What is the primary structural difference between eukaryotic and prokaryotic DNA?

Eukaryotic DNA is linear and contained within a membrane-bound nucleus. Prokaryotic DNA is a single circular loop found freely in the cytoplasm, often accompanied by smaller circular plasmids.

How do the cell walls of plants and bacteria differ in composition?

Plant cell walls are primarily composed of cellulose, providing structural support. Bacterial cell walls are made of peptidoglycan, which maintains cell shape and protects against osmotic pressure.

When comparing a $10$ $\mu m$ cell to a $1000$ $\mu m$ cell, what is the difference in order of magnitude?

The difference is two orders of magnitude. This is calculated by $1000 / 10 = 100$, and since $100 = 10^2$, the difference is $2$.

What is a common mistake when calculating the actual size of a cell from a microscope image?

A common error is failing to convert the measured image size (usually in mm) into the same units as the actual size (usually $\mu m$) before applying the magnification formula.

Why is it incorrect to say that all plant cells contain chloroplasts?

Only plant cells exposed to light, such as those in the leaves and stems, contain chloroplasts. Root cells, for example, do not contain them because they are underground and cannot perform photosynthesis.

What error occurs if a student identifies a cell with a cell wall as 'definitely a plant cell'?

This is an error because prokaryotes (bacteria) and fungi also possess cell walls. To be a plant cell, it must specifically have a cellulose cell wall and usually a permanent vacuole.

Define 'Plasmids' and state which cell type they are found in.

Plasmids are small, circular rings of DNA that are separate from the main genetic material. They are characteristic features of prokaryotic (bacterial) cells.

What is 'Cell Differentiation'?

Cell differentiation is the process by which a cell changes to become specialized for its job by developing different subcellular structures.

What is the standard unit of measurement for cells, and how does it relate to a meter?

The standard unit is the micrometer ($\mu m$). One micrometer is $1 \times 10^{-6}$ meters, or one-millionth of a meter.

Why do sperm cells contain a high density of mitochondria in their mid-piece?

Mitochondria are the site of aerobic respiration, which releases the energy required for the tail to rotate and propel the sperm toward the egg.

Eukaryotes & Prokaryotes | AQA GCSE Biology

1. Definition & Core Concepts

Eukaryotic cells are complex structures characterized by having their genetic material (DNA) enclosed within a distinct, membrane-bound nucleus. This category includes all animal and plant cells, as well as fungi and protists.
Prokaryotic cells are significantly simpler and smaller, lacking a nucleus; instead, their DNA exists as a single circular loop in the cytoplasm. Bacteria are the most common example of prokaryotic organisms.
Both cell types share fundamental components necessary for life: a cell membrane to control substance entry, cytoplasm where chemical reactions occur, and ribosomes for protein synthesis.

Comparison of a simple prokaryotic cell with a circular DNA loop and a eukaryotic cell with a defined nucleus containing DNA.

2. Structural Differences & Organelles

Genetic Storage: In eukaryotes, DNA is organized into linear chromosomes within the nucleus, whereas prokaryotes contain a single circular chromosome and often smaller rings of DNA called plasmids.
Specialized Organelles: Eukaryotes contain membrane-bound organelles like mitochondria (for aerobic respiration) and chloroplasts (in plants for photosynthesis), which are entirely absent in prokaryotes.
Cell Walls: While both can have cell walls, the composition differs; plant cell walls are made of cellulose, while bacterial cell walls are typically composed of peptidoglycan.

3. Scale, Measurement & Standard Form

Size Magnitude: Eukaryotic cells typically range from $10$ to $100$ $\mu m$ , making them significantly larger than prokaryotic cells, which usually measure around $1$ $\mu m$ .
Unit Conversion: Biological measurements often require converting between millimeters (mm) and micrometers ( $\mu m$ ). Since $1$ mm = $1000$ $\mu m$ , you multiply by $1000$ to go from mm to $\mu m$ and divide by $1000$ for the reverse.
Standard Form: To manage very small dimensions, scientists use standard form ( $A \times 10^n$ ). For example, $0.0015$ mm is expressed as $1.5 \times 10^{-3}$ mm to ensure precision and clarity.

4. Cell Specialization & Differentiation

Differentiation is the process by which an unspecialized cell acquires specific structural features to perform a particular function. This involves switching certain genes on or off to develop specific organelles or shapes.
Animal Specialization: Examples include sperm cells (with acrosomes for egg penetration and many mitochondria for swimming) and nerve cells (with long axons for rapid impulse transmission).
Plant Specialization: Examples include root hair cells (with large surface areas for water uptake) and xylem vessels (hollow tubes reinforced with lignin for water transport).

5. Key Distinctions

Feature	Prokaryotic Cells	Eukaryotic Cells
Size	Small ( $0.1$ to $5$ $\mu m$ )	Large ( $10$ to $100$ $\mu m$ )
Nucleus	Absent (DNA is free)	Present (DNA enclosed)
Organelles	No membrane-bound organelles	Mitochondria, Chloroplasts, etc.
DNA Shape	Circular loop + Plasmids	Linear chromosomes
Cell Division	Binary Fission	Mitosis

6. Exam Strategy & Tips

Magnification Formula: Always remember the triangle: $\text{Image Size} = \text{Actual Size} \times \text{Magnification}$ . When calculating, ensure the Image Size and Actual Size are in the same units before dividing.
Order of Magnitude: If one cell is $100$ $\mu m$ and another is $10$ $\mu m$ , the first is one order of magnitude ( $10^1$ ) larger. If it were $1000$ $\mu m$ , it would be two orders of magnitude ( $10^2$ ) larger.
Functional Reasoning: If an exam asks why a cell has many mitochondria, relate it to high energy demand (e.g., muscle contraction or active transport). If it lacks a nucleus, it might be to maximize space (like a red blood cell) or because it is a prokaryote.

Eukaryotes & Prokaryotes

Summary

1. Definition & Core Concepts

2. Structural Differences & Organelles

3. Scale, Measurement & Standard Form

4. Cell Specialization & Differentiation

5. Key Distinctions

6. Exam Strategy & Tips

Eukaryotes & Prokaryotes

Summary

1. Definition & Core Concepts

2. Structural Differences & Organelles

3. Scale, Measurement & Standard Form

4. Cell Specialization & Differentiation

5. Key Distinctions

6. Exam Strategy & Tips