AQA Biology Paper 1 Complete Notes

Specialised Cells and Their Adaptations

Your body contains billions of cells, but they're not all the same! Specialised cells have specific structures that help them perform their unique functions.

Sperm cells have a tail to swim toward an egg and a haploid nucleus (23 chromosomes) for fertilisation. Red blood cells lack a nucleus, giving them more space to carry oxygen, and their bi-concave shape increases surface area for better gas exchange.

Muscle cells contain protein fibres that contract to create movement and numerous mitochondria to supply energy. Nerve cells feature branched dendrites to connect with other neurons and a myelin sheath that insulates the axon, speeding up electrical impulse transmission.

Quick Tip: Remember that a cell's structure is always linked to its function - this connection appears frequently in exam questions!

Microscopy and Cell Investigation

Scientists use different types of microscopes to study cells, each with distinct advantages. Light microscopes use light beams and can view living samples but offer lower magnification and resolution than electron microscopes.

Electron microscopes provide much higher magnification and resolution by using electron beams instead of light. While they produce detailed images, they can only examine non-living samples and are very expensive.

When calculating the size of cells, remember the magnification equation: Magnification = Image Size ÷ Actual Size

You can prepare plant tissue slides by cutting a thin slice, adding a stain like iodine, carefully placing a coverslip, and focusing from low to high power. This technique allows you to observe cellular structures in detail.

Movement Across Cell Membranes

Substances move into and out of cells through different transport processes. Diffusion is the passive movement of particles from an area of high concentration to low concentration. It requires no energy and follows the concentration gradient.

Osmosis is a special type of diffusion where water molecules move through a partially permeable membrane from an area of higher water concentration to lower water concentration.

Active transport requires energy from respiration to move molecules against their concentration gradient. Plants use this process to absorb mineral ions from the soil when their concentration is lower in the surrounding soil.

Remember: In exams, you might need to explain why active transport is necessary in root hair cells or intestinal absorption!

Factors Affecting Diffusion and Osmosis

Three key factors affect how quickly diffusion happens in cells. A steeper concentration gradient means faster diffusion as particles move more rapidly from high to low concentration areas. Higher temperature increases the rate because particles have more kinetic energy. A larger membrane surface area allows more particles to pass through simultaneously.

You can investigate osmosis using potato cylinders placed in different concentrations of sucrose solution. By measuring mass changes before and after, you can determine how water moves by osmosis.

Plant root hair cells need active transport to absorb mineral ions from the soil when ion concentration outside is lower than inside the cell. Similarly, in your small intestine, active transport allows sugar absorption even when blood sugar levels are already high.

Exchange Surfaces and Gas Exchange

Your body has specialised structures for exchanging substances with the environment. Villi in the small intestine are perfectly adapted with their long, thin shape that increases surface area for absorption. Their one-cell-thick membrane provides a short diffusion pathway, and rich blood supply maintains a steep concentration gradient.

The lungs feature millions of tiny air sacs called alveoli that maximise surface area for gas exchange. Their moist, thin walls allow gases to diffuse quickly, and an extensive network of capillaries ensures efficient oxygen uptake and carbon dioxide removal.

Fish gills work similarly but extract oxygen from water. Their stacked gill filaments provide an enormous surface area, while water flowing in one direction and blood in the opposite direction creates an efficient counter-current exchange system.

Exam Alert: Be ready to compare different exchange surfaces and explain how their adaptations improve efficiency!

Investigating Osmosis Practically

When conducting an osmosis investigation, controlling variables is crucial. The independent variable is what you deliberately change (concentration of solution), while control variables are factors kept constant (potato cylinder size, temperature, time).

Here's how to carry out a reliable osmosis practical:

1. Cut potato cylinders of identical diameter using a cork borer
2. Ensure all cylinders are the same length using a ruler and scalpel
3. Measure initial mass of each cylinder
4. Place cylinders in different concentrations of solution for the same amount of time
5. Remove, blot dry, and measure final mass

The percentage change in mass tells you about water movement by osmosis. Cylinders in high concentration solutions lose mass as water moves out, while those in low concentration gain mass as water moves in.

The Cell Cycle and Mitosis

Cell division is essential for growth, repair and asexual reproduction. The cell cycle consists of three main stages that ensure cells reproduce accurately.

In Stage 1, the cell grows larger as its DNA replicates (chromosomes duplicate) and subcellular structures like mitochondria increase in number. This preparation ensures the cell has enough components to share between daughter cells.

During Stage 2 (mitosis), the nucleus divides as chromosomes align at the cell's centre, then identical sets are pulled to opposite ends of the cell. This forms two nuclei with identical genetic information.

In Stage 3, the cytoplasm and cell membrane divide to create two identical daughter cells. Each new cell contains the same genetic information as the original cell, allowing multicellular organisms to grow while maintaining consistent DNA throughout their tissues.

Key Point: Mitosis produces genetically identical cells, which is crucial for growth and tissue repair!

Stem Cells and Their Applications

Stem cells are undifferentiated cells that can develop into different types of specialised cells. They're incredibly valuable in medicine and research but come with various advantages and disadvantages.

Adult stem cells are found in specific tissues like bone marrow and can differentiate into limited cell types. They involve fewer ethical issues since adults can consent to their removal, but they're less versatile than embryonic stem cells.

Embryonic stem cells from early human embryos can develop into any cell type in the body. While this versatility could potentially treat numerous diseases or even grow replacement organs, they raise significant ethical concerns since embryos are destroyed to obtain them.

Scientists must carefully balance the tremendous medical potential of stem cells against ethical considerations and practical limitations like donor compatibility and infection risks.

Plant Meristems and Cell Organisation

Plant meristem cells are plant stem cells found in roots and shoots. They can differentiate into all plant cell types and are valuable for creating clones of entire plants. This ability makes them useful for preserving rare species or producing numerous identical plants with desirable traits like disease resistance.

In therapeutic cloning, cells from a patient are used to create a cloned embryo containing stem cells genetically identical to the patient. These stem cells could potentially grow replacement tissues or organs with minimal rejection risk.

Living organisms show levels of organisation:

• Cells are the basic units
• Tissues are groups of similar cells working together
• Organs combine different tissues for specific functions
• Organ systems are groups of organs working together
• Organisms are complete living things with all systems functioning together

Remember: This hierarchical organisation applies to both plants and animals!

The Digestive System

Your digestive system contains multiple organs working together to break down food into molecules small enough to be absorbed into your bloodstream.

The pancreas produces essential digestive enzymes (amylase, lipase, and protease) that break down different food components. The liver manufactures bile, which is stored in the gall bladder before being released to neutralise stomach acid and emulsify fats.

The small intestine is where most digestion products are absorbed into the blood, while the large intestine absorbs water and minerals. The journey begins in the mouth, where food is mechanically broken down and mixed with enzyme-containing saliva.

Food travels through the oesophagus to the stomach, which churns food and secretes hydrochloric acid and proteases. After nutrients are absorbed in the intestines, waste material passes through the rectum and exits through the anus.

Exam Tip: Know both the location and function of each digestive organ - you'll likely be tested on both!