Key concepts in biology revision notes

Enzyme Structure and Function

Enzymes are protein catalysts with specific 3D shapes. The active site is a key region that binds to a specific substrate. This interaction is often compared to a 'lock and key' mechanism, ensuring reaction specificity. The unique shape of the active site is vital for its function.

Denaturation Explained

Denaturation is the irreversible alteration of an enzyme's 3D structure, particularly its active site. This is caused by environmental changes like extreme temperature or pH. When denatured, the active site's shape changes, preventing substrate binding and leading to a loss of enzyme activity. The enzyme can no longer function as a catalyst.

Factors Causing Denaturation

High temperatures cause increased molecular vibrations, breaking the weak bonds (e.g., hydrogen and ionic bonds) that maintain the enzyme's tertiary structure. Extreme pH values (too acidic or too alkaline) alter the charges on amino acid side chains, disrupting these crucial bonds and consequently changing the active site's shape.

Consequences of Denaturation

A denatured enzyme can no longer bind effectively to its substrate because its active site has changed shape. This loss of affinity means the enzyme cannot catalyse its specific reaction, resulting in a significantly reduced or complete cessation of biochemical activity. Denaturation is usually permanent.

Enzyme Structure and Active Site

Enzymes are proteins with a complex 3D shape. A crucial part of this shape is the active site, a pocket or groove where a specific substrate molecule binds. The active site's shape is complementary to the substrate, much like a lock fits a specific key.

Mechanism of Action: Lock and Key, Induced Fit

Substrates bind to the enzyme's active site, forming an enzyme-substrate complex. The 'lock and key' model suggests a perfect fit. The 'induced fit' model refines this, stating that the enzyme's active site can slightly change its shape to better accommodate the substrate upon binding, optimising the reaction. Products are then released, and the enzyme is ready for another reaction.

Enzyme Specificity

Due to the unique and complementary shape of its active site, each enzyme is typically specific to only one type of substrate or a very small group of structurally similar substrates. This specificity ensures that the correct biochemical reactions occur within the cell.

Temperature and Enzyme Activity

Enzymes have an optimum temperature, typically around 37°C for human enzymes. Below this, activity increases with temperature as kinetic energy rises. Above the optimum, the enzyme denatures irreversibly, losing its active site shape and function.

pH and Enzyme Activity

Each enzyme has an optimum pH. Changes in pH outside this narrow range alter the charges on the amino acids within the enzyme, disrupting its tertiary structure and active site, leading to denaturation and loss of activity.

Substrate Concentration and Enzyme Activity

Increasing substrate concentration generally increases reaction rate until all enzyme active sites are occupied (saturation). At this point, the enzyme is working at maximum capacity, and further increases in substrate concentration have no effect.

Denaturation

Denaturation is the irreversible change in an enzyme's three-dimensional shape, particularly its active site, caused by extreme temperatures or pH. A denatured enzyme loses its catalytic function.

Understanding Osmosis and Mass Change

In osmosis, water moves across a partially permeable membrane. If a cell or tissue is placed in a solution with a higher water concentration (more dilute), water enters the cell, causing it to gain mass. Conversely, if placed in a solution with a lower water concentration (more concentrated), water leaves the cell, leading to a loss of mass.

Formula for Percentage Change in Mass

To calculate the percentage change in mass, use the formula: Percentage change in mass = ((Final mass - Initial mass) / Initial mass) × 100. A positive result indicates a gain in mass, while a negative result indicates a loss of mass.

Interpreting Results

A percentage gain suggests the tissue is in a hypotonic (more dilute) solution relative to its internal concentration, meaning water moved in. A percentage loss suggests the tissue is in a hypertonic (more concentrated) solution, meaning water moved out. A zero percentage change would imply the solution is isotonic (equal concentration).