Exchange between Organisms - Surface Area to Volume Ratio

Question 1

How do cells and organisms exchange material between themselves and their environment?

Answer 1

To enter or leave an organism, substances must pass across a plasma membrane. Single-celled and small multicellular organisms can satisfactorily exchange materials over their body surfaces using diffusion alone, especially if their metabolic rate is low.

Question 2

What happened to the surface area to volume ratio as organisms evolved and became larger?

Answer 2

Their surface area to volume ratios decreased and specialised respiratory surfaces evolved to meet the increasing requirement to exchange ever larger quantities of materials.

Question 3

What is required when large size is combined with a high metabolic rate?

Answer 3

There is a requirement for a mass transport system to move substances between the exchange surface and the cells of which the organism is composed.

Question 4

What do mass transport systems often involve in animals?

Answer 4

These systems often involve circulating a specialised transport medium (blood) through vessels using a pump (heart).

Question 5

How do plants exchange substances with their environment?

Answer 5

Plants do not move from place to place and have a relatively low metabolic rate and consequently reduced demand for oxygen and glucose. Coupled with their large surface area, essential for obtaining light for photosynthesis, they have not evolved a pumped circulatory system.

However, plants do transport water up from their roots to the leaves and distribute the products of photosynthesis. Their mass transport system comprises vessels too - xylem and phloem, but the movement of fluid within them is largely a passive process.

Question 6

What maintains the diffusion gradients?

Answer 6

The internal environment of a cell or organism differs from the environment around it. The cells of large multicellular animals are surrounded by tissue fluid, the composition of which is kept within a suitable metabolic range. In both plants and animals, it is the mass transport system that maintains the final diffusion gradients which allows substances to be exchanged across cell-surface membranes.

Question 7

How is the effectiveness of a gas-exchange surface increased?

Answer 7

The effectiveness of a gas-exchange surface is increased by having a large surface area, being thin, having an efficient blood supply and being ventilated.

Question 8

How is the surface area of the lungs increased in humans?

Answer 8

In humans, the surface area of the lungs is increased by alveoli and that of the small intestine by villi. The villi provide a large surface area with an extensive network of capillaries to absorb the products of digestion by diffusion and active transport.

Question 9

What does breathing in and breathing out involve?

Answer 9

Breathing in involves the ribcage moving out and up and the diaphragm become flatter. Breathing out involves these changes being reversed.

Question 10

How are water and mineral ions absorbed in plants?

Answer 10

In plants, water and mineral ions are absorbed by roots, the surface area of which is increased by root hairs.

Question 11

Why do plants have stomata?

Answer 11

Plants have stomata in their leaves through which carbon dioxide and oxygen are exchanged with the atmosphere by diffusion. The size of stomata is controlled by guard cells that surround them and help control water loss.

Question 12

What do the xylem and phloem tissue do in flowering plants?

Answer 12

In flowering plants, xylem tissue transports water and mineral ions from the roots to the stem and leaves, and phloem tissue carries dissolved sugars from the leaves to the rest of the plant.

Question 13

What is the circulatory system?

Answer 13

In animals, a circulatory system transports substances using a heart, which is a muscular organ with four main chambers - left and right atria and ventricles.

Question 14

How does blood flow around your body?

Answer 14

Blood flows from the heart to the organs through arteries and returns through veins. Arteries have thick walls containing muscle and elastic fibres. Veins have thinner walls and often have valves to prevent back-flow of blood.

Question 15

What is blood?

Answer 15

Blood is a tissue and consists of plasma in which red blood cells, white blood cells and platelets are suspended.

Question 16

What are red blood cells?

Answer 16

Red blood cells have no nucleus and are packed with haemoglobin. In the lungs haemoglobin combines with oxygen to form oxyhaemoglobin. In other organs, oxyhaemoglobin splits up into haemoglobin and oxygen.

Question 17

What are white blood cells?

Answer 17

White blood cells have a nucleus and form part of the body’s defence system against microorganisms.

Question 18

Why is a mass transport system needed?

Answer 18

The environment around the cells of multicellular organisms is called tissue fluid. The majority of cells are too far from exchange surfaces for diffusion alone to supply or remove their tissue fluid with the various materials needed to keep its composition relatively constant.

Therefore, once absorbed, materials are rapidly distributed to the tissue fluid and the waste products returned to the exchange surface for removal. This involves a mass transport system.

Question 19

How does the size and metabolic rate of an organism affect the amount of each material that is exchanged?

Answer 19

Organisms with a high metabolic rate exchange more materials and so require a larger surface area to volume ratio. In turn, this is reflected in the type of exchange surface and transport system that evolved to meet the requirements of each organism.

Question 20

What are some examples of things that need to be interchanged between an organism and its environment?

Answer 20

• respiratory gases (oxygen and carbon dioxide)
• nutrients (glucose, fatty acids, amino acids, vitamins, minerals)
• excretory products (urea and carbon dioxide)
• heat

Question 21

Except for heat, what are the two ways these exchanges can take place?

Answer 21

• passively (no metabolic energy is required), by diffusion and osmosis
• actively (metabolic energy is required), by active transport

Question 22

Where is the lowest oxygen concentration in a cell?

Answer 22

In a cell, the lowest oxygen concentration is inside the mitochondria, where oxygen is used up in respiration. Mitochondria also contain the highest concentration of carbon dioxide. This maintains the diffusion gradient for these gases in and out of the cell.

Question 23

What is needed for exchange to be effective?

Answer 23

Exchange takes place at the surface of an organism, but the materials absorbed are used by the cells that mostly make up its volume. For exchange to be effective, the exchange surfaces of the organism must be large compared with its volume.

Question 24

Why can’t simple diffusion be used in larger organisms?

Answer 24

Small organisms have a surface area that is large enough, compared with the volume, to allow efficient exchange across their body surface. The need for a short diffusion path limits the size of a single-celled organism to <1mm.

However, as organisms become larger, their volume increases at a faster rate than their surface area. Because of this, simple diffusion of substances across the outer surface can only meet the needs of relatively inactive organisms. Even if the outer surface could supply enough of the substance, it would still take too long for it to reach the middle of the organism if diffusion alone was the method of transport.

Question 25

What features have organisms evolved?

Answer 25

• a flattened shape so that no cell is every far from the surface (e.g. a flatworm or a leaf)
• specialised exchange surfaces with large areas to increase the surface area to volume ratio (e.g., lungs in mammals, gills in fish)

Question 26

What must be assumed to be able to calculate the surface area to volume ratio of cells with different shapes?

Answer 26

Cells or organisms may have to be assumed to have a uniform shape although in practice they almost never do.

Question 27

Why are all plasma membranes thin and not just cell-surface membranes?

Answer 27

Substances not only have to move into cells through the cell-surface membrane but also into organelles like mitochondria through the plasma membrane that surrounds them.

Question 28

What characteristics do exchange surfaces show to allow effective transfer of materials across specialised exchange surfaces by diffusion or active transport?

Answer 28

• a large surface area relative to the volume of the organism which increases the rate of exchange (a large SA:V ratio gives a short distance for diffusion)
• very thin so that the diffusion distance in short and therefore materials cross the exchange surface rapidly
• selectively permeable to allow selected materials to cross
• movement of the environmental medium, for example, air, to maintain a diffusion gradient (good ventilation)
• a transport system to ensure the movement of the internal medium, for example blood, in order to maintain a diffusion gradient (good blood supply)

Question 29

Where are specialised exchange surfaces located?

Answer 29

Being thin, specialised exchange surfaces are easily damaged and dehydrated. They are therefore often located inside an organism. Where an exchange surface is located inside the body, the organism needs to have a means of moving the external medium over the surface (e.g. a means of ventilating the lungs in a mammal).

Question 30

What are the solutions for larger organisms?

Answer 30

• become multicellular

- create a convoluted or highly irregular surface area

Question 31

What are the adaptations for earthworms?

Answer 31

• It is cylindrical so it’s SA:V ratio is smaller than flatworm’s but larger than compact organism of the same volume.
• Its skin is a respiratory surface which is kept moist by secreting mucus (need for moist surface restricts earthworm to damp environment of soil)
• It has a low oxygen requirement (because it is slow moving and has a low metabolic rate, so enough O₂ diffuses across its skin into blood capillaries beneath)
• The haemoglobin present in its blood carries oxygen around the body, in blood vessels, which maintains a diffusion gradient at the respiratory surface
• CO₂ is also carried in the blood and it diffuses out across the skin

Question 32

What is a terrestrial?

Answer 32

• a very large, warm blooded organism that needs a very high metabolic rate
• it has a very high demand for food, water, oxygen, and excretion of wastes

Question 33

What is Fick’s Law?

Answer 33

diffusion = (surface area x concentration gradient) / distance

Question 34

What does Fick’s Law state?

Answer 34

Fick’s law expresses the rate of diffusion of a given molecule across a membrane at a given temperature. This means you have to assume the temperature remains constant.

Question 35

How do the three factors affect the rate of diffusion?

Answer 35

• concentration gradient: the greater the concentration gradient, the greater the diffusion rate
• surface area: the larger the area across which diffusion occurs, the greater the rate of diffusion
• distance: thicker barriers slow diffusion rate, pores in a barrier enhance diffusion

Question 36

How does temperature affect the rate of diffusion?

Answer 36

Temperature increases the rate of diffusion due to increasing kinetic energy.

Question 37

How are microvilli adaptations that allow exchange?

Answer 37

They are extensions of the plasma membrane that increase the surface area of the membrane and thus increase the diffusion rate.

Question 38

How is a flattened body an adaptation that allows exchange?

Answer 38

Gaseous exchange across the surface of a flattened body. Flattening increases the S:V ratio and also decreases the distance over which diffusion has to occur within the body, e.g. flatworms.

Question 39

How are external gills an adaptation that allows exchange?

Answer 39

These increase the surface area but are unprotected and therefore easily damaged. Gaseous exchange usually takes place across the rest of the body surface as well as the gills, e.g. lugworm.

Question 40

How are highly vascularised internal gills an adaptation that allows exchange?

Answer 40

A ventilation mechanism draws water over the gill surfaces, e.g. fishes.

Question 41

How are highly vascularised lungs an adaptation that allows exchange?

Answer 41

The lungs are sacs connected to the pharynx. Air is drawn into them by a ventilation mechanism. Found in all air-breathing vertebrates.