Life Processes

Introduction to Life Processes

• Criteria for Life: Determining the difference between what is alive and what is not alive is often based on movement. While visible movements like a dog running, a cow chewing cud, or a human shouting are clear indicators, life persists even in the absence of visible action (e.g., a sleeping animal or a plant that is not visibly growing).
• Visible vs. Invisible Movement: Visible movement is insufficient as a defining characteristic. Molecular movements, which are invisible to the naked eye, are essential for life. Biologists maintain that these movements are necessary to maintain the organized structures of living organisms.
• Organized Structure and Maintenance: Living organisms are highly organized, consisting of tissues, cells, and organelles. The environment tends to break down this order over time. To stay alive, organisms must constantly repair and maintain these structures through molecular movement.
• The Virus Controversy: Viruses exhibit no molecular movement until they infect a host cell. This absence of independent molecular activity creates a scientific controversy regarding whether they are truly "alive."

Fundamental Life Processes

• Definition of Life Processes: The processes which together perform the maintenance job of preventing damage and breakdown in a living organism are called life processes. These must continue even when the organism is at rest or asleep.
• Nutrition: Since maintenance requires energy, organisms must transfer energy sources (food) from the environment to the body. Most food sources on Earth are carbon-based molecules.
• Respiration: To use the energy from food for molecular movement and growth, organisms need to break down molecules through chemical reactions. Oxidising-reducing reactions are the most common means. Respiration is the process of acquiring oxygen from outside the body and using it to break down food sources for cellular needs.
• Transportation: In multi-cellular organisms, specialized tissues take up food and oxygen in specific locations. A transportation system is required to carry these materials to all parts of the body.
• Excretion: Chemical reactions for energy generation create useless or harmful by-products (waste). Excretion is the biological process of removing these metabolic wastes from the body. In complex organisms, specialized tissues for excretion are developed, supported by the transportation system.
• Diffusion Limitations: While single-celled organisms can rely on simple diffusion for gas exchange and waste removal because their entire surface is in contact with the environment, multi-cellular organisms cannot. In complex bodies, all cells are not in direct contact with the surroundings, making specialized organs and transport systems necessary.

Nutrition: Autotrophic and Heterotrophic Strategies

• General Energy Needs: All organisms require energy and materials to maintain order, grow, and synthesize proteins. This is fulfilled through food.
• Autotrophic Nutrition:
  • Definition: Organisms that use simple inorganic material (carbon dioxide and water) and an external energy source (sunlight) to synthesize complex high-energy organic material.
  • Examples: Green plants and some bacteria.
  • Photosynthesis: The primary process where $CO_2$ and $H_2O$ are converted into carbohydrates in the presence of sunlight and chlorophyll. The chemical equation involves the reduction of carbon dioxide.
  • Starch and Glycogen: Plants store unused carbohydrates as starch for later use. Humans store similar energy reserves as glycogen.
  • Photosynthesis Steps:
    1. Absorption of light energy by chlorophyll.
    2. Conversion of light energy to chemical energy and the splitting of water molecules into hydrogen and oxygen.
    3. Reduction of carbon dioxide to carbohydrates.
  • Chloroplasts: These are cell organelles containing chlorophyll, visible as green dots in a leaf cross-section.
• Stomatal Function:
  • Stomata: Tiny pores on leaf surfaces used for massive gaseous exchange and transpiration.
  • Guard Cells: Control the opening and closing of stomatal pores. They swell when water flows into them (opening the pore) and shrink to close the pore.
• Heterotrophic Nutrition:
  • Definition: Organisms that utilize complex substances prepared by other organisms. They use bio-catalysts called enzymes to break these down.
  • Holozoic: Taking in whole material and breaking it down internally (e.g., animals).
  • Saprophytic: Breaking down food outside the body and absorbing it (e.g., fungi like bread moulds, yeast, mushrooms).
  • Parasitic: Deriving nutrition without killing the host (e.g., Cuscuta, ticks, lice, leeches, tapeworms).

Human Nutrition: The Alimentary Canal

• Structure: A long tube extending from the mouth to the anus. Food is moved via peristaltic movements (rhythmic contractions) along the gut.
• Mouth:
  • Teeth: Crush food into small, uniform particles.
  • Saliva: Secreted by salivary glands; contains the enzyme salivary amylase, which breaks down starch (complex molecule) into sugar.
  • Tongue: Mixes food with saliva and aids in chewing.
• Stomach:
  • Oesophagus: Transports food from the mouth to the stomach.
  • Gastric Glands: Located in the stomach wall; they release Hydrochloric Acid ($HCl$), Pepsin, and Mucus.
  • $HCl$: Creates an acidic medium to facilitate the action of pepsin.
  • Pepsin: A protein-digesting enzyme.
  • Mucus: Protects the stomach's inner lining from acid action.
• Small Intestine:
  • Length: Varies by diet; longer in herbivores (to digest cellulose) and shorter in carnivores (meat is easier to digest).
  • Secretions: Receives bile from the liver and pancreatic juice from the pancreas.
  • Bile: Makes the acidic food alkaline and emulsifies fats (breaks large fat globules into smaller ones).
  • Pancreatic Juice: Contains trypsin (protein digestion) and lipase (breaking down emulsified fats).
  • Intestinal Juice: Final digestion converts proteins into amino acids, carbohydrates into glucose, and fats into fatty acids and glycerol.
  • Villi: Finger-like projections that increase surface area for absorption. They are rich in blood vessels.
• Large Intestine and Anus: Absorbs water from unabsorbed food. Remaining waste is removed via the anus, regulated by the anal sphincter.
• Dental Caries: Tooth decay caused by bacteria producing acids from sugar, which demineralizes enamel and dentine. Plaque (bacterial masses) prevents saliva from neutralizing the acid.

Respiration: Energy Pathways

• Glycolysis: The first step in all organisms is the breakdown of glucose (a $6$-carbon molecule) into pyruvate (a $3$-carbon molecule) in the cytoplasm.
• Anaerobic Respiration:
  • In Yeast: Pyruvate is converted into ethanol and carbon dioxide (Fermentation) in the absence of oxygen.
  • In Muscle Cells: During sudden activity/lack of oxygen, pyruvate is converted into lactic acid (a $3$-carbon molecule), causing cramps.
• Aerobic Respiration: Occurs in the mitochondria. Pyruvate reacts with oxygen to produce three molecules of $CO_2$, $H_2O$, and a significant amount of energy.
• ATP (Adenosine Triphosphate):
  • Energy Currency: Synthesized from ADP and inorganic phosphate during respiration.
  • Energy Release: Breaking the terminal phosphate linkage using water releases approximately $30.5\,kJ\,mol^{-1}$.
  • Usage: Fuels endothermic reactions like muscle contraction, protein synthesis, and nerve impulse conduction.

Respiration in Different Organisms

• Plants: Exchange gases via stomata and inter-cellular spaces. Evolution of gas depends on the time of day:
  • Night: $CO_2$ elimination is the major activity.
  • Day: $CO_2$ produced in respiration is used for photosynthesis; $O_2$ release is the major event.
• Aquatic Organisms: Breathe oxygen dissolved in water ($O_2$ levels are low). They have a faster breathing rate than terrestrial animals. Fish force water past gills to extract $O_2$.
• Terrestrial Organisms: Use atmospheric oxygen through organs with high surface areas. The surface must be fine, delicate, and protected within the body.
• Human Respiratory System:
  • Route: Nostrils (filtered by hair/mucus) → Throat (rings of cartilage prevent collapse) → Lungs.
  • Alveoli: Balloon-like structures at the end of bronchioles. They provide a surface area of about $80\,m^2$ for gas exchange. Walls are covered in blood vessels.
  • Mechanism: Breathing involves lifting ribs and flattening the diaphragm to expand the chest cavity, creating a vacuum to suck air into the lungs.
  • Residual Volume: Lungs always retain some air to allow continuous gas exchange.
  • Respiratory Pigment: Haemoglobin, found in Red Blood Corpuscles (RBCs), has a high affinity for oxygen. Without it, diffusion to the toes would take $3$ years.
  • $CO_2$ Transport: Carbon dioxide is more soluble in water and is mostly transported dissolved in blood plasma.
• Health Warning: Smoking destroys cilia (hair-like structures that remove dust/germs), leading to infection, cough, and lung cancer. Tobacco chewing is a major risk factor for oral cancer.

Transportation in Humans: The Circulatory System

• Blood Composition:
  • Plasma: Fluid medium transporting food, $CO_2$, and nitrogenous wastes in dissolved form.
  • Red Blood Corpuscles (RBCs): Carry oxygen.
  • Platelets: Circulate to plug leaks and clot blood at injury sites.
• The Heart:
  • Chambers: Four chambers (Left Atrium, Left Ventricle, Right Atrium, Right Ventricle) to prevent mixing of oxygenated and deoxygenated blood.
  • Pathway: Oxygenated blood (from lungs) → Left Atrium → Left Ventricle → Body. Deoxygenated blood (from body) → Right Atrium → Right Ventricle → Lungs.
  • Wall Thickness: Ventricles have thicker muscular walls than atria because they pump blood to distant organs.
  • Valves: Ensure one-way blood flow.
• Double Circulation: Blood passes through the heart twice in one complete cycle. This is present in mammals and birds to meet high energy needs for body temperature regulation. Fishes have a $2$-chambered heart with single circulation.
• Blood Pressure:
  • Systolic Pressure: Contraction phase ($120\,mm \text{ of Hg}$).
  • Diastolic Pressure: Relaxation phase ($80\,mm \text{ of Hg}$).
  • Measurement: Using a sphygmomanometer.
• Blood Vessels:
  • Arteries: Carry blood away from the heart at high pressure; have thick, elastic walls.
  • Veins: Collect blood and bring it back to the heart; have thinner walls and valves.
  • Capillaries: One-cell thick vessels where exchange of material occurs.
• Lymph: A colorless fluid (tissue fluid) that escapes from capillaries. It carries digested fats and drains excess fluid from tissues back into the blood.

Transportation in Plants

• Energy Needs: Plants have low energy needs because they do not move and contain many dead cells. Transport can be slow.
• Xylem:
  • Function: Transports water and minerals from roots to leaves.
  • Mechanism: Roots actively take up ions, creating an osmotic gradient. Root pressure pushes water up. For tall trees, Transpiration Pull (suction created by evaporation from leaves) is the primary force during the day.
• Phloem:
  • Function: Transports products of photosynthesis (translocation of sucrose, amino acids) from leaves to storage organs and growing points.
  • Mechanism: Requires energy from ATP. Sucrose is loaded into phloem, increasing osmotic pressure, which moves material to tissues with lower pressure.

Excretion: Managing Metabolic Waste

• Human Excretory System: Consists of a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra.
• Nephrons: The functional filtration units of the kidney. Each consists of a cluster of capillaries associated with a cup-shaped Bowman’s capsule.
• Urine Formation:
  • Filtration: Nitrogenous wastes (urea/uric acid) are filtered from the blood.
  • Selective Re-absorption: Glucose, amino acids, salts, and water are re-absorbed in the tube. Roughly $180\,L$ of filtrate is produced daily, but only $1$-$2\,L$ is excreted.
  • Regulation: Re-absorption depends on the amount of excess water and dissolved waste in the body.
• Artificial Kidney (Hemodialysis): A device used during kidney failure to remove nitrogenous waste from blood via diffusion through semi-permeable tubes. Unlike real kidneys, there is no re-absorption.
• Excretion in Plants:
  • Oxygen: Released as a waste product of photosynthesis.
  • Transpiration: Removes excess water.
  • Storage: Wastes are stored in cellular vacuoles, old xylem (as resins/gums), or in leaves that eventually fall off.
  • Soil: Some waste is excreted directly into the surrounding soil.

Questions & Discussion

1. Why is diffusion insufficient to meet the oxygen requirements of multi-cellular organisms like humans?
   • In multi-cellular organisms, the body volume is large and most cells are not in direct contact with the environment, so diffusion cannot reach every cell quickly enough.
2. What criteria do we use to decide whether something is alive?
   • Movement (visible or molecular), growth, respiration, and the presence of maintenance processes.
3. What are outside raw materials used for by an organism?
   • Food (carbon sources) for energy and growth, oxygen for breaking down food, and minerals/water for body building.
4. What processes would you consider essential for maintaining life?
   • Nutrition, respiration, transportation, and excretion.
5. How is the small intestine designed to absorb digested food?
   • It has a large surface area due to extensive coiling and the presence of finger-like projections called villi, which are richly supplied with blood vessels.
6. Why is it necessary to separate oxygenated and deoxygenated blood in mammals and birds?
   • To ensure a highly efficient supply of oxygen to maintain constant body temperature (thermoregulation).
7. Describe the structure and functioning of nephrons.
   • Nephrons contain a capillary cluster in a Bowman's capsule for filtration, followed by a long tubule where selective re-absorption of useful substances occurs.
8. What is Organ Donation?
   • A generous act of donating organs (corneas, kidneys, heart, etc.) from a donor to a recipient whose organs have failed. It can be done by living donors (for some organs) or after death.