Cells, Transport and Biological Molecules
Cell structure and organelles
All living things are built from cells. Plant and animal cells share a cell membrane, cytoplasm, nucleus, mitochondria and ribosomes. Plant cells also have a cellulose cell wall, chloroplasts and a large permanent vacuole; animal cells do not. Chloroplasts absorb light for photosynthesis, while mitochondria release energy by respiration. Magnification = image size ÷ actual size, so a 40 mm image of a 0.05 mm cell has magnification ×800.
Movement in and out of cells
Diffusion is the passive net movement of particles from high to low concentration. Osmosis is the diffusion of water through a partially permeable membrane from a dilute solution to a more concentrated one. Active transport moves substances against the concentration gradient and needs energy from respiration.
Biological molecules and food tests
Carbohydrates include glucose (monosaccharide), sucrose (disaccharide) and starch or cellulose (polysaccharides). Proteins are chains of amino acids; lipids are made from glycerol and fatty acids. Benedict's reagent gives a brick-red precipitate with reducing sugars when heated; iodine turns blue-black with starch; Biuret turns purple with protein; ethanol gives a cloudy white emulsion with lipid.
Worked example
A potato cylinder is placed in distilled water and gains mass. The water outside is more dilute than the cell sap, so water enters the cells by osmosis through the partially permeable membranes. In concentrated salt solution the reverse happens and the cylinder loses mass.
Traps that cost marks
The cell wall is not the cell membrane and does not control entry in the same selective way. Osmosis involves water only, not dissolved solute. Benedict's test needs heating; iodine does not. Always state net movement for diffusion and name the membrane as partially permeable in osmosis answers.