BIOLOGY AS Edexcel Unit 2


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COMPARING THE STRUCTURE AND FUNCTION OF STARCH AND

Starch and cellulose are both polysaccharides (lots of the same molecule)

Structural features:

Starch-- made up of many alpha glucose units (H on top of the OH at the right end of the molecule) -- contains glycosidic bonds -- has branched molecules as amylopectin.

Cellulose-- made up of many beta glucose units (OH on top of the H at the right end of the molecule) -- contains glycosidic bonds -- never has branched molecules.

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FUNCTIONS OF CELLULOSE MICROFIBRILS AND PLANT FIBR

Cellulose microfibrils

many hydrogen bonds that make it strong --- arrangement and glue gives them strength and flexibility to plant cell walls --- fully permeable to water

Fibres

  • sclerenchyma and xylem (specialised cells) give support --- allow transport of water and mineral ions --- due to lignin being strong and waterproof.
  • purpose of fibres= stiffen to provide mechanical support + allow water and mineral (inorganic ions) transport

xylem vessels= form tubes for water and mineral ion transport- due to long cylinders made up of columns of cells whose walls have broken down- have open ends --- stiffened walls support plant (due to tough lignin)

sclerenchyma fibres= short structures with tapered ends in columns of stiffened cell walls that provide support (due to tough lignin)- but have closed ends so no water and mineral transport

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XYLEM AND LIGNIN

  • large cells with thick cell wall
  • walls waterproofed with lignin
  • lignin impregnates cell wall: cells become lignified
  • lignin reinforces walls preventing inward collapse
  • at same time tonoplast breaks down and there is autolysis of cell contents 
  • end walls between cells lost or perforated 
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TISSUES FOUND IN PLANTS

Three main types of tissue:

  • dermal tissue (epidermis) --- cross section of leaf
  • vascular tissue --- cross section of stem
  • ground tissue --- cross section of root

The vascular bundle in each cross section contains xylem vessels, phloem sieve tubes --- on outside of bundle there is sclerenchyma fibres.

In young plants vascular tissue is in bundles near outside of stem --- with age the separate bundles merge to form continuous rings- this is called secondary thickening- formation of new tissue by the repeated lateral division of cells in the cambium of a woody plant, adding successive layers of new growth --- this increases the girth of the stem or root, and the growth can be seen as annual rings (tree rings). Much of the woody part of a woody plant is the result of secondary thickening. 

phloem= transports sugars made by photosynthesis in leaves up and down plant.

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THE IMPORTANCE OF WATER AND INORGANIC IONS TO PLAN

Water: cohesion and surface tension --- inflate turgor- keeps plants upright (turgid=full) --- thermal properties --- used in chemical reactions such as photosynthesis --- density and freezing properties --- used as medium for biochemical reactions to occur in the cells --- solvent properties --- used as transport for mineral ions and organic molecules (sucrose)

Mineral ion:

nitrate: needed to make amino acids; proteins, DNA, RNA + plant hormones

nitrate deficient= can survive without as cells have own supply of proteins (sometimes yellow leaves)

calcium: forms calcium pectate in cell walls --- involved in membrane permeability

calcium deficient= stunted growth due to its role in cell structure and permeability

magnesium: component of chlorophyll --- helps formation of DNA --- activator of certain plant enzymes

magnesium deficient= yellow older leaves as plant can't make chlorophyll (also sign of nitrate deficient)

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HOW WATER IS TRANSPORTED THROUGH XYLEM VESSELS

1) water evaporates from spongy cells into substomatal cavity

2) diffuses out through stomata down a diffusion gradient

3) evaporation from plant = transpiration

4) tiny channels between cellulose microfibrils act as capillaries 

5) water drawn up by capillary action, caused by surface tension

6) water pulled up xylem vessels and through cell walls in continuous stream

7) cohesive forces between water molecules(H bonds)

8) water linked by cohesion and pulled up by tension= cohesion- tension theory

(the stream of water passing through the plant is known as the transpiration stream)

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HOW PLANT FIBRES ARE EXPLOITED BY HUMANS

The useful properties

cellulose- arranged in microfibrils --- produces rope that does not stretch but is strong and and flexible

lignified plant fibres- fibres very resistant to chemical and enzyme breakdown --- makes plant fibres such as wood a good building material

How they are extracted- the process is called 'retting'

need to extract fibres by taking apart the plant --- can be done 2 ways; mechanically pulling out the fibres or digesting the surrounding tissue.

cellulose especially when combined with lignin is very resistant to chemical and enzyme breakdown but the surrounding polysaccharides that hold fibres together dissolve away.

more lignin hared to separate fibres, so to produce fibre pulp from tree need caustic alkali. Bacteria and fungi can also break down stems piled in heaps.

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USES FOR PLANT FIBRES OTHER THAN TEXTILES

-can make mesh mats that absorb heavy metals and also hydrocarbons such as when there are oil spills in the sea 

-added to other materials to form biocomposites --- e.g when rapeseed fibres are mixed with plastic the product is stronger than plastic alone --- the biocomposites are renewable, more biodegradable and easier and safer to handle than composites containing artificial fibres

-researchers at Warick Uni built a car from plant fibres (Henry Ford produced a car entirely from hemp in 1941)

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PLANT CHEMICAL DEFENCES AGAINST ATTACK

Plants are easy targets to be eaten by animals, bacteria and fungi and if this is not beneficial to the plant such as aiding with seed dispersal, they need to have a chemical defence mechanism --- this is often in the form of a bitter taste so the animal doesn't bite it again, but some of the chemicals can even kill the animal --- if the chemicals kill the animal the plant will avoid future attacks. 

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PLANTS AND MEDICINE

Willow (salix) Foxglove (digitalis purpurea)

antibacterial

like prescription drugs, all plant medicines don't work 100% of the time

connection between hot countries and spices, garlic, oregano, thyme, cinnamon etc. --- several hypotheses: flavour- to disguise taste of spoiled food; cause sweating/ cooling; micronutrients

William Withering- discovered foxgloves to treat oedema/ dropsy reduce blood pressure, restored regular heartbeat --- but need to be regulated as medication needs to be specific, too much is toxic.

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DEVELOPING AND TESTING NEW DRUGS

  • disease targeted --- treatment and ideas developed --- search for possible drugs inc computer molecule designs and screening of chemicals
  • possible drugs made and tested on cell cultures
  • animal testing
  • clinical trials

Phase 1= tested on a few healthy people varied doses --- see if it behaves in same what as predicted from the tests on the live animals

Phase 2= 100- 300 patients with condition --- check if its is safe for patient and has desired effect

Phase 3= 1000-3000 patients- double blind trial- neither patients nor those recording change (doctors) know whether it is the drug or placebo (maybe existing treatment)

  • if successful they are granted license and are continued to be monitored to collect data on effectiveness and safety
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HISTORIC DRUG TESTING- example of William Witherin

  • discovered foxglove could be used to treat dropsy (swelling brought about by heart failure
  • made a chance observation- one of his patients was cured
  • but when he gave it to another patient she a lost died
  • realised he had to get the dose right- too much poisoned patients, too little had no effect
  • through crude method of trial and error that he discovered the right amount to give to patients
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Comments

Georgina Quayle

this is just some of topic 4 about plants

Sajchina

omg!!!!!!!!! this is so good thank you so much :D

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