Cell Structure and Types

Every cell contains essential structures that keep life ticking. The nucleus houses your genetic material (DNA), whilst the cytoplasm is where all the chemical reactions happen. Ribosomes build proteins, and mitochondria release energy through aerobic respiration.

There are two main cell types you need to know. Prokaryotic cells are simple, single-celled organisms like bacteria - their DNA floats freely in the cytoplasm rather than being locked in a nucleus. Eukaryotic cells are more complex and include all animal and plant cells.

Plant cells have extra bits that animal cells don't. The cell wall provides support, chloroplasts carry out photosynthesis using the green substance chlorophyll, and the vacuole contains cell sap made of sugar and salts.

Quick tip: Remember that mitochondria are the "powerhouses" of the cell - they're where energy gets released!

Microscopy and Cell Observation

Light microscopes use light and lenses to magnify specimens, letting you see individual cells and larger structures like nuclei. Electron microscopes use electrons instead of light, giving much higher magnification and resolution - basically sharper, clearer images.

When preparing slides, you'll add a stain like iodine to make structures more visible. Always start with the lowest power objective lens and use the coarse adjustment knob first, then fine-tune with the fine adjustment.

The key formula you'll need is: magnification = image size ÷ real size. This helps you calculate how much bigger something appears compared to its actual size.

Cell Differentiation and Stem Cells

Cell differentiation is how cells become specialised for specific jobs. Stem cells are undifferentiated cells that can potentially turn into any type of cell - they're found in early embryos and adult bone marrow.

Different cell types have specific adaptations: muscle cells are long with lots of mitochondria for energy, sperm cells have tails for swimming, and nerve cells are long with branched ends for carrying electrical signals.

Stem cell therapy offers exciting medical possibilities but raises ethical questions. Scientists can use stem cells to potentially treat diabetes, paralysis, and other conditions, though there are risks like viral contamination.

Cell Division and Chromosomes

Chromosomes contain your genetic material as coiled DNA molecules. Human cells have 23 pairs of chromosomes. Mitosis is how cells divide to create two identical cells for growth and repair.

The cell cycle involves DNA duplication, chromosome alignment, and separation before the cell splits completely. This process is crucial for growing, healing wounds, and replacing old cells.

Plant cells can differentiate throughout their lives, whilst most animal cells lose this ability early on. This is why plants can regrow from cuttings but animals can't regrow entire limbs.

Movement of Substances

Three key processes move substances in and out of cells. Diffusion is the movement of particles from high to low concentration - bigger concentration gradients and higher temperatures speed this up. Osmosis is specifically water movement through partially permeable membranes.

Active transport works against the concentration gradient and requires energy from respiration. Root hair cells use this to absorb minerals from soil even when there's a higher concentration inside the cell than outside.

You can investigate osmosis using potato cylinders in different sugar solutions. Mass increases show water moving in, whilst mass decreases show water moving out.

Remember: Diffusion and osmosis are passive (no energy needed), but active transport requires energy!

Gas Exchange Systems

Single-celled organisms can exchange gases directly across their membrane because they have a large surface area compared to their volume. Larger organisms need specialised exchange surfaces because their surface area to volume ratio is much smaller.

Alveoli in lungs are perfectly adapted for gas exchange with their huge surface area (75m²), thin walls, moist lining, and excellent blood supply. Similarly, villi in the small intestine have adaptations that speed up absorption of digested food.

Fish gills work differently - water flows in one direction whilst blood flows the opposite way. This counter-current flow maintains a concentration gradient, ensuring maximum oxygen transfer from water to blood.

Plant Exchange Systems

Leaves are brilliant exchange surfaces with several clever adaptations. Stomata are tiny holes on the leaf's underside that allow CO₂ in and oxygen out. Guard cells control stomata size to prevent excessive water loss.

The leaf's flattened shape increases surface area, whilst air spaces inside provide more area for gas exchange. Water vapour evaporates from cells and escapes through stomata - this is part of transpiration.

Root hair cells have long projections that increase surface area for absorbing water and minerals from soil. They use both diffusion and active transport depending on concentration gradients.