Transport in plants Flashcards
Why do plants need a transport system?
Large multicellular organisms with low SA:V.
Photosynthesis and gas exchange at leaves but can’t absorb water from the air.
Roots absorb water and minerals but not photosynthesise.
What are the main things plants take up?
They need C, H and O2.
Water and minerals, CO2, transport water up and sugars up and down.
What are the 2 types of vascular tissue?
- Xylem transports water and mineral ions
- Phloem transports sugars up and down from sources to sinks.
How is a vascular bundle arranged?
The middle of a young root - central core of xylem in an X. with phloem between the arms.
Strength to withstand the pull on the roots.
Surrounded by an endodermis to deliver water to the xylem vessels.
Where is phloem and xylem found in the stem?
On the outer edge. In young stems of woody plants, the bundles are separate and form a ring in older stems.
A layer of cambium is found between the phloem and xylem.
Where are xylem and phloem found in the leaf?
The midrib and veins of a leaf are formed by vascular bundles. The xylem is on top.
What drives water transport?
Evaporation of water at the leaves drives transpiration - energy from the sun, so is a passive process.
How can water move through plants?
Apoplast pathway - through spaces between cellulose in cell walls
Symplast pathway - through cytoplasm
Vacuolar pathway - similar to symplast but also through vacuoles.
How is the movement of water driven by osmosis?
Water moves from areas with a higher water potential to areas with lower.
Plant cell with high water potential = turgid - pressure on cell wall, reducing influx of water.
Plant cell with low water potential = flaccid - lose water, leading to plasmolysis.
What is the structure of xylem?
Dead cells joined end to end - continuous. Fibres support the plant. Hollow tubes grouped in vascular bundles. Lined with waterproof coat made of lignin (polysaccharide).
What are the adaptations of xylem?
- gaps without lignification = bordered pits so water can move between vessels
- dead cells aligned - continuous column
- narrow enough so water travels in unbroken column
- no end walls so water can move continuously
- narrow tubes reduce chances of air bubbles
- lignin spirals strengthens walls to stop them collapsing under negative pressure
- no cell contents to obstruct flow
How does water move in the leaves?
Most transpiration occurs form the stomata.
- Water from xylem to cells of spongy mesophyll (or apoplast pathway).
- Water evaporates within leaf.
- Water vapour moves out through open stomata.
What changes does the evaporation of water have to the water potential of the leaves?
- Water evaporates into the air spaces then diffuses down a water vapour potential gradient through stomata
- Loss of water from mesophyll lowers water potential - water diffuses in by osmosis
- A water potential gradient is established
What is transpiration?
The loss of water via evaporation from the aerial parts of a plant mainly through stomata.
What are the benefits of a constant water supply to the leaves?
Maintain cell turgidity
For metabolic processes eg. photosynthesis
Transport dissolved mineral ions in xylem
Evaporating water cools the leaves
What is cohesion?
The attraction between water molecules due to hydrogen bonds.
What is the cohesion-tension theory?
Cohesion cause water to be attracted in an unbroken column across mesophyll and the xylem. So water evaporating out the xylem pull more water up.
= transpiration pull, putting xylem under tension.
Water also adheres to hydrophilic polysaccharides in the xylem - pull water up the sides.
What is adhesion?
The attraction of water molecules to hydrophilic substances.
What factors affect transpiration?
Light intensity - stomata open if light.
Temperature - high temp increases transpiration by increasing evaporation and diffusion rate, and decreasing water vapour potential of air.
Humidity - high, decreases transpiration because lower potential difference.
Air movement - decreases humidity so increases transpiration rate.
Water availability - if not enough, stomata close.
How are root hair cells adapted to the uptake of water/mineral ions?
Ions actively transported from soil, reducing water potential of cytoplasm. So water enters by osmosis.
What is the root endodermis?
A layer of cells surrounding the medulla and xylem vessels (contain starch). Location for active process driving movement of water.
What is the Casparian strip?
In the endodermis - blocks apoplast pathway so water etc. pass into the cytoplasm.
- Creates a checkpoint for plant immune systems before transport.
- Blocks water from going back into the cortex from the xylem.
- Water and ions have to pass into the cytoplasm before the xylem.
What is root pressure?
The action of the roots loading water into the xylem –> positive pressure at base of stem.
Ions actively transported from endodermis to xylem and water follows by osmosis. Casparian strip stops water going back to roots.
Generates force that can push water a few metres up xylem.
What is transpiration pull?
Loss of water at leaves drags water up xylem.
Water molecules are cohesive - stick.
