Cell Structure Revision Notes

Vacuole:

• Surrounded by a single membrane called the tonoplast
• Contains cell sap

Plant stem structure and function:

Epidermis:

• Outer layer of the stem
• Secrete cutin, a waxy substance that helps to prevent water loss from the stem surface and protects against the entry of pathogens
• Cells may also form hairs, either as an extension of a single cell or from several modified epidermal cells

Parenchyma and collenchyma:

• Parenchyma are unspecialised cells but can be modified to make them suitable for storage and photosynthesis
• Collenchyma cells have thick primary cell walls which are even thicker at their corners- gives tissue strength
• Found around the outside of the stem and give plenty of support but remain living so they stretch as the plant grows

Sclerenchyma:

• Strong secondary walls with lignin on cell walls- makes the fibres strong and flexible
• Short structures with tapered ends that are closed

Xylem:

• Long cylinders made up of a column of cells whose end walls have been broken down
• Ends open and tough lignin present in walls
• Used for support and water and mineral ion transport

Function of starch, cellulose and cellulose microfibrils and plant fibres in plant cells:

• Starch is the energy storage molecule- made up of many alpha glucose units which can be used in respiration
• Cellulose is a major component of plant cell walls formed of many beta glucose units
• Cellulose microfibrils have many hydrogen bonds that together make it strong. The arrangement of cellulose microfibrils and the glue that binds then together give strength and flexibility to plant cell walls, whilst allowing them to be fully permeable to water
• Fibres give support and some allow the transport of water and mineral ions, due to lignin being strong and waterproof

Useful properties of cellulose and plant fibres:

• Properties of cellulose, as microfibrils, produce rope that does not stretch, but is flexible and has great strength
• Lignified plant fibres are very resistant to chemical and enzyme breakdown. This property makes plant fibres, such as wood, a good choice as building material

Plant fibres, starch and sustainability

• Fossil fuels are not sustainable as they are non-renewable resources
• Both starch and plant fibres are renewable resources as they come from plants and more can be produced the next growing season
• Starch can be processed into bioplastic to replace oil based plastics whilst plant fibres can be used for rope
• Both can be burned to release heat energy

Transpiration:

• Loss of water vapour from the surface of the plant, mainly from the leaves
• Once in the leaves, water moves out by osmosis from the xylem into the veins of the leaves into the mesophyll cells
• Water evaporates from the cellulose walls of the spongy mesophyll cells into the air spaces
• The water vapour moves through open stomata into the external air along a diffusion gradient

The uptake of water by plants:

• In the vacuolar pathway, water moves by osmosis across the vacuoles of the cells of the root system. The water moves down a concentration gradient from the soil solution to the xylem
• In the symplast pathway, water moves down the concentration gradient from the root hair cells to the xylem. However, in this pathway, the water moves through the interconnected cytoplasm of the cells of the root system through the plasmodesmata
• In the apoplast pathway, water is pulled by the attraction between water molecules across adjacent cell walls from the root hair cell to the xylem. As water is drawn into the xylem, attraction between the molecules ensures that water is pulled across from the adjacent cell wall and so on. Water moves across the cells of the root in the cell walls until it reaches the Casparian strip

Plants need water for:

• Inflating cells (turgor) which helps keep plants upright
• As a component of chemical reactions such as in photosynthesis
• As the medium in which biochemical reactions occur in cells
• As a transport liquid for mineral ions and organic molecules such as sucrose

Mineral ion deficiency:

• Nitrate: needed to supply nitrogen for making proteins, DNA, RNA. Without it, yellow leaves and stunted growth
• Calcium: needed to form calcium pectate in plant cell walls and to be involved in membrane permeability. Without it, the leaves are crinkly.
• Magnesium: needed as it is a component of chlorophyll and helps in the formation of DNA. Without it, turns yellow and is stunted
• Phosphates: needed for ATP and structural support. Without it, leaves will have purple veins and stunted growth.

William Withering’s digitalis soup:

• Learned of a remedy for treating a heart condition
• Isolated the active ingredient from the 20 or so ingredients in the remedy- it was the digitalis from the foxglove
• He trialled different doses on patients to find the most effective treatment
• He carefully recorded all his findings

A new medicine has to be:

• Effective: it cures or prevents the disease it is designed for, or relieves the symptoms
• Safe: non-toxic and without unacceptable side effects
• Stable: able to be stored for some time and under normal conditions
• Easily taken into and removed from your body: able to get to its target in your body and to be got rid of once it has done its job
• Capable of being make on a large scale: able to be manufactured in a very pure form, in large quantities and relatively cheaply.

Contemporary drug testing protocols:

• Placebo: name given to tablet/treatment that appears identical in all ways to the drug but is chemically inactive
• Double blind trial: patients are randomly divided into 2 groups. One group receives the drug, and the other receives the placebo. Neither the patients nor the people recording the changes in the patients know who has received the drug or who has received the placebo. This reduces the chance of bias
• Three phase testing:

• Phase 1: a few healthy people used with a range of doses to check drug is safe and that it behaves in the manner predicted by animal tests
• Phase 2: 100-300 patients with the condition to check it is patient safe and to check that it works on the condition as anticipated
• Phase 3: 1000-3000 patients used, and normally involves a double blind trial. Used to collect as much data as possible including effectiveness compared to placebo or standard treatment and frequency of any side effects

Modern day testing vs Withering:

• Similarities: both isolated a possible drug/treatment and both initially tested on a small number of patients and then a larger group of patients
• Differences: only modern tests test on animals before phase 1 and only modern tests on healthy people first. Only modern day trials have double blind trials.