Eukaryotic Cells: The Building Blocks of Complex Life

Eukaryotic cells are the sophisticated cells that make up plants, animals, and fungi - basically anything that's not bacteria. The key difference? They've got a proper nucleus wrapped in a membrane, plus loads of specialised compartments called organelles.

Animal cells and plant cells share many organelles but have some crucial differences. Both contain a nucleus (the control centre), mitochondria (the powerhouses), ribosomes (protein factories), and the plasma membrane (the cell's boundary). The endoplasmic reticulum comes in two forms: RER (rough ER) makes proteins, whilst SER (smooth ER) handles fats and steroids.

Plant cells get some exclusive extras that make them unique. They've got a tough cell wall for support, chloroplasts for photosynthesis, and a massive vacuole for storage. Animal cells, meanwhile, have lysosomes (cellular recycling centres) and centrioles (important for cell division) that plants typically don't.

Quick Tip: Remember that chloroplasts and cell walls are plant-only features, whilst lysosomes and centrioles are animal-only!

Organelles: Your Cellular Toolkit

Each organelle has a specific job that keeps the cell running smoothly. The cell wall acts like armour, providing protection and structural support to plant cells. The plasma membrane is found in all cells and controls what gets in and out of the cell.

The nucleus is absolutely crucial - it contains all your chromosomes and genetic material, making it the cell's command centre. The Golgi apparatus works like a post office, sorting and modifying proteins and lipids before shipping them where they need to go.

Energy production happens in the mitochondria, which generate chemical energy through aerobic respiration. Ribosomes are the protein-making machines, built from RNA and protein. Lysosomes contain powerful digestive enzymes that break down waste and worn-out cell parts.

Chloroplasts are the green organelles in plants that capture sunlight for photosynthesis. Vacuoles store cell sap and waste products, whilst centrioles help organise chromosomes during cell division.

Memory Hack: Think of mitochondria as cellular power stations - they're literally where your cells make energy!

Prokaryotic Cells and Viruses: Life's Simpler Forms

Prokaryotic cells are the simpler cousins of eukaryotic cells - they lack a proper nucleus and their genetic material floats freely in the nucleoid region. Bacteria are the most common prokaryotes you'll encounter.

These cells are surprisingly well-equipped despite their simplicity. They've got a cell wall, plasma membrane, and cytoplasm just like other cells. Many have a flagellum for swimming around, pili for attaching to surfaces, and small rings of DNA called plasmids. Some even wear a protective capsule like a cellular raincoat.

Prokaryotes reproduce through binary fission - essentially splitting into two identical daughter cells. It's much simpler than the complex cell division in eukaryotes.

Viruses aren't technically alive since they can't reproduce on their own. Instead, they hijack other cells through a sneaky process: attachment, penetration, uncoating, replication, assembly, and finally virion release to spread to new victims.

Key Point: Prokaryotes are complete living cells, whilst viruses are basically genetic material in a protein coat that needs a host to reproduce.

Methods of Studying Cells: Getting Up Close

Understanding cells requires some serious magnification since most are invisible to the naked eye. The measurement scale goes from centimetres to millimetres to micrometres to nanometres - each step is 1000 times smaller than the last.

Magnification tells you how much bigger an image appears compared to the real object. Resolution is actually more important - it's how well you can distinguish between two tiny objects that are really close together. You can magnify something loads, but if the resolution is rubbish, you'll just get a bigger blurry mess.

Scientists use fractionation and ultracentrifugation to separate different organelles from cells. This lets them study each type individually. The organelles separate by size and density, with nuclei being the largest and heaviest, followed by chloroplasts, mitochondria, lysosomes, ER, and finally ribosomes as the smallest.

These techniques are essential for understanding how cells work and what happens when things go wrong.

Remember: Good resolution beats high magnification every time - clarity is more important than size!

Calibrating the Eyepiece Graticule: Measuring the Microscopic

Getting accurate measurements under a microscope requires calibrating your eyepiece graticule - basically teaching your microscope what size things actually are. Without calibration, you're just guessing at measurements.

The process is straightforward but requires precision. First, you insert a stage micrometre and find its scale using low power. Then you align both scales at zero and find where they line up again - this gives you your conversion factor.

The maths is simple: divide the number of micrometre units by eyepiece units, then multiply by 10 (since each stage unit equals 10 micrometres). You'll need to repeat this for each lens power since magnification changes the scale.

Here's what typical results look like: low power might give you 250 µm per eyepiece unit, medium power 100 µm, and high power around 25 µm. The higher the magnification, the smaller the real distance each eyepiece unit represents.

Pro Tip: Always recalibrate when changing lenses - each magnification has its own conversion factor!

The Fluid Mosaic Model: How Membranes Work

Cell membranes aren't just simple barriers - they're dynamic, flexible structures that control everything entering and leaving cells. The fluid mosaic model explains how phospholipids, cholesterol, glycoproteins, and glycolipids work together.

Cholesterol is the membrane's stability manager. It wedges between phospholipids and binds to their hydrophobic tails, making them pack more tightly. This reduces fluidity and adds rigidity, helping maintain cell shape, especially in animal cells.

Transport across membranes happens in several ways. Simple diffusion lets small molecules pass through down concentration gradients. Facilitated diffusion uses protein channels to help larger molecules move from high to low concentration. Osmosis specifically describes water movement through partially permeable membranes.

Active transport is different - it uses energy to pump molecules against concentration gradients, from low to high concentration. This is essential for maintaining the precise chemical conditions cells need to function properly.

Key Insight: Think of membranes as selective bouncers at a club - they control who gets in, who gets out, and sometimes they'll even drag people inside!

Mitosis and the Cell Cycle: How Cells Multiply

Cell division isn't just splitting in half - it's a carefully choreographed process that ensures each new cell gets exactly what it needs. The cell cycle has distinct phases: G1, S, G2, and M (mitosis).

During G1, cells grow and duplicate everything except chromosomes. S phase is when DNA replication happens - each chromosome gets copied. G2 is quality control time, checking for errors before division begins.

Mitosis itself has five stages. Interphase sees continued growth and DNA replication. Prophase is when chromosomes condense and the mitotic spindle starts forming. During metaphase, chromosomes line up perfectly in the cell's centre.

Anaphase is the dramatic moment when chromosome copies separate and head to opposite ends of the cell. Telophase sees the cell finishing division as normal structures return and cytokinesis physically splits the cell in two.

Memory Aid: Remember IPMAT - Interphase, Prophase, Metaphase, Anaphase, Telophase!

Cancer: When Cell Division Goes Wrong

Cancer happens when normal cell division controls break down, leading to uncontrolled growth. Two key types of genes normally prevent this: proto-oncogenes and tumour suppressor genes.

Proto-oncogenes are like the accelerator pedal - they produce proteins that stimulate cell division when needed. If they mutate, they become stuck in the "on" position, causing cells to divide uncontrollably.

Tumour suppressor genes act like the brakes, producing proteins that slow or stop cell division. When these genes mutate, they become inactivated, removing the cell's ability to control its own growth.

Chemotherapy and radiotherapy work by deliberately damaging DNA in cancer cells. Since cancer cells divide rapidly, they're more vulnerable to this damage than normal cells, though healthy cells can also be affected.

Important: Cancer isn't just one disease - it's hundreds of different diseases that all involve uncontrolled cell division.

Vaccines and Antibodies: Your Body's Defence System

Your immune system is like a highly trained army that recognises and destroys threats. Antibodies are Y-shaped proteins that bind to specific antigens on pathogens, marking them for destruction.

Phagocytosis is how white blood cells called phagocytes literally eat pathogens. They use receptor binding points to recognise threats, change shape to engulf them, then use lysosomes containing lysozyme enzymes to break down and destroy the invaders.

Antigenic variability makes some pathogens particularly tricky. Viruses like flu constantly change their surface antigens, meaning your antibodies from last year's infection might not recognise this year's virus strain.

The secondary immune response is faster and stronger than the primary response. This is why vaccines work - they train your immune system to recognise threats so it can respond quickly if you encounter the real pathogen later.

Cool Fact: Your immune system remembers every pathogen it's ever encountered, which is why you usually only get chickenpox once!

Types of Immunity and Medical Applications

Immunity comes in different forms depending on how you acquire it. Natural immunity develops when your body fights off an actual infection, whilst artificial immunity comes from vaccines that expose you to harmless versions of pathogens.

Active immunity means your immune system produces its own antibodies after exposure to antigens. Passive immunity involves receiving ready-made antibodies from another source, like a mother passing antibodies to her baby through breast milk.

Monoclonal antibodies are identical antibodies produced in labs that target specific antigens. They're incredibly useful for immunoassays (like pregnancy tests), cancer treatment (targeting tumour cells specifically), and preventing transplant rejection.

Antibiotics don't work against viruses because viruses lack the cell walls that antibiotics target. This is why you can't treat viral infections like colds or flu with antibiotics - they're completely useless against viruses.

Real-world Application: ELISA tests use antibodies to detect everything from pregnancy hormones to COVID-19 - they're one of medicine's most versatile tools!