Hoja de repaso: Fundamentals of Living Organisms and Processes

📋 Course Outline

1. Characteristics and variety of living organisms
2. Levels of organisation and organelles
3. Cell structure and biological molecules
4. Movement of substances into and out of cells
5. Nutrition, respiration and gas exchange
6. Transport, excretion and coordination responses
7. Reproduction in flowering plants and humans
8. Inheritance and genetic variation
9. Organisms in the environment and feeding
10. Cycles within ecosystems and human impacts
11. Food production using microbes and fermenters
12. Selective breeding, genetic modification and cloning

📖 1. Characteristics and variety of living organisms

🔑 Key Concepts & Definitions

• Living organism : A living organism is a biological system that carries out life processes such as growth, reproduction, and response to stimuli.
• Characteristics of living organisms : Characteristics of living organisms are observable features that distinguish living things from non-living matter.
• Variety of living organisms : Variety of living organisms is the wide range of different organisms found in nature, including differences in structure and function.
• Classification : Classification is the grouping of organisms based on shared features to show relationships and variety.

📝 Essential Points

• Living organisms show life processes such as growth and reproduction.
• Living organisms respond to changes in their environment.
• Living organisms maintain internal conditions through regulation.
• Living organisms are made of cells, which are the basic units of life.
• Variety includes differences in body structure and how organisms carry out essential functions.
• Classification groups organisms by shared characteristics to organise the variety of life.

💡 Memory Hook

Life = G-R-R-A-C: Growth, Reproduction, Respiration/response, Regulation, Cells.

📖 2. Levels of organisation and organelles

🔑 Key Concepts & Definitions

• Cell : A cell is the basic structural and functional unit of living organisms.
• Organelle : An organelle is a specialised structure inside a cell that carries out a specific function.
• Tissue : A tissue is a group of similar cells working together to perform a particular job.
• Organ : An organ is a structure made of different tissues that work together for a shared function.
• Organ system : An organ system is a group of organs that coordinate to carry out a major body function.

📝 Essential Points

• Cells are organised into higher levels: cells form tissues, tissues form organs, and organs form organ systems.
• Organelle functions are separated so processes can happen efficiently within the same cell.
• Different organelles have different structures that match their roles in the cell.
• Organ systems work together to maintain overall body functions such as transport, digestion, and respiration.
• The level of organisation increases in complexity from cell to organ system.

💡 Memory Hook

Cell → Tissue → Organ → Organ system (CTOO): cells build tissues, tissues build organs, organs build systems.

📖 3. Cell structure and biological molecules

🔑 Key Concepts & Definitions

• Cell wall : A structural layer that provides support and protection for cells, preventing them from bursting easily.
• Cytoplasm : The fluid inside a cell where many chemical reactions occur, including some respiration steps.
• Mitochondria : Organelles that carry out respiration to release energy, especially in aerobic conditions.
• Lactic acid : An acidic product formed when lactose is fermented, lowering pH and changing milk texture.
• Ethanol : A fermentation product made during anaerobic respiration by yeast.

📝 Essential Points

• Only plant cells have cell walls; animal cells do not have cell walls.
• Anaerobic respiration by yeast occurs in the cytoplasm only.
• Aerobic respiration occurs in both cytoplasm and mitochondria.
• Lactobacillus ferment lactose into lactic acid, which sours and thickens milk to form yoghurt.
• Lowering pH during yoghurt making inhibits harmful microorganisms, acting as a preservative.
• During bread making, yeast produces ethanol and carbon dioxide under anaerobic conditions.

💡 Memory Hook

Plant cells = cell wall; yeast anaerobic = cytoplasm only; yoghurt = lactic acid lowers pH; bread = ethanol + CO2.

📖 4. Movement of substances into and out of cells

🔑 Key Concepts & Definitions

• Partially permeable cell membrane : A cell membrane is partially permeable, meaning some substances can pass through while others are blocked.
• Diffusion : Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
• Osmosis : Osmosis is the net movement of water from a more dilute solution to a more concentrated solution through a partially permeable membrane.
• Active transport : Active transport is the movement of substances across a cell membrane using energy to move against a concentration gradient.

📝 Essential Points

• Substances move into and out of cells because the cell membrane controls what can pass through.
• Diffusion moves substances down a concentration gradient, so they spread from where they are more concentrated to where they are less concentrated.
• Osmosis is specifically the movement of water, not other solutes, across a partially permeable membrane.
• Active transport is required when a substance must move against its concentration gradient.
• If oxygen is not available for respiration, cells cannot release enough energy for processes such as growth and contraction.

💡 Memory Hook

Membrane = gate: diffusion/osmosis go with gradients; active transport fights the gradient for entry.

📖 5. Nutrition, respiration and gas exchange

🔑 Key Concepts & Definitions

• Aerobic respiration : Aerobic respiration is the process where cells release energy from food molecules using oxygen.
• Oxygen concentration gradient : An oxygen concentration gradient is the difference in oxygen concentration across a cell membrane that drives oxygen diffusion into cells.
• Carbon dioxide concentration gradient : A carbon dioxide concentration gradient is the difference in carbon dioxide concentration across a cell membrane that drives carbon dioxide diffusion out of cells.
• Cell wall : A cell wall is a rigid outer layer that supports the cell and helps control what can pass through.
• Cellulose fibres : Cellulose fibres are strong fibres in the plant cell wall that create gaps allowing water and dissolved substances to pass.

📝 Essential Points

• Aerobic respiration uses oxygen and breaks down food molecules to release energy, producing carbon dioxide and water.
• A low oxygen concentration inside cells is maintained because oxygen is continually used in aerobic respiration.
• Because oxygen is continually used, oxygen keeps diffusing into the cell down its concentration gradient.
• A high carbon dioxide concentration inside cells is maintained because aerobic respiration continually produces carbon dioxide.
• Because carbon dioxide is continually produced, carbon dioxide keeps diffusing out of the cell down its concentration gradient.
• Plant cell walls are freely permeable because water and dissolved substances can pass through gaps between cellulose fibres.

💡 Memory Hook

Aerobic respiration: O2 goes in, CO2 goes out (use O2 → low O2 inside; make CO2 → high CO2 inside).

📖 6. Transport, excretion and coordination responses

🔑 Key Concepts & Definitions

• Osmosis : Osmosis is the movement of water through a partially permeable membrane from higher water potential to lower water potential.
• Hypotonic solution : A hypotonic solution has a higher water potential than the cell, so water tends to move into the cell.
• Hypertonic solution : A hypertonic solution has a lower water potential than the cell, so water tends to move out of the cell.
• Water potential gradient : A water potential gradient is the difference in water potential that drives net water movement by osmosis.

📝 Essential Points

• In a hypotonic (higher water potential) sucrose solution, potato cylinders increase in length and mass due to water entering by osmosis.
• In a hypertonic (lower water potential) sucrose solution, potato cylinders decrease in length and mass due to water leaving by osmosis.
• The direction of water movement depends on the relative water potential of the solution compared with the potato cells.
• To plan the experiment, state the sucrose concentration used for the solution and relate it to the concentration in potato cells.
• To improve reliability, repeat the experiment and calculate a mean for the results.
• To reduce measurement error, weigh potatoes directly and avoid mass changes from towel drying or blotting.

💡 Memory Hook

Hypo = water goes in (mass/length up); Hyper = water goes out (mass/length down).

📖 7. Reproduction in flowering plants and humans

🔑 Key Concepts & Definitions

• Flowering plant reproduction : Plant reproduction in which flowers produce gametes that can develop into seeds and fruits.
• Human reproduction : Human reproduction in which gametes from male and female combine to form a zygote that develops into a baby.
• Gamete : A gamete is a reproductive cell that carries genetic information and can fuse with another gamete.
• Fertilisation : Fertilisation is the fusion of male and female gametes to form a zygote.

📝 Essential Points

• Flowering plants reproduce using flowers, which are adapted for producing gametes and enabling fertilisation.
• Fertilisation in flowering plants leads to seed formation, and the seed can develop into a new plant.
• In humans, fertilisation occurs when a sperm and an egg fuse to form a zygote.
• A zygote develops into an embryo and then a baby through growth and development in the female reproductive system.
• Gametes are produced by reproductive organs and are specialised for carrying genetic material to the next generation.
• Fertilisation is the key step that links gamete fusion to the start of development in both plants and humans.

💡 Memory Hook

Fertilisation is the “join” step: gametes fuse → zygote → new organism (seeds in plants, baby in humans).

📖 8. Inheritance and genetic variation

🔑 Key Concepts & Definitions

• Gene : A gene is a section of DNA that codes for a specific characteristic.
• Allele : An allele is an alternative version of a gene that can produce different traits.
• Chromosome : A chromosome is a DNA structure that carries many genes and is found in the nucleus.
• Genetic variation : Genetic variation is the differences in DNA between individuals of the same species.

📝 Essential Points

• Genes are inherited from parents and determine inherited characteristics.
• Different alleles of the same gene can lead to different phenotypes.
• Chromosomes contain strands of DNA that act as instructions for cell structure and function.
• Genetic variation increases the chance that some individuals are better suited to their environment.
• Variation can arise from differences in the alleles individuals inherit from their parents.

💡 Memory Hook

Alleles are the “versions” of genes; genes are the “instructions” on chromosomes.

📖 9. Organisms in the environment and feeding

🔑 Key Concepts & Definitions

• Osmosis : Osmosis is the movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane.
• Diffusion : Diffusion is the random movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient.
• Active transport : Active transport is the movement of particles from low to high concentration against the concentration gradient using energy (ATP).
• Concentration gradient : A concentration gradient is the difference in concentration between two areas that drives diffusion and affects the rate of movement.
• Hypotonic solution : A hypotonic solution has a higher water potential than the cell, so water moves into the cell by osmosis.

📝 Essential Points

• Water movement by osmosis goes from higher water potential to lower water potential across a permeable membrane.
• In a hypotonic solution animal cells swell and may burst because water enters by osmosis.
• In an isotonic solution there is equal net movement of water in and out of the cell, so there is no net size change.
• In a hypertonic solution animal cells shrink/crenate because water leaves by osmosis, while plant cells can undergo plasmolysis.
• Rate of movement increases with a steeper concentration gradient, higher temperature, larger surface area, and a shorter/thinner membrane distance.

💡 Memory Hook

Osmosis = water potential down; Diffusion = particles down concentration; Active transport = particles up concentration (ATP).

📖 10. Cycles within ecosystems and human impacts

🔑 Key Concepts & Definitions

• Cardiac cycle : The cardiac cycle is the repeating sequence of heart muscle contractions and relaxations that moves blood through the heart.
• Double circulation : Double circulation is the two-part blood pathway where blood passes through the heart twice in one full circuit.
• Ventilation : Ventilation is the movement of air into and out of the lungs that keeps gases available for gas exchange.
• Alveoli : Alveoli are tiny air sacs in the lungs where oxygen diffuses into blood and carbon dioxide diffuses out.
• Lactobacillus : Lactobacillus is a bacterium used in yoghurt production that helps convert milk components during incubation.

📝 Essential Points

• The heart’s blood flow can be shown as a step-by-step flow diagram to label the correct sequence of chambers and vessels.
• Humans use double circulation, meaning oxygenated and deoxygenated blood are routed through separate pathways.
• Inhalation is part of ventilation and involves air being drawn into the lungs so oxygen can reach the alveoli.
• Smoking introduces harmful substances such as nicotine, tar, and carbon monoxide that can affect breathing and gas exchange.
• Yoghurt production uses Lactobacillus, and equipment sterilisation prevents unwanted microbes from contaminating the milk.
• Milk is pasteurised at 85–95°C to kill unwanted bacteria, then cooled to 40–45°C before adding Lactobacillus and incubating.

💡 Memory Hook

Cardiac cycle = pump rhythm; double circulation = two loops; ventilation = air in/out; alveoli = gas swap; yoghurt = Lactobacillus after pasteurise+cool.

📖 11. Food production using microbes and fermenters

🔑 Key Concepts & Definitions

• Microorganisms in food production : Microorganisms are used by humans to make foods and other useful substances by carrying out chemical reactions.
• Bacteria in yoghurt : Bacteria can be used to convert milk into yoghurt by digesting milk components and producing acidic products.
• Lactobacillus : Lactobacillus is the specific bacterium used to ferment milk into yoghurt by digesting lactose and forming lactic acid.
• Yeast fermentation : Yeast fermentation is anaerobic respiration in which yeast converts sugars into ethanol and carbon dioxide.
• Fermenter : A fermenter is a controlled vessel that supports microbial growth and fermentation to produce food products.

📝 Essential Points

• Bacteria reproduce rapidly, so the amount of chemicals they produce can increase quickly.
• Yoghurt production uses sterilisation to remove unwanted bacteria and reduce chemical contamination.
• Milk is pasteurised at 85–95°C to kill unwanted bacteria that could compete with Lactobacillus for lactose.
• After pasteurisation, milk is cooled to 40–45°C and Lactobacillus is added for incubation over several hours.
• Lactobacillus digests milk proteins and ferments lactose, converting lactose to lactic acid.
• Lower pH from lactic acid helps prevent harmful microorganisms, acting as a preservative and extending shelf life versus fresh milk.

💡 Memory Hook

Yoghurt: sterilise → 85–95°C → cool to 40–45°C → Lactobacillus makes lactic acid → cool to 5°C to stop.

📖 12. Selective breeding, genetic modification and cloning

📅 Key Dates

Date	Event
04 September 2025 14:12	Lab Safety (Course Guide) appears in the source content
26 August 2020 17:55	Edexcel IGCSE Biology Syllabus map appears in the source content
12 September 2018 15:36	Complete syllabus appears in the source content
26 August 2020 18:10	Shell Syllabus appears in the source content
01 December 2025 12:44	Shell Block test appears in the source content
03 December 2025 14:47	Autumn trial reflection appears in the source content
11 May 2026 10:30	Summer Trial revision posters appears in the source content
04 May 2022 11:03	2c) Biological molecules appears in the source content

📊 Synthesis Tables

Cell organisation levels

Level	Definition	Example
Cell	A cell is the basic structural and functional unit of living organisms.	Red blood cell, nerve cell
Tissue	A tissue is a group of similar cells working together to perform a particular job.	Muscle tissue, epithelial tissue
Organ	An organ is a structure made of different tissues that work together for a shared function.	Heart, stomach
Organ system	An organ system is a group of organs that coordinate to carry out a major body function.	Digestive system, circulatory system

⚠️ Common Pitfalls & Confusions

1. Mixing up diffusion and osmosis: diffusion is particles down a concentration gradient, while osmosis is water only through a partially permeable membrane.
2. Saying animal cells have cell walls: only plant cells have cell walls; animal cells do not.
3. Confusing respiration locations: aerobic respiration occurs in both cytoplasm and mitochondria, but anaerobic respiration by yeast occurs in the cytoplasm only.
4. Thinking oxygen concentration stays high inside cells: aerobic respiration continually uses oxygen, so oxygen stays low inside and keeps diffusing in.
5. Reversing hypotonic vs hypertonic: hypotonic solutions cause water to move into cells (mass/length increase), while hypertonic solutions cause water to move out (mass/length decrease).
6. Forgetting the key link step in both plants and humans: fertilisation is the fusion of gametes to form a zygote, which then develops.
7. Confusing fermentation products: yeast anaerobic respiration produces ethanol and carbon dioxide, while Lactobacillus fermentation produces lactic acid that lowers pH and thickens milk.

✅ Exam Checklist

1. Define a living organism and list the key characteristics (growth, reproduction, response to stimuli, regulation, cells) in the correct terminology.
2. Explain classification as grouping organisms by shared features to show relationships and variety.
3. State the organisation sequence from cells to tissues to organs to organ systems and match each level to its definition.
4. Identify what an organelle is and explain why organelles have specialised structures and functions within a cell.
5. Distinguish plant vs animal cells for cell wall presence, and describe the role of cellulose fibres and gaps in the plant cell wall.
6. Recall the correct locations for respiration: aerobic in cytoplasm and mitochondria, anaerobic by yeast in cytoplasm only.
7. Use the correct definitions for diffusion, osmosis, and active transport, including the direction relative to concentration gradients and the need for energy (ATP).
8. Describe how oxygen and carbon dioxide gradients are maintained during aerobic respiration and gas exchange (O2 in, CO2 out).
9. Apply hypotonic, isotonic, and hypertonic solutions to predict water movement and cell/tissue changes (animal swelling/bursting; crenation; plant plasmolysis).
10. For potato osmosis experiments, state the sucrose concentration used, relate it to potato cell concentration, and interpret length/mass changes accordingly.
11. Explain flowering plant reproduction: flowers produce gametes, fertilisation leads to seed formation, and seeds develop into new plants.
12. Explain human reproduction: sperm and egg fuse to form a zygote, which develops into an embryo and then a baby.
13. Define gene, allele, chromosome, and genetic variation, and explain how alleles lead to different phenotypes.
14. Describe feeding relationships and cycles within ecosystems at the level of the course content, including how human impacts affect ecosystems (as covered in the course guide).