Understanding Cells, Organisms & Disease
Understanding Cells, Organisms & Disease
Items in this lesson
Understanding Cells, Organisms & Disease
Learning Objective
At the end of the lesson you will understand how cells function, how living organisms are organised, how infections affect us, and how bioenergetics powers life.
What Are Cells?
Cells are the basic building blocks of all living organisms. In GCSE Biology, you learn that all living things are made of one or more cells, which is explained by the Cell Theory. Cells can vary in shape and size depending on their function. For example, some cells are specialised to carry oxygen, while others are adapted to transmit signals. Most cells contain key structures such as the cell membrane, cytoplasm, and genetic material. In multicellular organisms, specialised cells work together in tissues and organs to carry out important life processes.
Types of Cells
There are two main types of cells: prokaryotic and eukaryotic.
Prokaryotic Cells
Prokaryotic cells, such as Bacteria, are very small and simple in structure. They do not have a nucleus, so their genetic material (DNA) is found loose in the cytoplasm. They also lack membrane-bound organelles. Most have a cell membrane, cytoplasm, a cell wall, and sometimes plasmids (small rings of DNA). Some prokaryotic cells also have a flagellum to help them move.
Eukaryotic Cells
Eukaryotic cells are larger and more complex, and are found in plants, animals, fungi, and protists. They contain a nucleus, which stores the DNA and controls the cell’s activities. Eukaryotic cells also have membrane-bound organelles such as mitochondria and ribosomes. Plant cells may also contain chloroplasts and a large vacuole, which help with photosynthesis and maintaining structure.
Specialised Cells
Animal Cells
Examples: nerve, muscle, and red blood cells. Each performs unique roles, like carrying oxygen or contracting.
Animal cells can specialise for different roles in the body. For example, nerve cells are long and adapted to transmit electrical impulses quickly around the body. Muscle cells are able to contract and relax, allowing movement. Red blood cells are adapted to carry oxygen; they have a biconcave shape to increase surface area and contain haemoglobin to bind oxygen.
Examples: palisade and root hair cells. These cells absorb light or water for the plant
Plant cells also become specialised for particular functions. Palisade cells are found in leaves and are packed with chloroplasts to absorb light for photosynthesis. Root hair cells have long extensions that increase surface area, allowing them to absorb more water and minerals from the soil efficiently.
Plant Cells
Inside an Animal Cell
Key parts: nucleus, cytoplasm, cell membrane, mitochondria, and ribosomes. Each part has an important job.
Inside a Plant Cell
Key parts: cell wall, chloroplasts, vacuole and more. Chloroplasts help plants make food by photosynthesis.
How Do Cells Multiply?
Cells divide by mitosis to allow organisms to grow, replace damaged cells, and repair tissues. During mitosis:
One parent cell divides to form two identical daughter cells.
Each new cell has the same genetic material (DNA) as the original.
This ensures that the new cells can perform the same functions as the parent cell.
Mitosis is essential for growth in multicellular organisms and for healing after injury.
Levels of Organisation
Examples
The Heart is an organ made mainly of muscle tissue, which is made up of specialised muscle cells. The heart works as part of the circulatory system to pump blood around the body.
In living organisms, cells are organised into different levels of structure:
Cells group together to form tissues
Tissues work together to form organs
Organs work together in organ systems
Organ systems make up a complete organism
This organisation allows complex life processes to take place efficiently.
From Cells to Organisms
Key Organ Systems
Examples include respiratory, digestive, circulatory, and nervous systems. Each has specific functions.
Defining Pathogens
A pathogen is a microorganism that causes disease in living organisms. Pathogens can spread easily and make us ill by damaging cells or producing toxins.
How Infection Spreads
Pathogens can spread in several ways, allowing diseases to pass from one organism to another:
Air – Pathogens can be carried in droplets when people cough or sneeze
Water – Contaminated water can contain harmful microorganisms
Direct Contact – Touching infected people or surfaces can spread pathogens
Contaminated Food – Eating food that contains pathogens can cause illness
These methods of transmission allow pathogens to spread quickly and cause disease in populations.
Preventing the Spread of Pathogens
There are several ways to reduce the spread of pathogens and prevent disease:
Good Hygiene – Regular handwashing and cleaning surfaces reduce the spread of microorganisms
Vaccination – Vaccines protect people by helping the immune system recognise and fight pathogens
Safe Food Preparation – Cooking food properly and storing it safely kills harmful pathogens
Clean Water – Treating and filtering water removes harmful microorganisms
Isolation – Staying away from others when ill helps prevent diseases from spreading
These methods help protect individuals and reduce the spread of disease in the population.
How Our Bodies Defend Us
The immune system protects the body from pathogens and helps prevent disease.
How it works:
White Blood Cells – These cells help defend the body by engulfing pathogens and destroying infected cells
Antibodies – Proteins produced by white blood cells that are specific to each pathogen; they bind to and help destroy them
Antitoxins – Chemicals that neutralise toxins released by some pathogens
Together, these defences recognise, attack, and remove harmful microorganisms from the body, helping to keep us healthy.
Stops Infections: Vaccinations
Vaccination
Vaccines protect us against specific diseases by stimulating the immune system.
Vaccines contain a harmless form of a pathogen or parts of it
This triggers the immune system to produce antibodies
The body also creates memory cells that remember the pathogen
If the real pathogen enters the body later, the immune system responds quickly and effectively
This means the person is protected and less likely to become ill.
Antibiotics vs. Viruses
Antibiotics are medicines used to kill Bacteria or stop them from growing, helping to treat bacterial infections. However, they do not work against viruses, so they cannot be used to treat illnesses such as colds or flu. Overusing antibiotics can lead to antibiotic resistance, where bacteria mutate and become resistant to the drugs. This means infections become harder to treat and can spread more easily, making it important to use antibiotics only when necessary.
Example Mrsa
MRSA is a type of bacterial infection caused by strains of Staphylococcus aureus that have become resistant to many antibiotics. This makes MRSA infections difficult to treat. It commonly occurs in hospitals, where it can spread through direct contact or contaminated surfaces. MRSA can cause skin infections, abscesses, and in more serious cases, infections in the bloodstream or lungs. Good hygiene, careful use of antibiotics, and infection control measures are important in preventing its spread.
What is Bioenergetics?
Bioenergetics is the study of how energy flows through living organisms and how cells produce and use energy. It focuses on processes like respiration, where cells break down glucose to release energy, and photosynthesis, where plants capture energy from sunlight to make food. Understanding bioenergetics helps explain how organisms grow, move, and carry out all life processes efficiently.
Photosynthesis: Making Food
Plants use sunlight, carbon dioxide, and water to produce glucose and oxygen in a process called photosynthesis. Chlorophyll in the chloroplasts captures the light energy, which is then used to convert carbon dioxide and water into glucose, a sugar that provides energy and building material for growth. Oxygen is released as a by-product, which is essential for respiration in animals and other organisms.
Respiration: Using Food
Cells release energy from glucose through a process called respiration. During respiration, glucose is broken down, usually in the presence of oxygen, to produce energy, carbon dioxide, and water. The energy released is used to power essential processes in the cell, such as muscle contraction, nerve impulses, and the functioning of organs. This energy is vital for growth, movement, and maintaining life.
Types of Respiration
Occurs without oxygen
Glucose is only partially broken down, so less energy is released
Produces lactic acid as a waste product in animals (or ethanol and carbon dioxide in plants/yeast)
Example: Sprinting or high-intensity exercise
Anaerobic Respiration
Aerobic Respiration
Requires oxygen
Glucose is fully broken down to release a large amount of energy
Produces carbon dioxide and water as waste products
Example: Running, walking, or any steady exercise
Lesson Recap
Cells are the fundamental units of life, forming the building blocks of tissues, organs, and entire organisms. Understanding how cells are organised, how pathogens cause infection, and how energy flows in living things through bioenergetics is essential to understanding how organisms grow, survive, and respond to their environment.
Quick Quiz: What Have You Learned?
