Cell Types and Structure

Prokaryotic cells (like bacteria) are the simpler ones - they're basically just floating their DNA around without a proper nucleus! Their chromosomal DNA forms one long circular loop that controls everything the cell does. They also have bonus DNA called plasmids - these little loops can carry handy genes like drug resistance and get passed between bacteria like trading cards.

Eukaryotic cells (animals and plants) are the complex ones with a proper nucleus that houses their DNA. Animal cells have a cell membrane that acts like a bouncer, deciding what gets in and out, plus mitochondria (the powerhouses) where cellular respiration happens. The cytoplasm is like a gel where most chemical reactions occur.

Plant cells have everything animal cells have, plus some extras. They've got a rigid cell wall made of cellulose for support, and chloroplasts containing chlorophyll for photosynthesis. Think of plant cells as animal cells with added superpowers!

💡 Remember: Prokaryotes = no nucleus (bacteria), Eukaryotes = nucleus present (plants and animals)

Light Microscopy

Light microscopes have been helping scientists since the 1590s, and they're still brilliant for viewing things like nuclei and chloroplasts. The total magnification equals eyepiece lens magnification × objective lens magnification - simple maths that'll definitely come up in exams!

Electron microscopes (invented in the 1930s) are the heavy hitters with much higher magnification and resolution. While light microscopes are portable and can view living tissue, electron microscopes can reveal tiny details like plasmids and viruses that light microscopes simply can't see.

Resolution is crucial - it's how well a microscope can distinguish between two points close together. Higher magnification isn't always better though; if your specimen is quite big, you might not see the whole thing and focusing becomes trickier.

The key formula you need: Image size = Magnification × Real size. Master this and you'll ace those calculation questions!

💡 Top tip: Start with the lowest magnification first, then work your way up for detailed views

Using Microscopes and Standard Form

Preparing specimens properly is half the battle! Place a drop of water on a clean slide, add your specimen with tweezers, then add a stain if needed. Iodine stains cytoplasm whilst methylene blue stains DNA - different stains highlight different structures.

When using the microscope, always start with the lowest-powered objective lens. Use the coarse adjustment knob to get roughly in focus, then fine-tune with the fine adjustment knob. This step-by-step approach prevents you from crashing the lens into your slide!

Standard form makes those tiny measurements manageable. Remember: if the decimal point moves left, the power of 10 is positive; if it moves right, it's negative. So 0.017 becomes 1.7 × 10⁻².

You can still work out magnification as long as you can measure the image and know the real size of the specimen. This flexibility is handy when dealing with photographs or drawings in exams.

💡 Memory trick: Think "left = positive, right = negative" for standard form powers

Enzymes and Reactions

Enzymes are biological catalysts that speed up reactions without getting used up themselves. They're incredibly picky - each enzyme only works with one specific substrate because of their unique active site shape. Think of it like a lock and key system.

The 'lock and key' hypothesis explains enzyme specificity perfectly. If the substrate doesn't fit the active site's shape exactly, no reaction happens. This is why enzymes have "high specificity" for their substrate - they're basically biological perfectionists!

Every different biological reaction has its own enzyme. Respiration, photosynthesis, and protein synthesis all depend on specific enzymes to work properly. Together, these reactions make up your cell's metabolism.

Enzymes solve a major problem for living things. Chemical reactions usually need high temperatures to speed up, but enzymes let cells control reactions at body temperature without damaging themselves.

💡 Key point: Each enzyme is a protein coded by a different gene - that's why they all have unique shapes

Cellular Respiration

Respiration isn't just breathing - it's the process of transferring energy from glucose breakdown in every single cell. This exothermic reaction transfers energy to make ATP, which stores energy for cellular processes.

Aerobic respiration uses oxygen and is incredibly efficient, producing 32 ATP molecules per glucose molecule. The equation is: Glucose + Oxygen → Carbon dioxide + Water. This happens when there's plenty of oxygen available.

Anaerobic respiration works without oxygen but produces much less energy - just 2 ATP molecules per glucose. It's not the best energy transfer method, but it's better than nothing when oxygen runs out!

Temperature and pH affect respiration rate because the process is controlled by enzymes. Cells can also respire using other organic molecules (carbohydrates, proteins, lipids) as substrates when glucose isn't available.

💡 Remember: Aerobic = with oxygen (efficient), Anaerobic = without oxygen (less efficient but still useful!)

Diffusion and Active Transport

Diffusion is the movement of particles from higher to lower concentration - basically particles spreading out to even things up. Only very small molecules like glucose, water, and amino acids can diffuse through cell membranes.

Cell membranes are clever barriers that hold cells together whilst letting stuff in and out. Big molecules like starch and proteins can't squeeze through the tiny gaps, so they need other transport methods.

Active transport is diffusion's opposite - it moves particles against the concentration gradient using energy from respiration. This is essential when nutrients in your gut have lower concentration than in your blood, but your body still needs to absorb them.

Plants use active transport to grab minerals from soil even when there's more minerals in their roots than in the soil. Without this energy-requiring process, plants couldn't get essential nutrients and we'd struggle with malnutrition.

💡 Think of it: Diffusion is like rolling downhill (natural), active transport is like cycling uphill (needs energy!)

Osmosis and Water Potential

Osmosis is basically water diffusion across a partially permeable membrane. Water molecules move from higher water concentration to lower water concentration, trying to even things out on both sides.

Water potential tells you how concentrated a solution is. Pure water has the highest water potential, whilst all solutions have lower water potential than pure water. Think of it as water's "eagerness" to move.

Plant cells love being turgid (plump and swollen) because it creates turgor pressure against the cell wall, supporting the plant's structure. When plants don't get enough water, cells become flaccid and the plant wilts.

Animal cells are more vulnerable because they lack cell walls. They can actually burst if surrounded by solutions with higher water potential than their contents - that's why maintaining water balance is crucial.

💡 Plant power: Turgid cells = happy, supported plants. Flaccid cells = droopy, wilted plants

Exchanging Substances

Single-celled organisms have it easy - substances diffuse straight across their cell membrane because distances are short and they have a large surface area to volume ratio. Everything they need can get in and waste can get out quickly.

Multicellular organisms face bigger challenges. Cells deep inside are far from the outside environment, and larger organisms have a smaller surface area to volume ratio, making simple diffusion too slow.

Three main factors affect substance movement: concentration gradient $bigger difference = faster movement$, temperature $warmer = more energy = faster movement$, and surface area $more area = faster exchange$.

Multicellular organisms solve these problems with specialised exchange organs and transport systems. Animals use the circulatory system whilst plants use xylem and phloem vessels. The liver also produces urea as a waste product from protein breakdown, which gets filtered out by kidneys.

💡 Size matters: Small organisms can rely on diffusion, but big organisms need specialised transport systems