Eukaryotic and prokaryotic cells, sub-cellular structures, cell specialisation and differentiation, microscopy and magnification, chromosomes and the cell cycle (mitosis), stem cells, and transport by diffusion, osmosis and active transport.

What this topic is asking

AQA wants you to compare eukaryotic and prokaryotic cells, state the function of each sub-cellular structure, explain cell specialisation and differentiation, use the magnification equation, describe the cell cycle and mitosis, explain what stem cells do, and describe how substances move by diffusion, osmosis and active transport.

Cell types and structures

Animal cells contain a nucleus (controls activities and holds DNA), cytoplasm (where reactions happen), a cell membrane (controls what enters and leaves), mitochondria (aerobic respiration) and ribosomes (protein synthesis). Plant cells have all of these plus a cellulose cell wall for strength, chloroplasts containing chlorophyll for photosynthesis, and a permanent vacuole of cell sap that keeps the cell turgid. A bacterial cell is far smaller (about 1 micrometre versus 10 to 100 micrometres for many eukaryotic cells), and instead of a nucleus its single loop of DNA floats in the cytoplasm with smaller rings called plasmids. Knowing the orders of magnitude matters: AQA expects you to handle sizes written in standard form, such as $1 \times 10^{-6}$ m for a bacterium.

Specialisation, differentiation and microscopy

Cells differentiate to become specialised for a function. Examples to know precisely: sperm cells have a tail and many mitochondria for swimming, nerve cells are long with branched ends to carry impulses, muscle cells contain many mitochondria for contraction, root hair cells have a large surface area for absorbing water and minerals, and xylem and phloem cells are adapted for transport. In animals, most differentiation happens early in development and most mature cells can then only divide for repair; in plants, many cells keep the ability to differentiate throughout life.

The cell cycle, mitosis and stem cells

The cell cycle has three stages: the cell grows and increases the number of sub-cellular structures (such as ribosomes and mitochondria) and the DNA replicates to form two copies of each chromosome; mitosis then divides the nucleus, separating one copy of each chromosome to each end; finally the cytoplasm and membranes divide to form two genetically identical daughter cells. Mitosis is used for growth, repair of damaged tissue, and asexual reproduction. Stem cells are undifferentiated cells that can divide to form many cell types. They are found in early embryos (can become any cell type), in adult bone marrow (can form a limited range, such as blood cells) and in plant meristems (at root and shoot tips, where they form any plant tissue throughout life). Therapeutic uses include treating diabetes or paralysis, but there are ethical concerns about using embryos and a risk of viral contamination or rejection.

Transport in and out of cells

• Diffusion: the net movement of particles from a region of higher concentration to a region of lower concentration, down a gradient. It is passive (it needs no energy from respiration). The rate increases with a steeper concentration gradient, a higher temperature and a larger surface area to volume ratio. Examples include oxygen and carbon dioxide moving across the alveoli and gases moving in and out of leaves.
• Osmosis: the diffusion of water across a partially permeable membrane from a dilute solution (high water potential) to a more concentrated solution (low water potential). This is what moves water into root hair cells and what causes plant cells to become turgid.
• Active transport: the movement of substances against a concentration gradient, from low to high concentration, which requires energy released by respiration. Examples include mineral ions moving into root hair cells from dilute soil water, and glucose being reabsorbed in the small intestine when its concentration there is already low.

Exchange surfaces are adapted to maximise diffusion: they are thin (a short diffusion path), have a large surface area, and (in animals) a good blood supply and ventilation to maintain a steep gradient.