AFCT General Science: Life and Biological 2 — Questions and Answers

Question 1: What is the primary function of mitochondria in a cell?

Protein synthesis
Cell division
Producing ATP (energy) through cellular respiration (Correct answer)
Storing genetic information

Correct answer: Producing ATP (energy) through cellular respiration

Mitochondria are known as the 'powerhouses of the cell' because they produce adenosine triphosphate (ATP) through cellular respiration, converting the chemical energy in glucose into a form cells can use.

Mitochondria are double-membrane organelles that perform aerobic cellular respiration. The inner membrane is highly folded into cristae, increasing surface area for the electron transport chain. The process begins when pyruvate (from glycolysis in the cytoplasm) enters the mitochondrial matrix for the Krebs cycle, which produces NADH and FADH2. These electron carriers donate electrons to the electron transport chain on the inner membrane, which pumps hydrogen ions to create a concentration gradient. ATP synthase uses this gradient to produce ATP through oxidative phosphorylation. One glucose molecule can yield approximately 36-38 ATP molecules. Mitochondria have their own DNA (mtDNA, inherited maternally) and ribosomes, supporting the endosymbiotic theory that they were once free-living bacteria engulfed by ancestral eukaryotic cells.

Question 2: What is the process by which plants convert sunlight into chemical energy?

Cellular respiration
Fermentation
Photosynthesis (Correct answer)
Transpiration

Correct answer: Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria use sunlight energy to convert carbon dioxide and water into glucose and oxygen, using chlorophyll in chloroplasts.

Photosynthesis occurs in chloroplasts and consists of two stages. The light-dependent reactions take place in the thylakoid membranes, where chlorophyll absorbs light energy to split water molecules (photolysis), releasing oxygen as a byproduct and producing ATP and NADPH. The light-independent reactions (Calvin cycle) occur in the stroma, where CO2 is fixed into glucose using the ATP and NADPH from the light reactions. The overall equation is: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2. Chlorophyll primarily absorbs red and blue light, reflecting green (which is why plants appear green). Photosynthesis is the foundation of most food chains and is responsible for the oxygen in Earth's atmosphere. C3, C4, and CAM plants have evolved different strategies to optimize photosynthesis under varying environmental conditions.

Question 3: What molecule carries genetic instructions for the development and function of living organisms?

RNA
ATP
DNA (Correct answer)
Protein

Correct answer: DNA

DNA (deoxyribonucleic acid) is the molecule that carries the genetic instructions used in growth, development, functioning, and reproduction of all known living organisms and many viruses.

DNA is a double-stranded helical molecule composed of nucleotides, each containing a phosphate group, deoxyribose sugar, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C). Base pairing rules dictate that A pairs with T and G pairs with C, held together by hydrogen bonds. The sequence of these bases encodes genetic information in units called genes. In eukaryotes, DNA is organized into chromosomes within the nucleus. Humans have approximately 3 billion base pairs across 23 pairs of chromosomes, containing about 20,000-25,000 protein-coding genes. DNA replication ensures each daughter cell receives an exact copy during cell division. The genetic code is transcribed into mRNA and then translated into proteins by ribosomes. Watson and Crick described the double helix structure in 1953, building on X-ray crystallography data from Rosalind Franklin.

Question 4: What is the function of white blood cells in the human body?

Carrying oxygen to tissues
Clotting blood at wound sites
Defending the body against infections and foreign invaders (Correct answer)
Transporting nutrients through the bloodstream

Correct answer: Defending the body against infections and foreign invaders

White blood cells (leukocytes) are key components of the immune system. They identify, attack, and destroy pathogens such as bacteria, viruses, and other foreign substances that enter the body.

White blood cells (leukocytes) comprise several types with specialized immune functions. Neutrophils (60-70% of WBCs) are the first responders to bacterial infections, engulfing and destroying pathogens through phagocytosis. Lymphocytes include B cells (produce antibodies for humoral immunity), T cells (cell-mediated immunity, including helper T cells that coordinate immune response and killer T cells that destroy infected cells), and natural killer cells. Monocytes differentiate into macrophages in tissues, engulfing large particles and presenting antigens. Eosinophils combat parasites and moderate allergic reactions. Basophils release histamine and heparin in inflammatory responses. Normal WBC count is 4,500-11,000 per microliter of blood. Elevated counts (leukocytosis) often indicate infection, while low counts (leukopenia) increase infection susceptibility.

Question 5: What is natural selection?

The random mutation of genes in a population
A process where organisms with traits better suited to their environment survive and reproduce more successfully (Correct answer)
The deliberate breeding of organisms for desired traits
The migration of species to new environments

Correct answer: A process where organisms with traits better suited to their environment survive and reproduce more successfully

Natural selection, proposed by Charles Darwin, is the mechanism of evolution where individuals with heritable traits better adapted to their environment have higher survival and reproductive rates, passing those advantageous traits to offspring.

Natural selection operates through four principles: variation (individuals in a population differ in their traits), inheritance (many of these traits are heritable), differential reproduction (individuals with advantageous traits produce more offspring), and accumulation (favorable traits become more common over generations). For natural selection to occur, there must be variation, that variation must be heritable, and it must affect fitness (survival and reproduction). Natural selection does not create new traits; it acts on existing genetic variation produced by mutations, recombination, and gene flow. Types include directional selection (favoring one extreme), stabilizing selection (favoring average), and disruptive selection (favoring both extremes). Natural selection is not random, though the mutations it acts upon are. Combined with genetic drift, gene flow, and mutation, it drives evolution.

Question 6: What is the role of the cell membrane?

To produce energy for the cell
To control what enters and exits the cell (Correct answer)
To store the cell's DNA
To synthesize proteins

Correct answer: To control what enters and exits the cell

The cell membrane (plasma membrane) is a selectively permeable barrier that controls the passage of substances into and out of the cell, maintaining the cell's internal environment.

The cell membrane is a phospholipid bilayer with embedded proteins, described by the fluid mosaic model. Phospholipids have hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward, creating a selectively permeable barrier. Small nonpolar molecules (O2, CO2) and water pass through freely. Larger or charged molecules require transport proteins. Passive transport (no energy required) includes diffusion, osmosis, and facilitated diffusion through channel or carrier proteins. Active transport uses ATP to move substances against their concentration gradient via protein pumps (e.g., sodium-potassium pump). Endocytosis engulfs large particles, and exocytosis expels them. Membrane proteins also serve as receptors (receiving chemical signals), enzymes, and cell identification markers (glycoproteins). Cholesterol in the membrane maintains fluidity across temperature changes.

AFCT General Science: Life and Biological 2

What is the primary function of mitochondria in a cell?