Mitosis

The M phase of the cell cycle is the process known as mitosis. Some scientists classify cytokinesis as separate from mitosis, but still in the M phase. Mitosis is split into 4 distinct phases plus cytokinesis.

Prophase is the longest phase of mitosis. Chromatin condenses to form chromosomes, centrioles move to the poles, spindles form, the chromosomes attach to the spindle fibers at the centromere and the nuclear envelope breaks down.

Metaphase is usually brief. Chromosomes line up across the center of the cell and microtubules connect the centromere of each chromosome to the two spindles at the poles.

Anaphase is the next phase. The spindles split the sister chromatids and when they reach the poles and stop moving, anaphase is over.

Telophase if the final phase. The distinct, condensed chromosomes disperse, the nuclear envelope reforms around the chromosomes, and the spindles break apart.

Cytokinesis is the last step of cell division. This involves the division of cytoplasm and the cell membrane pinches shut, separating the two daughter cells.

This Krispy Kreme illustration is actually a great example of what this looks like:

Click through to see it full size.

Diffusion

After discussing the cell, it would only be fitting to talk about how the cell gets nutrients in and waste out.  The cell is surrounded by the cell membrane, which acts as the gate for everything that needs to pass into and out of the cell.

Particles will move freely across the cell membrane until equilibrium is reached.  Equilibrium means that the concentration (amount) of a substance is the same on both sides.  When the concentration of a substance is greater on the outside of the cell membrane, the particles will move across the membrane from the higher side to the lower side until both sides are equal.  That process, where particles move from an area of higher concentration to an area of lower concentration,  is called diffusion.  In the case of water, diffusion gets a special name: osmosis.  Osmosis is the diffusion of water.

Diffusion and osmosis occur naturally with the random movement of particles, so they cost the cell no energy.

Some particles are too big to properly move through the cell membrane, so the particles are helped across the membrane.  Facilitated diffusion is when particles move across a cell membrane through a protein channel.  Because the particles are still moving with the concentration, not against it, facilitated concentration costs the cell no energy.  However, sometimes the opposite of facilitated diffusion occurs.  Active transport is the process where a cell uses energy to move particles from a lower concentration to a higher concentration.  Again proteins are used to make this process work, but instead of protein channels, they’re called protein pumps.

Below you’ll find an animated gif that shows the process of diffusion of 2 blue particles, which we’ll call water, and the facilitated diffusion of 1 green particle, which we’ll call glucose.  The animation was originally done in Flash, but I couldn’t host a SWF or even a movie file for free, so I had to convert it to a GIF.  Maybe if these videos catch on I’ll do the YouTube thing, but for now I hope this works.

If you have problems, I’ve uploaded the video to Facebook.

The Cell

The fundamental unit of life is the cell.  Without at least 1 cell, an organism can’t live.  Even you and I started out as a single cell at one point.  So I present another artistic masterpiece, an animal cell I drew just for you.

As you see the cell is labeled an Animal Cell.  There are many types of cells and this is what a generic animal cell looks like.  Muscle cells and nerve cells will have a different look, but will have the same parts.  Likewise, cells of plants are similar, but not identical to animal cells.  And bacteria cells are even more different!  But since we’re animals, we’ll start with the animal cell.

You can think of the cell like a little city.  Every part inside of the cell has a purpose; it’s there to do a job.   The parts that make up a cell are called organelles.  Here are some of the important cell parts and their jobs:

Cell membrane – like the walls of a castle, the cell membrane allows in things that are needed (like food) and allows out things that aren’t needed (like waste).  The cell membrane is the cell’s first line of defense to keep unwanted intruders out (like viruses!).

Cytoplasm – sort of like jelly, the cytoplasm is the cell’s internal environment and keeps the organelles from bumping into each other.

Endoplasmic reticulum (E.R.) – specific jobs vary from one cell type to another, but ultimately helps with transport of proteins, much like a conveyor belt.

Golgi apparatus – modifies and transports proteins from the E.R., as well as creation of lysosomes and transport of lipids.  The Golgi apparatus is like a factory, it builds and ships.

Lysosome – these sacs are full of enzymes for digesting food that come into the cell.

Mitochondrion – like power plants, the mitochondria turn sugar into energy that’s usable by the cell.  Your house uses electricity, the cell uses a molecule called ATP.

Nuclear envelope – is like a second wall around the nucleus.  Just like the King has a second set of guards closer to him, the nuclear envelope is even more selective than the cell membrane, and not much gets through.  There are holes in the nuclear envelope and usually only nucleic acids and some proteins are allowed in and out.

Nucleolus – primarily makes ribosomes.  That’s all you need to know!

Nucleus – the mayor’s office of the cell.  Everything that happens in the cell is ultimately controlled by the nucleus and the DNA inside.   The lack of a nucleus is a easy way to tell that the cell you’re looking at is a prokaryote.

Ribosome – take amino acids and make proteins.  It’s like a little machine that runs nonstop until it reaches the end of its job!  There’s a neat little animation on Wikipedia.