All galaxies have a dense core of stars. Spiral galaxies, however, have arms that are often visible. There are two kinds of spiral galaxies: barred, where there appears to be a line running from arm to arm through the core, and <span>ordinary, where there is no bar. @Badbunnyhunny</span>
Answer:
4. Membrane and proteins secreted by the cell
Explanation:
Endoplasmic reticulum (ER) is one of the organelle that makes up the endomembrane system of eukaryotes. The ER is of two types viz: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).
The RER is called "rough" because of the ribosome organelles that are attached to its surface. The attachment of this ribosomes (site of protein synthesis) also defines its function. The RER is principally involved in the production of protein. Hence, the Rough Endoplasmic Reticulum functions mainly in the production of membrane and proteins secreted by the cell.
Answer:
The theory of evolution by natural selection, first formulated in Darwin's book "On the Origin of Species" in 1859, is the process by which organisms change over time as a result of changes in heritable physical or behavioral traits. Changes that allow an organism to better adapt to its environment will help it survive and have more offspring.
Evolution by natural selection is one of the best substantiated theories in the history of science, supported by evidence from a wide variety of scientific disciplines, including paleontology, geology, genetics and developmental biology.
The theory has two main points, said Brian Richmond, curator of human origins at the American Museum of Natural History in New York City. "All life on Earth is connected and related to each other," and this diversity of life is a product of "modifications of populations by natural selection, where some traits were favored in and environment over others," he said.
More simply put, the theory can be described as "descent with modification," said Briana Pobiner, an anthropologist and educator at the Smithsonian Institution National Museum of Natural History in Washington, D.C., who specializes in the study of human origins.
The theory is sometimes described as "survival of the fittest," but that can be misleading, Pobiner said. Here, "fitness" refers not to an organism's strength or athletic ability, but rather the ability to survive and reproduce.
