VCE Biology Unit 1 Exam Revision Comprehensive Notes

Unit 1 Study Checklist: Cells, Plasma Membranes, and Energy Transformations

Cellular Fundamentals and Organelle Function

• Cell Classification     * Prokaryotes: Unicellular organisms including bacteria and archaea. They lack a membrane-bound nucleus and membrane-bound organelles. They contain circular DNA (genophore) and ribosomes.     * Eukaryotes: Organisms with complex cells containing a membrane-bound nucleus and specialized membrane-bound organelles. They can be unicellular or multicellular.     * Comparison of Animal and Plant Cells         * Plant Cells: Contain a cell wall (cellulose), chloroplasts for photosynthesis, and a large central vacuole for storage and structural support.         * Animal Cells: Lack cell walls and chloroplasts; possess centrioles and often contain many small, temporary vacuoles or lysosomes.

• Cellular Organelles     * Nucleus: The control centre containing genetic material (DNA). It is enclosed by a double-membrane nuclear envelope with nuclear pores.     * Nucleolus: Located within the nucleus; the site where ribosomal RNA ($rRNA$) is produced.     * Mitochondria: Site of aerobic cellular respiration and $ATP$ production. Features a double membrane: the inner membrane is folded into cristae to increase surface area, enclosing the matrix.     * Chloroplasts: Site of photosynthesis in plants and algae. Structure includes grana (stacks of thylakoids) containing chlorophyll and the stroma (fluid).     * Ribosomes: Non-membrane-bound structures responsible for protein synthesis. They can be free in the cytosol or attached to the Rough Endoplasmic Reticulum.     * Endoplasmic Reticulum (ER)         * Rough ER: Studded with ribosomes; involved in the synthesis and transport of proteins.         * Smooth ER: Lacks ribosomes; involved in the synthesis of lipids and steroid hormones, and detoxification.     * Golgi Complex (Apparatus): A stack of flattened cisternae that modifies, sorts, and packages proteins and lipids into vesicles for secretion or delivery to other organelles.     * Lysosomes: Membrane-bound sacs containing approximately $50$ digestive enzymes responsible for breaking down debris and foreign material.     * Peroxisomes: Organelles that carry out the oxidation of long-chain fatty acids using the enzyme catalase.     * Vacuoles: Storage facilities for fluids, enzymes, and nutrients. In plants, they provide turgor pressure.     * Cell Wall: Provides structural support and protection; present in plants (cellulose) and fungi (chitin).     * Cytosol vs. Cytoplasm: Cytosol is the fluid part of the cell, while cytoplasm includes the cytosol and all organelles except the nucleus.

• Surface Area to Volume Ratio ($SA:V$)     * As a cell increases in size (while maintaining shape), its volume increases much faster than its surface area.     * A high $SA:V$ (found in small cells) allows for the efficient exchange of materials (nutrients in, wastes out) across the plasma membrane.     * Beyond a certain size, the two-way exchange rate is insufficient to meet cellular needs.

Plasma Membrane Structure and Transport Mechanisms

• Fluid Mosaic Model     * Phospholipid Bilayer: The primary component, consisting of hydrophilic (polar) phosphate heads facing outward and hydrophobic (non-polar) fatty acid tails facing inward. This arrangement provides flexibility.     * Proteins         * Integral Proteins: Span the entire width of the membrane (trans-membrane).         * Peripheral Proteins: Attached to the exterior or interior surfaces.         * Channel and Carrier Proteins: Facilitate the movement of specific molecules.     * Cholesterol: Regulates membrane fluidity.     * Glycoproteins/Glycolipids: Act as cell identity tags or receptors.

• Transport Processes     * Simple Diffusion: Passive movement of small, non-polar molecules (e.g., $O_2$, $CO_2$, alcohol, urea) from an area of high concentration to an area of low concentration directly through the lipid bilayer.     * Facilitated Diffusion: Passive movement of substances (e.g., glucose, ions) down a concentration gradient via specific channel or carrier proteins.     * Osmosis: The net movement of water molecules from a solution of low solute concentration (high water potential) to a solution of high solute concentration (low water potential) across a selectively permeable membrane.     * Active Transport: The movement of molecules (e.g., sodium ions, hydrogen ions in the stomach) against a concentration gradient using protein pumps. This process requires energy in the form of $ATP$.     * Bulk Transport         * Exocytosis: Export of materials out of the cell via vesicles fusing with the plasma membrane.         * Endocytosis: Intake of large particles or fluids by the membrane forming a pit and pinching off to form a vesicle.

• Tonicity     * Hypotonic: A solution with a lower solute concentration than the cytosol. Water enters the cell. Animal cells may lyse (burst); plant cells become turgid.     * Hypertonic: A solution with a higher solute concentration than the cytosol. Water leaves the cell. Animal cells shrivel; plant cells undergo plasmolysis (membrane pulls away from the cell wall).     * Isotonic: Equal solute concentration. There is no net movement of water.

Energy: Photosynthesis and Cellular Respiration

• Photosynthesis     * Purpose: Transformation of sunlight energy into chemical energy stored in glucose.     * Equation: $6CO_2 + 12H_2O \xrightarrow{\text{Light/Chlorophyll}} C_6H_{12}O_6 + 6O_2 + 6H_2O$     * Light-Dependent Stage (Grana)         * Inputs: Light energy, $H_2O$, $NADP^+$, $ADP + P_i$.         * Outputs: $O_2$ (waste), $NADPH$, $ATP$.     * Light-Independent Stage / Calvin Cycle (Stroma)         * Inputs: $CO_2$, $NADPH$, $ATP$.         * Outputs: Glucose ($C_6H_{12}O_6$), $NADP^+$, $ADP + P_i$, $H_2O$.     * Factors Affecting Rate: Light intensity, wavelength/color of light (chlorophyll absorbs blue and red best, reflects green), and temperature.

• Cellular Respiration     * Purpose: To release chemical energy from glucose to produce $ATP$, the direct energy source for cells.     * Aerobic Respiration Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \approx 30-38\,ATP$     * Stages of Aerobic Respiration         1. Glycolysis: Occurs in the cytosol. Breaks glucose into two pyruvate molecules. Produces $2\,ATP$ and $NADH$.         2. Krebs Cycle: Occurs in the mitochondrial matrix. Produces $CO_2$, $2\,ATP$, $NADH$, and $FADH_2$.         3. Electron Transport Chain: Occurs on the inner mitochondrial membrane (cristae). Uses $O_2$ to produce water and roughly $26-34\,ATP$.     * Anaerobic Respiration (Fermentation)         * Occurs in the absence of oxygen; takes place entirely in the cytosol.         * In Mammals: Glucose is converted to lactic acid ($C_3H_6O_3$). Yields $2\,ATP$ per glucose.         * In Yeasts: Glucose is converted to ethanol ($C_2H_5OH$) and carbon dioxide ($CO_2$). Yields $2\,ATP$ per glucose.

Cell Cycle, Apoptosis, and Development

The Cell Cycle and Mitosis

• Interphase: The longest phase where the cell grows and DNA replicates. Contains checkpoints ($G_1, G_2$).
• Mitosis Stages     1. Prophase: Chromosomes condense, nuclear membrane breaks down, spindle fibers form.     2. Metaphase: Chromosomes align along the cell equator.     3. Anaphase: Sister chromatids are pulled apart to opposite poles.     4. Telophase: New nuclear membranes form around the two sets of chromosomes.
• Cytokinesis: Division of the cytoplasm following mitosis, resulting in two identical diploid daughter cells.
• Checkpoints: Ensure DNA is undamaged and correctly replicated (regulated by proteins like $p53$). Malfunction can lead to tumors.

Apoptosis (Programmed Cell Death)

• Purpose: To remove old, damaged, or unnecessary cells during development and to prevent cancer.
• Pathways     * Intrinsic (Mitochondrial) Pathway: Triggered by internal cell damage.     * Extrinsic (Death Receptor) Pathway: Triggered by external signaling molecules.
• Stages: Cell shrinkage, membrane blebbing, fragmentation of the nucleus, and formation of apoptotic bodies which are engulfed by phagocytes.

Stem Cells and Differentiation

• Definition: Undifferentiated cells capable of self-renewal and differentiating into specialized cell types.
• Step Potency     * Totipotent: Can form any cell type, including embryonic and extra-embryonic (e.g., zygote).     * Pluripotent: Can form any of the three germ layers (e.g., Embryonic Stem Cells from the Inner Cell Mass of a blastocyst).     * Multipotent: Can form multiple related cell types (e.g., adult stem cells in bone marrow forming blood cells).     * Unipotent: Can only form one cell type (e.g., skin stem cells).
• Germ Layers     1. Ectoderm: Develops into skin and nervous system.     2. Mesoderm: Develops into muscle, bone, and circulatory system.     3. Endoderm: Develops into internal linings (digestive and respiratory systems).
• Ethical Considerations: Debate surrounding the use of embryonic stem cells due to the destruction of human embryos; alternative includes Induced Pluripotent Stem Cells ($iPSCs$).

Biological Systems and Homeostasis

Plant Systems

• Vascular Tissues     * Xylem: Transports water and minerals from roots to leaves. Consists of dead, lignified cells (vessels and tracheids). Uses transpiration pull, cohesion (water sticking to water), and adhesion (water sticking to walls).     * Phloem: Transports organic solutes (sucrose) via translocation. Consists of living sieve tube elements and companion cells.
• Leaf Structure: Includes stomata (pores for gas exchange), guard cells (regulate opening/closing), and mesophyll (photosynthetic cells).

Animal Systems

• Digestive System: Purpose is to break down food physically (teeth, stomach churning) and chemically (enzymes like amylase, protease, lipase) for absorption. Includes organs like the liver (bile production), gallbladder (bile storage), and pancreas (enzyme production).
• Excretory System: Removes nitrogenous wastes ($N$-wastes) such as ammonia (toxic), urea (mammals), or uric acid (birds). The kidneys use nephrons for filtration and reabsorption.
• Endocrine System: Uses hormones (chemical signals) to regulate long-term body processes. Includes the pituitary, thyroid, and pancreas.
• Respiratory System: Facilitates gas exchange ($O_2$ in, $CO_2$ out) via alveoli. Immediate effects of exercise include increased breathing rate and tidal volume.

Homeostasis and Regulation

• Definition: The maintenance of a relatively constant internal environment despite external changes.
• Stimulus-Response Model: Stimulus $\rightarrow$ Receptor $\rightarrow$ Control Centre $\rightarrow$ Effector $\rightarrow$ Response.
• Negative Feedback: The response counteracts the original stimulus to restore balance.
• Thermoregulation     * Body Temp Falls: Thermoreceptors in hypothalamus $\rightarrow$ Shivering (skeletal muscles), Vasoconstriction (skin arterioles), and increased metabolism via thyroxine.     * Body Temp Rises: Sweating, Vasodilation (increased blood flow to skin), and behavioral changes (seeking shade).
• Blood Glucose Regulation     * High Glucose: Pancreatic Beta cells release Insulin; liver converts glucose to glycogen.     * Low Glucose: Pancreatic Alpha cells release Glucagon; liver converts glycogen to glucose.
• Water Regulation (Osmoregulation)     * When solute concentration rises, the pituitary gland releases Antidiuretic Hormone (ADH), signaling kidney tubules to reabsorb more water, resulting in concentrated urine.

Questions & Discussion

Cellular Questions

• Question: Are all eukaryotes multicellular?     * Answer: False. Organisms like yeast and amoebae are unicellular eukaryotes.
• Question: Which has a higher $SA:V$ ratio: Sphere A (large) or Sphere B (small)?     * Answer: Sphere B. Smaller objects have higher relative surface area.
• Question: What is the difference between cytosol and cytoplasm?     * Answer: Cytosol is the fluid; cytoplasm is the fluid plus organelles.
• Question: Do prokaryotic cells have DNA?     * Answer: True, but it is not enclosed in a nucleus.

Transport & Metabolism Questions

• Question: By what process does the stomach move hydrogen ions to produce acid?     * Answer: Active transport, which requires $ATP$.
• Question: Where does the major production of $ATP$ occur?     * Answer: The mitochondria (specifically via the Electron Transport Chain).
• Question: What happens to water molecules in the light reactions of photosynthesis?     * Answer: They are split into hydrogen ions, electrons, and oxygen gas ($O_2$).

Systemic & Homeostasis Questions

• Question: Why do arteries have thicker walls than veins?     * Answer: To withstand the higher pressure of blood being pumped directly from the heart.
• Question: What is the effector in the regulation of carbon dioxide concentration?     * Answer: The respiratory muscles (diaphragm and intercostals).
• Question: What is the source of water for animals that do not drink?     * Answer: Metabolic water (water produced as a byproduct of aerobic respiration) and water in food.
• Question: Explain how a saline drip should be relative to blood plasma.     * Answer: It must be isotonic to prevent red blood cells from shrinking (hypertonic) or bursting (hypotonic).

Experimental Design

• Independent Variable: The factor intentionally changed.
• Dependent Variable: The factor measured in response to changes.
• Controlled Variables: Factors kept constant to ensure a fair test.
• Case Study (2,4-dinitrophenol): A toxin that allows electron transport but prevents $ADP$ phosphorylation to $ATP$. Energy is released as heat instead of being captured in $ATP$.