organelle A tiny cell structure that carries out a specific function or job within the cell.

Cell wall A rigid layer of nonliving material that surrounds the cells of plants and some other organisms.

Cell membrane The outside layer of the cell; controls which substances enter or leave a cell.

The control center of the cell that directs the cell’s activities. Nucleus The control center of the cell that directs the cell’s activities.

Material in cells that contains DNA and carries genetic information. Chromatin Material in cells that contains DNA and carries genetic information.

Cytoplasm The thick gel-like material between the cell membrane and the nucleus of a cell.

Mitochondrion Rod-shaped cell structures that produce most of the energy needed to carry out the cell’s functions.

Endoplasmic Reticulum A cell structure that forms a maze of passageways in which proteins and other materials are carried from one part of the cell to another.

A tiny structure in the cytoplasm of a cell where proteins are made. Ribosome A tiny structure in the cytoplasm of a cell where proteins are made.

Golgi Body A structure in a cell that receives proteins and other newly formed materials from the endoplasmic reticulum, packages them, and distributes them to other parts of the cell.

Chloroplast A structure in the cells of plants and some other organisms that captures energy from sunlight and uses it to produce food.

A water-filled sac inside a cell that acts as a storage area. Vacuole A water-filled sac inside a cell that acts as a storage area.

Lysosome A small round cell structure that contains chemicals that break down large food particles into smaller ones.

An organism whose cells lack a nucleus and some other cell structures. Prokaryote An organism whose cells lack a nucleus and some other cell structures.

An organism with cells that contain nuclei and other cell structures. Eukaryote An organism with cells that contain nuclei and other cell structures.

The Origin of Life Scientists think that early Earth had a different atmosphere than it has today. 3.6 billion years ago the main gases were nitrogen, water vapor, carbon dioxide, and methane. There was no oxygen. Today the main gas in the atmosphere is nitrogen. The first life forms probably lived in the oceans. They were unicellular and did not need oxygen.

Urey and Miller These two scientists designed an experiment in which they recreated the conditions of early Earth. They placed water (to represent the oceans), and a mixture of gases (nitrogen, methane, carbon dioxide) into a flask. They sent an electric current (to represent lightning) into the flask. Within a week a dark fluid appeared containing small chemicals that could join to form proteins.

Fossils of bacteria-like organisms have been found. The first cells were probably heterotrophs that used chemicals in their surroundings for energy. Early ancestors of today’s autotrophs had an important effect on the atmosphere. As they made their own food, they produced oxygen as a waste product.

Essay question: Compare and contrast plant, animal, and bacterial cells. Plant, animal and bacterial cells are all living things. They have a cell membrane, ribosomes,cytoplasm, and genetic material. The differences are that plant and animal cells are eukaryotes containing the genetic material in the nucleus; bacterial cells are prokaryotes because their genetic material is not enclosed in a nucleus. Plant cells and bacterial cells both have a cell wall. Plant cells, unlike animal and bacterial cells, have chloroplasts. The plant cell vacuole is also larger than in an animal cell. Bacterial cells are often much smaller than plant and animal cells.