- Created by: PriyalW
- Created on: 27-02-20 14:01
4.7 Structures in a Plant Cell
Chloroplasts These are where photosynthesis occurs.
Amyloplasts Storage vacuoles where starch is found.
Vacuole This is made up of water, minerals, enzymes, and waste products, the vacuole keeps the plant cell turgid (firm). The vacuole also breaks down unwanted substances.
Tonoplast The membrane surrounding the vacuole.
Plasmodesmata These are channels linking adjacent plant cells, allowing for the communication and transport of substances between cells.
Pits These are parts of the cell wall where the wall is thinner, allows for the transport of substances between cells.
Middle lamella The layer between plant cells, acts as an adhesive layer sticking the plant cells together.
4.7 & 4.9 Plant Cell Walls
Plant cell walls are made up of cellulose. Cellulose is a polysaccharide made up of β-glucose. The glucose is joined in a condensation reaction, and a 1,4 glycosidic bond forms, the condensation reactions form a long unbranched molecule.
A cellulose chain contains 1000-10000 glucose units, these chains stay as a straight line and hydrogen bonds form between the -OH groups in neighbouring cellulose chains, this forms bundles called microfibrils. The large number of hydrogen bonds means that the microfibril has a strong structure.
The microfibrils contain 60-70 cellulose molecules. The microfibrils wind in a helical arrangement around the cell and are stuck together with a polysaccharide glue. This glue is made up of short, branched polysaccharides called hemicelluloses and pectins. These bind to the surface of the cellulose and to each other, holding the microfibrils together.
The microfibrils are laid at different angles to each other in a matrix of hemicelluloses and pectins, this makes the structure strong, but pliable.
Core Practical 6
CP6- Identify sclerenchyma fibres, phloem sieve tubes and xylem vessels and their location within stems through a light microscope.
Rhubarb, microscope slide, coverslip, mounted needles, forceps, watch glass, methylene blue (1%), glycerol (50%), paper towel.
1.Place a piece of rhubarb on a watchglass, use forceps to pick out a vascular bundle and place of a microscope slide.
2. Use mounted needle to tease the bundles apart. Cover with a drop of methylene blue and leave for 5 minutes.
3. Draw off the extra stain with some paper. Place a drop of dilute glycerol on the fibres and mount under a coverslip. Press down with your thumb to separate the tissue. Do not move the slide.
4. Examine under low, medium, high magnification.
Xylem vessels form tubes for transport of water and minerals. The xylem is made up of of large cells with thick cell walls, they form a column of cells, acting as a tube for the transport of water mineral ions. Because the xylem vessels transport water, they need to be waterproof. The polymer lignin enters the cellulose cell wall, so the cells become lignified, meaning that entry of water and solutes is restricted. The tonoplast in the cells breaks down, autolysis occurs and organelles in the cells are lost, meaning that dead, empty cells form the tube. The microfibrils and lignin in the cell walls of the xylem vessels give them strength and therefore supports the plant.
What is lignin? Found in sclerenchyma and xylem cell walls, it is a hydrophobic polymer that hardens, and strengthens the cellulose cell wall.
As water evaporates from cell walls in the plant, its replaced by water using capillary action in the cell walls, drawing water to the cells. Water moves up the xylem vessels and through the cell walls in a continuous stream called the transpiration stream.
The xylem provides a method for the movement of water, this is a mass flow system for the transport of inorganic ions.
The phloem is made up of a column of long, narrow cells, these cells are alive, unlike those in xylem. The ends of the sieve tube have holes that are aligned with those of the neighbouring cell, this allows for the transport of materials. The ends of the sieve tube are called the sieve plates, amd the section of the phloem sieve tube between the plates is called the sieve tube element.
The transport of organic molecules within the phloem is called translocation. The main substances that are transported are sugars and amino acids. When minerals are taken from the roots to the leaves, the xylem is used, however, as this is one-way, the phloem is used when minerals are transported from part of the plant to another. Growth regulators and viruses may also travel in the phloem.
Alongside a sieve tube is a companion cell, as the sieve tube doesn't have a nucleus, mitochondria, ribosomes, rER, the companion contains these to perform the metabolic functions that maintain the sieve tube.
Mass transport is used in the sieve tubes to transport the solutes through the phloem, this works because loading the tubes with solutes increases the concentration, drawing water into the tube via osmosis.
4.11 Parenchyma and Sclerenchyma
Parenchyma is a plant tissue found throughout the plant, the parenchyma cells fill in the gaps between more specialised cells, and sometimes they have certain specialised functions, such as storage. The parenchyma cells have a role in supporting the plant.
The parenchyma can be modified to form scleremchyma. The sclerenchyma have strong, thick cell walls made up of cellulose microfibrils, which are strengthened with lignin. The lignin is deposited on cell walls of fibres formed by sclerenchyma in a spiral or ring pattern. This makes the fibres strong but flexible.
Once lignified, the cells die as lignin makes the cell waterproof so water cannot enter the cell. The cells in the sclerenchyma fibres are dead and empty. Sclerenchyma are found around the vascular bundles in older stems. As the sclerenchyma fibres can be completely impregnated with lignin, this makes them strong.
4.12 Importance of Ions to Plants
Nitrate These ions are needed to make amino acids, and other important biological molecules such as chlorophyll, nucleic acids, ATP, and some plant growth substances. This can cause yellow leaves due to a lack of chlorophyll pigment.
Calcium A lack of calcium causes stunted growth, this is because calcium has a role in the structure of the cell wall and in the permeability of the cell membrane.
Magnesium A lack in magnesium means that the plant is unable to make chlorophyll, this means that the leaves become yellow with reddish-brown tints.
4.12 Importance of Water to Plants
Cohesion and Surface Tension Hydrogen bonding between water results in strong cohesive forces between water, this keeps the water together as a continuous column in xylem. Surface tension caused by the cohesive forces, means that water can move up the capillary-like tubes in the cells.
Solvent Water can dissolve substances, once dissolved, the substances can move around freely. Water is also involved in reactions in plants, such as hydrolysis and condensation.
Thermal Properties Water has a high specific capacity, meaning a lot of energy is needed to cause a small increase in temperature. Water warms and cools slowly , this helps plants to avoid rapid changes in the internal temperatures, enabling them to maintain a fairly steady temperature.
Density and Freezing Properties Water expands when it freezes, as the water cools, the molecules slow down, this allows for the maximum number of hydrogen bonds to form. These hydrogen bonds hold the water molecules further apart than in liquid water, making ice less dense, this allows ice to float, allowing organisms to survive in the liquid water underneath ice.
Core Practical 7
CP7- Investigate plant mineral deficiencies
Independent=minerals present, dependent=physical characteristics of plant, control= volume of solution used, species of plant, amount of light recieved, time, temperature, volume of water used.
Mung beans, ruler, test tubes and rack, Sach's solution, Sach's solution lacking in either Mg, K, Ca, N, or P, black paper, rubber band.
Germinate mung beans for 3 days. Set up tubes containing the Sach's solution and the soluions lacking in each of the minerals. Weight the mung beans individually. Place the mung bean with its roots in solution and plug with some cotton wool at the top. Wrap some black paper around the tubes to prevent algal growth. Leave under a light bank, to keep light intensity and temperature constant, topping up the solutions as required. After 4 weeks weigh the mung beans. Calculate the percentage increase in mass for each of the beans. Observe any physical characteristics.
Core Practical 8
CP8- Determine the tensile strength of plant fibres.
Independent=type of fibre used, dependent=amount of mass that can be added before the fibre snaps.
Celery, clamp, clamp stand, Hoffman's clip, weights (1N increments), white tile, knife, ruler.
Remove a fibre from the celery using the knife and white tile. Record the length and width of the fibre. Connect onto a clamp and Hoffman's clip, and gradually add mass until the fibre snaps. Record the mass that was added before the fibre snapped. Repeat at least 5 times. Work out the cross-sectional area of the fibre. Divide the mass by the cross-sectional area, this will give the tensile strength.