Cell Division and Structure

Your body is constantly growing and repairing itself through a process called mitosis. Think of it like your cells making perfect copies of themselves - each new cell gets exactly the same 23 pairs of chromosomes as the original.

Before mitosis happens, your cells go through a growth phase where they get bigger and duplicate their DNA. Then during mitosis, the chromosomes line up in the middle and get pulled apart so each new cell gets an identical copy.

Stem cells are like the ultimate shape-shifters of your body. These undifferentiated cells can turn into any type of cell you need - muscle cells, nerve cells, blood cells, you name it! Scientists are dead excited about using stem cells to treat diseases like diabetes.

Quick Tip: Remember that mitosis creates two identical daughter cells - it's how you grow and replace damaged cells throughout your life.

When looking at cells under a microscope, you'll see different structures depending on whether it's a plant or animal cell. Animal cells have a nucleus (the control centre), mitochondria (the powerhouses), and ribosomes (protein factories). Plant cells have all that plus chloroplasts for photosynthesis and a tough cell wall for support.

Gas Exchange and Diffusion

Here's the thing about gas exchange - as organisms get bigger, simple diffusion through their surface just isn't enough anymore. That's why you've got lungs instead of just breathing through your skin like a tiny organism would!

Diffusion is basically particles moving from where there's loads of them to where there's fewer. The bigger the concentration difference, the faster it happens. Temperature speeds things up too because particles have more energy to move about.

Your lungs are brilliant at gas exchange because they've got a massive surface area (thanks to millions of tiny air sacs called alveoli), thin walls for easy diffusion, and an excellent blood supply to carry gases around.

Remember: Fish gills work the same way as your lungs - they're all about maximising surface area for efficient gas exchange.

Osmosis is just diffusion but specifically for water moving through a partially permeable membrane. Active transport is different though - that's when cells use energy to move substances against the concentration gradient, like when your intestines absorb nutrients.

Circulatory System and Heart

Your heart is basically two pumps stuck together. The right side pumps deoxygenated blood to your lungs, while the left side (which is thicker and stronger) pumps oxygenated blood around your entire body.

This double circulatory system is dead clever - blood goes through your heart twice in one complete circuit. First to pick up oxygen from the lungs, then to deliver it everywhere else. The whole system depends on valves that stop blood flowing backwards.

Enzymes are like molecular scissors that speed up chemical reactions in your body. They work best at around 37°C (your body temperature) and have a specific pH they prefer. If it gets too hot, the enzyme changes shape and stops working - we call this denaturing.

Key Point: Your heart rate and breathing rate increase during exercise because your muscle cells need more oxygen for respiration.

Food tests help us identify what's in our meals. Benedict's test shows sugars (going from blue to brick red), iodine detects starch $going from brown to blue-black$, and biuret finds proteins (going from blue to purple).

Blood Vessels and Disease

Arteries carry blood away from your heart under high pressure, so they need thick, elastic walls. Veins bring blood back with valves to prevent backflow. Capillaries are where the magic happens - they're just one cell thick so substances can easily diffuse in and out.

Your blood contains red blood cells (packed with haemoglobin to carry oxygen), white blood cells (your immune system's soldiers), platelets (for clotting), and plasma (the liquid bit that carries everything else).

Cardiovascular disease is when your arteries get clogged with fatty deposits, making them narrow and restricting blood flow. This can lead to heart attacks if the heart muscle doesn't get enough oxygen.

Important: Smoking, poor diet, lack of exercise, and stress all increase your risk of heart disease.

Treatments include stents (tiny tubes that keep arteries open), drugs that thin the blood or lower cholesterol, and in severe cases, heart transplants. Each treatment has pros and cons - stents work quickly but can cause blood clots, while drugs are less invasive but may have side effects.

Plant Transport and Cancer

Plants move water up from their roots through xylem vessels (dead cells strengthened with lignin) and transport sugars around in phloem tubes (living cells that can move substances both ways).

Transpiration is basically plants sweating - water evaporates from leaves and gets replaced by water drawn up from the roots. The rate depends on temperature, humidity, air flow, and light intensity.

Guard cells are like bouncers for the leaf - they control the stomata (tiny pores) that let gases in and out. When the plant has plenty of water, they open the stomata. When water's scarce, they close them to prevent water loss.

Plant Fact: Most stomata are on the underside of leaves where it's cooler and shadier, reducing water loss.

Cancer happens when cells divide uncontrollably. Benign tumours stay in one place and aren't usually dangerous. Malignant tumours are the nasty ones - they invade other tissues and can spread around the body. Risk factors include smoking, obesity, radiation, and some genetic mutations.

Guard Cell Function

Guard cells are absolutely crucial for plant survival. They've got a clever shape with thick inner walls and thinner outer walls that helps them change shape effectively.

When there's plenty of water available, guard cells become turgid (swollen with water) and curve apart, opening the stoma. This allows carbon dioxide in for photosynthesis and oxygen out as a waste product.

When water is scarce, guard cells become flaccid (lose water and go floppy), which closes the stoma tightly. This prevents further water loss but also stops gas exchange.

Smart Design: Guard cells are typically found on the underside of leaves where it's cooler and less windy, reducing unnecessary water loss.

Infectious Diseases and Defence

Communicable diseases are caused by pathogens - bacteria, viruses, fungi, or parasites that can spread between people. Bacteria reproduce rapidly and produce toxins. Viruses hijack your cells to make copies of themselves.

Examples include measles (viral, spread by droplets), gonorrhoea (bacterial STD), malaria (caused by parasites spread by mosquitoes), and rose black spot (fungal disease affecting plants).

Your body has several defence mechanisms. Your skin acts as a barrier, your stomach acid kills many pathogens, and mucus in your airways traps particles before they reach your lungs.

Defence Strategy: White blood cells are your immune system's army - they engulf pathogens, produce antibodies, and make antitoxins to neutralise harmful substances.

Antibiotics like penicillin kill bacteria but don't work on viruses. Overusing antibiotics can lead to antibiotic resistance, where bacteria evolve to survive the treatment. That's why you must always finish the full course your doctor prescribes.

Vaccination and Immunity

Vaccinations are like giving your immune system a practice run. They contain weakened or dead pathogens that can't make you ill but still trigger your white blood cells to produce antibodies.

The brilliant thing is that your immune system remembers these pathogens. If you encounter the real disease later, your body can rapidly produce the right antibodies before you get properly ill.

Clinical trials test new drugs in three stages. First on cells and tissues in labs, then on animals, and finally on human volunteers. This ensures drugs are safe and actually work before being released to the public.

Herd Immunity: When most people in a community are vaccinated, it protects those who can't be vaccinated due to illness or age.

Some clinical trials are double-blind, meaning neither patients nor doctors know who's getting the real drug versus a placebo. This prevents bias from affecting the results and gives more reliable data about the treatment's effectiveness.

Respiration and Exercise

Respiration isn't just breathing - it's the chemical reaction that releases energy from glucose in every cell of your body. The energy gets stored in molecules called ATP that cells can use for everything from muscle contraction to active transport.

Aerobic respiration uses oxygen and produces carbon dioxide and water as waste products. Anaerobic respiration happens when there's not enough oxygen (like during intense exercise) and produces lactic acid in animals or ethanol in yeast.

During exercise, your heart rate and breathing rate increase to supply more oxygen to your muscles. If you exercise really intensely, you build up an oxygen debt - that's why you keep breathing heavily even after you stop.

Exercise Tip: The fitter you are, the quicker your heart rate returns to normal after exercise because your cardiovascular system is more efficient.

Metabolism includes all the chemical reactions in your body. It needs energy for building larger molecules from smaller ones, muscle contraction, maintaining body temperature, and active transport in cells.

Photosynthesis and Limiting Factors

Photosynthesis is how plants make their own food using sunlight, carbon dioxide, and water. The equation is: carbon dioxide + water → glucose + oxygen (but only with light and chlorophyll present).

The rate of photosynthesis depends on limiting factors - light intensity, carbon dioxide concentration, temperature, and the amount of chlorophyll. Whichever factor is in shortest supply limits how fast photosynthesis can happen.

You can investigate this using pondweed - as light intensity increases, more oxygen bubbles are produced, showing faster photosynthesis. But eventually, the rate levels off when light is no longer the limiting factor.

Graph Skills: When plotting limiting factors, you'll see the rate increase then level off - the level bit shows when that factor is no longer limiting the reaction.

At low temperatures, enzymes work slowly. At high temperatures, they denature and stop working altogether. That's why there's an optimum temperature for photosynthesis around 25°C in most plants.