Topic 9: Plant Biology Flashcards
Water loss by transpiration
- Transpiration - the loss of water vapour from the stems and leaves of plants.
- Inevitable consequence of gas exchange in the leaf.
- Leaves must absorb CO2 for use in photosynthesis and excrete oxygen as a waste product.
- A large S.A needed - provided by the moist spongy mesophyll tissue in the lower parts of the leaf with lots of air spaces to increase the S.A.
- Unless the air spaces are fully saturated, water evaporates from the moist cell walls - ensures that the air spaces have a high relative humidity.
- The epidermis of moist plant leaves secretes wax to form a waterproof coating to the leaf called the waxy cuticle to prevent excessive transpiration and block gas exchange.
- Pores are needed in the epidermis for CO2 to enter and O2 to leave - stomata.
Measuring transpiration rates
The rate of transpiration is hard to measure directly - instead the rate of water uptake is measured with a potometer - as the plant transpires, it draws water out of the capillary tube to replace the losses - measured by the movement of the air bubble - repeat measurements are taken for reliability.
Investigating factors affecting transpiration rates
- Temperature - using a heat lamp, infrared thermometer. Energy in form of heat.
- Humidity - using a bag, spray, silica gel and hygrometer. Concentration gradient.
- Wind speed - fan, change distance / rate of rotations, anemometer. Start - reduced concentration gradient, at the peak the stomata close.
Water uptake in roots:
Plants absorb water and minerals from the soil using roots.
The SA is increased by branching of roots and root hair cells.
Absorb potassium, phosphate, nitrate and other mineral ions.
Concentration of these ions is usually much higher inside the root cells so they are absorbed by active transport.
Root hair cells have mitochondria and protein pumps.
Water therefore can follow by osmosis into the root hair cells.
Adaptations of plants to saline soils
Halophytes - plants that are adapted to saline soils.
Saline soils - in coastal habitats where water moves up in soil and evaporates, leaving dissolved ions at the surface.
In saline soils, the concentration of ions is so high that most plants are unable to survive, but some are adapted (halophytes).
To prevent water loss halophytes have a higher solute concentration, however, sugars in vacuoles (not ions) to protect cell activities.
Halophytes use different methods to get rid of excess sodium such as active transport back into the soil, excretion from special glands in the leaf and accumulating the ion in leaves and then shedding them.
Many are also succulents.
Adaptations of plants in deserts
Xerophytes - plants that are adapted to grow in dry habitats.
Giant cactus is an example.
Adaptations: vertical stems absorb sunlight early and late, thick waxy cuticle to reduce transpiration, stomata open during the night, spines instead of leaves, hairs on the underside of the leaf, smaller air spaces, few stomata, leaves roll up to reduce SA for transpiration.
Xylem and phloem in stems
Vascular tissue contains vessels used for transporting materials.
Two types of vascular tissue: xylem and phloem.
Order from the outside in stems:
- Epidermis, cortex, phloem, cambium, xylem, pith.
Structure and function of xylem
Xylem - provides support and transports water. In flowering plants, xylem are the main transport route for water.
Few cross walls.
From roots to the leaves in xylem to replace water losses from transpiration - transpiration stream.
Pulling forces help water to move up - adhesive property of water.
Tension can be transmitted from one water molecule to the next - cohesive property of water molecules due to hydrogen bonding.
Sometimes, the pressure in xylem is low - lignin supports the vessels and prevents inward collapse.
Models of water transport in xylem
- Water has adhesive properties - water adheres to glass so rises up the capillary tube but mercury does not.
- Water is drawn through capillaries in cell walls - strip of paper (made of cellulose) water rises up in pores.
- Evaporation of water can cause tension - porous pot is similar to leaf cell walls as water adheres to it and many narrow pores - as water evaporates from the pot more water is drawn into the pot to replace the losses.
The function of phloem
Plants need to transport organic compounds like sugars such as sucrose, from one part to another - phloem is needed.
There are several cell types in phloem tissue.
The movement of organic compounds takes place in phloem sieve tubes.
Sugars and amino acids are loaded into phloem sieve tubes by active transport in sources (stems, leaves, seeds, tubes).
Sugars and other organic compounds are unloaded from phloem sieve tubes in parts called sinks (roots).
The incompressibility of water allows transport along hydrostatic pressure gradients - high concentrations of solutes such as sugars in the phloem sieve tubes at the source lead to water uptake by osmosis and high hydrostatic pressure.
The low concentrations of phloem sieve tubes at the sink lead to exit of water by osmosis and low hydrostatic pressure - pressure gradient makes sap inside phloem sieve tubes flow from sources to sinks.
Loading phloem sieve tubes
- The main sugar carried - sucrose.
- Active transport is used to load it into the phloem but not by pumping sucrose directly - instead, protons are pumped out of phloem cells by active transport to create a proton gradient.
- Co-transporter proteins in the membrane of phloem cells then use this gradient to move a sucrose molecule into the cell by simultaneously allowing protons out down the concentration gradient.
- Some sucrose is loaded directly into phloem sieve tubes by this process.
- To speed up this process adjacent phloem cells also absorb sucrose by co-transport and then pass it to sieve tubes via plasmodesmata.
The structure of phloem sieve tubes
- Phloem sieve tubes from cells that break down their nuclei and most cytoplasmic organelles, but remain alive. - Large pores in the cross walls called sieve tubes allow sap to flow in both directions.
- Plasmodesmata - narrow cytoplasmic connections with adjacent companion cells.
- Lumen of sieve tubes with no organelles.
- Protein fibres.
- Cell membrane holds sap inside sieve tubes and has pumps for loading and unloading sucrose.
- Cell wall resists high pressure inside sieve tubes.
Measuring phloem transport rates
- Using aphids’ stylets (insert into phloem sieve tubes to obtain sap as food) - cutting the insect off its stylet which is inserted into the phloem tissue - sap emerges.
- Can also use radioactive isotope of CO2 - supply it to the leaf - radioactive sucrose is made in the leaf - loaded into phloem - time taken for this radioactive sucrose to emerge from stylet at different distances from the leaf - rate of movement of phloem sap.
Detecting traces of hormones
Plant hormones were discovered in the 20th century but research into their effects was hampered by low concentrations in plant tissues.
Chemically diverse hormones - different methods needed.
ELISA, gas chromatography, liquid chromatography.
Gene expression and transcription.
Indeterminate growth in plants
- Meristem tissue - regions where small undifferentiated cells continue to divide and grow.
- Apical meristems (at the apex) - flowering plants have meristems at the tip of the root and stem.
- Growth in apical meristems allows stems and roots to elongate + flowers and leaves.
- The growth of plants is indeterminate - apical meristems can continue to increase the lengths of stem and root throughout the life of a plant and can produce any number of extra branches of the stem or root or flowers and leaves.
