugars to be transported around the plant - get rid of waste substances - have a smal SA:V ratio with a high metabolic rate -
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What are the two types of tissues involved in transport in plants?
1) Xylem 2) Phloem
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What does Xylem and Phloem transport?
Xylem = Water and mineral ions Phloem = Solutes (sucrose)
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How are Xylem vessels adapted for transporting water & mineral ions?
- long tube-like structure (vessel elements) joined end to end - no end walls making an uniterrupted hollow tube - cells are dead so no cytoplasm - walls are thickened with lignin to help support the vessel - pits in lignin for lateral water movement
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How are the sieve tube elements in phloem adapted for transporting solutes?
-Are living cells that form tubes to transport the solutes - joined end to end to form sieve tubes - end walls are 'sieves' that allow solutes to pass through - have no nucleus, thin layer of cytoplasm and few organelles
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Why do phloem have companion cells?
Because the sieve tube elements don't have a nucleus or other organelles means that they can't survive by themselves so the companion cells carry out the living functions for both themselves and their sieve cells.
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How does water enter a plant from the soil?
Water moves from the soil of a high water potential through the root hair cells, which have a lower water potential via osmosis.
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What 2 pathways does water travel through the roots by?
The symplast and the apoplast pathways
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What is the symplast pathway?
The pathway where water travels through the cytoplasm of the cells which are connected by the plasmodesmata. Water moves through this pathway via osmosis
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What is the apoplast pathway?
Pathway where water travels through the cell walls where it simply diffuses through them as well as the spaces between them. The water carries solutes & moves from areas of high hydrostatic pressure to areas of low hydrostatic pressure (mass flow)
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What is mass flow?
The movement of fluids down a pressure gradient
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What happens when the apoplast pathway meets the Casparian *****?
The water has to take the symplast pathway so the water goes through a cell membrane, which are partially permeable and are able to control which substances in the water get through. From here the water moves into the xylem.
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What is the Casparian *****?
A waxy ***** in the cell walls of the endodermis in the root, which is impermeable to water, blocking the apoplast pathway
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Which pathway is used the most and why?
The apoplast pathway because it provides the least resistance
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How does water leave through the leaves? (1)
The xylem vessels transport water all around the plant and at the leaves, water leaves the xylem & moves into cells mainly via the apoplast pathway. Water evaporates from the cell walls into the spaces cells.
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How does water leave through leaves? (2)
When the stomata open, the water diffuses out down the water potential gradient into the surrounding air. The loss of water from a leaf is called transpiration
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Does water move up a plant against the force of gravity?
Yes
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What is the movement of water from roots to leaves called?
The transpiration stream
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What are the mechanisms that move water up a plant?
1) Cohesion 2) Tension 3) Adhesion
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How does cohesion and tension help water move up plants?
Water evaporates via transpiration = a tension (suction) pulling more water into the leaf. Water mols are cohesive, when some are pulled into leaf others follow = column of water in xylem moves upwards. Water enters stem via root cortex cells
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How does adhesion help water move up plants?
The water mols are also attracted to the walls of the xylem vessels helping water to rise up through the xylem vessels
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Cohesion and tension....
...allow the mass flow of water over long distances up the stem
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What is transpiration a consequence of?
Gas exchange
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How does transpiration occur?
A plant opens its stomata to let in CO2 to produce glucose (via photosynthesis), but this lets water out - higher conc of water inside leaf than the air, so water moves out the leaf down a water potential gradient when the stomata open.
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What are the four factors that affect transpiration rate?
1) Light 2) Temperature 3) Humidity 4) Wind
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How does light affect the transpiration rate?
The lighter it is, the faster the rate as the stomata open when it gets light, so CO2 can diffuse in for photosynthesis. When it's dark the stomata usually close so there's little transpiration
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How does temperature affect the transpiration rate?
The higher the temp the faster the rate. Warmer water mols have more kinetic energy so they evaporate faster, increasing the water potential gradient inside and outside of the leaf = water diffusing out of leaf faster
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How does humidity affect the transpiration rate?
The lower the humidity the faster the rate because if the air around the plant is dry, the water potential gradient leaf and air is increases, increasing transpiration
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How does wind affect the transpiration rate?
The windier it is the faster the rate. Lots of air movement blows away water mols from around the stomata, increasing the water potential gradient, increasing the rate of transpiration
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What piece of equipment can you use to estimate the transpiration rate?
A potometer
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What are Xerophytic plants?
Plants that are adapted to reduce water loss - are typically found in dry climates
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What are the first 2 adaptations of xerophytes to reduce water loss via transpiration?
1) Stomata are in sunken pits so they're sheltered from the wind. 2) A layer of 'hairs' on the epidermis traps moist air around the stomata, reducing the water potential gradient between the leaf and the air.
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What are the next 2 adaptations of xerophytes to reduce water losee via transpiration?
3) Some plants roll their leaves in hot/windy conditions to trap moist air & reduce exposed SA, protecting stomata from the wind. 4) Thick waxy layer on the epidermis reduces water loss by evaporation because it's waterproof
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What are the last 2 adaptations of xerophytes to reduce water loss via transpiration?
5) (Cacti) have spines as leaves to reduce the SA for water loss. 6) (Cacti) close their stomata at the hottest times of the day when transpiration rates are the highest
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What are hydrophillic plants?
Plants that have adapted to survive in water with low oxygen levels
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What is the first adaptation of hydrophillic plants to survive in water with low oxygen levels?
1) Air spaces in tissues helps the plant to float & store oxygen for respiration, e.g. water lillies, air spaces in roots & stem allow O2 to move from floating leaves to parts underwater
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What are the next 2 adaptations of hydrophillic plants to survive in water with low oxygen levels?
2) Stomata are usually only present on the upper surface of floating leaves to help maximes gas exchange. 3) Often have flexible leaves & stems - these are supported by the water around them. Flexibility helps prevent damage by water currents
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What is translocation?
The movement of dissolved substances, e.g. sucrose and amino acids to where they're needed in a plant
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What is another word for dissolved substances?
Assimilates
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Does translocatin require energy?
Yes
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Where does translocation happen in a plant?
In the phloem
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Where does translocation move substances from and to?
From 'sources' to 'sinks'
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What is a 'source' in translocation?
Where a substance is made (so there's a high concentration)
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What is a 'sink' in translocation?
The area where a substance is used up (so it's at a lower concentration than the source)
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Where is the source and the sinks for sucrse?
Source = Leaves Sink = other parts of the plant, especially food storage organs & meristems (areas of growth) in the roots, stems and leaves
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Can some parts of a plant be BOTH a sink and a source?
Yes, e.g. sucrose can be stored in the roots. During growing season, sucrose is transported from roots -> leaves to provide energy for growth, so the roots are the source and the leaves are a sink.
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How do enzymes maintain a concentration gradient in translocation?
They change the concentration gradient from the source to the sink by changing the dissolved substances at the sink (by breaking them down/make into something else), which makes sure there's always a lower concentration at the sink than the source.
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Example of enzymes maintaining a concentration gradient in translocation?
In potatoes, sucrose is converted into starch in the sinks = lower conc. of sucrose at the sink than in the phloem, ensuring a constant supply of new sucrose reaches the sink from the phloem. Invertase breaks down sucrose -> glucose - used by plant
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What is the best hypothesis to explain phloem transport?
The Mass Flow Hypothesis
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How does mass flow work: (1)
1) Active transport actively loads solutes (e.g. sucrose) into sieve tubes at the source (leaves). 2) Lowers water potential inside sieve tubes so water enters by osmosis from xylem & companion cells. 3) Creates high pressure in sieve tubes at source
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How does mass flow work: (2)
4) At the sink, solutes are removed from phloem to be used 5) This increases the water potential inside sieve tube, so water leaves the tubes by osmosis 6) This lowers the pressure inside the sieve tubes
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How does mass flow work: (3)
7) Results in a pressure gradient from the source to the sink 8) This gradient pushes solutes along the sieve tubes to where they're needed
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How do substances enter the phloem?
By active loading
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What does active loading do?
Move substances into the companion cells from surrounding tissues and from the companian cells into the sieve tubes AGAINST a concentration gradient
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Why does sucrose have to be actively loaded into the companion cells & sieve tubes?
Sucrose concentration is usually higher in the companion cells than the surrounding tissue cells and higher in the sieve tube cells than the companion cells.
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How is sucrose moved to where it needs to go?
By active transport and co-transporter proteins
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How does active loading work in translocation? (1)
In the companion cell, ATP is used to actively transport hydrogen ions (H+) out of the cell into surrounding tissue cells. This sets up a concentration gradient - there arer more H+ ions in the surrounding tissue than in the companion cell.
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How does active loading work in translocation? (2)
A sucrose mol binds to the co-transporter protein at the same time. The movement of H+ ions is used to move sucrose mol into the cell, against its conc. grad. Sucrose mols are then transported out of the companion cells into sieve tubes the same way
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Where does the ATP come from?
ATP is one of the products from respiration. The breakdown of ATP supplies the initial energy needed for the active transport of the H+ ions
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Card 2

Front

What are the two types of tissues involved in transport in plants?

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1) Xylem 2) Phloem

Card 3

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What does Xylem and Phloem transport?

Back

Preview of the front of card 3

Card 4

Front

How are Xylem vessels adapted for transporting water & mineral ions?

Back

Preview of the front of card 4

Card 5

Front

How are the sieve tube elements in phloem adapted for transporting solutes?

Back

Preview of the front of card 5
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