GCSE Biology: Cells and Photosynthesis Notes (AQA & CCEA)

Cell Structure Basics

Your body and every living thing around you is made up of cells - think of them as tiny building blocks of life. Animal and plant cells share some common parts, but plants have some special extras that make them pretty unique.

Both types of cells have a nucleus (the control centre containing DNA), cell membrane (the bouncer that controls what gets in and out), mitochondria (the powerhouses for energy), and cytoplasm (where chemical reactions happen). It's like they both have the same basic toolkit for staying alive.

Plant cells get some bonus features though. They've got a tough cell wall for extra support, a large permanent vacuole that keeps them rigid, and most importantly, chloroplasts containing chlorophyll to capture light for photosynthesis. These extras are what let plants stand tall and make their own food.

Key Point: Only the green parts of plants have chloroplasts - that's why stems and roots are often different colours!

Bacterial Cells - The Minimalists

Bacterial cells are the rebels of the cell world - they're microscopic, single-celled organisms that do things completely differently. Unlike plant and animal cells, they don't bother with a proper nucleus or most of the fancy organelles.

Instead, bacterial cells keep it simple with their DNA floating freely in the cytoplasm, a cell wall for protection, and sometimes a whip-like flagellum for swimming around. Some even carry extra bits of DNA called plasmids - think of these as bonus genetic material.

These tiny organisms might look basic, but don't underestimate them. Bacteria are everywhere and can survive in conditions that would kill other cells. They're also incredibly important for things like digestion, medicine production, and breaking down waste.

Remember: Bacterial cells are prokaryotes (no nucleus), while plant and animal cells are eukaryotes (have a nucleus).

How Substances Move In and Out of Cells

Getting the right stuff in and the wrong stuff out of cells is absolutely crucial for survival. The main way this happens is through diffusion - imagine dropping food colouring in water and watching it spread out naturally.

Diffusion is simply particles moving from where there's loads of them to where there's fewer - it's completely random but always follows this pattern. Your cell membrane is selectively permeable, meaning it acts like a really picky bouncer, letting some substances through while blocking others.

Three things speed up diffusion: a bigger concentration gradient (larger difference between inside and outside), higher temperature (particles move faster when heated), and larger surface area (more space for particles to cross). This is why cells can't grow too big - they'd struggle to get enough nutrients and oxygen through diffusion alone.

As cells grow, their volume increases faster than their surface area, creating a problem. That's why large organisms need specialised systems like lungs for gas exchange and blood for transport - diffusion just isn't fast enough over long distances.

Quick Tip: Remember the surface area to volume ratio - it's why elephants need complex systems but bacteria can survive with just simple diffusion!

From Cells to Complex Organisms

Life has an amazing way of building complexity from simple beginnings. It all starts with cells - the basic building blocks - then gets organised into increasingly complex structures that work together perfectly.

Tissues are groups of similar cells doing the same job (like muscle tissue for movement). These tissues combine to form organs like your heart or a plant's leaf, which are structures with specific functions. Multiple organs then work together as organ systems - think of your circulatory system with heart, blood vessels, and blood all cooperating.

Finally, all the organ systems combine to create a complete organism - whether that's you, your pet, or the tree in your garden. It's like building with Lego, starting with individual bricks and ending up with something amazing and functional.

Stem cells are special because they're undifferentiated - they haven't decided what type of cell to become yet. Embryonic stem cells can become any cell type, while adult stem cells are more limited. They're incredibly valuable in medicine for treating diseases and growing replacement tissues.

Amazing Fact: Your body contains about 37 trillion cells, all working together to keep you alive!

Stem Cells - The Body's Repair Kit

Stem cells are like biological Swiss Army knives - simple, undifferentiated cells that can divide and transform into almost any specialised cell type your body needs. They're absolutely crucial for growth, repair, and medical breakthroughs.

Embryonic stem cells are the ultimate multi-taskers, capable of becoming any cell type in the body. You'll find them in early embryos, unused IVF embryos, and umbilical cords. They grow rapidly and are easy to work with in labs, making them incredibly valuable for research.

Adult stem cells are more specialised, found in specific organs like bone marrow, blood, and heart tissue. They can only produce certain types of cells and are trickier to grow, but they're still essential for your body's natural repair processes.

Plant stem cells hang out in growing tips called meristems at roots and shoots. Under the right conditions, these cells can even reverse their specialisation - something animal cells usually can't do.

Medical Marvel: Stem cells are already treating leukaemia, potentially curing paralysis, and might one day grow replacement organs that perfectly match your tissue type!

Photosynthesis - Nature's Food Factory

Photosynthesis is probably the most important process on Earth - it's how plants make food from sunlight, and it produces all the oxygen you breathe. This endothermic process happens in chloroplasts and can be summed up in one beautiful equation.

The word equation is simple: Carbon dioxide + Water → Glucose + Oxygen (with light and chlorophyll needed). The chemical version shows the exact amounts: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

To test if photosynthesis has happened, scientists use the starch test with iodine solution. Plants convert glucose to starch for storage, and iodine changes from orange to blue-black when starch is present. Before any experiment, plants must be destarched by keeping them in darkness for 48 hours.

Testing different conditions proves what photosynthesis needs. Variegated leaves (with white and green parts) show that chlorophyll is essential - only green parts produce starch. Using soda lime to remove CO₂ stops photosynthesis completely, proving carbon dioxide is vital.

Cool Experiment: You can measure photosynthesis rates by counting oxygen bubbles produced by pondweed in different light intensities!

Measuring Photosynthesis Rates

Want to see photosynthesis in action? The classic pondweed experiment lets you measure how fast plants produce oxygen under different conditions. It's like watching a plant's productivity in real-time!

Set up pondweed (Elodea) in water under a bright lamp, and you'll see oxygen bubbles streaming from cut stems. Count the bubbles produced in 60 seconds to measure the photosynthesis rate - more bubbles mean faster photosynthesis.

For fair testing, you must control other variables: keep temperature constant (use a heat sink), use the same piece of pondweed, and maintain identical apparatus setup. Only change the one factor you're investigating, like light intensity or CO₂ concentration.

This experiment clearly demonstrates that CO₂ is essential for photosynthesis. Plants in containers with soda lime (which absorbs CO₂) produce no starch and show no colour change with iodine. Meanwhile, plants with sodium hydrogen carbonate (which releases CO₂) produce loads of starch and turn blue-black with iodine.

Pro Tip: If bubbles aren't appearing, try moving the lamp closer or cutting the pondweed stem underwater to get a fresh surface!

Leaf Structure - Perfect Design for Photosynthesis

Leaves are basically solar panels with attitude - every single feature is perfectly designed to capture light and exchange gases efficiently. Understanding leaf structure shows you how evolution creates the ultimate photosynthesis machine.

The waxy cuticle acts as a transparent waterproof jacket, letting light through while preventing water loss and blocking disease-causing microbes. Palisade mesophyll cells are tightly packed with chloroplasts near the top surface where light is strongest, maximising photosynthesis.

Spongy mesophyll has fewer chloroplasts but creates air spaces for gas exchange - it's like the leaf's breathing system. Guard cells control the stomata (tiny pores) that allow CO₂ in and O₂ out, while xylem brings water up and phloem carries sugars away.

For maximum light absorption, leaves have large surface areas, stay thin so all cells receive light, and pack loads of chlorophyll near the surface. The thin, transparent cuticle reduces water loss without blocking light - it's engineering perfection.

Amazing Design: Stomata are usually on the lower leaf surface to reduce water loss from direct sunlight!

Gas Exchange and Limiting Factors

Plants need to breathe too, but their gas exchange system is brilliantly adapted for both photosynthesis and respiration needs. Stomata on the lower leaf surface allow O₂ and CO₂ to enter and leave, controlled by guard cells that can open and close them.

Intercellular air spaces in spongy mesophyll let gases circulate freely to reach all photosynthesising cells. It's like having tiny corridors throughout the leaf for gas delivery and removal.

Three main factors limit photosynthesis rates: light intensity (low light slows everything down), temperature (cool weather can limit reactions even with bright light), and CO₂ levels (once light isn't limiting, CO₂ becomes the bottleneck).

Here's the clever bit - photosynthesis and respiration happen simultaneously in plants. Photosynthesis (CO₂ + water → glucose + oxygen) and respiration (oxygen + glucose → CO₂ + water + energy) are essentially opposite processes, and their products fuel each other.

Key Insight: Plants respire 24/7 like animals, but only photosynthesise during daylight hours when they have energy from light!

Light Conditions and Gas Exchange

Plants experience three distinct situations depending on light levels, and each creates different patterns of gas exchange that you can actually measure and observe.

In darkness, only respiration occurs - plants take in oxygen and release CO₂, just like animals do. CO₂ levels increase around the plant because there's no photosynthesis to use it up.

At low light levels (dawn/dusk), both processes happen simultaneously. This creates the compensation point where photosynthesis and respiration rates balance perfectly - there's no net gas exchange because CO₂ production equals CO₂ consumption.

In bright light (midday), photosynthesis massively outpaces respiration. Plants absorb loads of CO₂ for photosynthesis and release tons of oxygen. CO₂ levels around the plant decrease significantly.

Hydrogen carbonate indicator reveals these changes through colour: red shows normal atmospheric CO₂, yellow indicates increased CO₂ (more acidic), and purple shows decreased CO₂ levels. It's like a traffic light system for monitoring plant activity!

Experiment Tip: Always ensure fair testing by using equal volumes of indicator, same leaf areas, and destarched plants for accurate results!