B3.1/B3.2 GAS EXCHANGE AND TRANSPORT IN (PLANTS) and some water A1.1

PLANTZZZ

outline the function of leaves in plants

• photosynthesis

  • leaves contain chlorophyll, captures sunlight to convert carbon dioxide and water into glucose and oxygen

  • important for providing environment with oxygen and food

• gas exchange

  • occurs in the stomata to allow for exchange of gases

  • vital for photosynthesis and respiration

outline the structure of a leaf

outline the adaptations of leaves for gas exchange

• branched veins, containing xylem to transport water throughout the plant

• epidermis provides a protective single layer of cell which produces a waxy cuticle which is hydrophobic (lipid layer) to prevent water loss by evaporation from the plant

• stomatal guard cells can open and close to regulate gas exchange

• spongy mesophyll transports water from xylem and provides large surface area for evaporation, also creating air spaces which enhances gas exchange (diffusion of gases between environment and cells)

outline the adaptations of guard cells in leaves

• can open and close, allowing for controlling the regulation of gas exchange and transpiration

  • open: allow diffusion of CO₂ in for photosynthesis which allows for transpiration of H₂O out (excess oxygen from photosynthesis is also released)

  • closed: prevents water loss, no CO₂ into the leaf = no photosynthesis occurring

outline the adaptations of xylem in leaves

rigid walls:

• thickened cellulose made up of polymers called lignin prevents vessels from collapsing from the low internal pressure when the plant is transpiring

continuous/hollow tubules:

• continuous, hollow tubes that facilitate the upward movement of water and dissolved minerals from the roots to the leaves

• hollow structure allows for efficient transport without obstruction

pits:

• lignified walls have pits which are impermeable to water allowing for water to enter and exit

outline the structure of a stem

outline the structure of a root

outline the functions of structures in the stem/root

epidermis:

• provides physical protection

• reduces water loss with waxy cuticle

• may contain specialised structures

cortex:

• stores carbohydrates/nutrients

• provides structural support

• facilitates transport of water and nutrients

xylem:

• transports water and mineral ions from the root to the rest of the plant

phloem:

• transports organic nutrients produced (glucose, during photosynthesis)

• regulates plant metabolism

outline transpiration plants

• process by which water is transported from the roots of a plant to its leaves where it will be evaporated into the atmosphere

  • passive process, does not require ATP

  • transports water / nutrients (mineral ions)

what this does to the plant:

• provide cooling effect

  • prevent overheating/regulate temp

• allow for nutrient transport

  • movement of dissolved nutrients in water from roots to leaves through capillary action in xylem vessels

• allow for water regulation

  • maintain turgor pressure in plant cells which is essential for structural integrity of plant

outline the factors affecting the rate of transpiration

• temperature: increased temperature increases transpiration rate

  • increases the evaporation and diffusion due to the higher kinetic energy of water molecules at elevated temperatures

• humidity: increased humidity decreases transpiration rate

  • this decreases the water vapor concentration gradient between the air spaces and the atmosphere, decreasing overall diffusion of water vapor from the leaf to the surrounding air

• wind: increased wind increases the transpiration rate

  • removes the boundary layer of humid air that surrounds the leaf, lowering the humidity near the stomata which increases the concentration gradient for water vapor

• light: increased light increases transpiration rate

  • presence of light increases photosynthesis, stimulating guard cells to open for carbon dioxide

explain the process of transpiration

• water is evaporated into vapor in the spongy mesophyll by light energy absorbed by the leaf

• the vapour diffuses out of the leaf via stomata

• this creates a negative pressure gradient in the leaf, drawing h2o out of xylem vessels by capillary action (which is the passive movement of water against gravity), creating tension

• this tensions draws h2o up the xylem vessels creating a continuous stream via mass flow

  • water molecules are pulled up the xylem by cohesion, the force that holds polar water molecules together through hydrogen bonds

  • water molecules are also adhesive, sticking to the polar xylem walls, which helps facilitate their movement upward

• negative pressure that pulls water upward from the roots to the xylem is generated by actively transporting mineral ions into the root cell

  • creates a high concentration gradient inside the cells and a low concentration gradient in the soil, allowing water to diffuse into the roots via osmosis, which contributes to positive pressure

explain the generation of root pressure in xylem vessels by active transport of mineral ions

• mineral ions are actively transported into the root which decreases internal water potential

  • water moves from high water potential to low water potential via osmosis so from the soil to root

  • the influx of water into the root causes a high root pressure creating a positive pressure potential to push the water upwards through the xylem in roots and stems

• root pressure tends to be generated when transpiration is insufficient

  • night (lower temperatures, decreased light intensity)

  • high humidity levels (decrease concentration gradient)

  • spring season, before leaves have opened

• root hairs are lateral extensions of root epidermal cells to increase surface area for water/nutrient uptake

outline translocation and what this does for the plant

• translocation is the bidirectional transport of organic compounds from source to sink via the phloem

  • source: plant organ where organic compounds are synthesised or stored

  • sink: plant organ that uses or stores organic compounds

outline the role of phloem in translocation

vascular tissue in plants that translocates phloem sap which is a fluid containing water, minerals and carbon compounds

• composed of living cells (sieve tube elements and companion cells)

outline the adaptations of phloem sieve tubes and companion cells

phloem sieve tubes

• specialised cells connected end to end to form phloem sieve tubes

• these cells have a reduced cytoplasm and limited organelles and no nucleus to maximise space for transporting the sap

companion cells:

• provide metabolic support for sieve tube cells as it synthesises ATP and enzymes required for its processes

• contains many mitochondria to synthesise ATP to load materials at source and unload at sink

• contain multiple plasmodesmata which are connection channels between companion and sieve cells for the transport of materials

explain the process of translocation

1. loading at source

• solutes (sucrose) are actively transported from the source into companion cells via proton pumps which create a proton gradient

• these solutes then travel thru the plasmodesmata into the sieve tube cells, lowering the water potential

• water from the xylem is transported into phloem sieve tubes through osmosis, increasing the hydrostatic pressure in phloem

2. hydrostatic pressure

• this process creates a high hydrostatic pressure at the source than the sink therefore water will flow down the sieve tube cells along with the solutes from a high to low pressure gradient

3. unloading at sink

• after travelling down phloem, solutes are unloaded by companion cells into sinks through active or passive transport

• as the concentration of solutes decrease, the water pressure also decreases resulting in water to transfer from the phloem to the xylem by osmosis, decreasing the hydrostatic pressure in the phloem

• solutes in the sink (root cells) are metabolised or stored in vacuoles

outline how water is transported in the xylem

• transpiration creates negative pressure/tension that pulls water upwards → can be explained by 2 properties: adhesion and cohesion

• water is cohesive

  • water’s cohesiveness, caused by hydrogen bonds, causes molecules to form an unbroken column, allowing for the movement from roots to leaves

• water is adhesive with xylem walls

  • water is attracted to the hydrophilic walls of xylem due to adhesion, allowing for the anchoring of water columns to the xylem walls

• as a result, cohesion keeps the water molecules connected, forming a column, allowing for the adhesion of molecules up the hydrophilic walls, moving the entire column upwards all together, creating a continuous flow of water from the roots to leaves against gravity

using water as an example, explain its role as a medium for metabolism and for transport in plants and animals

• water is a universal solvent → it is polar

  • like dissolves like = hydrophilic/polar molecules and charged molecules such as mineral ions all dissolve in water

  • this allows for the efficient transport of molecules in water