Page 2 Biological and Biochemical Foundations of Living Systems Study Guide for the MCAT

Processes to Understand

Several processes are critical for biology. These processes include important biological principles for understanding how chemical reactions occur, how genetic information is stored and transmitted, and how molecules are made and utilized by cells.

Enzyme Activity

Enzymes are proteins that catalyze biological reactions by reducing the activation energy. Enzymatic activity is highly regulated and specific because enzymes work on particular substrates. Many enzymes require cofactors such as metal ions or vitamins to function. By interacting with cofactors, the enzyme structure can change; the 3D structure of enzymes is important for their activity and for recognizing their substrate.

Genetic Transmission of Information

The central dogma is that DNA is transcribed into RNA and RNA is translated into proteins. DNA stores the genetic information; RNA acts as a messenger; proteins make all the biomolecules needed for viability. RNA is made using one strand of DNA as a template. Single stranded messenger RNA is then translated into protein based on its sequence using transfer RNAs (codon and anticodon usage).

Genetic Diversity

Genetic diversity is an advantage of sexual reproduction. Organisms that reproduce sexually can inherit different alleles, or genes variants, encoded on chromosomes. Because each parent provides one copy of each chromosome to an offspring, each offspring is a mix of their parents DNA and can be homozygous or heterozygous. Genes encoded on the same chromosome are linked and inherited together, unless the chromosome has undergone recombination. Different genotypes result in different phenotypes because alleles can be dominant or recessive.


Living organisms break down carbohydrates and fatty acids to create energy, in the form of ATP, to fuel the processes needed for life. ATP hydrolysis is coupled with other chemical reactions to make them thermodynamically favorable. For this section, it is important to understand equilibrium constants, free reactions, exothermic reactions, endothermic reactions, and oxidation-reduction reactions.

Fuel Molecule Catabolism

Fuel molecules can include carbohydrates, fatty acids, and proteins. It is important to understand how fuel molecules are generated and utilized as an energy source. Important processes to understand include gluconeogenesis, glycolysis, fermentation, pentose phosphate pathway, glycogen synthesis and breakdown, and the citric acid cycle. You should know the substrates, endproducts, and the regulation of each of these major biochemical processes.

Cell Functions (include differentiation and specialization)

All cells in the human body contain the same genomic DNA, yet cells can differentiate and perform specialized functions. Stem cells are the precursors for all the different types of cells. Through differential expression of genes, cells produce proteins that are are needed for their particular function. For example, skin cells are different from heart muscle cells because skin cells express skin-specific genes that result in skin-specific proteins.

The Cell Cycle

Cells divide using a process called the cell cycle, which consists of the following phases: G0, G1, S, G2, and M. During interphase (G1, S, and G2), cells grow and acquire the nutrients needed for cell division. DNA replication occurs in the S phase. During the M phase, mitosis and cytokinesis occur. The mitotic phase is divided into prophase (chromatin condenses and the nuclear envelope dissolves), metaphase (sister chromatids line up in the center of the cell), anaphase (sister chromatids separate and move to opposite poles), telophase (nuclear envelope reforms), and cytokinesis (cell divide completely). The cell pauses at many cell cycle stages (checkpoints) to repair DNA and ensure that all the cell components are arranged correctly for cell division. If a cell cannot repair itself, it will undergo apoptosis to prevent abnormal cells from proliferating.

Cancer and Its Properties

Cancer occurs when control of the cell cycle is lost. This can occur when oncogenes are overexpressed or duplicated. Alternatively, loss of or mutations to tumor suppressor genes can also result in cancer.

Other Body Systems

The human body has different organ systems working together to maintain homeostasis. These organ systems must communicate with each other to respond effectively to changes in the environment. For this section, it is important to know all the organs in each system and how they contribute to the system’s function.

Endocrine System

The endocrine system consists of glands (pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus, and adrenal glands) that generate hormones, which are secreted chemical signals that internally regulate the body’s responses. Hormones can act on distant tissues or nearby cells. The structure of hormones is variable and can include peptides, steroids, neuroamines, or prostaglandins.

Nervous System

The nervous system controls voluntary and involuntary responses. Humans have central (brain and spinal cord) and peripheral nervous systems. Nerve cells or neurons are the major component of the nervous system. Neurons emit and receive electrical and chemical signals. Motor or efferent neurons send signals from the central nervous system to the body. Afferent or sensory nerves of the body send information to the brain. Sympathetic (flight or fight responses) and parasympathetic (housekeeping functions) are two types of responses.

Interaction of the Nervous and Endocrine Systems

The nervous and endocrine systems interact to regulate metabolism, reproduction, development, growth, behavior, and other important functions. Important examples of these interactions include neurons controlling the release of hormones and hormones altering neuronal responses because neurons express hormone receptors.

Respiratory System

The respiratory system is important for the intake or oxygen (inhaling) and removal of carbon dioxide (exhaling). In the lungs, oxygen is exchanged for carbon dioxide by diffusion in the alveoli. Additionally, the lungs contain mucus and hair cells that protect the host by trapping bacteria and particulates.

Digestive System

Ingestion of food is the start of the digestive process because saliva contains enzymes that start to break down food particles. In the stomach, food is further degraded in the acidic environment by additional enzymes. Food particles are absorbed and the pH neutralized in the small intestine. Water is absorbed from the lumen in both the small and large intestines. Undigested material is excreted through the rectum. The liver and pancreas make bile and enzymes, respectively, that are important for digesting food properly. The enteric nervous system regulates the muscles that control the movement of the food through the gastrointestinal tract (peristalsis); hormones also regulate the digestive system.

Lymphatic System

Lymphatic organs (thymus, spleen, and bone marrow), lymphatic vessels, and lymph are major components of the lymphatic system. Development of B and T cells occurs in the lymphatic organs. The lymphatic system is where antibody production occurs. The lymphatics absorb interstitial fluid, which contains proteins such as antibodies and antigens and returns this mixture to the circulatory system.

Excretory System

The excretory system eliminates waste and harmful materials from the body, normally in the form of urine and sweat. This system is important for maintaining blood pressure, salt levels, and pH. The kidneys filter blood using nephrons, which remove the impurities as urine and keep essential compounds such as water. The liver detoxifies harmful compounds and produces bile.

Reproductive System

Humans reproduce sexually, so the male and female reproductive systems are quite different. The female reproductive system includes the vagina, cervix, uterus, fallopian tubes, and ovaries. The male reproductive system includes the scrotum, testes, spermatic ducts, sex glands, and penis. Ova and sperm are produced through gametogenesis by females and males, respectively. The ovum and sperm fuse (fertilization) in the uterus and then the zygote is implanted. The embryo continues to gestate until birth approximately 9 months after fertilization.

Muscle System

The muscle system is important for movement, circulation, and thermogenesis (shivering). Striated, cardiac, and smooth muscles are the three types of muscles. Motor neurons stimulate muscles, which results in sarcomere shortening using myosin and actin, ATP consumption, and calcium release from the sarcoplasmic reticulum.

Skeletal System

The skeletal system provides structure and protects important organs. It also is the site of hematopoiesis and calcium storage. Two types of skeletons include endoskeletons and exoskeletons. Bones, cartilage, ligaments, and tendons make up the skeletal system.

Skin System

The skin system comprises skin, nails, and hair. The skin serves as a barrier to protect from disease and abrasions. Skin is also important for retaining water and regulating temperature (sweating and subcutaneous fat). Different layers of skin include the dermis and epidermis.

Circulatory System

The circulatory system functions to disperse oxygen and other important biomolecules such as hormones throughout the body and to remove waste and carbon dioxide. It is also important for thermoregulation. The heart pumps oxygenated blood from the lungs using arteries, capillaries, and veins carry the deoxygenated blood back to the lungs. Blood consists of red blood cells, white blood cells, plasma, and platelets.

Immune System

The immune system protects the body from diseases by distinguishing between self and foreign agents. Humans have innate and adaptive immune responses. Important adaptive cells for antibody production include T-lymphocytes (present antigen) and B-lymphocytes (produce antibodies). Innate phagocytic macrophages can present antigen to B-cells and bridge innate and adaptive responses. Immune responses are highly regulated and failures can result in cancer or autoimmune diseases.

Helpful Vocabulary

As you study, be sure you are familiar with these terms, as well. For drill and practice, you can make flashcards with the definition on one side and the term on the other. Doing so will further enable you to score well on this test.

gene, exons, introns, Krebs cycle, oxidative phosphorylation, lytic infection, lysogenic infection, DNA, RNA, ATP, ADP, diploid, haploid, resting potential, action potential, fluid mosaic cell membrane, Myelin sheath, Schwann cells, Nodes of Ranvier, operon, Michaelis–Menten, zymogen, allosteric enzymes, competitive inhibition, non-competitive inhibition, uncompetitive inhibition, gene penetrance, gene co-dominance, delta (this delta should be the Greek symbol-but this isn’t an option for Google docs)G, ketone bodies, gel electrophoresis, Southern blot, Northern blot, Western blot, nucleotide, nucleoside, nucleic acid, carbohydrate, protein, amino acid, lipid, steroid, hormone, fatty acid, phospholipid, glycolysis, gluconeogenesis, restriction enzymes, polymerase, rRNA, tRNA, mRNA, plasmid, autosomal chromosome, sex chromosome, organelle, plasma membrane, nucleus, Golgi apparatus, endoplasmic reticulum, capsid, genome, meiosis, mitosis, free energy, telomere, centromere