Human Biology: Cells, Tissues and Body Systems
Core concepts
Human Biology begins with the relationship between structure and function at every level of organisation: organelles, cells, tissues, organs, systems and the whole organism. You must know the principal organelles and their roles - the nucleus as the site of DNA and transcription, ribosomes for translation, rough and smooth endoplasmic reticulum for protein processing and lipid synthesis, the Golgi apparatus for modification and packaging, mitochondria for aerobic ATP production, and the cell membrane as a selectively permeable phospholipid bilayer with embedded proteins. Understand the fluid mosaic model, and the transport processes: simple diffusion, facilitated diffusion, osmosis, active transport and vesicle-mediated transport.
At tissue level, distinguish epithelium (covering and lining, with surface specialisations such as microvilli and cilia), connective tissue (including blood, bone and adipose), muscle (skeletal, smooth, cardiac) and nervous tissue. Link tissues to organ function: alveoli and villi maximise surface area; nephrons combine filtration, reabsorption and secretion; the small intestine couples structure to absorption.
Homeostasis is the unifying theme: negative feedback maintains a set point, while positive feedback amplifies change. Always identify the stimulus, receptor, control centre, effector and response.
Common traps
Questions often use the word "diffusion" when osmosis is correct, or assume osmosis requires a membrane permeable to solute. Remember that osmosis is the movement of water across a selectively permeable membrane down a water potential gradient. Students also confuse the roles of the Golgi apparatus and the endoplasmic reticulum, or claim that active transport moves substances "down" a concentration gradient. Another trap is describing a feedback loop without naming the effector or the corrective response.
Worked example
A question asks why a red blood cell placed in distilled water swells and bursts. Answer by comparing water potential: the cytoplasm has a lower (more negative) water potential than the distilled water, so water moves by osmosis across the selectively permeable membrane into the cell. The cell has no cell wall to resist the increasing hydrostatic pressure, so the membrane ruptures.
Rule of thumb
Name the level of organisation, the structure involved, and the functional consequence - marks are awarded for linking structure to function explicitly.