Biology Review for MCAT

Introduction to Cellular Biology and Organisms

  • Definitions:
      - Sensory: Afferent (carries signals to the central nervous system).
      - Motor: Efferent (carries signals from the central nervous system).

Structure of Cells

Prokaryotic Cells

  • Nucleoid:
      - Definition: The region within a prokaryotic cell where DNA is located.
      - Key Point: Prokaryotes do not have a nucleus; they have nucleoids.
      - Function: Contains the majority of the cell's genetic material.

Eukaryotic Cells

  • Nucleus:
      - Definition: Membrane-bound structure that contains the cell's DNA in eukaryotic cells.
      - Nucleolus:
        - Definition: A region within the nucleus not surrounded by a membrane; responsible for ribosome production.
        - Function: Synthesizes ribosomal RNA (rRNA) and assembles ribosomes.
      - Ribosomes:
        - Function: Essential for protein synthesis, translating mRNA by connecting amino acids to form polypeptides.

Organelles

  • Peroxisomes:
      - Function: Degrade toxic substances and fatty acids.

  • Endoplasmic Reticulum:
      - Rough ER:
        - Structure: Studded with ribosomes.
        - Function: Modifies proteins post-transcription.
      - Smooth ER:
        - Function: Lipid synthesis and detoxification processes.

  • Golgi Apparatus:
      - Function: Modifies, sorts, and packages proteins; only found in eukaryotic cells.

Vesicular Transport and Golgi Apparatus

  • COP I and COP II Proteins: Responsible for vesicular transport within the cell.
      - COP II: Transport from rough ER to Golgi (anterograde).
      - COP I: Returns vesicles from Golgi to ER (retrograde).
  • Clathrin: Facilitates transport between Golgi and plasma membrane.

Bacterial Classification

  • Bacterial Shapes:
      - Bacilli: Rod-shaped.
      - Cocci: Spherical.
      - Spirilla: Spiral-shaped.

  • Oxygen Requirement:
      - Obligate Aerobes: Require oxygen for survival.
      - Obligate Anaerobes: Oxygen is toxic to them.
      - Facultative Anaerobes: Can survive with or without oxygen.
      - Aerotolerant Anaerobes: Do not utilize oxygen but are not harmed by it.

  • Gram Staining:
      - Purpose: To determine the cell wall composition of bacteria.
      - Gram Positive: Stains purple; thick peptidoglycan layer.
      - Gram Negative: Stains pink; thinner peptidoglycan layer plus outer membrane.

Eukaryotic vs. Prokaryotic Cells

  • Eukaryotic Cells:
      - Contain a mitochondria-based electron transport chain.
      - Larger ribosomes (80S).
      - Undergo mitosis for reproduction.
  • Prokaryotic Cells:
      - Electron transport chain occurs in the cell membrane.
      - Smaller ribosomes (70S).
      - Reproduce via binary fission.

Genetics and Recombination in Bacteria

  • Plasmids: Extra-chromosomal DNA in bacteria, often carrying beneficial traits such as antibiotic resistance.
  • Genetic Recombination Methods:
      - Transformation: Uptake of naked DNA from the environment.
      - Conjugation: Transfer of DNA between bacteria via direct contact (conjugation bridge).
      - Transduction: DNA transferred by bacteriophages.
      - Transposons: Segments of DNA capable of inserting and excising themselves from the genome.

Viruses and Their Classification

  • Basic Structures:
      - Capsid: Protein coat of a virus.
      - Envelope: Lipid bilayer surrounding some viruses.
      - Virions: Completed virus particles.
      - Bacteriophages: Viruses that infect bacteria.

  • Nucleic Acid Types:
      - Single-stranded vs. double-stranded (DNA or RNA).
      - Positive Sense RNA: Directly translated into protein.
      - Negative Sense RNA: Requires conversion to positive sense before translation.

  • Types of Viral Reproduction:
      - Lytic Cycle: Virus replicates and causes host cell lysis.
      - Lysogenic Cycle: Viral DNA integrates into the host genome, replicating along with it.

Cell Cycle and Mitosis

  • Cell Cycle Stages:
      - G1 Phase: Cell grows and synthesizes mRNA and proteins.
      - S Phase: DNA replication.
      - G2 Phase: Further growth and organelle replication.
      - M Phase: Mitosis and cytokinesis occur.

  • Mitosis Phases:
      - Prophase: Chromatin condenses, spindle apparatus begins to form.
      - Metaphase: Chromosomes align at the cell's equator.
      - Anaphase: Sister chromatids are pulled to opposite poles.
      - Telophase: Nuclear membranes reform, chromosomes decondense.

Meiosis

  • Meiosis Stages:
      - Meiosis I: Reductional division, homologous chromosomes separate.
        - Prophase I: Crossing over occurs, leading to genetic diversity.
      - Meiosis II: Equational division, similar to mitosis but starts with haploid cells.

  • Consequences of Nondisjunction: Results in aneuploidy, e.g., Down syndrome (trisomy 21).

The Reproductive System

Male Reproductive System

  • Semen Composition: Contains sperm and seminal fluid for nourishment.
  • Pathway: Sperm produced in testis → Epididymis (maturation) → Vas deferens → Ejaculatory duct → Urethra.
  • Hormones Involved: FSH stimulates Sertoli cells for sperm production; LH stimulates Leydig cells for testosterone production.

Female Reproductive System

  • Ovaries: Responsible for oocyte production; regulated by FSH and LH.
  • Pathway: Ovaries → Fallopian tubes (fertilization site) → Uterus.
  • Hormones: Estrogen (promotes development of female characteristics) and progesterone (prepares the endometrium).

Embryogenesis and Development

  • Stages of Development:
      - Fertilization occurs in the ampulla of the fallopian tube.
      - Morula: Solid mass of cells formed.
      - Blastula: Hollow sphere of cells; inner cell mass formed (embryo).
      - Gastrulation: Formation of ectoderm, mesoderm, and endoderm layers.

  • Neurulation: Development of nervous system from ectoderm induced by the notochord.

Stem Cells and Circulation

  • Stem Cells:
      - Totipotent: Can become any cell type.
      - Pluripotent: Can become any cell type except placental structures.
      - Multipotent: Can become multiple cell types within a certain lineage.

  • Fetal Circulation:
      - Umbilical Vein: Carries oxygen-rich blood to the embryo.
      - Umbilical Arteries: Carry deoxygenated blood away from the embryo.
      - Fetal Hemoglobin: Higher affinity for oxygen than adult hemoglobin.

The Nervous System

  • Neurons:
      - Afferent Neurons: Sensory, carry information to the brain.
      - Efferent Neurons: Motor, send commands to muscles.
      - Interneurons: Connect afferent and efferent neurons.

  • Synapses:
      - Neurotransmitters transmit signals across the synaptic gap.
      - Reuptake: Process of neurotransmitters returning to the presynaptic neuron.
      - Enzymatic Breakdown: Dismantling neurotransmitters in the synapse.

Endocrine System Overview

  • Hormone Types:
      - Peptide Hormones: Water-soluble, utilize surface receptors.
      - Steroid Hormones: Lipid-soluble, act on intracellular receptors.

  • Key Hormones:
      - Insulin: Decreases blood glucose.
      - Glucagon: Increases blood glucose.

  • Hypothalamus and Pituitary: Regulate hormonal activity through releasing factors (e.g., GnRH, CRH).

The Respiratory System

  • Pathway of Air: Nostrils → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli.
  • Gas Exchange: Occurs via diffusion in the alveoli between air and blood.
  • Respiratory Control: Regulated by the medulla oblongata based on CO2 levels.

Cardiovascular System

  • Structure: Comprised of heart, blood vessels, and blood.
  • Blood Pathway: Right Atrium → Right Ventricle → Pulmonary Arteries (to lungs) → Pulmonary Veins (to left atrium) → Left Ventricle → Aorta → Body.
  • Heart Conduction: Initiated by SA Node → AV Node → Bundle of His → Purkinje Fibers.

Blood Composition

  • Components:
      - Erythrocytes: Red blood cells, lack nucleus, carry oxygen.
      - Leukocytes: White blood cells, immune response.
      - Platelets: Cell fragments involved in clotting.

  • Blood Types based on antigens present on red blood cells (A, B, AB, O) and Rh factor.

Immune System

  • Innate vs. Adaptive Immunity:
      - Innate: Non-specific defenses (skin, mucus, phagocytes).
      - Adaptive: Specific responses (T cells, B cells, antibodies).

  • Key Components:
      - Antibodies: Produced by B cells, target specific antigens.
      - T Cells: Kill infected cells and coordinate the immune response.
      - Cytokines: Signaling molecules between cells of the immune system.

Genetics and Evolution

  • Mendelian Genetics: Principles of inheritance, including dominant and recessive alleles.
  • Natural Selection: Mechanism by which evolution occurs, influencing allele frequency within populations.

Conclusion

  • Comprehensive understanding of cellular biology, organ systems, and genetic principles is crucial for mastering MCAT Biology.

Study Tips

  • Utilize diagrams and visuals.
  • Engage in spaced repetition for memorization.
  • Practice applying concepts through practice exams to enhance retention and understanding.