Biological and Biochemical Foundations of Living Systems: Content Category 2A

Assemblies of molecules, cells, and groups of cells within single cellular and multicellular organisms

The processes necessary to maintain life are executed by assemblies of molecules, cells, and groups of cells, all of which are organized into highly-specific structures as determined by the unique properties of their component molecules. The processes necessary to maintain life require that cells create and maintain internal environments within the cytoplasm and within certain organelles that are different from their external environments.

Cell membranes separate the internal environment of the cell from the external environment. The specialized structure of the membrane, as described in the fluid mosaic model, allows the cell to be selectively permeable and dynamic, with homeostasis maintained by the constant movement of molecules across the membranes through a combination of active and passive processes driven by several forces, including electrochemical gradients.

Eukaryotic cells also maintain internal membranes that partition the cell into specialized regions. These internal membranes facilitate cellular processes by minimizing conflicting interactions and increasing surface area where chemical reactions can occur. Membrane-bound organelles localize different processes or enzymatic reactions in time and space.

Through interactions between proteins bound to the membranes of adjacent cells, or between membrane-bound proteins and elements of the extracellular matrix, cells of multicellular organisms organize into tissues, organs, and organ systems. Certain membrane-associated proteins also play key roles in providing identification of tissues or recent events in the cell’s history for purposes of recognition of “self” versus foreign molecules.

The content in this category covers the composition, structure, and function of cell membranes; the structure and function of the membrane-bound organelles of eukaryotic cells; and the structure and function of the major cytoskeletal elements. It covers the energetics of and mechanisms by which molecules, or groups of molecules, move across cell membranes. It also covers how cell–cell junctions and the extracellular matrix interact to form tissues with specialized functions. Epithelial tissue and connective tissue are covered in this category.

Topic Level Key

The abbreviations found in parentheses indicate the course(s) in which undergraduate students at many colleges and universities learn about the topics and associated subtopics. The course abbreviations are:

BC = first-semester biochemistry
BIO = two-semester sequence of introductory biology
GC = two-semester sequence of general chemistry
OC = two-semester sequence of organic chemistry

Please note topics that appear on multiple content lists will be treated differently. Questions will focus on the topics as they are described in the narrative for the content category.

Plasma Membrane (BIO, BC)

• General function in cell containment

• Composition of membranes

  • Lipid components (BIO, BC, OC)

    • Phospholipids (and phosphatids)

    • Steroids

    • Waxes

  • Protein components

  • Fluid mosaic model

• Membrane dynamics

• Solute transport across membranes

  • Thermodynamic considerations

  • Osmosis

    • Colligative properties; osmotic pressure (GC)

  • Passive transport

  • Active transport

    • Sodium/potassium pump

• Membrane channels

• Membrane potential

• Membrane receptors

• Exocytosis and endocytosis

• Intercellular junctions (BIO)

  • Gap junctions

  • Tight junctions

  • Desmosomes

Membrane-Bound Organelles and Defining Characteristics of Eukaryotic Cells (BIO)

• Defining characteristics of eukaryotic cells: membrane bound nucleus, presence of organelles, mitotic division

• Nucleus

  • Compartmentalization, storage of genetic information

  • Nucleolus: location and function

  • Nuclear envelope, nuclear pores

• Mitochondria

  • Site of ATP production

  • Inner and outer membrane structure (BIO, BC)

  • Self-replication

• Lysosomes: membrane-bound vesicles containing hydrolytic enzymes

• Endoplasmic reticulum

  • Rough and smooth components

  • Rough endoplasmic reticulum site of ribosomes

  • Membrane structure

  • Role in membrane biosynthesis

  • Role in biosynthesis of secreted proteins

• Golgi apparatus: general structure and role in packaging and secretion

• Peroxisomes: organelles that collect peroxides

Cytoskeleton (BIO)

• General function in cell support and movement

• Microfilaments: composition and role in cleavage and contractility

• Microtubules: composition and role in support and transport

• Intermediate filaments, role in support

• Composition and function of cilia and flagella

• Centrioles, microtubule organizing centers

Tissues Formed From Eukaryotic Cells (BIO)

• Epithelial cells

• Connective tissue cells

Additional Review: Khan Academy MCAT Collection Tutorials

To support your studies, see the following video tutorials below from the Khan Academy MCAT Collection. The videos and associated questions were created by the Khan Academy in collaboration with the AAMC and the Robert Wood Johnson Foundation.