This topic covers what cells contain, how they are viewed, how they divide, and the three ways substances move in and out of them.
Eukaryotic cells, meaning animal, plant and fungal cells, have their genetic material enclosed in a nucleus. Prokaryotic cells, meaning bacteria, do not: their DNA is a single loop free in the cytoplasm, with extra small rings called plasmids.
Prokaryotic cells are much smaller, typically a few micrometres across against a few tens for eukaryotic ones. Bacteria also have a cell wall, but it is not made of cellulose.
Both have a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes. Plant cells also have a cellulose cell wall, a permanent vacuole of cell sap, and usually chloroplasts.
Ribosomes are where proteins are made; mitochondria are where most of the reactions of aerobic respiration take place; chloroplasts are where photosynthesis happens.
Cells become specialised so that their structure suits their function:
Differentiation is the process by which a cell acquires its specialised structure and function.
In animals, most cells differentiate early and afterwards divide mainly to repair and replace. In plants, many cells keep the ability to differentiate throughout life, so a cutting can grow into a whole new plant.
A light microscope magnifies up to about ×2000 and has limited resolution: the smallest distance between two points that can still be told apart. An electron microscope has much higher magnification and much higher resolution, so sub-cellular structures can be seen in detail.
magnification = image size ÷ real size. Note the units: 1 mm = 1000 µm.
The nucleus contains chromosomes, made of DNA, each carrying a large number of genes. In diploid body cells they occur in homologous pairs, 23 pairs in a human body cell.
The cell cycle has two main stages. First the cell grows, the number of sub-cellular structures increases and the DNA replicates so each chromosome forms an exact copy of itself. Then, in mitosis, one set of chromosomes is pulled to each end of the cell, the nucleus divides, and finally the cytoplasm and membranes divide to form two genetically identical cells.
Mitosis is used for growth, repair, replacement of worn-out cells, and asexual reproduction.
A stem cell is an undifferentiated cell that can divide to produce more cells of the same type, and can then differentiate into other types.
Human embryonic stem cells can form almost any kind of cell; adult stem cells, such as those in bone marrow, form a more limited range. In plants, the meristem tissue at the tips of roots and shoots can differentiate throughout the plant’s life.
Stem cells could treat conditions such as diabetes and paralysis, and therapeutic cloning could produce cells genetically very similar to the patient, greatly reducing the risk of immune rejection. Against that: the procedures are difficult and expensive, there is a risk of viral transmission, and some people have ethical or religious objections to using embryos.
Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration, down a concentration gradient. It is a passive process, needing no energy from the cell.
Oxygen and carbon dioxide move by diffusion in gas exchange, and urea diffuses from cells into the blood plasma.
The rate depends on four important factors. A steeper concentration gradient, a higher temperature, a larger surface area of membrane and a shorter diffusion distance all make diffusion faster.
Fick’s law puts those factors together: the rate of diffusion is proportional to the surface area multiplied by the concentration difference, and inversely proportional to the diffusion distance.
So doubling the surface area doubles the rate, while doubling the thickness of the membrane halves it.
Osmosis is the net movement of water across a partially permeable membrane, from a dilute solution to a more concentrated one. Equivalently, water moves from a region of higher water concentration to a region of lower water concentration.
It is a special case of diffusion, and it is also passive. A plant cell in pure water takes water in and becomes firm, or turgid; in a concentrated solution it loses water, and the membrane pulls away from the cell wall, which is plasmolysis. An animal cell has no wall, so it may burst in water or shrivel in a concentrated solution.
Active transport moves substances against a concentration gradient, from a lower to a higher concentration, across a membrane.
Because it moves substances against their concentration gradient, it requires energy from respiration. Cells carrying out high rates of active transport often contain many mitochondria. Root hair cells absorb mineral ions from very dilute soil water this way, and the small intestine absorbs glucose when its concentration in the gut is already lower than in the blood.
Bacteria are grown on nutrient agar in a Petri dish. Aseptic technique keeps unwanted microbes out: sterilise the equipment and the medium, pass the inoculating loop through a flame, and hold the lid at an angle rather than lifting it off.
Tape the lid so it cannot fall off, but do not seal it completely, since that would encourage harmful anaerobic bacteria. In schools the cultures are incubated at no more than 25 °C, which reduces the chance of growing pathogens that thrive at body temperature.
To measure how well an antibiotic works, place a disc soaked in it on the agar; the clear zone of inhibition around it shows where bacteria have not grown, and its area can be calculated with πr².
