Variation, Homeostasis, and Micro-Organisms
Classification and biodiversity
Biodiversity refers to the total number and variety of species in a specific area. It can be analyzed using sampling methods, and organisms are classified based on their physical characteristics.
Biodiversity is crucial for humans as it provides food, industrial materials, and potential new medicines. This is why studying and protecting Earth's biodiversity is essential. The population of organisms constantly changes due to several factors, including:
Competition for resources
Predation
Disease
Pollution
Classification
There are millions of species on Earth, and scientists organize them into groups to make identification easier. These groups start broadly, with few shared traits, and become more specific as similarities increase.
The largest groups are called kingdoms, and there are five main ones:
Animals
Plants
Fungi
Protoctists (single-celled organisms)
Bacteria
The Animal Kingdom
Animals are classified into two main groups:
Vertebrates – animals with a backbone, such as mammals, reptiles, amphibians, bony fish, and birds.
Invertebrates – animals without a backbone, including molluscs, annelids, arthropods, and nematodes.
The Plant Kingdom
Plants are divided into two groups:
Flowering plants – reproduce with flowers, such as sunflowers and grasses.
Non-flowering plants – reproduce using spores, like ferns and mosses.
The Fungi Kingdom
Fungi are distinct from plants and animals. They absorb nutrients from organic material rather than producing their own food through photosynthesis.
Some fungi, like mushrooms, are visible, while others, like yeast, are microscopic. They play a vital role in decomposing dead organisms and recycling nutrients in ecosystems.
Protoctists and Bacteria
Protoctists: are mostly single-celled organisms that don’t fit into the animal, plant, or fungi categories. Some, like algae, can photosynthesize, while others, like amoebas, consume food.
Bacteria: are microscopic, single-celled organisms that lack a nucleus. Some are beneficial, helping with digestion and decomposition, while others can cause disease.
Naming Species
The classification of organisms follows a specific hierarchy, from broad groups to specific species:
Kingdom – The largest classification level, grouping organisms with fundamental similarities (e.g., Animals, Plants).
Phylum – Divides kingdoms based on major body structures and functions (e.g., Chordata includes vertebrates).
Class – Further narrows phyla based on more specific traits (e.g., Mammalia includes warm-blooded animals with hair).
Order – Groups organisms within a class that share additional similarities (e.g., Carnivora includes meat-eating mammals).
Family – A more refined grouping of related organisms within an order (e.g., Felidae includes cats).
Genus – A category that includes species with very close similarities (e.g., Panthera includes lions and tigers).
Species – The most specific classification, consisting of organisms that can breed and produce fertile offspring (e.g., Panthera leo for lions).
The binomial system names organisms using their genus and species, ensuring universal recognition and avoiding confusion from regional common names. Latin is used to maintain consistency.
Morphological and Behavioral Adaptations
Organisms develop adaptations to improve their chances of survival in their environments. These adaptations can be structural (morphological) or behavioral.
Morphological Adaptations: are physical traits that help an organism survive in its habitat.
The Fennec fox, which lives in the desert, has large ears that help release body heat, keeping it cool.
In contrast, the Arctic fox has small ears and thick fur to retain heat and stay warm in its cold environment.
Behavioral Adaptations: involve actions or habits that help an organism cope with its surroundings.
The pangolin is nocturnal, meaning it is active at night. This behavior helps it avoid the extreme heat of its desert-like habitat, allowing it to hunt comfortably when temperatures are lower.
Cell Division and Stem Cells
Mitosis
Chromosomes carry genetic information in the form of DNA. They are located in the nucleus of the cell and contain a sequence of genes. Genes exist in pairs and each one provides instructions for making a specific protein.
Mitosis is a process of cell division that ensures new cells receive identical genetic information.
In human body cells, there are 46 chromosomes, arranged in 23 pairs. Each chromosome in a pair contains the same types of genes.
Cell Division
Cells divide for three main reasons:
Growth of an organism
Replacement of worn-out cells
Repair of damaged tissue
During these processes, mitosis occurs, producing two new cells, called daughter cells. These daughter cells must have identical genetic material to the original cell, ensuring the chromosome number remains unchanged.
Cancer
Cells typically divide through mitosis only when necessary, such as for growth or replacing damaged cells. However, when a cell becomes cancerous, it divides uncontrollably, forming new cells even when the body does not need them. This uncontrolled growth leads to the formation of a tumor.
Causes of Cancer
Substances that cause cancer are known as carcinogens. These agents damage DNA, leading to mutations. A single mutation is not enough to cause cancer; multiple mutations are usually required. This is why the risk of developing cancer increases with age.
Genetic factors can also make some people more likely to develop certain types of cancer. Any factor that increases the chance of developing a disease is called a risk factor, and different cancers have different risk factors.
Meiosis
Sexual reproduction requires two parents. During this process, the nucleus of a male gamete fuses with the nucleus of a female gamete, forming a new cell called a zygote.
In humans, gametes contain 23 chromosomes, which is half the number found in body cells. When male and female gametes combine, they form a zygote, which develops into an embryo with a complete set of 46 chromosomes—half inherited from each parent. This process is known as fertilization.
Gametes are produced through a type of cell division called meiosis.
In animals, gametes are sperm and egg cells.
In flowering plants, gametes are pollen and egg cells.
The Process of Meiosis
The genetic material is copied.
The cell divides twice, creating four gametes, each with a single set of chromosomes (haploid cells).
Each gamete is genetically unique, leading to variation.
Stem Cells
Stem Cells in Humans
An embryo develops from a fertilized egg. In the early stages of development, the embryo contains stem cells, which are undifferentiated cells that have not yet specialized. If removed from the embryo, these cells can develop into any cell type, making them embryonic stem cells.
Some stem cells remain in the body throughout life and are known as adult stem cells. These are found in limited numbers in various parts of the body, including:
Brain
Eyes
Blood
Heart
Liver
Bone marrow
Skin
Muscle
Unlike embryonic stem cells, adult stem cells can only differentiate into related cell types. For example, bone marrow stem cells can develop into blood cells and immune system cells but not other types of cells.
Using Human Stem Cells
Stem cells can divide and produce new cells that develop into different cell types. Because of this, they have potential medical uses, including treating diseases and replacing damaged or lost cells. They could help in conditions such as:
Type 1 diabetes
Multiple sclerosis (MS), which can cause paralysis
Spinal cord or brain injuries leading to paralysis
Stem cells used for treatment can come from two main sources:
Embryonic stem cells – Can turn into a wider variety of cell types but are difficult to obtain. The best source is a five-day-old embryo.
Adult stem cells – Have a more limited range of differentiation. A common example is bone marrow transplants, where bone marrow stem cells develop into different types of blood cells.
Bone marrow transplants are used in cases of:
Blood cancers, such as leukemia and lymphoma
Blood cell destruction, often due to cancer treatments
DNA, Genes, and Chromosomes
Key Concepts in Genetics
Gamete – A reproductive cell. In humans, these are sperm in males and eggs in females.
DNA – A complex molecule made of two intertwined strands forming a double helix. It carries genetic instructions that determine an organism’s traits. Except for identical twins, every individual has unique DNA.
Chromosomes – Thread-like structures found in the nucleus of a cell, composed of long strands of DNA. They occur in pairs, with one chromosome inherited from each parent.
Gene – A segment of DNA located on a chromosome that provides the code for making a specific protein. Genes are the fundamental units of heredity and can be passed down to offspring.
Alleles – Different forms of the same gene. For example, the gene for eye color may have variations for blue or brown eyes.
Genotype and Phenotype
Genotype – The genetic makeup of an individual, consisting of the alleles inherited for a particular trait.
Phenotype – The physical expression of a trait, such as hair color or eye color, resulting from an individual’s genotype.
Dominant and Recessive Alleles
Dominant allele – A version of a gene that is always expressed if present. It is represented by a capital letter (e.g., A). For example, the allele for brown eyes is dominant, meaning a person needs only one copy of it to have brown eyes.
Recessive allele – A gene variant that is only expressed when two copies are present, with no dominant allele. Represented by a lowercase letter (e.g., a). The allele for blue eyes is recessive, so both alleles must be blue for a person to have blue eyes.
Homozygous and Heterozygous
Homozygous – Having two identical alleles for a particular characteristic (e.g., AA or aa).
Heterozygous – Having two different alleles for a trait (e.g., Aa).
Structure of DNA
Deoxyribonucleic Acid (DNA)
DNA molecules are complex and large, carrying the genetic instructions that determine an organism’s traits.
Genes
A gene is a segment of DNA that provides instructions for making a specific protein. It is the fundamental unit of heredity and can be copied and passed down to future generations.
Chromosomes
Inside the cell nucleus, chromosomes are long, thread-like structures made of DNA. Each chromosome contains many genes.
In 1953, James Watson and Francis Crick identified the structure of DNA using data from other scientists. X-ray images revealed that DNA is composed of two strands twisted into a double helix.
Base Pairs
Each DNA strand has a backbone made of alternating sugar and phosphate molecules. Between these strands are bases, which pair in a specific way:
Thymine (T) pairs with Adenine (A)
Guanine (G) pairs with Cytosine (C)
The sequence of bases in a gene determines the order of amino acids in a protein. While many traits are influenced by multiple genes, some are controlled by a single gene, such as fur color in animals or red-green color blindness in humans. Different versions of a gene are called alleles.
Sex Determination
Inheritance of Biological Sex
Human cells contain 23 pairs of chromosomes. Of these, 22 pairs (autosomes) control general traits, while the 23rd pair determines biological sex:
Males have one X and one Y chromosome (XY).
Females have two X chromosomes (XX).
Genetic Diagram and Probability
A Punnett square illustrates how sex chromosomes combine in offspring:
A mother always passes an X chromosome (XX).
A father can pass either an X or a Y chromosome (XY).
The two possible outcomes are:
XX (Female) – An X chromosome from both parents results in a girl.
XY (Male) – An X chromosome from the mother and a Y chromosome from the father result in a boy.
Since each combination is equally likely, the probability of having a male or female child is 50%.
Variation
Variation refers to the differences between individuals of the same species. These differences can be caused by inherited traits, environmental factors, or a mix of both.
Causes of Variation
While individuals in a species share similarities, they are never exactly the same. Variation can be:
Genetic: Passed down from parents through DNA.
Environmental: Influenced by surroundings and experiences.
A combination of both:Traits affected by both genetics and environment.
Genetic Variation
Children inherit traits from both parents but are not identical to either one. This happens because they receive half of their genetic material from each parent.
Sperm and egg cells each carry half the normal number of chromosomes (haploid). When they combine during fertilization, they form a zygote, which has a full set of chromosomes (diploid) and contains all the genetic information needed to develop.
Examples of genetic variation in humans include:
Blood type
Skin color
Eye color
Gender (determined by inherited genes)
Environmental Variation
Traits can also be influenced by external factors such as climate, diet, lifestyle, or culture. For example:
Gaining or losing weight depends on food intake.
A plant growing in the shade may become taller to reach more sunlight.
Other examples of environmental variation include:
Scars or accents influenced by personal experiences.
The color of hydrangea flowers, which depends on soil type (blue in acidic soil, pink in alkaline soil).
Homeostasis
Homeostasis is the process of keeping the body's internal environment stable and at the optimal level. This is controlled by the nervous system and hormones.
Conditions Inside the Body
The body must maintain stable internal conditions because metabolism works best within a specific temperature and pH range.
Hormones
Hormones are chemical messengers made in endocrine glands and carried through the blood to specific organs. They work differently from the nervous system, which sends electrical signals.
One important hormone is insulin, which helps regulate blood sugar levels.
Regulating Blood Glucose
Cells need glucose for respiration to produce energy. The amount of glucose in the blood must stay balanced. The pancreas produces insulin, which controls blood sugar levels and ensures they remain stable.
Controlling Temperature
The skin plays a key role in regulating body temperature. It helps maintain a temperature of 37°C, which is the optimal temperature for the body's enzymes to function properly.
Role of Hair in Temperature Control
When too warm:
Hair lies flat as the hair erector muscle relaxes.
A thin layer of air is trapped above the skin.
More heat is lost to the environment.
When too cold:
Hair stands up as the hair erector muscle contracts.
A thicker layer of air is trapped above the skin.
The trapped air acts as insulation, reducing heat loss.
Vasoconstriction and Vasodilation
The body maintains stable internal conditions to function properly. Blood sugar levels and body temperature are carefully regulated to stay within a safe range. Factors like lifestyle choices, including drug and alcohol use, can disrupt this balance, affecting homeostasis.
Vasoconstriction happens when the body is too cold. Blood vessels near the skin narrow, reducing heat loss through the skin's surface.
Vasodilation occurs when the body is too hot. Blood vessels near the skin widen, allowing more heat to escape.
Shivering happens when you are cold. It is caused by involuntary muscle contractions, which require energy from respiration. This process generates heat to warm the body.
Sweating helps cool the body when it is too hot. Sweat is produced by sweat glands, travels through sweat ducts, and reaches the skin's surface. As it evaporates, it takes excess heat away, cooling the body.
Diabetes, Alcohol, and Obesity
Sometimes, the body's ability to maintain balance is disrupted by lifestyle choices or diseases, requiring medical help.
Diabetes
Diabetes is a condition where blood sugar levels stay too high. It can be managed with insulin, which helps lower blood glucose by converting it into glycogen in the liver. There are two types of diabetes:
Type 1 Diabetes
Caused by damage to pancreatic beta cells, which produce insulin.
Can be inherited or triggered by certain viruses.
People with type 1 diabetes produce little or no insulin.
Managed by:
Eating a low-sugar/carbohydrate diet
Injecting insulin
Possible pancreas tissue transplant
Type 2 Diabetes
Caused by the body becoming resistant to insulin.
Linked to obesity and poor lifestyle choices.
Can be controlled with diet and exercise.
Lifestyle and Disease Risk
Eating too much for a long time increases the risk of type 2 diabetes. Other harmful lifestyle choices include:
Excessive alcohol consumption
Drug misuse
Alcohol
Slows reaction time, which can be dangerous (e.g., when driving).
Can be addictive, leading to withdrawal symptoms.
Long-term use damages organs like the liver and circulatory system.
Micro-organisms
Micro-organisms are found everywhere. Most are harmless, and some are even beneficial, such as those in our gut that aid digestion or those that help decompose dead organisms and recycle nutrients.
However, some microorganisms cause disease. These harmful microorganisms are called pathogens.
Types of Pathogens
There are four main types of pathogens:
Viruses – Example: HIV, which can lead to AIDS.
Bacteria – Example: Salmonella, which causes food poisoning.
Fungi – Example: Athlete’s foot, a common skin infection.
Protists – Example: Malaria, spread by mosquitoes.
Pathogens follow a simple life cycle: they infect a host, reproduce (or replicate, in the case of viruses), spread to new hosts, and continue the cycle. They have special adaptations that help them survive and spread, making them effective at causing disease.
Diseases caused by pathogens are called communicable diseases, meaning they can be passed from one person to another.
Bacterial Cells
Bacterial cells have distinct features:
No nucleus – their DNA is in a loose loop in the cytoplasm.
A cell wall made of murein, different from the cellulose found in plant cell walls.
A cell membrane and cytoplasm.
Some contain plasmids (small loops of DNA).
No chloroplasts or mitochondria.
Exist as single cells.
Some have a slime capsule for protection.
Divide by binary fission, splitting in two and growing to full size before repeating the process.
May have pili to attach to surfaces or flagella to help them move.
Viruses
Viruses are not considered living organisms because they do not carry out all seven life processes: movement, respiration, sensitivity, nutrition, excretion, reproduction, and growth.
Instead of species, viruses are classified into strains. They consist of a short strand of genetic material (DNA or RNA) enclosed in a protein coat.
Their simple structure allows them to invade host cells, take over their functions, and replicate, causing infections.
Micro-organisms and Their Applications
Handling microorganisms safely requires careful techniques to prevent contamination and ensure accurate results. Studying them helps scientists understand their rapid growth and explore their potential uses, such as producing medicines, breaking down waste, or making food products like yogurt and cheese.
Bacterial Growth
Bacteria reproduce quickly, doubling in number about every 20 minutes through a process called binary fission. However, this rate depends on factors like nutrient availability, temperature, and moisture.
Methods of Growing Bacteria
Bacteria can be cultured using different methods, including:
Nutrient broth solution – A liquid medium containing essential nutrients like carbohydrates for energy and nitrogen for protein production.
Agar plates – A solid medium made by pouring molten agar into sterile Petri dishes, which allows bacteria to form visible colonies.
Aseptic Techniques for Safe Handling
To maintain pure cultures, proper sterilization and handling techniques must be used:
Sterilizing equipment: An inoculating loop, used to transfer bacteria, is heated until red-hot before and after use.
Minimizing exposure: The lid of the Petri dish is lifted only slightly to prevent airborne contamination.
Securing the dish: The lid is taped shut, labeled, and dated to maintain safety and track experiments.
Controlled incubation: Bacteria are incubated at 25°C for 24–48 hours in school labs to prevent the growth of harmful pathogens, which thrive at body temperature (37°C).
Sterilizing after use: All plates and equipment must be properly disposed of or sterilized to eliminate bacteria.
Counting Bacteria
Bacteria are too small to see individually, but when grown on agar, they form visible colonies. Scientists can estimate the number of bacteria in a sample by counting these colonies.
To ensure accurate results, a serial dilution is performed, reducing bacterial concentration step by step until individual colonies can be counted. This method helps determine the number of bacteria in the original sample.
Spreading Disease
Diseases spread from one person to another through different methods of transmission:
Direct contact: Can be sexual (e.g., during intercourse) or non-sexual (e.g., shaking hands).
Water: Contaminated water can carry diseases like cholera.
Air: Infected people can release tiny droplets containing viruses when they sneeze, spreading diseases like the common cold.
Unhygienic food preparation: Undercooked or reheated food can cause bacterial infections like E. coli food poisoning.
Vectors: Organisms such as mosquitoes or badgers can spread diseases like malaria or tuberculosis.
Antibiotics and Antibiotic Resistance
Antibiotics
Antibiotics are substances that slow down or stop the growth of bacteria. They are used to cure bacterial infections but are ineffective against viral diseases. Common antibiotics include penicillin and amoxicillin.
Penicillin: was discovered by Sir Alexander Fleming in 1928 when he noticed that a naturally occurring Penicillium mold killed bacteria in a petri dish.
How antibiotics work?
Antibiotics target bacterial cells without harming host cells. They have greatly reduced death rates from bacterial infections since their introduction. Different antibiotics work on different bacteria, so a variety is needed to treat different infections.
Antibiotic Resistance
Since the discovery of penicillin, antibiotic use has increased dramatically. However, overuse has led to the rise of antibiotic-resistant bacteria.
Causes of Antibiotic Resistance:
Overuse of antibiotics
Not completing the prescribed course
Use of antibiotics in farming
These factors contribute to the development of "superbugs" that no longer respond to antibiotics.
The Discovery of New Drugs
Natural sources of drugs were originally derived from natural sources, such as plants and microorganisms.
Aspirin: Derived from willow bark, initially used by the ancient Greeks to reduce fever and pain. The active ingredient, salicylic acid, was later modified to be less irritating to the stomach.
Digitalis: extracted from foxglove plants and used to treat heart conditions.
Today, most plant-based drugs are synthesized in laboratories by pharmaceutical companies, which use them as the basis for developing new medications.
Drug Testing Process
Before a new drug is prescribed, it must undergo rigorous testing to ensure safety and effectiveness. This process involves preclinical and clinical trials.
Preclinical Testing
Computer models and lab-grown human cells – Many substances fail at this stage due to toxicity or ineffectiveness.
Animal testing – Required in the UK for new medicines (but banned for cosmetics and tobacco). Scientists monitor animals for side effects.
Clinical Trials
Once animal testing is complete, human trials begin in phases:
Testing on healthy volunteers – Ensures the drug is safe.
Testing on patients with the illness – Determines effectiveness.
Preventing Bias in Clinical Trials
To ensure accurate results, researchers use control strategies:
Placebo/Control Group: One group receives the drug, another gets a placebo (inactive substance).
Blind Trials: Patients do not know if they are receiving the drug or placebo.
Double Blind Trials: Neither doctors nor patients know who receives the drug, eliminating bias.