Cell Biology

Cells are the basic units of life. Eukaryotic cells, such as animal and plant cells, contain genetic material inside a nucleus, while prokaryotic bacterial cells are smaller and have circular DNA free in the cytoplasm, often with additional plasmids.

Animal cells typically contain a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. Plant cells contain these structures plus a cellulose cell wall, chloroplasts and a permanent vacuole filled with cell sap.

Cell structures are linked to function. The nucleus contains genetic material, the cell membrane controls movement of substances, mitochondria are the main site of aerobic respiration, ribosomes make proteins, chloroplasts carry out photosynthesis and the cellulose cell wall strengthens plant cells.

Cells become specialised for particular functions. Examples include sperm cells, nerve cells and muscle cells in animals, and root hair cells, xylem cells and phloem cells in plants. Their structures are adapted to help them perform their roles efficiently.

Cell differentiation is the process by which an unspecialised cell develops into a specialised cell. Most animal cells differentiate early in development, whereas many plant cells, especially those in meristems, can continue differentiating throughout life.

Microscopy allows cells and sub-cellular structures to be studied. Light microscopes can show whole cells, while electron microscopes have much greater magnification and resolution, allowing much smaller structures to be seen in greater detail.

Magnification is calculated using $magnification=\frac{image\ size}{real\ size}$. Image size and real size must be in the same units. Useful conversions include $1\text{ mm}=1000\text{ μm}$ and $1\text{ μm}=1000\text{ nm}$. Resolution describes the ability to distinguish two nearby points as separate.

Microorganisms can be cultured in nutrient broth or on nutrient agar. Aseptic technique reduces contamination by using sterile media and equipment, sterilised inoculating loops and minimal opening of Petri dishes. School cultures are generally incubated at about $25^\circ C$.

Bacteria reproduce by binary fission and can multiply rapidly under suitable conditions. Population size can be estimated using $N=N_0\times2^n$, where $N_0$ is the starting population and $n$ is the number of divisions. Zones of inhibition around antibiotics or antiseptics can be compared, and circular areas can be calculated using $A=\pi r^2$.

Chromosomes are long DNA molecules found in the nucleus and contain many genes. Before mitosis, DNA is replicated. During the cell cycle the cell grows, copies its DNA and then divides to produce two genetically identical daughter cells.

Mitosis is important for growth, development, repair and some forms of asexual reproduction. Stem cells are undifferentiated cells that can divide and differentiate. Embryonic stem cells can form many cell types, adult bone marrow stem cells form several blood cell types, and plant meristem cells can form new plant tissues.

Stem cell therapy may help replace damaged cells and could be used to treat conditions such as some forms of paralysis or diabetes, but it also raises possible medical, ethical and religious concerns. Therapeutic cloning can produce genetically matched cells, while plant stem cells are used to clone rare plants and crops with useful characteristics.

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Its rate is affected by concentration gradient, temperature, surface area and diffusion distance. Larger multicellular organisms need specialised exchange surfaces and transport systems because their surface area to volume ratio is lower.

Efficient exchange surfaces usually have a large surface area, a thin membrane and, in animals, a good blood supply and ventilation. Surface area to volume ratio can be expressed as $SA:V=\frac{surface\ area}{volume}$, and smaller organisms generally have a larger ratio than larger organisms.

Osmosis is the diffusion of water through a partially permeable membrane from a dilute solution to a more concentrated solution. Percentage change in mass can be calculated using $\%\ change=\frac{final\ mass-initial\ mass}{initial\ mass}\times100$. Active transport moves substances against a concentration gradient using energy from respiration, for example mineral ion uptake by root hair cells and glucose absorption in the small intestine.