General Science
Biology & the Human Body
Biology is the study of living things. For General Science, understand basic ideas behind cells, DNA and genetics, reproduction and cell division, microorganisms, plants, ecosystems, evolution and adaptation, and the major systems of the human body.
Section 1
Cells & Organelles
The cell is the basic unit of life. Every organism is made of one or more cells. Humans have trillions of cells that work together.
Key structures
The cell membrane surrounds a cell and controls what enters and leaves. Cytoplasm is fluid-like material where many reactions occur. The nucleus contains most DNA and helps control cell activities. Mitochondria release usable energy from food. Ribosomes build proteins.
Plant cells also have a cell wall for support and protection, chloroplasts that perform photosynthesis, and a large central vacuole that stores water, nutrients, and wastes. Animal cells do not have cell walls or chloroplasts.
Example 1
Which structure most directly controls what enters and leaves a cell? A. Nucleus | B. Cell membrane | C. Ribosome | D. Mitochondrion
Solution
- 1The cell membrane forms the boundary of the cell.
- 2It regulates movement into and out of the cell.
Why: The membrane acts as a selective barrier between the cell and its surroundings.
Example 2
A cell requires large amounts of energy. Which organelle would it likely contain in large numbers? A. Mitochondria | B. Cell walls | C. Chloroplasts | D. Vacuoles
Solution
- 1Mitochondria release usable energy from food through cellular respiration.
- 2Cells with high energy demands need more energy-producing organelles.
Why: High-energy cells generally contain many mitochondria.
Section 2
DNA, Genes, Chromosomes & Cell Division
DNA contains genetic instructions. A gene is a section of DNA related to a trait or function. Chromosomes are organized DNA structures that contain many genes.
Most human body cells have 46 chromosomes arranged in 23 pairs. Mitosis produces genetically similar body cells for growth and repair. Meiosis produces sperm and eggs with half the chromosome number: 23. Fertilization combines sperm and egg and restores 46 chromosomes.
Example 1
Which process produces new cells for growth and tissue repair? A. Photosynthesis | B. Meiosis | C. Mitosis | D. Digestion
Solution
- 1Growth and tissue repair need new body cells.
- 2Mitosis produces genetically similar body cells.
Why: Mitosis is the normal division process used for growth and replacement.
Example 2
Why do human sperm and egg cells normally contain only 23 chromosomes? A. They are missing DNA. | B. Their chromosome number is reduced during meiosis. | C. They are produced by mitosis. | D. Chromosomes disappear during reproduction.
Solution
- 1Meiosis produces reproductive cells with half the normal chromosome number.
- 2When sperm and egg combine, 23 + 23 = 46.
Why: Half the normal number prevents chromosome count from doubling every generation.
Section 3
Microorganisms, Bacteria & Viruses
Not all microorganisms are harmful. Bacteria are single-celled living organisms; some cause disease, while others help digestion, decompose material, or help produce foods.
Viruses are smaller than cells and contain genetic material inside a protein coat. They cannot reproduce on their own; they need a host cell. Bacteria can reproduce independently under suitable conditions. Antibiotics work against certain bacterial infections, not viruses.
Example 1
What is a major difference between bacteria and viruses? A. Bacteria require host cells, but viruses do not. | B. Viruses require host cells, while bacteria can reproduce independently. | C. Both are identical types of cells. | D. Viruses are always beneficial.
Solution
- 1Viruses rely on host cells for reproduction.
- 2Bacteria are living cells that can reproduce independently under suitable conditions.
Why: Host-cell dependence is a major difference between viruses and bacteria.
Example 2
Why are antibiotics generally not useful for viral infections? A. Viruses are too large. | B. Antibiotics target bacterial processes, not viruses. | C. Viruses contain no genetic material. | D. Antibiotics work only outside the body.
Solution
- 1Antibiotics interfere with bacterial structures or processes.
- 2Viruses work differently and reproduce inside host cells.
Why: Treatments that work against bacteria do not automatically work against viruses.
Section 4
Plants & Photosynthesis
Plants make food through photosynthesis. They use light energy, carbon dioxide, and water to produce glucose and oxygen.
Chlorophyll is the green pigment in chloroplasts that absorbs light energy. Roots anchor a plant and absorb water and minerals. Stems support the plant and transport materials. Xylem moves water and minerals mainly upward from roots; phloem moves sugars to different parts. Stomata are leaf openings for gas movement.
Example 1
Which substance is released by plants as a product of photosynthesis? A. Nitrogen | B. Oxygen | C. Calcium | D. Salt
Solution
- 1Photosynthesis uses carbon dioxide and water.
- 2It produces glucose and oxygen.
Why: Oxygen is released as one product of photosynthesis.
Example 2
Which plant tissue carries water from roots toward leaves? A. Phloem | B. Xylem | C. Chlorophyll | D. Stomata
Solution
- 1Xylem transports water and minerals from roots through the plant.
Why: Xylem is the plant's main water-transport tissue.
Section 5
Ecosystems, Food Chains & Adaptation
An ecosystem includes living organisms and their nonliving environment, such as water, sunlight, temperature, and soil.
Producers make their own food; plants are common producers. Consumers get energy by eating organisms. Decomposers, including many fungi and bacteria, break down dead material and return nutrients. Food chains show energy movement, such as grass → rabbit → fox; food webs connect many chains.
An adaptation is an inherited trait that helps survival or reproduction. Through natural selection, helpful inherited traits can become more common in a population over many generations.
Example 1
What is the main role of decomposers in an ecosystem? A. Produce sunlight | B. Break down dead material and recycle nutrients | C. Hunt all consumers | D. Prevent plants from growing
Solution
- 1Decomposers break down dead organisms and wastes.
- 2This returns nutrients to the environment.
Why: Decomposition recycles materials that other organisms can use.
Example 2
Green insects survive more often than brown insects on green plants because predators see them less easily. What is most likely over many generations? A. Green coloration may become more common. | B. All insects immediately become green. | C. Predators stop eating insects. | D. Every brown insect changes color during its lifetime.
Solution
- 1Green insects have a survival advantage and are more likely to reproduce.
- 2If the trait is inherited, it can become more common over generations.
Why: Natural selection favors inherited traits that improve survival or reproduction.
Section 6
Circulatory, Respiratory & Digestive Systems
Major body systems work together. The circulatory system transports materials, the respiratory system exchanges gases, and the digestive system breaks food into absorbable substances.
The heart pumps blood, which carries oxygen, nutrients, hormones, and wastes. Red blood cells carry oxygen, white blood cells help defend against infection, and platelets help blood clot. In the respiratory system, gas exchange occurs in tiny lung air sacs called alveoli: oxygen enters blood and carbon dioxide leaves it.
Most nutrient absorption occurs in the small intestine. The large intestine mainly absorbs water and helps form solid waste.
Example 1
Which blood component primarily carries oxygen? A. Platelets | B. Red blood cells | C. White blood cells | D. Plasma proteins only
Solution
- 1Red blood cells contain hemoglobin, which binds oxygen.
Why: Their primary role is transporting oxygen from lungs to body tissues.
Example 2
Where does most gas exchange between lungs and blood occur? A. Trachea | B. Alveoli | C. Esophagus | D. Large intestine
Solution
- 1Alveoli are tiny air sacs surrounded by blood vessels.
- 2Oxygen and carbon dioxide are exchanged across their thin walls.
Why: Their thin walls allow efficient movement of gases between air and blood.
Section 7
Nervous, Endocrine, Muscular & Skeletal Systems
These systems help the body detect information, coordinate responses, move, maintain structure, and regulate long-term processes.
The nervous system uses rapid electrical and chemical signals through the brain, spinal cord, and nerves. The endocrine system uses hormones, chemical messengers carried in blood, to regulate growth, metabolism, reproduction, blood sugar, and stress.
Muscles create movement, posture, and heat by pulling on bones. The skeleton supports the body, protects organs, helps movement, stores minerals, and contains marrow where blood cells are produced.
Example 1
Which system uses hormones as chemical messengers? A. Endocrine | B. Skeletal | C. Digestive | D. Respiratory
Solution
- 1Hormones are produced by endocrine glands and travel through the bloodstream.
Why: The endocrine system regulates many processes using hormones.
Example 2
What is one major function of the skeletal system? A. Producing oxygen | B. Supporting the body and protecting organs | C. Digesting food | D. Producing all hormones
Solution
- 1Bones provide structural support and protect organs such as the brain, heart, and lungs.
Why: The skeleton provides the body's framework and protects internal structures.
Section 8
Immune System, Skin & Reproduction
The immune system protects against harmful organisms and substances. Skin provides a first line of defense. The reproductive system produces sperm and eggs.
White blood cells are important immune cells. Antibodies are proteins that recognize and bind specific foreign substances called antigens. Vaccines prepare the immune system to respond more quickly if it later encounters a specific pathogen.
Skin blocks pathogens, prevents excess water loss, regulates temperature, and detects sensations. When sperm and egg combine during fertilization, they form a zygote. Hormones help control many reproductive processes.
Example 1
What is one important function of skin? A. Carry oxygen through blood | B. Act as a barrier against pathogens | C. Produce sperm cells | D. Digest proteins
Solution
- 1Skin creates a physical barrier between internal tissues and the environment.
Why: Skin is part of the body's first line of defense against infection.
Example 2
What is a major purpose of vaccination? A. Immediately destroy every pathogen in the environment | B. Prepare the immune system to respond to a specific pathogen | C. Replace red blood cells | D. Stop all cell division
Solution
- 1Vaccines help the immune system recognize a pathogen or antigen.
- 2This enables a faster and stronger response later.
Why: Vaccination develops immune memory rather than acting as a physical barrier.
Quick Review
Quick Review
- Cell membrane controls entry and exit; nucleus contains DNA; mitochondria release energy; ribosomes build proteins; chloroplasts perform photosynthesis.
- Genes are sections of DNA; chromosomes contain many genes; mitosis supports growth and repair; meiosis produces reproductive cells.
- Bacteria are living single cells; viruses require host cells; antibiotics target bacteria, not viruses.
- Heart pumps blood; alveoli exchange gases; small intestine absorbs nutrients; nervous system sends rapid signals; endocrine system uses hormones.
The Big Picture
- Biology questions often connect structure to function: mitochondria → energy, ribosome → protein, red blood cell → oxygen transport, platelet → clotting, alveoli → gas exchange, small intestine → nutrient absorption, xylem → water transport, chloroplast → photosynthesis.