Practicals / LAB

Question 1

Lab 1 [leaf structure]

What are the main structures of a leaf?

Answer 1

• Impermeable waxy cuticle (prevents water loss through evaporation/diffusion of gases)
• Upper epidermis (is transparent to allow maximum light through to cells with chloroplasts)
• Palisade mesophyll layer (cells are stacked vertically to fit in as many cells as possible. These cells contain the most chloroplasts - needed for photosynthesis)
• Spongy mesophyll layer (air spaces provide an increased surface area for gas exchange and allow gases to diffuse)
• Lower epidermis, guard cells, stomata (guard cells open + close stomata to control water loss + entry/exit of gases. Walls of guard cells are thicker on the side adjacent to the stomata to enable opening + closing)

Question 2

Lab 2 [Cell staining + making microscope slides]

calibrating

Question 3

Lab 5 [food tests]

Strach test?

Answer 3

Put 2 ml of the sample into a boiling tube and add 1 ml of iodine solution

= A blue- black colour indicates the presence of starch

Question 4

Protein test?

Answer 4

Put 2 ml of the sample into a boiling tube and add 1 ml of Biuret reagent

= A lilac colour indicates the presence of protein

Question 5

Reducing sugar test?

Answer 5

• Put 2 ml of the sample into a boiling tube, add 2 ml of distilled water and 4 ml of Benedicts reagent
• Heat in a water bath at 100°C for 5 minutes
• = A brick red colour indicates the presence of a reducing sugar
• The reason for the 2 ml distilled water is so the volume is the same as with the non-reducing sugar which included an acid + an alkali

Question 6

Non-reducing sugar test?

Answer 6

• Put 2 ml of the sample into a boiling tube, add 1 ml of hydrochloric acid
• Heat in a water bath at 100°C for 5 minutes to hydrolyse the sugar
• Cool + add 1 ml sodium hydroxide solution to neutralise the acid
• Add 4 ml of Benedicts reagent + heat at 100°C for 5 minutes - = A brick red precipitate indicates the presence of a non-reducing sugar

Question 7

exam question

Starting with sucrose solution, how would you make a neutral pH solution of glucose and fructose?
What is the name of this process? (5 marks)

Answer 7

• Heat (1 mark)
• with hydrochloric acid (1 mark)
• then neutralise with the same quantity (1 mark)
• of sodium hydroxide solution (as the acid) (1 mark)
• The process is called hydrolysis (1 mark)