Pearson Science Double Award Biology 2: Structures and Functions

Topic 2 connects organelles to whole-organism systems. Use the same reasoning throughout: identify a structure, state its function and explain how it supports a biological process.

These notes follow the Double Award points in Pearson 4SD0 Issue 4. Practical methods are developed in the separate practical-skills hub, while this page owns mechanisms and interpretation.

Organisation and cells

Biological organisation runs from organelles to cells, tissues, organs and organ systems. The nucleus contains genetic material and controls cell activities. Cytoplasm is the site of many reactions, the cell membrane controls exchange, mitochondria carry out aerobic respiration, and ribosomes make proteins. Plant cells additionally have a cellulose wall, chloroplasts and a large permanent vacuole.

Molecules, enzymes and movement

Carbohydrates contain carbon, hydrogen and oxygen. Proteins also contain nitrogen. Starch and glycogen are built from simple sugars, proteins from amino acids, and lipids from fatty acids and glycerol.

Enzymes are biological catalysts with specifically shaped active sites. Rising temperature initially increases successful collisions, but excessive heat changes the active-site shape and causes denaturation. Moving away from optimum pH can also alter the active site.

Diffusion is net movement down a concentration gradient. Osmosis is net water movement through a partially permeable membrane from a more dilute solution to a more concentrated solution. Active transport moves substances against a gradient using energy from respiration. Exchange is faster with a larger surface-area-to-volume ratio, steeper gradient, higher temperature and shorter distance.

Plant and human nutrition

Photosynthesis converts light energy to chemical energy. Carbon dioxide and water form glucose and oxygen using light absorbed by chlorophyll. Light intensity, carbon dioxide and temperature can limit rate. A leaf supports photosynthesis through a broad, thin surface, chloroplast-rich palisade cells, air spaces, stomata and veins. Magnesium ions are needed for chlorophyll and nitrate ions for amino acids.

A balanced human diet supplies carbohydrate, protein, lipid, vitamins A, C and D, calcium, iron, water and fibre in suitable proportions. Requirements change with age, activity and pregnancy.

Food travels through the mouth, oesophagus, stomach, duodenum, ileum, colon and rectum by peristalsis. Amylase and maltase convert starch to glucose, proteases produce amino acids, and lipases produce fatty acids and glycerol. Bile is made by the liver, stored in the gall bladder, neutralises stomach acid and emulsifies lipids. Villi give the small intestine a large, thin, well-supplied absorption surface.

Respiration, gas exchange and transport

Respiration transfers energy into ATP for cellular work. Aerobic respiration uses oxygen and releases more energy than anaerobic respiration. Animal anaerobic respiration produces lactic acid; plant and yeast anaerobic respiration produces ethanol and carbon dioxide.