Cell biology

Section 4.1

12 spec leafs

Notes and three levels of exam-style practice for each registered specification leaf in this section.

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4.1.1.1

Eukaryotes and prokaryotes

Plant and animal cells are eukaryotic: they have cytoplasm enclosed by a cell membrane and genetic material enclosed in a nucleus.
Bacterial cells are prokaryotic and much smaller; they have cytoplasm and a cell membrane surrounded by a cell wall, but no nucleus.
A bacterium's genetic material is a single DNA loop, and it may also contain one or more small rings of DNA called plasmids.
When comparing cell sizes, convert centi-, milli-, micro- and nano-units consistently and use standard form or orders of magnitude rather than comparing the unconverted numbers.

Tier 1 · Easy

2 marks

ORIGINAL

State two differences between the genetic material in a typical animal cell and in a bacterial cell.

Tier 2 · Standard

3 marks

ORIGINAL

A cell has cytoplasm, a cell membrane, a cell wall, a single DNA loop and several plasmids. Identify the type of cell and explain your answer.

Tier 3 · Hard

4 marks

ORIGINAL

An animal cell is $3.0\times10^{-5}\,\mathrm{m}$ long and a bacterial cell is $3.0\times10^{-6}\,\mathrm{m}$ long. Calculate how many times longer the animal cell is and state the difference in orders of magnitude.

4.1.1.2

Animal and plant cells

The nucleus contains genetic material, the cell membrane controls movement into and out of the cell, and the cytoplasm is where most chemical reactions occur.
Mitochondria are the site of aerobic respiration and ribosomes are the site of protein synthesis; cells needing much energy or protein may contain many of the relevant structures.
Plant cells often also contain chloroplasts for photosynthesis and a permanent vacuole filled with cell sap, while plant and algal cells have a cellulose cell wall that strengthens the cell.
In the light-microscope practical, observe, draw and label plant and animal cells with clear single lines and include a magnification scale; estimate relative sizes when an exact boundary cannot be measured reliably.

Tier 1 · Easy

2 marks

ORIGINAL

State the function of a ribosome and the function of a mitochondrion.

Tier 2 · Standard

4 marks

ORIGINAL

A micrograph shows a cell with a nucleus, a cell wall, chloroplasts and a permanent vacuole. Identify the cell as plant or animal and explain the functions of two structures that support your identification.

Tier 3 · Hard

6 marks

ORIGINAL

A student uses a light microscope to compare onion epidermal cells with cheek cells. Describe how the student should produce useful biological drawings and explain two visible differences expected between the cells.

4.1.1.3

Cell specialisation

A specialised cell has structures that allow it to perform a particular function in a tissue, organ, organ system or whole organism.
Sperm cells have a tail for movement and many mitochondria, nerve cells have a long extension and branched connections, and muscle cells contain many mitochondria for contraction.
Root hair cells have a large surface area for uptake, xylem cells form hollow strengthened tubes for water transport, and phloem cells form tubes that transport dissolved sugars.
In an explanation, link each structural feature to the function it improves; naming a feature without the resulting advantage does not complete the reasoning.

Tier 1 · Easy

2 marks

ORIGINAL

Explain one way in which a sperm cell is specialised for its function.

Tier 2 · Standard

4 marks

ORIGINAL

A root hair cell has a long projection and many mitochondria. Explain how both features help the plant obtain mineral ions from the soil.

Tier 3 · Hard

5 marks

ORIGINAL

A new plant cell is found to be dead at maturity, hollow from end to end and strengthened with a waterproof material. Explain which transport tissue it belongs to and how the three features support its function.

4.1.1.4

Cell differentiation

Differentiation is the process by which a cell changes to become specialised for a particular function as an organism develops.
During differentiation, a cell acquires different sub-cellular structures that enable it to carry out its specialised function.
Most animal cells differentiate at an early stage, whereas many plant cells retain the ability to differentiate throughout life.
In mature animals, cell division is mainly restricted to repair and replacement; do not confuse differentiation, which changes cell type, with mitosis, which produces cells.

Tier 1 · Easy

2 marks

ORIGINAL

Define cell differentiation.

Tier 2 · Standard

3 marks

ORIGINAL

Compare differentiation in animals with differentiation in plants.

Tier 3 · Hard

5 marks

ORIGINAL

After damage, a mature animal replaces skin cells, while a plant grows a new root containing several cell types. Explain the roles of cell division and differentiation in both responses.

4.1.1.5

Microscopy

Magnification tells how many times larger an image is than the real object, whereas resolution is the ability to distinguish two points that are close together.
Electron microscopes have much higher magnification and resolving power than light microscopes, so they reveal finer sub-cellular detail and have increased understanding of cell structure.
Use $\text{magnification}=\frac{\text{image size}}{\text{real size}}$ and rearrange it only after converting image size and real size to the same units.
Use centi-, milli-, micro- and nano-prefixes correctly and express very large or small answers in standard form where appropriate; high magnification alone does not guarantee a detailed image if resolution is poor.

Tier 1 · Easy

2 marks

ORIGINAL

State why an electron microscope can show more detail in a cell than a light microscope.

Tier 2 · Standard

3 marks

ORIGINAL

A cell image is $24\,\mathrm{mm}$ long and the real cell is $12\,\mathrm{\mu m}$ long. Calculate the magnification.

Tier 3 · Hard

5 marks

ORIGINAL

A micrograph taken at a magnification of $\times 1500$ shows a cell as $30\,\mathrm{mm}$ long. Calculate the real length in micrometres. Explain why a second microscope with the same magnification might produce a more useful image.

4.1.1.6

Culturing microorganisms (biology only)

In separate Biology, bacteria can reproduce by binary fission as often as once every $20\,\mathrm{minutes}$ when nutrients and temperature are suitable, and they can grow in nutrient broth or as colonies on agar gel.
Use aseptic technique: sterilise culture media and Petri dishes, flame the inoculating loop, minimise opening of the lid, secure it with adhesive tape and store the plate upside down so condensation does not fall onto the agar.
School cultures are generally incubated at $25\,{}^\circ\mathrm{C}$ to reduce growth of harmful pathogens; zones of inhibition in the required practical can be compared using $A=\pi r^2$ .
Higher tier: express a calculated bacterial population in standard form; count the number of division intervals and multiply the starting population by $2^n$ , rather than multiplying it by $n$ .

Tier 1 · Easy

2 marks

ORIGINAL

A culture begins with $100$ bacteria. The mean division time is $20\,\mathrm{minutes}$ . Calculate the population after $60\,\mathrm{minutes}$ under suitable conditions.

Tier 2 · Standard

5 marks

ORIGINAL

Separate Biology: describe four aseptic steps used when preparing an uncontaminated bacterial culture on an agar plate and explain why the plate is incubated at $25\,{}^\circ\mathrm{C}$ in a school laboratory.

Tier 3 · Hard

4 marks

ORIGINAL

Higher tier: a separate-Biology bacterial culture starts with $500$ cells and has a mean division time of $20\,\mathrm{minutes}$ . Calculate the population after $2.0\,\mathrm{hours}$ and give the answer in standard form.

4.1.2.1

Chromosomes

The nucleus of a cell contains chromosomes made from DNA molecules.
Each chromosome carries a large number of genes.
In body cells, chromosomes are normally found in pairs.
Do not use chromosome, DNA and gene as interchangeable terms: chromosomes are made of DNA, and genes are carried on chromosomes.

Tier 1 · Easy

2 marks

ORIGINAL

State where chromosomes are found and what they are made of.

Tier 2 · Standard

4 marks

ORIGINAL

Describe the relationship between a nucleus, chromosomes, DNA and genes in a body cell.

Tier 3 · Hard

4 marks

ORIGINAL

A student says, 'A chromosome is a gene inside a DNA molecule.' Evaluate this statement and give a corrected description.

4.1.2.2

Mitosis and the cell cycle

Before division, a cell grows, increases the number of sub-cellular structures such as ribosomes and mitochondria, and replicates its DNA to make two copies of each chromosome.
During mitosis, one set of chromosomes is pulled to each end of the cell and the nucleus divides.
Finally, the cytoplasm and cell membranes divide to form two genetically identical cells.
Mitosis supports growth and development and the repair or replacement of cells; the separate named phases within mitosis are not required for this specification.

Tier 1 · Easy

2 marks

ORIGINAL

State two processes that occur before mitosis in the cell cycle.

Tier 2 · Standard

4 marks

ORIGINAL

Describe how one cell produces two identical cells during the cell cycle.

Tier 3 · Hard

6 marks

ORIGINAL

Cells at the tip of a growing root contain many ribosomes and mitochondria, copied chromosomes and dividing nuclei. Explain why these observations show that the cells are at different stages of the cell cycle and why the process is important to the plant.

4.1.2.3

Stem cells

A stem cell is undifferentiated, can produce many more cells of the same type and can differentiate to produce certain other cell types.
Embryonic stem cells can differentiate into most human cell types, adult bone-marrow stem cells can form several cell types including blood cells, and plant meristems can form any plant cell throughout life.
Stem-cell treatment may help conditions such as diabetes or paralysis; therapeutic cloning produces an embryo genetically matched to the patient, reducing the chance of rejection.
Stem-cell use must be evaluated by balancing potential medical benefits against risks such as viral transfer and ethical or religious objections; meristems also allow rapid cloning of rare or useful crop plants.

Tier 1 · Easy

2 marks

ORIGINAL

Define a stem cell.

Tier 2 · Standard

4 marks

ORIGINAL

Compare embryonic stem cells, adult bone-marrow stem cells and plant meristem cells.

Tier 3 · Hard

6 marks

ORIGINAL

Evaluate the use of therapeutically cloned embryonic stem cells to treat a patient with paralysis.

4.1.3.1

Diffusion

Diffusion is the spreading out of particles in a solution or gas, producing a net movement from higher concentration to lower concentration; oxygen, carbon dioxide and urea can cross cell membranes this way.
A steeper concentration gradient, a higher temperature and a larger membrane surface area each increase the rate of diffusion.
Single-celled organisms have a large surface area to volume ratio, but larger multicellular organisms need specialised exchange surfaces and transport systems to meet all their cells' needs.
Effective exchange surfaces such as small intestine, lungs, fish gills, roots and leaves have a large area and thin barrier; animal surfaces also have an efficient blood supply and gas-exchange surfaces are ventilated.

Tier 1 · Easy

2 marks

ORIGINAL

Dissolved dye particles spread through a beaker of still water. Name this process and give its complete definition.

Tier 2 · Standard

4 marks

ORIGINAL

Explain how three features of an air sac in a mammalian lung increase the rate of oxygen diffusion into the blood.

Tier 3 · Hard

6 marks

ORIGINAL

Cube A has side length $1\,\mathrm{mm}$ and cube B has side length $4\,\mathrm{mm}$ . Calculate the surface area to volume ratio of each cube and explain why a large multicellular organism needs exchange surfaces and a transport system.

4.1.3.2

Osmosis

Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
In the required practical, place equal-sized plant-tissue samples in a range of salt or sugar concentrations, control time and temperature, blot them consistently, and measure initial and final mass.
Calculate percentage mass change using $\frac{\text{final mass}-\text{initial mass}}{\text{initial mass}}\times100$ ; a positive result is a gain and a negative result is a loss.
Plot solution concentration against percentage mass change: the concentration at zero change estimates the tissue's internal concentration, while repeats and a mean reduce the effect of random variation.

Tier 1 · Easy

2 marks

ORIGINAL

A plant cell is placed in a solution more dilute than its cell contents. State the direction in which water moves and name the process.

Tier 2 · Standard

3 marks

ORIGINAL

A potato cylinder has an initial mass of $4.00\,\mathrm{g}$ and a final mass of $4.60\,\mathrm{g}$ . Calculate its percentage change in mass and state whether it gained or lost water.

Tier 3 · Hard

6 marks

ORIGINAL

A plant-tissue sample falls in mass from $5.00\,\mathrm{g}$ to $4.25\,\mathrm{g}$ in a concentrated sugar solution. Calculate the percentage change, explain the result by osmosis, and state two variables that should be controlled when comparing several sugar concentrations.

4.1.3.3

Active transport

Active transport moves substances from a more dilute solution to a more concentrated solution, against the concentration gradient, using energy transferred by respiration.
Root hair cells use active transport to absorb mineral ions from very dilute soil solutions so that plants obtain ions needed for healthy growth.
Cells of the small intestine can actively absorb sugar molecules from a lower concentration in the gut into blood with a higher sugar concentration; the sugar can then be used in respiration.
Diffusion is passive net movement down a concentration gradient, osmosis is passive movement of water through a partially permeable membrane, and only active transport moves against the gradient using energy.

Tier 1 · Easy

2 marks

ORIGINAL

Define active transport.

Tier 2 · Standard

4 marks

ORIGINAL

The concentration of nitrate ions is lower in soil water than inside a root hair cell. Explain how the plant can still absorb nitrate ions and why the process is useful.

Tier 3 · Hard

6 marks

ORIGINAL

Compare diffusion, osmosis and active transport, including the substances moved, direction relative to a concentration gradient, membrane requirement and use of energy.