Unit 9 - Transport in Plants Flashcards
Why do plants need transport systems?
- metabolic demands
- large size
- low surface area to volume ratio
Define cotyledon.
Organ found in seeds that acts as a food store for the developing embryo and forms the first leaves when the seed germinates.
Define dicotyledonous plant.
A plant which produces seeds with two cotyledons.
Define herbaceous dicot.
A plant with soft tissues and a short life cycle.
Define arborescent dicot.
A plant with hard lignified tissues and a long life cycle.
Define vascular bundle in plants.
The arrangement of the transport tissues in a plant. In herbaceous dicots, the xylem and phloem form vascular bundles, with cambium tissue in between them (the cells in this tissue can differentiate into xylem or phloem).
Where are the xylem and phloem found in the roots of a plant?
In the middle - this helps withstand the pressure from the stems and leaves being blown by the wind.
Where are the xylem and phloem found in the stem of a plant?
Around the edge with the phloem on the outside and the xylem facing inwards, surrounded by parenchyma tissue. This gives strength and support.
Where are the xylem and phloem found in the leaves of the plant?
The main vein (midrib), which supports the leaf’s structure. Branching veins spread throughout the leaf.
What is the role of xylem in the plant?
To transport water and mineral ions from the roots of the plant to the rest of the plant.
What is the structure of xylem?
Xylem is a non-living tissue. The vessels are made of long cells fused together into a hollow column (the cells have died and the contents and end walls decayed). Extra mechanical strength is given by xylem fibres which don’t transport water - they are long cells with lignified secondary walls. Lignin is a polymer which is laid in the xylem walls as rings, spirals or as tubes with bordered pits and gives strength to the vessels.
What is parenchyma?
A tissue which packs around the xylem vessels. It stores food and contains tannin - a bitter tasting chemical protecting the xylem from being eaten by herbivores.
What is the role of phloem in the plant?
To transport sugars and amino acids made in photosynthesis from the leaves around the plant (both up and down).
What are the sieve tube elements of the phloem?
Unlignified cells joined to form a long hollow vessel. As the plant grows, pores appear in the walls between the cells to form sieve plates and the tonoplast, nucleus and other organelles break down.
What are the companion cells of the phloem?
Cells which form with the sieve tube elements and are linked to them by plasmodesmata. They function to provide energy for the sieve tube elements.
Why is water transport important in multicellular plants?
- turgor pressure supports stems and leaves and drives cell expansion
- evaporation keeps plants cool
- forms aqueous solutions for transport
- raw material for photosynthesis
What adaptations do root hair cells have so they can absorb water from the soil?
- large SA:V ratio
- thin cell-surface membrane and cell wall
- high solute concentration in cytoplasm to maintain the water potential gradient
- very small
Why does water move into the root hair cells from the soil?
Soil water has a high water potential because it has a low concentration of dissolved minerals, while root hair cells have a high concentration of dissolved solutes in the cytoplasm and vacuolar sap, lowering the water potential. This means that water will move by osmosis into the root hair cells.
What are the three ways that water can move across the cortex from the root hair cells to the xylem vessels?
By the apoplast pathway, the symplast pathway or the vacuolar pathway.
What is the apoplast pathway?
When water moves through the intercellular spaces and the cell walls of the parenchyma to reach the endodermis.
What is the endodermis in the root of a plant?
The layer of cells surrounding the vascular bundle.
What is the symplast pathway?
When water moves across the parenchyma through the cytoplasm, which forms plasmodesmata to connect the cells.
What is the vacuolar pathway?
When water moves across the parenchyma from vacuole to vacuole. The water passes via the plasmodesmata.
What is the Casparian strip?
A waterproof layer of suberin (a waxy material) which covers the endodermal cells.
How does the Casparian strip affect the movement of water across the root?
It acts as an apoplast block by forcing water in the apoplast pathway into the cytoplasm or vacuoles.
What is an advantage of the apoplast block created by the Casparian strip?
Water is forced to pass through the selectively permeable cell-surface membranes. This prevents potentially toxic solutes in the soil water from reaching the xylem.
How do the endodermal cells increase the rate of osmosis into the xylem?
They move mineral ions into the xylem by active transport, which decreases the water potential in the xylem vessels, increasing the water potential gradient.
How does the active transport of mineral ions into the xylem by the endodermis help with transpiration?
It creates root pressure, a minor factor for pushing water up the xylem vessel.
Define transpiration.
The loss of water vapour through the leaves and stem of a plant
Define the transpiration stream.
The process moving water from the roots of a plant through the xylem to the leaves and stem, culminating in transpiration.
What is the cohesion-tension theory?
The combined effects of cohesion between water molecules and adhesion of the water molecules to carbohydrates on the xylem walls leads to capillary action, helping the water rise up the xylem.
What is the transpiration pull?
Water being drawn up the xylem in a continuous stream to replace that lost in evaporation. This results in tension in the xylem, helping move water through the roots.
What evidence is there for the cohesion-tension theory?
- changes in the diameter of trees
- when a xylem vessel is broken air moves in
What are stomata?
Pores created by the change in shape of guard cells on the surface of a leaf. When they open, water can leave the leaf and carbon dioxide can enter the leaf.
How do guard cells open the stomata?
When environmental conditions are favourable, the guard cells actively transport potassium ions into the cells, which lowers the water potential, allowing water to move in by osmosis. This increases the turgor of the guard cells so they change shape, opening the stomata.
How do the guard cells close the stomata?
When water is scarce, hormonal signals from the roots trigger turgor loss by causing a drop in levels of potassium ions (which increases the water potential), so water will diffuse out of the guard cells, closing the stomata. This allows the plant to conserve water.
What effect does an increase in light intensity have on the rate of transpiration and why?
It increases the rate. This is because light is needed for photosynthesis, so as light intensity increases, the number of open stomata increases.
What effect does an increase in relative humidity have on the rate of transpiration and why?
It will decrease the rate of transpiration because the atmosphere’s water potential will increase, decreasing the water potential gradient.
What effect will an increase in temperature have on the rate of transpiration and why?
It will increase the rate of transpiration because the relative humidity will decrease and the kinetic energy of the water molecules will increase.
What effect will an increase in air movement have on the rate of transpiration and why?
It will increase the rate of transpiration because water vapour will be moved away from the leaf more quickly, maintaining the water potential gradient.
What is translocation?
Transport of organic solutes in the phloem.
What are assimilates?
Products of photosynthesis which are transported through translocation - mainly sucrose and amino acids.
What is a source?
The part of a plant where assimilates are made and transported from.
Give three sources of assimilates.
- green parts of leaves and stem
- storage organs e.g. tubers and tap roots at the start of the growing period
- food stores in seeds at germination
What is a sink?
The part of a plant where assimilates are transported to, to be stored or used in respiration.
Give five main sinks for assimilates.
- roots
- meristems
- developing seeds
- fruits
- storage organs
How are assimilates actively loaded into the phloem?
Hydrogen ions are actively transported into the source. They bind to sucrose and diffuse through a co-transporter protein, increasing the sucrose concentration in the companion cells. Sucrose then diffuses into the sieve tube elements.
How do assimilates move from source to sink?
Mass flow:
A hydrostatic pressure gradient between the source and the sink is created by osmosis into the sieve tube elements near the source and osmosis out of the sieve tube elements near the sink into the sink cells. This causes water to move down the gradient towards the sink, carrying dissolved assimilates which then diffuse into the sink cells.
What evidence is there for mass flow, specifically active transport?
- Scientists can see adaptations of the companion cells due to advances in microscopy
- The flow is much faster than if by diffusion alone, suggesting an active process is involved
- If the mitochondria in the companion cells are poisoned, translocation stops
Define habitat.
Habitat = the place in which an organism lives
Define adaptation.
Adaptation = a positive characteristic of an organism that has been favoured by natural selection
Define mesophyte.
Mesophyte = a plant which is able to take up sufficient water to replace that lost in transpiration
Define xerophyte.
Xerophyte = a plant living in an area where loss of water from transpiration is greater than water uptake by the roots
Define hydrophyte.
Hydrophyte = a plant living either partially or completely submerged in water - these plants have problems with obtaining oxygen
What adaptations do xerophytes have?
- thick waxy cuticle
- reduced SA/SA:V ratio of leaves
- reduced number of stomata
- sunken stomata
- hairy leaves
- curled leaves
- leaf loss in winter
- either long, deep roots or widespread, shallow roots with a large SA
How does a thick waxy cuticle help xerophytes conserve water?
The waxy cuticle is lipid-based and so acts as a barrier to evaporation. The shiny surface also reflects heat, lowering the temperature to reduce transpiration.
How do sunken stomata help xerophytes conserve water?
Sunken stomata create humidity around the stomata, decreasing the water potential gradient to reduce transpiration.
How do hairy leaves help xerophytes conserve water?
Hairy leaves trap layers of air, reducing air movement. This reduces the water potential gradient because water is carried away more slowly, decreasing transpiration.
How do curled leaves help xerophytes conserve water?
Curled leaves trap the stomata in still, humid air, which reduces the water potential gradient and therefore transpiration.
What is a succulent?
A succulent is a xerophyte which stores water in the parenchyma tissue in stems and roots.
How do long, deep roots help xerophytes conserve water?
Long, deep roots can access water a long way below the ground’s surface.
How do widespread, shallow roots with a large surface area help xerophytes conserve water?
Widespread, shallow roots allow the plant to absorb as much water as possible after a rain shower before it evaporates.
What adaptations do hydrophytes have?
- no waxy cuticle (or very thin)
- wide, flat leaves
- high number of always open stomata
- reduced structure
- arenchyma
- large SA of stems and roots
- air sacs
- small roots
- hydathodes
Why do hydrophytes have wide, flat leaves?
Wide, flat leaves spread across the surface of the water to absorb as much light as possible.
In hydrophytes, what is arenchyma?
Arenchyma is a specialised parenchyma tissue which contains many air spaces. It makes the plant’s leaves and stems more buoyant and provides a pathway for oxygen to travel to underwater tissues.
Why do hydrophytes have air sacs?
Air sacs allow leaves and flowers to float to the surface of the water.
In hydrophytes, what are hydathodes?
Hydathodes are structures which release water droplets. These may then evaporate from the leaf surface.
Where are cacti found?
Cacti are found in deserts in the western US.
What adaptations do cacti have?
- hairy leaves
- spines
- sunken stomata
- fleshy body
- grow slowly
- few flowers
- thick stems
- wide, extensive and deep roots
Where is marram grass found?
Marram grass is found in coastal sand dunes.
What adaptations does marram grass have?
- curled leaves
- sunken stomata
- hairy leaves
- thick waxy cuticle
