AQA Biology Paper 1 Revision Guide - Year 10/11

Cell Structure Basics

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Cell Structure and Types

All living things are made up of cells, but not all cells are the same! Eukaryotes (like animals and plants) have a proper nucleus, whilst prokaryotes (bacteria) keep their DNA floating freely in the cytoplasm.

Animal cells contain the essential components: nucleus (the control centre), mitochondria (powerhouses), ribosomes (protein factories), and cytoplasm (the jelly-like filling). The cell membrane acts like a selective barrier, controlling what enters and exits.

Plant cells have all the animal cell parts plus some extras that make them special. They've got a tough cell wall for structure, chloroplasts for photosynthesis, and a large vacuole filled with cell sap. These adaptations help plants make their own food and stand upright.

Quick Tip: Remember that bacteria are prokaryotes - they're much simpler than plant and animal cells, with their DNA just floating around in a region called the nucleoid.

Microscopes and Cell Division

Light microscopes are your go-to tools for observing cells - they're cheap, easy to use, but limited to 0.2µm resolution. Electron microscopes offer incredible detail (0.1nm resolution) but they're expensive and complex to operate.

Understanding magnification is crucial: it's simply image size divided by object size. The units scale from nanometres (nm) to micrometres (µm) to millimetres (mm) - each step is 1000 times bigger than the last.

Mitosis is how cells create identical copies of themselves. The process involves three main stages: growth (cell gets bigger), DNA replication (chromosomes are copied), and division. During mitosis, chromosomes line up in the middle, then get pulled apart to create two genetically identical daughter cells.

Remember: Humans have 23 pairs of chromosomes (46 total) - one set from each parent!

Stem Cells and Specialisation

Stem cells are the body's master cells with two key abilities: they divide by mitosis to make more cells, and they can differentiate into specialised cell types. Think of them as blank templates that can become anything the body needs.

Embryonic stem cells are the most versatile - they can become any type of cell in the body. Adult stem cells, found in bone marrow, are more limited and mainly produce different types of blood cells (red, white, and platelets).

Plants have their own stem cells in regions called meristems. These cells keep dividing throughout the plant's entire life, creating new specialised cells like xylem (water transport) and phloem (food transport) as needed.

Differentiation is the process where a stem cell develops specific structures and functions. For example, red blood cells lose their nucleus to carry more oxygen, whilst nerve cells grow long projections to transmit signals quickly.

Key Point: The more specialised a cell becomes, the fewer jobs it can do - but it becomes incredibly efficient at its specific role!

Stem Cells in Medicine

Stem cell therapy offers hope for treating serious diseases by replacing damaged cells with healthy ones. Scientists extract embryonic stem cells, grow them in labs, then stimulate them to become the specific cell types patients need.

This technology could potentially treat Type 1 diabetes (damaged pancreas cells), paralysis (damaged nerve cells), and sickle cell anaemia (misshapen red blood cells). The process involves careful cultivation and differentiation of stem cells before transplanting them into patients.

However, there are significant challenges. Rejection by the immune system remains a major risk, even with immunosuppressive drugs. There's also limited supply of embryonic stem cells and potential virus transmission from donor cells.

The biggest concerns are tumour development (stem cells divide rapidly and can form cancers) and ethical objections to using embryonic cells, which some consider potential human life.

Alternative Option: Adult stem cells avoid ethical issues but can only differentiate into blood cells, limiting their therapeutic applications.

Diffusion and Transport

Diffusion is the natural movement of particles from high concentration to low concentration - imagine perfume spreading across a room. It's a passive process requiring no energy and works through partially permeable membranes like cell walls.

Three factors speed up diffusion: steeper concentration gradients (bigger difference in concentration), higher temperature (particles move faster), and larger surface area (more space for movement).

Osmosis is a special type of diffusion involving only water movement. Water moves from areas of high water concentration to low water concentration through partially permeable membranes.

Active transport works against the concentration gradient, moving substances from low to high concentration. This requires energy from cellular respiration, which is why cells doing active transport (like root hair cells) are packed with mitochondria.

Memory Aid: Solutions can be isotonic (same concentration), hypertonic (more solutes), or hypotonic (fewer solutes) - this determines which way water moves!

Specialised Exchange Surfaces

Efficient exchange surfaces share common features that maximise transfer rates. They have large surface areas, thin walls for shorter diffusion distances, and good permeability to the substances they exchange.

Animal exchange surfaces like alveoli (lungs) and villi (small intestine) also need excellent blood supply to carry materials away quickly. Plant surfaces like root hair cells and leaves are designed for maximum contact with soil and air.

These adaptations ensure rapid exchange of essential materials. Root hair cells have long projections that increase surface area for water and mineral absorption, whilst alveoli have incredibly thin walls for efficient gas exchange.

The key principle is maximising the rate of diffusion by optimising all the factors that affect it - surface area, concentration gradients, and diffusion distances.

Real-World Connection: Athletes train at high altitude to increase their red blood cell count, improving oxygen transport efficiency!

Photosynthesis Basics

Photosynthesis happens in chloroplasts using the green pigment chlorophyll to absorb light energy. The equation is: Carbon dioxide + Water → Glucose + Oxygen (using light energy).

Plants use the glucose they make in five important ways: cellular respiration for energy, making cellulose for strong cell walls, storing starch for later use, creating amino acids for proteins, and producing oils and fats as energy reserves.

Leaf structure is perfectly designed for photosynthesis. The waxy cuticle reduces water loss, stomata (controlled by guard cells) allow gas exchange, and mesophyll cells packed with chloroplasts capture light energy efficiently.

The upper palisade mesophyll contains most chloroplasts for maximum light absorption, whilst the spongy mesophyll has air spaces for gas movement. Veins transport water in and sugar out.

Amazing Fact: A single leaf can contain millions of chloroplasts, each one acting like a tiny solar panel!

Factors Affecting Photosynthesis and Respiration

Light intensity, temperature, carbon dioxide concentration, and chlorophyll amount all affect photosynthesis rates. When one factor becomes limiting, increasing others won't help - you need to address the bottleneck.

Greenhouses create optimal conditions by trapping heat, providing artificial light, and pumping in CO₂ (often from paraffin heaters). They also exclude pests, though setup costs are expensive.

Cellular respiration releases energy from glucose and is always exothermic (releases heat). Aerobic respiration uses oxygen efficiently: Glucose + Oxygen → Carbon dioxide + Water.

When oxygen runs short during intense exercise, anaerobic respiration kicks in: Glucose → Lactic acid. This incomplete breakdown creates less energy and causes muscle cramps from lactic acid buildup.

Exercise Connection: The burning feeling in your muscles during intense workouts is lactic acid - you need extra oxygen afterward to convert it back to glucose!

Disease and Infection

Pathogens are microorganisms causing communicable diseases that spread through air, contaminated food/water, or direct contact. Prevention involves good hygiene, eliminating vectors, vaccination, and quarantine measures.

Viruses aren't technically alive - they're just genetic material in a protein coat. They hijack host cells, forcing them to make viral copies until the cell bursts and releases new viruses.

Measles spreads through coughing/sneezing, causing fever and rashes that can be fatal. HIV transmits through bodily fluids, weakening the immune system and potentially leading to AIDS with flu-like symptoms.

Tobacco Mosaic Virus affects plants, creating discoloured leaf patches where photosynthesis can't occur. This reduces the plant's ability to make food and can seriously impact growth and survival.

Prevention Tip: Simple hygiene measures like regular handwashing and covering coughs can dramatically reduce disease transmission!