FUNCTIONS OF CELLS IN EXCRETION, GROWTH AND SENSITIVITY IN LIVING THINGS

The functions of cells is not only limited to only Nutrition, Respiration as there are many functions cells plays in our bodies which I will be looking at today

Source - Pixabay

EXCRETION

The metabolic reactions that occur all the time in cells make waste products. Many of these waste products are toxic (poisonous), for example the carbon dioxide, lactic acid and ethanol produced during cellular respiration. It is important that they are removed from the cell as quickly as possible.

However, some metabolic waste products are useful. Oxygen is a waste product of photosynthesis. It is necded by the plant cells that photosynthesise. However, so much oxygen is produced that it can be removed trom the cell to be used by other cells or other organisms. The carbon dioxide produced in respiration is needed by plant and algal cells for photosynthesis.

lf there is too much of a substance in a cell or in the body of an organism, some of it may be broken down, so that it can be removed. For example excess amino acids (the building blocks of proteins) cannot be stored in the cells. So, excess amino acids are broken down in the liver cells to produce molecules that can be used in the Krebs cycle to give energy. The rest of the molecule is turned into nitrogenous waste, which necds to be excreted from the cells, and then from the body. In humans this is urea, which is excreted by the kidneys and removed in the urine. In some other organisms, the nitrogenous waste can be uric acid (such as birds) or even ammonia.

Excretion is the removal of metabolic waste products from the cell, however they have been formed. Excretion usually takes place by a process of diffusion down a concentration gradient from the cell to the surroundings. However, sometimes materials are actively moved out of the cell. For example, the contractile vacuole in protozoans such as Amoeba not only removes excess water from the cell, but also carries small amounts of urea and carbon dioxide out in solution.

GROWTH

One of the reasons why cells and organisms need food is for growth. Growth in a biological sense does not just mean getting bigger. Growth is a permanent increase in the dry mass of an
organism. lt involves an irreversible increase in the size or length of an organism, and it also involves a permanent increase in the number or
size of the cells. Growth takes place as the result of cell division (mitosis), cell enlargement and cell differentiation.

Mitosis is simple cell division resulting in two new cells that are genetically identical to the original cell. It is the process by which all cells divide, except when special sex cells are made for reproduction.

The chromosome pairs in the nucleus of the cell are copied. Each individual chromosome becomes a pair of chromatids. These then move apart to opposite ends of the cell. The cytoplasm divides and the result is two identical daughter cells. Once cell division has taken place, the cells make new biological material so they can enlarge. They also often differentiate, to form specialised cells that carry out specialised jobs in the body.

You can see mitosis taking place in areas of rapidly dividing cells, such as in the root tip of a growing plant.

Growth is usually controlled by chemical messages called hormones, which affect the rate of cell division or enlargement. In humans, growth is controlled by a number of hormones including growth hormone and the sex hormones oestrogen and testosterone. In insects, growth is controlled by juvenile hormone and ecdysone. In plants, growth is controlled by a number of hormones, including auxins and giberellin.

The rate of growth of an organism is also affected by external factors such as the amount of food available, the availability of nutrients and water, the humidity level and, for plants, the amount of light available for photosynthesis. The rate at which an organism grows can be measured and used to produce a growth curve. This can be done with plants and animals.

It is important that biologists can measure growth in plants, such as seedlings, so that they can estimate the growth rate of a plant. This can help them to establish which crop plants are going to give a high yield. It can also be used to measure how diferent factors affect the rate of growth, as increase in length of stem or roots, or the increase in numbers of leaves as ways of measuring growth in plants.

However, measuring the increase in dry mass is a better way of mcasuring growth because it is more representative of the whole structure. It also gives a better guide to eventual yield. Measuring dry mass is also more reliable than measuring fresh mass because fresh mass can vary as the cells take up or lose water. Although dry mass measurements are more reliable, they are also more difficult and time-consuming. Also, when you measure the dry mass, you have to kill some of the plants. This means you
need to grow a large sample so that a percentage of them can be dried and weighed at intervals.

This is the method used: A large batch of seeds is planted. The seedlings are grown under the same conditions. At intervals, a sample is harvested, cleaned of any soil from the roots, and dried in an oven, until the seedlings reach constant mass. If a graph is plotted of dry mass against time, the rate of growth can be estimated.

SENSITIVITY

It is very important for all living organisms to be aware of changes in their environment. This is true for individual cells as well as whole organisms.

Sensitivity to their surroundings is how organisms avoid danger or difficult conditions, find food or the best conditions to make food and find a mate or another organism. Chemical sensitivity often comes from receptors in the cell membrane that respond to particular chemicals. Chemicals in the cell may respond to the presence of light.

Some cells, particularly in single-celled organisms, have special areas which are sensitive to light. Membranes may respond to pressure.

Single-celled organisms such as Amocba,
Chlamydomonas and Euglena all show sensitivity to different factors in their environment and react by moving away or towards things.

Plants need to be sensitive to their environment too. They need to respond to factors, such as light, water and gravity, to make sure that they grow the right way up, and that they make as much food by photosynthesis as possible.

Plants achieve their sensitivity through a system of hormones, chemical messages that are produced in one part of an organism and have an effect elsewhere. Plant hormones have several effects on plants, for example, they co-ordinate flowering, cell division and cell elongation. These are essentially growth processes, and plant responses ot this type are called growth responses. Since growth is a slow
process, most plant responses are slow.

Plants need light for photosynthesis, and they grow towards the light. When a seed germinates, the roots grow downwards and the shoots grow upwards. These responses to gravity are vital if the new plant is to be anchored firmly in the soil, and the shoots and leaves are to be held above the ground in the sunlight.

Plant responses to stimuli that come from one direction are known as tropisms. The following investigations will allow you to observe some tropisms for yourself.

The shoots of seedlings that were uncovered will grow upwards normally. Those in the lightproof box grow towards the hole. Since the hole was the only source of light into the box, the curving
growth of the shoots was a response to light reaching them from one side only. This type of response, in which shoots grow towards the light, is termed positive phototropism.

Animals are also sensitive to their environment. They often have specialised cels that sospond factors such as light, chemicals, touch and taste. Animal responses to a stimulus often involves movement. They move towards things they want or like, such as food or a mate, and move away from things they dont like. For example, an earthworm lives underground in the dark. If it is in the light,
This means it is above ground and can easily be seen and eaten by birds and other predators

Earthworms have a clear response to light called phototaxis. They move away from light as fast as they can.

REFERENCES

. https://www.britannica.com/science/excretion
. https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookEXCRET.html
. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/cell-growth
. https://www.sciencedirect.com/topics/mathematics/sensitivity-function
.