Cell biology
Cell structure, microscopy and magnification, cell division by mitosis and stem cells, and transport by diffusion, osmosis and active transport.
Learn
Cell biology, explained point by point
Everything the GCSE specification expects you to be able to do, and how to actually do it - the same lesson a signed-in student studies from.
Compare eukaryotic and prokaryotic cells and name their sub-cellular structures
Eukaryotic cells (animal, plant) keep their DNA inside a nucleus; prokaryotic cells (bacteria) are much smaller and have their DNA loose in the cytoplasm, sometimes with extra rings called plasmids. Learn the parts and their jobs: the nucleus controls the cell, mitochondria release energy in respiration, and ribosomes build proteins. Plant cells add a cell wall, a permanent vacuole and chloroplasts for photosynthesis.
Use a microscope and calculate magnification
A light microscope magnifies enough to see cells; an electron microscope magnifies far more, revealing tiny structures like ribosomes. Magnification links three quantities: magnification = image size ÷ real size. So if a cell is drawn 30 mm across but is really 0.06 mm, the magnification is 30 ÷ 0.06 = 500×. Keep both sizes in the same units before dividing.
Describe mitosis and the cell cycle
The cell cycle is how one cell becomes two identical cells for growth and repair. First the cell grows and copies its DNA and organelles, then mitosis separates the two sets of chromosomes, and finally the cell splits in two. Each new cell is genetically identical to the parent - that is why a graze heals with the same kind of skin.
Explain what stem cells are and why they are useful
Stem cells are unspecialised cells that can divide and turn into different cell types. Embryonic stem cells can become almost any cell, while adult stem cells (like those in bone marrow) are more limited, and plant stem cells sit in the meristems at growing tips. They offer treatments - replacing damaged cells, for example - though using embryos raises ethical questions worth being able to discuss.
Explain how substances move by diffusion, osmosis and active transport
Diffusion is the spreading of particles from where they are crowded to where they are sparse - like oxygen moving into a cell. Osmosis is the same idea for water moving across a partially permeable membrane, from dilute to concentrated. Active transport goes the other way, low to high concentration, so it needs energy from respiration - that is how roots absorb minerals from dilute soil.
Link the surface area to volume ratio to how quickly substances are exchanged
Small objects have a large surface area compared with their volume, so substances diffuse in and out fast enough. As things get bigger the ratio falls, so large organisms need exchange surfaces - like lungs or gills - with big surface areas, thin walls and a good blood supply to keep diffusion quick.
Cell biology key terms
The words the specification and the mark schemes use, each defined the way an examiner wants it.
- Eukaryotic cell
- A cell whose genetic material is enclosed in a nucleus. Animal and plant cells are eukaryotic.
- Prokaryotic cell
- A smaller, simpler cell with no nucleus: the DNA is a single loop, sometimes with extra small rings called plasmids. Bacteria are prokaryotes.
- Diffusion
- The spreading out of particles from an area of higher concentration to an area of lower concentration. It needs no energy input.
- Osmosis
- The diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
- Active transport
- The movement of substances from a more dilute solution to a more concentrated solution, against the concentration gradient. It requires energy from respiration.
- Mitosis
- The part of the cell cycle in which one cell divides to form two genetically identical cells, used for growth and repair.
- Stem cell
- An undifferentiated cell that can divide to produce more cells like itself, and can differentiate into specialised cell types.
- Differentiation
- The process by which a cell becomes specialised for a particular job, such as a nerve cell or a root hair cell.
Practice
Try a Cell biology question
A GCSE-style original question from this topic. Have a go before you open the working - deciding on an answer first is what makes the working stick.
A cell has an image size of 40 mm and a real size of 20 µm. Work out the magnification. (1 mm = 1000 µm.)
- 20000
- 2
- 2000
- 800000
Show the answer and the working
Answer: 2000
Convert to the same unit first: 40 mm = 40000 µm, then 40000 ÷ 20 = 2000.
- Magnification = image size ÷ real size, but the units must match.
- Convert 40 mm to µm: 40 × 1000 = 40000 µm.
- 40000 ÷ 20 = 2000, so the magnification is ×2000.