Transport in Plants Flashcards
why do plants need transport systems?
- plants have a small SA:V ratio so they can’t rely on diffusion alone to get the substances they require
- the products of photosynthesis need to get around to cells around the whole plant, not just the leaves
- mineral ions absorbed by roots need to be transported to all cells to make proteins
What is a cotyledon?
An embryonic leaf in a germinating seed.
What are dicotyledonous plants? Give an example.
Plants with two cotyledons, such as trees and geraniums.
What are examples of woody dicotyledonous plants and what are the characteristics of them?
Trees and shrubs.
They are long-lived and have a woody stem.
What are examples of herbaceous dicotyledonous plants and how do herbaceous plants differ from woody plants?
Geraniums and other fast-growing plants.
They grow quickly, can be short-lived, and do not have a woody stem.
What does the xylem transport?
What does the phloem transport?
Water and mineral ions from the roots to the leaves.
Organic molecules, such as sugars from photosynthesis, throughout the plant, in both directions.
What are vascular bundles?
Groups of xylem and phloem vessels.
what does the cross section of a root of a dicotyledonous plant look like?
the vascular bundle in the root is called the stele
the outermost cells of the root is called the epidermis, where root hair cells grow.
the thick layer of cells inside the epidermis is called the cortex, which contains parenchyma cells. (parenchyma stores nutrients and tannins which are bitter to keep herbivores from eating the plant)
the endodermis surrounds the vascular bundle
the xylem is in the center, surrounded by phloem vessels.
the xylem being positioned in the centre provides mechanical strength and prevents uprooting by strong winds.
(LOOK AT A LABELLED DIAGRAM AND MEMORISE IT!!)
what does the cross section of a stem of a dicotyledonous plant look like?
the vascular bundles are arranged in a ring around the edge of the stem.
the centre of the stem is called the pith, which consists of parenchyma cells.
The epidermis and cortex are found around the edge of the stem
The xylem is closer to the centre, and the phloem is towards the outer edge.
the vascular bundle arrangement helps the stem withstand bending due to wind.
(LOOK AT A LABELLED DIAGRAM AND MEMORISE IT!!)
what does the cross section of a leaf of a dicotyledonous plant look like?
the central vascular bundle in a leaf is called the midrib.
the midrib provides support and transport.
smaller vascular bundles connected to the main one also supports the leaf.
the xylem is at the upper part, and the phloem is at the lower part.
photosynthesis takes place in the palisade mesophyll, located in the upper half of the leaf.
(LOOK AT A LABELLED DIAGRAM AND MEMORISE IT!!)
What are xylem vessels made of?
Long, hollow dead cells with no end walls, strengthened by lignin. little cytoplasm
How does lignin help xylem function?
Lignin strengthens the vessels, prevents collapse, and can be arranged in rings, spirals, or bordered pits for lateral water movement to other cells in the plant.
What other supporting tissues are found in the xylem?
Xylem parenchyma (stores food and tannin) and additional supporting fibers.
(LOOK AT A LABELLED DIAGRAM OF XYLEM AND PHLOEM AND MEMORISE IT!!)
What are sieve tube elements?
Living cells in the phloem with perforated end walls called sieve plates, allowing the movement of substances.
What is the role of companion cells in the phloem?
They provide energy for active transport and support the function of sieve tube elements. They are the ‘life support’ for sieve tube elements as the sieve tubes have lost all organelles and are dead. companion cells and sieve tube elements are joined by plasmodesmata. companion cells have organelles (specifically cytoplasm, nucleus and mitochondria)
how do xylem and phloem differ in transport direction?
Xylem moves substances upward only, while phloem moves substances both up and down the plant.
How can xylem vessels be observed in plants?
By staining plant stems with a dye that binds to lignin and examining them under a microscope.
Why is water essential for plants?
Water is vital in plants for several reasons:
It maintains cell turgidity, preventing wilting.
It drives cell expansion via turgor pressure.
It acts as a solvent for mineral ions and nutrients.
It transports substances through the xylem and phloem.
It regulates temperature through transpiration.
How does water enter root hair cells from the soil?
Soil has a higher water potential than root hair cells.
Root hair cells contain solutes that lower their water potential.
Water moves by osmosis from the soil into the root hair cells, following a concentration gradient.
This process is passive and does not require energy.
What are the two main pathways water takes across the root, and how do they work?
Symplast Pathway:
Water moves through the cytoplasm of cells.
It passes from cell to cell via plasmodesmata.
This pathway is regulated because water crosses selectively permeable membranes.
Apoplast Pathway:
Water moves through cell walls and intercellular spaces.
It does not enter the cytoplasm, meaning it moves freely.
This is the fastest route, but it is blocked at the Casparian strip in the endodermis.
What is the function of the Casparian strip in water transport?
The Casparian strip is a waterproof band of suberin in the endodermis.
It blocks the apoplast pathway, forcing water to enter the symplast pathway.
This ensures selective absorption of minerals before water enters the xylem.
Minerals are actively transported into endodermal cells, lowering the water potential and drawing in water by osmosis.
What is root pressure, and what evidence supports its role?
Root pressure is the force generated by osmosis and active transport in the root that pushes water up the xylem.
It occurs when minerals are actively transported into the xylem, lowering water potential and drawing in water.
Evidence includes:
ATP is required – if ATP production is inhibited, root pressure decreases.
Temperature dependency – root pressure increases with temperature, suggesting enzymatic activity.
Oxygen levels impact it – oxygen deprivation reduces root pressure, supporting the role of respiration.
Guttation – small droplets of water appear on leaves in humid conditions, caused by root pressure forcing water out
What is transpiration, and how does it drive water movement?
Transpiration is the loss of water vapour from the leaves via stomata.
It creates a water potential gradient, drawing water up from the roots.
The transpiration stream is the continuous movement of water from the roots to the leaves via the xylem.
This process is passive and relies on:
Cohesion (water molecules stick together).
Adhesion (water molecules stick to the xylem walls).
Tension (negative pressure pulls water upwards).
How do stomata control the rate of transpiration?
Stomata are small openings mainly found on leaf surfaces.
Guard cells regulate their opening and closing:
In high light and low CO₂, they open for gas exchange.
In dry conditions, they close to reduce water loss.
Stomata respond to environmental factors like humidity, temperature, and wind speed.
