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Plant cells have a cell wall in addition to a cell membrane, whereas animal cells have only a cell membrane. Plants use cell walls to provide structure to the plant. Plant cells contain organelles called chloroplasts, while animal cells do not. Chloroplasts allow plants to make the food they need to live using photosynthesis.

Introduction to magnetism: Wikipedia article that provides an overview of the concept.

The force that acts across the air gaps between magnets is the same force that creates wonders such as the Aurora Borealis. In fact, magnetic effects pervade our lives in myriad ways, from electric motors to medical imaging and computer memory. In this chapter, we introduce magnets and learn how they work and how magnetic fields and electric currents interact.

Overview of animal and plant cells. Topics include cell walls, vacuoles, chloroplasts, peroxisomes, lysosomes, mitochondria, etc.

This online resource introduces the geocentric and heliocentric models. Exoplants, planets and their motions, planetary orbits, the role of gravity and how the solar system was formed are also covered. 

This fascinating video details the journey that a group of friends made in Nevada. Their aim was to illustrate the solar system to scale.