Cells and Microscopes (intro)

Every living thing — a tree, a fish, your dog, you — is built from tiny units called cells. A cell is so small that you cannot see it with your eyes alone. We need a tool called a microscope to make it look bigger. Once we look closely we discover that each cell is a busy little factory, with its own parts that each do a different job. Cells are the basic building blocks of life.

What is a Cell?

A cell is the smallest part of a living thing that can still be alive on its own. Some living things, like bacteria, are made of just ONE cell. Bigger living things like humans are made of TRILLIONS of cells working together. Cells take in food, get rid of waste, grow, and make copies of themselves — exactly like the whole organism does, just at a tiny scale.

The Main Parts of a Cell (Organelles)

• Cell membrane — a thin skin around the cell that controls what goes in and out.
• Nucleus — the control center. It holds the DNA and tells the rest of the cell what to do.
• Mitochondria — tiny power stations. They release energy from food so the cell can work.
• Ribosomes — small dots that build proteins, the body's tools and building blocks.
• Cytoplasm — a jelly-like fluid that fills the cell and holds the organelles in place.
• Vacuole — a storage bag for water, food, or waste (huge in plant cells).

Plant Cells vs Animal Cells

Plant cells and animal cells share most of the same organelles, but plant cells have two extra parts. They have a strong CELL WALL outside the membrane that gives the cell a fixed shape, and they have green CHLOROPLASTS that catch sunlight to make food in a process called photosynthesis. Animal cells have neither — they have only the soft membrane and they get their food by eating other living things.

• Both have: cell membrane, nucleus, mitochondria, ribosomes, cytoplasm, vacuole.
• Plant only: cell wall (rigid outer frame) + chloroplasts (make food from sunlight).
• Animal cells are usually round and flexible; plant cells are usually box-shaped.

The Microscope

A microscope is a tool that uses lenses and light to make tiny things look much bigger. The thing you want to look at goes onto a glass SLIDE. The slide sits on the STAGE. A LIGHT under the stage shines up through the slide. You look down through the EYEPIECE at the top, and below the eyepiece an OBJECTIVE LENS does most of the magnifying. A FOCUS KNOB on the side moves the lens up and down so the picture becomes sharp.

• Eyepiece — where you put your eye.
• Objective lens — the lens close to the slide that does most of the magnifying.
• Stage — the flat platform that holds the slide.
• Slide — a thin glass plate that carries the sample.
• Light source — shines up through the slide so you can see it.
• Focus knob — turns to make the image sharp.