Cells, Organ Systems and Ecosystems (WJEC)
What is a Cell?
A cell is the basic unit of life. It carries out functions needed for an organism to survive and grow.
All living things are made of cells, which can work alone (unicellular organisms like bacteria) or together in groups (multicellular organisms like humans and plants).
Cells come in different types, but they all have similar structures to help them function properly.
Animal and Plant Cells
Both animal and plant cells have:
Cytoplasm – a jelly-like fluid where chemical reactions happen. It contains organelles and enzymes.
Nucleus – stores genetic material (DNA) and controls cell activities.
Cell membrane – a thin layer that controls what enters and leaves the cell.
Mitochondria – releases energy by breaking down food during respiration.
Plant cells have extra parts:
Cell wall – a rigid outer layer made of cellulose that gives the cell strength and prevents it from bursting.
Vacuole – a large sac filled with cell sap (water, sugars, and minerals) that helps keep the cell firm.
Chloroplasts – contain chlorophyll, which captures sunlight for photosynthesis to make food.
Microscopy
Magnification – how much bigger an image looks compared to the actual object.
Resolution – the smallest details that can be seen clearly.
Light microscopes use light to magnify cells so we can study them.
Electron microscopes provide much greater detail, allowing scientists to see tiny structures inside cells.
Advances in microscopes have helped scientists understand cells better, leading to discoveries about how cells function and how diseases develop.
Cell Differentiation
Cells change to become specialized for specific jobs (e.g., red blood cells carry oxygen, sperm cells help with reproduction).
Some genes turn on or off to decide the cell's function.
Once a cell is specialized, it usually cannot change back or become a different type.
In humans, most differentiation happens before birth, but some cells, like skin or blood cells, can still divide and specialize throughout life.
Organisation and Movement Across Cell Membranes
Organisation
Tissue: A group of similar cells working together for a specific function.
Muscle tissue, Xylem tissue
Organ: A group of tissues working together for a specific function.
Brain, Heart
Organ system: A group of organs working together for a specific function.
Digestive system, Nervous system
Organism: A living thing that can function independently.
Human, Rose
Movement Across Cell Membranes
Cell membranes are selectively permeable, allowing only certain substances to pass through. There are three ways substances move across cell membranes:
Diffusion
Osmosis
Active transport
Diffusion
The movement of molecules from high concentration to low concentration until evenly spread.
This process does not require energy (passive transport).
Small molecules like oxygen (O₂) and carbon dioxide (CO₂) move by diffusion.
Factors affecting diffusion:
Temperature – Higher temperature = faster diffusion due to more molecular movement.
Concentration gradient – The steeper the gradient, the faster the diffusion.
Osmosis
The movement of water molecules from a high water concentration (low solute) to a low water concentration (high solute) across a partially permeable membrane.
A form of diffusion specific to water movement.
Active Transport
Movement of molecules from low concentration to high concentration, against the concentration gradient.
Requires energy from respiration.
Examples:
Root hair cells absorbing minerals from soil.
Glucose uptake into the bloodstream in the small intestine.
Enzymes
Enzymes are biological catalysts that speed up reactions without being changed themselves. They allow reactions to occur at lower temperatures.
Enzymes are proteins made of amino acids. Their shape determines their function.
The active site is where the substrate binds. Each enzyme fits only a specific substrate (high specificity).
Factors affecting the rate of enzyme-controlled reactions
Temperature
Ph
How Enzymes Work – 'Lock and Key' Hypothesis
The substrate collides with the enzyme’s active site.
The enzyme-substrate complex forms.
The enzyme breaks down the substrate into products.
The products are released, and the enzyme is ready to work again.
Respiration and the Respiratory System
Cellular Respiration
Respiration is a process in which cells release energy from food (like glucose) using enzymes. This happens through a series of chemical reactions in the cytoplasm and mitochondria.
The energy is stored as ATP, which is needed for movement, maintaining body temperature, and active transport.
There are two types of respiration:
Aerobic respiration (with oxygen): Glucose + Oxygen → Carbon dioxide + Water + Energy
Produces more energy (ATP) because glucose is fully broken down.
Anaerobic respiration (without oxygen): Glucose → Lactic acid + Energy
Happens during intense exercise when oxygen is low. Produces less energy and causes lactic acid buildup, leading to muscle fatigue.
The body needs extra oxygen after exercise to break down lactic acid in the liver. This is called "oxygen debt."
The Respiratory System
Why do we need a respiratory system?
Large organisms need a respiratory system because:
They have a small surface area compared to their size.
Diffusion alone is not enough to supply oxygen and remove carbon dioxide.
How air moves through the system:
Nose → Trachea → Bronchi → Bronchioles → Alveoli → Blood (capillaries)
The respiratory system is lined with mucus, which traps harmful substances. Tiny hair-like structures called cilia move the mucus to the throat, where it is swallowed. This helps protect the lungs from infections.
Breathing (Ventilation)
Breathing moves fresh air into the lungs (inhalation) and stale air out (exhalation).
Inhalation:
Ribs move up and out.
Diaphragm flattens.
Chest space increases, and air moves in.
Exhalation:
Ribs move down and in.
Diaphragm returns to its dome shape.
Chest space decreases, and air moves out.
A bell jar model can demonstrate how the lungs expand and contract. However, the model has some limitations because it does not fully mimic the movement of ribs and the structure of the lungs.
Gas Exchange in the Alveoli
Alveoli are tiny air sacs in the lungs where oxygen enters the blood, and carbon dioxide is removed. They are adapted for efficient gas exchange:
Large surface area for gas exchange.
Surrounded by many capillaries for a good blood supply.
Thin walls for quick diffusion.
Moist surfaces to help gases dissolve and move faster.
Differences Between Inhaled and Exhaled Air
Inhaled air has more oxygen (21%) and less carbon dioxide (0.04%).
Exhaled air has less oxygen (16%) and more carbon dioxide (4%).
Nitrogen levels stay the same (79%).
Exhaled air contains more water vapor.
To test for carbon dioxide, bubble the air through lime water. If carbon dioxide is present, the lime water turns milky.
Digestion and the Digestive System
Digestion is the process of breaking down large food molecules into smaller ones that the body can use. This is necessary because:
Large molecules cannot pass through the gut wall.
Small molecules dissolve in the blood and are transported around the body.
Food contains three main types of biological molecules:
Fats – made of glycerol and fatty acids.
Carbohydrates – made of simple sugars like glucose.
Proteins – made of amino acids.
Once food is broken down, the small molecules are used to build new molecules or to produce energy for the body.
Enzymes in Digestion
Enzymes help speed up the breakdown of food. Different enzymes work on different types of food:
Carbohydrases – break down carbohydrates into simple sugars. Found in the mouth, pancreas, and small intestine.
Proteases – break down proteins into amino acids. Found in the stomach and small intestine.
Lipases – break down fats into glycerol and fatty acids. Found in the pancreas and small intestine.
How the Digestive System Works
The digestive system includes organs that break down food, absorb nutrients, and remove waste.
Mouth – Food is chewed (mechanical digestion). Saliva contains amylase, which starts breaking down starch.
Stomach – Produces protease to break down proteins. Contains hydrochloric acid to kill bacteria.
Pancreas – Produces enzymes that help digest carbohydrates, proteins, and fats in the small intestine.
Small intestine – Completes digestion and absorbs nutrients into the blood.
Large intestine – Absorbs water from undigested food.
Liver – Produces bile.
Gallbladder – Stores bile, which helps digest fats.
Rectum – Stores waste (faeces).
Anus – Where waste is removed from the body.
Peristalsis – helps food through the gut
Diet and Health
A balanced diet includes different types of nutrients:
Carbohydrates – Provide energy.
Proteins – Needed for growth and repair.
Fats – Store energy.
Minerals and Vitamins – Help the body function properly.
Fibre – Helps move food through the gut. .Water – Needed for chemical reactions and transporting nutrients.
Testing for Food Nutrients
Glucose test – Add Benedict’s solution and heat. The colour changes from green to red, depending on the sugar amount.
Protein test – Add Biuret solution. If protein is present, the colour changes from pale blue to purple.
Starch test – Add iodine. If starch is present, the colour changes from yellow-brown to blue-black.
Circulatory System and Circulation
The circulatory system is a network of organs and blood vessels that moves blood around the body. It carries oxygen, nutrients, and other substances to cells and removes waste like carbon dioxide.
Components of Blood
Blood is a liquid that transports important substances. It has four main parts:
Red blood cells transport oxygen from the lungs to the body and take carbon dioxide back to the lungs. They contain haemoglobin, which binds to oxygen.
White blood cells protect the body by fighting infections. Phagocytes engulf harmful microbes, while lymphocytes produce antibodies.
Plasma is the yellow liquid that carries nutrients, hormones, and waste products.
Platelets help the blood clot to stop bleeding.
The Double Circulatory System
Mammals have a double circulatory system, meaning blood flows through the heart in two loops.
Pulmonary circulation. The right side of the heart pumps deoxygenated blood to the lungs, where it picks up oxygen and releases carbon dioxide.
Systemic circulation. The left side of the heart pumps oxygenated blood to the rest of the body, delivering oxygen and collecting carbon dioxide.
Structure of the Heart
The heart is a muscular organ that pumps blood. It has four chambers.
Left atrium and left ventricle
Right atrium and right ventricle
The septum separates the left and right sides of the heart.
Blood flows through the heart.
Oxygenated blood from the lungs enters the left atrium via the pulmonary vein.
It moves to the left ventricle, which pumps it to the body through the aorta.
Deoxygenated blood from the body enters the right atrium via the vena cava.
It moves to the right ventricle, which pumps it to the lungs through the pulmonary artery.
Heart Valves
Valves prevent blood from flowing backward, ensuring one-way movement. There are two main types.
Atrioventricular valves. Between the atria and ventricles, stopping blood from going back into the atria.
Semilunar valves. Between the ventricles and arteries, stopping blood from flowing back into the heart.
Blood Vessels
There are three types of blood vessels.
Arteries carry blood away from the heart under high pressure. They have thick walls and a narrow opening to maintain pressure.
Veins carry blood back to the heart under low pressure. They have thinner walls, a wider opening, and valves to prevent backflow.
Capillaries are tiny vessels that connect arteries and veins. Their thin walls allow oxygen and nutrients to pass into cells and waste products to leave.
Cardiovascular Disease
Cardiovascular disease refers to diseases affecting the heart and blood vessels. A buildup of fatty deposits in arteries can reduce blood flow. This may lead to blood clots, increasing the risk of heart attacks or strokes.
Risk Factors for Cardiovascular Disease
High blood pressure damages arteries, increasing the risk of blockages.
High cholesterol leads to fatty deposits in blood vessels.
Smoking reduces oxygen in the blood, making the heart work harder.
Obesity puts strain on the heart.
Lack of exercise increases the risk of obesity and heart problems.
Poor diet. Eating too much fat and salt can raise blood pressure and cholesterol.
Family history. Some people inherit a higher risk of heart disease.
Treating Cardiovascular Disease
Lifestyle changes. Eating a healthy diet, exercising, reducing stress, and avoiding smoking and alcohol.
Medication. Statins lower cholesterol levels but can have side effects like liver damage.
Angioplasty. A small balloon and a stent are inserted into an artery to keep it open. However, this requires surgery and may cause scar tissue or blood clots.
Plants and Photosynthesis
Photosynthesis
Photosynthesis is a process where green plants and algae use light energy to make food. It happens inside chloroplasts in plant cells.
Requirements for Photosynthesis
Carbon dioxide provides carbon and oxygen for making glucose.
Water provides hydrogen for making glucose.
Light provides the energy for the process.
Chlorophyll, a green pigment in chloroplasts, absorbs light energy.
Process of Photosynthesis
Plants take in carbon dioxide and water. Using light energy, they produce glucose and release oxygen as a waste product.
The word equation for this process is:
Carbon dioxide + Water → Glucose + Oxygen
Importance of Photosynthesis
It produces glucose, which plants use for energy in respiration.
Glucose helps plants grow by forming complex molecules.
These molecules pass through food chains to feed other organisms.
It releases oxygen, which is needed for respiration in living things.
Factors Affecting Photosynthesis
Temperature. Higher temperatures increase the speed of reactions, but too much heat (above 45°C) damages enzymes and slows photosynthesis.
Light intensity. More light increases the rate of photosynthesis, but after a certain point, another factor becomes limiting.
Carbon dioxide levels. More carbon dioxide increases photosynthesis, but eventually, other factors limit the process.
Uses of Glucose in Plants
Used in respiration to release energy.
Stored as starch or oils.
Converted to sucrose for transport to other parts of the plant.
Used to make cellulose for cell walls.
Combined with minerals from the soil to make proteins.
Transport in Plants
Plants have a system to move water, minerals, and food.
Xylem carries water and minerals from roots to leaves in a process called the transpiration stream.
Phloem transports sugars from leaves to other parts of the plant.
Absorption of Water and Minerals
Water moves into root hair cells by osmosis (movement from high to low water concentration).
Water then moves from cell to cell until it reaches the xylem.
Minerals are absorbed by active transport, which requires energy.
Root hair cells are adapted for absorption because they have:
Long extensions to increase surface area.
Many mitochondria provide energy for active transport.
Transpiration
Transpiration is the process of water loss from leaves.
Water evaporates from the mesophyll layer inside the leaf.
It then diffuses out through the stomata.
More water is pulled up the xylem to replace it.
This creates a continuous flow of water from the roots to the leaves.
Factors Affecting Transpiration
Temperature. Higher temperatures increase water evaporation and transpiration.
Humidity. High humidity slows transpiration because the air is already full of moisture.
Wind. Wind removes water vapor, maintaining a high concentration gradient and increasing transpiration.
Ecosystems, Nutrient Cycles, and Human Impact on the Environment
Food Chains and Food Webs
The sun is the main source of energy for all living things. Plants and algae, called producers, absorb sunlight and turn it into energy through photosynthesis.
This energy is passed through food chains when animals eat plants and when predators eat other animals.
When organisms die, decomposers break them down and return nutrients to the soil.
A food chain shows how energy moves between organisms. Each step in a food chain is called a trophic level. A simple food chain looks like this:
Producer → Primary consumer → Secondary consumer → Tertiary consumer
Producers, like plants, are always at the beginning because they make energy from sunlight.
Primary consumers eat plants, while secondary and tertiary consumers are predators.
Decomposers can break down dead organisms at any level.
Food webs link different food chains, showing how organisms in an ecosystem depend on each other.
Energy Loss in a Food Chain
Not all energy is passed from one level to the next. Some energy is lost because:
Most of the sun's energy is reflected and not absorbed.
Animals use energy for movement and body heat.
Some parts of food are not digested and pass out as waste.
Because so much energy is lost, food chains usually have only four or five levels. There is not enough energy to support more levels.
Pyramids of Numbers and Biomass
Feeding relationships can be shown as pyramids.
A pyramid of numbers shows how many organisms are at each level.
A pyramid of biomass shows the dry mass of living material at each level.
Pyramids of biomass are usually pyramid-shaped because energy is lost at each level. Pyramids of numbers may look different because they count organisms without considering their size.
Nutrient Cycles
Nutrients are constantly recycled between living things and the environment. There is a limited amount of nutrients on Earth, so they must be reused.
The Carbon Cycle
Plants remove carbon dioxide from the air during photosynthesis.
Animals eat plants, passing carbon through the food chain.
Plants and animals release carbon dioxide back into the air when they respire.
When organisms die, decomposers break them down and release carbon dioxide.
Burning fuels, like wood and coal, also adds carbon dioxide to the air.
The Nitrogen Cycle
Dead plants and animals decompose, releasing nitrogen into the soil as ammonia.
Bacteria in the soil turn ammonia into nitrates, which plants absorb to make proteins.
Some bacteria, called nitrogen-fixing bacteria, take nitrogen from the air and turn it into nitrates.
Animals eat plants, passing nitrogen through the food chain.
Other bacteria, called denitrifying bacteria, turn nitrates back into nitrogen gas, returning it to the atmosphere.
Human Impact on the Environment
Humans need land for food and housing, which affects nature. More farmland means fewer natural habitats and reduced biodiversity. It is important to balance human needs with protecting ecosystems and endangered species.
Intensive Farming
Intensive farming is a method that increases food production using machinery, chemicals, and small spaces for animals.
Advantages:
Produces more food to feed the growing population.
Maximizes space and increases profit.
Disadvantages:
Reduces biodiversity by destroying habitats.
Excess fertilizer can pollute rivers and kill wildlife.
Chemicals can harm other animals or enter the food chain.
Keeping animals in small spaces is seen as unethical.
Overuse of antibiotics can lead to antibiotic resistance.
Eutrophication
Fertilizers and sewage can wash into lakes and rivers, causing a process called eutrophication.
Fertilizer or sewage enters the water.
Extra nutrients cause algae to grow quickly.
The algae block sunlight, stopping underwater plants from making oxygen.
Plants die and decompose.
Decomposers use up even more oxygen.
Without oxygen, fish and other animals die.