life sciences notes

1. Mitosis and the Cell Cycle

Definition and Biological Significance
- Mitosis is a type of cell division where a single parent cell divides to produce two genetically identical daughter cells, each containing the same diploid ( $2n$ ) chromosome number as the parent.
- Primary Functions:
  - Growth: Allows multicellular organisms to develop from a single zygote into complex organisms by increasing cell number.
  - Repair and Replacement: Replaces damaged, worn-out, or dead cells (e.g., skin epithelial renewal, healing of wounds).
  - Asexual Reproduction: Enables unicellular organisms (e.g., amoeba, yeast) and certain multicellular organisms (e.g., vegetative propagation in plants) to reproduce without gametes.
The Cell Cycle Overview
- Interphase ( $90\%$ of cell cycle):
  - $G_1$ Phase (First Gap): Rapid cell growth, high metabolic rate, synthesis of organelles and proteins.
  - $S$ Phase (Synthesis): DNA replication occurs; single-stranded chromosomes duplicate to form two identical sister chromatids joined at the centromere.
  - $G_2$ Phase (Second Gap): Further growth, accumulation of energy stores, synthesis of spindle proteins, final preparation for division.
- M-Phase (Mitotic Phase): Consists of Karyokinesis (nuclear division) followed by Cytokinesis (cytoplasmic division).
Stages of Mitosis (Karyokinesis)
- Prophase:
  - Chromatin network condenses, coils, and shortens into distinct, visible chromosomes.
  - Each chromosome consists of two identical sister chromatids held together at a centromere.
  - Nucleolus disappears and the nuclear envelope breaks down.
  - Centrosomes (containing centrioles in animal cells) separate and migrate to opposite poles.
  - Spindle fibres radiate from centrosomes, forming the mitotic spindle.
- Metaphase:
  - Chromosomes align singly along the equatorial plate (metaphase plate) of the cell.
  - Spindle fibres attach securely to the centromere (kinetochore) of each chromosome.
- Anaphase:
  - Centromeres divide simultaneously, separating sister chromatids.
  - Spindle fibres contract and shorten, pulling daughter chromosomes (formerly chromatids) centromere-first toward opposite poles.
  - Cytokinesis begins toward late anaphase.
- Telophase:
  - Daughter chromosomes reach the opposite poles and begin to uncoil back into fine chromatin threads.
  - Nuclear envelope reassembles around each set of chromosomes at each pole.
  - Nucleoli reappear inside each newly formed nucleus.
  - Spindle fibres disintegrate completely.
Cytokinesis Comparison
- Animal Cells: A cleavage furrow forms in the cell membrane along the equator. The plasma membrane pinches inward until the cytoplasm divides into two separate daughter cells.
- Plant Cells: A rigid cell wall prevents pinching. Instead, membrane-bound vesicles from the Golgi body coalesce at the equator to form a cell plate. Cellulose is deposited, forming a middle lamella and new primary cell walls.
Cancer and Uncontrolled Mitosis
- Mutations in genes controlling the cell cycle (e.g., proto-oncogenes and tumor suppressor genes) lead to uncontrolled, rapid mitotic divisions.
- Produces an abnormal mass of undifferentiated cells known as a tumour.
  - Benign Tumour: Cells remain localized within a capsule, do not invade neighboring tissue, and do not metastasize.
  - Malignant Tumour: Cells invade adjacent healthy tissues, enter the bloodstream or lymphatic system, and establish secondary tumours elsewhere (metastasis).
- Carcinogens: Agents that induce mutations causing cancer (e.g., ionizing radiation, UV light, tobacco smoke tar, asbestos, certain viruses such as HPV).

2. Photosynthesis

Definition and Importance
- The biochemical process whereby autotrophic organisms (green plants, algae) capture radiant energy from sunlight and convert it into chemical potential energy stored within organic molecules (glucose).
- Supplies the base organic energy for almost all terrestrial and aquatic food chains and produces oxygen essential for aerobic organisms.
Chemical Equation
- Word equation: Carbon Dioxide + Water + Radiant Energy $\rightarrow$ Glucose + Oxygen
- Balanced chemical equation:
  $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$
Chloroplast Structure and Adaptations
- Double Membrane: Semi-permeable outer and inner membranes enclosing the organelle.
- Stroma: Gel-like fluid containing enzymes, ribosomes, circular DNA, and starch granules; site of the light-independent phase.
- Thylakoids and Grana: Disc-like thylakoid sacs stacked into grana to provide a large surface area for chlorophyll molecules and electron transport proteins; site of the light-dependent phase.
The Two Phases of Photosynthesis
- Light-Dependent Phase (in Thylakoid membranes / Grana):
  - Chlorophyll absorbs radiant energy, energizing electrons.
  - Photolysis: Radiant energy splits water molecules:
    $2H_2O \rightarrow 4H^+ + 4e^- + O_2$
  - Oxygen ( $O_2$ ) is released through stomata as a byproduct.
  - Energy released during electron flow phosphorylates ADP to form $ATP$ (photophosphorylation).
  - Hydrogen ions ( $H^+$ ) are picked up by coenzyme NADP to form NADPH.
- Light-Independent Phase / Calvin Cycle (in Stroma):
  - Does not directly require light, but depends on $ATP$ and NADPH produced in the light phase.
  - Carbon Fixation: Carbon dioxide ( $CO_2$ ) entering via stomata is combined with a 5-carbon compound via the enzyme RuBisCO.
  - Energy from $ATP$ and hydrogen from NADPH reduce the fixed carbon to produce glucose ( $C_6H_{12}O_6$ ).
  - Excess glucose is converted into starch for osmotic-neutral storage.
Limiting Factors of Photosynthesis
- Light Intensity: Rate increases proportionally with increasing light until another factor becomes limiting or the chlorophyll reaches light saturation.
- $CO_2$ Concentration: Higher levels increase rate until the carbon-fixing enzymes work at maximum capacity.
- Temperature: Governed by enzyme activity. Increases up to an optimum ( $25\,^{\circ}\text{C} - 35\,^{\circ}\text{C}$ ); temperatures above the optimum cause enzyme denaturation, dropping the rate sharply.

3. Cellular Respiration

Definition and Importance
- The biochemical process occurring in all living cells in which organic food molecules (primarily glucose) are catabolized in a controlled stepwise manner to release energy in the form of adenosine triphosphate ( $ATP$ ).
Chemical Equation for Aerobic Respiration
- Balanced chemical equation:
  $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + 36\text{ to }38\,\text{ATP}$
Mitochondrion Structure and Function
- Outer Membrane: Smooth and permeable to small molecules.
- Inner Membrane: Folded into deep convolutions called cristae, vastly increasing the surface area for the electron transport system and ATP synthase enzymes.
- Matrix: Fluid space enclosed by the inner membrane containing enzymes for the Krebs cycle, circular DNA, and mitochondrial ribosomes.
Phases of Aerobic Cellular Respiration
- Phase 1: Glycolysis (Cytoplasm / Cytosol):
  - Occurs anaerobically (does not require $O_2$ ).
  - 6-carbon glucose molecule is broken down into two 3-carbon pyruvate molecules.
  - Net gain: $2\,\text{ATP}$ (via substrate-level phosphorylation) and $2\,\text{NADH}$ .
- Phase 2: Krebs Cycle / Citric Acid Cycle (Mitochondrial Matrix):
  - Requires oxygen indirectly; pyruvate enters the mitochondrion and undergoes oxidative decarboxylation to form Acetyl-CoA.
  - Acetyl-CoA ( $2C$ ) joins oxaloacetate ( $4C$ ) to form citrate ( $6C$ ), followed by a cyclic series of decarboxylation and dehydrogenation reactions.
  - Carbon dioxide ( $CO_2$ ) is released as waste.
  - Yield per glucose molecule: $2\,\text{ATP}$ , $6\,\text{NADH}$ , and $2\,\text{FADH}_2$ .
- Phase 3: Oxidative Phosphorylation (Inner Membrane / Cristae):
  - High-energy electrons from $NADH$ and $FADH_2$ are passed through electron transport chains.
  - The released energy pumps protons ( $H^+$ ) across the inner membrane, creating a proton gradient that drives ATP synthase to produce $32\text{ to }34\,\text{ATP}$ .
  - Role of Oxygen: Oxygen serves as the terminal electron acceptor, binding with depleted electrons and free protons ( $H^+$ ) to form metabolic water ( $H_2O$ ): $O_2 + 4e^- + 4H^+ \rightarrow 2H_2O$
Anaerobic Respiration (Fermentation)
- Occurs in the total absence of oxygen in the cytoplasm; consists only of glycolysis followed by fermentation to regenerate $NAD^+$ :
  - Lactic Acid Fermentation (in animal skeletal muscle during strenuous exercise):
    $\text{Glucose} \rightarrow 2\,\text{Lactic Acid} + 2\,\text{ATP}$
  - Accumulation of lactic acid causes muscle fatigue and oxygen debt.
  - Alcoholic Fermentation (in yeast and plant cells):
    $\text{Glucose} \rightarrow 2\,\text{Ethanol} + 2CO_2 + 2\,\text{ATP}$
  - Economically important in brewing and baking industries.

4. Animal Nutrition and the Digestive System

Essential Digestive Processes
- Ingestion: Taking food into the alimentary canal through the mouth.
- Digestion: Mechanical breakdown (teeth, churning) and chemical breakdown (hydrolytic enzymes) of insoluble polymers into soluble monomers.
- Absorption: Movement of digested nutrients across the intestinal epithelium into blood or lymph.
- Assimilation: Incorporation of absorbed nutrients into body tissues for metabolic processes.
- Egestion: Elimination of undigested and unabsorbed waste material through the anus.
How to Draw and Label the Human Digestive System
- Step-by-Step Drawing Strategy:
  - Begin with an outline of the head and neck in profile: draw the mouth cavity, tongue, and pharynx.
  - Draw a straight, vertical muscular tube (the oesophagus) descending through the thoracic cavity and penetrating the diaphragm.
  - Connect the oesophagus to the stomach, drawn as a J-shaped muscular sac situated on the left side beneath the diaphragm. Show the cardiac sphincter at the entrance and the pyloric sphincter at the exit.
  - Draw the liver as a large, wedge-shaped organ positioned on the right side superior to the stomach, with a small sac-like gall bladder tucked beneath it.
  - From the pyloric sphincter, draw the C-shaped loop of the duodenum (first part of small intestine). Position the leaf-like pancreas nestled in the curvature of the duodenum. Draw bile and pancreatic ducts converging to enter the duodenum.
  - Draw the coiled small intestine (jejunum and ileum) occupying the central abdominal region.
  - Surround the small intestine with the large intestine (colon): ascending colon on the right, transverse colon horizontally across, and descending colon down the left, leading to the S-shaped sigmoid colon, rectum, and anus.
  - At the junction of the ileum and ascending colon, draw the blind pouch (caecum) with the small worm-like projection (vermiform appendix).

5. The Respiratory System and Gaseous Exchange

How to Draw and Label the Human Respiratory System
- Step-by-Step Drawing Strategy:
  - Draw the upper airway: nasal cavity, pharynx, epiglottis, and the larynx (voice box).
  - Extend downward into the trachea (windpipe), drawing distinct horizontal C-shaped bands representing cartilaginous rings that keep the airway patent.
  - At the carina (mid-chest), bifurcate the trachea into the left and right primary bronchi, entering the left and right lungs respectively.
  - Show branching within each lung into secondary and tertiary bronchi, narrowing into fine bronchioles, terminating in clusters of microscopic alveoli.
  - Draw the lobed lungs (right lung has 3 lobes; left lung has 2 lobes to accommodate the cardiac notch for the heart).
  - Enclose the lungs with two parallel lines representing the inner visceral pleura and outer parietal pleura, separated by the pleural cavity filled with lubricating pleural fluid.
  - Beneath the lungs, draw the dome-shaped muscular diaphragm sheet, and outline the ribs and intercostal muscles along the thoracic wall.
How to Draw and Label an Alveolus
- Drawing Details:
  - Draw a large circular or cup-shaped sac.
  - Line the inner surface of the sac with a single layer of extremely thin, flattened cells with prominent flat nuclei: the alveolar squamous epithelium.
  - Draw a very thin, moist liquid film lining the internal lumen of the alveolus (representing surfactant and moisture layer).
  - Wrap a close-fitting pulmonary capillary around the outer wall of the alveolus.
  - Line the capillary wall with a single layer of flattened endothelial cells (capillary endothelium).
  - Draw the microscopic space between the alveolar and endothelial walls as a fused basement membrane.
  - Inside the capillary lumen, draw biconcave disc-shaped red blood cells (erythrocytes) passing in single file.
  - Label directional diffusion arrows:
  - Arrow for $O_2$ moving from the alveolar air space into the erythrocyte.
  - Arrow for $CO_2$ moving from blood plasma into the alveolar air space.
The 6 Requirements of an Efficient Gaseous Exchange Surface (Alveolar Adaptations)
- 1. Extremely Large Surface Area:
  - The human lungs contain approximately 300 to 500 million alveoli, providing an enormous aggregate surface area ( $\approx 70\text{ to }100\,\text{m}^2$ ) for the simultaneous diffusion of vast volumes of respiratory gases.
- 2. Very Thin Diffusion Barrier:
  - The barrier comprises only two cell layers: the alveolar squamous epithelium and the capillary endothelium, separated by an ultra-thin basement membrane ( $< 1\,\mu\text{m}$ in thickness). This short diffusion pathway maximizes diffusion rate in accordance with Fick's Law.
- 3. Moist Surface:
  - The inner alveolar surface is lined with a thin film of water containing pulmonary surfactant. Gases cannot diffuse across biological membranes in gaseous state; $O_2$ dissolves in this moisture film before diffusing into the blood. Surfactant lowers surface tension, preventing alveolar collapse during expiration.
- 4. Well Ventilated:
  - Continuous mechanical breathing (inhalation and exhalation) continually refreshes alveolar air, bringing in fresh $O_2$ and removing accumulated $CO_2$ . This maintains a steep concentration gradient between alveolar air and blood.
- 5. Highly Vascularized / Efficient Transport System:
  - An extensive, dense meshwork of pulmonary capillaries covers each alveolus. Continuous blood flow carries newly oxygenated blood away and brings deoxygenated blood from tissues, preserving a steep concentration gradient.
- 6. Well Protected:
  - Because gaseous exchange surfaces must be thin and delicate, they are internalized deep within the thoracic cavity, shielded from mechanical trauma and desiccation by the bony thoracic cage (ribs, sternum, vertebrae) and pleural membranes.
Transport of Oxygen ( $O_2$ ) from Alveolus to Body Cells and its Cellular Function
- Alveolar Gas Exchange:
  - Inspired air in the alveoli has a high partial pressure of oxygen ( $pO_2 \approx 104\,\text{mmHg}$ ), while deoxygenated blood arriving at the pulmonary capillaries from the pulmonary artery has a low $pO_2$ ( $\approx 40\,\text{mmHg}$ ).
  - $O_2$ dissolves in the moisture layer, diffuses down its concentration gradient across the alveolar squamous epithelium, basement membrane, and capillary endothelium into the blood plasma, and rapidly enters the red blood cells (erythrocytes).
- Oxygen Transport in the Blood:
  - Approximately $1.5\%$ of $O_2$ dissolves physically in blood plasma.
  - Approximately $98.5\%$ binds reversibly with the iron-containing pigment haemoglobin inside erythrocytes to form oxyhaemoglobin:
    $\text{Hb} + 4O_2 \rightleftharpoons \text{Hb}(O_2)_4$
- Systemic Delivery to Tissues:
  - Oxygenated blood travels via the pulmonary veins to the left side of the heart and is pumped through the systemic aorta to capillary beds in metabolizing tissues.
  - Tissue cells actively consume $O_2$ during aerobic respiration, establishing a low $pO_2$ ( $< 40\,\text{mmHg}$ ) and high $pCO_2$ in tissues.
  - Under these conditions (low $pO_2$ , lower pH / Bohr effect), oxyhaemoglobin readily dissociates, releasing free $O_2$ .
  - $O_2$ diffuses out of the erythrocytes, across the capillary endothelium, into tissue interstitial fluid, and into the cells down its concentration gradient.
- Metabolic Function of Oxygen in Cells:
  - Inside the cells, oxygen enters the mitochondria.
  - It functions as the terminal electron acceptor in the electron transport chain of oxidative phosphorylation.
  - By capturing low-energy electrons and binding with hydrogen ions ( $H^+$ ), it forms water and keeps electron flow moving. Without $O_2$ , electron transport ceases, proton pumping stops, oxidative phosphorylation fails, and ATP synthesis plummets, resulting in cellular death.
Effects of Smoking on the Gaseous Exchange System
- Components of Tobacco Smoke:
  - Nicotine: Addictive stimulant that constricts peripheral blood vessels, elevates heart rate and blood pressure, and contributes to cardiovascular disease.
  - Carbon Monoxide ( $CO$ ): Odourless, toxic gas that binds preferentially to haemoglobin with an affinity over 200 times greater than oxygen, forming carboxyhaemoglobin. This permanently blocks $O_2$ binding sites, drastically reducing the blood's oxygen-carrying capacity.
  - Tar: Sticky, brown particulate matter that condenses in the lungs, containing numerous known chemical carcinogens (e.g., benzopyrene).
  - Irritants: Chemicals that induce inflammation and stimulate mucus secretion.
- Chronic Bronchitis:
  - Tar and irritants paralyze and destroy the ciliated epithelial cells lining the trachea and bronchi.
  - Goblet cells and submucosal glands undergo hypertrophy, hypersecreting thick mucus.
  - Without functional cilia, mucus cannot be propelled upward toward the pharynx.
  - Mucus pools in the lower respiratory tract, trapping bacteria and leading to persistent inflammation, chronic infections, and the characteristic violent "smoker's cough".
- Emphysema:
  - Chronic inflammation attracts macrophages and neutrophils, which release the proteolytic enzyme elastase.
  - Elastase hydrolyzes and breaks down the structural protein elastin in the alveolar walls.
  - Delicate alveolar septa break down and fuse into enlarged, irregular, inelastic air cavities.
  - Consequences:
  - Drastic reduction in the total functional surface area available for gas exchange.
  - Loss of pulmonary elasticity; bronchioles collapse during exhalation, trapping air in the lungs and leading to severe dyspnoea (shortness of breath) and permanent respiratory impairment.
- Lung Cancer:
  - Carcinogens in tar penetrate bronchial epithelial cells, damaging DNA and mutating critical tumor-suppressor genes (e.g., $p53$ ).
  - Results in uncontrolled, rapid mitosis of abnormal epithelial cells, forming a malignant bronchial carcinoma that invades lung parenchyma and metastasizes to other organs.

6. Essay: The Interdependence Between Cellular Respiration and Photosynthesis (30 Marks)

Introduction

All living organisms require a continuous supply of metabolic energy to maintain cellular homeostasis, drive endergonic biochemical reactions, and sustain life. The biosphere is sustained fundamentally by two reciprocal biochemical pathways: photosynthesis and cellular respiration. Photosynthesis, executed by photoautotrophs such as plants and cyanobacteria, converts radiant energy from the sun into chemical potential energy stored within organic macromolecules. In contrast, cellular respiration, executed by both autotrophs and heterotrophs, catabolizes these organic compounds to release usable chemical energy in the form of adenosine triphosphate ( $ATP$ ). These two processes are biologically complementary: the products of one serve as the essential substrates of the other, establishing a reciprocal cyclical relationship that stabilizes atmospheric gas concentrations and powers global biogeochemical cycles.

Biochemical Reciprocity and Stoichiometry

The structural interdependence of photosynthesis and cellular respiration is rooted in their complementary biochemical equations:

Photosynthesis:
$6CO_2 + 6H_2O + \text{Radiant Energy} \rightarrow C_6H_{12}O_6 + 6O_2$
Aerobic Cellular Respiration:
$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{Chemical Energy (ATP)}$

During the light-dependent phase of photosynthesis, photolysis of water in the thylakoid lumen produces molecular oxygen ( $O_2$ ) as a byproduct, alongside hydrogen ions and ATP. This oxygen is released through the stomata into the atmosphere. Aerobic cellular respiration depends completely on this atmospheric oxygen: in the inner mitochondrial membrane, oxygen functions as the terminal electron acceptor during oxidative phosphorylation, accepting electrons and protons to form metabolic water ( $H_2O$ ). Conversely, the Krebs cycle and link reaction of cellular respiration continuously release carbon dioxide ( $CO_2$ ) as a decarboxylation byproduct. This $CO_2$ diffuses out of cells, enters the atmosphere, and provides the vital carbon source for the light-independent phase (Calvin cycle) of photosynthesis, where it is fixed into glucose by the enzyme RuBisCO.

Flow of Matter versus Flow of Energy

A fundamental distinction within their interdependence is the difference between how matter and energy move through ecosystems:

Cyclical Flow of Matter: Carbon, hydrogen, and oxygen atoms cycle endlessly between the chloroplast and the mitochondrion, and between biotic communities and the abiotic environment. The carbon fixed into glucose by a plant may be consumed by an animal, oxidized during cellular respiration to carbon dioxide, released into the atmosphere, and refixed by another plant's photosynthetic machinery. Water follows an identical cyclical route, being split in the chloroplast and resynthesized at the cytochrome oxidase complex of the mitochondrion.
Unidirectional (Non-cyclic) Flow of Energy: Unlike matter, energy does not cycle. Radiant energy from thermonuclear reactions in the sun enters ecosystems and is converted by chlorophyll into chemical potential energy stored in covalent bonds of glucose. During respiration, that potential energy is transferred into high-energy phosphate bonds of $ATP$ . Every energy transformation is inherently inefficient according to the Second Law of Thermodynamics; significant portions of energy are continuously dissipated into the environment as low-grade metabolic heat. Consequently, ecosystems require a constant, uninterrupted influx of solar energy to drive photosynthesis, which in turn fuels respiration.

Ecological and Planetary Balance

At an ecological and planetary scale, the balance between photosynthesis and cellular respiration regulates Earth's climate and atmospheric composition:

Atmospheric Homeostasis: Photosynthesis is the primary planetary sink for $CO_2$ and the sole major source of atmospheric $O_2$ . Cellular respiration acts as a counterweight, consuming oxygen and liberating carbon dioxide. Over geological time, the near-equilibrium between global gross primary production and global total respiration has stabilized atmospheric oxygen at approximately $21\%$ and carbon dioxide at trace levels, preventing toxic hypoxia and runaway greenhouse warming.
Anthropogenic Disruption: Modern industrial activities, extensive deforestation, and the combustion of fossil fuels (ancient photosynthetic products) have elevated atmospheric $CO_2$ beyond the rate at which photosynthetic terrestrial and marine sinks can sequester it. This uncoupling of the global balance enhances the greenhouse effect, driving global warming and ocean acidification.

Conclusion

Photosynthesis and cellular respiration are inextricably intertwined biochemical pillars of the biosphere. One process constructs organic matter and liberates oxygen, while the other breaks down that matter to harness vital energy, producing the carbon dioxide and water needed to initiate the cycle again. Without the continuous operation of photosynthesis, heterotrophs would quickly perish from energy starvation and oxygen depletion; without the complementary recycling of carbon dioxide via cellular respiration and decomposition, autotrophs would exhaust available atmospheric carbon. Life on Earth exists as an integrated planetary engine driven by the continuous flow of solar energy through this balanced metabolic loop.