Topic 9 Plant biology

Objectives

Expected outcomes

· Required content

· Explain how plants transport water from the roots and through the stem in order to replace the lost water

· Define transpiration as the loss of water for the leaf by evaporation

· State that transpiration is an inevitable consequence of gas exchange in the leaf

· Describe the pathway that water takes as it leaves the leaf

· Describe adaptations that the plant has in order to reduce water loss

· Discuss why excessive water loss may be damaging to the plant

· Transpiration is the loss of water from the leaves by the evaporation of water from the leaves of plants. It is said to be an inevitable consequence of gas exchange in the leaf.

· Plants require the intake of carbon dioxide for the process of photosynthesis and excrete oxygen as a byproduct of this process. Gas exchange takes place through the stomata- as the protective waxy cuticle prevents the gasses from diffusing directly.

· As water moves into the leaf it evaporates from the cell wall of the spongy mesophyll cells and then diffuses through the air spaces and out of the stomata opening where it is lost from the plant. In order to reduce this water loss, plants have developed a pair of guard cells at each stoma, which have the job of controlling the size of the aperture of the stomata. The presence of the hormone abscisic acid stimulates the closing of stomata.

· Excessive water loss from a plant can result in reduced enzyme function leading to reduced metabolisms and wilting due to plasmolysis within the plant cells. The opposite of this would be turgor- when a plant cell has a high water content it will exert pressure ion the cell wall and become turgid. The plant cell uses salt such as sodium to maintain an osmotic balance. It will pump sodium in or out of the cell by active transport.

· Describe the structure and formation of the xylem tube

· Describe the structure and function of the xylem

· Describe how the xylem is formed

· Discuss the types of lignification used by plants with regard to the xylem

· State the function of the xylem

· Xylem are hollow continuous tubes (vascular tissue) that are developed to efficiently transport water and dissolved minerals. The xylem has thickened cell walls that are impregnated with the waterproof chemical lignin (lignified), this process also improves the strength of the xylem allowing it to remain standing at low water pressures.

· Xylem vessels form between adjacent procambium cells (each unit referred to as a tracheid) which are derived from the parenchyma tissue, much of the cellular material is removed as the xylem form, so the xylem effectively consists of a hollow tube (lumen) made from lignified non-living cells and as a result the movement of water must be a passive process.

· There are different types of lignification that may occur, this includes annular, spiral, reticulated and pitted vessels these structures provide extra support and help water move along the xylem.

· Explain the mechanisms, including the cohesion of water molecules, that assist with the transport of water through the xylem

· Explain how the evaporation of water and its adhesive properties generates tension forces in the cell walls of the leaf

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· Recap the properties of water and identify which of these are essential for water moving in the transpiration stream

· Explain how water moves through the xylem through cohesion, tension, adhesion and evaporation

· Describe factors that influence the rate that water is lost from the leaves

· Along with providing the plant with support, the water is important for transport and for metabolism. Water will move in a continuous column from the root to the leaves through the transpiration stream, passing through the xylem vessels. Water moves through the xylem due to its properties of cohesion and adhesion.

· Water molecules are di-polar and have a negatively charge oxygen and two positively charged hydrogens. This allows the formation of hydrogen bonds between neighboring water molecules (cohesion) and other substances (adhesion). Water moves up the xylem through the process of capillary action, the water adheres to the cell walls of the xylem and as it moves up the tube the water molecules pull along other water molecules due to cohesion (surface tension).

· At the other end of the tubes water evaporates the loss of this water reduces the pressure in the xylem causing water to be pulled along (transpiration pull) in order to replace the molecules that have been lost.

· Transpiration can occur over long distances and is a passive process using the energy from the sun. Factors that can influence the rate of transpiration include temperature- the higher the temperature the greater evaporation the faster transpiration, the higher temperature also speeds up the diffusion of the water molecules as they leave through the stomata of the leaf they move further away from the leaf due to their kinetic energy, light- as light intensity increases a greater number of stomata are opened resulting in faster transpiration, wind speed- more wind means faster evaporation and the lower the humidity, therefore faster transpiration. Humidity- the higher the humidity the more saturated the air is and the slower transpiration is. In addition the root pressure can also cause water to move up the plant,

· Explain how the active uptake of minerals in the roots influences water potential and therefore the rate of osmosis

· Describe what is meant by water potential

· Discuss how plants are able to manipulate the water potential gradient by the active intake of minerals

· Discuss the route taken by water from the soil to the xylem

· The plant must take in minerals such as potassium, phosphate and nitrate in order to grow and effectively metabolize. These minerals are found in the soil and are usually bound to soil particles. The minerals dissolve in water and will diffuse along a concentration gradient towards the roots- we call this mass flow. These minerals will enter the plant through the roots, which have root hairs and branching in order to increase surface area and maximize uptake.

· This uptake is done by active transport so large numbers of mitochondria are found in the root hair cells in order to provide the necessary ATP. Some minerals have an electrochemical charge, so the plant pumps H+ ions out of the root in order to displace them, this creates electrochemical gradient which causes positively charged minerals to enter the root and negatively charged minerals will enter the root linked to the H+ ions.

· Water potential is defined as tendency for water to move from one place to another by osmosis. Water will move from the soil (high water potential) to the root (low/negative water potential) by osmosis, in order to assist with this the plant will actively uptake mineral salts which will maintain a hypertonic state within the root tissue and thus maintains a water potential gradient between the soil and the root. Roots have a large surface area and have root hairs to further increase absorption the hairs have protein pumps in the membranes and lots of mitochondria to provide the ATP for the active transport of solutes.

· Once in the root the water will move through the cortex to the xylem. It may move directly through the cytoplasm of the cells (symplastic pathway) or it may move around the cells and through the cell walls (apoplastic pathway). Water absorbed through the root hairs will pass through the cortex and into the endodermis (Casparian strip) a waxy waterproof barrier that blocks the apoplastic pathway. It is then directed through other endodermal cells in to the xylem where it becomes part of the transpiration pull, moving along due to root pressure and capillary action.

· Describe the adaptations of xerophytes

· Define xerophyte

· Describe how plants have evolved to be adapted to a desert

· Name a variety of xerophytes and identify the techniques that they use in order to reduce water loss

· A xerophyte is a plant that is adapted to survive in very dry conditions such as a desert- (eg prickly pear cactus) plants adapted to dry conditions, these have a thick waxy cuticle, few or small stomata, stomata that only open at night, modified leaves (spines) with a small surface area, water storage tissue in the stem/leaves/roots, A deep and extensive root system for greater water uptake. Marram grass is another xerophyte, this plant has rolled leaves to increase humidity, stomata arranged in pits to trap moist air. Small hairs on the leaf surface also reduce air movement.

· Create a model showing the transpiration stream

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· Biological models are used to imitate biological systems as they allow one particular aspect to be studied independently as opposed to studying multiple factors. Eg capillary tubes can be used to measure the uptake of water by the xylem due to its adhesive and cohesive properties, blotting paper and porous pot can also be used to imitate the flow of water through the xylem or the evaporation of water from the leaf.

· Identify and draw xylem tissue from a microscope image

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· Use a potometer to measure the rate of transpiration

· A potometer is a device that can be used to measure the rate of transpiration.

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· Measure the aperture of stomata form a leaf cast

· Determine stomatal frequency from observations

· By painting the underside of a leaf with clear nail varnish, we can make a leaf cast. We can then use a microscope and micrometer to measure the stomatal frequency (number of stomata per cm2) and aperture of the stomata.

9.2 Plant Biology

Objectives

Expected outcomes

Required content

· Explain the need for plants to transport substances from a source to a sink

· Identify the need for substances other than water and dissolved minerals to be transported within the plant

· Define translocation

· Define and source and sink

· State the role of the phloem

· Plants need to be able to transport organic substances (sucrose or amino acids) from the site of production (source) to where they will be used or stored (sink) for example a plant will manufacture sucrose in the leaf and transport it to the roots where it is stored as starch- starch may also be stored as granules in the chloroplasts, or in seeds equally the carbohydrates may be converted and stored as lipids- molecules with a higher energy content.

· These amino acids and sucrose are transported in the phloem through the process of translocation- this is an active process requiring the use of ATP. It is possible for a source to become a sink depending upon the plant’s needs, this is facilitated by the fact that materials can move through the phloem in either direction.

· Examples of sources would be photosynthetic leaves, green stems, storage organs such as roots and tubers after winter. Sinks include fruits, seeds and developing roots and tubers.

· Describe the structure of the phloem tube

· Sketch a diagram of the phloem and identify the key features

· Identify the function of the sieve plates

· Compare the structure of the phloem to that of the xylem

· Phloem tubes are made up of sieve tubes, between the cells there are perforated cell walls called sieve plates these allow movement between cells. The sieve tubes are made up of living modified cells that lack a nucleus and vacuole.

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· Longitudinal section (section through the vertical axis) transverse section (cross section through the horizontal axis)

· State the role of the companion cell and link to the loading of organic molecules in to the phloem by active transport

· Identify phloem and companion cells for diagrams and microscope images

· State the role of the companion cell

· Describe the process of loading and unloading of the phloem by the companion cell

· It is more efficient for the plant to transport sugar in the form of sucrose, as this molecule is not metabolized during respiration and therefore can be transported efficiently. Companion cells are associated with the sieve tubes and are important for the loading and unloading of the phloem vessel.

· The sugars are transported from the mesophyll cells where they are produced to the companion cells by the apoplastic pathway. At the companion cell H+ ions are actively pumped out of the cell through a hydrogen pump- ATP is needed for this. These ions then flow back in to the cell down a concentration gradient through a co-transport protein as it moves an energy change occurs and this is used to co-transport sucrose.

· Some plants use the symplastic route to get the sugar from the mesophyll cell to the companion cell, in this case the sugar passes through small channels (plasmodesmata) linking the cells together. In order to maintain the flow, the sucrose will be converted in to an oligosaccharide (a small carbohydrate chain).

· Explain how a hydrostatic pressure gradient is formed

· Explain mass flow and link to the transport of substance within the xylem

· Explain how substances require the generation of hydrostatic pressure in order to move through the phloem

· Describe the concept of mass flow

· Link the transport of substances in the xylem to the transport of substances in the phloem

· The buildup of sucrose in the companion cell reduced osmotic pressure and as a result water moves into the cell by osmosis. The rigidity of the cell wall and the buildup of the uncompressible water molecules cause the buildup of hydrostatic pressure.

· The pressure gradient results in mass flow, the movement of water and dissolved substances from the high pressure source to the low pressure sink. At the sink the sucrose is unloaded from the phloem and utilized or converted to starch, this maintains the pressure gradient. Water returns to the xylem at the sink. Companion cells have large numbers of mitochondria in order to provide the ATP needed to pump out the hydrogen ions.

· These cells also provide necessary metabolites to the phloem sieve tubes. Plasmodesmata connect the companion cells with the phloem sieve tubes. Infoldings in the membrane of the companion cell also increase surface area and speed up the loading and unloading of the phloem.

· Analyse data showing phloem transport rates

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9.3 Plant Biology

Objectives

Expected outcomes

Required content

· State the role that the meristems and undifferentiated cells play in the growth of a plant

· Link growth to mitosis occurring at the apex of the shoot

· Define apical meristem and undifferentiated cell

· Discuss the role that meristems and undifferentiated cells have in relation to plant growth

· State the plants grow from the root tip and the apex of the stem

· Explain that the growth of the stem/root is due to mitosis

· An apical (primary) meristem is the tip of the stem or root where growth occurs by cell division (mitosis). Plants go through indeterminate growth (unlimited).

· The meristem contains undifferentiated cells, which go through cell divisions leading to growth. Some plants may develop lateral (sideways) meristems also.

· The mitosis that occurs in the meristems results in the extension of the stem and root these cells also absorb water and nutrients, which leads to further elongation. Leaves and flowers are also derived from the shoot meristem, however the cells that produce these become differentiated.

· Discuss the role played by plant hormones in the regulation of growth

· Describe the role of Auxin efflux pumps and explain how they can create a concentration gradient within plant tissue

· Identify Auxin as a growth hormone in plants

· Identify the areas of a plant where Auxin is produced and describe how it is distributed in the plant

· Describe the role played by Auxin efflux pumps and explain how a concentration gradient may be created

· Auxins are a group of hormones that are involved in the growth of plants, they are weak acids. They are responsible for the development of roots, growth of leaves and development of the fruit. Indole-3-acetic acid (IAA) is an example of an auxin. IAA is produced in the apical meristem and is transported down the stem to the area where needed, auxins can be transported in the phloem.

· High levels of auxins can actually inhibit growth- particularly at the auxiliary buds (found at the base of each leaf) Therefore auxins released in the apical meristem can prevent growth in the auxiliary buds- we call this apical dominance. Auxins are distributed within the plant by the auxin efflux pumps, these help to develop a concentration gradient of auxin this is achieved the presence of transport proteins (PIN 3) in the cell membrane this protein uses energy to transport the hormone between cells. It occurs in one direction and is referred to as polar auxin transport.

· Describe the effect of Auxin and explain how it can influence growth and gene expression

· Discuss the connection between Auxin and gene expression within a plant

· Auxins activate proton pumps, which pump hydrogen ions from the cytoplasm towards the cell wall. The presence of these ions lowers the pH and weakens the bonds between the cellulose layers that make up the cell. wall.

· This softens the cell wall and makes it more flexible. This in turn results in the elongation or growth of a cell. Auxins are believed to bind to receptors within the cell that controls gene transcription. It is believed that this stimulate the production of the PIN 3 protein needed for the transport of auxins between cells. Auxins can therefore influence gene expression.

· Uses of plant auxins

· Describe the commercial uses of Auxin

· Discuss the use of plant hormones as weed killers

· Auxins can be used in rooting powders to stimulate root growth in plant cuttings they can be used to stimulate uniform flowering and for fruit development. It is known that auxins can actually inhibit growth in some plants, this has led to the development of synthetic hormone herbicides, these target specific type of plants to disrupt metabolism and interfere with growth- making the leaves grow faster.

· They tend to work better on di-cotyledons than they do on monocotyledons. It is believed that these herbicides can stimulate the production of ethylene which may cause leaf loos, these chemicals are used as defoliants (eg agent orange)

· Describe the response of the plant shoot to light as a tropism

· Define tropism

· Describe the process of phototropism

· Describe an investigation to see the effects of phototropism

· Explain how the growth of the plant is initiated during a tropic response

· A tropism is a directional growth response to a stimulus. Light and gravity are key stimuli involved in this response. The response to light is called phototropism, stems grow towards light- we call this positive phototropism. Cells on the brighter side of the stem detect the light, they release the hormone auxin (IAA) this is transported to the darker side of the stem where it stimulates cell growth.

· The stem will bend towards the light as a result therefore maximizing photosynthesis. A typical method of demonstrating this is to place a plant into a box and cut a hole in one side. Light is shone through the hole and after some time the stem will move towards the light.

· Give examples of further tropisms

· Describe the process of geotropism

· Describe how a clinostat can be used to demonstrate geotropism

· Geotropism also known a s gravitropism, is a plants response to gravity. The roots are positively geotropic and will grow in the direction of gravity, the stem is negatively geotropic and will grow against gravity. Organelles in the cells called statoliths (calcium containing vesicles) accumulate in response to gravity, this results in the generation of PIN 3 transport proteins and therefore the distribution of auxins to where they are needed. A clinostat is a device that rotates slowly, this can be used to demonstrate the effects of gravity on a germinating seedling.

· Discuss the applications of micro-propagation

· Micro-propagation is a technique that is used to produce large numbers of cloned plants. Sterile tissue (explant) is taken from the apical meristem of the desired plant and placed on to sterile agar gel containing auxins and cytokines. It works on the principle that the tissue contains totipotent stem cells. A callus (mass of undifferentiated cells) is formed. These cells can then be developed to form clones.

· To form roots a ratio of auxin to cytokines of 10:1+ is needed, then roots will develop, a ratio of less than 10:1 will result in the formation of the shoot. Micro-propagation allows the production of a monoculture with a desired characteristic eg disease resistance.

· Discuss the practical implications of micro-propagation such as the rapid bulking up of disease resistant crops and the conservation of rare species

· Rapid bulking means the production of large amount of identical plants. This technique can be used to produce virus free plants (as viruses are transported in the vascular bundles and not in the meristems). It can be used to produce large numbers of orchids. It can also be used to conserve rare species and produce large numbers of disease resistant crops.

· Other plant hormones

· Cytokines which also stimulate cell division and gibberellin important for the maturation of fruits.

· Can plants communicate- discuss the use of chemical by plants for both internal and external communication- do they have language?

· Can plants communicate? I is believed that plants are able to communicate with each other through a network of underground fungi.

· Compare the structure of monocotyledons and dicotyledons

· monocotyledon seeds contain one cotyledon/seed leaf;

· dicotyledon seeds contain two cotyledons/seed leaves;

· monocotyledons have parallel veins;

· dicotyledons have net-like (branched) veins;

· monocotyledon stems have scattered vascular bundles;

· dicotyledon stems have vascular bundles around edge;

· monocotyledon roots are adventitious/fibrous;

· dicotyledon roots are from a tap root or are branched;

· monocotyledon flower parts/petals are (usually) in threes;

· dicotyledon flower parts/petals are (usually) in fours or fives;

Objectives

Expected outcomes

Required content

· Describe the structure of a flower

· Identify the key parts of an insect pollinated flower

· Pedicel- the stalk holding up the flower

· Receptacle- where the flower sits

· Petal- attracts insects

· Nectary- produces the nectar which attracts the insects

· Sepal- protects the developing flower

· Anther- produces and stores large amounts of pollen (male gamete). Filament holds anther in position, both together these are referred to as the stamen which is the male part of the flower.

· Ovary- where the ovules form, stigma- catches the pollen, style- holds the stigma in position. Together these form the carpel which are the female parts of the flower.

· Wind pollinated plants do not need to attract pollinators and rely on the wind to distribute the pollen. These plants tend to produce larger amounts of very light pollen. The anthers hang outside the flower to catch the wind as do the stigma which are often feathery- thus increasing surface area to catch the pollen.

· Describe how flowering involves a change in gene expression within the shoot apex

· Recap the key structures and functions of a flower

· Discuss how the formation of a flower requires a change in gene expression within the shoot apex

· As a plant is developing, we say it is in its vegetative phase. As it matures it will enter its reproductive phase. At this point the meristems will start to produce the flowers and sexual organs. This requires a change in gene expression in the apical meristem of the shoot and this occurs as cells become differentiated. Daylight plays a role in the activation and inhibition of genes.

· For example, phytochrome is a light detecting pigment found in green plants. It is responsible for the activation of the FT (flowering) gene. The mRNA that is produced a s a result of this gene expression is transported from the site of production through the phloem to the shoot meristem where flower production is stimulated.

· Phytochrome

· A photoreceptive chemical located in green plants

· Phytochrome can exist in two interconverting forms. PFR responds to light in the far red wavelength (740nm) and is converted to PR. PR is rapidly converted back to PFR in the presence of red light (660nm) PFR will also convert to PR in the absence of light- but at a much slower rate. PFR is the active form of the pigment and there are binding sites for this form within the cytoplasm (PR will not bind to these so is considered as inactive).

· Note daylight is higher in red light than far red light so there is more PFR during daylight.

· Link the length of light and dark periods to flowering in some plants

· Discuss how the seasons influence the time at which a plant flowers

· Explain with examples how the length of light and dark periods influence flowering

· Give examples of short day and long day flowering plants- state that flowering in the short day chrysanthemum is stimulated by long nights rather than short days

· Factors that can trigger this changes in plant structure could be length of daylight (dark period) and temperature. Short day plants will respond to the long dark periods by flowering in the autumn examples of this would be the chrysanthemum, cotton and rice. Long day plants respond to short dark periods by flowering in the spring or early summer examples of this would be red clover, wheat and pea plants. (note it is actually the length of the dark period and not the length of the light period that stimulates the flowering we call this photoperiodism).

· During the day PFR is converted to form PR, at the end of a short night (long day) therefore in long day plant there will be a high level of PFR remaining in the plant, this acts as a flowering promoter and bonds to the receptors in the cells and activates the FT gene.

· In short day plants PFR actually inhibits the FT gene, at the end of a long dark period much of the active PFR has been converted to the inactive PR as a result the FT gene is active.

· Discuss methods used to encourage short day plants to flower out of season

· It is more profitable for flower producers to be able to control when plants flower. By artificially controlling exposure to light, it is possible to control levels of PFR.

· Long day plants are exposed to artificial light during the night of the non-flowering season, which increases PFR and stimulates flowering out of season, the Siam tulip is one example of this.

· Explain that the success of plant reproduction is due to successful fertilization

· Recap the process of plant fertilization and the growth of the pollen tube

· Explain how seeds form and how that the ovary develops into a fruit

· Identify the main parts of a seed

· Fertilization occurs only after successful pollination. Plants are adapted for wind pollination or for pollination by a vector (pollinator) such as an insect.

· The pollinator and the flower form a mutualistic symbiotic relationship. The pollinator gets the nectar and the flower gets pollinated. Insect pollinated flowers have various adaptations to ensure successful pollination.

· Once pollen has landed on the stigma of the carpel, a pollen tube containing the male DNA will grow down the style towards the waiting ovule that is located in the ovary the pollen tube must pass through the micropyle for fertilization to occur. The seed will form from the fertilized ovule and the ovary will form the fruit. The type of fruit will determine the method of distribution.

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· Testa – a thick protective seed coat, Plumule- develops in to stem, Radicle- forms the root, Cotyledon- embryonic leaf, Hilum- scar where the seed was attached to the ovary. The Micropyle is a small opening through which the pollen tube grows during fertilization and water enters during germination. Note that self-pollination reduces variation and therefore limits natural selection. It makes organisms more susceptible to disease and more likely to suffer from genetic disease due to inbreeding.

· Explain that the success of plant reproduction is due to successful seed dispersal

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· Discuss the need for seed dispersal

· Identify the adaptations of seeds that improve their chances of dispersal over a wide area

· Compare methods of dispersal, including wind, animal and water dispersal

· Seeds need to be dispersed in order to reduce competition between parent plants and their offspring and to colonize larger or new areas./. Various methods exist to assist the dispersal of the seeds and these include water, vectors (animals) and wind. The seeds are usually adapted in order to ensure efficient dispersal.

· Wind dispersed eg sycamore and dandelion, shaped to maximize air resistance light, featherlike to increase surface area.

· Vectors- fruits (apple), tough acid resistant testa, sticky or with hooks to stick to fur eg burdock.

· Water- hard seed coat and low density eg mangroves.

· Seed dispersal tends to correspond with the ripening of the fruit

· Design investigations to test hypotheses about factors affecting germination

· During the process of germination water is needed to rehydrate the seed, soften the testa and activate the enzymes for metabolism eg amylase which hydrolyses starch to form maltose of maltase which converts maltose to glucose, initiating respiration and the development of cellulose cell walls.

· Gibberellin is produced which stimulates the production of amylase. The germinating seed will use its energy reserves for respiration.

· Oxygen is needed for respiration and the temperature must be favorable. Note that light is not needed for seed germination. Investigations can be conducted to test the effect of these factors on seed germination. Some seeds need a cold spell to germinate, some need exposure to fire in order to germinate and others need to pass through the gut of an animal, all of these adaptations are in place to prevent the seed from geminating at the wrong time.