GCSE AQA Biology: Cell Biology Topic

Getting Started with Cell Biology

This is your introduction to one of the most important topics in GCSE Biology. Cell biology forms the foundation for understanding how all living things work, from bacteria to humans.

You'll be exploring both the structure and function of cells, learning how they divide and specialise, and discovering the latest research in stem cell technology. The content here applies to both separate science and combined science students, with some higher tier material clearly marked.

Quick Tip: Bold content throughout these notes indicates higher tier material - make sure you know what level you're studying!

Cell Structure Basics

Every living thing is made of cells, but not all cells are the same. There are two main types: eukaryotic cells (like yours) and prokaryotic cells (like bacteria).

Animal and plant cells are eukaryotic and contain a cell membrane, cytoplasm, and a nucleus with DNA. Bacterial cells are prokaryotic - they're much smaller and have a cell wall, cell membrane, cytoplasm, and a single circular strand of DNA plus small DNA rings called plasmids.

These different parts are called organelles - think of them as tiny organs within the cell that each have specific jobs. Understanding the size differences between cells requires using orders of magnitude - if something is 1000 times bigger, we say it's 10³ times bigger.

Remember: Prokaryotic = no nucleus (bacteria), Eukaryotic = has nucleus (plants and animals)

Animal and Plant Cell Structures

Each organelle in your cells has a specific job that keeps you alive. The nucleus controls the cell and contains DNA, whilst the cytoplasm is where chemical reactions happen thanks to enzymes. Your cell membrane acts like a bouncer, controlling what gets in and out.

Mitochondria are your cell's power stations where aerobic respiration occurs, providing energy. Ribosomes are the protein factories, building the proteins your body needs to function properly.

Plant cells have everything animal cells have, plus some extras. Chloroplasts contain chlorophyll and carry out photosynthesis to make food. The permanent vacuole contains cell sap and helps keep the plant rigid, whilst the cellulose cell wall provides extra strength and support.

Memory Trick: Plants need extra support to stand up, which is why they have cell walls and vacuoles that animals don't need!

Bacterial Cells and Cell Specialisation

Bacterial cells are much simpler than plant and animal cells. They have cytoplasm, a cell membrane, and a cell wall made of peptidoglycan (not cellulose). Their DNA floats freely in the cytoplasm as a single circular strand, plus small plasmids.

Cell specialisation happens through differentiation - cells gain new structures to become experts at specific jobs. In animals, most cells differentiate once early in life, but plants can keep differentiating throughout their lives.

Sperm cells are streamlined with lots of mitochondria for swimming and enzymes to break into egg cells. Nerve cells have long extensions called dendrites and axons to carry electrical signals quickly. Muscle cells contain special proteins (myosin and actin) that slide over each other to create movement.

Key Point: Specialised cells trade being good at everything for being excellent at one specific job!

Plant Specialisation and Differentiation

Plant cells specialise just like animal cells do. Root hair cells have a massive surface area to absorb water and minerals from soil - they're basically the plant's drinking straws with lots of mitochondria for active transport.

Xylem cells transport water up the plant. They're dead, hollow tubes reinforced with lignin deposited in spirals to withstand water pressure. Phloem cells carry food from photosynthesis around the plant using sieve plates - broken-down cell walls that let substances flow through.

Stem cells are the source of all specialised cells. They differentiate by switching genes on or off to produce different proteins. In animals, most cells lose this ability after early development, but adult stem cells in bone marrow can still make new blood cells. Plants are more flexible - many cells can re-differentiate if needed.

Fascinating Fact: Plants can essentially "change their mind" about what type of cell they want to be, but most animal cells are stuck with their choice!

Microscopy and Magnification

You can't see cells without microscopes because they're incredibly tiny. Light microscopes use lenses to magnify images up to x2000 with a resolving power of 200nm - perfect for viewing cells and large organelles.

Electron microscopes revolutionised biology by using electrons instead of light. They achieve magnifications up to x2,000,000! Scanning electron microscopes create 3D images, whilst transmission electron microscopes show detailed 2D images of organelles like ribosomes and mitochondria.

The key calculations you need are: magnification = eyepiece lens × objective lens, and size of object = image size ÷ magnification. When dealing with tiny measurements, standard form makes the numbers manageable - for example, 0.00015 becomes 1.5 × 10⁻⁴.

Exam Tip: Always check your units match when calculating object sizes - convert everything to the same unit first!

Growing Microorganisms Safely

Scientists grow microorganisms in labs using nutrients to study them properly. This process, called culturing, can be done in nutrient broth or on agar gel plates. The culture medium contains everything bacteria need: carbohydrates for energy, minerals, proteins, and vitamins.

Sterile technique is absolutely crucial. Petri dishes and agar must be sterilised to prevent contamination. Inoculating loops are flamed to kill unwanted microorganisms, and plates are sealed (but not completely) to prevent airborne contamination whilst allowing oxygen in.

Plates are stored upside down at 25°C - any higher and dangerous bacteria that thrive at body temperature might grow. Bacteria can divide every 20 minutes through binary fission, so populations explode quickly. The formula is: bacteria at start × 2^number of divisions = final bacteria count.

Safety First: The 25°C temperature limit isn't random - it prevents growing bacteria that could harm humans!

Testing Antibiotics and Calculations

You can test antibiotic effectiveness using paper discs soaked in different antibiotics placed on bacterial cultures. The zone of inhibition (clear area where bacteria died) shows how effective each antibiotic is - bigger zones mean more effective antibiotics.

Always include a control disc soaked in sterile water to prove only the antibiotic causes bacterial death. Antibiotic-resistant bacteria will survive and grow right up to the disc, whilst non-resistant bacteria die, creating the clear zone.

Measuring these zones requires calculating cross-sectional areas using πr², where r is the radius. This same formula applies when measuring bacterial colonies. Standard form often comes in handy when dealing with the massive numbers of bacteria produced during experiments.

Real-World Connection: This exact method helps doctors choose the right antibiotics for treating infections in hospitals!

Chromosomes and Cell Division

Your genetic information lives in chromosomes - coiled DNA structures in the nucleus. Each chromosome contains many genes (sections of DNA coding for proteins). You have 23 pairs of chromosomes (46 total) in body cells, but gametes (sex cells) have only 23.

The cell cycle has three main stages. Interphase involves growth, organelle production, and DNA replication creating the characteristic 'X' shape. Mitosis sees chromosomes line up and get pulled apart by cell fibres. Cytokinesis splits the cytoplasm to form two identical daughter cells.

Mitosis is essential for growth, replacing damaged cells, and asexual reproduction. Unlike sexual reproduction, asexual reproduction needs only one parent organism that simply replicates its cells to produce genetically identical offspring.

Memory Aid: Mitosis = Making Identical Twins Of Sister cells - both daughter cells are exactly the same!

Stem Cells and Their Applications

Stem cells are undifferentiated cells that can divide and specialise into different cell types. Embryonic stem cells can become any cell in the body, making them incredibly valuable for treating diseases like diabetes, Alzheimer's, and spinal injuries.

Adult stem cells from bone marrow can produce various blood cells. Plant meristems found in root and shoot tips can differentiate into any plant cell throughout the plant's life, making plant cloning possible for preserving rare species or desired characteristics.

Therapeutic cloning could produce embryos with identical genes to patients, providing stem cells that wouldn't be rejected. However, this raises ethical concerns since obtaining embryonic stem cells destroys the embryo, and we don't fully understand how to control differentiation yet.

Debate Point: The potential to cure devastating diseases must be weighed against the ethical concerns of embryo destruction - there's no easy answer!