Movement of Substances Into & Out of Cells (2d)

Diffusion

Passive movement of particles from a region of higher to lower concentration, down a concentration gradient

• random, but results in spreading out of particles until they are at even conc. throughout the available space

• passive since it doesn’t require energy, particles have kinetic energy

Diffusion through the Cell Membrane

• gases and small particles move into/out of living cells by diffusion when they cross the cell membrane

• partially permeable: allows some gases and molecules but not others

• small particles like oxygen can diffuse freely, larger molecules like glucose can’t

Examples of Diffusion in Living Organisms

• leaf: conc. of CO₂ in chloroplasts is lower than atmosphere when photosynthesising, so CO₂ diffuses into the cell

• lungs: conc. of oxygen inside the alveoli is higher than conc. of oxygen in the capillaries so oxygen diffuses into the blood to be transported

• liver: conc. of urea (waste product) is higher in the liver cells than the blood flowing through the liver, so urea diffuses out of the liver cells into the blood

Osmosis

The movement of water from a region of higher water potential (dilute solution) to lower water potential (concentration solution) through a semi-permeable membrane down a concentration gradient

• diffusion of water since it’s down the conc. gradient

• partially permeable membranes prevent the movement of large molecules (eg: sugars) but allow movement of small water molecules

Water Potential

• water moves from high to low water potential

• the more solute (eg: sugar or salt) a solution contains, the lower its potential (lower its water concentration)

• the lesser the solute, the higher the potential

• Pure water has the highest water potential

Osmosis in Animal Cells

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

• in strong sugar solution (lower water potential) → the cell loses water, becoming crenated (shrivelled)

• in distilled water (higher water potential) → the cell gains water, eventually bursting since it lacks a cell wall to maintain structure

Osmosis in Plant Cells

due to cell wall, plants are protected from bursting

• in a strong sugar solution (lower water potential), the cell loses water, the vacuole shrinks and the cell membrane pulls away from the cell wall, making the cell flaccid or plasmolysed

• in distilled water (higher water potential), the cell gains water, the vacuole expands, the membrane pushes against the cell wall, making the cell turgid

• turgid cells provide structural support and prevent wilting

Active Transport

The movement of particles across a cell membrane from a region of lower to higher concentration, requiring energy released by respiration

• energy is needed because particles move against the conc. gradient → released in cellular respiration

• involves protein pumps embedded in the cell membrane

Examples of Active Transport

• absorption of products of digestion into the bloodstream form the lumen of the small intestine

• absorption of mineral ions from the soil to the root hair cells of plants

Factors influencing Diffusion

• surface area to volume ratio

• concentration gradient

• temperature

• distance

Factors influencing Diffusion: Surface Area to Volume Ratio

• surface area = total area through which substances can diffuse

• as a cell/organism increases in size, its ratio decreases

• this means that less surface area is available for each unit of volume, so diffusion alone becomes less efficient at supplying cells with essential substances and removing waste products

• to overcome this, larger organisms have evolved specialised exchange surface (alveoli in lungs/ villi in the small intestine) that increase s.a and often have thin walls and good blood supply to maintain steep conc. gradient for efficient diffusion

• many organisms adapted for diffusion have increased s.a in some way- eg: root hair cells in plants (which absorb water and mineral ions) and cells lining the ileum in animals (which absorb the products of digestion)

Diffusion Distance

• shorter the distance that molecules have to travel, the faster diffusion can occur

• this is why alveoli in the lungs and capillary walls are only one cell thick- to minimise diffusion distance for gases

• shorter diffusion distance → oxygen and carbon dioxide diffuse rapidly and efficiently between the air in the alveoli and blood in the capillaries

Temperature

• higher the temperature, faster the molecules move (have more k.e)

• more collisions against the cell membrane, faster rate of movement across them

Concentration gradient

• greater the difference in conc. on either side of the membrane, the faster the movement across it will occur

• because on the side with the higher concentration, more random collisions against the membrane will occur

Factors influencing Diffusion Practical: Agar Blocks

• Agar is clear, jelly-like

• when molten agar is mixed with NaOH (an alkali) and phenolphthalein, pink agar is produced

• Phenolphthalein is colourless when pH <8.3 and pink in alkaline

• pink agar will turn colourless in an acidic solution

• as acid diffuses into the blocks, neutralisation occurs between the acid and the alkaline NaOH in the agar block

Factors influencing Diffusion Practical: Apparatus

• Pink agar (contains sodium hydroxide and phenolphthalein indicator)

• White tile

• Scalpel (knife)

• Dilute hydrochloric acid

• Beakers

• Thermometer

• Water baths 

• Ice

• Forceps

• Stopwatch

Factors influencing Diffusion Practical: Safety Hazards

• dilute HCl

• scalpel

• agar prepared with NaOH and phenolphthalein indicator

Factors influencing Diffusion Practical: Method

• using scalpel, cut agar to different side lengths (0.5, 1, 2cm)

• calculate SA:V ratio

• pour 50ml dilute HCl into a beaker

• using forceps, place the 0.5cm cube into the beaker and start a timer

• observe, record time when pink agar turns colourless, record time

• repeat with same side length to increase reliability

• repeat experiment for different side lengths

Factors influencing Diffusion Practical: Results and Analysis

• when agar cubes are placed in the HCl, acid diffuses and reacts with the NaOH → pink indicator turns colourless

• time taken to turn colourless can be compared

• smaller cubes, acid has more sa relative to size → diffusion distance is shorter

• as size increases, larger cubes have less sa per unit volume, takes longer for the acid to diffuse to te center → diffusion distance is longer

• larger cubes take longer to lose their colour

• rate of diffusion remains constant, but total rate of exchange depends on cube’s SA:V ratio

• experiment models how small organisms with high SA:V ratio rely on diffusion alone for gas exchange, larger organisms cannot

Factors influencing Diffusion Practical: Limitations

determining endpoint introduces human error

• improvement: measure distance in mm after a set period of time → provides qualitative measurement that’s easier to compare

difficult to cut cubes into the same size and small differences affect SA”V ratio and rate of diffusion

• improvement: use a ruler and sharp scalpel to cut as accurately as possible

Factors influencing Osmosis Practical: Apparatus

• Potatoes

• Cork borer

• Knife

• Sucrose solutions (from 0 Mol/dm³ to 1 mol/dm³)

• Test tubes

• Balance

• Paper towels

• Ruler

• Test tube rack

Factors influencing Osmosis Practical: Method

• prepare a range of sucrose solutions ranging from 0 Mol/dm³ (distilled water) to 1 mol/dm³

• set up 6 labelled test tubes with 10cc of each sucrose solution

• using the knife, cork borer and ruler, cut 6 equally sized cylinders of potato

• blot with paper towel and weigh on the balance

• put 1 piece into each conc. of sucrose solution

• after 4 hours, remove, blot and reweigh

Factors influencing Osmosis Practical: Results and Analysis

• potato in distilled water gains most mass because of a high conc. gradient → water moves in by osmosis, increasing turgor pressure, making the potato firm

• potato in strongest solution loses the most mass as water moves out by osmosis → making cells flaccid and the potato turns soft

• if there was no net movement, conc. was same as in the cytoplasm of the potato cells

Factors influencing Osmosis Practical: Limitations

slight differences in cylinders make the results unreliable

• solution: for each sucrose conc., repeat with several cylinders. repeating = anomalies can be identified and ignored when calculating mean

Factors influencing Osmosis Practical: CORMMS

• C - changing the concentration of sucrose solution

• O - the potato cylinders will all be taken from the same potato or potatoes of the same age

• R - repeat the investigation several times to ensure results are reliable

• M1 - measure the change in mass of the potato cylinders

• M2 - ...after 4 hours

• S - control the volume of sucrose solution used, the dimensions of the potato cylinders and each cylinder must be blotted before it is weighed each time