AQA Biology: Comprehensive Unit 1 Revision Book

Cell Biology Introduction

You're about to explore the amazing world of cells - the fundamental units that make up all living things on Earth. From bacteria living in extreme environments to the cells in birds soaring over Mount Everest, there's incredible diversity in the microscopic world.

Cells fall into two main groups: prokaryotic cells (found in bacteria) and eukaryotic cells (found in animals, plants, and other complex organisms). While they share some similarities, these cell types have crucial differences that determine how complex an organism can become.

Did you know? If Earth's entire 4.6 billion year history was compressed into just one hour, humans would only appear in the final few seconds!

Prokaryotes vs Eukaryotes

Prokaryotes are the old-timers of life - they've been around for about 3.5 billion years and include all bacteria. These single-celled organisms are incredibly small (1-10 micrometres long) and lack a proper nucleus to store their DNA.

Eukaryotes arrived much later, roughly 2 billion years ago, but they're generally much larger and more complex. All animals and plants are eukaryotes, and their cells contain a nucleus that safely stores genetic material in organised structures called chromosomes.

The key differences are straightforward: prokaryotes have no nucleus and are always single-celled, while eukaryotes have a nucleus and can form complex multicellular organisms. Bacterial cells typically contain cytoplasm, ribosomes, a cell wall, and DNA floating freely in a single loop, plus sometimes extra DNA rings called plasmids.

Scale check: About 100-1000 bacteria could fit in a straight line across just 1 millimetre!

Animal Cell Structure

Animal cells are eukaryotic powerhouses that contain several key components working together to keep you alive. Unlike bacterial cells, they're much larger and have specialised structures called organelles that each perform specific jobs.

The cell surface membrane acts like a selective barrier, controlling what enters and leaves the cell. Glucose and oxygen diffuse in for respiration, while waste products like carbon dioxide diffuse out. Inside, the cytoplasm is mostly water and provides the space where most chemical reactions happen.

Mitochondria are your cellular power stations - these organelles release energy from glucose through respiration. Active cells like muscle cells are packed with mitochondria because they need loads of energy. Ribosomes are the protein factories, building all the proteins your cells need to function.

Energy fact: Mitochondria have folded inner surfaces to maximise the area available for energy release - just like having more surface area speeds up dissolving sugar in tea!