Human Eye Anatomy

Ever wondered how you can instantly switch from reading your phone to spotting a mate across the school yard? Your eye is basically a super-sophisticated camera that automatically adjusts itself thousands of times a day.

The pupil acts like a camera aperture - it gets smaller in bright light (constricted) and bigger in dim light (dilated) thanks to the iris muscles. The cornea and lens work together to bend light rays and focus them perfectly on the retina, which is packed with light-sensitive cells.

Accommodation is your eye's autofocus feature. When you look at something close, your ciliary muscles contract to make the lens short and fat. For distant objects, these muscles relax and the lens stretches out thin. If this system doesn't work properly, you end up short-sighted (myopia) or long-sighted (hyperopia).

Quick Test Tip: Remember that short-sighted people can't see far away clearly, and long-sighted people struggle with close-up vision!

Circulatory System & Gas Exchange

Your heart is basically two pumps working side by side - the right side deals with deoxygenated blood heading to your lungs, whilst the left side pumps fresh, oxygenated blood around your body. It's like having a two-lane motorway system in your chest.

Blood isn't just red liquid - it's a transport network carrying everything your body needs. Red blood cells ferry oxygen around, white blood cells fight off infections, and platelets patch up cuts. All of this floats in plasma, which carries nutrients, hormones, and waste products.

Gas exchange happens through diffusion - oxygen moves from areas where there's loads of it (like your lungs) to areas where there's less (like your muscle cells). Different organisms have evolved clever ways to maximise this exchange, from fish gills to human lung alveoli.

Remember: The heart has four chambers and four valves - they all work together to keep blood flowing in one direction only.

Nervous System

When you touch something hot and instantly pull your hand away, that's your nervous system in action - and it's faster than any text message you'll ever send. This electrical network controls everything from blinking to thinking.

The reflex arc is your body's emergency response system. A stimulus (like heat) gets detected by receptors, which send electrical impulses along sensory neurons to your spinal cord. The signal then shoots back through motor neurons to effectors (your muscles), causing an instant response.

Synapses are the tiny gaps between nerve cells where the magic happens. Electrical signals can't jump across these gaps, so they get converted into chemical messages using neurotransmitters. It's like your neurons are having a chemical conversation.

The endocrine system works alongside your nervous system but uses hormones instead of electrical signals. It's slower but longer-lasting - perfect for controlling things like growth and mood.

Exam Gold: The reflex arc bypasses your brain completely - that's why you react before you even realise what's happened!

DNA Basics

DNA is basically your body's instruction manual, written in a four-letter code that determines everything from your eye colour to how tall you'll grow. Every cell in your body (except red blood cells) contains 46 chromosomes arranged in 23 pairs.

Genes are specific sections of DNA that code for particular proteins - think of them as individual recipes in your genetic cookbook. Your genome is the entire collection of genetic material that makes you uniquely you.

You inherit different versions of genes called alleles from each parent. If you get the same allele from both parents, you're homozygous for that trait. Different alleles make you heterozygous. Dominant alleles only need one copy to show up, whilst recessive alleles need two copies.

Your genotype is the genetic code you carry, whilst your phenotype is what actually shows up - like having brown eyes or being able to roll your tongue.

Memory Trick: Think of DNA as a twisted ladder (double helix) where the rungs are made of complementary base pairs - A always pairs with T, C always pairs with G.

DNA Structure & Function

DNA is a polymer made up of smaller units called nucleotides - each one contains a phosphate, a sugar, and one of four bases (A, T, G, C). These bases follow strict pairing rules in the double helix structure.

The genetic code gets read in groups of three bases called triplets. Each triplet codes for a specific amino acid, and chains of amino acids fold up to make proteins. Every protein has a unique shape that determines its particular function in your body.

The base sequence is crucial - change just one letter and you might completely alter the protein that gets made. This is why DNA copying has to be incredibly accurate during cell division.

Complementary base pairing ensures that when DNA copies itself, each new strand is identical to the original. It's like having a perfect photocopier that never makes mistakes.

Key Point: The sequence of bases in DNA is like a recipe - change the ingredients and you'll get a completely different dish (protein)!

DNA Mutations

Mutations are changes in the DNA base sequence - basically typos in your genetic code. Most of the time these changes don't matter, but sometimes they can have dramatic effects on the proteins that get made.

Substitution mutations happen when one base gets swapped for another - like changing an A to a T. This might change one amino acid in the final protein, which could alter how it works.

Insertion and deletion mutations are often more serious because they shift the entire reading frame. It's like removing one letter from the middle of a sentence - everything after that point becomes gibberish.

Think About It: Some mutations can actually be beneficial - they're the raw material for evolution and help organisms adapt to changing environments.

Meiosis & Mitosis

Mitosis is your body's photocopying service - it produces two identical diploid cells from one parent cell. This is how you grow and replace damaged tissues, creating cells with the full set of 46 chromosomes.

Meiosis is completely different - it's all about making gametes (sex cells) for reproduction. This process creates four genetically unique haploid cells, each with only 23 chromosomes.

During meiosis, chromosomes swap sections with each other and then get randomly distributed into different cells. This shuffling creates genetic variation, which is why you're not identical to your siblings (unless you're identical twins).

When a sperm and egg fuse during fertilisation, the diploid chromosome number gets restored and the new organism starts developing through mitosis.

Exam Essential: Mitosis = growth and repair (identical cells). Meiosis = reproduction (genetically different gametes).

Cell Types & Structure

Animal cells are the building blocks of complex organisms like yourself. These eukaryotic cells have a nucleus containing genetic material, plus mitochondria for energy production and ribosomes for making proteins.

Plant cells have all the same bits as animal cells, plus some extras. The cell wall provides structure and support, chloroplasts carry out photosynthesis, and a large vacuole helps maintain the cell's shape.

Bacterial cells are much simpler prokaryotic organisms. They lack a proper nucleus, instead having circular DNA floating in the cytoplasm. Many bacteria also have plasmids (extra DNA rings) and flagella for swimming around.

The cell membrane in all cell types acts like a selective bouncer, controlling what gets in and out of the cell. Meanwhile, the cytoplasm is where most chemical reactions happen.

Quick Comparison: Think of eukaryotic cells as having separate rooms (organelles) for different functions, whilst prokaryotic cells are more like studio flats with everything in one space.

Cell Differentiation & Specialisation

Differentiation is how a single fertilised egg eventually becomes a complex organism with hundreds of different cell types. It's like having one blueprint that can be read in different ways to build different structures.

Stem cells are the ultimate shape-shifters - they can divide and become any type of specialised cell your body needs. As organisms develop, cells gradually lose this flexibility and become committed to specific roles.

Sperm cells are perfect examples of specialised cells. They're basically swimming genetic delivery systems, complete with a flagellum for propulsion, loads of mitochondria for energy, and digestive enzymes to break through egg cell barriers.

Every specialised cell has a structure that perfectly matches its function - it's like each cell type has evolved to be the best possible tool for its particular job.

Remember: Specialisation = trading flexibility for efficiency. A heart muscle cell can't become a brain cell, but it's absolutely brilliant at contracting rhythmically!

Respiration

Cellular respiration is how your cells extract energy from food - it's basically controlled burning that happens inside every living cell. This exothermic reaction transfers energy from glucose to make ATP, your body's universal energy currency.

Aerobic respiration happens when there's plenty of oxygen around. It completely breaks down glucose into carbon dioxide and water, releasing loads of energy. The equation is: glucose + oxygen → carbon dioxide + water + ATP.

Anaerobic respiration kicks in during intense exercise when your muscles can't get enough oxygen. In human muscle cells, this produces lactic acid, which makes your muscles ache. Yeast does anaerobic respiration too, but produces ethanol and carbon dioxide instead.

Your body uses the energy from respiration for everything - building new molecules, muscle contraction, and maintaining your body temperature. Without respiration, you'd be like a phone with a dead battery.

Exam Focus: Aerobic respiration is much more efficient than anaerobic - you get about 19 times more ATP per glucose molecule when oxygen is available!