Cells & Immunity Overview

You're diving into one of biology's most fascinating topics! Understanding cell structure and immunity will help you grasp how life works at its most fundamental level. This Paper 1 content forms the backbone of A-level biology, so mastering these concepts now will make everything else click into place later.

Quick Tip: Think of cells as mini-factories - each organelle has a specific job, just like different departments in a company!

Eukaryotic Cell Structure Basics

Eukaryotic cells are the building blocks of complex life, and they're surprisingly well-organised! The key feature that sets them apart is having their DNA safely tucked away in a membrane-bound nucleus, surrounded by cytoplasm filled with specialised organelles.

Animal and plant cells share many features, but plants have some extra kit. Both have the cell-surface membrane, mitochondria, nucleus, ribosomes, and the endoplasmic reticulum system. Plants get bonus points with chloroplasts for photosynthesis, tough cell walls for support, and large vacuoles for storage.

The Golgi apparatus acts like a postal service, whilst lysosomes are the cellular clean-up crew. Think of it as each organelle having a specific job that keeps the whole cell functioning smoothly.

Remember: Algal and fungal cells are similar to plant cells, but fungi swap cellulose cell walls for chitin ones!

Nucleus and Mitochondria

The nucleus is basically the cell's control room, wrapped in a double membrane with nuclear pores acting like security checkpoints. Inside, you'll find the nucleoplasm containing linear DNA bound to histones, plus the nucleolus where ribosomes are made.

This powerhouse organelle handles all the genetic heavy lifting. It stores your DNA as chromatin, copies it during DNA replication, and kicks off protein synthesis through transcription to produce mRNA.

Mitochondria are the cell's energy factories, sporting a double membrane with the inner one folded into cristae for maximum surface area. The matrix inside contains its own circular DNA and 70S ribosomes - pretty independent!

These organelles pump out ATP through aerobic respiration, providing energy for everything from protein synthesis to active transport. More energy needed? More mitochondria present!

Top Fact: Mitochondria have their own DNA because they were probably ancient bacteria that moved in with early cells!

Chloroplasts and Endosymbiosis

Chloroplasts are plants' solar panels, capturing light energy with their double membrane structure. Inside the stroma, you'll find stacks of thylakoids called grana, connected by lamellae - think of them as solar cell arrays maximising light absorption.

Like mitochondria, chloroplasts have their own circular DNA and 70S ribosomes, plus they store starch granules and lipid droplets for energy. Their main gig is photosynthesis, converting light into organic compounds like carbohydrates.

Here's where it gets mind-blowing: endosymbiosis theory suggests that mitochondria and chloroplasts were once free-living bacteria. An ancestral cell basically invited them to move in permanently, creating the first eukaryotic cells through this biological flatshare arrangement.

Think About It: The bacteria living inside you as organelles still have their own genetic code - they're like permanent houseguests!

Cellular Clean-Up Crew

Peroxisomes are the cell's detox units, packed with enzymes in a simple lipid bilayer membrane. They specialise in breaking down toxic substances like hydrogen peroxide (H₂O₂) and other nasty metabolites that could damage the cell.

Lysosomes are like cellular recycling centres, filled with hydrolytic enzymes ready to break things down. These membrane-bound sacs release lysozymes and other enzymes to digest pathogens, worn-out organelles, or cellular waste.

Both organelles are essential for keeping cells healthy and functioning. Without peroxisomes, toxic substances would build up, whilst without lysosomes, cells would be cluttered with cellular rubbish and vulnerable to infections.

Memory Hook: Lysosomes = "lysis" (breaking down) + "soma" (body) - they literally break down stuff in the cell body!

Endoplasmic Reticulum and Vacuoles

The endoplasmic reticulum comes in two flavours, and they've got very different jobs! Rough ER (rER) is studded with ribosomes that churn out proteins, which then get processed, folded, and packaged into vesicles for transport to places like the Golgi apparatus.

Smooth ER (sER) keeps things clean and focuses on lipid synthesis - think cholesterol and steroid hormones. It's like having separate production lines for different products in the same factory.

Plant cell vacuoles are massive storage units surrounded by the tonoplast membrane. They maintain turgor pressure to keep plants upright and store cell sap containing sugars, amino acids, pigments, and waste products. Without them, plants would wilt faster than lettuce in the sun!

Quick Comparison: Animal cells might have tiny temporary vacuoles, but plant vacuoles are the real storage champions!

Golgi Apparatus and Transport

The Golgi apparatus is essentially the cell's packaging and shipping department. Made of flattened membrane sacs, it takes proteins from the rough ER and gives them the final touches - adding carbohydrates to create glycoproteins or modifying lipids into glycolipids.

Once everything's properly modified and packaged, the Golgi creates Golgi vesicles - small membrane sacs that act like delivery trucks. These vesicles transport proteins and lipids to their final destinations, whether that's the cell surface or somewhere else entirely.

The Golgi also produces lysosomes, making it a pretty versatile organelle. It's basically quality control, packaging, and shipping all rolled into one efficient system.

Think of It: The Golgi apparatus is like Amazon's fulfilment centre - products come in, get processed, packaged, and shipped out!

Cell Walls and Membranes

Cell walls are like cellular armour, providing serious structural support. In plants and algae, they're made of cellulose (a tough polysaccharide), whilst fungi use chitin instead. This rigid barrier prevents cells from bursting when water rushes in through osmosis.

The cell-surface membrane is where the real action happens - it's a phospholipid bilayer with hydrophilic heads facing outward toward water and hydrophobic tails tucked safely inside. Embedded proteins act like gatekeepers and communication devices.

This selectively permeable barrier controls what gets in and out of the cell. Surface molecules and receptors enable cell recognition and signalling, making it crucial for everything from immune responses to hormone detection.

Key Point: The cell wall provides strength, but the cell membrane provides intelligence - controlling molecular traffic and communication!

Ribosomes - The Protein Factories

Ribosomes might be small, but they're absolute workhorses! Made of ribosomal RNA and proteins arranged in two subunits (large and small), they're the sites where protein synthesis actually happens through translation.

These tiny structures grab onto mRNA and use tRNA to build polypeptide chains amino acid by amino acid. They're not membrane-bound organelles, so they can float freely in the cytoplasm or attach to the rough ER for protein production.

Here's the thing - almost every cell has ribosomes because proteins are essential for life. However, cells with high protein production rates $like muscle cells or antibody-producing cells$ pack in way more ribosomes to meet demand.

Remember: More protein needed = more ribosomes present. It's simple supply and demand at the cellular level!

Centrosomes and Cell Division

Centrosomes are the cell's construction managers, organising microtubules that form the cellular scaffold. In animal cells, they contain paired centrioles, but plant cells manage without them - showing that plants really are built differently!

During cell division (mitosis and meiosis), centrosomes become absolutely crucial. They organise spindle fibres that grab chromosomes and pull them apart, ensuring each new cell gets the right genetic material.

Think of centrosomes as the foreman on a construction site - they don't do the heavy lifting themselves, but they organise all the structural elements that make cell division possible.

Key Difference: Animal cells have centrioles in their centrosomes, but plant cells prove you don't always need the full kit to get the job done!