topic 2 C) D) - biological molecules and movement of substances

chemical elements in carbohydrates

carbon, oxygen, hydrogen

chemical elements of lipids

carbon, oxygen, hydrogen

chemical elements of proteins

carbon, oxygen, hydrogen and nitrogen (sometimes sulphur)

what are carbohydrates

• small simple sugars or more complex larger molecules

• monosaccharide = simple sugar e.g. glucose, fructose

• disaccharide = two monosaccharides joined together e.g. maltose from 2 glucose

• large polysaccharide = lots of monosaccharides joined together e.g. starch, glycogen, cellulose from glucose (insoluble storage molecules)

what are lipids

• made up of triglycerides

• basic unit is one glycerol molecule chemically bonded to three fatty acid chains

• divided into fats (solids at r.t) and oils (liquids)

what are proteins

• formed from long chains of amino acids

• different proteins have different a.a sequences and therefore shapes

• shape of a protein determines its function

• e.g. enzymes, haemoglobin, ligaments, keratin

how to prepare a sample of food for food tests

• break up food using pestle and mortar

• transfer to test tube and add distilled water

• mix with water by stirring with glass rod

• filter mixture using funnel and filter paper, collect the solution

test for glucose

• add benedicts solution

• heat in boiling water bath for 5 minutes

• take out and observe colour

• positive = from blue to orange/brick red

test for starch

• add drops of iodine solution to food sample

• positive = from orange/brown to blue/black

test for protein

• add drops of Biuret solution

• positive: colour change from blue to violet/purple

test for lipids

• mix food sample with 4cm³ of ethanol and shake

• allow time for sample to dissolve in ethanol

• strain into another test tube

• add ethanol solution to an equal volume of cold distilled water

• positive test forms cloudy emulsion

what are enzymes

• proteins that act as biological catalysts to speed up rate of reaction without being changed or used up

• biological because they are made in living cells

• essential to maintain reaction speeds of all metabolic reactions at a rate that can sustain life

how do enzymes work

• enzymes are specific to one particular substrate because the active site of the enzyme (where the substrate attaches) is complementary to the shape of the substrate

1. enzymes and substrates randomly move about in solution

2. when an enzyme and its complementary substrate collide, an enzyme-substrate complex forms and reaction occurs

3. a product forms from the substrate and is released from the active site

factors affecting enzyme action

• temperature

• pH

how does temperature affect enzyme action

• enzymes work fastest at their optimum temperature (37 in the human body)

• heating to higher temperatures will break the bonds that hold the enzyme together and it will lose its shape and denature

• substrates cannot fit into denatured enzymes as the shape of the active site is lost

• increasing temp towards optimum increases activity of enzymes as more kinetic energy means molecules move faster and therefore collide more with substrate molecules

practical for effect of temperature on enzymes

amylase is an enzyme that digests starch into maltose

• add 5cm³ starch solution to a test tube and heat to a set temperature

• add drop of iodine to each of the wells of a spotting tile

• use a syringe to add 2cm³ amylase to the starch solution and mix

• every minute, transfer a droplet of solution to a new well of iodine which should turn blue-black

• repeat until the iodine stop turning blue-black (means amylase has broken down all starch)

• record time taken for the reaction to be completed

• repeat for a range of temps from 20-60

how does pH affect enzyme action

• if pH is too high or low, the bonds that hold amino acid chain together to make the protein can be destroyed, changing shape of active site

• moving too far away from optimum pH will cause denaturing

• optimum for most is 7, however some eg. stomach will have lower and some in alkaline e.g. duodenum will have higher

practical investigating effect of pH on enzymes

• amylase digests starch into maltose

• add a drop of iodine to each of the wells of the spotting tile

• use a syringe to place 2cm³ of amylase into a test tube

• add 1cm³ buffer solution at pH2 to the test tube using a syringe

• use another test tube to add 2cm³ of starch solution to amylase and buffer solution, then start stopwatch whilst mixing using pipette

• every 10 seconds transfer a droplet of solution to a new well of iodine solution (should turn blue-black)

• repeat every 10 seconds until iodine stops changing colour

• record time taken for reaction to be completed

• repeat with different pH buffers from pH3-7

what is diffusion

the movement of molecules from a region of higher concentration to a region of lower concentration

• moves down concentration gradient, molecules move with random movement

• results in even concentration throughout

• molecules move into or out of living cells by diffusion when they cross the cell membrane (partially permeable)

• allows living organisms to: gain nutrients in digestive system, gain oxygen in lungs, remove waste products in lungs and kidneys

what is osmosis

movement of water molecules from a region of higher water concentration (dilute solution) to region of lower water concentration through a partially permeable membrane

• diffusion of water moving down concentration gradient

• without a cell wall, osmosis can have severe effects on animal cells

• in lower water concentration, cell loses water and shrivels

• in higher water concentration, cell gains water and eventually bursts

• due to cell wall, plant cells are protected from bursting

• in lower water concentration, cell loses water, vacuole shrinks, cell membrane pulls away from wall and makes cell flaccid

• in higher water concentration cell gains water, vacuole expands, membrane pushes against cell wall, making cell turgid

what is active transport

The movement of particles across a cell membrane from a region of lower concentration to a region of higher concentration

• energy is needed as particles move against concentration gradient

• a.t across cell membrane involves protein carrier molecules that are in the cell membrane

• e.g. absorption of products of digestion into the bloodstream from the lumen to the small intestine

• absorption of mineral ions form soil to root hair cells

factors influencing diffusion

• SA:V ratio - larger SA:V ratio = faster rate of substance movement

• diffusion distance - smaller the distance, faster the transport

• temperature - higher temp = faster movement

• concentration gradient - greater difference in concentration, faster movement

practical investigating factors affecting diffusion

• beetroot: cells have dark purple/red pigment

• heating above 45 can damage cell membrane, causing pigment to leak out

• speed at which pigment leaks tells rate of diffusion

  1. use a knife to cut 2 equally sized cubes of beetroot

  2. rinse

  3. put 5cm³ water into 2 test tubes A and B

  4. keep A at room temp and transfer test tube B to hot water bath at 90

  5. leave test tube for 2 mins, then add a piece of beetroot

  6. after 10 mins, observe colour of liquid in both

  7. higher temp = more leaked pigment

practical investigating factors influencing osmosis

1. prepare a range of sucrose solutions ranging from 0mol/dm³ to 1

2. set up 6 labelled test tubes w 10cm³ of each of the sucrose solutions

3. use knife cork borer and ruler to cut 6 equal sized cylinders of potato

4. blot each one with paper towel and weight

5. put 1 piece into each concentration

6. after 4 hours, remove, blot and reweigh

   • potato in distilled water will gain most mass due to high conc. gradient, causing water to move in and make cell firm

   • potato in strongest sucrose solution will lose most mass as water moves out of cells by osmosis, making cell flaccid