The Cell and Tissues of the Body
The cell and tissues domain tests how cell structure supports transport, metabolism and division, and how the four basic tissue types - epithelial, connective, muscle and nervous - combine to form organs. Questions often ask you to predict movement across membranes, classify epithelia, or identify a tissue from its matrix and cells.
Core rules and concepts
- The plasma membrane is a phospholipid bilayer with embedded proteins; small nonpolar molecules diffuse directly, while ions and polar molecules need channels or carriers.
- Osmosis is water movement across a selectively permeable membrane toward higher solute concentration. Tonicity compares a solution with intracellular fluid: hypotonic causes swelling, hypertonic causes shrinkage.
- Mitochondria produce most ATP by oxidative phosphorylation; ribosomes synthesize protein; rough endoplasmic reticulum folds and modifies proteins; smooth ER makes lipids and detoxifies; Golgi packages and sorts.
- Epithelia are classified by cell shape (squamous, cuboidal, columnar) and layer number (simple or stratified). Simple epithelia favor exchange; stratified epithelia resist abrasion.
- Connective tissue has cells, fibers and ground substance. Collagen resists tension, elastin recoils, and reticular fibers form supportive networks.
- Tight junctions seal the space between cells, desmosomes anchor cells to each other, and gap junctions allow small molecules and ions to pass.
Worked example
A red blood cell with an internal osmolarity of about 300 mOsm is placed in 200 mOsm saline. The solution is hypotonic relative to the cell, so water moves into the cell by osmosis. The cell swells and may lyse. If placed in 400 mOsm saline, water leaves and the cell crenates. The reasoning depends on comparing solute concentration outside with inside, not on the absolute number of particles.
Traps that cost marks
Do not call osmosis “diffusion of solute”; it is diffusion of water. Do not describe the epidermis as vascular. Do not confuse microvilli, which increase surface area, with cilia, which move materials. Remember that dense connective tissue heals slowly because it is poorly vascularized.