Amazing Cells

INSIDE A CELL

Nucleus

The nucleus safely stores the cell’s DNA blueprint. It also separates the cell’s DNA from the activity of the cytoplasm, keeping it safe from interference. Proteins work together to copy segments of DNA into RNA during transcription.

Cytoskeleton

The cytoskeleton functions as a system of roads for the transport of cargo by motor proteins. It also acts as a springy skeleton that gives the cell it’s shape as well as organize the events of cell division.

Endoplasmic Reticulum

The ER makes up roughly half the membrane in a cell. So there’s a lot of surface area to perform chemical reactions and store important enzymes in the smooth ER. Freshly made proteins and fats are packed into vesicles and sent from the ER to the Golgi apparatus.

Mitochondria

Mitochondria have their own set of unique proteins embedded in their membrane folds enabling them to participate in a variety of cellular activities. Mitochondria are responsible for generating the cell’s energy. Using sugar and oxygen, protein complexes in the inner membrane manufacture energy molecules that are used throughout the cell.

Golgi Apparatus

Vesicles filled with proteins and other macromolecules fuse with the Golgi apparatus. Here these molecules are tagged with specific labels that transport proteins will use to deliver them to the proper place in the cell.

Lysosomes and Other Specialized Vesicles

When proteins have reached the end of their usefulness, they are transported to the lysosome to be degraded. Specialized motor proteins carry tethered proteins along microtubules to their destination.

DIRECTING TRAFFIC: HOW VESICLES TRANSPORT CARGO

Generally, molecules, such as proteins, are too big in order to pass through membranes. Instead, large molecules are loaded into vesicles. Vesicles are constantly forming, usually at the plasma membrane, the ER, and the Golgi. Once formed, vesicles deliver their contents to locations within or outside of the cell. A vesicle forms when the membrane bulges out and pinches off. It travels to its destination then merges with another membrane to release its cargo. In this way proteins and other large molecules are transported without having to cross a membrane. When the coat proteins assemble at the membrane, they force the lipid bilayer to begin to blend. As they gather at the membrane, coat proteins, may also select the cargo that is packaged into the forming vesicle. As more oat proteins are added, they shape the surrounding molecule into a sphere. Once a coated vesicle pinches off, the coats falls off, and the cargo-filled vesicle is ready to travel to its’ destination.

THE EVOLUTION OF THE CELL

Some of the oldest cells on Earth are single-cell organisms called bacteria. Fossil records indicate that mounds of bacteria once covered young Earth. Some began making their own food using carbon dioxide in the atmosphere and energy they harvested from the sun. This process (called photosynthesis) produced enough oxygen to change Earth's atmosphere. Soon afterward, new oxygen-breathing lifeforms came onto the scene. With a population of increasingly diverse bacterial life, the stage was set for some amazing things to happen. Mitochondria and chloroplasts have striking similarities to bacteria cells. They have their own DNA, which is separate from the DNA found in the nucleus of the cell. And both organelles use their DNA to produce many proteins and enzymes required for their function. A double membrane surrounds both mitochondria and chloroplasts, further evidence that each was ingested by a primitive host. The two organelles also reproduce like bacteria, replicating their own DNA and directing their own division. Mitochondrial DNA (mtDNA) has a unique pattern of inheritance. It is passed down directly from mother to child, and it accumulates changes much more slowly than other types of DNA. Because of its unique characteristics, mtDNA has provided important clues about evolutionary history. For example, differences in mtDNA are examined to estimate how closely related one species is to another.

CELL SIZE AND SCALE

The smallest objects that the unaided human eye can see are about 0.1 mm long. That means that under the right conditions, you might be able to see an amoeba proteus, a human egg, and a paramecium without using magnification. A magnifying glass can help you to see them more clearly, but they will still look tiny. Smaller cells are easily visible under a light microscope. It's even possible to make out structures within the cell, such as the nucleus, mitochondria and chloroplasts. Light microscopes use a system of lenses to magnify an image. The power of a light microscope is limited by the wavelength of visible light, which is about 500 nm. The most powerful light microscopes can resolve bacteria but not viruses. To see anything smaller than 500 nm, you will need an electron microscope. Electron microscopes shoot a high-voltage beam of electrons onto or through an object, which deflects and absorbs some of the electrons. Resolution is still limited by the wavelength of the electron beam, but this wavelength is much smaller than that of visible light. The most powerful electron microscopes can resolve molecules and even individual atoms.

MEMBRANES ORGANIZE CELLULAR COMPLEXITY

Membranes organize proteins and other molecules enabling the cell to run much more efficiently than if everything were floating freely. Mitochondrial membranes, for example, keep protein assembly lines together for efficient energy production. And the lysosome safely holds enzymes that would destroy essential proteins if released into the cytoplasm. Membrane-enclosed vesicles form packages for cargo so that they may quickly and efficiently reach their destinations. In this way, membranes divide the cell into specialized compartments, each carrying out a specific function inside the cell. Phospholipids provide the framework for all membranes in the cell. Phospholipids are made up of a phosphate head region and a lipid tail region. The two ends of a phospholipid have very different chemical properties. The head end is attracted to water, while the tail end moves away from water. Phospholipids with their embedded proteins form a dynamic, fluid environment. Individual proteins and phospholipids flow freely. Complexes of proteins and specific subtypes of phospholipids form "rafts" that move through the membrane. Organelles stretch and bend and even flow through the cell. Fluid membranes allow cells to be dynamic and responsive to their environment.