Life Science: GED Test Prep
Life science questions on the GED Science Exam cover the topics studied in high school biology classes. In this article, you will review the basics of biology and learn the answers to some of the key questions scientists ask about the nature of life and living beings.
Life science explores the nature of living things, from the smallest building blocks of life to the larger principles that unify all living beings. Fundamental questions of life science include:
- What constitutes life?
- What are its building blocks and requirements?
- How are the characteristics of life passed on from generation to generation?
- How did life and different forms of life evolve?
- How do organisms depend on their environment and on one another?
- What kinds of behavior are common to living organisms?
Before Anthony van Leeuwenhoek looked through his homemade microscope more than 300 years ago, people didn't know that there were cells in our bodies and that there were microorganisms. Another common misconception was that fleas, ants, and other pests came from dust or wheat. Leeuwenhoek saw blood cells in blood, microorganisms in ponds, and showed that pests come from larvae that hatch from eggs laid by adult pests. However, it took more than 200 years for Leeuwenhoek's observations to gain wide acceptance and find application in medicine.
Today we know that a cell is the building block of life. Every living organism is composed of one or more cells. All cells come from other cells. Cells are alive. If blood cells, for example, are removed from the body, given the right conditions, they can continue to live independently of the body. They are made up of organized parts, perform chemical reactions, obtain energy from their surroundings, respond to their environments, change over time, reproduce, and share an evolutionary history.
All cells contain a membrane, cytoplasm, and genetic material. More complex cells also contain cell organelles. Here is a description of cell components and the functions they serve. Also, refer to the figures.
- The cell wall is made of cellulose, which surrounds, protects, and supports plant cells. Animal cells do not have a cell wall.
- The plasma membrane is the outer membrane of the cell. It carefully regulates the transport of materials in and out of the cell and defines the cell's boundaries. Membranes have selective permeability—meaning that they allow the passage of certain molecules, but not others. A membrane is like a border crossing. Molecules need the molecular equivalent of a valid passport and a visa to get through.
- The nucleus is a spherical structure, often found near the center of a cell. It is surrounded by a nuclear membrane and it contains genetic information inscribed along one or more molecules of DNA. The DNA acts as a library of information and a set of instructions for making new cells and cell components. In order to reproduce, every cell must be able to copy its genes to future generations. This is done by exact duplication of the DNA.
- Cytoplasm is a fluid found within the cell membrane, but outside of the nucleus.
- Ribosomes are the sites of protein synthesis. They are essential in cell maintenance and cell reproduction.
- Mitochondria are the powerhouses of the cell. They are the site of cellular respiration (breakdown of chemical bonds to obtain energy) and production of ATP (a molecule that provides energy for many essential processes in all organisms). Cells that use a lot of energy, such as the cells of a human heart, have a large number of mitochondria. Mitochondria are unusual because unlike other cell organelles, they contain their own DNA and make some of their own proteins.
- The endoplastic reticulum is a series of interconnecting membranes associated with the storage, synthesis, and transport of proteins and other materials within the cell.
- The Golgi complex is a series of small sacs that synthesizes, packages, and secretes cellular products to the plasma membrane. Its function is directing the transport of material within the cell and exporting material out of the cell.
- Lysosomes contain enzymes that help with intracellular digestion. Lysosomes have a large presence in cells that actively engage in phagocytosis—the process by which cells consume large particles of food. White blood cells that often engulf and digest bacteria and cellular debris are abundant in lysosomes.
- Vacuoles are found mainly in plants. They participate in digestion and the maintenance of water balance in the cell.
- Centrioles are cylindrical structures found in the cytoplasm of animal cells. They participate in cell division.
- Chloroplasts exist in the cells of plant leaves and in algae. They contain the green pigment chlorophyll and are the site of photosynthesis—the process of using sunlight to make high energy sugar molecules. Ultimately, the food supply of most organisms depends on photosynthesis carried out by plants in the chloroplasts.
- The nucleolus is located inside the nucleus. It is involved in the synthesis of ribosomes, which manufacture proteins.
In a multicellular organism, individual cells specialize in different tasks. For example, red blood cells carry oxygen, white blood cells fight pathogens, and cells in plant leaves collect the energy from sunlight. This cellular organization enables an organism to lose and replace individual cells, and outlive the cells that it is composed of. For example, you can lose dead skin cells and give blood and still go on living. This differentiation or division of labor in multicellular organisms is accomplished by expression of different genes.