Lesson Explainer: Absorbing Minerals Biology

In this explainer, we will learn how to recall examples of micronutrients and macronutrients required by plants and explain how plants absorb these essential nutrients from the environment.

All living organisms require nutrition—primarily, to keep them alive! Without essential nutrients, crucial life processes like respiration, growth, and repair of damaged tissues will not take place. Unlike humans, plants do not have complex digestive systems to break down their food and provide their cells with the needed nutrients. Plants require some of their nutrition in the form of minerals, which are inorganic substances that cannot be synthesized by a plant and so must be taken in via absorption. Plants obtain essential minerals from the soil generally via absorption into their highly specialized roots.

Definition: Nutrients

Nutrients are substances that an organism requires for energy, building materials, and controlling biological processes.

Definition: Mineral

A mineral is an inorganic nutrient that cannot be synthesized by an organism and so must be ingested or absorbed.

The biological molecules within the cells of all living things on Earth are made up largely of three key elements: carbon, hydrogen, and oxygen. However, there are also other elements and minerals that are essential for a healthy organism, and these may differ between different species. In plants, the essential minerals required are divided into two main groups, macronutrients and micronutrients. Table 1 gives some examples of minerals that are classified as macronutrients or micronutrients.

Macronutrients are those minerals that are needed in relatively large amounts. The prefix macro- actually means “large” or “long.” For instance, nitrogen is a key component of the amino acids that will be joined together to form proteins. Proteins are one of the four major biological macromolecules in living organisms; therefore, nitrogen is needed in considerable amounts. Without enough nitrogen, the plant is likely to have leaves that turn yellow and potentially die. As mentioned, carbon, hydrogen, and oxygen are key components of almost all organic compounds in plants, so a lack of these elements will result in very poor growth, wilting of a plant, and eventual death. Carbon alone makes up around 45% of the dry mass of a typical plant! Magnesium is a component of chlorophyll, which is the green pigment in plants that absorbs sunlight to provide the light energy needed for photosynthesis. Without enough magnesium, the leaves of the plant will turn yellow and photosynthesis will not take place. In plants that produce flowers, a lack of availability of nutrients can also prevent flower growth.

Micronutrients are those elements needed in much smaller amounts (no more than a few milligrams per litre) and can be referred to as trace elements. Most micronutrients act as “cofactors” in enzymatic reactions. A cofactor is a nonprotein component of an enzyme that helps the enzyme catalyze a specific reaction. For instance, iron is a cofactor for proteins that are involved in important metabolic processes like cellular respiration and photosynthesis.

Definition: Cofactor

A cofactor is a nonprotein molecule or ion that is required for the proper functioning of an enzyme.

Example 1: Comparing Macronutrients and Micronutrients

Nutrients required by plants can be divided into two groups: macronutrients and micronutrients. Which of the following best explains the difference between the two?

  1. Micronutrients are rarely found in soil and more difficult for plants to obtain than macronutrients.
  2. Macronutrients are required in large amounts by the plant, whereas micronutrients are required in very small quantities.
  3. Macronutrients are large molecules compared to micronutrients, which are much smaller-sized molecules.
  4. Macronutrients take longer to break down in a plant than micronutrients.

Answer

All living organisms require nutrients to keep them alive, functioning, and healthy. A nutrient is any substance that is required by an organism to survive, grow, and reproduce. Humans obtain nutrients from the food they eat and digest, but plants must absorb the majority of their nutrients from the soil.

The two major classes of nutrients plants require can be divided into macronutrients and micronutrients. Macronutrients include elements like carbon, hydrogen, oxygen, potassium, phosphorus, and magnesium. These are the nutrients that the plant requires in relatively large amounts. For instance, all the major biological molecules are primarily composed of carbon, hydrogen, and oxygen in different ratios, so a plant will need to take in a considerable amount of these elements. Micronutrients include elements like iron, zinc, and chlorine. Micronutrients are primarily used as cofactors in enzyme-controlled reactions. They are not required in such large quantities, and plants generally only need to absorb small amounts of these nutrients from the soil.

Therefore, the difference between macronutrients and micronutrients is that macronutrients are required in large amounts by the plant, whereas micronutrients are required in very small quantities.

Some of these nutrients will exist as ions in the soil. Ions are electrically charged atoms, or groups of atoms, that form an electrically charged molecule. Positively charged ions, such as K+, Ca2+, and Mg2+ are called cations. Negative ions, such as SO42, NO3, Cl, and HPO42 are called anions.

Key Term: Ions

Ions are electrically charged atoms, or groups of atoms, that form an electrically charged molecule. Ions that have a positive charge are called cations, and those with a negative charge are called anions.

If the concentration of ions in the surrounding soil is higher than the concentration of ions within the plant roots, the ions will move into the roots using a process called diffusion. Diffusion is the movement of particles from an area of high concentration to low concentration. It is a passive process, which means it does not require a supply of energy. Figure 1 demonstrates how ions move by diffusion from the soil and into the root hair cells.

Definition: Diffusion

Diffusion is the movement of molecules from a region of high concentration to a region of low concentration.

Example 2: Recalling How Ions Move into Plant Roots form High Concentrations in the Soil

If there is a high concentration of potassium ions in the soil, they move to an area of low concentration in the roots. What term is given to this process?

  1. Diffusion
  2. Synthesis
  3. Osmosis
  4. Digestion
  5. Active transport

Answer

Much like humans, plants need to obtain a certain amount of different nutrients to stay alive and healthy. Potassium is a crucial nutrient to plants as it plays an important role in biological processes like protein synthesis and photosynthesis.

When particles or molecules are found in large concentrations in a particular area, they tend to “spread out” and move into areas where there is a lower concentration of these particles or molecules. This trend is demonstrated in the diagram below.

Eventually, the concentration of molecules will even out, as shown in the next diagram.

This process is known as diffusion. Diffusion is a passive process, which means it does not require energy, and it is the movement of particles from an area of high concentration to an area of low concentration. In the question, we are told that potassium is in high concentrations in the soil. So, it will move by diffusion into the roots where it is in a lower concentration.

Therefore, the correct answer is A: diffusion.

The cell wall of root hair cells is relatively thin, to allow the movement of ions and water. However, some ions may not make it all the way into the cell cytoplasm and vacuole. Underneath the cell wall is a cell membrane. The cell membrane is semipermeable, which means they allow some ions to pass through but prevent others from doing so.

Key Term: Semipermeable (Partially Permeable)

A structure is semipermeable if it allows some molecules or substances to pass through but not others.

The membranes of root hair cells also demonstrate selective permeability. While semipermeable membranes tend to allow, or not allow, substances through based on size or the charge of the molecule, if a membrane is selectively permeable, it will select substances to allow into the cell based on the cell’s current needs.

Key Term: Selectively Permeable

A membrane is selectively permeable if it can regulate the substances that pass across it.

If there is a higher concentration of a certain ion inside the cell than in the surrounding soil, it will not be able to diffuse into the root hair cell. However, if the plant still requires more of the ion, it can be moved into the cell via active transport.

Definition: Active Transport

Active transport is a process requiring energy by which particles move across a plasma membrane from an area of low concentration to high concentration.

Active transport, as the name suggests, is an active process—this means it requires energy to take place. In contrast, diffusion is a passive process, as it does not require energy to occur. Energy for active transport is supplied in the form of ATP, which is an energy-carrying molecule that is found in all living cells.

Let’s have a look at some experimental data that helps demonstrate active transport of minerals.

The Nitella genus is composed of multiple species of green algae that grow in water. Algae also require essential minerals, which are divided into two main groups: macronutrients and micronutrients. In Figure 2, we can see that the concentrations of chlorine, potassium, magnesium, calcium, and sodium are higher within the cells of the Nitella, compared to the surrounding water.

However, we know that these ions are essential to maintaining a healthy and functioning alga. Potassium, magnesium, and calcium are all macronutrients, so these especially should be absorbed by the alga in relatively large amounts. As they are in such low concentrations in the surrounding water, it is not possible for the ions to diffuse into the alga cells—remember, diffusion is the movement of substances from an area of high concentration to an area of low concentration. Instead, these ions will need to be actively transported into the cells to keep the algae functioning. Figure 3 provides a simple diagram to outline how this happens.

We can also see that ions are selectively absorbed. This refers to the plant taking in more of some ions than others, depending on their nutritional need. In the graph shown in Figure 2, for instance, the Nitella algae are absorbing a higher volume of chlorine ions than sodium ions.

Example 3: Understanding the Process by Which Ions Move against Their Concentration Gradients

The graph shows a comparison between the cells of the algae Nitella and the surrounding water. By what process could the Nitella obtain more calcium (Ca2+) from the surrounding water?

  1. Diffusion
  2. Synthesis
  3. Osmosis
  4. Digestion
  5. Active transport

Answer

Plants and algae must obtain many of their required nutrients by absorbing them from their surroundings. Plants on land will absorb nutrients from the soil and into their roots, whereas algae found in aquatic environments will take in nutrients from the surrounding water.

There are two major ways elements and minerals can be taken in by plants and algae. Diffusion is the movement of substances down their concentration gradient, from an area where they are in a relatively high concentration to an area where they are in a low concentration. Diffusion is a passive process, which means it does not require energy to occur. The diffusion of water molecules across a semipermeable membrane is known as osmosis. Active transport, however, is the movement of substances against their concentration gradient. This means substances will move from an area of low concentration to an area of high concentration. Active transport, as the name suggests, is an “active” process and requires an input of energy to occur.

In the graph, we can see that the calcium ions are in a much higher concentration within the cells of the Nitella algae than in the surrounding water. However, the algae may still require more calcium ions to carry out essential life processes. To obtain more calcium ions, the plant would need to move them against their concentration gradient. As we have just seen, the way to do this is by using active transport.

Therefore, the process the Nitella could use to obtain more calcium (Ca2+) from the surrounding water is option E, active transport.

Let’s summarize the key points from this explainer.

Key Points

  • All living things, including plants, require nutrients to stay healthy, functioning, and alive.
  • Plants that do not obtain enough nutrients may have stunted growth and discolored leaves, wilt, and potentially die.
  • Macronutrients are essential nutrients that plants need in relatively large amounts, whereas micronutrients are those that are needed in small amounts.
  • Plants and algae can absorb nutrients from their environment by diffusion or active transport.

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