Cell Theory and Cell Structure (Video)

0:08 Seeing cells for the first time

First, we will see how cells were first observed. Nucleic acids and proteins carry out their work inside cells. The cell itself, the structure that holds those molecules, is the subject of cell theory. In 1665 Robert Hooke examined thin slices of cork with a compound microscope. He saw small box-like compartments and named them cells, and he published the observations in Micrographia. A compound microscope uses two or more lenses in series. If you look at a thin slice of cork under a compound microscope, you see the compartments Hooke drew. In the 1670s Antonie van Leeuwenhoek built single-lens microscopes that magnified more than 200 times. He described living single-celled organisms, including bacteria, and called them animalcules.

1:10 The cell theory

Second, we will state the cell theory. The next step was to ask whether all living things are made of cells. In 1838 Matthias Schleiden proposed that all plants are made of cells. In 1839 Theodor Schwann extended the idea to animals. In 1855 Rudolf Virchow stated that cells arise only from pre-existing cells. He popularised the Latin form of the statement, omnis cellula e cellula. These proposals give the three statements of the classical cell theory. All living organisms are made of one or more cells. The cell is the basic unit of structure and function in living things. All cells arise from pre-existing cells.

1:54 Prokaryotic cells

Third, we will look at prokaryotic cells. With the cell established as the basic unit, the next question is how cells differ. Cells come in two basic types, prokaryotic and eukaryotic. The difference is whether a membrane encloses the DNA in a nucleus. Bacteria and archaea are prokaryotes. Their DNA is usually a single circular chromosome in the nucleoid, a region of the cytoplasm that no membrane encloses. A typical bacterial cell is about 1 to 5 micrometers long. A micrometer is one thousandth of a millimeter. The cell has a plasma membrane, a cell wall, and cytoplasm with 70S ribosomes. Bacteria often carry plasmids, small circular DNA molecules that replicate separately from the chromosome, and many have flagella and pili.

2:47 Eukaryotic cells and the nucleus

The fourth part covers eukaryotic cells and the nucleus. Eukaryotic cells are the second type, and they occur in animals, plants, fungi and protists. They are larger, typically about 10 to 100 micrometers across, and they contain membrane-bound organelles. An organelle is a structure inside the cell with a specialized function. The nucleus holds the linear chromosomes inside a double membrane, the nuclear envelope. Nuclear pores pass through the envelope. The nucleolus inside the nucleus assembles ribosomal subunits. Proteins made for membranes or for export pass through two further organelles. The rough endoplasmic reticulum carries ribosomes and makes proteins for membranes and for secretion. The smooth endoplasmic reticulum makes lipids. The Golgi apparatus modifies, sorts and packages proteins into vesicles.

3:45 Energy organelles and the cytoskeleton

Fifth, we will look at the energy organelles and the cytoskeleton. The energy for this work comes mostly from mitochondria, which carry out aerobic respiration and make most of the cell's ATP. ATP is the molecule that supplies energy for many reactions in the cell. Mitochondria have a double membrane, and the inner membrane folds into cristae. Cristae increase the surface area of the inner membrane. Chloroplasts in plant and algal cells carry out photosynthesis. They also have a double membrane, and inside they hold stacks of thylakoids, which are flattened membrane sacs. Mitochondria and chloroplasts contain their own circular DNA and 70S ribosomes. This is evidence for the endosymbiotic theory, which holds that they descend from engulfed bacteria. Other structures handle digestion and support. Lysosomes contain digestive enzymes. The cytoskeleton, made of microfilaments, intermediate filaments and microtubules, gives the cell its shape and moves materials.

4:48 Plant and animal cells compared

The last part compares plant and animal cells. These organelles also let you tell a plant cell from an animal cell. Plant cells have a cellulose cell wall, chloroplasts and a large central vacuole. A vacuole is a fluid-filled compartment bounded by a membrane. Animal cells lack all three structures but have centrioles. For a worked comparison, take a bacterial cell 2 micrometers long and an animal cell 20 micrometers across. The animal cell is ten times longer. Volume scales with the cube of length, so ten cubed gives about a thousand times the volume. Before the question, here is what this video covered. Microscope observations from Hooke onwards led to the cell theory, which states that living organisms consist of cells and that cells arise only from pre-existing cells. Prokaryotic cells have no nucleus, and their DNA lies in the nucleoid. Eukaryotic cells are larger, and they keep their DNA in a nucleus and contain membrane-bound organelles such as mitochondria and the Golgi apparatus. Here is a quiz question for you. Which features of mitochondria and chloroplasts support the endosymbiotic theory? Pause the video and work it out. The answer follows. Mitochondria and chloroplasts carry their own circular DNA and bacterial-type 70S ribosomes, they have two membranes, and they divide by binary fission, as bacteria do.