Cell Structures and DNA

Every living thing is made of cells, but they're not all the same. Plant cells have extra bits like cell walls for protection and chloroplasts for making food, whilst animal cells are more flexible with just a cell membrane. Bacterial cells are much simpler - they don't even have a proper nucleus!

All cells contain DNA, which is like a instruction manual written in four letters: A, C, G, and T. This DNA is twisted into a double helix shape and packed into chromosomes inside the nucleus (or floating freely in bacteria).

When cells need to multiply, they go through mitosis - a carefully choreographed process where the cell copies its DNA and splits into two identical cells. Stem cells are special because they can turn into any type of cell your body needs, which is why scientists are excited about using them to treat diseases like Parkinson's.

Quick Tip: Remember that magnification = image height ÷ actual height - you'll need this formula for microscopy calculations!

Moving Substances In and Out of Cells

Your cells are constantly moving stuff around, and there are three main ways this happens. Diffusion is the easiest - substances naturally move from areas where there's loads of them to areas where there's less, like oxygen moving into your blood in your lungs.

Osmosis is specifically about water moving through partially permeable membranes. Think of plant roots soaking up water from soil - the water moves from where there's more of it to where there's less.

Sometimes cells need to work against the natural flow, which is called active transport. This takes energy but lets cells grab important stuff like glucose from your gut even when there's already plenty inside.

Your body is organised in levels: cells group together to make tissues, different tissues combine to form organs, and organs work together in organ systems. It's like building with increasingly complex Lego sets!

Remember: Diffusion and osmosis happen naturally, but active transport needs energy - just like pushing a ball uphill!

Enzymes and Digestion

Enzymes are like molecular scissors that break down your food so your body can actually use it. The three main digestive enzymes you need to know are lipase (breaks down fats), protease (tackles proteins), and amylase (sorts out starch).

Each enzyme works using the lock and key model - they have a specific shaped active site that only fits certain substances, like a key that only opens one lock. Once the enzyme has done its job, it can be used again and again.

Enzymes are incredibly fussy about their working conditions. They have an optimum temperature (around 37°C for human enzymes) and optimum pH (varies by enzyme). Get too hot or too acidic/alkaline and they become denatured - their shape changes and they stop working completely.

The beauty of enzymes is that they speed up reactions that would otherwise take ages, making digestion efficient enough to keep you alive and energised.

Top Tip: If enzyme conditions go wrong, the active site changes shape permanently - it's like melting a key!

Heart and Blood

Your heart is basically two pumps stuck together. The right side pumps blood to your lungs to pick up oxygen, whilst the left side pumps that oxygen-rich blood around your body. It's got its own pacemaker to keep the rhythm steady.

Your blood travels through three types of vessels with very different jobs. Arteries have thick walls and narrow lumens because they handle high-pressure blood leaving the heart. Capillaries are just one cell thick so substances can easily diffuse in and out. Veins have thin walls, wide lumens, and valves to stop blood flowing backwards.

Blood itself contains red blood cells (no nucleus so more space for oxygen), white blood cells (your immune system's army), and platelets (tiny cell fragments that help blood clot when you're injured).

Problems like blocked arteries can be treated with stents - tiny tubes that keep blood vessels open and prevent heart attacks.

Memory Trick: Arteries take blood Away from the heart - both start with 'A'!

Disease and Health

Cardiovascular disease happens when fatty deposits build up in your coronary arteries, potentially forming blood clots that block oxygen getting to heart cells. If too many cells die, you get a heart attack. Risk factors include smoking, high blood pressure, and too much salt or fat in your diet.

Health isn't just about not being ill - it's your overall physical and mental wellbeing, affected by diet, exercise, community, and your genes. Epidemiology studies look at large populations to spot patterns and identify risk factors.

Cancer occurs when cells start dividing uncontrollably, forming lumps called tumours. Benign tumours like warts grow slowly and are generally harmless. Malignant tumours are the dangerous ones - they grow fast, spread aggressively, and can move around your body.

Major cancer risk factors include smoking, poor diet, too much sun exposure, and unprotected sex. The good news is that many of these risks are within your control.

Key Point: Many diseases are preventable through lifestyle choices - your health is largely in your hands!

Plant Structure and Disease Spread

Plant leaves are perfectly designed for photosynthesis. The palisade mesophyll is where most photosynthesis happens, whilst the spongy mesophyll has spaces for gas exchange. Guard cells and stomata control water loss through transpiration.

Xylem tubes carry water up from roots to leaves, whilst phloem tubes transport food and ions back down. Transpiration increases with bright light, high temperature, and wind, but decreases in high humidity.

Pathogens are microorganisms that cause disease - viruses, bacteria, fungi, and protists. They spread through air, touch, blood, sexual fluids, or vectors like mosquitoes.

Measles is a viral infection spread by liquid droplets in air. It causes cold-like symptoms, red eyes, high temperature, and a distinctive rash. Thanks to the MMR vaccine, it causes little damage nowadays.

Plant Tip: Think of xylem going 'up' and phloem going 'down' - like a two-way motorway for plant transport!

Viral and Bacterial Infections

HIV is a virus spread through unprotected sex, sharing needles, childbirth, infected blood, or breastfeeding. It attacks white blood cells, weakening your immune response and potentially leading to AIDS if untreated.

TMV (Tobacco Mosaic Virus) spreads between plants through direct contact, causing reduced chlorophyll levels and lower crop yields - a major problem for farmers.

Salmonella bacteria spread through eating infected food, causing diarrhoea, stomach cramps, vomiting, fever, and potentially dangerous dehydration.

Gonorrhoea is a bacterial infection spread through sexual contact. Symptoms include thick, smelly discharge, pain when urinating, and bleeding. It can cause blindness in babies if passed on during birth.

Safety Note: Many of these diseases are preventable through safe practices - protection is always better than treatment!

More Diseases and Immune Defence

Rose black spot is a fungal disease spread by spores, causing large discoloured areas on plants and potentially killing them. Malaria is caused by parasites spread by female mosquitoes, causing high fever, sweats, chills, headaches, and can be lethal.

Your body has amazing natural defences. Stomach acid kills bacteria, mucus and hairs in your nose trap pathogens, skin acts as a barrier, and tears wash your eyes clean.

Your immune system fights back using white blood cells that consume pathogens, produce antitoxins to neutralise toxins, and create antibodies to target specific diseases.

Vaccination works by introducing small amounts of dead or inactive pathogens, allowing your immune system to develop antibodies safely. This creates herd immunity and has wiped out some diseases completely, though vaccines don't always work and can have side effects.

Amazing Fact: Your immune system remembers diseases it's seen before - that's why you usually only get chickenpox once!

Antibiotics and Drug Development

Antibiotics kill bacteria, but there's a growing problem. Bacteria divide quickly, so mutations develop rapidly through natural selection. Non-resistant bacteria die off, but resistant ones survive and multiply, eventually creating superbugs that current antibiotics can't touch.

New drugs must be rigorously tested for toxicity (safe dose levels), efficacy (how well they work compared to existing treatments), and proper dosage requirements.

Many medicines come from natural sources. Alexander Fleming discovered penicillin in 1928 when he noticed mould on a bacterial plate had stopped bacteria growing. Aspirin comes from willow bark, and digitalis (a heart drug) comes from highly toxic flowers.

The race is on to develop new antibiotics faster than bacteria can develop resistance - it's an ongoing battle between human ingenuity and bacterial evolution.

Historical Note: Fleming's discovery of penicillin was partly accidental - sometimes the best discoveries come from unexpected observations!

Photosynthesis and Respiration

Photosynthesis is how plants make food: water + carbon dioxide → oxygen + glucose (using light energy). It's an endothermic reaction that stores energy, and the glucose gets stored as starch in places like potatoes.

Aerobic respiration is the opposite: glucose + oxygen → water + carbon dioxide + energy. This exothermic reaction releases the energy your cells need to function.

When there's no oxygen available, cells switch to anaerobic respiration. In humans, this produces lactic acid that builds up in muscles, creating an oxygen debt that makes you breathe hard afterwards. You get less energy this way than with oxygen.

Yeast does anaerobic respiration differently: glucose → carbon dioxide + ethanol. This is why bread rises (CO₂ bubbles) and how alcoholic drinks are made.

Energy Tip: Aerobic respiration gives you much more energy than anaerobic - that's why you can't sprint forever!