Transport in Plants (OCR A Level Biology): Flashcards

Compared with animal cells, plant cells and tissues have a much lower metabolic rate, reducing their demand for oxygen for aerobic respiration.

• The xylem transports water and mineral ions

• The phloem transports sucrose and other nutrients

• Bring substances quickly from one exchange site to another

• Maintain the diffusion gradients at exchange sites and between cells and their fluid surroundings

• Ensure effective cell activity by keeping the fluid environment of cells within a suitable metabolic range

• They have adaptations giving a high surface area : volume ratio for absorption and diffusion of gases

• Leaves and stems possess chloroplasts, which produce oxygen and use up carbon dioxide

• There is a low demand for oxygen because plant tissues have a low metabolic rate

Glucose is produced in the leaves by photosynthesis, and must then be transported to other parts of the plant.

False — plant cells and tissues have a much lower metabolic rate than animal cells.

True

Vascular tissue that transports dissolved minerals and water up the plant, while also providing structural support and a site for food storage.

Vascular tissue that transports organic compounds (particularly sucrose) from the source to the sink, and can move these compounds both up and down the plant.

• Transport of dissolved minerals and water up the plant

• Structural support

• Food storage

To transport organic compounds, particularly sucrose, from the source (e.g. a leaf) to the sink (e.g. the roots).

This transport can occur both up and down the plant.

In vascular bundles, which also contain other supporting tissues.

• Sieve tube elements

• Companion cells

• Parenchyma – for storage

• Fibres – for strengthening

Because the different organs are subjected to different stresses, so the arrangement of the vascular tissue is adapted to withstand the strains acting on each organ.

The vascular bundle is found in the centre of the root.

• Xylem forms the central core, helping the root withstand the pulling strains as water is transported upwards

• Phloem is located on the edges of the central core

The vascular bundles are located around the outside of the stem.

• Xylem is found on the inside (closest to the centre of the stem) to help support the plant

• Phloem is found on the outside (closest to the epidermis)

The vascular bundles form the midrib and veins, spreading from the centre of the leaf in parallel lines.

• Xylem is found on the upper side of the bundles (closest to the upper epidermis)

• Phloem is found on the lower side of the bundles (closest to the lower epidermis)

In the roots, the xylem tissue forms the central core of the vascular bundle, helping the root withstand pulling strains.

In leaves, xylem tissue is found above phloem tissue within the vascular bundle.

They are the main conducting cells of the phloem, forming the tubes through which organic compounds are transported.

False — phloem can transport organic compounds both up and down the plant, from source to sink.

False — in the stem, xylem is found on the inside (closest to the centre) while phloem is on the outside, closest to the epidermis.

Vascular tissue in a plant that transports dissolved minerals and water, provides structural support, and stores food.

• Transports dissolved minerals and water around the plant

• Structural support

• Food storage

• Tracheids

• Vessel elements

• Xylem parenchyma

• Sclerenchyma cells (fibres and sclereids)

Tracheids and vessel elements.

Long, narrow, tapered cells with pits.

• Large cells with thickened cell walls

• No end plates when mature

A water-conducting cell of the xylem that is large, has thickened cell walls, and has no end plates when mature.

Tracheids and vessel elements are both types of water-conducting cell.

Xylem tissue is a vascular tissue that transports dissolved minerals and water around the plant.

Look for the thicker cell walls and the larger diameter.

Fibres and sclereids.

Both are types of water-conducting cell.

True

False — xylem transports water and dissolved minerals; it is phloem that transports the products of photosynthesis.

To transport organic compounds (assimilates), particularly sucrose, from the source (e.g. leaf) to the sink (e.g. roots).

Transport can occur both up and down the plant.

The organic compounds, particularly sucrose, that are made by the plant and transported in the phloem.

• Source = the part of the plant where organic compounds are made or loaded into the phloem (e.g. a leaf)

• Sink = the part of the plant where organic compounds are delivered to be used or stored (e.g. the roots)

They are dissolved in water to form sap.

• Sieve tube elements — the main conducting cells

• Companion cells — control the metabolism of their associated sieve tube element

• Parenchyma cells for storage

• Fibres for strengthening

The main conducting cell of phloem; these cells line up end to end to form a continuous tube through which sap is transported.

• Control the metabolism of their associated sieve tube element

• Play a role in the loading and unloading of sugars into the phloem

Mature phloem tissue contains living cells, whereas mature xylem tissue is dead (showing no organelles).

• Sieve tube elements have no lignin (xylem has lignified walls)

• Sieve tube elements have sieve plates (xylem does not)

• Sieve tube elements are living and have associated companion cells with nuclei and dense cytoplasm (xylem is dead with no organelles)

Sieve tube elements line up end to end to form a continuous tube through which sap moves.

Organic compounds are dissolved in water to form sap, which is transported in the phloem.

True

False — sieve tube elements have no lignin; it is xylem that has lignified walls.

A plant whose seeds contain two cotyledons (seed leaves). Dicots also have a network of veins, leaves with broad blades and petioles (stalks), and a tap root with lateral branches.

A seed leaf found inside a seed. Dicotyledonous plants have two cotyledons.

A dicot with a relatively short life cycle (one growing season) and non-woody tissue.

• Seeds containing two cotyledons (seed leaves)

• A network of veins

• Leaves with broad blades and petioles (stalks)

• A tap root with lateral branches

• To meet their metabolic demands (glucose, hormones and mineral ions are required for various processes within the plant)

• To efficiently move substances up and down the plant

• To compensate for their relatively small SA:V ratio (they generally cannot rely on diffusion alone)

A network of vessels (vascular tissue) running through the leaves, stem and roots that transports substances around the plant.

Vascular tissue that transports water and mineral ions from the roots to the rest of the plant.

Vascular tissue that transports substances from the source (e.g. leaf) to the sink (e.g. root).

Structures in which the xylem and phloem are arranged together.

The bundles are laid out differently in the leaves, stem and roots.

• Leaves

• Stem

• Roots

Xylem transports water and mineral ions from the roots to the rest of the plant.

In phloem, substances are transported from the source (e.g. leaf) to the sink (e.g. root).

• Do not draw individual cells

• Read the question carefully, as you may only need to draw a portion of the image

• Keep the proportions between tissues to scale

• Include the magnification on the drawing

• Use clear, continuous lines and a sharp pencil; do not shade

• Count the number of eyepiece graticule divisions across the specimen

• Multiply this number by the calibrated value of one division (in µm) to give the actual width

True

False — that describes phloem; xylem transports water and mineral ions from the roots to the rest of the plant.

The loss of water vapour from a plant to its environment by evaporation and diffusion, mainly through the stomata in the leaves.

The movement of water from the roots to the leaves through the plant.

The stomata in the leaves.

Around 99% of the water absorbed is lost through evaporation from the stem and leaves.

When the stomata open to allow gaseous exchange (e.g. uptake of carbon dioxide), water vapour inevitably evaporates and diffuses out at the same time. Transpiration is therefore an unavoidable consequence of having open stomata.

• Cooling the plant via evaporative cooling

• Aiding the uptake of mineral ions through the transpiration stream

• Providing support through turgor pressure, keeping leaves spread out (large surface area) and supporting the stem of non-woody plants

The evaporation of water vapour from the leaves, together with the cohesive and adhesive properties of water molecules, pulls water up through the xylem.

The water potential gradient: water moves from the soil (high water potential), through the plant's cells, to the atmosphere (low water potential).

The rate of transpiration depends on the concentration gradient of water vapour between the inside of the leaf and the surrounding air; a larger one gives a faster rate of diffusion.

Air currents sweep water molecules away from the leaf surface, maintaining the concentration gradient and increasing the rate of transpiration.

When air is still, water molecules accumulate near the leaf, creating high local humidity, lowering the concentration gradient and reducing transpiration.

Higher temperature increases the kinetic energy of water molecules, so they leave the leaf down the concentration gradient faster, increasing the rate.

However, if temperature gets too high the stomata close to prevent excess water loss, sharply reducing transpiration.

Stomata close in the dark, greatly reducing transpiration.

When light is sufficient the stomata open and transpiration increases, but once they are open any further increase in light intensity has no effect on the rate.

High humidity means a large concentration of water molecules in the air around the leaf. This reduces the concentration gradient between the inside of the leaf and the outside air, so the rate of transpiration decreases.

At a certain humidity an equilibrium is reached, with no concentration gradient and no net loss of water vapour.

True — still air creates a local region of high humidity, lowering the concentration gradient and reducing transpiration.

False — water moves down a water potential gradient, from high water potential in the soil to low water potential in the atmosphere.

The loss of water vapour from the surface of a plant (mainly the leaves) by evaporation from mesophyll cell surfaces and diffusion out through the stomata.

• Mineral ions are taken up from the soil by active transport or diffusion, depending on soil mineral concentrations

• The mineral ions lower the water potential of the root hair cells

• Water then enters the cells from the soil by osmosis

• The apoplast pathway

• The symplast pathway

The route by which water travels across a plant tissue through the cell walls and intercellular spaces, without crossing any cell membranes (so it does not involve osmosis).

The route by which water travels across a plant tissue through the cytoplasm and vacuoles of cells, moving from cell to cell by osmosis and via the plasmodesmata.

Because the water travels within the freely permeable cell walls and intercellular spaces and does not cross any cell membranes.

• Water moves into root hair cells from the soil by osmosis, increasing the water potential of the root hair cell

• Water moves down its water potential gradient into neighbouring root cells, increasing their water potential

• Water continues to move across the root from high to low water potential

A waxy band (made of suberin) that surrounds the cells of the endodermis and forms an impassable barrier to water.

• It blocks the cell walls of the endodermis cells, preventing water from entering the xylem via the apoplast pathway

• This forces water into the symplast pathway

• This is thought to help the plant control which mineral ions reach the xylem

• Water evaporates from the surface of cells in the leaves, lowering the water potential of leaf cells

• Water is drawn out of the xylem into leaf cells by osmosis down its water potential gradient

• More water molecules are drawn upwards in a continuous column due to forces of cohesion between water molecules

• Cohesion: attractive forces between water molecules hold them together in a continuous column

• Adhesion: attractive forces between water molecules and the sides of the xylem help draw the water column upwards

The upward movement of water in the xylem is known as the transpiration stream.

• Water vapour diffuses out of the leaf air spaces into the surrounding environment down a water vapour potential gradient

• This creates a water potential gradient between the mesophyll cells and the air spaces, so more water moves from the mesophyll cells into the air spaces (evaporating from the cell surface)

• Losing water lowers the water potential of the mesophyll cells, so water moves in by osmosis from neighbouring cells and the xylem

False — apoplast water travels through the freely permeable cell walls and does not cross cell membranes.

True

Apparatus used to investigate the effect of environmental factors on the rate of transpiration, by measuring the rate of water uptake by a plant shoot.

• Air movement (airflow)

• Humidity

• Temperature

• Light intensity

To prevent air from entering the xylem.

If air enters the apparatus, the readings will be inaccurate.

Any moisture on the leaves will affect the rate of transpiration.

In a potometer, the rate of transpiration is measured by tracking the movement of a single air bubble along the capillary tube.

The further the bubble travels in the same time period, the faster transpiration is occurring (and vice versa).

• Cut a shoot underwater and place it in the tube

• Set up the airtight apparatus, sealing gaps with vaseline, and dry the leaves

• Allow a single air bubble to form in the capillary tube

• Set up the environmental factor being investigated and allow the plant to adapt for 5 minutes

• Record the starting location of the bubble, leave for a set time, then record the end location

• Reset the bubble using the tap below the reservoir and repeat

To allow the rate of transpiration to stabilise in response to the new environmental conditions before readings are taken.

Set up a fan or hairdryer directed at the plant.

Change the distance of a light source from the plant.

Keep all factors the same other than the one variable being investigated.

Spray water inside a plastic bag and wrap it around the plant to increase humidity.

Change the temperature of the room (e.g. a cold room or a warm room).

True — water uptake is used as a measure of the rate of transpiration.

False — high humidity decreases the rate of transpiration.

The transport of assimilates from source to sink within phloem tissue, requiring an input of metabolic energy (ATP).

The liquid transported within phloem sieve tubes. It consists mainly of sucrose, along with water and other dissolved substances such as amino acids, hormones and minerals.

A part of the plant where assimilates are made or released, e.g.

• Green leaves and green stem (photosynthesis)

• Storage organs such as tubers and tap roots (unloading stored substances)

• Food stores in germinating seeds

A part of the plant where assimilates are required or stored, e.g.

• Meristems (apical or lateral) that are actively dividing

• Roots that are growing and/or actively absorbing mineral ions

• Sites where assimilates are stored, such as developing seeds, fruits or storage organs

• Sucrose is a disaccharide, allowing efficient energy transfer and greater energy storage

• It is less reactive than glucose because it is a non-reducing sugar, so no intermediate reactions occur during transport

• Sucrose also has less of an osmotic effect than glucose

The loading and unloading of sucrose can be slowed down or stopped at high temperatures or by respiratory inhibitors, showing that it depends on energy (ATP) from respiration.

• Collecting and studying sap from plants with 'clotting' sap (e.g. castor oil plants)

• Using aphids: after the aphid inserts its stylet, the head is removed and the sap that continues to flow is collected

• Using radioactively labelled metabolites (e.g. carbon-14 labelled sugars) traced during translocation

Symplastic pathway: through the cytoplasm and plasmodesmata; a passive process as sucrose moves by diffusion.

Apoplastic pathway: through the cell walls; an active process requiring ATP.

• Companion (transfer) cells pump hydrogen ions out of their cytoplasm into the cell walls via a proton pump, using ATP (active process)

• The high concentration of hydrogen ions in the cell wall causes them to move back down their concentration gradient into the companion cell

• They pass through a cotransporter protein, which simultaneously carries sucrose into the companion cell against the sucrose concentration gradient

• Sucrose then moves into the sieve tubes via the plasmodesmata

During apoplastic loading, hydrogen ions move back into the companion cell through a cotransporter protein, which simultaneously carries sucrose against its concentration gradient.

• Infoldings in the cell surface membrane increase the surface area available for active transport of solutes

• Many mitochondria provide the ATP needed to power the proton pump

Sucrose is converted into other molecules, keeping its concentration low at the sink. This is a metabolic reaction requiring enzymes, e.g. invertase, which hydrolyses sucrose into glucose and fructose.

Because assimilates always move from source to sink, and sinks can be located either above or below the source in the plant.

False — the apoplastic pathway is an active process requiring ATP; only the symplastic pathway is passive.

True

The movement of phloem sap (containing sucrose and other organic solutes) up and down the plant, driven by a hydrostatic pressure gradient between source and sink.

Phloem sap contains sucrose and other organic solutes.

• Sucrose is a disaccharide, so it contains more energy than a monosaccharide

• Sucrose is less reactive than glucose because it is a non-reducing sugar

A hydrostatic pressure gradient between the source and the sink.

• Sucrose is actively loaded into the sieve elements at the source

• This lowers the water potential in the sieve tube

• Water then moves in by osmosis, which increases the hydrostatic pressure at the source

• Solutes are unloaded from the sieve elements at the sink

• This causes water to follow by osmosis (leaving the sieve tube)

• This lowers the hydrostatic pressure at the sink

The difference in hydrostatic pressure creates a hydrostatic pressure gradient.

Phloem sap flows by mass flow from the area of high hydrostatic pressure (source) to the area of low hydrostatic pressure (sink).

At the source, sucrose is actively loaded into the sieve elements, which lowers the water potential in the sieve tube, causing water to enter by osmosis.

Mass flow means solutes move faster than they could move by diffusion alone.

No.

The source is not always the leaves and the sink is not always the roots. Phloem sap can move either up or down the plant depending on the time of year.

It is dependent on water moving in and out of the xylem vessels by osmosis.

It moves by mass flow, both up and down the plant.

True

False — phloem sap moves up or down the plant depending on the time of year.

A plant adapted to living in dry and arid conditions.

A plant adapted to living in freshwater.

To maximise water conservation, allowing survival in dry and arid conditions.

Xerophytes have physiological and structural (xeromorphic) adaptations that maximise water conservation.

Marram grass (Ammophila arenaria).

• Rolled/curled leaves

• Hairs on the epidermis

• Sunken stomata (in pits or grooves)

• Thick waxy cuticle

Receiving enough carbon dioxide during the day and enough oxygen at night, because water contains less oxygen and carbon dioxide than air.

Their cells possess a cell wall, which prevents too much water from being absorbed.

They are thin and flat and contain large air spaces to give them buoyancy.

This keeps them close to the surface of the water, where there is more light for photosynthesis.

There is little need to prevent water loss in an aquatic environment, so the cuticle can be very thin.

On the upper surface of the leaf.

Only small roots are needed, as nutrients can also be extracted from the surrounding water through the plant's tissues.

The water lily.

• Rolled/curled leaves — trap a layer of moist air and reduce the exposed surface area for evaporation

• Hairs on the epidermis — trap moist air, reducing the water potential gradient and so reducing transpiration

• Sunken stomata — trap moist air, reducing the water potential gradient out of the leaf

• Thick waxy cuticle — reduces evaporation of water from the leaf surface

It allows gas exchange to occur with the air instead of the water.

The xylem is significantly reduced, as there is no need to transport water throughout the plant.

False — hydrophytes have a very thin waxy cuticle, as there is little need to prevent water loss.

True