Animal cell
Before you start
Every living thing we know is made of cells, and a human body contains tens of trillions of them. A muscle cell, a nerve cell and a cell lining the stomach look very different, yet they share the same basic parts. Every cell must solve the same problems: keep its inside separate from the outside, store its DNA, make proteins and release energy. DNA is the cell’s written list of which proteins to make. Later, organ pages such as the heart and the stomach show what cells achieve together, and the cell division page shows how one cell becomes two. Both start from the parts we meet here; you do not need those pages first.
A diagram full of names can make it feel as though everything must be remembered at once. Follow just one protein being prepared for release. Proteins are molecules that cells make and use; this one needs instructions, processing and a route out. The nucleus holds the information, ribosomes attached to the endoplasmic reticulum make the protein, and the endoplasmic reticulum and Golgi apparatus help process and transport it. We keep its route in the diagram. If a structure becomes hard to recognise, return to its part in this piece of work before returning to its name.
The setupThis is an enlarged animal cell, cut open so that we can see inside. Each colour marks one kind of structure. Click a structure in the model, or choose its name from the list, to read what it does; Look closer moves the view in to its inner detail. The cutaway switch closes the cell again so that you can see its outer surface, and Show the secretion process follows one protein on its way out of the cell.
What to do and noticeBegin at the boundary: select the cell membrane, then the nucleus, and find the two membranes of the nuclear envelope and their small pores. Next, select the rough endoplasmic reticulum and the Golgi apparatus and use Look closer on each. Only then play the secretion process, and keep your eye on the gold cargo: it is the same protein from start to finish, carried inside small membrane sacs between the structures you have just located.
Predict firstWhat information does a cell need before making a protein?
Nucleus
A cell needs to make proteins, including enzymes that help reactions take place. A protein contains smaller units called amino acids joined in a particular order. DNA stores information for this order; a gene is a section of DNA information used to make a product with a job to do. Let us follow a gene for one protein. The cell makes a messenger RNA copy, which a ribosome uses to put amino acids in order. Most DNA stays in the nucleus while the RNA leaves through a nuclear pore.
Find the inner and outer membranes first, then their small pores. The threads inside represent chromatin, DNA associated with proteins; they are not part of the envelope.
Follow one set of protein instructions
The cell model shows where structures are. This separate construction shows what “DNA information is used to make protein” means. DNA stores the instructions, and the working copy is made of messenger RNA. Advance one step at a time; nothing moves on by itself.
Each numbered square stands for the information specifying one amino-acid unit, and each circle stands for the unit itself. An amino acid is one small bead in the chain of a protein. Matching colours let us follow the order from the information to the chain. These are instruction markers, not pictures of DNA or RNA molecules. Distances and the number of units are chosen to make each step visible.
Follow the same protein from its instructions to its release outside the cell, then look at how energy is supplied for these activities.
Prerequisites: A cell is a basic unit that carries out life processes. Its plasma membrane separates inside from outside. Organelles are internal structures with particular jobs; we will introduce their names as we follow those jobs.Find the boundary and the instructions
Predict firstWhat information does a cell need before making a protein?
Open the cutaway, select the nucleus and focus on it. Its inner and outer membranes have pores through which some molecules can pass. Inside, DNA is a long molecule storing information. A protein contains smaller units called amino acids joined in a particular order; the relevant section of DNA holds information for making it. The cell first makes a messenger RNA copy of that information, and the RNA leaves through a pore. The copy leaves; the DNA has not become protein and does not have to leave the nucleus.
How to observe
- Rotate the cross-section and select an organelle to focus on
- Look at the internal structure of the endoplasmic reticulum, the Golgi apparatus and the mitochondrion
- Follow a vesicle and watch the secreted protein reach the outside of the cell
What this model simplifies
A structural sketch of a generic animal cell: colours, numbers, distances and timings are all simplified for teaching and are not microscope measurements of any particular cell. The transport route illustrates the process and is not the pathway of every protein.
