Biology Paper 2: Key Topics Revision

Cell Types and Structures

Your body is made up of trillions of cells, and they're not all the same! There are two main types: prokaryotic (simple cells like bacteria) and eukaryotic (complex cells found in animals and plants).

Animal cells are like tiny factories with specific parts doing different jobs. The nucleus acts as the control centre, containing all your genetic material that determines everything from your eye colour to your height. Mitochondria are the powerhouses where energy is made through respiration - think of them as cellular batteries. Ribosomes are the protein-making machines, whilst the cell membrane acts like a bouncer, controlling what gets in and out of the cell.

Plant cells have all the same parts as animal cells, plus some extras that make them special. The cell wall made of cellulose gives plants their rigid structure (which is why trees don't collapse!). Chloroplasts contain chlorophyll and carry out photosynthesis, turning sunlight into food. The permanent vacuole stores cell sap and helps keep the plant upright.

Quick Tip: Remember that plant cells are basically animal cells with extra bits - they've got everything animals have, plus cell walls, chloroplasts, and vacuoles!

Bacterial Cells and Cell Division

Bacterial cells are much simpler than animal and plant cells - they're the rebels of the cell world! Unlike other cells, bacteria don't have a proper nucleus. Instead, their DNA floats freely in the cytoplasm as a single circular strand. They might also have small rings of DNA called plasmids, which are like bonus genetic material.

Understanding how cells divide is crucial because it's how you grow and heal. Mitosis is the process where one cell becomes two identical cells. It happens in three main steps: first, the cell grows and copies its DNA; then chromosomes line up in the middle and get pulled apart; finally, the whole cell splits into two identical daughter cells.

Transport processes are how substances move in and out of cells. Diffusion is like perfume spreading through a room - particles move from crowded areas to less crowded ones. Osmosis is diffusion but only for water molecules across special membranes. Active transport is different - it uses energy to move particles against the flow, like swimming upstream.

Remember: Mitosis is essential for growth and replacing damaged cells - without it, you couldn't heal from cuts or grow taller!

Exchange Surfaces and Microscopes

Single-celled organisms have it easy - they can get everything they need directly through their cell membrane. But you're made of billions of cells, so you need specialised exchange surfaces to get oxygen, nutrients, and waste materials where they need to go.

These surfaces have clever adaptations: thin membranes create short diffusion distances (like having express lanes), and large surface areas allow lots of diffusion to happen at once. It's like having more checkout tills in a supermarket - everything moves faster.

Microscopes are essential tools for studying cells. Light microscopes are the ones you'll use in class - they're cheap, easy to operate, but have limited magnification. Electron microscopes are the supercars of the microscope world - incredibly detailed images but expensive and complex to use.

The key formula you need to remember is: magnification = image size ÷ actual size. This helps you work out how much bigger something appears compared to its real size.

Top Tip: When using the magnification formula, make sure all your measurements are in the same units - don't mix millimetres with micrometers!

Cell Specialisation and Organisation

Differentiation is how cells become specialists at particular jobs. It all starts with a zygote (fertilised egg) that divides into stem cells - these are like cellular blank slates that can become anything.

Specialised cells have specific shapes and features for their jobs. Sperm cells have long tails for swimming and loads of mitochondria for energy. Muscle cells are packed with mitochondria for contraction. Nerve cells are long and thin with branched endings to connect with other nerves. It's like having different tools for different jobs.

Your body is organised like a hierarchy: cells group together to form tissues (like muscle tissue), tissues combine to create organs (like your stomach), and organs work together in organ systems (like your digestive system). Each level has a specific function and works with the others.

Stem cells are incredibly important because they can divide and turn into any type of cell your body needs. In plants, stem cells are found in areas called meristems, where new growth happens.

Key Point: Think of cell organisation like a school - individual students (cells) form classes (tissues), classes make up year groups (organs), and year groups create the whole school (organ system)!

The Digestive System and Enzymes

Your digestive system is basically a sophisticated food processor that breaks down large, insoluble food into small, soluble molecules your cells can actually use. It's made up of several organs working as a team.

Key players include: glands that produce digestive juices, your stomach that creates acid to kill bacteria and provide the right conditions for enzymes, the small intestine where nutrients get absorbed into your blood, and your liver that produces bile to help digest fats.

Enzymes are biological catalysts - they speed up reactions without getting used up themselves. They work using the lock and key hypothesis: each enzyme has a uniquely shaped active site where specific substrates fit perfectly, like a key in a lock.

Temperature and pH massively affect enzyme activity. Most enzymes work best at body temperature (37°C) and neutral pH. Get too hot or too acidic/alkaline, and enzymes become denatured - they change shape permanently and stop working, like a melted key that won't fit the lock anymore.

Exam Tip: Remember that enzymes are proteins, so anything that affects protein structure (extreme temperature or pH) will destroy enzyme function permanently!

Digestive Enzymes and Food Tests

Three main types of digestive enzymes break down your food. Carbohydrases (like amylase) convert carbohydrates into simple sugars - amylase in your saliva starts breaking down starch the moment you start chewing. Proteases break proteins into amino acids, and lipases split fats into fatty acids and glycerol.

You can test for different food types using simple chemical tests. Benedict's test turns brick red for sugars, iodine test goes blue-black for starch, Biuret test turns purple for proteins, and the emulsion test creates a cloudy layer when lipids are present.

Bile is produced in your liver and stored in your gallbladder. It has two crucial jobs: neutralising stomach acid so intestinal enzymes can work properly, and emulsifying fats - breaking large fat droplets into smaller ones to give lipase enzymes more surface area to work on.

When investigating enzyme activity, you can measure reaction rates using the formula: rate = 1000 ÷ time. The classic experiment involves testing how quickly amylase breaks down starch at different pH levels, using iodine to detect when all the starch has been digested.

Practical Tip: In enzyme experiments, always control variables like temperature and concentration - only change one factor at a time to get reliable results!

Heart and Circulation

Your heart is an incredible muscular pump that never takes a break, beating around 70 times per minute for your entire life. It works as a double circulatory system with two separate circuits: one pumping blood to your lungs for oxygen, another pumping oxygenated blood around your body.

The heart has four chambers: two atria (top chambers) that receive blood, and two ventricles (bottom chambers) that pump it out. The left ventricle has thicker muscular walls because it needs to pump blood all around your body, whilst the right ventricle only sends blood to your lungs.

Valves ensure blood flows in the right direction and prevent backflow. The heart has its own blood supply through coronary arteries. A natural pacemaker in the right atrium controls your heartbeat by sending electrical impulses across the heart muscle.

The heartbeat cycle works like this: blood flows into both atria, the atria contract pushing blood into the ventricles, then the ventricles contract powerfully, sending blood to the lungs and body whilst valves snap shut to prevent backflow.

Did You Know: Your heart will beat approximately 2.5 billion times in your lifetime - that's why keeping it healthy through exercise and good diet is so important!

Blood Vessels and Breathing

Your body contains three types of blood vessels, each perfectly designed for their job. Arteries carry blood away from your heart under high pressure, so they have thick muscular walls with elastic fibres to withstand the pressure. Veins carry blood back to your heart under low pressure, so they have thin walls, wide lumens, and valves to prevent backflow. Capillaries have walls just one cell thick, allowing easy exchange of substances between blood and tissues.

Your lungs are in your thorax, protected by your ribcage. Ventilation happens when your ribcage moves up and out, and your diaphragm moves down, increasing chest volume and drawing air in. The opposite happens when you exhale.

Gas exchange occurs in tiny air sacs called alveoli. Oxygen diffuses from the alveoli into your bloodstream whilst carbon dioxide diffuses the opposite way. Alveoli are perfectly adapted: they're tiny but numerous (huge surface area), have walls one cell thick (short diffusion pathway), and are surrounded by capillaries (good blood supply).

Calculate blood flow rate using: volume of blood ÷ number of minutes. Calculate breathing rate by counting breaths per minute.

Amazing Fact: You have approximately 300 million alveoli in your lungs, creating a surface area roughly the size of a tennis court for gas exchange!

Blood Components and Immune System

Blood is much more than just a red liquid - it's a complex transport system. Plasma is the liquid part that carries everything: glucose, amino acids, hormones, waste products like carbon dioxide and urea, plus immune system components.

Red blood cells are oxygen delivery specialists. Their biconcave shape maximises surface area, they have no nucleus (more room for oxygen), and they're packed with haemoglobin that binds oxygen to form oxyhaemoglobin. They're like tiny oxygen taxis shuttling between your lungs and tissues.

White blood cells are your immune system's army, defending against pathogens (disease-causing microorganisms). Some produce antibodies that clump pathogens together, others engulf and digest invaders, and some produce antitoxins to neutralise toxins.

Platelets are small cell fragments that help blood clot at wound sites. When you cut yourself, platelets stick together forming a plug, which then hardens into a scab. This prevents excessive bleeding and stops pathogens entering your body whilst new skin grows underneath.

Health Tip: White blood cells need good nutrition to function properly - eating a balanced diet with plenty of vitamins helps keep your immune system strong!

Coronary Heart Disease

Coronary heart disease is a non-communicable disease where the coronary arteries supplying your heart muscle become blocked with fatty deposits. This reduces oxygen supply to your heart and can lead to heart attacks.

Stents are tiny metal mesh tubes inserted into blocked arteries to keep them open. They're effective at reducing heart attack risk and have quick recovery times, but there's a small risk of complications during surgery, infections, and blood clots forming near the stent.

Statins are drugs that reduce levels of 'bad' LDL cholesterol whilst increasing 'good' HDL cholesterol. They significantly reduce stroke and heart attack risk, but must be taken continuously and can have side effects. They work slowly, preventing further fatty deposits rather than removing existing ones.

Faulty heart valves can become stiff (won't open properly) or leaky (allow backflow). They can be replaced with biological valves from pigs or cattle, which work very well but only last 12-15 years, or artificial valves that last longer but may require blood-thinning medication.

Prevention: The best treatment for coronary heart disease is prevention through regular exercise, healthy diet, not smoking, and managing stress - your future self will thank you!