Animal Cell Model: 3D Organelles, Microscope & Protein Labs

An organelle is a specialized structure inside a cell that performs a specific job, much like an organ does for the whole body. Most organelles in an animal cell are wrapped in their own membrane — the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes are examples. A few important structures, like ribosomes, are not membrane-bound.

The nucleus is the control center of the cell. It stores the cell's DNA, and the genes in that DNA direct which proteins the cell makes. A double membrane called the nuclear envelope surrounds it, and pores in that envelope let messenger RNA and other molecules pass in and out. Inside is the nucleolus, where ribosomes are assembled.

Mitochondria carry out aerobic cellular respiration: they use oxygen to release the energy stored in glucose and capture it in molecules of ATP, the cell's usable energy currency. Their inner membrane is folded into cristae that increase the surface area for the reactions of the electron transport chain. Cells that need lots of energy, like muscle cells, contain thousands of mitochondria.

The rough endoplasmic reticulum, studded with ribosomes, makes and folds proteins, while the smooth ER makes lipids. Both pinch off vesicles that carry their cargo to the Golgi apparatus. The Golgi then modifies, sorts, and packages those molecules and ships them in new vesicles to their destinations inside or outside the cell.

A lysosome is a membrane sac filled with acidic digestive (hydrolytic) enzymes. It breaks down worn-out organelles, food particles brought into the cell, and invading bacteria, then recycles the useful building blocks. Its membrane keeps those powerful enzymes safely separated from the rest of the cell.

Animal and plant cells share most organelles — nucleus, mitochondria, ER, Golgi, and ribosomes. Plant cells additionally have a rigid cell wall outside the membrane, chloroplasts for photosynthesis, and a large central vacuole. Animal cells lack all three, but they have centrioles and typically many small lysosomes.

Membranes let the cell create separate compartments, each with its own chemistry. The acidic interior of a lysosome, the energy-converting space inside a mitochondrion, and the protected DNA of the nucleus all depend on a membrane keeping conditions different from the surrounding cytosol. Membranes also vastly increase usable surface area for reactions, which is why the inner mitochondrial membrane and the ER are so folded.

Gently rub the inside of your cheek with a clean cotton bud, smear it on a slide and add a drop of methylene blue stain. Lower a coverslip slowly at an angle to avoid air bubbles. Start with the 4× objective and the coarse focus knob, then move to 10× and 40× and use only the fine focus. The cells look flat and irregular, and the stain shows each nucleus dark blue.

Water moves into the cell by osmosis because the inside of the cell is more concentrated than pure water. Animal cells have no cell wall to resist this, so the red blood cell swells into a sphere and bursts (hemolysis). In a very salty solution the opposite happens: water leaves, and the cell shrinks and crinkles. Blood plasma is about the same concentration as 0.9% salt, so cells keep their shape.

The gene is copied into mRNA in the nucleus. Ribosomes on the rough ER read it and build the protein, which is folded inside the ER. A vesicle carries it to the Golgi apparatus, which modifies and packs it into another vesicle. That vesicle fuses with the cell membrane and releases the protein outside (exocytosis). Mitochondria supply the ATP energy for these steps.

Most animal cells are about 10–30 micrometres across — around 50 typical cells in a row would make one millimetre. Some are much bigger (a large fat cell can be about 0.1 mm, just visible to the eye) and some nerve cells have extensions about a metre long. Because they are so small, you need a light microscope to see them and an electron microscope to see their organelles in detail.