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IB Categories Archives: Topic 9: Plant biology

9.3 – Reproduction in Angiospermophytes

9.3 – Reproduction in Angiospermophytes

9.3.1 – Draw and label a diagram showing the structure of a dicotyledonous animal-pollinated flower

Sepal

  • Cover the flower structure while the flower is developing
  • Sometimes modified into petals

Petals

  • surround the male and female flower parts
  • function is to attract animal pollinators

Anther

  • Where meiosis occurs to produce haploid pollen

Filament

  • a stalk that supports the anther

Stamen

  • male reproductive structure
  • consists of the anther and filament

Pistil, or carpel

  • female reproductive structure
  • consists of the stigma, style and ovary

Stigma

  • the surface on which pollen lands
  • receives pollen from the anther
  • the pollen grows down to the ovary

Ovary

  • where meiosis occurs to produce haploid ovules

Style

  • pollen grows in a tube down through the style
  • connects the stigma to the ovary

9.3.2 – Distinguish between pollination, fertilisation and seed dispersal

Pollination

The transfer of pollen grains from the mature anther to the receptive stigma

Fertilisation

The fusion of the male gamete with the female gamete to form a zygote

Seed Dispersal

Seeds are moved away moved away from the vicinity of the parental plant before germination to reduce competition for limited resources. Mechanisms for this include fruits, winds, water and animals.

9.3.3 – Draw and label a diagram showing the external and internal structure of a named dicotyledonous seed

Broad Bean – Vicia faba

  • Testa – seed coat to protect the plant embryo and the cotyledon food stores. Formed from the ovule wall.
  • Radicle – the embryonic root. Attached to and sandwiched between the cotyledons
  • Plumule – the embryonic stem.
  • Cotyledons – contain food store for the seed. Two in the seed
  • Micropyle – a hole in the test from pollen tube fertilisation, through which water can enter the seed prior to germination. A hand lens is needed to see it.
  • Scar – where the ovule was attached to the carpel wall, or ovary/ fruit.

 

9.3.4 – Explain the conditions needed for the germination of a typical seed

Many seeds have a dormant period, in which they do not germinate as soon as they are dispersed. This happens for a number of reasons, including:

  • incomplete seed development – the embryo is immature, which is overcome in time
  • presence of a plant growth regulator – such as abscisic acid, which inhibits development, and disappears with time
  • impervious seed coat – eventually made permeable by abrasion with coarse soil or the action of microorganism
  • requirement for pre-chilling – under moist condition, before the seed can germinate, sometimes below 5°C for 50 days (the equivalent of winter in temperate climates

Once dormancy is overcome, germination will occur some certain essential external conditions are met

  • water – uptake occurs so that the seed is fully hydrated and the embryo is able to be physiologically active
  • oxygen – must be present in high enough partial pressure to sustain aerobic respiration. Growth requires continuous supply of metabolic energy in the form of ATP that is best generated by aerobic cell respiration in all the cells
  • suitable temperature – close to optimum temperature for the enzymes involved in the mobilisation of stored food reserves, translocation of organic solutes in the phloem, and the synthesis of intermediates for cell growth and development.

Seeds also may require fire, freezing, passing through the digestive system of a seed dispersing animal, washing to remove inhibitors, light or the erosion of the seed coat.

 

9.3.5 – Outline the metabolic processes during germination of a starchy seed

  • Water absorption, causing the testa to split, combined with adequate oxygen and favourable temperature. Cotyledon cells are activated
  • Gibberellic acid (GA) is formed by the embryo in the embryo’s cotyledon
  • GA passes to the food stored in the cotyledons, and stimulates the production of amylase
  • Starch is broken down into maltose, which is then broken down into glucose
  • Glucose is released for energy to sustain respiration and growth. It may also be polymerised into cellulose for cell wall formation

9.3.6 – Explain how flowering is controlled in long-day and short-day plants, including the role of phytochrome

Flowering Cues

The blue-green pigment called phytochrome is present in very low concentrations, which is a highly reactive protein. It is a photoreceptor pigment in the leaves, able to absorb light of a particular wavelength, and changes its structure as a consequence.

Forms of Phytochrome

PR is a blue pigment that mainly absorbs red light, which has a wavelength of 660nm. PFR is a blue-green pigment that mainly absorbs far-red light, with a wavelength of 730nm. This inhibits flowering in short day plants, and promotes flowering in long day plants. PR is converted into PFR when exposed to light (or just red light). It is converted back into PR in the dark, or just far-red light

Long-Day Plants

These bloom when days are longest and nights are shortest, such as midsummer. They include radishes, spinach, lettuce, barley, wheat and clover. They need sufficient exposure to light. In daylight, PR is converted into PFR, but during the short night, the PFR does not have a long time to convert back into PR. At the end of the night period, the concentration of PFR is high. The high PFR concentration is the trigger for flowering. The short night is the critical element.

Short-Day Plants

These only flower if the period of darkness is longer than a certain critical length. They typically flower in the spring or autumn when day length is short. In daylight or red light, PR is converted into PFR, which only requires brief exposure. At the end of the night period the concentration of PFR is low, which is the trigger for flowering. The long night is the critical element.

 

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9.2 – Transport in Angiospermophytes

9.2 – Transport in Angiospermophytes

9.2.1 – Outline how the root system provides a large surface area for mineral ion and water uptake by means of branching and root hairs

The roots of plants have numerous branches and hairs to increase their surface area. This means that they are in greater contact with the soil. Water and minerals are absorbed from the soil solution through these root hairs.

The monocotyledon roots are highly fibrous and branching structures. This increases their surface area for water absorption. Dicotyledon plants have a min tap root, with additional root branching off it. This enables them to access deeper water and minerals. They have shallow roots close to the surface, and then the tap root penetrates deeper into the ground.

Root hairs are found behind the growing tip of each root, and are an extension of individual epidermal cells. They are used to increase the surface area for water absorption.

 

 

The cells with hairs have a greater cell wall size for increased nutrients and water absorption.

 

 

 

9.2.2 – List ways in which mineral ions in the soil move to the root

Diffusion This requires a concentration gradient. The minerals are generally in low concentration in the soil. The move through the route called the symplast.

This requires a concentration gradient. The minerals are generally in low concentration in the soil. The move through the route called the symplast.

Fungal Hyphae

This is a form of mutualism. The fungi provide minerals in a form that the plants can use, such as nitrates. They form networks called mycelium to increase surface area within the root to concentrate the minerals. In return, fungi receive sugar from the plants.

Mass Flow of Water

Ions are carried through the apoplast, through spaces in the cellulose wall. This way, the water actually does not go near the living content of the cells. There is a hydrostatic pressure gradient instead of an osmotic gradient, and negative pressure potential.

9.2.3 – Explain the process of mineral ion absorption from the soil into the roots by active transport

This process requires metabolic energy for transport.

The process occurs against a concentration gradient. The ions move from a region of high concentration to a region of lower concentration. The cells tend to hoard essential ions.

This is also a selective process, in which specific ions can be absorbed based on the needs of the plant

It also uses protein pumps which select specific ion to be transported to the other side of the membrane. The process requires ATP. If a certain pump is not present, then substance will not be transported.

All these ions are found in the soil solution

9.2.4 – State that terrestrial plants support themselves by means of thickened cellulose, cell turgor and lignified xylem

 

Thickened Cellulose

The cellulose is found on the cells located on the outer sections of the stem. These may also be called collenchyma cells.

 

Cell Turgor

Turgidity exerts pressure on the surrounding cells. When water enters by osmosis, this increases the volume of the cells. More pressure, the turgor pressure, is exerted on the cell walls, providing mechanical support for the tissue. When the plant wilts, it is because there is not enough water to create this cell turgor.

 

Lignified Xylem

The xylem tissue is strengthened by extra cellulose, and hardened further with lignin. This is a chemical substance which increases the strength of the xylem and provides most of the support in woody stems.

 

 

9.2.5 – Define transpiration

Transpiration is the loss of water vapour from the leaves and stems of plants

9.2.6 – Explain how water is carried by the transpiration stream, including the structure of xylem vessels, transpiration pull, cohesion, adhesion and evaporation

Structure of Xylem Vessels

Xylem cells develop into long, hollow tubes. The inside of the lateral walls of the vessel has cellulose deposited in it, and is hardened by lignin. This is a very tough tissue so it can resist negative pressure, or suction. Water moves up through the xylem vessel. It is a passive process, requiring no energy.

Transpiration Pull

Transpiration is the evaporation of water vapour through stomata of green plant leaves. As this water is lost, the osmotic pressure increases, causing suction for water uptake.

Cohesion

This is based on the hydrogen bonding of water. It means that the column of water does not break under tension, such as negative pressure or suction. The column of water is a continuous stream from the roots to the leaves. The water molecules are attracted through hydrogen bonding, which results in the pull.

Adhesion

Water molecules adhere to the xylem vessel walls as a result of the properties of water. Combined with cohesion, the water column is able to run the entire length of the xylem at a continuous rate. The water replaces that lost in transpiration.

Evaporation

Water evaporates through the stomatal pore down a humidity gradient. This pulls more water by mass flow into the spongy mesophyll space. There is a gradient of negative pressure potential from the stomatal pore, through the leaf and down the xylem

The leaf absorbs light on its large surface area and heat is produced. The water in the spongy mesophyll tissue enters the vapour phase and evaporates through the stomata. This is then replaced by water from the xylem, drawing it up using transpiration pull. The water enters at the root. The plant will actively pump minerals into the xylem to create osmotic pressure

Uptake in the roots

The water enters through osmosis, as the soil has a lower solute concentration of minerals than the cytoplasm of the epidermal cells. Water moves across the cortex using the water potential gradient. Symplastic movement is a form of osmosis through the plasmodesma. Apoplastic movement is a form of capillary action, moving through the connecting cell walls. At the end of the endodermis, there is a casparian strip, which acts as a barrier to the movement of water into the xylem by the apoplastic pathway. Instead, solute and water must go through the plasma membrane before it can enter the stele. As a result, the uptake of minerals is controlled by the plasma membrane.

ABA and Water Stress

ABA [abscisic acid] is a growth regular in the stems, fruits and leaves of plants. It is important in the leaves during physiological stress, such as drought, where it causes the stomata to close to maintain water levels. The main factors that decrease the availability of water to the plant

The main factors that decrease the availability of water to the plant are:

  • Light, as this causes the stomata to open, causing more water loss, as well as raising the temperature of the plant and turning the water into vapour.
  • Temperature, causes more water to evaporate and raises the level of transpiration
  • Wind increases the rate of transpiration because it increases the concentration gradient between the inside and outside of the leaf
  • Humidity decreases the rate of transpiration as it lowers the concentration gradient between the inside and the outside of the leaf

Organic Solute Transport in the Stem

Called translocation, which is when organic foods, including sugars and amino acids, move through the phloem. The sugars are produced in the leaves during photosynthesis, which is used to assist growth or storage. Amino acids are made in the root tips from nitrates in the soil, then taken to where they are needed for photosynthesis. Other chemicals may also move through translocation.

Any plant that has many pores connected to the phloem in called a sieve plant. The sieve tubes carry the solutes from the pores to the phloem using ATP, then they use hydrostatic pressure to be taken away for storage. Remember that the phloem itself is living, undergoing metabolism. The phloem allows for transport in any direction by mass flow.

Movement Through the Leaf

Leaves have tiny pores on them called stomata, to allow gas exchange. There are also some in the stems. They are mainly found in the lower epidermis of dicot leaves. Stomata are made of two, elongated guard cells that are joined to the other cells and each

Stomata are made of two, elongated guard cells that are joined to the other cells and each other, but they can separate to make a pore. When the cells increase in turgor pressure because of water uptake, they open, and then close again when the water is lost. In general, the guard cells stay closed when it is dark, but open when it is light.

In general, the guard cells stay closed when it is dark, but open when it is light. Alternatively, if the plant is wilting from lack of water, they will close. The function of the stomata is to control transpiration to stop too much water loss.

Since leaves have such a large surface area, they are able to absorb more light, which is consequently converted into heat energy. The temperature of the leaf rises, turning water in the spongy mesophyll into vapour. Guard cells then open to allow this vapour to be released, and cooling the leaf. The space is then filled with more vapour through symplastic and apoplastic movement. This in turn draws the water through the xylem from the roots.

Plant Example that you Need to Know: Xerophytes

Xerophytes are plants [such as cacti] which live in permanently dry and arid conditions.
Xerophytes have evolved to have many features that allow them to retain water under their conditions.

These include:

  • Thick cuticle on leaf and stem epidermis – to prevent water loss
  • Hairs on the epidermis – traps moisture to reduce diffusion
  • Fewer Stomata – Less opportunity for water escape
  • Stomata in pits or groves – these trap water to slow diffusion
  • Rolled or folded leaves – this occurs when the leaf becomes flaccid due to lack of water to reduce the rate of transpiration
  • Superficial roots – absorbs condensation from the surface at night
  • Deep roots – absorbs water from the deep water table
  • Alternative photosynthesis pathway – called the C4 pathway, which takes places as well as the C3 pathway to enhance carbon fixation so that the stomata can be closed
  • CAM metabolism – stores CO2 in the form of acids during the night for fixation during the light. Called crassulacean acid metabolism.
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9.1 – Plant Structure and Growth

9.1 – Plant Structure and Growth

9.1.1 – Draw and label plan diagrams to show the distribution of tissues in the stem and leaf of a dicotyledonous plant

Stem Cross-Section of a Dicotyledonous Plant

  • Epidermis – Surface of the stem made of a number of layers with a waxy cuticle to reduce water loss
  • Cortex Tissue – Forming a cylinder of tissue around the outer edge of the stem. Often contain cells with secondary thickening in the cell walls, which provides additional support
  • Vascular Bundle – contains xylem, phloem and cambium tissue
  • Xylem – a longitudinal set of tubes that conduct water from the roots, upwards through the stem to the leaves
  • Phloem – Transports sap through the plant tissue in a number of possible directions
  • Vascular Cambium – a type of lateral meristem that forms a vertical cylinder in the stem. This produces the secondary xylem and phloem through cell division in the vertical plane
  • Pith – found in the centre of the stem. Composed of thin walled cells called parenchyma. This degenerates in some plants to leave a hollow stem

  • Cuticle – Waxy layer which reduces water loss through the upper epidermis
  • Upper epidermis – A flattened layer of cell that forms the surface of the leaf and makes up the cuticle
  • Palisade Layer – The main photosynthetic region of the leaf
  • Vascular bundle – Contains the transport system and vascular meristem tissue (xxylem and p-phloem)
  • Spongy Mesophyll – Contains spaces the allow the movement of gases and water through the leaf tissue
  • Lower Epidermis – Bottom surface layer of tissues which contains the guard cell that form each stoma

9.1.2 – Outline three differences between the structures of dicotyledonous and monocotyledonous plants

 

9.1.3 – Explain the relationship between the distribution of tissues in the leaf and the functions of these tissues

  • Phloem – transports the products of photosynthesis (sugars, amino acids) to the rest of the plant
  • Xylem – Transports water and minerals into the leaf tissue from the stem and roots
  • Epidermis – Produces a waxy cuticle for the conservation of water. The guard cells of the stomata regulate inward gas exchange. It is tough and transparent to allow light absorption.
  • Stomata – The site of inward diffusion of CO2
  • Palisade Layer – Main photosynthetic region
  • Spongy Layer – Creates the spaces and surfaces for the movement of water and gases
  • Lower Epidermis – Contains stomatal pores which allow gas exchange with the leaf, mainly carbon dioxide.
  • Vascular Bundles – Spread through the leaf like a network. No mesophyll cell is ever more than a few cells from a vascular bundle. They also support the leaf
  • The thin, flat structure of the leaf results in a large surface area to maximise light absorption in the chloroplasts of the mesophyll cells.
  • The air spaces between the mesophyll cell allow gas exchange and are the pathway for
    diffusion

9.1.4 – Identify modifications of roots, stems and leaves for different functions: bulbs, stem tubers, storage roots and tendrils

Bulbs

  • Onions 

– have scaly outer leaves

– inner leaves filled with food reserves

– the heart contains the terminal bud, the lateral bud and the stem, which eventually                                  becomes a new plant by bulb division short lateral stem

– short lateral stem

Stem Tubers

  • Potatoes

– once the leaves have manufactured sugar from photosynthesis, it is converted into starch

– the potato is attached underground to the lateral stem

– the tuber [potato] is packed with starch and some protein

– this is then able to sprout into a new potato plant

  • Cacti

– leaves are spines to prevent water loss in transpiration

– stem is enlarged for water storage and carries out photosynthesis

  • Strawberries

– an example of runner stems spreading out from the main body of the plant o forms new                       roots where it touches the ground to independently establish small plantlets

– adapted to seek out water sources

Storage Roots

  • Carrots

– An example of a modified tap root

– the root has become swollen with food reserves

– also stores water

– serves to stabilise the plant in the soil

Tendrils

  • Stem tissue modified as a tendril

– these will grow around other stems or support structures

– develop to support the weak stems of climbing plants, such as grape plants

  • Leaf tissue modified as a tendril

– these also serve to support the plant

– an example is sweet peas

9.1.5 – State that dicotyledonous plants have apical and lateral meristems

  • Plants grow from the meristems
  • Meristematic cells divide by mitosis to allow growth of the plant
  • Lateral meristem

– forms from the cambium cells in the centre of the vascular bundles

– Causes secondary growth by adding vascular tissue

– Increases the girth of the stem

– Stem circumference and strength increase

  • Apical meristems

– Occur at the tips of the stem and root

– Case primary growth

– Cell division

– Cell enlargement

– Cell differentiation and specialisation

9.1.6 – Compare growth due to apical and lateral meristems in dicotyledonous plants 

9.1.7 – Explain the role of auxin in phototropism as an example of the control of plant growth

  • The concentration of auxin is greatest among cells undergoing cell division
  • Growth of the plant stem is inhibited by light
  • The growth response is regulated by auxin

– Auxin causes the cells to expand on the shaded side

  • This causes the shoot to grow towards the light source

Tropism – bending growth movement towards or away from a directional stimulus
Phototropism – bending growth towards the unilateral source of light
Auxins – type of plant growth hormones, or growth regulating factors

 

  • The cylindrical shoot is enclosed in a sheath of cells called the coleoptile
  • Darwin’s experiments showed that the tip is sensitive to light; the apical meristem
  • Chemicals are transmitted to the rest of the plant to affect growth; it is transported to the zone of cell growth
  • Greater concentration of auxin results in a larger degree of bending

  • Light stimulus leads to growth on the opposite side of the stem
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