Chapter Notes

Life Processes

To determine if something is alive, we look for characteristics that distinguish it from non-living things. Obvious signs like running, chewing, or shouting indicate life. Even when an organism is asleep, we know it's alive because we can observe processes like breathing. For plants, growth over time is a key indicator. However, relying solely on visible movement is not sufficient, as some life processes are invisible to the naked eye.

The most fundamental characteristic of life, according to biologists, is molecular movement. Living organisms are highly organized structures, from tissues down to cells and their components. This order is constantly threatened by the environment and tends to break down. To counteract this, living creatures must continuously repair and maintain their structures by moving molecules around. This constant molecular activity is essential for life. Viruses, for example, show no molecular movement on their own and only become active when they infect a host cell, which is why their status as "truly alive" is debated.

WHAT ARE LIFE PROCESSES?

The essential functions that living organisms perform to maintain their structure, repair damage, and stay alive are called life processes. These processes must continue even when the organism is resting or asleep.

Key life processes include:

• Nutrition: The process of taking in food (a source of energy) from the outside and using it for maintenance and growth.
• Respiration: The process of breaking down food sources to release energy for cellular needs. This often involves taking in oxygen.
• Transportation: The system for moving food, oxygen, and waste materials from one part of the body to another.
• Excretion: The process of removing harmful waste by-products generated by metabolic activities.

In single-celled organisms, these processes can occur through simple diffusion because their entire surface is in contact with the environment. However, in complex multi-cellular organisms, specialized tissues and organ systems are required to perform these functions efficiently, as not all cells are in direct contact with the surroundings.

NUTRITION

All living organisms need energy to maintain order in their bodies, even when not performing any activity. They also require raw materials from the outside to grow, develop, and synthesize substances like proteins. The source of this energy and these materials is the food they consume.

How do living things get their food?

While the need for energy is universal, organisms fulfill it in different ways. This leads to two main modes of nutrition:

• Autotrophic Nutrition: Organisms make their own food from simple inorganic sources like carbon dioxide ($CO_2$) and water ($H_2O$). Green plants and some bacteria are autotrophs.
• Heterotrophic Nutrition: Organisms obtain energy by consuming complex substances prepared by autotrophs. These complex substances are broken down into simpler ones using biological catalysts called enzymes. Animals and fungi are heterotrophs.

Autotrophic Nutrition

Autotrophs meet their carbon and energy needs through photosynthesis. This is the process where they take in inorganic substances from the outside (carbon dioxide and water) and convert them into stored energy in the form of carbohydrates, using sunlight and chlorophyll.

The overall chemical equation for photosynthesis is: $6 \mathrm{CO}_{2}+12 \mathrm{H}_{2} \mathrm{O} \xrightarrow[\text { Sunlight }]{\text { Chlorophyll }} \underset{\text { (Glucose) }}{\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}}+6 \mathrm{O}_{2}+6 \mathrm{H}_{2} \mathrm{O}$

The carbohydrates produced are used for energy. Any excess carbohydrates not used immediately are stored as starch in plants, serving as an internal energy reserve.

Key Events in Photosynthesis:

1. Absorption: Light energy is absorbed by the green pigment chlorophyll, which is contained in cell organelles called chloroplasts.
2. Conversion: The absorbed light energy is converted into chemical energy. This energy is used to split water molecules ($H_2O$) into hydrogen and oxygen.
3. Reduction: Carbon dioxide ($CO_2$) is reduced to form carbohydrates (like glucose).

These steps don't always happen one after another. For example, desert plants often take up $CO_2$ at night and create an intermediate compound, which is then converted into carbohydrates during the day when sunlight is available.

Raw Materials for Photosynthesis:

• Carbon Dioxide ($CO_2$): Plants obtain $CO_2$ from the atmosphere through tiny pores on the surface of their leaves called stomata. These pores are opened and closed by guard cells. When guard cells swell with water, the pore opens; when they shrink, the pore closes. This helps the plant regulate gas exchange and prevent excessive water loss.
• Water ($H_2O$): Terrestrial plants absorb water from the soil through their roots.
• Sunlight: The energy source, absorbed by chlorophyll.
• Other Materials: Plants also absorb other essential minerals from the soil, such as nitrogen, phosphorus, iron, and magnesium. Nitrogen is crucial for synthesizing proteins and other compounds.

Heterotrophic Nutrition

Heterotrophic organisms are adapted to their environment and have different strategies for obtaining food.

• Saprotrophic Nutrition: Some organisms, like fungi (bread moulds, yeast, mushrooms), break down food material outside their bodies and then absorb the nutrients.
• Holozoic Nutrition: Others, like animals, take in whole food and break it down inside their bodies. The type of nutritive apparatus depends on the food source (e.g., a cow for stationary grass vs. a lion for a mobile deer).
• Parasitic Nutrition: Some organisms derive nutrition from other living plants or animals without killing them. Examples include cuscuta (amar-bel), ticks, lice, leeches, and tape-worms.

How do Organisms obtain their Nutrition?

• Single-celled organisms: In simple organisms like Amoeba, food is taken in using temporary finger-like extensions called pseudopodia, which form a food-vacuole around the food particle. Inside the vacuole, enzymes break down the complex food into simpler substances that diffuse into the cytoplasm. In Paramoecium, which has a definite shape, food is taken in at a specific spot, moved there by the rhythmic movement of cilia covering its surface.
• Multi-cellular organisms: As organisms become more complex, different parts specialize. In humans, this specialization is seen in the digestive system.

Nutrition in Human Beings

The human digestive system consists of the alimentary canal, a long tube extending from the mouth to the anus, with various specialized regions.

1. Mouth: Food is crushed by the teeth into smaller particles. Salivary glands secrete saliva, which wets the food and contains the enzyme salivary amylase. This enzyme begins the digestion of starch (a complex carbohydrate) into simple sugar. The tongue helps mix the food with saliva.

2. Oesophagus (Food-pipe): The food is moved from the mouth to the stomach through the oesophagus. The walls of the alimentary canal have muscles that contract rhythmically to push the food forward. These movements are called peristaltic movements.

3. Stomach: This large, muscular organ expands when food enters. The stomach walls contain gastric glands that release:

   • Hydrochloric acid (HCl): Creates an acidic medium, which is necessary for the enzyme pepsin to work.
   • Pepsin: A protein-digesting enzyme.
   • Mucus: Protects the inner lining of the stomach from the corrosive action of the acid.

4. Small Intestine: This is the longest part of the alimentary canal and the site of complete digestion of carbohydrates, proteins, and fats. The length varies; herbivores have longer small intestines to digest cellulose, while carnivores have shorter ones as meat is easier to digest.

   • The small intestine receives secretions from the liver and pancreas.
   • Bile Juice (from the Liver): Makes the acidic food from the stomach alkaline for pancreatic enzymes to act. It also contains bile salts that break down large fat globules into smaller ones, a process called emulsification, which increases the efficiency of fat-digesting enzymes.
   • Pancreatic Juice (from the Pancreas): Contains enzymes like trypsin (for digesting proteins) and lipase (for breaking down emulsified fats).
   • Intestinal Juice: The walls of the small intestine secrete enzymes that complete the digestion process, converting proteins into amino acids, complex carbohydrates into glucose, and fats into fatty acids and glycerol.

5. Absorption in the Small Intestine: The inner lining of the small intestine has numerous finger-like projections called villi, which vastly increase the surface area for absorption. The villi are richly supplied with blood vessels that transport the absorbed food to every cell in the body. This food is then used for obtaining energy, building new tissues, and repairing old ones.

6. Large Intestine: Unabsorbed food is sent here, where its walls absorb most of the remaining water.

7. Anus: The rest of the waste material is removed from the body via the anus. The exit is regulated by the anal sphincter.

RESPIRATION

The food taken in during nutrition is used in cells to provide energy for life processes. Respiration is the process of breaking down glucose to release this energy. It begins in the cytoplasm of all cells.

Breakdown of Glucose by Various Pathways

1. First Step (in Cytoplasm): Glucose (a 6-carbon molecule) is broken down into a 3-carbon molecule called pyruvate. This process occurs in the cytoplasm and does not require oxygen.

2. Further Breakdown of Pyruvate: The fate of pyruvate depends on the presence or absence of oxygen.

   • Anaerobic Respiration (Absence of Oxygen):

     • In organisms like yeast, this process is called fermentation. Pyruvate is converted into ethanol and carbon dioxide.
     • In our muscle cells during strenuous activity (when there's a lack of oxygen), pyruvate is converted into lactic acid (a 3-carbon molecule). The build-up of lactic acid causes muscle cramps.

   • Aerobic Respiration (Presence of Oxygen):

     • This process takes place in the mitochondria.
     • Pyruvate is completely broken down to produce carbon dioxide, water, and a large amount of energy.

ATP: The Energy Currency

The energy released during cellular respiration is immediately used to synthesize a molecule called ATP (Adenosine Triphosphate) from ADP (Adenosine Diphosphate) and an inorganic phosphate (P).

$\mathrm{ADP}+(\mathrm{P}) \xrightarrow{\text { Energy }} \mathrm{ADP} \sim \mathrm{P} = \mathrm{ATP}$

ATP stores this energy and acts as the "energy currency" for the cell. When the cell needs energy for activities like muscle contraction, protein synthesis, or nerve impulse conduction, ATP is broken down, releasing a fixed amount of energy ($30.5 \text{ kJ/mol}$).

Respiration in Plants and Animals

• Plants: Gas exchange occurs through stomata in leaves, and the large intercellular spaces ensure all cells are in contact with air. The direction of gas diffusion depends on the plant's needs.

  • At night: No photosynthesis occurs, so $CO_2$ elimination is the major exchange activity.
  • During the day: $CO_2$ generated during respiration is used for photosynthesis. The major event is the release of oxygen.

• Aquatic Animals: These animals use oxygen dissolved in water. Since the amount of dissolved oxygen is low, their breathing rate is much faster than that of terrestrial animals. Fish take in water through their mouths and force it past the gills, where dissolved oxygen is taken up by the blood.

• Terrestrial Animals: These animals breathe oxygen from the atmosphere. They have respiratory organs with a large surface area for efficient gas exchange. This surface is typically fine and delicate, so it is placed inside the body for protection.

Respiration in Human Beings

1. Air is taken in through the nostrils, where it is filtered by fine hairs and mucus.
2. The air then passes through the throat and into the lungs. The throat contains rings of cartilage that ensure the air passage does not collapse.
3. Within the lungs, the passage divides into smaller and smaller tubes, which finally terminate in balloon-like structures called alveoli.
4. The alveoli provide a massive surface area for the exchange of gases. Their walls are very thin and are surrounded by an extensive network of blood capillaries.
5. Oxygen from the inhaled air diffuses across the alveolar and capillary walls into the blood. In the blood, the respiratory pigment haemoglobin in red blood cells binds to oxygen and transports it to all the tissues in the body.
6. Carbon dioxide, which is more soluble in water than oxygen, is mostly transported in the dissolved form in our blood from the body tissues back to the lungs, where it diffuses into the alveoli and is exhaled.

TRANSPORTATION

Transportation in Human Beings

The human circulatory system is responsible for transporting food, oxygen, and waste materials. It consists of the heart, blood, and blood vessels.

• Blood: A fluid connective tissue.

  • Plasma: The fluid medium that transports food, carbon dioxide, and nitrogenous wastes in dissolved form.
  • Red Blood Corpuscles (RBCs): Carry oxygen with the help of haemoglobin.
  • Platelets: Cells that circulate in the blood and help in clotting at the site of an injury to prevent blood loss.

• Our Pump - The Heart: A muscular organ about the size of a fist. It has four chambers to prevent the mixing of oxygen-rich and carbon dioxide-rich blood.

  • Right Atrium: Receives de-oxygenated blood from the body.
  • Right Ventricle: Pumps de-oxygenated blood to the lungs.
  • Left Atrium: Receives oxygenated blood from the lungs.
  • Left Ventricle: Pumps oxygenated blood to the rest of the body.
  • Valves ensure that blood flows in only one direction. The ventricles have thicker muscular walls than the atria because they have to pump blood out to various organs.

• Double Circulation: In humans, blood travels through the heart twice for each complete circuit of the body. One cycle pumps blood to the lungs (pulmonary circulation), and the other pumps it to the rest of the body (systemic circulation). This double circulation allows for a highly efficient supply of oxygen, which is necessary for warm-blooded animals like mammals and birds that use energy to maintain a constant body temperature.

• The Tubes - Blood Vessels:

  • Arteries: Carry blood away from the heart to various organs. They have thick, elastic walls because the blood is under high pressure.
  • Veins: Collect blood from different organs and bring it back to the heart. They have thinner walls and contain valves to prevent the backflow of blood, which is no longer under high pressure.
  • Capillaries: On reaching a tissue, arteries divide into extremely thin vessels called capillaries. Their walls are one-cell thick, allowing for the exchange of materials between the blood and surrounding cells. Capillaries then join together to form veins.

• Lymph: Also known as tissue fluid, lymph is another fluid involved in transportation. Some plasma, proteins, and blood cells escape from capillaries into the intercellular spaces to form lymph. It is similar to plasma but colorless and contains less protein. Lymph carries digested fats from the intestine and drains excess fluid from the tissues back into the blood.

Transportation in Plants

Plants need a transport system to move water and minerals from the roots and food (energy stores) from the leaves to all other parts. Since plants don't move and have many dead cells, they have low energy needs and can use a relatively slow transport system.

• Xylem: Transports water and minerals from the soil upwards.
• Phloem: Transports the soluble products of photosynthesis (food) from the leaves to other parts of the plant. This process is called translocation.

Transport of Water Water movement in the xylem is driven by two main forces:

1. Root Pressure: Cells in the roots actively take up ions from the soil. This creates a concentration difference, causing water to move into the root xylem via osmosis. This creates a pressure that pushes water upwards, especially at night.
2. Transpiration Pull: The loss of water in the form of vapor from the aerial parts of the plant (mainly leaves) is called transpiration. This evaporation creates a suction force, or pull, that draws the column of water up through the xylem from the roots. During the day, when stomata are open, transpiration pull is the major driving force for water movement.

Transport of Food and Other Substances Translocation in the phloem is an active process that requires energy.

1. Material like sucrose, produced during photosynthesis, is transferred into the phloem tissue using energy from ATP.
2. This increases the osmotic pressure in the phloem, causing water to move into it.
3. This pressure moves the material in the phloem to tissues with less pressure, such as storage organs (roots, fruits) or growing organs (buds). This allows the plant to move food according to its needs.

EXCRETION

Metabolic activities in organisms generate waste products that are often harmful and need to be removed. Excretion is the biological process of removing these harmful metabolic wastes from the body.

Excretion in Human Beings

The human excretory system includes a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra.

• Kidneys: Located in the abdomen on either side of the backbone. Their function is to filter waste products from the blood to produce urine.
• Nephrons: The basic filtration units of the kidney. Each kidney contains a large number of nephrons packed together. A nephron consists of a cluster of thin-walled blood capillaries associated with a cup-shaped structure called a Bowman's capsule and a long coiled tube.

How Urine is Produced:

1. Filtration: Blood enters the capillaries in the Bowman's capsule, where water, glucose, amino acids, salts, and nitrogenous wastes (like urea) are filtered out.
2. Selective Re-absorption: As this initial filtrate flows along the coiled tube, useful substances like glucose, amino acids, salts, and a major amount of water are selectively re-absorbed back into the blood. The amount of water re-absorbed depends on how much excess water is in the body and the amount of dissolved waste to be excreted.
3. Urine Formation: The fluid remaining after re-absorption is urine. It flows from the kidneys through the ureters into the urinary bladder, where it is stored. When the bladder is full, the urge to urinate leads to its release through the urethra.

Excretion in Plants

Plants use different strategies for excretion compared to animals.

• Gaseous Wastes: Oxygen (a by-product of photosynthesis) and carbon dioxide (from respiration) are removed through stomata.
• Excess Water: Removed by transpiration.
• Other Wastes:
  • Many waste products are stored in cellular vacuoles.
  • Wastes may be stored in leaves that eventually fall off.
  • Some waste products are stored as resins and gums, especially in old xylem tissue.
  • Plants can also excrete some waste substances into the soil around them.