Sec 3 Biology Cell Structure & Organisation notes

A cell is a tiny bag of life. The cell membrane forms its outer boundary — a thin, partially permeable skin that decides which substances pass in and out. Inside sits the jelly-like cytoplasm, where most of the cell’s chemical reactions happen, suspended in which floats the nucleus: the control centre that stores DNA and directs what the cell makes and does.

Plant cells add three features animal cells never have. A cellulose cell wall — a tough, freely permeable box around the membrane — resists the outward push of a swelling cell and gives the plant its rigid support. Chloroplasts, green oval organelles, hold the pigment chlorophyll, which absorbs light energy to power photosynthesis. And a large permanent vacuole filled with cell sap presses outward against the wall, keeping the cell firm (turgid). Animal cells, if they have vacuoles at all, keep them small and temporary.

A good light microscope shows the structures of the previous section. An electron microscope resolves details hundreds of times finer, revealing the working organelles inside the cytoplasm. The syllabus names four.

• Mitochondria — oval, sausage-shaped organelles with folded inner membranes. They are the sites of aerobic respiration, releasing energy from glucose so the cell can work. Cells burning lots of energy (muscle, liver) are crowded with them.
• Ribosomes — tiny dots, often dotted along the endoplasmic reticulum. They are the sites of protein synthesis, assembling amino acids into the proteins the cell needs.
• Endoplasmic reticulum (ER) — a network of folded membranes threading through the cytoplasm that transports materials, including newly made proteins, around the cell.
• Golgi body — a stack of flattened membrane sacs that packages substances (for example digestive enzymes) into vesicles ready for secretion out of the cell.

Exam questions love the pairing "which organelle, and why?" — so tie every organelle to its job in one line: mitochondria = energy release, ribosomes = protein assembly, ER = transport within the cell, Golgi = packaging for secretion. Writing the function, not just the name, is where the marks live.

So the compare-and-contrast answer is short: a plant cell has a cellulose cell wall, chloroplasts and a large permanent vacuole that the animal cell lacks. Every other named structure is shared. Note the trap in "chloroplasts" — not all plant cells have them: root cells live underground in the dark and contain none, which is why roots cannot photosynthesise.

When a question shows you a labelled diagram, check the three plant-only features first. If the diagram has any one of them, it is a plant cell; if it has none, it is an animal cell (bacteria and fungi have their own walls — but that is a different story from cellulose).

Multicellular organisms divide the work: each cell type is specialised, with a structure tailored to one job. The syllabus names three examples you must be able to explain.

• Energy demand

  Muscle cell

  Packed with mitochondria because contraction burns energy constantly — aerobic respiration in those mitochondria releases the energy the muscle needs to pull.

• Absorption

  Root hair cell

  A long, tube-like extension of the cell pushes between soil particles, hugely enlarging the membrane surface in contact with soil water for faster uptake of water and mineral ions.

• Transport

  Red blood cell

  Loses its nucleus before maturing, so more haemoglobin — the oxygen-binding pigment — fits inside; its biconcave dent gives a large surface for fast oxygen loading.

The examiner’s phrasing is always "explain how the structure is adapted to its function". Structure first (what it looks like), function second (what that shape achieves). A one-line "because" sentence — more surface, more haemoglobin, more energy — is the mark.

Fresh cells are mostly water and nearly transparent, so under the light microscope their parts blur together. A temporary stain (such as iodine or methylene blue) colours specific structures — the nucleus takes up stain strongly — giving the contrast needed to identify them. Staining does not change the cell’s size or magnification; it only makes structures visible.

Because cells are far smaller than millimetres suggest, biologists quote sizes in micrometres (µm): 1 mm = 1000 µm. Magnification compares the image with reality: magnification = image size ÷ actual size. Rearranged, actual size = image size ÷ magnification. Always convert lengths to the same unit before dividing.

A single cell can manage every life process alone — but a multicellular organism wins by specialisation and division of labour. Similar cells doing one shared job form a tissue (cardiac muscle tissue, for example). Different tissues combine into an organ (the heart, built of muscle, nerve and lining tissues). Organs cooperating form an organ system (the circulatory system: heart plus vessels), and all the systems together make the organism.

The payoff is efficiency. Your digestive system illustrates it end to end: enzyme-secreting cells (with their ribosome-rich cytoplasm) make the enzymes, muscle tissue churns the food, and the folded absorbing lining collects the products. No single general-purpose cell could digest, absorb and deliver food nearly as well as the specialised assembly line can.