Water Loss

What is water loss?

Water is lost through openings in the organism’s body.

Water loss in relation to the Surface area to volume ratio

Surface area to volume ratio (SA: Vol) plays an essential role in water loss. Let’s take a look at an example in Figure 1. Both shapes have the same volume (100 m³); however, the ratio is higher in the first shape due to its larger surface area. Therefore, the first shape will have a higher SA: Vol ratio. This is good for unicellular organisms and specialised structures in multicellular ones.

In gas and nutrient exchange, a large surface area is beneficial. However, the dangers of a large surface area - you guessed it, more water loss! In the case of Figure 1, more water loss will occur in the first shape. Organisms must balance gas and nutrient exchange, water loss and other requirements.

Fig. 1 - SA:Vol comparison between the two shapes

Water loss in insects

Apart from the alimentary canal (mouth to anus), there are three ways that an insect will lose water. These are:

1. Through the body wall.
2. Through the spiracular system (openings in the exoskeleton).
3. Partial loss by (1) and (2).

Adaptations to limit water loss in insects

Insects have developed adaptations to limit water loss, including the waterproof exoskeleton, spiracles, and small SA: Vol.

Small SA: Vol

Smaller insects, such as a fly, will have a larger SA: Vol ratio and lose more water than a grasshopper. However, a small SA: Vol ratio would also cause trouble for the gas exchange. This is why insects have a tracheal system to facilitate gas exchange. Having a larger surface area will reduce water loss.

Exoskeleton

The body of an insect is covered by a cuticle made of chitin. The external surface (epicuticle) covering the insect’s body is waxy and water-resistant. It does not contain chitin. The cuticle (can also be referred to as an exoskeleton) is impermeable and stops water from evaporating (Figure 1b)). This helps to avoid dehydration. Of course, if it was utterly impermeable, gas exchange could not occur and that is why the tracheal system has spiracles.

Spiracles

Spiracles are small openings in the tracheae at the body surface. They are usually found around the abdominal area (Figure 1a)). The gas exchange will take place at the spiracles. When the spiracles are open, water loss will occur. To limit this, when the insect is at rest, the spiracles will remain closed. When the insect is at rest, the need for oxygen is reduced due to lower respiration. Spiracles contain valves to open and close.

Spiracles are further adapted to reduce water loss by containing hairs inside them. The hairs can retain some water, maintaining humidity. This reduces water loss. Prevention of water loss will also ensure that the gas exchange surfaces remain moist.

Fig. 2 - A) Tracheal system of an insect. B) Exoskeleton of an arthropod

Water loss in plants

Plants have two transport systems - xylem and phloem. We will focus on the xylem, which is responsible for transporting water and inorganic minerals from the roots to the plant parts above the ground.

Water is lost by transpiration. Transpiration through the stomata into the air is the driver for the water movement. Transpiration reduces the water potential in the leaves and, in turn, in the plant. This allows water to move from the roots up the plant.

Fig. 3 - An overview of water movement through the plant

Limiting water loss in plants

Unlike insects, plants have a large surface area-to-volume ratio. This is a must because plants need a large ratio for photosynthesis. The large surface area of leaves is needed to catch the solar energy and facilitate the exchange of oxygen and carbon dioxide.

The main ways vascular plants (well-developed plants) limit water loss include the cuticle, leaf hairs, stomata and mutualistic relationship between microorganisms. The cuticle is waxy and water-repellent, which keeps the water “locked” in the plant. Similarly to insects’ spiracles, the stomata can be opened and closed. Instead of valves, such as in the spiracles, stomata have guard cells. Guard cells can increase (lower water loss and less gas exchange) or decrease in size (higher water loss and more gas exchange).

Mutualistic microorganisms help the plant’s roots to stay moist in exchange for the plant’s sugars.

Xerophytes

Xerophytes are plants that have adapted to live in very dry environments. They have developed adaptations to limit water loss to avoid desiccation in these harsh environments (e.g. dunes).

Reduction of the surface area

Smaller leaves equal smaller surface area for water loss. Small and circular leaves, such as pine needles, considerably reduce water evaporation.

Thick cuticle

Although all plants have a waxy cuticle, water can still escape. Thicker the cuticle, less water loss can occur. Pine needles have thick waxy cuticles.

Rolled leaves

Plants that do not have needle-like leaves will roll the leaves up. Stomata in the leaves are present in the lower epidermis. The Rolling of the leaves will trap the water vapour and, in turn, will create a high water potential (high concentration of free water molecules). The water potential between the outside and the inside of the leaf will become equal (no potential gradient). This will result in no water loss. Plants such as Marram grass do incorporate this adaptation.

Hairy leaves and stomata in pits and grooves

Stomata in pits and surrounded by hairs will reduce transpiration. A thick layer of hairs on the leaf surface and within the pits will trap moisture. There will be a reduction of the water potential gradient to lose less water, for example, the heather plant (hairy leaves) and pine tree (pitted stomata).

Crassulacean acid metabolism (CAM) physiology

CAM is a carbon fixation pathway that has evolved as an adaptation in some xerophytes to reduce water loss. The plant will photosynthesise during the day and only exchange gases during the night.

Halophytes

We will also briefly cover halophytes. Halophytes are plants that have adapted to grow in salty conditions, such as marshlands. Halophytes will:

• Alter their flowering schedules and will only flower during the rainy season to minimise the exposure to salt.
• Excrete salt - halophytes will excrete the salt through certain parts, such as the stem. They can contain salt glands that have adapted to get rid of the salt within the plant actively.
• Root adaptations - in some halophytes, the roots are adapted to exclude around 95% of the salt from the soil.
• Tissue positioning - some will concentrate the salt in particular leaves. The salt will then drop from the leaf (abscission).
• Cellular sequestration - halophytes can isolate salts within their cell walls and vacuoles.

Fig. 4 - A comparison between xerophytes and halophytes

Water loss in humans

We will lose water through our breath, skin, faeces, urine and sweat. On average, a human body will take in about 2.5 L of water a day and lose the same amount. If you become dehydrated, your body will “tell you” you are thirsty and need to drink water (Figure 5). You have probably also noticed your urine is more concentrated when you drink less water. This is also a good indicator that you need to have some water soon!

Unlike insects and plants, we do not have a waxy cuticle to trap water vapour or decrease water loss by evaporation. When it is hot, the body will sweat to keep cool. We will also breathe out water vapour and cannot alter how much water we will lose.

Kidneys will regulate the amount of water and ions removed from the body. This is osmoregulation. Osmoreceptors that modulate osmolality (number of dissolved particles in the fluid) are found in the brain’s hypothalamus. The pituitary gland, attached to the hypothalamus, will control the water present in the bloodstream and how much urine is produced by the kidneys by releasing hormones.

Fig. 5 - Overview of the thirst response in humans