Living things eat, grow, get rid of waste products and reproduce. All living things are
made of cells. In even the tiniest unit of any living thing, there is a cell. Cells have special
structures called organelles. The organelles help cells do the work of moving materials
around, dividing to make more cells and making proteins for the body’s needs.
Cells get energy through a process called cellular respiration. During this process,
cells convert sugar (called glucose) and oxygen into water and carbon dioxide. Carbon
dioxide is the gas we breathe out. This whole process releases energy for the cell to use.
The energy is stored as ATP. The cell keeps ATP in storage, like “back up power.” It can be
taken out to be used as needed. By storing ATP, the cell always has the energy it needs.
Living things can have just one cell or many. Single-celled organisms include things
like bacteria, yeast, and some types of algae. They do the same things that living things do.
However, they must do it all within just one cell. Multi-cellular organisms have billions of cells
that work together to provide for the organism’s needs.
Plant and animal cells both have organelles. Some types of organelles are the same in
both plant and animal cells. Other types of organelles, however, are only found in plant cells
or animal cells.
All cells have a control center called a nucleus. The nucleus stores a special molecule
called DNA. The organism’s traits are controlled by the coding found in its DNA.
All cells have a cell membrane that surrounds the cell to protect it and control what goes
in or out. Materials can move through the membrane by diffusion or osmosis. Diffusion
is when materials move in or out of a cell from a place of high concentration to one of low
concentration. Osmosis is a special kind of diffusion that allows water to pass through the
membrane. However, in osmosis, many other materials are not allow to pass through. Plant
cells have an extra layer called a cell wall that surrounds each cell’s membrane. The cell
wall is much stiffer to help the plant’s stems stand up and support leaves and fl owers.
Cytoplasm is a thick gelatin-like fl uid that fi lls the space between a cell’s nucleus and
its cell membrane. Organelles fl oat in and are supported by the cytoplasm. Ribosomes are
organelles that make proteins. Lysosomes, which are found mostly in animal cells, break
apart nutrients. The Golgi apparatus (GOAL-gee ap-a-RAT-us) prepares proteins be sent
to various parts of the body. Vacuoles are like bags of fl uid that cells use to store things
until they are needed or until they can be disposed of. Mitochondria generate energy for the
cell. The endoplasmic reticulum, or ER, is a system of tubes and passages for transporting
materials. Chloroplasts, which are found only in plants, allow food to be made using sunlight
and carbon dioxide. All the organelles work together to make sure that the cells, and
ultimately the living organism, can do all the things that are necessary for survival.