Key concepts in biology study guide

Cells are the fundamental units of life, and their internal structures, known as subcellular organelles, are specialised to perform specific functions essential for the cell's survival and organism's biology. Eukaryotic cells, found in animals and plants, are characterised by a true nucleus and membrane-bound organelles. Prokaryotic cells, like bacteria, are simpler, lacking a true nucleus and most membrane-bound organelles. In animal cells, the nucleus houses generic material and controls cell activities, the cell membrane regulates substance passage, mitochondria generate energy, and ribosomes synthesise proteins. Plant cells share many animal cell structures but also possess a rigid cell wall for support, chloroplasts for photosynthesis, and a large central vacuole for storage and turgor. Bacterial cells feature a cell membrane and ribosomes like other cells, but their genetic material is in a nucleoid region (chromosomal DNA) with additional DNA in plasmids, and many have flagella for movement. Each structure's shape and location are integral to its role in maintaining cell viability and contributing to the overall function of the organism.

Cells require mechanisms to transport substances across their membranes, both into and out of the cell, to maintain an internal environment suitable for biochemical processes. These mechanisms can be broadly categorised into passive processes, which do not require cellular energy, and active processes, which do. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. This occurs directly through the cell membrane for small, uncharged molecules like oxygen and carbon dioxide. Osmosis is a specific type of diffusion that involves the net movement of water molecules across a selectively permeable membrane from an area of higher water potential to an area of lower water potential. Active transport, conversely, moves substances against their concentration gradient, from a region of lower concentration to a region of higher concentration. This process requires energy, typically supplied by ATP, and involves specific carrier proteins embedded within the cell membrane. Each method is vital for different cellular functions, from nutrient uptake to waste removal.