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Water Loss

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Biology

Water is an essential element not only for us humans but for other organisms too. In plants, for example, water plays a crucial role in transporting minerals up the plant, photosynthesis, general cooling of the plant and others! Naturally, when the water travels up the plant, it has to go somewhere. Water loss will occur through the evaporation of water (transpiration in plants).

Why is water important?

Water is vital to most organisms on the planet; hence, water is often conserved to prevent loss. A few examples of why water is essential:

  • Temperature regulation - Temperature within an animal’s body needs to be maintained for homeostasis (maintaining internal body stability).
  • pH regulation of the internal body
  • Hydrolysis for ATP breakdown and energy production
  • Digestion - water forms a lining within the digestive tract and animal’s stomach from the corrosive properties of acid.
  • Photosynthesis - uses water to produce oxygen and sugars.
  • Movement of ions and sugars in the plant’s xylem.

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.

Figure 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.

Figure 2a) Tracheal system of an insect.

Figure 2b) 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.

Stomata: openings in the plant epidermis.

Figure 2. 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 insect’s spiracles, the stomata can be open 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.

Figure 3. 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 4). 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.

Figure 4. Overview of the thirst response in humans.

The pituitary gland is also found in all vertebrates, although its structure will vary within different groups.

Water Loss - Key takeaways

  • Water is vital to most living organisms on the planet. Water is used for body temperature and pH regulation, photosynthesis, movement of ions and sugars and others.
  • Water loss mainly occurs through evaporation (transpiration in plants).
  • Living organisms have varied adaptations to control water loss. These adaptations include specialised surfaces and organs and a lower SA: Vol ratio.
  • Some living organisms, such as xerophyte and halophyte plants, are adapted to live in extremely harsh environments. They will have additional adaptations to water conservation and reduction in loss.

Water Loss

Stomata contain guard cells, which by increasing and decreasing in size, can decrease/increase the size of the stomatal opening. In xerophytes, stomata can be hidden away in pits and grooves and hair appendages are present to trap water vapour.

The main ways vascular 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. Stomata opens and closes when the guard cells increase/decrease in size. Mutualistic microorganisms help the plant’s roots to stay moist in exchange of the plant’s sugars. Xerophytes and halophytes are especially adapted to prevent water loss and will have additional adaptations to conserve water.

The human body cannot control the amount of water that is lost through the skin.

Control of water loss will happen through the kidneys. Kidneys control how much water and ions are removed from the body by osmoregulation.

In all vertebrates, the pituitary gland, which is attached to the hypothalamus, will control the water present in the bloodstream and how much urine is produced by the kidneys. It will vary across different groups of animals.

Final Water Loss Quiz

Question

Water loss mostly occurs by evaporation in humans. How do plants mostly lose water?

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Answer

By transpiration.

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Question

Give two examples on why water is important in plants.

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Answer

Photosynthesis and movement of ions.

Show question

Question

The body of an insect is covered by a waxy cuticle that is made of cellulose. True or False?


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Answer

False.

Show question

Question

Fill in the blanks about spiracles in insects. 


Spiracles are small openings in the ________, at the body ________ of an insect. They are usually found around the ________ area. Oxygen and carbon dioxide are ________ at the spiracles. Water loss will occur when the spiracle is _______. In order to limit the water loss, when the insect is resting, it will ______ its spiracles. Due to the lowered activity, the need for _______ is reduced. Spirales have _______ which will _______ and ________. Spiracles also have _______ that will trap the water ________. This maintains the ________.  Preventing water loss will ensure that the gas exchange surfaces will remain _______.


Show answer

Answer

Spiracles are small openings in the tracheae, at the body surface of an insect. They are usually found around the abdominal area. Oxygen and carbon dioxide are exchanged at the spiracles. Water loss will occur when the spiracle is open. In order to limit water loss, when the insect is resting, it will close its spiracles. Due to the lowered activity, the need for oxygen is reduced. Spiracles also have valves which will open and close. Spiracles also have hairs that will trap the water vapour. This maintains the humidity. Preventing water loss will ensure that the gas exchange surfaces will remain moist.

Show question

Question

What would happen if the insect’s cuticle was utterly impermeable?


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Answer

Water loss will be reduced, however, the gas exchange would not take place and the insect will die.

Show question

Question

Plants transport water and ions via phloem. True or False?


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Answer

False.

Show question

Question

 Give examples of four adaptations in xerophytes to limit water loss.


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Answer

Small SA:Vol ratio, thick cuticle, rolled leaves, CAM physiology. (any other adaptation).

Show question

Question

Define a halophyte.


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Answer

Halophyte is a plant that has adapted to live in salty conditions.

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Question

Explain how rolling of the leaves will reduce water loss from the plant. Give an example of a plant that rolls its leaves.


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Answer

Rolling of the leaves will trap water vapour and create a high water potential. When the water potential is the same in both internal and external environments, therefore, there will be no loss of water. Plants such as Marram grass have this adaptation.

Show question

Question

What is meant by CAM physiology in plants?


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Answer

CAM is a carbon fixation pathway which has evolved to reduce water loss. A plant using CAM, will photosynthesise during the day and exchange gases during the night.

Show question

Question

Why would a halophyte alter its flowering season? Give an example of a cue to flower.


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Answer

Halophytes live in salty conditions. Flowering will require extra energy which is used for salt exclusion etc. A halophyte will flower during the rainy season, when the salt content in the soil is lower.

Show question

Question

 Fill in the blanks about water loss control in humans.


 _______ regulate the amount of water and ions ________ from the body. This process is referred to as ___________. __________ that modulate __________ are found in the _________ of the brain. The_________ gland, which is attached to the __________, will control how much _________ is produced by the kidneys and water _________ in the bloodstream. It will control this by releasing ___________.

Show answer

Answer

Kidneys regulate the amount of water and ions removed/excreted from the body. This process is referred to as osmoregulation. Osmoreceptors that modulate osmolality are found in the hypothalamus of the brain. The pituitary gland, which is attached to the hypothalamus, will control how much urine is produced by the kidneys and water concentration/content in the bloodstream. It will control this by releasing hormones.

Show question

Question

What is an example of a symptom that you need to drink more water?


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Answer

Dry mouth.

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Question

What is the most external surface of the insect’s cuticle called?


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Answer

Epicuticle.

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Question

Large surface area to volume ratio is great for reducing water loss in an insect. True or False?


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Answer

False.

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