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We begin this unit with a discussion of what it means to be alive.  This is a tricky question.  Seeing a dog chasing a car, we all know that the dog is alive and the car is not.  By why is the car considered to be nonliving?  Recent discoveries of organisms living in places we never thought life could exist make us reconsider what it means to be alive.  So, rather than a definition, biologists prefer to describe the properties of living things.

Life Functions - Things that organisms must do in order to (1.) be considered alive, and (2.) to continue living.

  1. Nutrition - Organisms must take in nutrients (food, water, etc.)
  2. Transport - Organisms must have a way to move materials around inside the body, or in and out of the body.
  3. Respiration - Organisms must be able to break down nutrients for energy.
  4. Excretion - Organisms must be able to get rid of wastes.
  5. Sense & Respond - Organisms must be able to interact with (sense and respond to) their environment.
  6. Growth - Organisms must be able to grow by getting bigger and/or more developed.
  7. Regulation - Organisms must have a way to control and coordinate all the things that happen within their bodies.
  8. Reproduction* - Organisms must have the ability to produce offspring.  

*Note: Individuals don't have to reproduce to be alive, but there must be some reproduction within the species or they will die out.

In medieval times, it was common for people to believe that living things could come from nonliving or dead material.  This idea is called spontaneous generation, and was based on casual observations of the world around them.  For example, in the springtime people would notice frogs crawling up out of the mud in the bottom of a pond.  Their simple explanation was that the frogs developed right from the mud.

Experiments by Francesco Redi (1688), Lazzaro Spallanzani (1768), and Louis Pasteur (1864) eventually disproved the idea of spontaneous generation, which was replaced with the idea of Biogenesis.  This idea states that living things come from living things of the same type.

In our continuing effort to define "life", we find that there are some nonliving things, such as fire, that seem to carry out all the life functions.  So, we next consider the Cell Theory.

The Cell Theory

  1. All living things are made up of cells.
  2. All cells come from pre-existing cells.
  3. Cells are the smallest unit that perform all of the life functions.

Of course, any study of cells requires the use of microscopes.  Check out the "Microscopy" note for information. 

We must also consider the overall organization of living things:
  • Cell - The smallest unit of life that performs all the life functions.
  • Tissue - A group of cells that share the same structure and function.
  • Organ - A group of tissues that work together to perform a specific task within the body.
  • Organ System - A group of organs that work together to carry out a life function.
  • Organism - A complete living thing.

Cell Parts                                                                                                A Tour of the Cell     The Cell Song
Some basic information on organelles (specialized parts of cells that carry out specific functions, like mini-organs).  For more information, see the online videos.
  • Cell Wall – Gives structure and support.  (In plant cells only.)
  • Cell Membrane – Separates the cell from its environment.  Controls what goes in and out of the cell.
  • Cytoplasm – The fluid inside the cell.
  • Nucleus – Controls everything that happens inside the cell.  Contains DNA.
  • Nuclear Membrane – Separates the nucleus from the rest of the cell.  Controls what goes in and out of the nucleus.
  • Nucleolus – Makes ribosomes.
  • Endoplasmic Reticulum (ER) – Transports materials within the cell.
    • Rough ER – Has ribosomes on it.
    • Smooth ER – Does not have ribosomes.
  • Ribosomes – Make proteins.
  • Mitochondrion – The “powerhouse” of the cell.  Breaks down nutrients for energy.
  • Chloroplast – Has chlorophyll that the plant uses for photosynthesis (to make food from sunlight).
  • Cytoskeleton – Gives the cell structure and support, and helps moves things around inside the cell.
  • Centriole – Helps the cell divide.  (In animal cells only.)
  • Golgi Body – Packages materials to be sent out of the cell.
  • Vacuole – Stores food, water and wastes.  In plants, uses water pressure to provide support.  (If you don’t water your plants, they wilt.
  • Lysosome – A type of vacuole that stores digestive enzymes to break things down.

Cell Transport                                                                                        Cell Membrane     Transport Across Cell Membranes
Cells have to be able to move things in and out of themselves.  There are two basic types of cell transport: Active and Passive.

1. Passive transport is where materials flow in and out of the cell without using the cell's energy.  The material moves down the concentration gradient (downhill).

  • Diffusion is the process of particles moving from an area of high concentration to an area of low concentration.  In other words, particles like to spread out.
  • Osmosis is the diffusion of water across a membrane.  In other words, it is water flowing into or out of a cell.
  • Facilitated Diffusion is diffusion that needs the help of a protein to move substances across the cell membrane.
    • Channel Proteins act like tunnels for specific substances.
    • Carrier Proteins act like gates for specific substances.

2. Active transport is when the cell has to use its own energy to move materials into and out of itself.  Usually, material is moved against the concentration gradient (uphill).
  • Pumps, like the Sodium-Potassium Pump, are proteins that use the cell's energy (in the form of an ATP molecule) to carry materials into and out of the cell.
  • Endocytosis is when the cell engulfs a particle (or several particles), creating a vessicle from its cell membrane.
  • Exocytosis is when the cell fuses a vessicle with its cell membrane, dumping the vessicle's contents outside of the cell.

The Cell Cycle                                                                 Mitosis

Interphase - All the time the cell is performing its normal functions.  (The cell's regular day-to-day life.)

  • G1 Phase - Growth 1 - The cell grows.
  • S Phase - Synthesis - The cell replicates its chromosomes.
  • G2 Phase - Growth 2 - The cell continues growing.
Mitosis - The cell prepares to divide into two new cells.
  • Prophase
    • The nucleus disappears.
    • The centrioles migrate to the poles.
    • Spindle fibers form.
  • Metaphase
    • The chromosomes line up at the equator of the cell.
  • Anaphase
    • Spindle fibers shorten.
    • Chromosomes are pulled apart.
  • Telophase
    • Nuclei re-form.
    • The cell membrane pinches in.
Cytokinesis - The cell divides into two new daughter cells.

Fun Stuff & Animations

Online Lessons 

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