Year 8 Science | Victorian Curriculum 2.0
Cells & cell theory
Topic 01 | Biological sciences | Answer key

Year 8 answers

Fluency

Cell theory & parts

    1. (i) All living things are made of cells; (ii) the cell is the basic unit of structure and function; (iii) all cells come from pre-existing cells.
    2. (a) Nucleus, (b) mitochondrion, (c) chloroplast.
    3. Cell wall, chloroplasts, large central vacuole.
    4. Controls what enters and leaves the cell; separates the cell from its surroundings.
    5. The jelly-like fluid inside the cell where organelles sit and most chemical reactions happen.
    6. Unicellular: bacterium, yeast, Amoeba. Multicellular: human, tree, mushroom.
Fluency

Prokaryote vs eukaryote

    1. No — their DNA floats freely in the cytoplasm.
    2. Eukaryotes (typically 10-100 μ\mum vs 1-10 μ\mum for prokaryotes).
    3. Bacteria (e.g. E. coli), archaea.
    4. Animals, plants, fungi (also protists).
    5. Prokaryotes still carry DNA; it is simply not enclosed in a membrane-bound nucleus.
Fluency

Magnification

    1. 88 mm =8000= 8000 μ\mum. M=8000/40=×200M = 8000 / 40 = \times 200.
    2. Image =60×500=30 000= 60 \times 500 = 30\,000 μ\mum =30= 30 mm.
    3. 2424 mm =24 000= 24\,000 μ\mum. Real =24 000/300=80= 24\,000 / 300 = 80 μ\mum.
    4. 0.20.2 mm =200= 200 μ\mum; 45004500 μ\mum =4.5= 4.5 mm.
Reasoning

Explain and analyse

    1. Cell division needs DNA to be copied. Without a nucleus, a red blood cell has no DNA, so it cannot make new cells.
    2. Specialisation lets different cells do different jobs well (e.g. muscle contracts, nerves signal). This is more efficient than every cell trying to do everything.
    3. Without light, chloroplasts cannot photosynthesise. Chlorophyll may break down over time and the leaf may turn pale/yellow.
    4. Cell theory states all living things are made of cells. Bacteria are cells, so they are alive, even though they lack a nucleus. Having a nucleus is not a requirement for life.
Problem solving

Applied contexts

    1. Real size =30= 30 mm /1000=0.03/ 1000 = 0.03 mm =30= 30 μ\mum.
    2. 11 mm =1000= 1000 μ\mum. 1000/5=2001000 / 5 = 200 cells.
    3. Two of: plant cell has a cell wall (rigid, rectangular shape); plant cell has chloroplasts (green); plant cell has a large central vacuole.
    4. All cells come from pre-existing cells (cell theory, point 3). Surgical tools can carry bacterial cells that would reproduce inside a patient and cause infection. Sterilising destroys those cells.
Reasoning

Challenge

    1. (i) Mitochondria and chloroplasts are about the size of bacteria (prokaryotes). (ii) They have their own circular DNA, like prokaryotes. (iii) They divide by splitting in two, like bacteria.
    2. A fungal cell (has wall, no chloroplasts) or a bacterial cell (has wall, no chloroplasts). Check for a nucleus: fungi are eukaryotic and have one; bacteria do not.
    3. Volume of 11 cm3^3 = 101210^{12} μ\mum3^3. Volume per cell =203=8000= 20^3 = 8000 μ\mum3^3. Number of cells ≈1012/8000=1.25×108\approx 10^{12} / 8000 = 1.25 \times 10^8, i.e. about 125 million cells.
    4. A unicellular organism must carry out every life process in one cell — limiting its maximum size because surface area must keep up with internal volume. Cells inside a multicellular organism can be supplied by blood and specialise, so they can be larger or have extreme shapes (e.g. long nerve cells).
Year 8 Science study companion | Answer key