Unit 1 Biology: Cells and Multicellular Organisms Complete Study Guide

The Cell Theory

• Cell Theory remains the foundation of modern biology and consists of three fundamental statements:
  • 1. All living organisms are composed of cells.
  • 2. The cell is the basic unit of life.
  • 3. All cells arise from pre-existing cells.
• Cells are defined as the smallest structures capable of independently carrying out life processes.

Prokaryotic and Eukaryotic Cells

• Cells are categorized into two primary types based on their structural complexity and presence of a nucleus.

• Feature Comparison:

  • Nucleus: Prokaryotic cells have no true nucleus; Eukaryotic cells possess a true nucleus.
  • DNA: In prokaryotes, DNA is circular and free-floating in the cytoplasm. In eukaryotes, DNA consists of linear chromosomes housed within the nucleus.
  • Membrane-bound Organelles: Absent in prokaryotic cells; present in eukaryotic cells.
  • Size: Prokaryotic cells are smaller ($1\text{--}10\,\mu m$), while eukaryotic cells are larger ($10\text{--}100\,\mu m$).
  • Complexity: Prokaryotic cells are simpler; eukaryotic cells are more complex.
  • Ribosomes: Prokaryotic ribosomes are smaller ($70S$), whereas eukaryotic ribosomes are larger ($80S$).
  • Reproduction: Prokaryotes reproduce via binary fission; eukaryotes reproduce via mitosis or meiosis.
  • Cell Wall: Usually present in prokaryotes. In eukaryotes, they are present only in plants and fungi.
  • Examples: Bacteria are prokaryotes; Animals, plants, and fungi are eukaryotes.

• Prokaryotic Cell Structure Features:

  • Plasma membrane
  • Cytoplasm
  • Ribosomes
  • Nucleoid region (where circular DNA is located)
  • Plasmids (small circular DNA loops)
  • Cell wall
  • Flagellum (sometimes present for movement)

• Important Note: Prokaryotes explicitly lack a nucleus, mitochondria, endoplasmic reticulum (ER), and Golgi apparatus.

Eukaryotic Cell Organelles

• Nucleus:

  • Structure: Surrounded by a nuclear membrane; contains chromatin/chromosomes and an internal nucleolus.
  • Function: Controls cell activities, stores genetic information (DNA), and directs protein synthesis.

• Ribosomes:

  • Function: The site of protein synthesis.
  • Two Types:
    • Free Ribosomes: Synthesize proteins to be used inside the cell.
    • Attached Ribosomes: Found on the Rough ER; synthesize proteins for export from the cell.

• Rough Endoplasmic Reticulum (RER):

  • Structure: A membrane network covered in ribosomes.
  • Function: Synthesizes, folds, and transports proteins.

• Smooth Endoplasmic Reticulum (SER):

  • Function: Involved in lipid synthesis, detoxification of chemicals, and calcium storage.

• Golgi Apparatus:

  • Function: Modifies proteins and lipids, packages them into vesicles, and secretes substances.
  • Example: Packaging digestive enzymes into lysosomes.

• Lysosomes:

  • Structure: Small sacs containing hydrolytic (digestive) enzymes.
  • Function: Digest pathogens, break down waste products, and recycle old organelles.

• Mitochondria:

  • Structure: Features a double membrane; the inner membrane is folded into structures called cristae.
  • Function: The site of aerobic respiration and the production of $ATP$.

• Chloroplasts (Plants Only):

  • Structure: Contains chlorophyll, thylakoids, grana, and stroma.
  • Function: The site of photosynthesis.

• Vacuoles:

  • Plant Cells: Possess a large central vacuole that stores water and maintains turgor pressure.
  • Animal Cells: Contain small, temporary vacuoles.

Cell Specialisation and Differentiation

• Differentiation: The process by which unspecialised cells become specialised. This occurs when different genes are switched on or off.
• Examples of Specialised Cells:
  • Red Blood Cell: Adapted by having no nucleus and a biconcave shape to facilitate oxygen transport.
  • Muscle Cell: Contains many mitochondria to provide energy for contraction.
  • Root Hair Cell: Has a large surface area to maximize water absorption in plants.
  • Sperm Cell: Features a tail for swimming and many mitochondria to power movement.

Stem Cells: Characteristics, Potency, and Ethics

• Characteristics: Unspecialised cells that can divide by mitosis and differentiate into specialised cell types.
• Potency Types:
  • Totipotent: Can differentiate into any cell type, including placental tissue.
  • Pluripotent: Can differentiate into almost any body cell type.
  • Multipotent: Limited to differentiating into related cell types (e.g., blood stem cells).
  • Unipotent: Can only produce one specific cell type.
• Sources of Stem Cells:
  • Embryonic stem cells
  • Adult stem cells
  • Induced pluripotent stem cells ($iPSCs$)
• Ethical Issues and Concerns:
  • Potential destruction of embryos.
  • Issues surrounding informed consent.
  • Concerns regarding human cloning.
• Benefits:
  • Ability to treat various diseases.
  • Used for tissue regeneration and organ repair.

Hierarchical Organisation

• Levels of Organisation: Cell $\rightarrow$ Tissue $\rightarrow$ Organ $\rightarrow$ Organ System $\rightarrow$ Organism.
• Tissue Types and Functions:
  • Epithelial: Serving as a covering or lining.
  • Connective: Providing support and transport (e.g., blood).
  • Muscle: Facilitating movement.
  • Nervous: Used for communication and signaling.

Surface Area to Volume Ratio ($SA:V$)

• Formula: $SA:V = \frac{\text{Surface Area}}{\text{Volume}}$
• Importance: Cells exchange substances via the plasma membrane.
• Effect of Size: As cell size increases, the volume increases faster than surface area, making diffusion less efficient.
• Consequences for Large Cells: They cannot obtain nutrients quickly enough or remove wastes efficiently.
• Adaptations to Increase SA:
  • Microvilli
  • Flattened shapes
  • Folding of membranes

The Fluid Mosaic Model of the Cell Membrane

• Structure: Consists of a Phospholipid Bilayer.
  • Hydrophilic Phosphate Heads: Face the aqueous environment (water).
  • Hydrophobic Fatty Acid Tails: Face inward, away from water.
• Proteins:
  • Channel Proteins: Allow specific substances to pass through the membrane.
  • Carrier Proteins: Change shape to actively or passively move substances across.
• Cholesterol: Stabilises the membrane and prevents it from becoming too fluid or too rigid.
• Glycoproteins: Involved in cell recognition, acting as receptors, and immune responses.

Membrane Transport Mechanisms

• Passive Transport:

  • Requires no energy ($ATP$).
  • Movement occurs down a concentration gradient (High to Low).
  • Simple Diffusion: Movement of small, nonpolar molecules (e.g., oxygen, carbon dioxide).
  • Osmosis: Diffusion of water across a partially permeable membrane from a dilute solution to a concentrated solution.
  • Tonicity Effects on Animal Cells:
    • Hypotonic: Cell swells or bursts.
    • Hypertonic: Cell shrinks.
    • Isotonic: No net movement of water.
  • Facilitated Diffusion: Passive movement utilizing membrane proteins for ions, glucose, and large polar molecules.

• Active Transport:

  • Requires energy ($ATP$).
  • Moves substances against a concentration gradient (Low to High).
  • Protein Pumps: Carrier proteins that actively move molecules (e.g., the sodium-potassium pump).
  • Endocytosis: The membrane engulfs substances; includes phagocytosis (solids) and pinocytosis (liquids).
  • Exocytosis: Vesicles fuse with the plasma membrane to release substances from the cell.

Biological Macromolecules

• Carbohydrates:
  • Monomer: Monosaccharides (e.g., glucose).
  • Functions: Source of energy and energy storage.
• Proteins:
  • Monomer: Amino acids.
  • Functions: Enzymes, hormones, structural support, and transport.
• Lipids:
  • Components: Glycerol and fatty acids.
  • Functions: Long-term energy storage, insulation, and membrane structure.

Digestion and Exchange Surfaces

• Mechanical Digestion: Physical breakdown of food (e.g., chewing).
• Chemical Digestion: Enzymes breaking large molecules into smaller ones.
• Digestive Enzymes:
  • Amylase: Acts on starch to produce maltose.
  • Protease: Acts on protein to produce amino acids.
  • Lipase: Acts on lipids to produce fatty acids and glycerol.
• Exchange Surface Features: Large surface area, thin walls, moist environment, and a rich blood supply.
• Villi: Finger-like projections in the small intestine adapted with microvilli (increase SA), thin epithelium, and nearby capillaries.

Circulatory System and Nephron Structure

• Closed Circulatory System: Blood remains within vessels, allowing for high pressure, fast transport, and efficient exchange.
• Nephron Structure:
  • Glomerulus: A capillary knot that filters blood.
  • Bowman’s Capsule: Collects the resulting filtrate.
  • Proximal Convoluted Tubule: Reabsorbs glucose, amino acids, and water.
  • Loop of Henle: Reabsorbs water/salts and maintains the concentration gradient.
  • Distal Tubule: Provides further regulation.
  • Collecting Duct: Site of final water reabsorption; produces urine.
• Urine Formation Processes:
  • Glomerular Filtration: High pressure forces small molecules out; large proteins stay in the blood.
  • Selective Reabsorption: Returns useful substances to the blood.
  • Tubular Secretion: Adds extra wastes into the filtrate.

Enzyme Structure and Function

• Structure: Globular proteins with a specific active site.
• Function: Acts as a catalyst by lowering activation energy to speed up reactions.
• Models:
  • Lock-and-Key Model: The active site perfectly fits the substrate.
  • Induced-Fit Model: The active site changes shape slightly to fit the substrate better (considered more accurate).
• Factors Affecting Activity:
  • Temperature: Increasing temperature increases kinetic energy and collisions, but extremely high temperatures cause denaturation.
  • pH: Extreme pH values alter the shape of the active site.
  • Substrate Concentration: Rate increases until enzymes are saturated.
  • Inhibitors: Competitive inhibitors compete for the active site; non-competitive inhibitors change the active site shape.

Metabolism and Cellular Energy (ATP)

• Metabolism Types:
  • Catabolism: Breaks large molecules down to release energy.
  • Anabolism: Builds large molecules and requires energy.
• ATP Structure: Composed of Adenine, Ribose, and three phosphate groups ($Adenine + Ribose + P + P + P$).
• ATP Cycle:
  • Energy Release (ATP Hydrolysis): Breaking the final phosphate bond releases energy.
  • Formula: $ATP \rightarrow ADP + P_i + \text{energy}$
  • Energy Storage (Phosphorylation): Energy from respiration is used to add a phosphate group back to $ADP$.
  • Formula: $ADP + P_i + \text{energy} \rightarrow ATP$
• Why ATP is Important: It releases energy immediately, is easily recycled, stores manageable amounts of energy, and links catabolic and anabolic reactions.

Aerobic and Anaerobic Respiration

• Aerobic Respiration Overall Equation:
  • $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + 36\text{--}38\,ATP$
• Steps of Aerobic Respiration:
  • 1. Glycolysis: Occurs in the cytoplasm. Inputs: glucose and $ATP$. Outputs: pyruvate, $ATP$, and $NADH$.
  • 2. Krebs Cycle: Occurs in the mitochondrial matrix. Produces $CO_2$, $NADH$, $FADH_2$, and $ATP$.
  • 3. Electron Transport Chain (ETC): Occurs on the inner mitochondrial membrane. Electrons transfer through carriers; energy is used to produce the most $ATP$. Oxygen is the final electron acceptor.
• Anaerobic Respiration: Occurs without oxygen; produces much less $ATP$.
  • In Animals: Produces lactic acid.
  • In Yeast: Produces ethanol and $CO_2$.

Gas Exchange and Diffusion Gradients

• Alveoli Adaptations: Thin walls (short distance), large surface area (faster diffusion), moist lining (gases dissolve), and rich blood supply (maintains gradient).
• Diffusion Directions:
  • Alveoli $\leftrightarrow$ Capillaries: Oxygen moves from alveoli to blood; $CO_2$ moves from blood to alveoli.
  • Capillaries $\leftrightarrow$ Cells: Oxygen moves from blood to cells; $CO_2$ moves from cells to blood.

Plant Physiology: Photosynthesis and Transport

• Photosynthesis Overall Equation:
  • $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$
• Light-Dependent Reactions: Occur in thylakoid membranes. Inputs: light, water. Outputs: oxygen, $ATP$, $NADPH$.
• Light-Independent Reactions (Calvin Cycle): Occur in the stroma. Inputs: $CO_2$, $ATP$, $NADPH$. Output: glucose.
• Xylem vs Phloem:
  • Xylem: Transports water and minerals upward only; composed of dead cells with thick lignin walls.
  • Phloem: Transports sugars in both directions; composed of living cells with thin walls.
• Transpiration: The loss of water vapour from leaves. Explained by the Cohesion-Tension Theory, where water molecules stick together and are pulled upward.
• Factors Increasing Transpiration: High temperature, wind, and high light.
• Factors Decreasing Transpiration: High humidity.
• Stomata and Guard Cells: Stomata are pores for gas exchange. Guard cells control them; they open when water enters (turgid) and close when water is lost (flaccid).

Practical Skills and Experimental Design

• Microscope Calculation:
  • $\text{Total Magnification} = \text{Eyepiece} \times \text{Objective Lens}$
  • Field of view decreases as magnification increases.
• Diffusion Practical: Agar cubes with indicator in acid. Smaller cubes diffuse faster due to a larger $SA:V$ ratio and shorter distance.
• Experimental Terms:
  • Independent Variable: The variable changed.
  • Dependent Variable: The variable measured.
  • Controlled Variables: Kept constant.
  • Reliability: Improved by repeating trials.
  • Accuracy: How close a value is to the truth.
  • Validity: Ensuring only one independent variable is changed.