Size and Surface Area

Exchanging Substances: Organisms in their environment

• Cells need to take in oxygen (for aerobic respiration) and nutrients.

• Need to excrete products (carbon dioxide, urea)

• Stay same temperature, so heat exchanged

• Depends on surface area to volume ratio

Surface Area to Volume ratios

• Smaller animals have larger surface area to volume ratios

• Larger animals have smaller surface to volume ratios

• It’s a factor which affects exchange of substances in an organism

• For example:

Multicellular Organisms: Exchange Organs and Mass Transport Systems

• All cells need to be supplied with substances like glucose and oxygen

Also needs to remove remove waste to avoid damaging itself

Single-celled:

• Substances can diffuse directly into or out of the cell

• Substances travel across the cell membrane

• Diffusion rate is quick as small distances needed to travel

Multicellular:

• Diffusion across outer membrane is too slow as distances are too large and larger animals have low SA:V ratio where it’s difficult to exchange enough substances to supply cells

• Uses exchange organs (e.g. lungs) along with an efficient system

Effects of Size and Shape on Heat Exchange

• Metabolic activity creates heat inside cells, along with creating waste products.

Size:

• Rate of heat loss depends on surface area

• Large volume = small surface area → harder to lose heat

• Small volume = large surface area → easier to lose heat

• Therefore, smaller organisms need a high metabolic rate in order to generate enough heat to stay warm

Shape:

• Compact shape = small surface area (relative to volume) → minimises heat loss

• Less compact shape = large surface area → increases heat loss

• Compact or not, it depends on the temperature of their environment:

  

Behavioural and Physiological Adaptations

• Not all have size or shape to suit their climate, some have other adaptations

• Animals with high SA:V ratio tend to lose more water (evaporates)

• Small desert animals have kidney structure adaptations so they produce less urine

• To support high metabolic rate, small mammals in cold areas need to eat large amounts of high energy food (seeds and nuts)

• Large organisms in hot areas (elephants and hippos) have slow heat loss

• Elephants have large flat ears to increase SA → lose more heat → physiological

• Hippos spend much of the day in water → behavioural adaptation