Size and Surface Area

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  • Size and Surface Area
    • Exchange substances
      • Cells need to take in oxygen for aerobic respiration and nutrients
      • Need to excrete waste products like carbon dioxide/urea
      • Most organisms need to stay at roughly same temperature, heat needs to be exchanged
    • SA:V Ratio
      • Smaller animals have high SA:V ratio
      • Volume= L x W x H
      • SA= Area of each side added up
      • Always :1, simplify down
    • Exchange organs/ Mass transport systems
      • Organism needs to supply every one of its cells with glucose/oxygen for respiration
      • Also needs to remove waste products
      • Single celled organisms- substances diffuse directly across CSM. Diffusion rate quicker, shorter diffusion distance
      • Multicellular- diffusion too slow: Long diffusion distance, low SA:V ratio (can't supply enough substances for whole organism)
      • Multicellular animals need specialised exchange organs
      • Also need mass transport systems e.g circulatory, blood carries glucose/oxygen/hormones/antibodies/waste. Plants use xylem/phloem
    • Heat exchange
      • Rate of heat loss depends on SA
      • Small SA= less heat loss. Large SA= more heat loss
      • Smaller organisms need high metabolic rate to generate heat
      • Animals with compact shape have smaller SA, minimises heat loss
      • Less compact shape means larger SA, increases heat loss
      • Temp of animals environment determines shape: Arctic fox compact, African bat eared fox not, European fox in between
    • Behavioural/ Physiological adaptations
      • Animals w/ high SA:V ratio lose more water by evaporation from surface. Small desert mammals have kidney adaptation, produce less urine
      • Small mammals in cold regions eat high energy foods to support high metabolic rate
      • Smaller mammals have thick fur/hibernate in cold weather
      • Larger organisms in hot regions- Elephants have large, flat ears to increase SA, lose more heat. Hippos spend large part of day in water to keep cool


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