Science GCSE Revision: Cell Biology, Chemistry, and Physics

Cell Biology and Microscopy

• Sub-cellular Structures

  • Mitochondrion (pl. Mitochondria): The mitochondrion is the primary site of aerobic respiration within a cell. It is often referred to as the "powerhouse" of the cell because it generates energy for cellular processes.
  • Ribosome: The site of protein synthesis.
  • Cell Wall: A rigid structure made of cellulose that provides support and protection in plant cells.
  • Vacuole: A membrane-bound organelle found in plant cells that maintains turgidity and stores nutrients or waste products.

• Microscopy

  • Electron Microscope vs. Light Microscope: Electron microscopes offer significantly higher resolution and magnification compared to light microscopes. This allows scientists to see sub-cellular structures in much finer detail. However, unlike light microscopes, electron microscopes generaly cannot be used to observe living cells easily and are more expensive to maintain.
  • Magnification Formula: To calculate the magnification of an image, use the following ratio:       $Magnification = \frac{\text{Image size}}{\text{Real size}}$

• Specialized Cells and Adaptations

  • Sperm Cell: Structurally adapted for reproduction. It contains an acrosome at its tip, which contains digestive enzymes used to penetrate the egg cell membrane.
  • Ciliated Epithelial Cells: Located in the airways (trachea and bronchi), these cells have hair-like projections called cilia. Their function is to move mucus and trapped dust particles upward and away from the lungs to keep the respiratory system clear.

Enzymes and Biochemical Reactions

• The Enzyme Mechanism

  • Lock and Key Model: This model describes enzyme specificity. The substrate acts as a "key" that fits perfectly into the enzyme's specific "lock," known as the active site.
  • Synthesis: In enzyme-controlled reactions, synthesis refers to the process of building up small molecules into larger, more complex molecules.
  • Digestion: The breakdown of large insoluble molecules into small soluble ones. For example, Protease is the enzyme responsible for breaking down proteins into amino acids.

• Factors Affecting Enzyme Activity

  • Temperature: Every enzyme has an optimum temperature. If the temperature is raised significantly past this point, the enzyme becomes denatured. Denaturation involves a physical change in the shape of the active site, meaning the substrate can no longer fit, and the reaction stops.

Separation of Mixtures and Chromatography

• Definitions

  • Mixture: A substance consisting of two or more elements or compounds that are not chemically combined together.

• Separation Techniques

  • Filtration: The most suitable method for separating an insoluble solid from a liquid.
  • Simple Distillation: Used to obtain a pure liquid (such as pure drinking water) from a solution (such as sea water). It involves evaporating the liquid and then the condensing stage, where the gas cools down and turns back into a liquid.
  • Fractional Distillation: Used to separate mixtures of liquids with different boiling points.
    • Fractionating Column: This column provides a temperature gradient (hotter at the bottom, cooler at the top) so that different vapors condense at different levels.
  • Crystallization: Used to separate a soluble solid from a solvent by evaporating the liquid.

• Paper Chromatography

  • Phases: In paper chromatography, the stationary phase is the chromatography paper, and the mobile phase is the solvent.
  • Experimental Details: The start line on a chromatogram must be drawn in pencil because pencil lead (graphite) is insoluble and will not run or interfere with the results, whereas ink would dissolve and move with the solvent.
  • Purity: If a substance is pure, it will produce only one spot on a chromatogram.
  • $R_f$ Value Calculation: The Retention Factor ($R_f$) is calculated as:       $R_f = \frac{\text{Distance moved by spot}}{\text{Distance moved by solvent}}$

Atomic Structure and the Periodic Table

• Subatomic Particles

  • Proton: Relative Charge = $+1$.
  • Neutron: Relative Charge = $0$.
  • Electron: Relative Charge = $-1$.
  • Nucleus: Almost all of the mass of an atom is concentrated in the nucleus, which consists of protons and neutrons.

• Atomic Definitions

  • Atomic Number: The number of protons in the nucleus of an atom.
  • Isotopes: Atoms of the same element that have the same number of protons but different numbers of neutrons.
  • Relative Atomic Mass ($A_r$): The weighted average mass of the isotopes of an element.
    • Example (Chlorine): If a sample contains $75\%$ of ${^{35}Cl}$ and $25\%$ of ${^{37}Cl}$:           $A_r = \frac{(75 \times 35) + (25 \times 37)}{100} = 35.5$

• History and Organization of the Periodic Table

  • John Dalton: Viewed atoms as solid, indivisible spheres.
  • J.J. Thomson: Proposed the Plum Pudding Model, suggesting the atom was a sphere of positive charge with electrons embedded in it.
  • Ernest Rutherford: Conducted the alpha particle scattering experiment, proving the atom is mostly empty space with a dense, positively charged nucleus.
  • Dmitri Mendeleev: Organized the early Periodic Table by increasing atomic weight, leaving gaps for elements that had not yet been discovered.
  • Modern Periodic Table: Elements in the same group have the same number of electrons in their outer shell.
  • Group 1 (Alkali Metals): Reactivity increases as you go down the group.
  • Group 0 (Noble Gases): These elements are unreactive (inert) because they have a full, stable outer shell of electrons.

Electron Configuration and Ions

• Electron Arrangement

  • The first (innermost) shell can hold a maximum of 2 electrons.
  • Example (Sodium): Sodium has an atomic number of 11. Its configuration is $2, 8, 1$.

• Ionic Bonding and Ion Formation

  • Metal Atoms: Form positive ions by losing electrons from their outer shell. If an atom loses 2 electrons, the ion has a charge of $+2$.
  • Non-Metal Atoms: Form negative ions by gaining electrons.
    • Oxide Ion: $O^{2-}$.
    • Group 7 (Halogens): Form ions with a charge of $-1$.
    • Chloride Ion ($Cl^-$): Follows a stable configuration of $2, 8, 8$.
  • Ionic Lattice: Ions are held together by strong electrostatic forces of attraction between oppositely charged ions.

Forces, Mass, and Weight

• Mass vs. Weight

  • Mass: The amount of matter in an object, measured in kilograms ($kg$).
  • Weight: The force acting on an object due to gravity, measured in Newtons ($N$). Weight depends on the local gravitational field strength.
  • Weight Formula:$W = m \times g$       Where $W$ is weight, $m$ is mass, and $g$ is gravitational field strength.
  • Earth Calculation: On Earth, where $g \approx 10\,N/kg$, the weight of a $5\,kg$ object is:       $5\,kg \times 10\,N/kg = 50\,N$
  • Measurement: Weight is typically measured in a lab using a Newtonmeter (spring balance).

• Resultant Forces

  • Balanced Forces: Occur when forces acting on an object are equal in size and opposite in direction. The resultant force is zero ($0\,N$), and a moving object will continue at a constant velocity.
  • Unbalanced Forces: Result in acceleration or deceleration.
    • Calculation Example: A car with $1000\,N$ forward drive and $400\,N$ air resistance has a resultant force of:           $1000\,N - 400\,N = 600\,N\text{ forward}$
  • Friction: A force that opposes motion between two surfaces sliding past each other.

Motion and Newton's Laws

• Scalars and Vectors

  • Scalar: A quantity with magnitude (size) only (e.g., speed, mass, distance).
  • Vector: A quantity with both magnitude and direction (e.g., velocity, acceleration, force, displacement).
  • Displacement: The straight-line distance and direction from an object's starting point to its ending point.

• Motion Graphs

  • Distance-Time Graph: The gradient (slope) of the line represents the speed of the object.
  • Velocity-Time Graph:
    • A flat, horizontal line indicates the object is moving at a steady, constant speed.
    • The gradient of the line represents the acceleration.
    • The total area under the graph line represents the distance traveled.

• Newton's Second Law and Acceleration

  • Acceleration Formula:$Acceleration = \frac{\text{Change in velocity}}{\text{Time taken}}$
  • Newton's Second Law Formula:$F = m \times a$       Where $F$ is the resultant force, $m$ is mass, and $a$ is acceleration.
  • Calculation Examples:
    1. Mass calculation: If $F = 10\,N$ and $a = 2\,m/s^2$, then:             $m = \frac{F}{a} = \frac{10\,N}{2\,m/s^2} = 5\,kg$
    2. Speed calculation: If a cyclist travels $60\,m$ in $12\,s$:             $Speed = \frac{60\,m}{12\,s} = 5\,m/s$
    3. Acceleration calculation: A runner goes from $0\,m/s$ to $6\,m/s$ in $3\,s$:             $a = \frac{6\,m/s - 0\,m/s}{3\,s} = 2\,m/s^2$
  • Units: The unit for acceleration is $m/s^2$.
  • Deceleration: If an object is slowing down, its acceleration value is expressed as negative.