Biology Topic 9 (9.1-9.4)

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What is transpiration?

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What is transpiration?

The loss of water vapour from the stems and leaves of plants

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What are the key steps in transpiration?

  1. Light energy converts water in the leaves to vapour, which evaporates from the leaf via stomata.

  2. New water is absorbed from the soil by the roots. This creates a difference in pressure between the leaves and roots.

  3. Water will flow, via the xylem, along the pressure gradient to replace the water lost from the leaves (transpiration stream)

<ol><li><p>Light energy converts water in the leaves to vapour, which evaporates from the leaf via stomata.</p></li><li><p>New water is absorbed from the soil by the roots. This creates a difference in pressure between the leaves and roots.</p></li><li><p>Water will flow, via the xylem, along the pressure gradient to replace the water lost from the leaves (transpiration stream)</p></li></ol>
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Where does transpiration take place?

In the xylem

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What are stomata?

Pores on the underside of the leaf which facilitate gas exchange

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What is the relationship between transpiration and photosynthesis?

Photosynthetic gas exchange requires the stomata to be open so transpiration will be affected by the level of photosynthesis.

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What is cohesion?

The force of attraction between 2 particles of the same substance

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Cohesion in water molecules

Water molecules are polar and can form hydrogen bonds. Cohesive properties cause water molecules to be dragged up the xylem towards the leaves.

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What is adhesion?

The force of attraction between 2 particles of different substances

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Adhesion in water molecules

Xylem wall is polar and can form intermolecular associations with water molecules. Water molecules move up the xylem via capillary action and are pulled inward on the xylem walls to create tension

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What is the transpiration stream?

The flow of water through the xylem from the roots to the leaf against gravity

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How is the xylem structured to relate to its function?

  • It is a tube composed of dead cells that are hollow - allows for free movement of water

  • Dead cells - movement of water is passive and unidirectional

  • Cell wall contains pores (pits) - enables water to be transferred between cells

  • Walls have thickened cellulose and are reinforced by lignin - provide strength as water is transported under tension

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What are tracheids?

Tapered cells that exchange water via pits - slower rate of water transfer

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What are vessel elements?

End walls have become fused to form a continuous tube - faster rate of water transfer

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How can is lignin deposited in annular vessels?

The lignin forms a pattern of circular rings at equal distances from each other

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How is lignin deposited in spiral vessels?

The lignin is present in the form of a helix or coil

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How is water evaporated out of a plant?

  1. Some light energy absorbed is converted to heat - evaporates water in the spongy mesophyll

  2. Vapour diffuses out of the leaf via stomata - negative pressure gradient in the leaf

  3. Negative pressure gradient causes a tension force in leaf cells - draws water from the xylem (transpiration pull)

  4. Water is pulled from xylem under tension due to adhesive attraction

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What regulates water loss in plants?

The opening and closing of stomata

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How is water loss regulated in plants?

  • Guard cells control the opening of the stomata by becoming increasingly flaccid in response to cellular signals

  • Plant begins to wilt from water stress - dehydrated mesophyll cells release the hormone abscisic acid (ABA)

  • ABA triggers efflux of potassium from guard cells - decreasing water pressure in the cells (lose turgor)

  • This causes the stomatal pore to close - guard cells become flaccid and block the opening

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What factors affect the rate of transpiration?

Levels of photosynthesis, humidity, temperature, light intensity, wind

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How do plants optimise water and mineral ion uptake from the soil?

  1. Having a maximal surface area for absorption via their roots

  2. some plants use a fibrous, highly branching root system - increases surface area

  3. Others have a main tap root with lateral branches - penetrate the soil to access deeper water reservoirs

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What are root hairs and what is their role in absorption?

Root hair cells are cellular extensions on the epidermis of roots which increase the surface area for abroption

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What happens to materials absorbed by the root epidermis?

Diffuse across the cortex towards a central stele - where the xylem is located

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What is the Casparian strip?

An impermeable layer in the endodermis surrounding the stele. It prevents passive water and ion flow

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What is the function of the Casparian strip?

Water and minerals are pumped across it by specialised cells - this controls the rate of uptake

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What is translocation?

The movement of organic compounds from sources to sinks

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Where does translocation take place?

In the phloem

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What is a source?

Where the organic compounds are synthesised - photosynthetic tissues

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What is a sink?

Where the compounds are delivered to for use or storage

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What is an example of a source?


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Sink examples

Roots, fruits and seeds

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What are sugars transported as and why?

Sucrose because it is soluble but metabolically inert

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What is the fluid of the phloem called?

Plant sap

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What are phloem sieve tubes composed of?

sieve element cells and companion cells

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What are sieve element cells?

Long and narrow cells that are connected to form the sieve tube

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What are sieve tubes connected by?

sieve plates - porous to enable flow between cells

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How are sieve elements structured to relate to their function?

They have no nuclei and a reduced number of organelles to maximise space for the translocation of materials. They also have thick and rigid cell walls to withstand the hydrostatic pressure - facilitates flow

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What is the role of the companion cells?

Provide metabolic support for sieve element cells and facilitate the loading and unloading of materials at the source and sink

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How are the companion cells structured to relate to their function?

  • Have an infolding plasma membrane - increases SA:Vol ratio - more material exchange

  • Have lots of mitochondria - fuel active transport of materials between the sieve tube and source or sink

  • Have active transport proteins in the plasma membrane - move materials in or out of the sieve tube

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What are Plasmodesmata?

They are between sieve elements and companion cells and connect the cytoplasm of the 2 cells and mediate the exchange of metabolites

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What is the direction of materials in the phloem?


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How is movement of sap in the phloem mediated?

By the hydrostatic pressure from the xylem

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How do xylem and phloem vessels differ in terms of their cavity diameter?

Xylem have larger cavities compared to phloem

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In monocotyledons, how is the arrangement of the stele and the location of xylem and phloem vessels different from dicotyledons?


  • Stele is large

  • vessels form a radiating circle around the central pith

  • Xylem vessels located more internally

  • Phloem vessels external


  • Stele is very small

  • xylem is located centrally and the phloem surrounds it

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What shape may xylem vessels form in dicotyledons, and where is the phloem situated?

Xylem may form a X shape, and the phloem is situated in the surrounding groups

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How are vascular bundles arranged in monocotyledons, and where are phloem vessels positioned in the stem?

  • Vascular bundles are in a scattered arrangement throughout the stem.

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What are organic compounds produced at the source actively loaded into phloem sieve tubes by?

Companion cells

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How are organic compounds loaded into phloem sieve tubes?

  • Materials can pass into the sieve tube via interconnecting plasmodesmata (symplastic loading)

  • Materials can be pumped across the cell wall by membrane proteins (apoplastic loading)

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Is apoplastic loading of sucrose into the phloem sieve tubes an active or passive process?

It is active as it requires ATP

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How does apoplastic loading work?

  1. H+ ions actively transported out of phloem cells by proton pumps (hydrolysis of ATP)

  2. Concentration of H+ ions builds up outside the cell - creating proton gradient

  3. H+ ions diffuse back into the phloem via a co-transport protein - requires sucrose movement - results in a build up of sucrose in the sieve tub for transport from the source

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What are meristems?

Tissues in a plant consisting of undifferentiated cells capable of indeterminate growth

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What does meristematic tissue do?

Allow plants to regrow structures or even form new platns

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What are the two types of meristematic tissue?

Apical and lateral

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What are apical meristems?

Occur at shoot and root tips and are responsible for primary growth (plant lengthening). Produces new leaves and flowers

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What are lateral meristems?

Occur at the cambium and causes secondary growth (plant widening/thickening). Produced bark

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How does growth occur at the regions with apical meristems, and what processes are involved?

Growth is due to a combination of cell enlargement and repeated cell division. Included mitosis and cytokinesis

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What is the outcome of the differentiation of the dividing meristem in plants?

A variety of stem tissues and structures e.g. leaves and flowers

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Where does growth occur in the stem?

The nodes

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What is auxin?

A group of hormones produced by the tip of a shoot or root that regulate plant growth

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What do auxin efflux pumps do?

They set up concentration gradients within tissues - changing the distribution of auxin within the plant

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How do auxin efflux pumps control the direction of plant growth?

They do this by determining which regions of plant tissue have high auxin levels

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What is the mechanism of action of the auxin in the shoots?

Auxin stimulates cell elongation - high conc of auxin promotes growth

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What is the mechanism of action of the auxin in the roots?

Auxin inhibits cell elongation - high conc of auxin limits growth

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How does auxin influence cell growth rates?

By changing the pattern of gene expression with a plant’s cells

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How does auxin work in the shoots?

  • Auxin increases the flexibility of the cell wall to promote plant growth

  • Auxin activates a proton pump in the plasma membrane - causes secretion of H+ ions into the cell wall - results in a decrease in pH

  • this decrease in pH causes cellulose fibres within the cell wall to loosen (breaking bonds)

  • Auxin upregulates expression of expansins - increases elasticity of cell wall

  • AN influx of water causes the cell to increase in size

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What are tropisms?

Describe the growth or turning movement of a plant in response to a directional external stimulus

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What is phototropism?

A growth movement in response to a unidirectional light source

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What is geotropism?

A growth movement in response to gravitational forces

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What is hydrotropism?

A growth movement in response to a water gradient

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What is thigmotropism?

A growth movement in response to a tactile stimulus

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what controls phototropism and geotropism?

The distribution of auxin in the plant cells

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What happens in phototropism?

  • light receptors (phototropins) trigger the redistribution of auxin to the dark side of the plant

  • In shoots, high auxin conc. promotes cell elongation so the dark side of the shoot elongates and grows towards the light (positive phototropism)

  • In roots, high auxin conc. inhibits cell elongation so the dark side of the root becomes shorter and roots grow away from the light (negative phototropism)

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What happens in geotropism?

Auxin accumulates on the lower side of the plant in response to gravity

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What is micropropagation?

A technique used to produce large numbers of identical plants from a selected stock plant. Occurs in a lab

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Why do plants reproduce asexually from meristems?

Because they are undifferentiated cells capable of indeterminate growth

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What is vegetative propagation?

When a plant cutting is used to reproduce asexually in the native environment

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What are the steps in micropropagation?

  1. plant tissue selected from a stock and sterilised

  2. Tissue sample (explant) is grown on a sterile nutrient agar gel

  3. Explant treated with growth hormones (auxin) to stimulate shoot and root development

  4. the growing shoots can be continuously divided and separated to form new samples (multiplication phase)

  5. Roots and shoots are developed - transferred to soil

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Uses of micropropagation - rapid bulking

  • Cloned via micropropagation to conserve the fidelity of the selected characteristic

  • process more reliable than selective breeding - new plants are genetically identical to the stock plant

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Uses of micropropagation - virus-free stains

  • Viruses spread through infected plants via the vascular tissue - which the meristems do not contain

  • Propagating plants from the non-infected meristems allows for the rapid reproduction of virus-free plant strains

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Uses of micropropagation - propagation of rare species

  • Micropropagation used to increase numbers of rare or endangered plant species

  • also used to increase number of species that are difficult to bread sexually (orchids)

  • also used to increase numbers of plant species that are commercially in demand

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What are the uses for micropropagation?

Propagation of rare species, virus-free strains, rapid bulking

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What can axillary (lateral) buds do?

They have the potential to form new branching shoots complete with leaves and flowers

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What controls the growth of the stem and the formation of new nodes in plants?

Plant hormones released from the shoot apex

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What is apical dominance

The production of auxins prevents growth in lateral (axillary) buds

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What is the purpose of apical dominance?

Ensures that a plant will use its energy to grow up towards the light in order to outcompete other plants

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What are the different ways a plant can reproduce?

Vegetative propagation, spore formations, pollen transfer

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What happens in sexual reproduction in flowering plants?

The transfer of pollen (male gamete) to an ova (female gamete)

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The are the 3 phases in sexual reproduction in plants?

Pollination, fertilisation, seed dispersal

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What happens during pollination?

The pollen grains are transferred from an anther (male) to a stigma (female). Plants possess both male and female structures (monoecious) and can self-pollinate. Cross-pollination id preferable - improves genetic diversity

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What happens during fertilisation?

Fusion of male gamete nuclei with female nuclei - forms a zygote.

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Where is the male gamete stored?

In the pollen grain

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Where is the female gamete stored?

In the ovule

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What happens during seed dispersal?

  • Fertilisation of gametes results in the formation of a seed - moves away from the parental plant

  • Seed dispersal reduces competition for resources between germinating seed and parental plant plant

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What are the different seed dispersal mechanisms?

Wind, water, fruit, animals

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What is cross-pollination?

Transferring pollen grains from one plant to the ovule of a different plant

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What is a mutualistic reltionship?

Both species benefit from the interaction. Plant gains sexual reproduction and the animals gains a source of nutrition (nectar)

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Common examples of pollinators

Birds, bats, insects

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What are flowers?

Reproductive organs of angiospermophytes that develop from the shoot apex

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How are flower structures produced?

Changes in gene expression trigger to enlargement of the shoot apical meristem - this tissue then differentiates to form different flower structures

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What influences the activation of genes responsible for flowering?

Abiotic factors - linked to season

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