Key concepts in biology Study Guide

Cells, the fundamental units of life, contain specialised sub-cellular structures that perform specific roles crucial for their survival and function. These structures differ between eukaryotic cells (like animal and plant cells) and prokaryotic cells (like bacteria). In animal cells, the nucleus houses genetic material and controls cell activities; the cell membrane regulates substance passage; mitochondria generate energy through respiration; and ribosomes synthesise proteins. Plant cells share these but also possess a cell wall for structural support, chloroplasts for photosynthesis, and a permanent vacuole for maintaining turgor and storing substances. Bacterial cells, being prokaryotic, lack a nucleus and most membrane-bound organelles; their chromosomal DNA is free in the cytoplasm, and plasmids carry additional genetic information. They have a cell membrane, ribosomes for protein synthesis, and some may have flagella for movement.

Cells manage the movement of substances across their membranes through diffusion, osmosis, and active transport. Diffusion is the passive movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient. This process does not require energy and occurs for substances like oxygen and carbon dioxide. Osmosis is a specific type of diffusion that involves the movement of water molecules across a partially permeable membrane from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution). Active transport, in contrast, moves substances against their concentration gradient, from an area of lower concentration to an area of higher concentration. This process requires energy, typically supplied by cellular respiration, and often involves specific carrier proteins in the cell membrane. Examples include the absorption of mineral ions by plant root hair cells and glucose absorption in the small intestine.