Cell StructureAQA GCSE Biology: Revision notes

Both animal and plant cells are eukaryotic cells containing a membrane-bound nucleus and other membrane-bound organelles. Understanding the function of each structure is essential for GCSE Biology.

Animal Cell Structures:

• Cell membrane: Semi-permeable barrier controlling what enters and leaves the cell
• Nucleus: Contains DNA and controls cell activities; site of transcription
• Cytoplasm: Gel-like substance where chemical reactions occur; contains organelles
• Mitochondria: Site of aerobic respiration; produces ATP energy for the cell
• Ribosomes: Sites of protein synthesis; can be free-floating or attached to rough endoplasmic reticulum

Plant Cell Structures (in addition to animal cell structures):

• Cell wall: Rigid structure made of cellulose; provides support and prevents cell from bursting
• Chloroplasts: Site of photosynthesis; contains chlorophyll pigment
• Permanent vacuole: Large, fluid-filled sac; maintains turgor pressure keeping plant cells rigid; stores nutrients and water

Key Functional Differences:

Plant cells are typically larger than animal cells due to their large permanent vacuole.

Prokaryotic cells (bacteria and archaea) are fundamentally different from eukaryotic cells. Understanding these differences is crucial for answering comparison questions on the exam.

Key Differences:

Prokaryotic Cell Structures:

• Plasmids: Small, circular loops of DNA separate from chromosomal DNA; often carry antibiotic-resistance genes
• Flagella: Tail-like structures for movement; made of flagellin protein
• Pili: Hair-like structures (shorter than flagella); used for attachment and DNA transfer
• Capsule: Thick, jelly-like layer outside cell wall; protection and adhesion
• Nucleoid region: Area where chromosomal DNA is located (not membrane-bound)

Sub-cellular structures vary because different cells have different functions and energy demands. This variation reflects adaptation to function—a key principle in biology.

Animal Cells:

• Require ATP from mitochondria for movement, active transport, and protein synthesis
• Many mitochondria in muscle and nerve cells (high energy demand)
• No cell wall—allows flexibility for movement and shape change
• Small vacuoles—less storage need; cell shape can change
• No chloroplasts—obtain energy from food through respiration

Plant Cells:

• Large permanent vacuole—maintains shape and rigidity; reduces cell's growth cost
• Cell wall—provides structural support; reduces need for thick cell membrane
• Numerous chloroplasts—sites of photosynthesis to produce glucose
• Fewer mitochondria than animal cells (but still present for respiration)

Root Cells vs Leaf Cells (Plant Examples):

• Root cells: Few/no chloroplasts (no light for photosynthesis); many mitochondria (energy for active uptake of minerals)
• Leaf mesophyll cells: Many chloroplasts (photosynthesis); numerous mitochondria (energy for photosynthesis and active processes)

Prokaryotic Cells:

• No mitochondria—energy produced at cell membrane (less efficient than eukaryotic method)
• Simple structure—quick reproduction; small size enables rapid diffusion
• Plasmids—allow rapid adaptation to environment (e.g., antibiotic resistance)
• Flagella—enables movement to find nutrients

Summary Principle:

Structures are present or abundant only where needed for the cell's function. This is structural and functional adaptation.

Light microscopes are standard tools in GCSE Biology practical work. You must understand magnification calculations and how to use the microscope correctly.

Using a Light Microscope:

1. Start with the lowest magnification objective lens
2. Place the slide on the stage and secure with clips
3. Use the coarse focus knob to bring the object into rough focus
4. Use the fine focus knob to achieve sharp focus
5. Increase magnification by rotating to a higher-power objective lens
6. Re-focus with the fine focus knob only (never use coarse focus at high magnification)
7. Adjust the diaphragm to control light intensity for clear viewing

Calculating Magnification:

The formula is: Magnification = Image size ÷ Actual size

All measurements must be in the same units (usually micrometres or millimetres).

Worked Example:

If an onion cell appears 8 mm long under a microscope, but its actual size is 0.08 mm:

• Magnification = 8 mm ÷ 0.08 mm
• Magnification = 100×

Rearranging the Formula:

You may need to find image size or actual size:

• Image size = Magnification × Actual size
• Actual size = Image size ÷ Magnification

Drawing Cells from Observations:

• Draw only what you can see under the microscope
• Use labels and label lines (not pointing arrows)
• Keep drawings large and clear (at least half a page)
• Draw structures as they appear (cells are not always perfect circles)
• Add a scale or state the magnification used

Common Units and Conversions:

• 1 mm = 1000 micrometres (μm)
• 1 μm = 1000 nanometres (nm)
• 1 mm = 1,000,000 nm

Always convert to the same unit before calculating magnification.

Electron microscopes have advantages and disadvantages compared to light microscopes. GCSE questions often ask you to compare them or explain why electron microscopes are used for detailed cellular observations.

Light Microscope Limitations:

• Resolution (ability to distinguish two close objects) limited by the wavelength of light (~0.2 μm)
• Cannot view objects smaller than approximately 0.2 μm
• Cannot resolve fine details of organelles like mitochondria or ribosomes
• Magnification limited to approximately 1500× in practice

Electron Microscope Advantages:

Key Advantages of Electron Microscopes:

• Much higher resolution due to shorter wavelength of electrons
• Much greater magnification allows observation of smaller structures
• Can view ultra-fine details of mitochondrial cristae, ribosome structure, and viral particles
• Essential for studying cell ultrastructure

Disadvantages of Electron Microscopes:

• Expensive equipment and maintenance
• Requires highly skilled operators
• Samples must be dead (vacuum or staining kills cells)
• Cannot observe living cells
• Produces black-and-white images (not natural colour)
• Less portable than light microscopes

Exam Context: Electron microscopy has revealed the detailed structure of mitochondria (cristae for ATP production), rough endoplasmic reticulum (ribosomes attached), and other organelles impossible to see with light microscopes.