IB Biology SL 2025 Exam - Unit 2 Study Guide

Covering topics from Unit 2 that could be on the 2025 test!

What is the role of enzymes?

Enzymes convert substrates into products

What role do enzymes play in metabolism?

Almost all metabolic reactions are catalyzed by an enzyme, with each enzyme catalyzing one specific reaction (called specificity)

Living organisms need to make a large number of different enzymes

By controlling the amount of enzymes, the organism can control how fast or slow the metabolic processes happen

Anabolic Reactions

When monomers are constructed into macromolecules, building up energy using ATP

Protein synthesis by ribosomes

DNA synthesis

Synthesis of starch, cellulose, glycogen

Catabolic Reactions

When macromolecules break down into monomers, releasing energy (ATP) in the process

Digestion of food

Cell respiration

Digestion of complex carbon compounds

Enzymes as globular proteins with an active site for catalysis

The 3D shape of enzymes has chemical properties that allow them to be catalysts

Substrates must bind to the active site, which varies in size, and the enzymes perform chemical processes to create the products, which are released

A few amino acids are needed at the active site to create the chemical conditions (usually brought together by folding of the polypeptides)

Interactions between substrate and active site to allow induced-fit binding

When the substrate binds to the active site, interactions between the substrate and the active site cause bond angles and lengths to be altered, causing the 3D shape to change as well (induced-fit bonding)

These changes make it easier for bonds within them to break and new bonds to form, converting substrates into products

Role of molecular motion and substrate-active site collisions in enzyme catalysis

Since the substrates and enzymes move in random directions, these collisions only happen occasionally

While some enzymes have chemical properties to draw substances or change their alignment, successful collisions only happen when the substrate and active site are aligned

Typically, the enzymes move around since they are smaller. However, there are instances where the enzymes are immobilized and the substrate needs to move by itself

Relationships between the structure of the active site, enzyme-substrate specificity, and denaturation

The shape and chemical properties of an enzyme’s active site allow substrate molecules to bind, but not other molecules (enzyme-substrate specificity)

Some enzymes only bind to the same substrate (glucose only goes to glucokinase), while others can bind to a group (hexokinase can bind with any hexose sugar)

Since the shape of the active sites rely on relatively weak interactions between amino acids within the protein, heat and acidity can change these shapes, causing denaturation

Effects of temperature, pH, and substrate concentration on the rate of enzyme activity

The higher the temperature, the more kinetic energy both the substrates and enzymes have, therefore higher enzyme activity. However, there comes a point where the temperature is too high and the enzymes lose their shape

Usually enzymes have an optimal pH for activity. Once the pH falls outside their optimal range, enzyme activity slows because of denaturation

The higher the substrate concentration, the slower the rate of enzyme activity since more sites are occupied at once

Measurements in enzyme-catalysed reactions

Typically, measurements for catalyzed reactions are recorded in the change in the amount of the chemical divided by time (millimoles per second gives us mmol s^-1)

Effect of enzymes on activation energy

Since chemical reactions are not single-step processes, activation energy is needed for substrates to convert to products

Without enzymes, the activation energy is much greater in the exothermic process

With enzymes, the exothermic process releases the same net amount of energy, but requires less activation energy

What is ATP?

A nucleotide that consists of the base adenine, 5-carbon ribose sugar, and 3 phosphate groups

Often described as the energy currency of the cell

Soluble in water and is stable at pH levels near neutral

Cannot pass freely through the phospholipid bilayer, so the rate at which they leave or enter the cell can be controlled

The 3rd phosphate can be easily removed or reattached. When it’s removed, there is a small amount of energy released, but it’s enough to fulfill many processes

What does ATP support?

Synthesizing macromolecules - linking monomers together requires energy, therefore the conversion of ATP to ADP releases the energy needed.

Active Transport - Pumping ions or particles across a membrane against the concentration gradient requires energy

Movements - in order for cells to move, they need energy, therefore ATP

Energy transfers during interconversions between ATP and ADP

When ATP converts to ADP, there is a net release of energy (endergonic). This energy is small, but enough to fulfill many processes. The extra phosphate is sometimes linked to another molecule to fulfill a reaction

When ADP converts to ATP, energy is needed (exergonic). This can come from cell respiration, photosynthesis or chemosynthesis

Cell Respiration Process

Carbon compounds are oxidized to release energy and this energy is used to create ATP

Respiration requires oxygen and produces carbon dioxide (called gas exchange)

Glucose breaks down to pyruvate, which breaks down to acetyl, and finally carbon dioxide

Aerobic Cell Respiration

Glucose and oxygen convert to carbon dioxide and water, which converts ADP to ATP

C₆H₁₂O₆ + 6O₂ → 6CO₂ +6H₂O + energy (ATP)

The pyruvate enters the mitochondria to be oxidized further

Anaerobic Cell Respiration

Glucose simply breaks down, producing lactate in animals and some bacteria and ethanol in yeast and fungi

Lactate: C₆H₁₂O₆ → 2C₃H₆O₃ + ATP

Ethanol: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + ATP

Variables affecting rate of cell respiration

Oxygen uptake

Carbon dioxide production

Consumption of glucose or other respiratory substrates

Transformation of light energy to chemical energy when carbon compounds are produced in photosynthesis

Some organisms, such as plants, are able to make all carbon compounds they need only using light energy, carbon dioxide and water

Photosynthesis

The process where light energy is converted to chemical energy in carbon compounds

Carbon compounds include: carbohydrates, proteins, lipids and nucleic acids

Photosynthesis Equation

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6CO₂

Why does oxygen form during photosynthesis?

Oxygen is produced because of the photolysis that splits water molecules. This increases the concentration of oxygen in chloroplasts, so oxygen diffuses out

Water molecules need to split because the hydrogen is used for the reduction reaction that converts carbon dioxide to glucose

Equation: 2H₂O → 4e^- + 4H⁺ + O₂

Separation and identification of photosynthetic pigments by chromatography

Thin layer chromatography can be used to reveal the pigments of leaf tissue by putting it into solvent and the solution will separate the different types of pigments

Absorption of specific wavelengths of light by photosynthetic pigments

Specific pigments in plants absorb certain wavelengths of light while reflect others. This is why us humans view certain flowers in certain colors, since they reflect that specific wavelength while absorbing others for photosynthesis

Similarities and Differences between Absorption and Action Spectra

Absorption Spectrum - a graph showing the percentage of light absorbed at each wavelength by a pigment

Action Spectra - a graph showing the rate of photosynthesis at each wavelength of light

One can use both graphs to determine at what wavelength the most photosynthesis occurs

Techniques to investigate the effects of limiting factors on the rate of photosynthesis

Varying carbon dioxide concentration (at some point the rate will steady out)

Varying light intensity (at some point the rate will steady out)

Varying temperature (at some point the temperature will be too high and photosynthesis will slow down)

Carbon dioxide enrichment experiments as a means of predicting future rates of photosynthesis and plant growth

Sometimes, higher carbon dioxide concentrations can increase the rates of photosynthesis and plant growth.

FACE (free air carbon dioxide enrichment experiments) wants to increase the CO₂ concentration while keeping other factors the same to test this out

Catalyst

Substance that increases the rate of a chemical reaction but is not changed by the reaction

In regards to enzymes, if cells did not make enzymes, chemical reactions on life would be very slow

Active Site

The small area on the enzyme that allows for substrates to be turn into products

The shape and chemical properties of the active site and substrate match each other, which allows the substrate to bind with the enzyme while most other substances cannot

Gas Exchange

Oxygen entering cells through the plasma membrane and carbon dioxide leaving the cells at the same time. This happens via simple diffusion

Gas Exchange & Cell Respiration Relationship

Without gas exchange, cell respiration could not continue because there would be a lack of oxygen and a harmful excess of carbon dioxide in the cell

Without cell respiration, gas exchange would not continue because the use of oxygen and production of carbon dioxide create the concentration gradients

Features of Aerobic Cell Respiration

Oxygen is used

Carbohydrates such as glucose, lipids (fats and oils) and amino acids after deamination can be used

Carbon dioxide and water are waste products

Yield of ATP is high, more than 30 ATP molecules per glucose

Initial reactions are in the cytoplasm, but more occur in the mitochondria including use of oxygen

Features of Anaerobic Cell Respiration

Oxygen is not used

Only carbohydrates can be used

Carbon dioxide plus lactate or ethanol

Yield of ATP is low - 2 ATP per glucose

All reactions happen in cytoplasm

Photons

A particle or unit of light

Discrete quantities of energy, and the energy is related to its wavelength (longer wavelength = less energy)

Photons are absorbed by pigment molecules if the energy they hold causes an electron in an atom of the pigment molecule to jump to a higher energy level (excitation)

Examples of measuring rate reactions

Catalase - easily measured by volume of oxygen gas produced

Amylase - disappearance of a substrate. Iodine can be used to measure the presence or absence of starch

Trypsin - enzyme that catalyzes the breakdown of protein into amino acids. This is paired with biuret solution to measure the rate of reaction

Process in Mitochondrias