AQA GCSE Biology Biology Paper 1 (Higher), November 2021: Question 1

✅ Mark Scheme Accepted Answers (Any two):

• (Microscope) slide
• Cover slip
• Dye / stain (e.g., methylene blue, iodine)
• (Mounted) needle
• Pipette / dropper
• Scalpel
• Forceps / tweezers
• Swab (to collect cheek cells)

❌ Common Errors & What Not to Write

• Water: The mark scheme explicitly states ignore water. Water is a liquid medium, not a specific lab tool or reagent for staining.
• Knife: The mark scheme says ignore knife — use the scientific term scalpel.
• Microscope: The question asks what was used to prepare the cells, not to view them.

✅ Correct Answers

01.2 (Part A): Eyepiece or eyepiece lens [1 mark]

01.3 (Part B): To focus (the image / cells) [1 mark]

Also accepted for 01.3: to make the cells / image clearer, or to improve resolution.

❌ Common Misconceptions

• For Part A: Do not write "objective lens". Part A points directly to the top lens you look through (eyepiece). The objective lenses are on the rotating nosepiece below.
• For Part B: Do not write "magnification" or "zooming in". The focusing wheel moves the stage to focus the light rays; it does not change magnification.
• Do not just say "moves the stage up and down" — state the scientific function (to focus).

✅ Any One Valid Reason [1 mark]:

• No cells in the field of view
• Slide is not in the correct position / not over the light hole
• Mirror is not aligned / light is not turned on or plugged in
• Objective lens not clicked into place
• Objective lens is dirty
• The student is looking at an air bubble
• The microscope is not focused
• Cells were not stained (too transparent to see)

🧠 Exam Technique: Think Practically!

When asked why a sample can't be seen under a microscope, mentally walk through setting up the instrument:

1. Light: Is it on/reflected properly?
2. Lens: Is it locked into place and clean?
3. Stage: Is the specimen centered directly above the condenser lens?
4. Focus: Has the focal plane been reached?

✅ Differences to Include

• Nucleus: Red blood cells have no nucleus (mature), whereas plant cells have a nucleus.
• Cell Wall: Red blood cells have no cell wall; plant cells have a rigid cellulose cell wall.
• Chloroplasts / Chlorophyll: Red blood cells do not contain chloroplasts/chlorophyll; plant cells often do (e.g. leaf cells).
• Vacuole: Red blood cells lack a permanent vacuole; plant cells have a large permanent vacuole filled with cell sap.
• Haemoglobin: Red blood cells contain haemoglobin; plant cells do not.
• Shape & Size: Red blood cells are a biconcave disc and much smaller; plant cells are typically rectangular/angular and much larger.

💡 Similarities to Include

• Cell membrane: Both have a cell membrane that encloses the cytoplasm.
• Cytoplasm: Both contain cytoplasm where cellular reactions take place.
• Pigments: Both contain pigments (haemoglobin in red blood cells; chlorophyll in photosynthetic plant cells).

✅ Two Required Marking Points:

1. Mark 1: Water enters the cells by osmosis (or diffusion).
2. Mark 2: The plant cell has a cell wall which prevents it from bursting (or: the red blood cell has no cell wall so it swells and bursts).

❌ Common Errors in Osmosis Explanations

• Forgetting the mechanism: Many students write "water goes in" without naming osmosis. You must state the process by name to secure Mark 1.
• Confusing cell wall and cell membrane: Writing "the plant's cell membrane stops it bursting" scores 0 for Mark 2. It is specifically the rigid cellulose cell wall that withstands turgor pressure.
• Overcomplicating water potential: You do not need a lengthy definition of osmosis here; the mark scheme notes: "ignore explanations of osmosis". Focus on entry of water + presence/absence of cell wall.