1.11 Required Practicals: Cell Structure and Transport

A free GCSE Biology lesson for AQA (8461) from OBA School: required practical 1: microscopy · Required practical 3: osmosis in plant tissue. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Prepare a slide and use a light microscope to observe, draw and label plant and animal cells
• Work out magnification, real size and cell size using a scale bar or the field of view
• Plan an osmosis investigation, naming the independent, dependent and control variables
• Calculate percentage change in mass, plot a graph and use it to estimate the concentration of cell contents
• Evaluate a method, spot anomalies and suggest improvements

Key words

• Independent variable: The variable you change on purpose.
• Dependent variable: The variable you measure.
• Control variable: A variable kept the same so it does not affect the results.
• Anomaly: A result that does not fit the pattern. Leave it out of the mean.

Required practical 1: using a light microscope

Aim: to observe, draw and label plant cells (onion epidermis) and animal cells (cheek cells) and estimate their size.

Equipment: light microscope, glass slides, cover slips, forceps, mounted needle, pipette, iodine solution (for plant cells), methylene blue (for animal cells), onion, paper towel.

• 1; Method (onion cells) Put a drop of water on the middle of a clean slide.
• 2; Method (onion cells) Peel a thin, see-through layer of epidermis from the inside of an onion piece with forceps. Lay it flat on the drop.
• 3; Method (onion cells) Add a drop of iodine solution to stain the cells, so the nucleus and cell wall show up clearly.
• 4; Method (onion cells) Lower a cover slip at an angle using a mounted needle, so no air bubbles are trapped. Soak up extra liquid with paper towel.
• 5; Method (onion cells) Put the slide on the stage. Start with the lowest-power objective. Use the coarse focus to focus, then the fine focus.
• 6; Method (onion cells) Switch to a higher-power objective and refocus with the fine focus only. Draw and label a few cells.

• Iodine and methylene blue stain skin and are irritants; How to reduce it Wear eye protection and gloves; wipe up spills.
• Glass slides and cover slips can break and cut; How to reduce it Handle carefully; report breakages; do not use cracked slides.
• Cheek-cell samples could carry infection; How to reduce it Use a clean cotton bud once only, then put it straight into disinfectant.

Drawing rules: Use a sharp pencil; draw clear single lines with no shading; draw only what you see; label with ruled lines that do not cross; add a title and the magnification or a scale bar.

Microscope calculations

You can estimate cell size by counting how many cells fit across the field of view, if you know its diameter. You can also use a scale bar on a drawing or photo: measure the bar and the cell with a ruler and compare.

Worked example: using a scale bar

A scale bar on a photo is 25 mm long and is labelled 50 µm. A cell on the photo is 40 mm long.
Magnification = 25 mm ÷ 50 µm = 25 000 µm ÷ 50 µm = ×500
Real length of cell = 40 mm ÷ 500 = 0.08 mm = 80 µm

Answer: 80 µm

Required practical 3: osmosis in plant tissue

Aim: to investigate the effect of different sugar (or salt) solution concentrations on the mass of plant tissue.

Equipment: potato, cork borer, ruler, scalpel and tile, 6 boiling tubes, sugar solutions (0.0 to 1.0 mol/dm³), measuring cylinder, balance (to 0.01 g), paper towel, timer.

• 1; Method Use a cork borer to cut potato cylinders of the same diameter. Trim them to the same length (e.g. 3 cm) and remove any skin.
• 2; Method Blot each cylinder dry, weigh it and record its starting mass.
• 3; Method Add the same volume (e.g. 10 cm³) of each concentration to a labelled boiling tube. Put one cylinder in each tube.
• 4; Method Leave all tubes for the same time (e.g. 30 minutes) at the same temperature.
• 5; Method Remove each cylinder, blot it dry in the same way and weigh it again.
• 6; Method Calculate the percentage change in mass. Repeat the whole investigation and work out mean values.

• Independent; In this practical concentration of the sugar solution
• Dependent; In this practical percentage change in mass of the potato cylinder
• Control; In this practical size and surface area of cylinders; type and age of potato; volume of solution; time in solution; temperature; blotting method

Safety: Cork borers and scalpels are sharp: cut on a tile, away from your fingers. Wipe up spills of sugar solution so no one slips.

Processing and evaluating osmosis results

Cylinders start with different masses, so compare the percentage change: (final mass − starting mass) ÷ starting mass × 100. Plot it against concentration and draw a smooth curve of best fit.

Where the curve crosses 0% there is no net movement of water. That solution has the same concentration as the potato cell contents (it is isotonic). Above it, cylinders lose water and mass; below it, they gain.

• Blotting removes different amounts of surface liquid; Improvement blot every cylinder the same way (e.g. roll once on paper towel)
• Cylinders not all exactly the same size; Improvement measure length with a ruler; use one cork borer
• Water evaporates from the tubes; Improvement cover the tubes with cling film
• Few results / big gaps between concentrations; Improvement repeat 3 times and calculate a mean; add more concentrations near where the curve crosses zero

Common mistakes

• Saying the independent variable is “the potato”. It is the concentration of the solution.
• Comparing change in mass instead of percentage change, when the starting masses differ.
• Including an anomaly when you calculate a mean.
• Drawing cells with shading, sketchy lines or structures you cannot actually see.
• Using the coarse focus with a high-power lens. This can crack the slide.

Revision questions

Try these, then check your answers below each one.

Question 1. In the osmosis practical, why are the potato cylinders all cut with the same cork borer and trimmed to the same length? (Choose from: To make them easier to weigh, To keep their surface area the same, as a control variable, To make the potato more concentrated, So that the solution stays the same colour.)

Answer: To keep their surface area the same, as a control variable

Working: Cylinders of the same size have the same surface area, so this cannot affect the results.

Question 2. A potato cylinder changed from 2.45 g to 2.70 g. Calculate the percentage change in mass to 1 decimal place.

Answer: 10.2 %

Working: (2.70 − 2.45) ÷ 2.45 × 100 = 0.25 ÷ 2.45 × 100 = 10.2%.

Question 3. True or false? In a biological drawing, you should shade the nucleus to make it stand out.

Answer: False

Working: False. Use clear, single pencil lines with no shading.

Related lessons

• Previous lesson: 1.10 Exchanging Materials
• Next lesson: 2.1 Cell Division
• In this chapter: 1.1 The World of the Microscope
• In this chapter: 1.2 Animal and Plant Cells
• In this chapter: 1.3 Eukaryotic and Prokaryotic Cells

Continue in the full course

This lesson is section 1.11 (ARPBH0111) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.