Cells are the fundamental units of life, broadly categorised as Eukaryotic or Prokaryotic.

• Eukaryotic Cells: Found in animals, plants, fungi, and protists. Characterised by a membrane-bound nucleus containing linear DNA (chromosomes) and various membrane-bound organelles (e.g., mitochondria, chloroplasts). Generally larger and more complex.
• Prokaryotic Cells: Found in bacteria and archaea. Simpler and smaller. Lack a true nucleus; their genetic material is a single, circular DNA molecule located in the cytoplasm (nucleoid region). Lack complex membrane-bound organelles. May possess small, circular DNA molecules called plasmids.

Animal Cells (Eukaryotic)

• Nucleus: Contains DNA; controls cell activities.
• Cell Membrane: Controls what enters/leaves the cell (selectively permeable).
• Mitochondria: Site of aerobic respiration; release energy (ATP).
• Ribosomes: Site of protein synthesis.
• Cytoplasm: Jelly-like substance where reactions occur.

Plant Cells (Eukaryotic)

• Includes animal cell structures plus:
• Cell Wall: Rigid outer layer (made of cellulose); provides structural support and prevents bursting.
• Chloroplasts: Site of photosynthesis; contain chlorophyll (green pigment).
• Permanent Large Central Vacuole: Contains cell sap; maintains turgor pressure (keeps cell firm).

Bacterial Cells (Prokaryotic)

• Cell Wall: Provides support (made of peptidoglycan).
• Cell Membrane: Controls entry/exit.
• Cytoplasm: Contains ribosomes and genetic material.
• Chromosomal DNA: Single, circular loop in the nucleoid region.
• Plasmid DNA: Small, circular DNA rings (optional); may carry genes for antibiotic resistance.
• Ribosomes: Protein synthesis (smaller than eukaryotic ones).
• Flagella: Optional whip-like tail(s) for movement.

Comparison Table

Feature	Animal Cell (Eukaryotic)	Plant Cell (Eukaryotic)	Bacterial Cell (Prokaryotic)
Nucleus	Present	Present	Absent (Nucleoid region)
Genetic Material	Linear DNA	Linear DNA	Circular DNA
Plasmid DNA	Absent	Absent	Often Present
Cell Membrane	Present	Present	Present
Cell Wall	Absent	Present (Cellulose)	Present (Peptidoglycan)
Mitochondria	Present	Present	Absent
Chloroplasts	Absent	Present	Absent
Ribosomes	Present (Larger, 80S)	Present (Larger, 80S)	Present (Smaller, 70S)
Vacuole	Small/Temporary (if present)	Large, central, permanent	Absent
Flagella	Absent (except sperm)	Absent (in most higher plants)	Sometimes Present

In multicellular organisms, cells differentiate to become specialised, meaning they develop specific structures (adaptations) that allow them to perform a particular function effectively.

Sperm Cells

• Function: To transport the male's DNA to the egg cell for fertilisation.
• Adaptations:
  • Tail (Flagellum): Long tail whips side-to-side for propulsion (swimming).
  • Mitochondria: Numerous mitochondria packed in the mid-section provide abundant energy (ATP) for tail movement.
  • Acrosome: Cap-like structure on the head containing digestive enzymes to break down the outer layers of the egg cell.
  • Haploid Nucleus: Contains only one set of chromosomes (half the normal number) so that upon fertilisation, the resulting zygote has the correct diploid number.

Egg Cells (Ova)

• Function: To be fertilised by sperm and provide nutrients for the initial growth of the embryo.
• Adaptations:
  • Nutrient-Rich Cytoplasm: Contains a large store of energy-rich food (lipids, proteins) to nourish the developing embryo until implantation.
  • Haploid Nucleus: Contains one set of chromosomes.
  • Membrane Changes after Fertilisation: The cell membrane structure changes immediately after one sperm enters, forming a barrier (fertilisation membrane) to prevent other sperm from entering (polyspermy block).

Ciliated Epithelial Cells

• Function: Found lining surfaces such as the trachea and bronchi. They move mucus (containing trapped dust, bacteria, and viruses) upwards, away from the lungs, to be swallowed or expelled.
• Adaptations:
  • Cilia: Tiny hair-like projections on the cell surface that beat in a coordinated, wave-like rhythm to sweep the mucus along.
  • Mitochondria: Numerous mitochondria provide the energy (ATP) required for the continuous movement of the cilia.

Adaptations Summary Table

Cell Type	Function	Key Adaptations
Sperm Cell	Carry male DNA to egg	Acrosome: Enzymes to penetrate egg. Haploid Nucleus: Half chromosome set. Mitochondria: Energy for swimming. Tail: Propulsion.
Egg Cell (Ovum)	Be fertilised; provide early nutrients	Nutrient-rich Cytoplasm: Food store. Haploid Nucleus: Half chromosome set. Membrane changes post-fertilisation: Prevents polyspermy.
Ciliated Epithelial Cell	Move mucus along airways	Cilia: Hair-like structures beat to move substances. Mitochondria: Provide energy for cilia movement.

Microscopes magnify objects too small to see with the naked eye. Key developments, especially electron microscopy, have significantly advanced our understanding of cell biology.

Light Microscopes (LM)

• Use visible light passing through the specimen and glass lenses to magnify the image.
• Maximum useful magnification around $\times 1500$.
• Limited resolution (the ability to distinguish between two close points as separate entities), typically around 200 nm (0.2 $\mu m$). This means structures closer than 200 nm appear as a single object.
• Advantages: Relatively inexpensive, easy to use, can view living specimens (in colour).

Electron Microscopes (EM)

• Use a beam of electrons instead of light, and electromagnets instead of glass lenses. Electrons have a much shorter wavelength than light.
• Offer much higher magnification (e.g., $>\times 1,000,000$) and significantly better resolution (e.g., $<1$ nm).
• Allows visualisation of cellular ultrastructure (very fine details), such as ribosomes, internal membranes of mitochondria and chloroplasts, and viruses.
• Disadvantages: Very expensive, large, require complex specimen preparation (often involving heavy metal stains), specimens must be dead and observed in a vacuum, images are initially black and white.

Impact of EM

The superior resolution of electron microscopes allowed biologists to see sub-cellular structures in much greater detail than ever before. This enabled them to understand the relationship between the structure of organelles and their function (e.g., the folded inner membranes (cristae) in mitochondria provide a large surface area for respiration).

Comparison Table

Feature	Light Microscope (LM)	Electron Microscope (EM)
Radiation Source	Visible Light	Beam of Electrons
Lenses	Glass	Electromagnets
Max Magnification	$\approx \times 1,500$	$>\times 1,000,000$
Max Resolution	$\approx 200$ nm	$<1$ nm
Specimen State	Living or Dead	Dead only
Preparation	Simple	Complex
Environment	Air	Vacuum
Cost/Complexity	Lower	Very High
Image Type	Colour	Black and White

Biology deals with structures across a vast range of sizes. Understanding scale and using estimation are important skills.

• Scale: Recognising the relative sizes of objects is key. Cells are typically measured in micrometres ($\mu m$), while smaller organelles like ribosomes are measured in nanometres (nm). (See Topic 1.5 for units).
• Estimation: Making a sensible, approximate calculation of a value (like size or number) without precise measurement. It's about making an educated guess based on available information.
• Why Use Estimation?
  • Reasonableness Check: To quickly verify if a calculated answer (e.g., magnification, actual size from a formula) is plausible or if a mistake might have been made (e.g., unit conversion error). Is the answer in the right ballpark?
  • Difficult Measurements: To approximate the size or number of objects that are hard to measure directly using tools like an eyepiece graticule. This applies to very small organelles, irregularly shaped structures, or features without sharp, clear boundaries.
  • Field of View: Estimating how many cells fit across the diameter of the microscope's field of view can give a rough idea of cell size if the field diameter is known.
• How to Estimate Size (using a known reference in an image):
  1. Identify a structure within the image whose actual size is known or can be determined (e.g., a scale bar provided on the micrograph, or perhaps the known diameter of the entire cell).
  2. Visually compare the unknown structure (e.g., a nucleus) to the known reference structure (e.g., the cell diameter) *in the image*.
  3. Estimate the ratio: How many times smaller or larger does the unknown structure appear compared to the reference? (e.g., "The nucleus looks about one-third the width of the cell image").
  4. Calculate the estimated actual size: Multiply the actual size of the reference structure by the estimated ratio. (e.g., If cell actual diameter = 30 $\mu m$, estimated nucleus actual diameter $\approx \frac{1}{3} \times 30 \, \mu m = 10 \, \mu m$).

Accurate use of scientific units and handling very large or small numbers using standard form are essential skills in quantitative biology, especially microscopy.

Metric Prefixes for Length/Size

Prefix	Symbol	Power of 10 (relative to metre)	Meaning
(metre)	m	$10^0$	Base unit
milli	m	$10^{-3}$	one thousandth
micro	$\mu$	$10^{-6}$	one millionth
nano	n	$10^{-9}$	one billionth
pico	p	$10^{-12}$	one trillionth

Converting Between Units

• Each step involves multiplying or dividing by 1000.
• Going DOWN (to smaller units): Multiply by 1000 per step.
  • m $\to$ mm ($\times 1000$)
  • mm $\to$ $\mu m$ ($\times 1000$)
  • $\mu m$ $\to$ nm ($\times 1000$)
  • nm $\to$ pm ($\times 1000$)
• Going UP (to larger units): Divide by 1000 per step.
  • pm $\to$ nm ($\div 1000$)
  • nm $\to$ $\mu m$ ($\div 1000$)
  • $\mu m$ $\to$ mm ($\div 1000$)
  • mm $\to$ m ($\div 1000$)
• Example: $2$ mm $= 2 \times 1000 = 2000$ $\mu m$ $= 2000 \times 1000 = 2,000,000$ nm.
• Example: $50$ nm $= 50 \div 1000 = 0.05$ $\mu m$ $= 0.05 \div 1000 = 0.00005$ mm.

Standard Form (Scientific Notation)

• Used to express very large or very small numbers conveniently.
• Format: $A \times 10^n$
  • $A$ is a number such that $1 \le A < 10$.
  • $n$ is an integer (positive, negative, or zero) representing the power of 10.
• Large Numbers (n is positive): The decimal point moves $n$ places to the right. Example: $3,400,000 = 3.4 \times 10^6$.
• Small Numbers (n is negative): The decimal point moves $n$ places to the left. Example: $0.000072 = 7.2 \times 10^{-5}$.
• Calculations:
  • Multiplication: Multiply the $A$ values and add the powers ($n$). Adjust the result to standard form if needed.
    $(2 \times 10^5) \times (4 \times 10^{-2}) = (2 \times 4) \times 10^{(5 + (-2))} = 8 \times 10^3$.
  • Division: Divide the $A$ values and subtract the powers ($n$). Adjust the result if needed.
    $(9 \times 10^7) \div (3 \times 10^3) = (9 \div 3) \times 10^{(7 - 3)} = 3 \times 10^4$.

This practical involves using a light microscope to observe biological specimens, calculate magnification, and produce labelled scientific drawings.

Using a Light Microscope Safely and Effectively

1. Starting Point: Always begin with the lowest power objective lens clicked into position and the stage lowered using the coarse focus knob.
2. Placing the Slide: Place the prepared microscope slide onto the stage and secure it gently with the stage clips, ensuring the specimen is over the hole in the stage.
3. Initial Focusing (Low Power): Look through the eyepiece lens. Turn the coarse focus knob slowly (usually towards you to raise the stage) until the image comes into rough focus. Be careful not to raise the stage too high and hit the objective lens.
4. Fine Focusing: Use the fine focus knob to bring the image into sharp, clear focus. Adjust the diaphragm or lamp intensity for optimal lighting if necessary.
5. Increasing Magnification: Carefully rotate the revolving nosepiece to select the next higher power objective lens. The microscope should be roughly parfocal, meaning the image remains approximately in focus.
6. Refocusing (High Power): Use *only* the fine focus knob to sharpen the image at higher magnification. Never use the coarse focus knob at high power, as this can easily crash the objective lens into the slide, damaging both.

Preparing a Temporary Wet Mount (e.g., Onion Epidermis)

1. Place a clean microscope slide on a flat surface. Add one drop of water (or appropriate liquid like saline) to the centre.
2. Obtain a thin sample of the specimen. For onion, carefully peel a single, transparent layer of epidermis from the inner surface of an onion slice using forceps.
3. Place the specimen flat within the water drop, trying to avoid folds.
4. If necessary, add a drop of biological stain (e.g., iodine solution for onion cells to make the nucleus and cell walls more visible; methylene blue for cheek cells). Allow the stain to diffuse for a minute or two.
5. Take a clean coverslip. Hold it at an angle (approx. $45^{\circ}$) with one edge touching the side of the liquid drop on the slide.
6. Lower the coverslip slowly and carefully using a mounted needle or forceps to gently guide it down. This technique minimizes the trapping of air bubbles, which appear as distinct circles with dark edges under the microscope and can obscure the view.
7. Use a piece of tissue paper to gently blot away any excess liquid that has squeezed out from under the edges of the coverslip.

Magnification Calculations

• Total Magnification = (Magnification of Eyepiece Lens) $\times$ (Magnification of Objective Lens). (Eyepiece is usually $\times 10$).
• Image Size, Actual Size, Magnification Relationship (I AM triangle): Remember the formula triangle.
  • Magnification (M) $$ M = \frac{\text{Image Size (I)}}{\text{Actual Size (A)}} $$
  • Actual Size (A) $$ A = \frac{\text{Image Size (I)}}{\text{Magnification (M)}} $$
  • Image Size (I) $$ I = \text{Actual Size (A)} \times \text{Magnification (M)} $$
• Units Consistency: This is CRITICAL. Before calculating, ensure Image Size (I) (measured from your drawing or micrograph, e.g., in mm) and Actual Size (A) (the real size of the specimen, e.g., in $\mu m$) are converted to the *same units*. It's often easiest to convert the image size measurement to $\mu m$ (1 mm = 1000 $\mu m$). Magnification (M) itself has no units.

Producing Labelled Scientific Drawings

• Pencil Only: Use a sharp HB pencil for all lines and labels. Ensure lines are clean and not smudged.
• Size & Proportion: Drawing should be large enough (e.g., occupying at least half the page/space) to show details clearly and accurately represent the proportions of the structures observed.
• Lines: Use clear, solid, continuous lines for outlines (no sketching, feathering, or gaps). Avoid shading unless specifically representing stained areas (use stippling sparingly if needed).
• Accuracy & Detail: Draw only what you can clearly observe through the microscope at that magnification. Do not add details from memory or textbooks that are not visible. Represent the structures faithfully.
• Labels: Use a ruler to draw straight label lines. Lines should start exactly at the structure being labelled and extend horizontally to the margin. Do not use arrowheads on the end of the label line. Label lines should not cross each other. Write labels clearly, printed horizontally, usually outside the drawing area.
• Information: Include a clear, underlined title stating what the drawing shows (e.g., "Drawing of human cheek cells"). Include the total magnification used (e.g., $\times 100$, $\times 400$).

Enzymes are biological catalysts, typically proteins, that accelerate metabolic reactions by lowering the activation energy needed for the reaction to start. They remain unchanged at the end of the reaction.

Key Features & Function

• Active Site: A unique, three-dimensional pocket or groove on the enzyme's surface. Its specific shape is crucial for function and is determined by the enzyme's amino acid sequence and folding (tertiary structure).
• Substrate: The specific molecule(s) that bind to the active site and are acted upon by the enzyme.
• Specificity: Enzymes exhibit high specificity. This means that the shape of the active site is complementary to the shape of only one specific substrate (or a very small group of structurally similar substrates), like a key fitting into a lock.

Mechanism of Action

1. The substrate molecule collides with the enzyme's active site (requires sufficient kinetic energy).
2. The substrate binds to the active site, forming a temporary enzyme-substrate complex.
3. The enzyme facilitates the chemical reaction (e.g., breaking bonds in the substrate or joining substrates together), converting the substrate into product(s). The binding can strain bonds within the substrate, making the reaction easier.
4. The product(s) are released from the active site, as their shape is no longer complementary.
5. The enzyme's active site is now free to bind with another substrate molecule; the enzyme is regenerated.

Models of Enzyme Action

• Lock and Key Model: An earlier, simpler model suggesting the active site and substrate have fixed, rigid shapes that fit together perfectly, like a specific key fitting into a specific lock.
• Induced Fit Model (Currently Accepted): A more dynamic and accurate model. It proposes that the active site is somewhat flexible. The binding of the substrate induces a subtle change in the shape of the active site, allowing it to mould around the substrate for a tighter, more precise fit (like a glove adjusting to a hand). This induced fit optimizes the catalytic activity by positioning chemical groups correctly and potentially straining substrate bonds.

• Denaturation Definition: A process where the specific three-dimensional structure of a protein, particularly an enzyme, is altered, leading to a loss of its biological function. Crucially, this involves a change in the precise shape of the active site. This change is usually permanent (irreversible).
• Causes: Enzymes function optimally within specific environmental conditions. Denaturation is typically caused by exposure to conditions outside this optimal range, primarily:
  • High Temperatures: As temperature increases significantly beyond the optimum, the atoms within the enzyme molecule vibrate more vigorously. This increased energy breaks the relatively weak bonds (like hydrogen bonds and ionic bonds) that maintain the enzyme's specific tertiary structure (its precise 3D folded shape).
  • Extreme pH Values: pH measures acidity/alkalinity. Each enzyme has an optimum pH. At pH values far from this optimum (either too acidic or too alkaline), the concentration of H$^+$ or OH$^-$ ions interferes with the charges on the amino acid R-groups within the enzyme. This disrupts the ionic and hydrogen bonds responsible for maintaining the correct tertiary structure and active site shape.
• Mechanism & Consequence: The breaking of weak bonds causes the enzyme molecule to unravel or change shape. This alters the specific 3D conformation of the active site. Since the active site's shape is no longer complementary to the substrate molecule, the substrate cannot bind effectively (or at all). As a result, the enzyme loses its catalytic activity and cannot function.

The rate of enzyme-catalysed reactions is influenced by several factors, including temperature, pH, and substrate concentration.

Temperature

• Effect: As temperature increases, molecules gain kinetic energy, move faster, and collide more frequently. This increases the rate of enzyme-substrate complex formation and thus the reaction rate, up to a point.
• Optimum Temperature: Each enzyme has an optimum temperature at which it functions most efficiently (highest rate). For most human enzymes, this is around $37^{\circ}$C (body temperature).
• Above Optimum: At temperatures significantly above the optimum, the enzyme begins to denature. The increased vibrations break bonds, changing the active site shape. The rate decreases rapidly as the enzyme loses function.
• Graphical Representation: Rate vs. Temperature graph typically shows a curve rising to a peak (optimum) and then falling sharply.

pH

• Effect on Structure: pH affects the ionisation state (charges) of amino acid R-groups within the enzyme, particularly those forming the active site. These charges are crucial for maintaining the enzyme's tertiary structure and for binding the substrate.
• Optimum pH: Each enzyme has a specific optimum pH at which its conformation is ideal for maximum activity. This varies widely depending on the enzyme and its location (e.g., pepsin in the acidic stomach, pH $\approx 2$; trypsin in the alkaline small intestine, pH $\approx 8$).
• Away from Optimum: At pH values significantly above or below the optimum, the enzyme's structure is disrupted due to altered charges interfering with ionic and hydrogen bonds. This leads to denaturation (change in active site shape) and a decrease in reaction rate.
• Graphical Representation: Rate vs. pH graph is typically a bell-shaped curve, with the peak representing the optimum pH.

Substrate Concentration

• Effect (at constant enzyme concentration): At low substrate concentrations, the reaction rate is directly proportional to the substrate concentration. Increasing the substrate provides more molecules to bind with available active sites, leading to more frequent enzyme-substrate complex formation. In this phase, the substrate concentration is the limiting factor.
• Saturation Point: As substrate concentration continues to increase, the rate begins to level off and eventually reaches a maximum velocity ($V_{max}$). This plateau occurs because all the available enzyme active sites are occupied (saturated) with substrate molecules at any given moment. The enzymes are working at their maximum capacity.
• Limiting Factor at Plateau: Once saturation is reached, adding more substrate will not increase the reaction rate further. The enzyme concentration (the number of available active sites) becomes the limiting factor.
• Graphical Representation: Rate vs. Substrate Concentration graph shows an initial steep increase, then the slope gradually decreases until the rate becomes constant (plateau).

This practical investigates how changing pH affects the rate at which the enzyme amylase breaks down starch.

Background & Principle

• Enzyme: Amylase (catalyses the breakdown of starch).
• Substrate: Starch solution.
• Product: Maltose (a sugar).
• Testing for Starch: Iodine solution is used. It turns blue-black in the presence of starch and remains orange-brown/yellow in the absence of starch.
• Aim: To determine the effect of different pH values on the time taken for amylase to digest starch completely.

Method Outline

1. Set up Iodine: Place single drops of iodine solution into multiple wells of a spotting tile.
2. Prepare Solutions: Create a series of test tubes each containing amylase solution mixed with a buffer solution of a specific pH (e.g., pH 4, 5, 6, 7, 8, 9). Use the same concentration of amylase in each. Have a separate tube of starch solution.
3. Control Temperature: Place all test tubes (enzyme/buffer mixes and starch) into a water bath set at a constant, suitable temperature (e.g., $35^{\circ}$C) and allow them to reach this temperature (equilibration).
4. Start Reaction: Add a measured volume of the starch solution to one of the enzyme/buffer tubes (e.g., the pH 4 tube). Mix quickly and immediately start a timer. Keep the tube in the water bath.
5. Sampling: At regular intervals (e.g., every 20 seconds), use a clean pipette to transfer a drop of the reaction mixture from the test tube into one of the iodine wells on the spotting tile.
6. Observe & Record Endpoint: Note the colour. Continue sampling into fresh wells until the iodine solution no longer turns blue-black but remains orange-brown/yellow. Record the time taken to reach this endpoint (when all starch is digested).
7. Repeat for all pH values: Carry out steps 4-6 for each of the prepared pH buffer/amylase solutions.
8. Reliability: Repeat the entire experiment for each pH at least once more to check for consistency and calculate a mean time if appropriate.

Variables

• Independent Variable (what is changed): pH (controlled by using different buffer solutions).
• Dependent Variable (what is measured): Time taken for starch digestion (until iodine remains orange-brown).
• Calculating Rate: Rate can be calculated as $Rate = \frac{1}{Time}$ (in $s^{-1}$ or $min^{-1}$).
• Control Variables (kept constant): Temperature (using water bath), enzyme concentration, starch concentration, total volume of reaction mixture.

Expected Results & Conclusion

• The shortest time (fastest rate) will occur at the enzyme's optimum pH (e.g., often around pH 7 for salivary amylase).
• Longer times (slower rates) will be observed at pH values further away from the optimum due to the enzyme becoming less active or denatured.
• Plotting Rate ($1/$Time) against pH should yield a bell-shaped curve, peaking at the optimum pH.

Calculating the rate of enzyme activity allows for quantitative comparison under different conditions.

Method 1: Change in Quantity over Time

• This method is used when you can measure how much substrate is used up or how much product is formed over a specific time period.
• Formula: $$ Rate = \frac{\text{Change in Quantity}}{\text{Time Taken}} $$
• Example: If an enzyme produces 30 $cm^3$ of gas in 60 seconds, the rate is $\frac{30 \, cm^3}{60 \, s} = 0.5 \, cm^3/s$.
• Units: Depend on what is measured (e.g., g/min, mol/s, $cm^3 min^{-1}$).

Method 2: Time Taken to Reach an Endpoint

• This is often used in school practicals where it's easier to measure the time it takes for a specific observable event to occur (e.g., disappearance of starch, a colour change).
• Since rate is inversely proportional to time (faster reaction = less time), the rate can be expressed as the reciprocal of the time taken.
• Formula: $$ Rate = \frac{1}{\text{Time Taken}} $$
• Units: The unit will be "per unit time", such as $s^{-1}$ (per second) or $min^{-1}$ (per minute).
• Example: If it takes 40 seconds for starch to disappear in an amylase experiment, the rate is $\frac{1}{40 \, s} = 0.025 \, s^{-1}$.

Calculating Rate from Graphs (Quantity vs. Time)

• When plotting experimental data (e.g., volume of product vs. time), the graph is often a curve (steepest at the start, flattening over time as substrate is used up).
• The rate of reaction at any specific point in time is equal to the gradient (slope) of the curve at that exact point.
• Finding the Gradient on a Curve:
  1. Choose the time point on the x-axis.
  2. Draw a straight line that just touches the curve at that point without crossing it (this is a tangent). Make the tangent reasonably long.
  3. Choose two points on the tangent line (preferably far apart) and read their coordinates ($x_1, y_1$) and ($x_2, y_2$).
  4. Calculate the gradient: $$ Gradient = \frac{\text{Change in Y } (y_2 - y_1)}{\text{Change in X } (x_2 - x_1)} $$ This gradient represents the rate at that specific time.
• The Initial Rate: The rate at the very beginning of the reaction (time = 0) is usually the fastest. It's found by calculating the gradient of the tangent drawn to the curve at t=0. This is often used for comparisons because conditions (like substrate concentration) are well-defined at the start.

Enzymes are indispensable biological catalysts that regulate the speed of virtually all chemical reactions (metabolism) occurring within living organisms, enabling life processes to occur efficiently under physiological conditions (normal body temperature and pH).

Role in Breakdown (Catabolism / Digestion)

• Enzymes are essential for breaking down large, complex, often insoluble food molecules into smaller, simpler, soluble molecules that can be absorbed into the bloodstream and utilized by cells.
• Carbohydrases catalyse the breakdown of complex carbohydrates (polysaccharides like starch) into simple sugars (monosaccharides like glucose).
  • Example: Salivary and pancreatic amylase break down starch $\to$ maltose (a disaccharide). Enzymes like maltase then break down maltose $\to$ glucose.
• Proteases catalyse the breakdown of proteins $\to$ amino acids.
  • Example: Pepsin (in stomach) begins protein digestion. Trypsin (in small intestine) continues the process, breaking proteins and polypeptides into smaller peptides and amino acids.
• Lipases catalyse the breakdown of lipids (fats and oils) $\to$ fatty acids and glycerol.

Role in Synthesis (Anabolism)

• Enzymes are equally crucial for building up large, complex molecules from smaller, simpler precursor units. These processes require energy and are essential for growth, repair, storage, and creating cellular components.
• Carbohydrate Synthesis: e.g., Enzymes like glycogen synthase join glucose molecules to form glycogen (in animals) or starch (in plants) for energy storage.
• Protein Synthesis: Enzymes are involved at various stages as amino acids are joined together according to the genetic code (at the ribosomes) to form specific polypeptide chains, which then fold into functional proteins.
• Lipid Synthesis: Enzymes catalyse the reactions that join fatty acids and glycerol to form triglycerides (for energy storage) and phospholipids (for cell membranes).
• DNA Replication/Transcription: Enzymes like DNA polymerase and RNA polymerase are fundamental to copying and reading genetic information.

Overall Significance

• Without enzymes acting as catalysts, metabolic reactions would proceed far too slowly at the relatively low temperatures and neutral pH found within most organisms to sustain life.
• They allow precise control over metabolic pathways.
• Essential for: Energy release (respiration), nutrient absorption, muscle contraction, nerve impulse transmission, waste removal, defence against pathogens, growth, and repair.

Key Digestive Enzymes Summary Table

Enzyme Type	Specific Example	Substrate	Product(s)	Location (Example)
Carbohydrase	Amylase	Starch	Maltose	Saliva, Pancreatic Juice
Carbohydrase	Maltase	Maltose	Glucose	Small Intestine Lining
Protease	Pepsin	Proteins	Polypeptides	Stomach
Protease	Trypsin	Proteins/Polypeptides	Smaller Peptides/Amino Acids	Small Intestine (from Pancreas)
Lipase	Pancreatic Lipase	Lipids (Fats)	Fatty Acids & Glycerol	Small Intestine (from Pancreas)