O Level Biology Chapter 1 & 2: Cell Structure and Movement Into and Out of Cells

Fundamental Cell Biology and Cell Types

• Cell Theory Fundamentals:

  • All living organisms are composed of cells, which serve as the fundamental structural and functional units of life.
  • All new cells arise from pre-existing cells through cell division.

• Organism Classification by Cellularity:

  • Multicellular Organisms: Organisms composed of multiple specialized cells working in coordination.
  • Examples: Dogs, cows, humans.
  • Unicellular Organisms: Single-celled organisms that carry out all life processes within a single cell.
  • Examples: Yeast, bacteria.

• Primary Cellular Categorizations:

  • Eukaryotic Cells: Cells that possess a distinct nucleus enclosed by a nuclear membrane and contain membrane-bound organelles.
  • Prokaryotic Cells: Cells that lack a membrane-enclosed nucleus and contain no membrane-bound organelles.

• Comprehensive Comparison Between Animal and Plant Cells:

• Cell Wall:
  • Animal Cells: Absent.
  • Plant Cells: Present (composed of cellulose).
• Cell Membrane:
  • Animal Cells: Present.
  • Plant Cells: Present.
• Cytoplasm:
  • Animal Cells: Present.
  • Plant Cells: Present.
• Nucleus:
  • Animal Cells: Present.
  • Plant Cells: Present.
• Ribosomes:
  • Animal Cells: Present.
  • Plant Cells: Present.
• Mitochondria:
  • Animal Cells: Present.
  • Plant Cells: Present.
• Chloroplasts:
  • Animal Cells: Absent.
  • Plant Cells: Present (contain chlorophyll for photosynthesis).
• Vacuoles:
  • Animal Cells: Absent, or present only as small, temporary vacuoles.
  • Plant Cells: Present as a single large central permanent vacuole filled with cell sap.
• Storage Carbohydrate:
  • Animal Cells: Glycogen.
  • Plant Cells: Starch.
• Cell Shape:
  • Animal Cells: Irregular or variable shape.
  • Plant Cells: Fixed, regular, or rigid shape.

Structure and Function of Cell Organelles

• Cell Wall:
  • Characteristics & Structure: Found exclusively in plant, bacterial, and fungal cells (absent in animal cells). Plant cell walls are made of non-living cellulose fibers arranged in a tough layer surrounding the cell membrane. It is fully permeable.
  • Functions:
  1. Provides mechanical strength and structural support for plant cells.
  2. Prevents plant cells from bursting (lysing) when absorbing excess water via osmosis.
  3. Being fully permeable, it permits the unrestricted passage of water, solute molecules, and mineral salts.

• Cell Membrane:
  • Characteristics & Structure: Present in all cell types. Composed of a lipid bilayer integrated with proteins and carbohydrates. It forms a boundary around the cytoplasm and is partially permeable (selectively permeable).
  • Functions:
  1. Acts as a physical barrier separating internal cellular contents from the extracellular environment.
  2. Controls and regulates the movement of substances entering and leaving the cell.

• Cytoplasm:

  • Characteristics & Structure: Clear, jelly-like fluid composed primarily of water, containing dissolved nutrients, salts, and suspended particles/organelles.
  • Functions:
  1. Serves as the primary site for metabolic chemical reactions.
  2. Stores food materials in the form of storage granules (e.g., glycogen granules in animals, starch granules in plants).
  3. Contains and supports all subcellular organelles.

• Nucleus:

  • Characteristics & Structure: Spherical or oval organelle enclosed by a double membrane (nuclear membrane). Contains DNA (deoxyribonucleic acid), organized as chromatin or chromosomes, and contains a nucleolus.
  • Functions:
  1. Controls all metabolic activities and processes of the cell.
  2. Carries inherited genetic material responsible for transmitting hereditary traits.
  3. Regulates cell division, development, and growth.

• Vacuoles:
  • Characteristics & Structure: Fluid-filled spaces enclosed by a membrane. Plant cells have a large central permanent vacuole filled with a aqueous solution called cell sap (containing sugars, salts, and waste products).
  • Functions:
  1. Stores food materials, nutrients, and cellular waste products.
  2. Maintains turgor pressure against the cell wall, keeping plant cells firm, rigid, and supportive.

• Mitochondria (Singular: Mitochondrion):
  • Characteristics & Structure: Microscopic, rod-shaped or oval organelles enclosed by a double membrane (envelope) consisting of an outer membrane, an inner membrane, and an intermembrane space. The inner membrane features extensive internal folds.
  • Functions: Known as the powerhouse of the cell. Responsible for releasing energy from glucose during aerobic respiration to synthesize adenosine triphosphate (ATP).
  • Structural Adaptation: The extensive folding of the inner mitochondrial membrane drastically increases the surface area available for membrane-bound respiratory enzymes, thereby maximizing the rate of aerobic respiration and ATP release.

• Chloroplasts:
  • Characteristics & Structure: Double-membrane organelles found in photosynthetic plant cells. Contain internal stacks of membranes and a green light-absorbing pigment called chlorophyll.
  • Functions: Traps light energy and converts it into chemical energy (stored in glucose) during photosynthesis.

• Rough Endoplasmic Reticulum (RER):
  • Characteristics & Structure: Network of flattened, membrane-bound sacs (cisternae) studded with ribosomes on its outer surface.
  • Functions: Facilitates protein synthesis via the surface-bound ribosomes and transports synthesized proteins within the cell.

• Ribosomes:

  • Characteristics & Structure: Small, non-membrane-bound granular organelles found attached to the surface of the Rough Endoplasmic Reticulum or floating freely in the cytoplasm.
  • Functions: Responsible for protein synthesis by assembling individual amino acids into polypeptide chains/proteins.

• Vesicles:

  • Characteristics & Structure: Small, membrane-enclosed sacs (small vacuoles).
  • Functions: Transport materials and substances between cellular compartments or to the cell membrane for secretion.

• Variation in Organelle Abundance According to Function:

  • Cells that execute high energy-consuming processes (e.g., muscle cells, active transport root hair cells) contain elevated counts of mitochondria.
  • Cells specialized for high-volume protein synthesis and secretion contain elevated densities of ribosomes and rough endoplasmic reticulum.

• Chemical Composition of Cell Walls across Kingdoms:

  • Plant Cell Walls: Made of cellulose.
  • Bacterial Cell Walls: Made of murein (peptidoglycan).
  • Fungal Cell Walls: Made of chitin.

• Adenosine Triphosphate (ATP):

  • Universal energy-carrying molecule found in all living cells, functioning as the primary chemical energy currency for biological processes.

Levels of Biological Organisation

• Hierarchy of Biological Structure:
  • Organelles $\rightarrow$ Cells $\rightarrow$ Tissues $\rightarrow$ Organs $\rightarrow$ Organ Systems $\rightarrow$ Multicellular Organism.

• Organelle:

  • Definition: Specialized intracellular compartment or structure that performs a specific function within a cell.
  • Plant Examples: Chloroplasts, Ribosomes, Mitochondria.
  • Animal Examples: Mitochondria, Ribosomes.

• Cell:

  • Definition: The basic functional and structural unit of a living organism, containing organelles and modified to perform specific tasks.
  • Plant Examples: Palisade mesophyll cell, Root hair cell.
  • Animal Examples: Muscle cell, Red blood cell, Neurone.

• Tissue:

  • Definition: A group of similar specialized cells working together to perform a shared, specific function.
  • Plant Examples: Xylem tissue, Phloem tissue.
  • Animal Examples: Muscular tissue, Epithelial/Glandular tissue.

• Organ:

  • Definition: A distinct structure composed of a collection of different tissues working together to perform a specific physiological function.
  • Plant Examples: Leaf, Root, Stem.
  • Animal Examples: Heart, Stomach, Liver.

• Organ System:

  • Definition: A group of interrelated organs with coordinated functions working together to perform major body operations.
  • Plant Examples: Root system, Shoot system.
  • Animal Examples: Digestive system, Respiratory system, Circulatory system.

Specialized and Modified Cells

• Modified Animal Cells:

  • Red Blood Cell (Erythrocyte):
  • Primary Function: Transport of oxygen throughout the organism.
  • Adaptations:
    • Small, highly elastic cells with a biconcave disc shape to maximize surface area to volume ratio for efficient gas exchange.
    • Lack a nucleus and most organelles upon maturation to maximize internal volume for holding haemoglobin.
    • Cytoplasm is densely packed with haemoglobin, an iron-containing protein that reversibly binds oxygen.

• Sperm Cell:
  • Primary Function: Male reproductive gamete specialized to reach and fertilize an ovum.
  • Adaptations:
    • Anatomically divided into head, neck (mid-piece), and tail.
    • Head: Contains a haploid nucleus carrying male genetic material and an acrosome containing digestive enzymes to penetrate the outer protective layer of the ovum.
    • Mid-piece: Tightly packed with mitochondria to release ATP energy required for flagellar movement.
    • Tail (Flagellum): Long, whip-like structure providing propulsion for active swimming.

• Ovum / Egg Cell:

  • Primary Function: Female reproductive gamete for fertilization and early embryo nutrition.
  • Adaptations:
    • Large, spherical cell containing cytoplasm filled with food reserves/yolk.
    • Contains a haploid nucleus.
    • Surrounded by a specialized jelly coat that hardens immediately following fertilization to prevent entry of additional sperm (polyspermy).

• Neurone (Nerve Cell):

  • Primary Function: Specialized for carrying rapid electrical nerve impulses across long distances.
  • Adaptations:
    • Features extremely elongated nerve fibers called axons.
    • Contains a central cell body with radiating dendrites.
    • Axon is encased in an insulating myelin sheath to accelerate impulse conduction speed.

• Ciliated Epithelial Cell:

  • Primary Function: Continuous beating of cilia to waft mucus, trapped dust particles, and microbes away from the lungs.
  • Adaptations:
    • Contains tiny, hair-like projections called cilia on the apical cell surface.
    • Lines the upper respiratory tract (trachea and bronchi).

• Muscle Cell:

  • Primary Function: Contraction to produce movement.

  • Adaptations:

    • Spindle-shaped cells containing specialized contractile protein filaments.
    • Contains abundant mitochondria to generate ATP required for repeated muscle contractions.

  • Modified Plant Cells:

• Root Hair Cell:

  • Primary Function: Efficient absorption of water and mineral ions from the soil.
  • Adaptations:
    • Possesses an elongated, finger-like projection that greatly increases cell surface area for absorption.
    • Contains a large, concentrated cell sap vacuole to maintain a low water potential, driving water entry by osmosis.
    • Features a very thin cell wall to minimize diffusion distance.
    • Contains numerous mitochondria to release energy (ATP) via aerobic respiration for active transport of mineral ions against concentration gradients.

• Xylem Vessel Elements:

  • Primary Function: Provides structural support and transports water and dissolved mineral ions upward from roots to leaves.
  • Adaptations:
    • Consists of dead cells lacking cytoplasm, nuclei, or end cross-walls, forming continuous hollow tubes that offer no resistance to upward water flow.
    • Cell walls are heavily thickened and reinforced with lignin to prevent vessel collapse under suction pressure and provide mechanical support.
    • Features unlignified side pits allowing lateral movement of water to surrounding tissues.

• Palisade Mesophyll Cell:

  • Primary Function: Primary site of photosynthesis within the leaf.
  • Adaptations:
    • Columnar, vertically elongated cells packed closely together to catch maximum light near the upper leaf surface.
    • Features thin cell walls to provide a short diffusion path for gases ($\text{CO}_2$ and $\text{O}_2$).
    • Packed with abundant chloroplasts that can reorient within the cytoplasm to absorb maximum light energy.

Practical Work: Magnification, Calculations, and Microscopy

• Magnification Formula Triangle:
  • Symbols:
  • $I$ = Image size (or drawing size)
  • $A$ = Actual size of specimen
  • $M$ = Magnification factor
  • Mathematical Equations:
  • $\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}}$
  • $\text{Actual size} = \frac{\text{Image size}}{\text{Magnification}}$
  • $\text{Image size} = \text{Actual size} \times \text{Magnification}$

• Units of Measurement & Linear Conversions:
  • $1\,\text{m} = 1000\,\text{mm}$
  • $1\,\text{cm} = 10\,\text{mm}$
  • $1\,\text{mm} = 1000\,\mu\text{m}$ (micrometers)
  • $1\,\mu\text{m} = 1000\,\text{nm}$ (nanometers)
  • Conversion Rules:
  • To convert millimeters ($\text{mm}$) to micrometers ($\mu\text{m}$), multiply by $1000$.
  • To convert micrometers ($\mu\text{m}$) to millimeters ($\text{mm}$), divide by $1000$.
  • To convert centimeters ($\text{cm}$) to millimeters ($\text{mm}$), multiply by 10$.\n\n\n\n- **Worked Calculation Example**:\n - *Problem*: Given a measured drawing size I = 30\,\text{mm}$and a magnification$M = 3000\times$, calculate actual specimen size$A.\n - *Calculation*:\n - A = \frac{I}{M} = \frac{30\,\text{mm}}{3000} = 0.01\,\text{mm}\n - *Conversion to Micrometers*:\n - 0.01\,\text{mm} \times 1000 = 10\,\mu\text{m}\n\n- **Instruments for Visualizing Cells**:\n - **Unaided Human Eye**: Incapable of resolving individual microscopic cells.\n - **Hand Lens**: Provides low magnification (up to 10\times); cells appear only as tiny dots.\n - **Light Microscope**: Uses visible light passing through glass lenses to magnify specimens; allows visualization of cell outlines, nuclei, chloroplasts, and large vacuoles.\n - **Electron Microscope**: Uses high-voltage beams of electrons focused by electromagnetic lenses; provides vastly superior magnification (100,000\times or more) and resolution to reveal internal ultrastructure (e.g., mitochondrial cristae, ribosomes, membrane bilayers).\n\n\n\n# Cellular Transport Mechanisms: Diffusion\n\n- **Cellular Need for Transport**:\n - Living cells require constant influx of nutrients (e.g., glucose, oxygen) for metabolic oxidation and cellular structure assembly.\n - Cells must continuously expel metabolic waste products (e.g., carbon dioxide, urea) to prevent toxic accumulation that disrupts biochemical equilibrium.\n - Transport occurs passively (diffusion, osmosis) or actively (active transport).\n\n- **Definition of Diffusion**:\n - **Diffusion** is the net movement of particles (molecules or ions) from a region of their higher concentration to a region of their lower concentration down a concentration gradient as a result of their random kinetic movement.\n\n- **Fundamental Causes of Diffusion**:\n 1. Random, continuous thermal movement of individual particles.\n 2. Presence of a **concentration gradient** (a difference in concentration across space).\n - Diffusion continues until particles are uniformly distributed, establishing dynamic equilibrium.\n\n- **Permeability of Cell Membrane**:\n - The membrane's partially permeable nature permits small uncharged molecules (\text{O}_2$,$\text{CO}_2$,$\text{H}_2\text{O}) to diffuse freely, while blocking large complex molecules (e.g., proteins, starch).\n\n\n\n\n\n- **Concentration Gradient Directionality**:\n - *Down a concentration gradient*: Movement from higher concentration to lower concentration.\n - *Up/Against a concentration gradient*: Movement from lower concentration to higher concentration.\n\n- **Factors Determining the Rate of Diffusion**:\n 1. **Temperature**: Temperature elevation increases particle kinetic energy \rightarrow$faster velocity$\rightarrow$higher collision frequency$\rightarrow increased rate of diffusion.\n 2. **Surface Area**: Larger surface area available for exchange \rightarrow faster total rate of diffusion.\n 3. **Diffusion Distance**: Thinner exchange membranes/barriers \rightarrow$shorter distance$\rightarrow faster diffusion rate.\n 4. **Concentration Gradient**: Steeper difference in concentration between two points \rightarrow higher net diffusion rate.\n 5. **Particle Size**: Smaller, lighter molecules move faster than larger, bulkier particles \rightarrow higher diffusion rate.\n\n# Cellular Transport Mechanisms: Active Transport\n\n- **Definition of Active Transport**:\n - **Active Transport** is the movement of particles (molecules or ions) across a cell membrane from a region of their lower concentration to a region of their higher concentration (against a concentration gradient), using energy released from cellular respiration (ATP) through carrier proteins.\n\n- **Role and Mechanism of Carrier Proteins**:\n - **Carrier Proteins** are specialized transmembrane proteins embedded in the phospholipid bilayer.\n - Carrier proteins exhibit specificity: each carrier protein possesses a binding site complementary to a specific ion or solute molecule.\n - *Steps of Operation*:\n 1. Solute binds specifically to the carrier protein binding site on the side of lower concentration.\n 2. Respiration-derived ATP supplies energy, causing the carrier protein to undergo a conformational shape change / rotation.\n 3. Translocated solute is released on the opposite side of the membrane into the region of higher concentration.\n\n\n\n- **Physiological Importance of Active Transport**:\n 1. Active uptake of essential mineral ions (e.g., nitrates) into root hair cells from low concentrations in soil water.\n 2. Selective reabsorption of glucose into blood capillaries from fluid inside kidney tubules against concentration gradients.\n 3. Complete absorption of digested food materials (glucose, amino acids) from the small intestine lumen into intestinal epithelial cells.\n\n- **Factors Determining the Rate of Active Transport**:\n 1. **Mitochondrial Density**: Greater number of functional mitochondria \rightarrow$higher aerobic respiration rates$\rightarrow$increased ATP output$\rightarrow higher rate of active transport.\n 2. **Oxygen Concentration**: Higher oxygen availability \rightarrow$increased aerobic respiration rate$\rightarrow$higher ATP availability$\rightarrow accelerated active transport rate.\n 3. **Carrier Protein Abundance**: Higher density of operational carrier proteins embedded in the cell membrane \rightarrow greater transport capacity.\n\n- **Summary Comparison: Diffusion vs. Active Transport**:\n - **Gradient Direction**: Diffusion is down a concentration gradient (High to Low); Active Transport is against a concentration gradient (Low to High).\n - **Energy Input**: Diffusion is passive (no ATP needed); Active Transport is active (requires energy in the form of ATP from respiration).\n - **Membrane Requirement**: Simple diffusion does not require membrane carrier proteins; Active Transport strictly requires specialized carrier proteins.\n\n# Cellular Transport Mechanisms: Osmosis and Water Potential\n\n- **Water Potential (\Psi)**:\n - A measure of the kinetic energy and tendency of water molecules to move out of a solution.\n - Pure water possesses the highest possible water potential (assigned a standard reference value of zero).\n - Addition of solute particles lowers water potential (making it increasingly negative).\n\n- **Water Potential Gradient**:\n - The spatial difference in water potential between two solutions separated by a partially permeable membrane.\n\n- **Definition of Osmosis**:\n - **Osmosis** is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution) down a water potential gradient across a partially permeable membrane.\n\n\n\n\n\n- **Biological Significance of Osmosis in Plants**:\n 1. Primary mechanism for water uptake from soil via root hair cells.\n 2. Generates internal hydrostatic pressure (**turgor pressure**) necessary to maintain cell stiffness and provide mechanical support to non-woody plant tissues.\n\n- **Turgor Pressure Mechanics**:\n - As water enters plant cells, the vacuole expands, pushing cytoplasm against the inelastic, rigid cellulose cell wall.\n - The resisting force of the cell wall prevents further water uptake once equilibrium is reached, maintaining cell firmness.\n\n- **Effects of Osmosis on Plant Cells**:\n - **In High Water Potential Solutions (Dilute / Hypotonic Environment)**:\n - Water enters the vacuole via osmosis down a water potential gradient.\n - Cell volume expands, becoming **turgid**. Rigid cell walls counteract internal pressure, preventing cell lysis.\n\n\n\n - **In Low Water Potential Solutions (Concentrated / Hypertonic Environment)**:\n - Water exits the vacuole via osmosis into the surrounding solution down a water potential gradient.\n - Vacuole volume decreases, causing cytoplasm to pull away slightly from cell walls; the cell becomes **flaccid** (reversible loss of turgor).\n\n\n\n - **In Severe Low Water Potential (Extremely Concentrated Solutions)**:\n - Excessive water loss causes the plasma membrane to strip away entirely from the cellulose cell wall.\n - The cell becomes **plasmolysed**. Plasmolysis is an irreversible state causing permanent tissue wilting and cell death.\n\n\n\n- **Effects of Osmosis on Animal Cells**:\n - **In High Water Potential Solutions (Hypotonic)**:\n - Water diffuses inward via osmosis across the cell membrane down a water potential gradient.\n - Lacking a protective cell wall, animal cells swell extensively until the membrane bursts (**lysis**).\n - **In Low Water Potential Solutions (Hypertonic)**:\n - Water diffuses outward via osmosis into surrounding solution down a water potential gradient.\n - Animal cells lose water, shrink, and shrivel (**crenation**).\n - **Homeostatic Implication**: Animal cells must be continuously bathed in extracellular **tissue fluid** maintaining an identical water potential (isotonic) to protect cells from destruction via shrinking or bursting.\n\n\n\n# Practical Investigations in Cellular Transport\n\n- **Dialysis (Visking) Tubing Experiment for Osmosis Modeling**:\n - *Principle*: Dialysis (Visking) tubing is a synthetic partially permeable membrane containing microscopic pores that permit passage of small solvent molecules (water) while blocking large solute molecules (sucrose).\n - *Procedure*:\n 1. Take a section of dry dialysis tubing, add a few drops of water, and rub gently between fingers to open.\n 2. Tie a tight knot at one end to seal the base.\n 3. Fill tubing with a highly concentrated sucrose/sugar solution using a dropper pipette.\n 4. Insert a long narrow capillary/glass tube into the open end; tie thread tightly around tubing to form a seal.\n 5. Submerge the Visking tubing bag into a beaker of pure water, securing the vertical glass tube with a stand and clamp.\n 6. Mark the initial liquid meniscus level on the glass tube; record the height of the rising liquid column every 2\,\text{min}$$.
  1. Repeat the procedure across varying sucrose concentrations to measure relative osmotic water uptake rates.