Cell Structure Basics

Every living thing is built from cells, but not all cells are the same. Eukaryotic cells (like yours) have a proper nucleus wrapped in a membrane, whilst prokaryotic cells (bacteria) have their DNA floating freely in the cytoplasm.

Animal and plant cells share the basics: a cell membrane controlling what goes in and out, cytoplasm where chemical reactions happen, and a nucleus containing DNA. Bacterial cells are much simpler and smaller, with just a cell wall, membrane, cytoplasm, and loose DNA plus small DNA rings called plasmids.

Understanding orders of magnitude helps you grasp just how tiny these structures are. If something is 1000 times bigger, we write it as 10³ times bigger. The prefixes you'll use most are centi (0.01), milli (0.001), micro (0.000,001), and nano (0.000,000,001).

Quick tip: Remember that bacterial cells are prokaryotic (no proper nucleus) whilst animal and plant cells are eukaryotic (proper nucleus present).

Animal vs Plant vs Bacterial Cells

Plant cells have everything animal cells have, plus some extras that make them unique. Chloroplasts contain the green pigment chlorophyll for photosynthesis, a permanent vacuole filled with cell sap, and a cell wall made of cellulose for extra strength and rigidity.

Mitochondria are the powerhouses where aerobic respiration happens, giving cells energy. Ribosomes make proteins and sit on the rough endoplasmic reticulum. The cytoplasm isn't just empty space - it's packed with enzymes that speed up chemical reactions.

Bacterial cells are completely different. Their cell wall is made of peptidoglycan (not cellulose), and instead of a nucleus, their DNA floats freely as a single circular strand. They also have plasmids - small rings of extra DNA.

Remember: Only plant cells have chloroplasts, permanent vacuoles, and cellulose cell walls. These three features make them unique!

Specialised Cells in Action

Cells become specialised through differentiation - gaining new structures to do specific jobs. Most animal cells differentiate once early on, but plant cells can change throughout their lives.

Sperm cells are perfectly designed for reaching the egg. They have a streamlined head, long tail for swimming, lots of mitochondria for energy, and digestive enzymes in the acrosome to break through the egg's membrane. Nerve cells transmit electrical signals with their long axons and branched dendrites, plus mitochondria to make neurotransmitters.

Muscle cells contract using special proteins (myosin and actin) that slide over each other, powered by energy from numerous mitochondria. They store glycogen for quick energy release during contraction.

Plant root hair cells maximise water uptake with their large surface area and permanent vacuole. Xylem cells become hollow tubes reinforced with lignin spirals to transport water up the plant. Phloem cells have sieve plates for moving food around the plant.

Top tip: Look for the connection between structure and function - every specialised feature has a purpose that helps the cell do its job better.

Microscopy and Magnification

You can't see cells without microscopes because they're incredibly tiny. Light microscopes use two lenses (objective and eyepiece) and can magnify up to x2000 with a resolving power of 200nm. Robert Hooke first observed cork cells with one in 1665.

Electron microscopes revolutionised cell biology in the 1930s by using electrons instead of light. They magnify up to x2,000,000 with much better resolution. Scanning electron microscopes create 3D images, whilst transmission electron microscopes show detailed 2D views of organelles.

The key calculations you need are: magnification = eyepiece lens × objective lens, and size of object = size of image ÷ magnification. Always check your units match up!

Standard form makes huge or tiny numbers manageable. Write numbers between 1 and 10, then multiply by powers of 10. For example, 1.5 × 10⁻⁵ = 0.000015.

Exam hack: Practice rearranging the magnification formula - you might need to find the magnification or image size instead of object size.

Growing Microorganisms Safely

Scientists grow microorganisms in labs using nutrient-rich culture media containing carbohydrates, minerals, proteins and vitamins. You can grow them in nutrient broth solution or on agar gel plates where they form visible colonies.

Sterile technique is crucial to prevent contamination. Sterilise Petri dishes and media with autoclaves or UV light, flame inoculating loops, and seal plates with tape $but not completely - you need some air$. Store plates upside down to prevent condensation dripping onto the agar.

Keep cultures at 25°C, not 37°C (body temperature), to prevent dangerous human pathogens from growing. Bacteria multiply by binary fission every 20 minutes under good conditions. Use the formula: bacteria at end = bacteria at beginning × 2^(number of divisions).

Testing antibiotics involves placing antibiotic-soaked discs on bacterial plates. The inhibition zone (clear area where bacteria died) shows effectiveness - bigger zones mean better antibiotics. Calculate areas using πr².

Safety first: Always follow sterile technique - contaminated cultures can be dangerous and will ruin your results.

Cell Division and Chromosomes

Your genetic information lives in chromosomes - coiled DNA structures in the nucleus. You have 23 pairs (46 total) in body cells, inheriting one set from each parent. Genes are short DNA sections coding for proteins that control characteristics.

Mitosis is cell division that produces two identical cells. The cell cycle has three stages: Interphase (cell grows, DNA replicates, organelles increase), Mitosis (chromosomes line up and separate), and Cytokinesis (cytoplasm divides).

During mitosis, replicated chromosomes form the characteristic 'X' shape. Cell fibres pull one copy of each chromosome to opposite sides before the cell splits. This process is vital for growth, development, and replacing damaged cells.

Sex cells (gametes) are different - they only have 23 chromosomes (half the normal number) so that when sperm and egg fuse, the offspring gets the full 46.

Key point: Mitosis produces identical cells for growth and repair, whilst gametes have half the chromosomes for reproduction.

Stem Cells and Their Potential

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 medical treatments like diabetes, Alzheimer's, and spinal injuries.

Adult stem cells from bone marrow can form various blood cells but are more limited than embryonic ones. Plant meristems in root and shoot tips can differentiate throughout the plant's life, making plant cloning possible for research or conservation.

Therapeutic cloning could produce embryos with the patient's exact genes. Stem cells from these embryos wouldn't be rejected because they'd match the patient's genetic makeup perfectly.

However, stem cell research faces challenges. We don't fully understand differentiation control, and obtaining embryonic stem cells destroys the embryo, raising ethical concerns. The benefits of treating serious diseases must be weighed against these moral considerations.

Think about it: Stem cell therapy could cure currently incurable diseases, but the ethical debates around embryo use continue to influence research funding and regulations.