Studied by 3 peopleTranspiration
the continuous movement of water from the roots of a plant to the leaves where water is lost as water vapor.
Due to water's adhesive and cohesive properties, the water loss at the leave creates a constant pull/movement of water through the plant.
Spongy mesophyll
is the loosely packed mesophyll that provides the surface for gas exchange. It faces the bottom side of the leaf
Stomata
the small gaps in the lower epidermis that allow gas exchange. These can open and close with the help of guard cells to regulate/minimize water loss
Waxy cuticle
prevents excessive water loss from both the top and bottom side of the leaf
Xylem
is the main water transport vessel and can be recognized in a picture by its large hollow spaces
Phloem
is the main vessel for the transport of organic molecules(ie. Sugars and amino acids) through sap
Cambium
makes up the interface of xylem and phloem
Root
where the main part of water absorption occurs.
_ has a large surface area, therefore the plant is able to absorb lots of water from the soil via the process of osmosis, as the root cells are more saturated in sugars and ions compared to the soil
Mineral ions can only be absorbed through active transport since the concentration of ions is greater in the root than in the soul
There are mitochondria and protein pumps in the _ cells that allow the plant to take up sodium, potassium and other minerals through active transport
Stem
where the xylem is located(to transport water)
It is a tubular structure, made of dead plant cells that have fused together
The primary xylem originates in the root and is strengthened by a thick cell wall and lignin
The rest of the xylem is also often impregnated with lignin which helps the xylem cells resist the inward collapse that would occur in very low pressures
How water is pulled from the roots to the top of the plant
Occurs through the process of transpiration.
The mass flow of water is achieved due to three main properties:
Water pressure is high at the roots (water in) and low at the leaves (water out) creating a pressure gradient.
Adhesive property of water allows water molecules to adhere to the xylem wall and move upwards.
Cohesive property of water allows water molecules to stick to one another, creating a continuous stream
The cohesive and adhesive properties of water allow it to…
make a continuous stream and adhere to the walls of xylem (respectively)
Xylem cells are dead → allowing movement of water. Lumen is filled with water. The ring-shaped structures are thickenings of the cell wall surrounded by lignin. Pores in the outer cell wall allow for water to move in and out of the xylem to the surrounding tissues.
The cohesive and adhesive properties of water allow it to make a continuous stream and adhere to the walls of the xylem (respectively)
Saline Soil
Soil found in dry climates, where water is evaporated from the soil → leaves behind soil saturated with mineral ions.
Due to the soil’s high solute concentration in saline soil…
the water would not be able to move through osmosis in most plants → halophytes are plants adapted to saline soils as they keep the solute concentration inside their roots higher than the concentration of the soil
Halophytes do this by keeping a high concentration of potassium sugars in their cytoplasm
In the vacuoles, they can keep high levels of Na+ since there are no metabolic processes going on there → there is an active excretion of sodium out of the plant
In this way, halophytes maintain a high ion concentration so that the little water from the soil still enters the plant by osmosis
In dessert areas,
Xerophytes are plants adapted to live in dry habitats.
Cacti are examples of xerophytes and they thrive in desert environments
potometer
is a device designed to measure the rate of transpiration of plants
The plant is tightly attached to a long capillary tube that has a water source.
There is an air bubble inside the capillary that indicates how much water the plant has taken up.
As the water transpires off of the plant’s leaves, the water is taken up into the capillary, and the bubble moves.
In order to calculate the precise amount of water that was taken up, the measurements should be repeated several times.
Temperature’s effect on transpiration rate
Higher temperatures → water molecules evaporate more easily from the surface of the leaves; air molecules around the leaf move faster, so the relative humidity is also lower→ creating a higher gradient for water to move out of the leaf
At TOO HIGH temperatures: the leaves’ stomata may close and the transpiration decreases slightly.
Measured using a potometer→ using a heat lamp direced at the plant and a thermometer to measure the leaf temperature
humidity
Refers to the number of water molecules in the air
humidity’s effect on transpiration rate
Higher humidity → the smaller the concentration gradient between the moist mesophyll and the humid environment → transpiration decreases
Measured using potometer→ a plastic bag is placed around the plant so that no air escapes→ by spraying water in the bade or by adding silica bags you can increase/decrease the humidity around the plant → the relative humidity can be measured by using a hydrometer
Wind’s effect on transpiration rate
If the air around the leaf is still, the water vapor in the air will not move far away from the leaf and the humidity will increase.
The higher the wind speed, the lower the humidity will be around the leaf, and the higher the transpiration rate.
At too high wind speed → stomata may close
Measured using a potometer→ an electric fan can produce the movement of air around the plant and velocity can vary by moving the fan closer or further away from the plant→ an anemometer is used to measure the exact speed of air movement
phloem
is the vessel transporting organic compounds within the plant, a term often substituted by the term phloem sieve tube
source
(in the phloem) constitutes photosynthtic tissue(ie. leaves)
sink
(in the phloem) constitutes organs of storgate of organic material(ie. fruits, roots) \n
structure of the phloem
is made of live cells that brak down their nucleus and most of the organelles
The walls in between the cells fcontain many holes called the sieve plate, which both help maintain the structure of the tube, and allow for sap to pass through either direction
The membrane of phloem cells:
contains many protein pumps that load and unload sucrose
also contains plasmodesmata→ tight connections between the phloem cells and the companion cells in the vicinity to mediate exchange of materials
the function of the phloem
brings organic molecules to all parts of the plant
the “sources of the plant”
The photosynthetic parts of the plant produce organic “food” molecules
In the _→ sucrose is being loafers into the phloem through active transport. This is because the phloem tube is highly concentrated in organic molecules.
sinks
are the storage parts of the plants, such as the roots, tuber, or fruits
this is where the sucrose gets unloaded from the phloem. Companion cells exist between bothering the source and the sieve tube→ intermediate location for the organic products.
Hydrostatic Pressure
the pressure exerted by a fluid is maintained by different solute concentrations in the liquid and surrounding spaces
what increases hydrostatic pressure
In a plant, the sources are rich in sugars(causing osmosis) the movement of water from the cells surrounding the source, and therefore the xylem, toward the sieve tube
what lowers hydrostatic pressure
At the sources the solute concentrations are low at the sieve tube, as organic molecules are being stored away → water can therefore leave the phloem and enter the surrounding tissues and eventually the xylem
(a pressure gradient between the sieve tubes at sources and sinks. This allows for a constant movement of the sap at the sieve tube from the areas gy the sources toward the area of the sink. → organic molecules are constantly and efficiently being moved from the site of production to the site of storage)
how sucrose is loaded from the source into the phloem
active transport
Purpose of a Co-transporter
pumps sucrose into the phloem by making use of a hydrogen ion gradient that is created by a hydrogen protein pump.
This is a type of secondary active transport, where the co-transporter uses the energy stored in the hydrogen gradient to move the sucrose against its concentration gradient
how can companion cells help with active transport
they take sucrose from the source and pass it onto sieve tube
Aphids
small animals that feed on the sap of plants
they do this by piercing the plant with their stylets until they reach the phloem sieve tube
Measuring the rate of phloem transport: If scientists supply the plant with CO2 containing radioactive Carbon
the plant makes radioactive sucrose that can then be detected in the phloem
Once the aphid pierces the phloem, the stylet is cut off, and the sucrose-rich sap that comes out will be collected by the aphid
The time taken for the radioactive sucrose to move to different parts of the phloem can be used as a measure of phloem transport rate
Flowers
are the reproductive organs of many plants.
The stamen constitutes the male organs and the carpel constitutes the female organs
The seed
contains the embryo of a plant and supplies the food and water needed for its development. The seed is made of modified leaves called cotyledons
Fertilization
is the process of sygote formation, which occurs through the fusion of male gametes(in polen) and female gametes(in ovule)
Pollination
is the transfer of pollen(containing male gametes) from anthers to the stigma(which contains the female gamete). Pollination occurs as the pollen is transferred to the stigma, where the gametes germinate and grow in a pollen tube to the ovary
Seed Dispersal
this is the process where the seeds containing the plant embryo are dispersed in nature where they germinate and grow. Seed dispersal occurs as the fertilized ovules develop into fruits that contain seeds.
When Short day plants flower
flower during the winter
When long day plants flower
flower during the summer
the length of the dark period
is the factor determines when the plant will flower
Regulated by the molecule called phytochrome that exists in two forms:
Inactive red form(Pr)
Active far red form(Pfr)
Inactive red form(Pr)
absorbs red light causing it to become Pfr
Active far red form(Pfr)
absorbs far-red light causing it to become Pr
Sunlight contains
more red light than far-red light → during the day Pr gets converted to Pfr, reaching its highest levels of Pfr. At night(absence of sunlight) Pfr reverts back to Pr
Short-day plants(winter flowering)
Pfr inhibits the flowering of the plant, therefore a long night will cause the lowest amounts of Pfr, causing flowering
Long-day plants(summer flowering)
Pfr triggers flowering, therefore shorter nights will cause low amounts of Pfr changing to Pr and an accumulation of Pfr, causing flowering
Note:
Plants can be made to flower out of their season by varying the amount of time when they are exposed to light vs. dark
Most flowering plants depend on insects and other animals for pollination and consequent reproduction. → Mutualistic relationship→ the animal that pollinates the flower feeds on its nectar and in return transfers pollen from one flower to another(or within one plant)
Oftentimes, a specific species of insect interacts with one specific plant
Ie. there is a species of bee that feeds on vanilla orchid nectar and therefore only pollinates vanilla orchids by crossing from one plant to another in search for food.
Meristems
are the regions of plants containing undifferentiated cells that continuously divide and grow
Determinate Growth
can be observed in animal species, where the embryo doesn’t grow indefinitely, but has a determined number of legs, arms, organs, etc, to grow
Indeterminate Growth
is seen in plants, where the apical meristem can continually provide new cells for further growth of the plant(ie. meristems)
Auxin
is a plant hormone that controls growth in the shoot tip, by stimulating of inhibiting cell division, and setting the direction of growth(tropism)
Phototropism
is directional growth guided by the brightest source of light
Micropropogation
is a term used to describe the propagation of plants(growth) from a single small piece of plant tissue
Apical meristem
found in the shoots and roots of the plant
Are responsible for the lengthening of the plant(primary growth) and leaf development
Shoot of the plant
s defined as the stem with leaves- the shoot contains the cells of apical meristem which continuously divide
During these divisions, some of the cells get displaced to the sides of the meristem and therefore stop dividing → instead they grow and differentiate in order tot produce stem and leaf tissues
Leaves are produced when
the meristem cells get displaced to the side of the apical meristem and form bumps
These bumps(AKA Leaf primordia) are differentiated cells that continue to divide and grow until they form fully-grown leaves
Lateral maristems
are found at the cambium of the plant and are responsible for lateral growth(secondary growth)/thickening of the plant and production of bark in woody plants
phototrophins
are photosensitive pigments in plant shoot cells that respond to different light intensity
Once activated, the photopigments also activate photon pumps that pump H+ ions into the cell wall
In the cytoplamst, auxin
is a negatively charged molecule
The positive charge of the wall attracts auxin molecules to move towards the wall, where they bind a hydrogen ion and then diffuse into the next cell
Once in the cytoplasm again, the auxin
loses the H+ ions and becomes negatively charges
It can then move to the next cell, and therefore redistribute to the shadier side of the plant
Once in the shadier parts of the plant, the auxin binds its receptor and activates genes responsible for the secretion of more H+ ions into the cellulose cell wall
With the extra H+ ions in the cell wall
the acidity increases, and the cellulose loosens up, allowing the plant to elongate, which leads to the general movement of the plant towards the light.
This is because when the cells elongate, they’re causing a bending of the plant towards the opposite direction
In roots however, the auxin inhibits cell elongation
In order for the embryo shoot to emerge from the seed the seed needs to be exposed to optimal conditions such as the following:
Water → to rehydrate the plant seed
Oxygen → to start up cell respiration
Warmth → to provide optimal temperature for enzyme activity
You can test for the effects of water oxygen and warmth on the seed germination by
designing an experiment where two of them are constant and the third is changed.
Example:
Steps of Micropropagation
A small piece of plant tissue is taken from the shoot tip of a plant
The piece of tissue is sterilised and placed in a sealed flask containing nutrient rich agar with abundant auxin
The plant is allowed to grow into a shapeless lump of tissue called callus which can then be split for extended growth
A piece of the callus can be transferred to auxin poor agar, containing other hormones neded for shoot and root development
Once the plantlet develops, it can be transferred to soil to continue its growth into a proper plant
Advantages of Micropropagation
Speeds up the process of plant propagation, only uses small amounts of tissue
The shoot tips re usually virus free
The costs of plant production is decreased→ they don’t have to be taken from native habitat
Note
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