Cells as the basic units of living organisms

Welcome to the Secret Life of Cells!

Welcome to your study notes for Cambridge AS Level Biology. In this chapter, we are zooming in—way in—to look at cells. Think of a cell as a tiny, bustling city. Just like a city has a town hall, power stations, and post offices, a cell has different parts called organelles that work together to keep you alive. Whether you are a top-tier scientist in the making or someone who finds Biology a bit overwhelming, these notes are designed to make things clear and simple.

Why is this important? Every single thing you do—from breathing to reading this sentence—is possible because your cells are hard at work. Understanding how they work is the foundation for everything else in Biology!

1. Eukaryotic Cells: The Complex Builders

Most organisms you see around you (like plants and animals) are eukaryotic. This just means their cells have a nucleus and other parts wrapped in membranes. Don't worry if these names seem long; we will break them down one by one.

The "Control Center" and "Boundary"

Cell Surface Membrane: This is the thin outer layer. It’s like a security guard at a gate; it controls what enters and leaves the cell.
The Nucleus: This is the brain of the cell. It contains your DNA (the instruction manual for building you).
Nuclear Envelope: A double membrane that surrounds the nucleus, keeping the DNA safe.
Nucleolus: A dark spot inside the nucleus where ribosomes are made.

The "Factory and Shipping" Department

Rough Endoplasmic Reticulum (RER): Imagine a factory floor covered in dots. These dots are ribosomes. The RER’s job is to fold and transport proteins.
Smooth Endoplasmic Reticulum (SER): Similar to RER but has no ribosomes (it looks "smooth"). It makes lipids (fats) and steroids.
Golgi Body (or Golgi Apparatus): Think of this as the cell's Post Office. It collects proteins from the RER, packages them into little bags called vesicles, and sends them to where they are needed.
Ribosomes: Tiny machines that build proteins. In the main part of the cell, they are 80S (larger), but inside mitochondria and chloroplasts, they are 70S (smaller).

The "Power Stations"

Mitochondria: This is where aerobic respiration happens. They produce ATP, which is like "cellular cash"—the energy the cell spends to do work.
Did you know? Mitochondria have their own small circular DNA, which is a clue that they might have lived as separate bacteria billions of years ago!

The "Clean-up and Support" Crew

Lysosomes: These are spherical bags filled with digestive enzymes. They act like a trash disposal unit, breaking down old cell parts or bacteria.
Centrioles and Microtubules: Centrioles are small tubes (found in pairs) that help the cell divide. Microtubules are like the cell's skeleton, giving it shape and acting as tracks for moving things around.
Cilia and Microvilli: These are hair-like structures on the outside. Cilia move things across the cell surface (like clearing dust from your throat), while microvilli are folds that increase the surface area for absorbing food.

Quick Takeaway:

Every organelle has a specific job. If the Nucleus is the boss, the Mitochondria provide the power, and the Golgi Body handles the shipping.

2. Plant Cells: The Special Features

Plant cells have all the parts mentioned above, but they also have a few extra "upgrades" to help them make their own food and stay upright.

Chloroplasts: These are the green "solar panels" of the plant. They perform photosynthesis. Like mitochondria, they contain 70S ribosomes and small circular DNA.
Cell Wall: A tough outer layer made of cellulose. It’s like a sturdy wooden fence that keeps the cell from bursting if it takes in too much water.
Plasmodesmata: Tiny channels (gaps) in the cell walls that allow plant cells to "talk" to each other by swapping materials.
Large Permanent Vacuole and Tonoplast: A huge water-filled sac in the middle of the cell. The tonoplast is the name of the membrane surrounding this vacuole. It keeps the cell firm and pressurized.

Common Mistake to Avoid: Both plant and animal cells have mitochondria! Some students think plants only have chloroplasts, but they need mitochondria to get energy from the food they make.

3. Comparing Plant and Animal Cells

When looking at photomicrographs (photos from microscopes), you can tell them apart using this checklist:

Animal Cells:
• Often irregular in shape.
• Have centrioles.
• No cell wall, no chloroplasts, no large vacuole.

Plant Cells:
• Usually a fixed, rectangular shape.
• Have a cellulose cell wall.
• Have chloroplasts and plasmodesmata.
• Have a large permanent vacuole.

4. Prokaryotic Cells: The Simple Survivors

Prokaryotes (like bacteria) are much simpler and smaller (usually 1–5 µm in diameter) than eukaryotic cells. Think of them as a "studio apartment" compared to the "mansion" of a eukaryotic cell.

Key Features of Bacteria:
• Unicellular: They are always single-celled.
• Peptidoglycan Cell Walls: Their wall is made of a different material than plant walls.
• Circular DNA: They don't have a nucleus; their DNA just floats in the middle in a loop.
• 70S Ribosomes: Smaller than the ones in our cytoplasm.
• No double membranes: They do not have a nucleus, mitochondria, or chloroplasts.

Memory Aid:

PROkaryotic = NO nucleus.
EUkaryotic = DO have a nucleus.

5. Energy for the Cell: ATP

All cells need energy for things like movement, building proteins, and active transport. The syllabus states you must know that:
Cells use ATP from respiration for energy-requiring processes.
Think of ATP as a rechargeable battery. Respiration "charges" it, and the cell "drains" it to get work done.

6. Viruses: The Non-Cellular Outsiders

Are viruses alive? In Biology (9700), we describe them as non-cellular structures. This means they are not cells.

Structure of a Virus:
1. Nucleic Acid Core: This is their genetic material, which can be either DNA or RNA.
2. Capsid: A protective coat made of protein.
3. Outer Envelope: Some viruses (like the flu) have an extra layer made of phospholipids stolen from their host cell.

Quick Review: Viruses are much smaller than bacteria and cannot do anything on their own. They have to hijack a living cell to make copies of themselves.

Final Summary Checklist

Before you move on, make sure you can:
• Identify all the eukaryotic organelles on a diagram.
• Explain why mitochondria and chloroplasts have circular DNA and 70S ribosomes.
• Spot the differences between a bacterial cell and an animal cell.
• Describe a virus as a protein coat (capsid) with a DNA/RNA core.

Don't worry if this seems like a lot of names to remember! Try drawing the cells yourself and labeling them. Biology is a very visual subject, and "seeing" the cell helps the facts stick in your mind.