Responding to Change

Living things sense and respond to change to ensure survival and optimum functioning. No environment is perfect for sustaining life without the organism itself adapting to it.

What are the changes in our ecosystem?

Our environment is in a state of constant change. One day might bring you a heatwave, making you sweat and hyperventilate, another day might bring you a broken thermostat in the middle of the winter. While some animals in the desert are in a state of constant water deficit, others could be struggling to keep themselves dry. Taller plants in the rainforest get an abundance of light, while the lower branches get fractured bits of sunlight.

While adaptations assist organisms to successfully respond to long-term environmental conditions, organisms have to prepare for sudden, emergent changes that the environment can surprise them with. These changes require an instant response for the organism to protect itself. This is where the concept of homeostasis comes in.

Cells maintain homeostasis by responding to change

The five facets that constitute the internal conditions of the cell include:

• The water content.

• Temperature.

• pH level of the cell contents.

• The nutrition level – mainly the concentration of glucose in the cell.

• Pressure levels in comparison to the environment.

Changes in any of these parameters act as a trigger for cells. These changes are known as stimuli.

Response to stimuli

Internal and external changes in the organism’s environment that interfere with the organism’s ability to carry out reactions efficiently require a response. It is vital that a response is given. Let’s take a closer look at the process.

The role of stimuli in responding to change

Cells detect changes through a specialised monitoring and security system called receptors.

What are receptors?

When the receptors detect changes, they send signals to the control centres to remedy the changes.

What is a response?

What happens next? As discussed, receptors send chemical or electrical impulses to the organism’s control centre. The control centre, in turn, sends signals to the relevant systems that can identify and respond positively to change. These target organs are called effectors, and the reaction to the stimuli is called a response.

Fig. 1 - Response to change

Let’s look at glucose control in the body as an example of a cell responding to its environment.

Example of cells responding to their environment

Meet X, a healthy person with no history of disease. Let's follow a scenario:

1. X eats a doughnut.
2. A few hours later, glucose is absorbed through the intestines into the bloodstream, causing an increase in blood glucose concentration. Some of the glucose is absorbed by the cells through simple diffusion, but not enough. The amount left in the bloodstream remains higher than normal.
3. High blood glucose level stimulates the receptors in the pancreas, which in turn cause the production and release of a hormone called insulin. Insulin enters the bloodstream and stimulates the uptake of glucose molecules into the cells, thus providing them with the nutrition they require to carry out their functions.

However, what happens if X doesn’t eat anything else that day and there is no glucose in their bloodstream? Luckily, earlier, when X had just eaten, some of the glucose molecules were stored by the liver cells and packed away in the form of glycogen. When the blood glucose concentration is dangerously low, a hormone called glucagon is released. Glucagon stimulates the conversion of glycogen into glucose and its release into the bloodstream. This ensures the maintenance of the homeostasis of blood glucose.

Fig. 2 -Example of cells responding to their environment in the human body

Responding to change in plants

While animals have more dramatic responses to stimuli, plants have to modify their growth patterns according to the changes in the environment. This is due to their inability to move around. Plants need adequate sunlight and water for optimum survival. They grow towards or away from the stimuli through patterns of growth called tropism.

Tropisms are predominantly regulated by the hormone auxin, present in the plant’s roots and shoots. Auxin promotes cell division, and its concentration in various areas of the stem or root determines the direction of growth. Plant stems grow towards the light as they contain chloroplasts which are the site of photosynthesis reactions. The auxins are distributed towards the area of the plant, which receives light and stimulates growth.

The plant identifies ways to respond positively to change and lean towards the stimulus – this is positive phototropism. Roots demonstrate negative phototropisms by growing away from the well-lit into the shaded areas (usually underground, as deeper soil has higher water concentration).

Another hormone called ethylene causes the fruits of the plant to ripen.

Fig. 3 - Phototropism in roots and shoots of plants

Nervous coordination in responding to change

In animals, the response to change involves a more complex mechanism of action, which includes various groups of cells. The receptors flag the stimulus, which converts it into electrical and chemical signals, further transmitting these signals to the brain via nerve cells called neurons. Signals are transmitted amongst cells via action potential – changes in the potential of the cell membranes. All these changes occur in the nervous system, which consists of the brain, the spinal cord, nerves, and neurons.

Muscle contraction in responding to change

Some stimuli require the organism to move to respond to the stimulus. This involves the limbs and other muscle groups which are responsible for mobility. There is a synchronised contraction and relaxation of the various skeletal muscle groups, overseen by the brain and the spinal cord and transmitted through neurons. The movement of the sliding filaments and the consequent contraction and relaxation of the muscles are controlled by electric and chemical signals and can be voluntary or involuntary.