Every living thing is built from cells

Word	What it means
Cell	The smallest unit that can carry out all the processes of life.
Organism / single-celled / multicellular	A living thing / made of one cell, like an amoeba / made of many cells, each kind with its own job, like a poplar tree or you.
Cell membrane / cell wall	The thin, flexible layer around every cell, controlling what goes in and out / a stiff layer outside a plant cell's membrane.
Nucleus / cytoplasm	The cell's control centre, holding its genetic instructions / the jelly-like fluid filling the cell.
Mitochondria / chloroplasts / vacuole	Release energy from food / in plant cells, make food by photosynthesis / a sac storing water, food and wastes.
Magnification / field of view	How many times larger an image looks than the object / the circle you see through a microscope.
Micrometre (µm)	A thousandth of a millimetre.
Diffusion / osmosis	Particles spreading from where they are more concentrated to where they are less / the diffusion of water across a membrane.
Selectively permeable	Letting some substances through but not others, as a cell membrane does.
Tissue / organ / organ system	A group of similar cells doing one job / several tissues working together as one structure / several organs working together on one major job.
Stimulus / response	A change inside or around the body that it detects / what the body does about it.

The cell theory. In 1665 Robert Hooke looked at a thin slice of cork through an early microscope and named the tiny boxes he saw cells. By the mid-1800s, the evidence supported three statements:

1. All living things are made of one or more cells.
2. The cell is the basic unit of life.
3. Every cell comes from another cell.

One cell or many. An amoeba is one cell that does everything itself: it moves, wraps around its food, and splits in two to reproduce. In a grizzly bear each kind of cell has one job — nerve cells carry signals, muscle cells contract, red blood cells carry oxygen — and none could survive on its own.

total magnification = eyepiece × objective lens  ·  10× × 40× = 400×

Worked: how big is a cell? At 40×, a clear ruler shows the field of view is 4.5 mm across. At 400×, ten times the magnification, it is ten times narrower: 4.5 ÷ 10 = 0.45 mm, or 450 µm. About 5 cells fit end to end across it, so each is about 450 ÷ 5 = 90 µm long.

Using a microscope. Focus on low power first, then switch up and use only the fine focus, so the lens never hits the slide. The image is upside down and reversed: move the slide left and the cells seem to move right.

Part	Its job	Plant	Animal
Cell membrane	Controls what enters and leaves	Yes	Yes
Nucleus	Controls the cell; holds its genetic instructions	Yes	Yes
Cytoplasm	Fills the cell; many reactions happen in it	Yes	Yes
Mitochondria	Release energy from food	Yes	Yes
Cell wall	Stiff support outside the membrane	Yes	No
Chloroplasts	Make food by photosynthesis	In green parts	No
Vacuole	Stores water, food and wastes	One large one	Small ones, if any

Structure fits function. A plant has no skeleton. Its stiff cell walls, with full vacuoles pressing out against them, hold it up. When a plant goes without water, its vacuoles shrink and it wilts. An animal cell has no wall, so it can change shape: a white blood cell squeezes between other cells to reach an infection.

Worked: plant or animal? Cells from an onion bulb look boxy, with thick edges and a clear space filling most of each one, but no green anywhere. The walls and the large vacuole say plant. There are no chloroplasts because the bulb grows underground, in the dark, where it cannot make food. Missing chloroplasts do not make a cell an animal cell.

Diffusion. Open a bottle of perfume in one corner and soon the whole room smells of it. Its particles are always moving, so they spread from where they are more concentrated to where they are less, with nothing pushing them. In the lungs, oxygen is more concentrated in the air than in the blood arriving there, so it diffuses into the blood; carbon dioxide diffuses the other way.

Osmosis. A cell membrane is selectively permeable: water passes through it easily, but many dissolved substances cannot. So water moves across by itself, from the side with fewer dissolved particles to the side with more.

Worked: potato slices. Two potato slices each start at 10.0 g. One sits in fresh water for an hour, the other in strong salt water.

fresh water: 10.0 g → 10.6 g, a gain of 0.6 g  ·  salt water: 10.0 g → 9.1 g, a loss of 0.9 g

Fresh water has fewer dissolved particles than the potato's cells, so water moved in: the slice gained mass and went stiff. Strong salt water has more, so water moved out and the slice went limp. It was the water moving, not the salt, that changed the mass.

Everyday osmosis. Limp celery stiffens in a glass of water. Road salt can kill roadside grass by drawing water out of its roots.

cells → tissues → organs → organ systems → organism

The stomach, for example, is an organ built of muscle, nerve and other tissues.

System	Main organs	Job
Respiratory	Nose, trachea, bronchi, lungs	Takes in oxygen; gets rid of carbon dioxide
Circulatory	Heart, arteries, veins, capillaries, blood	Carries oxygen, food and wastes to and from every cell
Digestive	Mouth, esophagus, stomach, small and large intestines	Breaks food into particles small enough to enter the blood
Excretory	Kidneys and bladder; lungs and skin help	Removes wastes; the kidneys filter the blood to make urine
Nervous	Brain, spinal cord, nerves, sense organs	Detects changes and controls the body's responses

Worked: one breath, four systems. Oxygen diffuses from tiny air sacs in the lungs into the blood, and the heart pumps it to a leg muscle. There the muscle cells' mitochondria use it to release energy from food the digestive system supplied. Carbon dioxide returns in the veins and is breathed out, and the nervous system sets the pace.

Worked: responding to exercise. A student counts her pulse for 15 seconds and multiplies by 4:

resting: 18 × 4 = 72 beats/min  ·  after 2 minutes of jumping jacks: 33 × 4 = 132  ·  after 3 minutes' rest: 21 × 4 = 84

Exercise is the stimulus; a faster heart is the response. Working muscles use oxygen faster, so the nervous system speeds up the heart and breathing, then slows them once the need has passed. Shivering makes heat when you are cold; sweat cools you as it evaporates. Touch a hot pan and your hand jerks away before you feel the pain: in a reflex, the signal goes to the spinal cord and straight back to the muscles, without waiting for the brain.

Research and health. A vaccine trains the body's defences using a harmless form or part of a germ; vaccination wiped out smallpox, and the world was declared free of it in 1980. Insulin, which keeps people with type 1 diabetes alive, was first used in Toronto in 1922, purified by James Collip of the University of Alberta. Smoking damages the tiny hairs that sweep dirt out of the airways, and wildfire smoke carries fine particles deep into the lungs.

• Saying plant cells have chloroplasts instead of mitochondria. They have both.
• Mixing up the cell wall and the cell membrane. Every cell has a membrane; plant cells also have a wall outside it.
• Mixing up the levels. Similar cells make a tissue, tissues make an organ, organs make a system.
• Getting osmosis backwards. Water moves toward the side with more dissolved particles.
• Adding the lens powers. A 10× eyepiece and a 40× objective give 400×, not 50×.
• Treating each body system as separate. Oxygen reaches a muscle only if the respiratory and circulatory systems work together.