SA:V and Metabolic Processes

Why are cells generally small in size?

Cells are small to maintain a high surface area–to–volume (SA:V) ratio, which allows efficient diffusion of substances into and out of the cell through the membrane.

How does surface area–to–volume ratio change as cells increase in size?

As an object gets larger, its surface area relative to its volume decreases.

Why can large cells not efficiently rely on diffusion?

Large cells cannot efficiently use diffusion to bring nutrients in and remove wastes like carbon dioxide and metabolic wastes due to a low SA:V ratio.

Why do organisms increase surface area in certain tissues?

To increase diffusion of molecules or heat exchange with the environment.

How do organisms increase surface area in tissues?

By using thin sheets of tissue (such as gills or elephant ears) or highly folded surfaces (such as mitochondrial membranes or intestinal villi).

Why are whales so large?

Whales are warm-blooded mammals living in cold ocean water, and increased body size decreases SA:V ratio, reducing heat loss and increasing evolutionary fitness through energy savings.

Why are there no mouse-sized marine mammals?

Small mammals have a high SA:V ratio, lose heat rapidly in water, experience cold stress, and have decreased fitness; only larger sizes are viable.

Why are otters the smallest marine mammals?

Otters survive because their fur coats reduce heat loss despite their smaller size.

What is metabolism?

Metabolism is the sum total of all chemical processes within an organism, including inputs, processes, and outputs.

What is metabolic rate?

Metabolic rate is the amount of energy an organism expends over a given period of time.

What is basal metabolic rate?

Basal metabolic rate is the amount of energy consumed by an organism at rest at a comfortable temperature.

How can metabolic rate be measured?

By measuring oxygen consumption, carbon dioxide production, or heat production.

What are endotherms?

Endotherms generate body heat internally as a byproduct of metabolism and regulate body temperature around a set point.

What is a cost of being an endotherm?

Endotherms require much more food than ectotherms of similar weight.

What are advantages of endothermy?

Endotherms can remain active regardless of environmental temperature and dominate cold environments such as polar regions.

What are ectotherms?

Ectotherms have body temperatures that conform to environmental temperatures.

How does body size affect basal metabolic rate in mammals?

As endothermic mammals increase in size, their basal metabolic rate increases.

How does relative metabolic rate change with size in mammals?

Relative metabolic rate decreases as body size increases; smaller mammals require more energy per gram of tissue.

What is metabolic scaling?

Metabolic scaling refers to differences in relative metabolic rate across body sizes.

Why do shrews have higher metabolic demands than elephants?

Shrews have higher SA:V ratios, lose heat more easily, and must perform more cellular respiration, resulting in higher relative metabolic rates.