Transport In Plants Flashcards
(32 cards)
Why would single-called algae not need a specialised transport system
It has a very large surface area to volume ratio. Thus can rely on diffusion for the transport of molecules.
Why would multicellular organisms need a specialised transport system
They have a low surface area to volume ratio. Thus cannot rely on diffusion alone for the transport of molecules.
Many parts of the plant cannot carry out photosynthesis and undergo high rate of metabolic reactions (cells in root tissue absorb mineral ions by active transport). Therefore, due to these high metabolic reactions sugar must be transported to these tissues.
Mineral ions transported from the roots to other parts of the plant. (Nitrate ion to make amino acids)
Plants transport hormones from where they are synthesised to their target tissue
Describe what is meant by monocotyledonous and dicotyledonous herbaceous plant
Some plants have one cotyledon. These are called monocotyledonous plants.
Some plants have one cotyledon. These are called dicotyledonous plants.
Describe the process of a cotyledon forming
Seeds contain an embryonic leaf called a cotyledon. When the seed germinates, the cotyledon unfurls allowing the seedling to carry out photosynthesis.
What are some example of dicotyledonous plants
Herbaceous dicotyledonous plant (Geranium) - short lived ,fast growing and no woody stem
Woody dicotyledonous plant- (Oak tree , shrubs) - long-lived and woody stem
Explain the function of the xylem
Xylem vessel:
Carries water and mineral ions from the roots of the plant up the stem to the leaves
Xylem fibre:
Xylem fibres are not used to transport water. They instead provide mechanical support the plant
Explain the function of the phloem
In the leaves, the plant carries out photosynthesis which produces a sugar glucose. The glucose is used to form other compounds e.g different sugar or amino acids.These are called assimilates.
Transport organic molecules (assimilates) such a glucose produced by photosynthesis in the leaves.
What are the xylem and phloem vessel grouped together in
Vascular bundles
Describe the cross section of a root
Root hair cells grow from the epidermis. We then have a thick layer of cells called the cortex. This contains parenchyma cells
In the centre of the root there are vascular bundles (also also called a stele) which are surrounded by a layer of cells called the endodermis.
In the vascular bundle the xylem vessel is in the centre with the phloem vessel around the xylem.
Explain the adaptations of the xylem
Xylem vessels are mechanically strong therefore when they are grouped up in the centre of the root this helps the root from being pulled out of the soil.
If a xylem vessel is blocked or damaged then water can move through the pits to different vessels. The pits allow water to move out of the xylem (cells in leaves)
The spiral arrangement of lignin help to support the structure of the xylem vessel. When water is pulled up the xylem vessel this causes the pressure in the vessel to fall slightly. The lignin in the vessel wall helps to prevent the vessels from collapsing.
Xylem vessels contain parenchyma cells. These act as a store of starch.
Xylem vessels contain tannins which are bitter compounds that deter herbivores from eating the plant.
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Describe the cross section of a plant stem
Vascular bundles are arranged in rings around the edge of the stem. Within the vascular bundle the phloem vessel are located around the edge of the stem. The xylem vessel are found closer to the centre. (As the vascular bundles are around the edge of the stem this helps the stem to withstand bending due to the wind.)
Around the edge we have the epidermis and the cortex.
The centre of the plant stem is called the pith which consist of parenchyma cells.
Describe the cross section of a leaf
The midrib is in the centre and it provides both transport and support to the leaf. The leaf is also supported by smaller vascular bundles connected to the main one.
In the leaf, the xylem is at the upper part of the vascular bundle and the phloem is at the lower part.
Photosynthesis mainly takes place in the palisade mesophyll which is in the upper half of the leaf.
Explain the structure of the xylem
Xylem vessels:
-Formed from a column of dead cells. The walls break down, forming a long hollow tube.
-wall contain spirals of lignin (lignin is waterproof and strengthens the walls). -
-Pits (areas with no lignin) allow water and solutes to move between vessels.
-Xylem vessel contain parenchyma cells which function as storage (e.g. starch) and also contain tannins (bitter chemicals thought to protect from herbivores).
Xylem fibres:
-Narrow, elongated dead cells with thick lignified walls. (Provides structural support)`
Describe the structure of the phloem
Sieve Tube Elements:
-Living cells, but lack nucleus, vacuole, and most organelles. (Can’t make much ATP or protein — rely on companion cells.)
-Arranged end-to-end to form long tubes.
-End walls form sieve plates with large pores
Companion Cells:
-Full set of organelles, including nucleus and many mitochondria. (Supply sieve tube elements with ATP, proteins, and other essential molecules.)
-Connected to sieve tubes via plasmodesmata (microscopic channels).
Phloem Sap:
-The fluid transported in phloem.
-Contains sugars (mainly sucrose), amino acids, and other substances.
Supporting Tissues:
-Fibres – long, narrow, lignified cells.
-Sclereids – variable shape, also have lignified cell walls.
(Both provide structural support)
Describe the adaption of the phloem
Sieve tube elements:
-Sieve tube elements are long, narrow cells joined end to end . This a continuous tube for efficient transport of phloem sap (sugars and other solutes).
-No nucleus, vacuole or most organelles in sieve tube elements . This leaves space inside the cell for easier flow of phloem sap.
-End walls contain sieve plates. This allows solutes to flow easily between adjacent sieve tube elements.
Companions cells:
-Companion cells contain a nucleus and many mitochondria. This produces ATP and proteins needed for active transport and to support sieve tube elements, which can’t make their own.
-Companion cells connected by plasmodesmata (cytoplasmic channels). This allows movement of ATP, enzymes and nutrients into sieve tube elements to maintain their function.
Lignin:
-No lignin in sieve tubes. This keeps phloem tissue flexible and alive, allowing substances to move by active transport as needed. However, contains lignified fibres and sclereids. This provides mechanical support to the phloem since sieve tubes themselves are not strengthened by lignin.
Describe how root hairs cells are adapted for the absorption of water and minerals
The densely packed root hairs massively increase the surface area to volume ratio of the root. The surface of the root hair consists only of the cell wall and the cell membrane. This makes the surface extremely thin, increasing the rate of osmosis.
Water in the soil contain dissolved mineral ions (Mg^2+ which plants use to make chlorophyll). The concentration of these mineral ions are lower in the soil compared to inside the root hair cell. Therefore, root hair cells use active transport to move these mineral ions into the cell.
The root hair cell contain other dissolved compounds such as sugars. Therefore, the water potential inside the root hair cell is lower than in the soil. Thus water moves into the root hair cell by osmosis down the water potential gradient.
Describe the how water passes through the root to the xylem
Symplast pathway:
-Water enters the cytoplasm of the root hair cell via osmosis (soil has a higher water potential than the cell).
-Water moves cell to cell through plasmodesmata — small channels that connect the cytoplasm of neighbouring cells.
-The symplast pathway is driven by the water potential gradient between the root hair cells and xylem. (The water potential of the root hairs cells greater than the cortex cells. In the xylem the water potential is relatively low). Water moves by osmosis across the cortex down the water potential gradient
-This movement is slower because the cytoplasm contains organelles that can obstruct flow
Apoplast pathway:
-Water moves through the cell walls and spaces between cells.
-The cellulose fibres in the walls provide an open structure, offering less resistance.
-Water moves by mass flow, helped by cohesion (hydrogen bonding between water molecules).
-As water is pulled into the xylem, more water is pulled along behind it — a continuous stream.
-The apoplast pathway offers much less resistance to water flow than the symplast pathway.
Describe what happen before the water enters the xylem
-Water reaches the endodermis where water has traveled from either the apoplast or symplast pathway
Casparian Strip blocks the apoplast pathway:
-Endodermis contains a waxy, waterproof band of suberin in the cell walls — called the Casparian strip.
-This blocks water from continuing via the apoplast pathway.
-So now, all water must enter the symplast pathway — through the cell membrane and cytoplasm. This allows the cell membrane to select what enters the xylem,
Active transport of mineral ions into the xylem:
-Endodermal cells actively pump mineral ions to lower the water potential inside the xylem.
Water enters the xylem by osmosis:
Water moves from the endodermal cells into the xylem down the water potential gradient. This process generates root pressure ( helps move water up the xylem)
Root pressure is active and depends on respiration:
-If respiration is stopped root pressure stops. ( root pressure is evidence for active transport in water uptake)
Describe how transpiration takes place in plants
Water enters the leaf from the xylem in the vascular bundle.
The waxy cuticle on the leaf surface reduces water loss by evaporation.
Stomata (mostly on the lower leaf surface) open for gas exchange — CO₂ in, O₂ out (for photosynthesis).
Leaf cells are coated with a thin film of water that evaporates into the air spaces inside the leaf.
Water vapour then diffuses out through the open stomata, down a water potential gradient.
This combined process of evaporation and diffusion is called transpiration
Describe the cohesion-tension theory of water molecules
As water evaporates from leaf cells, their water potential decreases. As a result water moves by osmosis from nearby cells to replace the lost water.This creates a chain of water movement, continuing all the way to the xylem.
Continuous water loss from the xylem creates tension (suction effect).This pulls water up the plant — the process is called the transpiration stream.
Water molecules stick to each other by cohesion (due to hydrogen bonding) also stick to xylem walls by adhesion. (Cohesion + adhesion allows water to rise up narrow tubes). This is capillary action.
As water leaves the top of the xylem (due to transpiration), more water is pulled up — this is transpiration pull.
This whole mechanism — driven by transpiration, cohesion, adhesion, and tension — is called the cohesion-tension theory.
Describe evidence for the cohesion-tension theory of water molecules
If a plant stem is cut air is sucked into the xylem suggesting that the xylem vessel are under tension. However, the air prevents cohesion between water molecules so water movement stops.
If we measure the diameter of the tree trunk. We can see that this reduces when transpiration is at its maximum. This supports the idea that transpiration pull generates tension in the xylem.
Describe how to use a bubble potometer to measure the rate of water uptake in a plant (PAG 5 and PAG 11)
A bubble potometer consist of a fine capillary tube which is filled with water.The tube is connected to a plant that has been cut at the stem. The tube is connected to a syringe filled with water. Finally, we use a needle to place a small air bubble at the end of the capillary tube. As water evaporates from the leaves of the plant, water is drawn into the stem. This causes the air bubble to move towards the plant. By measuring how far the air bubble moves in a given time. We can calculate the rate of water uptake in the plant. We can then see the rate of water uptake changes if we change the conditions. (Different light intensities by increasing distance of plant to a light source or wind conditions fan compared to still air) In between experiments, we can reset the position of the air bubble by adding more water from the syringe.
The potometer only measures water uptake into the plan. Not all the water takes part in transpiration. (Some water taken in may be a reactant for photosynthesis. However, the vast majority of water taken in by the plant will take part in transpiration).
When setting up a potometer when taking our cutting from the parent plant air will be sucked into the xylem vessels. These air gaps would prevent water from being taken up the stem. Therefore, we must cut the stem of our plant into water (cut off the last 1cm). Water will flow into the xylem and we will avoid any air gaps.
We then place the potometer under the water and insert the cut end, again avoiding any air gaps.
Avoid damaging the plant and avoid getting water onto the underside of the leaves where most of the stomata are found
Ensure that the potometer is fully sealed. Thus we must smear some petroleum jelly around the connection between the stem and the tube.
We also need to allow the plant to adapt to its surroundings for 10 minutes before starting the experiment. (Acclimatise)
Describe how to use a mass potometer to measure the rate of water uptake in a plant (PAG 5 and PAG 11)
We place the plant on a pot ontop of a balance. As the plant loses water through transpiration, the total mass decreases. With this potometer we prevent evaporation of water from the soil. Otherwise, this would contribute to mass loss and give a false reading for transpiration. We do this by covering the soil with plastic wrap.
The mass potometer directly measures the rate of transpiration rather than the rate of water uptake.
This is much less destructive to the plant as it does not involve cutting the stem.
Describe the role of stomata in controlling the rate of transpiration in the plants
Guard cell structure:
-Each stoma is surrounded by two guard cells.
-Inner wall of guard cells is thicker; cellulose microfibrils are arranged in rings → causes specific changes in cell shape during water movement.
Opening of stomata (in light):
-Light stimulates K⁺ ions and other solutes to enter guard cells → lowers water potential.
-Water moves in by osmosis, guard cells become turgid.
-Due to the wall structure, guard cells curve, opening the stomatal pore.
-CO₂ diffuses in for photosynthesis → but water vapour also escapes (transpiration increases).
Closing of stomata at night
-In darkness, photosynthesis stops → no need for CO₂ → stomata close to reduce water loss.
During a drought:
-During drought, roots release a hormone (e.g. abscisic acid) → guard cells lose turgidity → stomata close. This excessive water loss, even though it reduces photosynthesis.
