Samacheer Class 8 Science - Organisation of Life

Cell differentiation is the process by which cells become specialized in structure and function during development. Our body develops from a single cell called the zygote, which undergoes mitotic division to produce many cells. As development progresses, these cells undergo changes in their structure, size, and content, acquiring specific characteristics that enable them to perform specialized functions. This process of acquiring specialized structures and functions is called differentiation. Through differentiation, different cell types such as nerve cells, muscle cells, and blood cells are formed, eventually leading to the formation of tissues, organs, and ultimately the complete fetus and adult organism.

Tissues are groups of similar cells that work together to perform a specific function. Based on their structure and function, tissues can be classified into four main types. Epithelial or covering tissue forms protective layers on the body surface and lines internal organs, providing protection and controlling the exchange of materials. Muscular or contractile tissue is responsible for movements and locomotion by contracting and relaxing. Connective tissue provides support and binds different structures of the body together, including bone, cartilage, and blood. Nervous tissue conducts nerve impulses throughout the body, enabling communication between different parts and coordinating body functions. Each tissue type has specialized cells adapted to perform its particular function efficiently.

Alveoli are tiny, thin-walled air sacs located at the ends of the bronchioles in the lungs. They are the functional units of the respiratory system where gas exchange occurs. The primary function of alveoli is to facilitate the exchange of oxygen and carbon dioxide between the air and the blood. Oxygen from the inhaled air diffuses across the thin alveolar walls into the surrounding capillaries, where it binds to hemoglobin in red blood cells for transport throughout the body. Simultaneously, carbon dioxide, a waste product of cellular metabolism, diffuses from the blood into the alveoli and is expelled during exhalation. The alveoli have a large surface area due to their numerous quantity, which maximizes the efficiency of gas exchange and is essential for maintaining proper oxygen levels and removing carbon dioxide from the body.

Osmoconformers and osmoregulators are two different strategies organisms use to manage water and salt balance in their bodies. Osmoconformers are organisms that try to maintain the osmolality of their body fluids matching with their surrounding environment. Most invertebrates and marine organisms such as starfish, jellyfish, and many fish species are osmoconformers. Since their internal osmolality is similar to their external environment, they do not need to expend much energy on osmoregulation. Osmoregulators, on the other hand, are organisms that maintain their internal osmolality at a relatively constant level, which can be extremely different from that of the surrounding environment, through active physiological processes. Most vertebrates, including mammals, birds, reptiles, and freshwater fish, are osmoregulators. They use various organs and mechanisms such as kidneys, gills, and specialized tissues to regulate the concentration of salts and water in their bodies, allowing them to survive in diverse environments ranging from freshwater to saltwater to deserts.

Metabolism is the sum of all chemical reactions and processes by which living organisms sustain their life and maintain their body functions. It encompasses all the biochemical activities that occur within cells and tissues to keep an organism alive, growing, and reproducing. Metabolism consists of two major components: anabolism and catabolism. Anabolism refers to the buildup or synthesis of complex substances from simpler molecules, requiring energy input. Examples include the synthesis of proteins from amino acids, the formation of glycogen from glucose, and the building of new tissues during growth. Catabolism refers to the breakdown of complex substances into simpler molecules, releasing energy that the organism can use for various life processes. Examples include the digestion of food, the breakdown of glucose during respiration to release energy, and the breakdown of stored fats. Together, these anabolic and catabolic processes allow organisms to obtain energy from food, build and repair tissues, maintain body temperature, and carry out all other essential life functions.

Aerobic respiration and anaerobic respiration are two different processes by which organisms release energy from food molecules. Aerobic respiration takes place in the presence of oxygen, while anaerobic respiration takes place in the absence of oxygen. In aerobic respiration, glucose is completely broken down, and the end products are carbon dioxide and water. This process releases a large amount of energy, approximately 38 ATP molecules per glucose molecule. Aerobic respiration is common in all higher plants and animals and occurs mainly in the mitochondria. In anaerobic respiration, glucose is partially broken down without oxygen, and the end products are carbon dioxide and either ethanol or lactic acid, depending on the organism. This process releases much less energy compared to aerobic respiration, approximately 2 ATP molecules per glucose molecule. Anaerobic respiration is common in certain microorganisms such as yeast and bacteria, and also occurs in human muscle cells during intense physical activity when oxygen supply is limited. Anaerobic respiration in muscles produces lactic acid, which causes muscle fatigue and soreness.

Epithelial tissue is a tissue that acts as a covering and protects the underlying tissues and organs. It is classified into several types based on the shape and arrangement of cells. Squamous epithelium consists of flat, thin cells arranged in a single layer and is found in areas where diffusion and filtration occur, such as in the alveoli of lungs and the lining of blood vessels. Cuboidal epithelium consists of cube-shaped cells and is found in ducts of glands and in the kidney tubules where secretion and absorption take place. Columnar epithelium consists of tall, rectangular cells and is found in the lining of the stomach and intestines where secretion and absorption of nutrients occur. Ciliated epithelium consists of columnar cells with hair-like structures called cilia on their free surface, and is found in the respiratory tract where cilia help move mucus and trapped particles. Glandular epithelium consists of cells specialized for secretion of substances such as sweat, mucus, saliva, and digestive enzymes, and forms the lining of glands throughout the body.

The human eye can be compared to a camera as both functions by gathering, focusing, and transmitting the light through the lens for creating an image of an object.
The iris of the eye controls the size of the pupil depending on the amount of light entering it.
The pupil is like the eyehole of a camera which allows light to come in.
Lens : It is a transparent, biconvex, and an adjustable part of an eye, made up of protein. The lens with the help of the cornea refracts light focused on the retina, therefore creating images on it.
The retina consists of photoreceptors and converts light rays into impulses to be sent to the brain. It is light sensitive.
The retina is compared to the film in a camera.

Homeostasis is a property of a human biological system where self-regulating processes tend to maintain a stable internal environment and balance for the survival of the organism. The body maintains homeostasis through various physiological and behavioral responses. Behavioral responses include actions such as seeking shelter, adjusting clothing, and changing activity levels to maintain body temperature and comfort. Physiological responses involve automatic adjustments in body functions such as heart rate, breathing rate, and hormone secretion. All the processes of integration and coordination of these functions are mediated by the nervous system and the hormonal (endocrine) system. The nervous system provides rapid responses through electrical signals, while the endocrine system provides slower but longer-lasting responses through hormones. Several organs and systems work together to maintain homeostasis. The liver helps regulate blood glucose levels and detoxifies harmful substances. The kidneys maintain water and electrolyte balance by filtering blood and producing urine. The brain, particularly the hypothalamus, acts as the control center that monitors internal conditions and triggers appropriate responses. The autonomic nervous system controls involuntary functions such as heart rate, digestion, and breathing. The endocrine system releases hormones that regulate metabolism, growth, reproduction, and stress responses. Together, these organs and systems ensure that the internal environment remains relatively constant despite changes in the external environment.

Osmosis is the movement of solvent particles across a semipermeable membrane from a dilute solution into a concentrated solution. The solvent moves to dilute the concentrated solution and equalize the concentration on both sides of the membrane. The movement of liquids in and out cells is dependent on the concentration of the solution surrounding it. There are 3 types of situations in which this could vary:
1. Isotonic:
Here the concentration of external and internal solution of the organism are the same.
2. Hypotonic:
Here the external solution concentration is less compared to the concentration of the inner solution of an organism. In this case water will rush into the organism.
3. Hypertonic:
Here the external solution concentration is greater than the concentration of the inner solution of an organism. In this case the water will rush out of the organism.
Example:
Red blood cells.
When red blood cells are placed in hypertonic solution, water flows out of the cell faster than it comes in. This results in shrivelling of RBC.
On the other hand if RBC is hypotonic, more water will flow into the cell than out. This results in swelling of the cell and followed by bursting.
If the RBC is placed in an isotonic solutions, the flow of water in and out of the cell will happen at the same rate.

Inhalation and exhalation are two complementary processes that together form the breathing cycle. During inhalation, the muscles of the diaphragm contract and pull downward, causing the diaphragm to move down. Simultaneously, the external intercostal muscles between the ribs contract, causing the ribs to move upwards and outwards. These movements increase the volume of the thoracic or chest cavity. As the volume increases, the pressure inside the lungs decreases, creating a pressure gradient that causes air to enter the lungs through the nose and mouth. During exhalation, the muscles of the diaphragm relax, and the diaphragm moves upward to its original position. The ribs move downwards and inwards as the external intercostal muscles relax. These movements decrease the volume of the thoracic cavity. As the volume decreases, the pressure inside the lungs increases, forcing air out of the lungs through the nose and mouth. In normal quiet breathing, exhalation is a passive process that does not require muscular effort. However, during forced exhalation such as during exercise or coughing, additional muscles called abdominal muscles contract to further decrease the thoracic volume and expel air more forcefully.

The process of taking air into the lungs is called inspiration or inhalation.
During inspiration, the sternum is pushed up and outward and the diaphragm is pulled down.
This increases the volume of the thoracic cavity and the pressure decreases.
The air outside the body flows into the lungs. Here exchange of gases takes place between the air and the blood.
The process of expelling air from the lungs is called expiration or exhalation. Upon exhalation, the lungs recoil to force the air out of the lungs.
The intercostal muscles relax, returning the chest wall to its original position. During exhalation, the diaphragm also relaxes, moving higher into the thoracic cavity.
This increases the pressure within the thoracic cavity relative to the environment.
Air rushes out of the lungs due to the pressure gradient. This movement of air out of the lungs is a passive event.

Instant energy is needed for performing day-to-day activities of the body and for responding to sudden physical demands. Energy is obtained through the intake of food, which provides fuel for all organ systems. Instant energy may be required in cases of extended physical activities like running, jumping, or playing sports, and also during physical ailments like tiredness, giddiness, or weakness. Glucose is the simplest form of carbohydrate and is the primary source of energy for cells. When glucose is consumed, it is quickly absorbed into the bloodstream and can be immediately utilized by cells for energy production through the process of cellular respiration. Glucose dissolves readily in blood and is rapidly transported to organs and tissues throughout the body, thus helping to provide instant energy. The digestion of carbohydrates such as starch and sucrose breaks them down into glucose molecules, which can then be absorbed and used immediately or stored as glycogen in the liver and muscles for later use. This is why glucose and other simple sugars are particularly effective for providing quick energy during physical exertion or when the body needs an immediate energy boost. Complex carbohydrates take longer to digest and provide sustained energy over a longer period.

Pickles are prepared usually by addition of excess salt. The salty solution creates a high concentration (hypertonic) in the external medium of the vegetable / fruit which is used for making pickles.
Therefore water comes out of the vegatable and it undergoes plasmolysis and begins to shrink.
Thus when water content is lost the pickle is able to retain its shelf life for a longer period.

In a closed and crowded place, the number of people present is much greater than in an open space. All of these people continuously breathe out carbon dioxide as a byproduct of cellular respiration. Therefore, the amount of carbon dioxide in the air accumulates and becomes much higher than normal. Simultaneously, the amount of oxygen available in the air decreases as it is consumed by the respiration of all the people present. This creates an imbalance in the composition of air, with high levels of carbon dioxide and low levels of oxygen. When we inhale this air with reduced oxygen content and high carbon dioxide content, our body receives insufficient oxygen for efficient cellular respiration. Additionally, the high concentration of carbon dioxide can stimulate the respiratory center in the brain to increase breathing rate, but the air available still contains inadequate oxygen. This combination of low oxygen and high carbon dioxide makes it difficult to breathe normally in a closed and crowded place. We may experience shortness of breath, discomfort, dizziness, or headache. Proper ventilation and air circulation are essential to maintain adequate oxygen levels and remove excess carbon dioxide in enclosed spaces.

Excessive usage of mobile phones can have several harmful effects on the eyes. Cell phone radiation can damage the eyes and cause early cataract formation, a condition where the lens becomes cloudy and vision is impaired. The radiation can also affect the retina, cornea, and other structures of the eye. Prolonged screen time strains the eye muscles, particularly the ciliary muscles that control focusing, leading to eye fatigue and discomfort. Excessive mobile phone usage also causes temporary problems such as dry and itchy eyes due to reduced blinking, blurry vision from constant focusing on a near object, and pain in the eyes. The blue light emitted by mobile phone screens can also contribute to eye strain and may affect sleep patterns. The teacher demonstrated several important values through her actions. She showed empathy by showing concern for Arjun's health and well-being despite his being a student. She demonstrated responsibility by taking initiative to address the problem and advising the parents to seek professional medical help rather than ignoring the issue. She also promoted personal care and awareness by educating the student about the importance of limiting screen time and taking care of one's health. Additionally, the teacher showed the value of preventive health care by intervening early to prevent further damage to Arjun's eyes.