Comprehensive Edexcel Combined Science: Biology Paper 1 Notes

Enzymes

Think of enzymes as your body's ultimate helpers - they're biological catalysts that speed up chemical reactions happening inside you every second. These protein molecules have a special active site where substances (called substrates) fit perfectly, like a lock and key mechanism.

When conditions aren't right, enzymes can denature, meaning their active site changes shape and stops working properly. Each enzyme has an optimum condition where it performs at its absolute best - usually around body temperature for human enzymes.

You've got three main types doing crucial jobs: amylase breaks down starch into glucose, protease tackles proteins into amino acids, and lipase splits lipids into fatty acids and glycerol. Without these molecular machines, digestion would take forever!

Quick Tip: Remember that enzymes are reusable - they don't get used up in reactions, so one enzyme can catalyse thousands of reactions!

Transport Across Membranes

Your cells are constantly moving stuff around, and there are three main ways this happens. Diffusion is the simplest - particles naturally move from areas of high concentration to low concentration, like perfume spreading across a room.

Several factors affect how fast diffusion works: concentration difference (bigger difference = faster movement), temperature (warmer = faster), surface area (more area = more movement), and distance (shorter = quicker). It's basically particles following the path of least resistance.

Osmosis is diffusion's water-loving cousin - it specifically moves water through semi-permeable membranes from high to low concentration. Active transport is different because it requires energy from mitochondria to move substances against the concentration gradient, using special transport proteins.

Remember: Diffusion and osmosis are passive (no energy needed), but active transport requires cellular energy to work against natural flow!

Cell Division and Growth

Mitosis is how your body creates new cells for growth and repair, producing two identical diploid daughter cells from one parent cell. Remember PMAT: Prophase (chromosomes condense, nuclear membrane disappears), Metaphase (chromosomes line up), Anaphase (chromosomes pull apart), Telophase (new nuclei form), then Cytokinesis splits the cell completely.

Meiosis is totally different - it creates sex cells (gametes) in your reproductive organs. Unlike mitosis, meiosis produces four unique haploid cells that are genetically different from the parent. This genetic shuffling is crucial for sexual reproduction.

When gametes from two parents fuse during fertilisation, they create a zygote with the full set of chromosomes again. Meiosis happens in testes and ovaries, while mitosis occurs throughout your body for growth and healing.

Key Difference: Mitosis makes identical copies for growth, meiosis makes unique gametes for reproduction!

Plant Growth and Cancer

Plants grow differently from animals, using special regions called meristems near root and shoot tips where rapid cell division happens. These areas contain stem cells that can differentiate into any specialised plant cell throughout the plant's entire life.

Animal stem cells are more limited - embryonic ones can become any cell type, but adult stem cells can only develop into related cell types. They're essential for growth and repair, but their potential decreases with age.

Cancer occurs when stem cell division goes completely out of control. Benign tumours grow slowly, stay contained within a membrane, and are usually easily removed. Malignant tumours are far more dangerous - they grow rapidly, spread throughout the body (metastasis), and invade neighbouring tissues.

Cancer can be caused by genetics, poor diet, viruses like HPV, UV radiation, or chemical carcinogens. Understanding these risk factors helps with prevention strategies.

Important: Not all tumours are cancerous - benign ones are generally treatable, while malignant ones require immediate medical attention!

Genetics Basics

DNA is life's instruction manual - a long polymer made of four bases (A, T, G, C) held together by hydrogen bonds and supported by a sugar-phosphate backbone. These bases pair specifically: A with T, G with C, creating the famous double helix structure.

Genes are sections of DNA that code for specific characteristics, and they come in different versions called alleles. You're homozygous when both alleles are the same (BB or bb) and heterozygous when they're different (Bb). Dominant alleles always show up in your appearance, while recessive ones only appear when there's no dominant allele present.

Your genotype is the actual alleles you have, while your phenotype is what you actually look like. Mutations happen when DNA copying goes wrong during cell division, creating variation in populations.

The Human Genome Project mapped all human DNA base pairs, helping us identify inherited diseases and develop better medicines.

Memory Tip: Think of alleles as different versions of the same app - they do similar jobs but might work slightly differently!

Inheritance Patterns

Inheritance works through meiosis splitting chromosomes so you get one copy from each parent. These can be dominant or recessive, determining which traits actually show up in your appearance.

Punnett squares are brilliant tools for working out the probability of inheriting certain characteristics. For example, sex determination works simply: females have XX chromosomes, males have XY, giving roughly 50:50 odds for each gender.

Pedigree diagrams track how genes pass through families over generations, helping identify patterns of inheritance for specific traits or genetic conditions.

Most characteristics are actually polygenic, meaning multiple genes control them rather than just one. Variation in populations comes from both genetic differences and environmental factors working together.

Exam Tip: Practice drawing Punnett squares for different crosses - they're a common question type and easy marks once you get the hang of them!

Classification and Selective Breeding

Scientists classify all life into three main domains based on genetic analysis: Archaea (primitive bacteria in extreme environments), Bacteria (regular bacteria), and Eukarya (organisms with nuclei, including us). The binomial system gives each species a two-part Latin name showing genus and species.

Selective breeding lets farmers and breeders create new varieties with desirable traits like disease resistance, higher yields, or better flavour. The process involves selecting parents with wanted characteristics, breeding them, then choosing the best offspring to continue the cycle over many generations.

This technique has pros and cons. Benefits include disease resistance, improved yields, and crops designed for specific environments. However, it raises ethical concerns about animal welfare, reduces genetic diversity, and can make populations vulnerable to new diseases.

The classification hierarchy goes: Kingdom, Phylum, Class, Order, Family, Genus, Species - each level getting more specific.

Real World: Selective breeding created everything from different dog breeds to sweeter apples - it's been shaping our world for thousands of years!

Evolution and Natural Selection

Darwin's theory of evolution explains how species change over time through five key steps: genetic variation from mutations, environmental changes creating competition, natural selection favouring the fittest, inheritance of advantageous traits, and gradual evolution over generations.

Evidence for evolution is everywhere. Bacteria rapidly develop antibiotic resistance, fossils show gradual changes over time, and human evolution traces back to Africa with key fossils like Ardi (4.4 million years old), Lucy (3.2 million years old), and Leakey's fossils (1.6 million years old).

Genetic engineering takes evolution into our own hands by directly modifying genomes. We can create crops resistant to pests, bacteria that produce human insulin, and organisms with entirely new capabilities.

Genetic engineering works faster than natural selection and has revolutionised medicine and agriculture. However, it's expensive, might affect natural species through crossbreeding, and could cause unexpected health problems.

Fascinating Fact: Antibiotic-resistant bacteria are evolution in action - they're adapting faster than we can develop new medicines!

Health and Disease Types

Communicable diseases spread between people via pathogens (disease-causing organisms), while non-communicable diseases can't be passed on and often result from genetics or lifestyle. Your immune system uses antibodies to recognise and destroy antigens on pathogen surfaces.

Different pathogens cause different diseases: viruses (HIV, Ebola), bacteria (cholera, TB), fungi (athlete's foot), and protists (malaria). Non-communicable diseases include genetic disorders, cardiovascular disease from blood clots, liver cirrhosis, and obesity from poor nutrition.

Your body has multiple defence layers. Physical barriers like skin, blood clotting, and mucus provide non-specific protection. Chemical defences include stomach acid and lysozymes in tears and saliva.

The specific immune response is your body's targeted attack system. When pathogens enter, lymphocytes produce specific antibodies, destroy the invaders, and create memory cells for faster future responses. BMI and waist-to-hip ratios help measure obesity-related health risks.

Body Wisdom: Your immune system has an incredible memory - once it fights off a pathogen, it remembers how to beat it faster next time!

Disease Prevention and Treatment

Physical barriers like condoms effectively reduce communicable disease spread, especially STDs. Antibiotics specifically target bacterial infections by slowing down or stopping bacterial cell activity - but they're useless against viruses.

Drug development follows strict stages for safety: laboratory testing on cells and tissues, animal testing, trials on healthy volunteers, then large-scale testing on infected patients. This process can take years but ensures medicines are safe and effective.

Immunisation gives you a head start against diseases by introducing dead or inactive pathogens. Your immune system creates antibodies and memory cells without you getting ill, so if you encounter the real disease later, your body responds immediately.

The immunisation graph shows how antibody levels spike quickly on second exposure compared to the initial slow response. This rapid secondary response is what protects you from getting the disease.

Prevention Power: Vaccines have eliminated diseases like smallpox and dramatically reduced others like polio - they're one of medicine's greatest successes!