CIE AS Biology: Transport in Plants

What is a dicotyledonous plant?

Dicotyledons is a type of plant that has a pair of leaves [cotyledons] in the embryo of the seed. They typically have short leaves with blades and narrow stalks.

Why do water molecules tend to go on the symplast pathway?

Due to differences in water potential in the vacuoles or cytoplasms of neighbouring cells

What is the symplast pathway?

From cytoplasm to cytoplasm via plasmodesmata or adjacent cell membranes down water potential gradient.

What is common in both the symplast and apoplast pathways?

In both water moves into the cell walls

Where is the vascular bundle located on a leaf?

Located in different areas of the leaf (mainly central) - phloem is on the upper side and xylem on the lower side

What are xerophytes?

Plants that are adapted to living in areas where water is in short supply

How are xerophytic leaves adapted for their function?

- SA:vol ratio is very low --> reduces rate of transpiration - low number of stomata in a leaf - thick waxy cuticle [made up of cutin] on BOTH sides of the leaf - stomata are sunken - trichores - rolled leafs --> maintains humid air around leaf + lower SA:vol ratio - hypodermis - leaves have less air spaces

What is the structure of a phloem sieve tube element?

- elongated cells - joined end-to-end forming end walls which are sieve plates with sieve pores - have at least one companion cell adjacent to it with cytoplasmic connections through plasmodesmata - has a cell wall, membrane, cytoplasm, ER and mitochondria - cytoplasm only lines the walls of the cell

What are the adaptations of the xylem?

- lignified cell walls --> vessels do not collapse when there's tension [negative pressure] due to transpiration - cellulose and lignin in cell walls --> water is attracted by adhesion to cause mass flow - cells do not contain organelles --> no protoplasm that would get in the way of water movement - small diameter of vessels --> develops capillary action, prevents air bubbles from forming in water, - pits in vessels --> prevents air bubbles, allows water to move between vessels to different parts of the plant

What are biotic factors that affect transpiration?

- size of the leaf [surface area] - thickness of the waxy cuticle - width of the stomata - whether the stomata are open or closed

How does the phloem load foods, such as sucrose into the sieve tube elements?

1) hydrogen ions are actively pumped [using a proton pump] from the companion cell cytoplasm to its cell wall 2) proton pump creates an excess of H+ ions in the apoplast pathway of the companion cell 3) the H+ ions diffuse passively back into the companion cell through a co-transporter protein --> transports sucrose and H+ ions at the same time 4) in the companion cell, the sucrose diffuses into the sieve tube through the plasmodesmata

How does water move up the xylem?

1) root pressure --> positive pressure that forms when lots of water move into the roots due to water potential difference 2) mass flow of water (capillary action) --> water moves in an unbroken column due to cohesive and adhesive forces 3) tension due to transpiration --> the removal of water from leaves by transpiration causes a tension in the xylem vessel and a difference in Ψw

How does the phloem transport foods down the sieve tube elements?

1) the loading of sucrose into the sieve tube element decreases the Ψw 2) water from the xylem diffuses across into the phloem --> causes a high hydrostatic pressure to build in the phloem 3) a pressure difference is generated between source and sink --> mass flow of water and dissolved substances through sieve tube to the sink 4) sucrose is actively unloaded and the water goes to the xylem and surrounding cells

How does water move from the xylem into the leaf cells?

1) water moves by osmosis from xylem to a leaf mesophyll cell 2) the water moves into the cell wall of the mesophyll cell, forming a thin film of water around the cell 3) water evaporates into an air space in the leaf 4) water DIFFUSES from the air space through an open stomata by transpiration

How does water move from the root hairs to the xylem?

1) water moves down the Ψw across the root 2) water molecules pass through the cells of the cortex via the apoplast & symplast pathway 3) water molecules reach the endodermis and are blocked by the band of suberin in the casparian strip 4) water molecules passes through the endodermis in unthickened parts of the cell wall 5) water moves into the xylem vessel elements through the pits in the vessel

What is the structure of xylem vessel elements?

Cells are dead and have deposits of lignin spirals. No cytoplasm or cell organelles

How are plant roots affected by waterlogged soil? Thus, how is the root pressure affected?

Roots can no longer respire due to lack of oxygen in soil, take up of water by osmosis decreases and root pressure decreases

What is the purpose of the casparian strip?

To prevent water from going back into the cortex from the xylem

What is the cohesion-tension theory?

Water molecules stick to one another + to walls of xylem forming a water column

What is sclerenchyma?

a specialized cell with a secondary cell wall containing lignin [stained red under microscope]

What is a plan diagram?

annotated diagram that shows what you can see through the microscope with a measured scale. It shows the relative proportions of tissues or cells.

What are assimilates?

assimilates are the organic products made in the plant by assimilation

What is the Casparian Strip made up of?

band of suberin --> thick waterproof and waxy

Where is the vascular bundle located in a transverse section of a root?

centre of the root - the xylem located in the centre of the vascular bundle and the phloem on the outer edges

Why does the diameter of a tree trunk increase and decrease during the course of a day?

change in water content --> change in water cohesive-tension forces

What is the structure of the phloem companion cell?

contains all eukaryotic cell organelles and linked with sieve tube element to form a single functional unit by plasmodesmata

What are some examples of xerophytic plants?

couch grass, marram grass, cacti

When sucrose is transported out of the roots, does the water potential in the phloem increase or decrease?

decrease

How does sucrose move from the companion cell to the sieve tube element?

diffusion through plasmodesmata

What is collenchyma?

elongated, thick cell walls, provide support for leaves and stem

What are tri chores?

epidermal hairs which maintains humid air around the leaf

How are ions absorbed in the root?

facilitated diffusion when there's a high conc of ions in the soil active transport when there's a low conc of ions in the soil

Where is the vascular bundle located in a transverse section of a stem?

in the cortex - the xylem is located on the inside of the vascular bundle and the phloem on the outside

Why do xylem vessel elements have pits?

lignin is not deposited where the plasmodesmata of the living cells were --> the pits located on the vessel elements allows water to move freely between vessels and prevent air bubbles from forming

Why are sunken stomata a good adaptation for xerophytic plants?

maintains humid air around leaf

When sucrose is actively transported into the phloem, does the water potential become more or less negative?

more

What is the apoplast pathway?

movement of water from cell wall to cell wall (and in intracellular spaces) due to cohesion-tension

How does humidity affect rate of transpiration?

negative correlation --> at higher humidity there is more water vapour outside the leaf --> higher water potential in the air --> reduces water potential gradient --> lower rate of transpiration

What is the endodermis?

one cell layer around the vascular bundle

What is the epidermis in a plant?

one cell layer that covers the outside of the plant

What is parenchyma?

packing tissue between specialised structures that undergoes metabolic reactions. The cells have thin cell walls and vary in size. The majority of cells outside/around the vascular bundle is parenchyma

How does light intensity affect the rate of transpiration?

positive correlation --> higher rate of photosynthesis so more H2O produced --> guard cells become turgid and stoma open --> higher SA for water to diffuse out of leaves --> higher rate of transpiration

How does wind speed affect transpiration?

positive correlation --> higher wind speed --> wind will blow any humid air around the leaf --> difference in water potential gradient --> higher rate of transpiration

How does temperature affect the rate of transpiration?

positive correlation --> kinetic energy of water molecules increase --> higher rate of diffusion

What is assimilation in plants?

range of processes where inorganic products are converted into organic products which is used in parts of the plant (e.g. photosynthesis)

From where does the phloem transport foods from?

source --> site in the plant where assimilates are produced or stored (e.g. storage organs, leaves)

What aspect of leaves need to be considered when measuring the rate of transpiration?

surface area

Where does water evaporate from during transpiration?

the cell wall of the mesophyll cell

How does water move from the soil into the root hair cells?

the root of a plant is covered in root hair cells which increase SA water is absorbed by osmosis into the vacuoles of root hair cells the root hair cells contain inorganic ions in the vacuole and cytoplasm to maintain low water potential

What is the cortex?

tissue between external of the vascular bundle in the root and stem [mainly consists of parenchyma tissue]

Where is the pith in a transverse section of a plant stem?

tissue located in the centre of the stem mainly consisting of parenchyma cells

What is the function of the xylem?

transports water and soluble mineral nutrients from the roots throughout the plant [in one direction]

How is transpiration measured?

using a potometer by measuring the amount of time taken for the air bubble to move a certain distance [measuring the rate of water uptake of the plant]