Fundamental Biological Processes: From Cells to Systems

Posted on Oct 19, 2025 in Biology

Metabolism and Energy

Diabetes: Type 1 vs. Type 2

In Type 2 diabetes, insulin is produced, but it does not affect the body’s cells. In contrast, with Type 1 diabetes, no insulin is produced at all. Management for diabetes can include exercise and careful control of diet.

Cellular Respiration and Muscle Contraction

Glucose serves as a respiratory substrate, used to produce ATP. This ATP is required to unbind the globular myosin heads from the actin-myosin cross-bridges, allowing them to re-bind further along the actin filament and cause muscle contraction.

Respiratory Quotients (RQ)

The respiratory quotient varies depending on the substrate being respired:

  • Carbohydrates: 1.0
  • Protein: 0.9
  • Lipids: 0.7

Chemiosmosis in Mitochondria

Chemiosmosis is the process of ATP synthesis in the mitochondria:

  1. NADH transports hydrogen ions (H+) and electrons (e-) to the inner mitochondrial membrane.
  2. Molecules are oxidized and reduced along the electron transport chain.
  3. Protons are pumped across the inner membrane, building a concentration gradient.
  4. Electrons, supplied by NAD and FAD, move through protein complexes.
  5. Protons flow back through ATP synthase, producing ATP from ADP and inorganic phosphate (Pi).
  6. Oxygen acts as the final electron acceptor.

Photosynthesis

The Role of Photosynthetic Pigments

Photosynthetic pigments absorb different wavelengths of light and reflect the wavelengths they do not absorb. In the photosystem, accessory pigments absorb light and pass the energy to the primary pigment, chlorophyll. This excites electrons in the magnesium (Mg) atom of chlorophyll, passing them along the electron transport chain.

Adaptations in Aquatic Organisms

An aquatic organism may live in water that filters out certain wavelengths of light. To survive, it may need to absorb blue light more effectively than terrestrial organisms, requiring different accessory pigments to maximize light absorption.

Chloroplast Structure Comparison

A comparison between two chloroplasts revealed key differences:

  • One contained only three stacked thylakoids, whereas a typical chloroplast has many thylakoids stacked into structures called grana.
  • The atypical chloroplast contained no intergranal lamellae, which are present in a normal one.
  • A normal chloroplast may also contain starch grains, which were absent in the other.

Biological Molecules

Comparing Glycogen and Chitin

Similarities:

  • Both are polymers with 6-membered rings.
  • Both contain 1,4 glycosidic bonds.

Differences:

  • Bonding: Glycogen forms both 1,4 and 1,6 glycosidic bonds, while chitin only forms 1,4 glycosidic bonds.
  • Structure: Glycogen is branched, whereas chitin is a straight-chained polymer.
  • Monomers: Glycogen is composed of alpha-glucose monomers. Chitin’s monomers contain a different chemical group on the second carbon atom.

Adaptations of Cellulose

Cellulose is adapted for its structural role due to its properties:

  • It forms strong, unbranched chains.
  • It has a fibrous structure.
  • It is insoluble in water.

Cell Biology

The Phospholipid Bilayer and Membrane Functions

The cell membrane consists of a phospholipid bilayer, where each phospholipid has a hydrophilic head and a hydrophobic tail. This structure has several key functions:

  • Compartmentalisation: It separates the contents of organelles from the cytoplasm.
  • Vesicle Formation: Membranes are used to form vesicles for transporting substances like enzymes.
  • Increased Surface Area: The inner mitochondrial membrane is folded to increase the surface area for processes like chemiosmosis.

Experimental Design and Critique

Critique of an Oxygen Uptake Study

A student’s hypothesis was deemed invalid. The observed percentage decrease in oxygen uptake was between 35-45%, which was above the predicted 20%. The data was not representative enough to validate the hypothesis because only one pregnant woman was used as a subject, which fails to show the effect across a whole pregnancy.

Measuring Transpiration with a Potometer

To compare the rate of transpiration in two different plant species, the following method should be used:

  1. Maintain constant conditions for light intensity, temperature, and wind speed.
  2. Ensure both plant species are healthy.
  3. Set up the potometer correctly.
  4. Allow the plants 2 minutes to acclimatize.
  5. Record the initial position of the air bubble.
  6. After a set length of time, record the new position of the bubble.
  7. Calculate the rate of transpiration using the formula: Rate = Distance moved by bubble / Time taken.
  8. Repeat the experiment for each plant and calculate an average result to ensure reliability.

Investigating Membrane Permeability with Beetroot

Method and Assumptions:

  1. The core assumption is that pigment release is dependent on the permeability of the cell membrane. Beetroot pieces are placed in different concentrations of ethanol.
  2. When cut, the beetroot pieces must be washed to remove surface pigment.
  3. Another assumption is that the absorption reading from a colorimeter is directly related to the amount of pigment released.

Controls and Improvements:

  • Controls: Use beetroot pieces of the same size, from the same plant, and dry each piece after washing. Use the same colorimeter for all readings.
  • Graph Critique: A presented bar graph lacked units for the concentration of ethanol and did not include a line of best fit.
  • Data Analysis: Calculating an average can provide more accurate, representative results and help identify and remove anomalies.

Seedling Gas Exchange Experiment

In an experiment measuring gas exchange in seedlings, soda lime was not used because the seedlings were photosynthesising and therefore needed to take up CO₂. At 1500 lux, the liquid in the manometer did not move (net gas exchange was 0), indicating that the rate of photosynthesis was equal to the rate of respiration. Above 1500 lux, there was a positive value for liquid movement, meaning the rate of photosynthesis was greater than the rate of respiration, as more O₂ was being produced than consumed.

Human Physiology and Nervous System

Key Tissues

Three important tissues in the body are:

  • Liver tissue
  • Pancreatic tissue
  • Muscle tissue

Mechanism of Neurotoxin Action

A neurotoxin can act as a non-competitive inhibitor by altering the tertiary structure of sodium (Na+) ion channels. This has the following effects:

  1. Sodium ions cannot enter the axon to depolarise the membrane to its threshold potential (-70mv).
  2. An action potential is not generated.
  3. The axon remains at its resting potential.
  4. As a result, neurones will not transmit impulses, and the body cannot respond to stimuli.

Cardiovascular System: Atrial Fibrillation and Control

In atrial fibrillation, the sinoatrial node (SAN) would still send signals to the atrioventricular node (AVN), so atrial systole would continue. However, atrial systole would occur more frequently than ventricular systole, causing an irregular heartbeat.

Common Biological Misconceptions Corrected

  • Gas Detected by Chemoreceptors: Chemoreceptors detect changes in the pH of the blood, which is influenced by CO₂ concentration, not baroreceptors.
  • Nerve Controlling Heart Rate: The accelerator nerve increases heart rate, not the vagus nerve (which decreases it).
  • Adrenaline’s Target: Adrenaline binds to receptors on the SAN, not the AVN.
  • Heart Muscle Type: The heart is made of cardiac muscle, not smooth muscle.

Innate Reflexes: Moro and Cranial Reflexes

The Moro Reflex

The Moro reflex is an involuntary response in infants:

  • The action of putting their arms out and back in rapidly would thrust the head forwards, counteracting a lack of head support.
  • Crying signifies to a carer that the baby is in distress so that the carer can support the baby’s head.
  • The response is a rapid reflex, reducing the risk of harm to the baby’s neck. Its purpose is to help an infant grab hold of something to prevent falling.

The Cranial (Blink) Reflex

This is an involuntary reflex to protect the eye:

  1. Receptors in the eye detect movement close to the eye as a stimulus.
  2. An impulse is sent through a sensory neurone to a relay neurone in the brainstem, bypassing conscious parts of the brain.
  3. The impulse travels down a motor neurone to the effectors (eyelid muscles).
  4. This causes the eyelid to shut. As a cranial reflex, it is not communicated down the spinal cord, which allows for a much faster response to a given stimulus.

Mechanics of Breathing: Diaphragm Function

If the diaphragm cannot lower (contract), the volume of the thoracic cavity cannot increase. This causes the pressure inside the lungs to increase, preventing a pressure gradient from forming between the air outside and the air inside the lungs. Consequently, not enough air can be inhaled.