3.1.3 Transport in Plants

Why do plants need a transport system (3 points)?

1. Metabolic demands - every cell needs oxygen and glucose for respiration, and mineral ions for cell structure 2. Size - very varied sizes 3. SA:V - diffusion works well in the leaves, but not enough to supply the whole plant

Why does simple diffusion not work for plants?

The large transport distance means that diffusion wouldn't be fast enough to meet the metabolic requirements of cells

Give 3 ways plants adapt to increase their SA:V ratio

Roots have root hair cells, leaves are flat and thin and plats have a branching body shape

What are the two key types of transport tissue?

Xylem and phloem

What other types of transport tissue may be seen in vascular bundles (depending on their location)?

Parenchyma, collenchyma, sclerenchyma and cambium

What is the role of parenchyma?

Packing tissue that fills spaces between other tissues. In roots, these cells may store starch.

What is the role of collenchyma?

They strengthen the vascular bundles and outer parts of stems, but allow flexibility. They have thick cellulose walls

What is the role of sclerenchyma?

They support tissue made of dead cells with heavily thickened cell walls containing lignin. These cells strengthen stems and leaf midribs, but are not always present (found in woody plants).

What is the role of the cambium?

Meristem tissue. It divides by mitosis to produce xylem and phloem

What does vascular tissue look like in the root?

There is a central core of xylem in an X shape, and the phloem is found between the xylem. The vascular bundle is surrounded by endodermis, with a layer of meristem tissue inside called the pericycle

How does the shape of the vascular tissue in the leaf help to support the plant?

It provides strength in the core to resist forces of pulling or uprooting

What does vascular tissue look like in the stem?

The vascular bundles are in a ring towards the edge of the stem, with the xylem inside of the phloem and cambium tissue in between.

How does the shape of the vascular tissue in the stem help to support the plant?

It provides flexibility for blowing the the wind, but strength to maintain structure

What does vascular tissue look like in the leaf?

Vascular bundles form the midrib and veins in the leaf, with the xylem towards the upper surface

What is the midrib?

The central vein in a leaf

How do the veins change in dicotyledonous plants compared to monocotyledonous plants?

In dicots the veins branch and are smaller towards the edge of the leaf. In monocots the veins are parallel.

When asked for a low power drawing (plan diagram), what should you do?

Don't draw individual cells, draw all tissues completely enclosed by lines and draw a correct interpretation of the distribution of tissues

When asked for a high power drawing, what should you do?

Draw only a few representative cells and draw the cell wall of all plant cells

What is the structure of the xylem?

It consists of lignified xylem vessels and pits, joined end to end to form tubes. There The cells are dead and there are no organelles.

What four cells make up the xylem?

Tracheids, vessel elements, xylem parenchyma and sclerenchyma cells

What are tracheids?

Long narrow tapered cells with no perforations in the cell walls. They are lignified.

How does the pattern of lignin change in a plant?

As the plant gets older, the patterns of lignin change - they can be spiral, annular (rings) or reticulate (broken rings)

What is the function of lignin in xylem vessels?

To prevent collapse

What is the function of pits in xylem vessels?

They are non-lignified and allow a small amount of lateral movement of water between xylem vessels

What is the structure of the phloem?

It consists of sieve tube elements and companion cells. The sieve tube elements contain cross walls called sieve plates, and are perforated by pores. The companion cells are linked to the sieve tube elements through plasmodesmata

Do sieve tube elements contain a nucleus, vacuole or ribosomes?

No, to maximise the the space for translocation

Do companion cells elements contain a nucleus and other organelles?

Yes, to provide metabolic support to sieve tube elements, and help with the loading and unloading of assimilates

When is a cells plasmolysed?

In a hypertonic solution - the water potential is lower outside the cell

When is a cell turgid?

In a hypotonic solution - the water potential is higher outside the cell

Why does water not continue to move in when a cell is turgid?

Pressure on the cell wall creates a pressure potential which prevents further entry of water

Name the 3 main pathways for water through a plant

Apoplast pathway, symplast pathway and vacuolar pathway

How does the apoplast pathway work?

Water moves by mass flow between cells through spaces in the cell walls. It is the path of least resistance

How does the symplast pathway work?

Water passes through the plasma membrane and into the cytoplasm, then uses the plasmodesmata to move between cells

How does the vacuolar pathway work?

It is similar to the symplast pathway but also involves entering the vacuole

What is transpiration?

The loss of water vapour from the upper plants of a plant

Where is most water lost during transpiration?

Through the stomata

Why does transpiration happen?

It transports mineral ions from the root to leaf, maintains cell turgidity throughout the plant and supplies the water needed for photosynthesis, growth and cell elongation

Name 5 factors affecting the rate of transpiration

Light intensity, temperature, humidity, wind and water availability

How does light intensity affect the rate of transpiration?

Stomata close in the dark, but once the stomata open any increase in light intensity has no effect on the rate of transpiration

How does temperature affect the rate of photosynthesis?

Increase in temp = increase in KE of molecules, therefore increase the rate of transpiration since diffusion occurs at a faster rate. If the temp is too hot the stomata close to prevent excess water, reducing the rate of transpiration

How does humidity affect the rate of transpiration?

High humidity reduces the concentration gradient between the leaf and the surrounding air, reducing transpiration rates. At a certain level of humidity equilibrium is reached so there is no net movement

How does wind affect transpiration rates?

Air currents can sweep away molecules from the leaf's surface, maintaining the concentration gradient and increasing the rate of transpiration

How can we measure transpiration?

A potometer. You can measure the movement of an air bubble

Explain how transpiration works

Water enters via the root hair cells, and moves through the root cortex to the endodermis by the symplast and vacuolar pathway (and a bit of the apoplast pathway). Movement up the stem is uni-directional and is driven by three processes: root pressure, transpirational pull and capillary action. Water then leaves the leaf mostly through the stomata, as it evaporates from cells, lowering the water potential and causing water to move directly into the cells by osmosis.

Why is water blocked by moving via the apoplast pathway in transpiration?

A layer of suberin in the casparian strip surrounding the endodermis blocks the water moving back into the cortex

What is the name given to a plant that is adapted to live with little water?

Xerophyte

Give 3 ways plants may reduce the rate of transpiration

Stomata in pits - moist air is trapped close to the air of water loss reducing the diffusion gradient

Thick waxy cuticle reduces water loss

Rolled leaves trap transpiration

Name an example of a xerophyte and list 3 adaptations

Marram grass - rolled leaves, thick waxy cuticle and stomata in pits covered by hairs

What is a hydrophyte and give an example

Hydrophytes are adapted to live in water. Their leaves contain air spaces for buoyancy and they have stomata on the upper surface. An example is the water lily

What is a hydathode?

A type of pore in the epidermis that releases water and sap.

What is translocation?

The process of moving assimilates from source to sink

What are assimilates?

Substances that will become incorporated into biological tissue

Give an example of a source and a sink for translocation

The source could be green leaves where photosynthesis has produced glucose. The sink could be growing roots

Why are carbohydrates transported as sucrose in plants?

It allows for efficient energy transfer and increased energy storage as sucrose is a disaccharide. It is also less reactive than glucose so no intermediate reactions can occur when it is being transported

Explain how sucrose is loaded

Through the apoplastic pathway, modified companion cells pump hydrogen ions via a proton pump into their cell walls. This is an active process so requires ATP. The hydrogen ion gradient between the surrounding cells and the companion cell causes the hydrogen ions to move back into the companion cells down the concentration gradient through a co-transporter protein, along with a sucrose molecule against the concentration gradient. The sucrose molecules then move into the sieve tubes via the plasmodesmata

Explain the mass flow hypothesis

The movement of sucrose into the sieve tube element reduces the water potential, causing water to move into the phloem by osmosis. This increases the hydrostatic pressure. There is a pressure gradient between the high hydrostatic pressure near the source cell and lower hydrostatic pressure near the sink cells. The solutes then move down the pressure gradient towards the sink. The solutes are unloaded at the sink, increasing the water potential and causing water to move out of the phloem.

How is sucrose unloaded?

The unloading of assimilates occurs at the sinks. The sucrose is actively transported out of the companion cell and then moves out of the phloem tissue via apoplastic pathways. To maintain a concentration gradient in the sink tissues, sucrose is converted into other molecules using enzymes