Cell Biology lessons
What is a cell?
A cube-shaped cell grows from 1 to 100 µm while oxygen diffuses in from its membrane.
A cell is the smallest unit of life. A bacterium is a single cell, and your body is built from about 30 trillion of them.
Cells are measured in micrometres (µm), thousandths of a millimetre. Most animal cells are 10 to 30 µm across, too small to see without a microscope.
Everything a cell needs comes in through its surface. As a cell grows, its volume outgrows its surface, so large living things are built from many small cells.
Practice: Surface area to volume
Work out the surface area, volume or surface area to volume ratio of a cube-shaped cell.
Cell theory and cell sizes
In 1665, Robert Hooke looked at cork through a microscope and named the tiny boxes he saw cells. By the 1850s, scientists had worked out cell theory.
It says every living thing is made of cells, and the cell is the basic unit of life. New cells only come from cells that already exist, by dividing.
Prokaryotic cells, like bacteria, have no nucleus and are usually 1 to 5 µm long. Eukaryotic cells, like yours, keep their DNA in a nucleus and are often ten times bigger.
Cells and their parts on a size scale from 10 nm to 1 mm, with what you need to see each one.
Practice: Order by size
Put cells and their parts in size order when the sizes are given in different units.
Inside an animal cell
Inside the membrane is the cytoplasm, a watery jelly where most of the cell's chemical reactions happen. In it sit organelles, small parts that each do a job.
The nucleus holds the DNA, the instructions for the whole cell. The mitochondria release energy from glucose in respiration.
The cell membrane controls what goes in and out. Lysosomes are sacs of enzymes that break down worn-out parts and invading bacteria.
A 3D animal cell, cut open, touring the membrane, nucleus, mitochondria and lysosomes.
Practice: Find the organelle
Tap the organelle on a cell diagram from its name or its job.
How a cell makes proteins
Ribosomes are tiny machines that build proteins, following instructions copied from the DNA. Some float in the cytoplasm, and many stud the rough endoplasmic reticulum (rough ER).
Proteins made on the rough ER pass inside it and fold into shape. Small bubbles of membrane called vesicles then carry them to the Golgi apparatus.
The Golgi finishes, sorts and packs the proteins. New vesicles take them where they are needed, and some leave the cell through the membrane.
A 3D tour that follows a protein from the nucleus to a ribosome, the rough ER, the Golgi and a vesicle.
Practice: Match each organelle to its job
Match each organelle to the job it does.
The cell membrane
The membrane is a double layer of phospholipids, called a bilayer. Each phospholipid has a head that is attracted to water and two tails that avoid it.
The heads face the water inside and outside the cell, and the tails hide in the middle. The whole layer is less than 10 nm thick.
Small molecules like oxygen slip straight through. Glucose and charged ions need protein channels and carriers, and big proteins cannot cross, so the membrane is partially permeable.
Molecules meet the phospholipid bilayer: oxygen slips through, glucose and ions use proteins, a big protein bounces off.
Diffusion and osmosis
Diffusion is the net movement of particles from where they are more concentrated to where they are less concentrated. It needs no energy from the cell, because the particles move on their own.
A bigger difference in concentration makes diffusion faster. So do warmth, a larger surface and a shorter distance.
Osmosis is the diffusion of water across a partially permeable membrane. Water moves from a dilute solution into a more concentrated one.
Particles wander across a membrane: dissolved particles spread out, then water moves towards the sugar it cannot reach.
Active transport
Active transport moves particles against their concentration gradient, from where they are scarce to where they are crowded. Carrier proteins in the membrane do the pumping.
This takes energy, which comes from ATP made in respiration. Cells that do a lot of active transport are packed with mitochondria.
Root hair cells use it to take in mineral ions from dilute soil water. Your gut uses it to absorb glucose into the blood.
A carrier protein in a root hair cell uses ATP to pump mineral ions in from the soil, against the gradient.
Practice: Which kind of transport?
Sort examples and facts into diffusion, osmosis and active transport.
Respiration in mitochondria
Aerobic respiration releases energy from glucose using oxygen. It goes on in every living cell, all the time, and most of it happens in the mitochondria.
Word equation: \text{glucose} + \text{oxygen} \to \text{carbon dioxide} + \text{water}
Balanced symbol equation: \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \to 6\text{CO}_2 + 6\text{H}_2\text{O}
A balanced equation has the same atoms on both sides, just rearranged. Here each side has 6 carbon, 12 hydrogen and 18 oxygen atoms.
The energy released is stored in a small molecule called ATP. Each glucose gives about 30 ATP, which power everything the cell does.
The atoms of one glucose and six oxygen molecules regroup into carbon dioxide and water, with live atom counts.
Practice: Balance the respiration equation
Set the numbers in front of each molecule until the atoms on both sides match.
The nucleus and chromosomes
The nucleus holds DNA, a very long molecule that carries the cell's instructions. Stretched out, the DNA in one of your cells would be about 2 metres long.
To fit, it winds around proteins called histones, then coils again and again. Each packed length of DNA is a chromosome.
Your body cells have 46 chromosomes, in 23 pairs. One chromosome of each pair came from each parent.
DNA packed level by level, from the double helix to a chromosome, with the width at each level.
The cell cycle
Cells make new cells by dividing in two. The cell cycle is the series of stages a cell goes through between one division and the next.
Most of the cycle is interphase. The cell grows, makes more organelles and copies every chromosome.
Then mitosis shares the copies between two new nuclei, and the cell splits. The two new cells are genetically identical.
A 24-hour cell cycle going round, with the DNA copied during interphase and the split in mitosis.
Mitosis
In prophase, the copied chromosomes coil up and become visible, and the nucleus breaks down. In metaphase, they line up across the middle of the cell.
In anaphase, fibres pull the two copies of each chromosome to opposite ends. In telophase, a new nucleus forms around each set.
Last, in cytokinesis, the cell pinches in two. Mitosis is how your body grows and repairs damage.
A cell with four chromosomes stepping through prophase, metaphase, anaphase, telophase and cytokinesis.
Practice: Name the stage
Name the stage of mitosis from a picture of the cell.
Plant, animal and bacterial cells
Plant cells have the same parts as animal cells, plus three more. A cell wall made of cellulose surrounds the membrane and keeps the cell's shape.
Chloroplasts absorb light for photosynthesis, though root cells have none. A large permanent vacuole full of cell sap keeps the cell firm.
Bacteria have a cell wall too, but no nucleus, mitochondria or chloroplasts. Their DNA floats in the cytoplasm as one big loop.
An animal, a plant and a bacterial cell side by side, lighting up the parts each one has.
Practice: Which cells have it?
Tick which cell types have each part in a comparison table.
Photosynthesis
Plants make their own food by photosynthesis. In the chloroplasts, a green pigment called chlorophyll absorbs light energy.
That energy turns carbon dioxide and water into glucose, and oxygen is given off. It is respiration in reverse, with the same molecules in the same numbers.
Balanced symbol equation (needs light): 6\text{CO}_2 + 6\text{H}_2\text{O} \to \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2
Plants respire all the time, day and night. In bright light they photosynthesise faster than they respire, so leaves give out oxygen.
A leaf through a day: as the light changes, photosynthesis and respiration trade places and the gases flip direction.
Practice: Build the equations
Drag the molecules into the word equations for photosynthesis and respiration.
Specialised cells
As a living thing develops, its cells become specialised, with shapes and parts suited to one job. This is called differentiation.
A red blood cell has no nucleus, which leaves room for more haemoglobin to carry oxygen. A nerve cell is long and thin to carry signals a long way.
A sperm cell has a tail to swim and is packed with mitochondria to power it. A root hair cell has a long thin hair that gives it a large surface to absorb water.
Six specialised cells, each with the feature that suits it to its job picked out.
Stem cells
Stem cells are not yet specialised. They can divide to make more stem cells, or differentiate into specialised cells.
Embryonic stem cells can become almost any type of cell. Adult stem cells, like those in bone marrow, make only a few types, such as blood cells.
Bone marrow transplants already use stem cells to treat blood cancers. Stem cell treatments for type 1 diabetes and Parkinson's disease are being tested in clinical trials.
A stem cell dividing: one daughter stays a stem cell while the other specialises, for embryonic and bone marrow stem cells.
Practice: Specialised and stem cells
Match each specialised or stem cell to the feature that suits its job.