Observing cell structure
· There are two types of microscopes: light (up to X1500 and low resolution); and electron (up to X500 000 and high resolution).
· Resolution is how well small, close objects can be seen separately. High resolution produces detailed images of cells (ultrastructure).
· Specimens need preparation to make structures visible. Light microscopes need stains (e.g. acetic orcein for DNA). Electron microscopes need lead salts to scatter electrons and produce images. The pictures produced are called micrographs.
The magnification of a micrograph is the observed size/actual size.
Cell structure and function
· Organelles; nucleus, nucleolus (makes ribosomes), mitochondria (make ATP for cellular energy), lysosomes (contain lytic enzymes), chloroplasts (plant cells only), centrioles (animal cells only, aid cell division), cilia and flagella (beat to produce cell movements).
· Ribosomes make proteins, rough endoplasmic reticulum transports the protein to Golgi apparatus which packages and secretes it out of the cell.
· Eukaryotic cells have a true nucleus and membrane-bound organelles. Prokaryotes have naked DNA and small organelles with no membranes around them.
· The fluid mosaic is a phospholipid bilayer with scattered proteins.
· The cell surface membrane is for transport (partially permeable) and recognition/signalling (e.g. receptor molecules for hormones).
· Passive transport (diffusion/facilitated diffusion) does not use energy, while active transport does and is against a concentration gradient.
Osmosis is diffusion of water from high water potential to low across a cell membrane. Endocytosis is bulk movement of fluid/particles into a cell. Exocytosis is movement to the outside of the cell.
The cell cycle
· The life cycle of a dividing cell is the cell cycle. It mostly involves copying and checking genetic information. The final small part involves mitosis which forms two new cells.
· Cells can continue a cell cycle or they differentiate.
· The main stages of mitosis are: Prophase (chromosomes thicken, become visible); Metaphase (chromosomes line up on the equator); Anaphase (chromatids separate); and Telophase (each set of chromatids forms a new nucleus).
· New cells produced in mitosis are genetically identical (same chromosome combinations) to each other and the parent cell. Cells formed by meiosis are not genetically identical.
· Some mitosis (e.g. bone marrow) produces stem cells. These can differentiate into many different cell types (e.g. red blood cells, neutrophils). They are specialised for their function (e.g. epithelial or guard cells).
Cells are organised into tissues (e.g. squamous or ciliated epithelium; xylem/phloem) which are organised into organ systems.
· The alveolus wall is an efficient exchange surface as it is only one cell thick. It is moist and is highly folded for a large surface area.
· Alveoli are supplied with a rich network of capillaries which carry blood close to the alveolus wall (the exchange surface).
· Muscles move air in and out of the lungs (ventilation). Along with the blood supply, this keeps up a concentration gradient of O2 & CO2.
These features increase the rate of diffusion of O2 into the blood from the alveolus and CO2 out of the blood into the alveolus.
Lung structure and function
· The trachea and bronchi have rings of cartilage which keep them open for airflow during ventilation.
· Smooth muscle contracts and narrows the bronchi and bronchioles and elastic tissue opens these airways. This controls airflow.
· Goblet cells in the lining of the trachea, bronchi and bronchioles secrete mucus which traps particles (e.g. pollen and bacteria).
· Ciliated epithelial cells in the lining beat upwards. This removes any swallowed mucus or particles, keeping the lungs clean.
Animal transport systems
· Large multicellular animals have a small surface area for their volume, resulting in a large distance for diffusion of gases.
· They need a special transport (blood) system to supply O2 and remove CO2, especially if very active (e.g. birds, mammals and fish).
· Single system (fish): heart – gills – body – heart. Double system (mammals): left heart – body – right heart – lungs – left heart.
Closed circulatory system (fish, birds and mammals): blood stays in blood vessels. Open system (insects): blood leaves vessels.
· The left ventricle wall (thick) pumps blood around the body. The right ventricle wall (thinner) only has to pump blood to the lungs.
· Atrial walls are very thin since they only have to pump blood a short distance into the ventricles.
· Cardiac cycle: chambers fill; ventricles contract (cuspid valves close, “lub” sound); atria contract (semi-lunar valves close, “dub” sound).
The Sino-Atrial Node (in right atrium) maintains beat rhythm. The AtrioVentricular Node and Purkyne fibres pass the beat on to ventricles.
· Blood contains cells, plasma proteins and dissolved substances.
· Tissue fluid is blood minus cells and plasma proteins.
· Haemoglobin (Hb) in red blood cells picks up O2 easily at the lungs where O2 pressure is high (Hb dissociation curve is to the left and S-shaped) and releases it at tissues where O2 pressure is low.
CO2 from tissues combines with Hb, making it release O2 (CO2 makes the dissociation curve move to the right). This Bohr effect means more O2 for active tissues. The CO2 diffuses into plasma.
Plant transport - water and xylem
· Water and dissolved minerals taken up by roots are carried by an open tube of dead cells (xylem vessels) up the stem to leaves.
· Water gets to the xylem across the root cortex by the apoplast (cell wall) and symplast (cytoplasm) pathways.
· The waterproof Casparian strip in endodermis cells forces water into the symplast. This creates root pressure in the xylem.
The transpiration stream pulls water up the xylem. Cohesion holds water molecules together; adhesion holds them to the xylem walls.
Plant transport - translocation and phloem
· Phloem is a living tissue which carries substances (e.g. sucrose, growth substances) around the plant. This is called translocation.
· Translocation occurs from where sucrose is produced (the source, e.g. leaves) to where they are used (the sink, e.g. roots, meristems).
· Translocation requires energy to actively pump (load) the sucrose into the source. Sucrose is used at the sink, and mass flow occurs.
Two cell types in phloem: sieve tubes (tube of cells) and companion cells (have organelles and move sucrose into sieve tubes).