Cambridge IGCSE Biology 0610/13 October/November 2025 Comprehensive Notes

Characteristics and Classification of Living Organisms

Biological life is identified by several fundamental characteristics, including movement, sensitivity, growth, and excretion. The sundew, a carnivorous plant, exhibits sensitivity and movement when capturing prey. Sensitivity is the ability to detect or sense stimuli in the internal or external environment and to make appropriate responses, such as detecting an insect on a leaf. Movement is defined as an action by an organism or part of an organism that causes a change of position or place, seen when its sticky hair-like structures bend over to trap an insect.

Arthropods are classified based on physical features such as their number of legs. Insects are defined by having exactly three pairs of legs, which distinguishes them from arachnids (four pairs), crustaceans, and myriapods. Dichotomous keys further aid in classification; for vertebrates, specific traits are definitive. Mammals are identified by the presence of fur, while birds are distinguished by feathers. Reptiles are characterized by having dry skin and scales, whereas amphibians typically possess moist skin and fish have gills and wet scales.

Cell Structure and Movement Processes

Cells contain specific components that facilitate genetic and metabolic functions. A plasmid is a small, circular piece of DNA found in bacteria, separate from the chromosomal DNA. Typical plant cells consist of a cell wall made of cellulose, a cell membrane, cytoplasm, a nucleus, a large central vacuole, and chloroplasts which are the site of photosynthesis. These are distinct from animal cells, which lack a cell wall and chloroplasts.

Calculating the actual size of biological structures is performed using the formula:

$\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}}$

For example, if an alveolus has a diameter of $90\,\text{mm}$ in a diagram with a magnification of $\times 225$, its actual size is:

$\text{Actual size} = \frac{90\,\text{mm}}{225} = 0.40\,\text{mm}$

Substances move across cell boundaries through diffusion, osmosis, and active transport. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. This rate decreases as the diffusion distance increases. In plants, oxygen moves from the soil into root hair cells via diffusion. Osmosis is the movement of water through a partially permeable membrane; in a laboratory setting, dialysis tubing is commonly used to model the properties of the cell membrane for osmosis investigations.

Biological Molecules and Enzyme Catalysis

Chemical tests are used to determine the composition of food. The Biuret test identifies the presence of protein, with a positive result indicated by a color change from blue to purple. The DCPIP test is used for Vitamin C, where a positive result is marked by the blue reagent becoming colorless.

Metabolic reactions are controlled by enzymes, which act as biological catalysts. Amylase is a specific catalyst that breaks down starch into sugars. In these reactions, the substance being acted upon is the substrate, and the substances formed are the products.

Plant Physiology and Nutrition

During photosynthesis, plants produce sugar that serves multiple roles. It can be used immediately in respiration to release energy, stored as insoluble starch, used to synthesize cellulose for cell walls, or converted to nectar. While sugar is a product of photosynthesis, it is not used to power the process of photosynthesis itself.

Gas exchange in plants is regulated by stomata, which are small pores mostly found on the underside of leaves. Each stoma is flanked by two guard cells that control its opening and closing. The surrounding tissue consists of mesophyll cells, which are critical for gaseous exchange and photosynthesis.

Human Anatomy and Physiology Systems

The human digestive system facilitates both physical and chemical breakdown of food. Physical digestion occurs in the stomach, where food is churned in the presence of an acidic pH (hydrochloric acid), which also helps kill pathogens and provides the optimal environment for certain enzymes.

In the circulatory system, the heart is a muscular pump with four chambers. The left atrium is the chamber responsible for receiving oxygenated blood directly from the lungs. Respiratory function occurs in the lungs, where oxygen and carbon dioxide are exchanged in air sacs called alveoli. Expired air differs from inspired air in that it has a lower oxygen concentration and a higher carbon dioxide concentration due to the gas exchange process.

Aerobic respiration is the chemical process that uses oxygen and glucose to produce energy, carbon dioxide, and water. The equation for this process is:

$\text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water}$

Coordination, Response, and Homeostasis

The human nervous system coordinates rapid responses through reflex arcs. These pathways involve sensory, relay, and motor neurones. Relay neurones are found exclusively within the central nervous system (CNS). Coordination in the eye is handled by various structures; the cornea is responsible for refracting (changing the direction of) light as it first enters the eye.

Chemical coordination is managed by the endocrine system. Endocrine glands secrete hormones like insulin directly into the bloodstream to regulate physiological processes such as blood glucose levels. Plant coordination involves growth responses called tropisms. Shoots demonstrate negative gravitropism by growing away from the stimulus of gravity (upwards), while roots demonstrate positive gravitropism by growing toward it.

Reproduction and Health

Reproduction can be asexual or sexual. Asexual reproduction, such as that seen in potato plants forming tubers, involves only one parent and produces offspring that are genetically identical to the parent. Sexual reproduction involves the fusion of male and female gametes. In flowers, the anther produces pollen, and the stigma receives it. Seed germination requires both water and oxygen to proceed.

In the human male reproductive system, the prostate gland produces seminal fluid to support sperm. During puberty, both males and females experience secondary sexual characteristics, such as the growth of sexual organs. Females additionally experience hip widening and fat deposition, while males develop facial hair.

Human health is maintained by preventing the spread of diseases. HIV is a virus that can be spread through direct contact, such as the exchange of body fluids or sharing needles. The transmission rate can be reduced through the screening of blood used for transfusions. Infections caused by bacteria can be treated with antibiotics, which are drugs that kill bacteria only and have no effect on viruses.

Genetics and inheritance

Inheritance involves the passing of genetic information through alleles, which are alternative forms of a gene. A genotype is the specific set of alleles an organism possesses. In monohybrid crosses involving a dominant allele for red flowers ($R$) and a recessive allele for white flowers ($r$), a cross between two heterozygous parents ($Rr \times Rr$) results in a phenotypic ratio of approximately $3:1$ (for example, $93$ red flowers to $28$ white flowers).

Ecology, Environment, and Biotechnology

Energy enters ecosystems via the Sun and is captured by producers like green plants or hazel trees. In a food chain ($\text{plants} \rightarrow \text{chickens} \rightarrow \text{foxes}$), energy is lost at each level primarily through respiration. Ecological pyramids represent this energy transfer; in a four-level pyramid where the producer is at level 4, levels 1 and 2 represent carnivores.

Human population growth typically follows a sigmoid curve, passing through lag, exponential (log), stationary, and death phases. Human environmental impact includes water pollution from untreated sewage, which introduces pathogens into ecosystems. Agriculture uses herbicides to kill weeds and insecticides for pests. Intensive farming can increase yields but leads to a loss of natural habitats.

Biotechnology utilizes the rapid reproduction of microorganisms. One bacterium dividing every $15\,\text{min}$ will undergo $6$ divisions in $90\,\text{min}$. The calculation for the total number of bacteria is:

$2^6 = 64$

Starting from a single bacterium, the increase in the number of bacteria is:

$64 - 1 = 63$