CELL ULTRASTRUCTURE
• Chloroplasts
• Amyloplasts
• Vacuole
• Tonoplast
• Plasmodesmata
• Pits
• Cellulose cell wall
• Middle lamella
CHLOROPLASTS
belong to a family of closely related plant organelles called plastids. They contain pigment chlorophyll.
They are the site of photosynthesis, where energy from the sun is used to make storage molecules. The chloroplasts have a double membrane within which is another membrane system arranged into flattened sacs called thylakoids. These can be stacked to form structures called grana. The chloroplasts measure about 2μm by 5μm.
AMYLOPASTS
also, plastids, which store starch in the cytoplasm.
CENTRAL VACUOLE
have varied roles depending on the type of plant cell. Generally they are used as a disposal site for metabolic by-products that could endanger the cell if they remained in the cytosol. They may also be used for storage of organic compounds such as proteins in seed cells, may be enriched with pigments to colour the cells in petals, or may contain compounds that are poisonous or unpalatable to animals.
They have a very important role in cell growth as they can enlarge with the cell having a minimal investment in new cytoplasm. The vacuole is part of the endomembrane system
TONOPLAST
This is the membrane of the central vacuole also part of the endomembrane system.
CELLULOSE CELL WALL
This protects the plant cell, maintains its shape and prevents excessive uptake of water. The strong cell walls together hold the plant upright. The cell wall is much thicker than the plasma membrane, ranging from 0.1μm to several micrometers.
• Cellulose is a polysaccharide and is in part responsible for the plant’s “strength”.
• Cellulose is a polymer of glucose but varies slightly from that which it forms with starch.
• a glucose forms starch and β glucose forms cellulose
• Cellulose is formed through a condensation reaction between the –OH group on the first carbon of one glucose group and the –OH on the fourth carbon of the second glucose group. Note that the second glucose molecule is
rotated through 180°.
• This forms a 1,4 glycosidic bond forms.
MICROFIBRILS
• Hydrogen bonds form between the –OH groups in the neighbouring cellulose chains, forming bundles called microfibrils.
• The individual hydrogen bonds are relatively weak compared to the glycosidic bonds. However, due to the large number of bonds, it produces a strong structure.
• Microfibril bundles are made up of about 60 – 70 cellulose molecules.
• A composite structure is created by the bundles winding around the cell and being stuck together by hemicelluloses and pectins.
These are short, branched polysaccharides.
• Pectins also make up the middle lamella, found between adjacent cells and is responsible for holding the cells in place.
• Young cells first construct primary cell walls, which are thin and flexible.
• When the cells matures and stops growing, it then strengthens its wall by adding a secondary cell wall between the plasma membrane and the primary cell wall.
MINERAL DEFICIENCIES IN PLANTS
• Plants get their minerals from the soil (they are dissolved in the soil water).
• They are then transported around the plant via the xylem.
• However the concentrations of these minerals in the soil is very low.
• This means that in order for the plant to get the minerals into the root hair cells they must move minerals up a concentration gradient by active transport.
• What does this mean?
The are two types of nutrients:
MICRONUTRIENTS
Micronutrients: Present in small amounts in plant tissues e.g. Iron, copper and zinc.
MACRONUTRIENTS
Macronutrients: Present in large amounts in plant tissues e.g. Carbon, hydrogen, oxygen, nitrogen, phosphorous, magnesium and potassium.
In plant fertilisers often a mixture of different micro and macronutrients is used.
CALCIUM (CA)
New leaves (top of plant) are distorted or irregularly shaped. Causes blossom-end rot.
NITROGEN (N)
General yellowing of older leaves (bottom of plant). The rest of the plant is often light green.
MAGNESIUM (MG)
Older leaves turn yellow at edge leaving a green arrowhead shape in the center of the leaf.
PHOSPHORUS (P)
Leaf tips look burnt, followed by older leaves turning a dark green or reddish-purple.
POTASSIUM (K)
Older leaves may wilt, look scorched. Interveinal chlorosis begins at the base, scorching inward from leaf margins.
SULFUR (S)
Younger leaves turn yellow first, sometimes followed by older leaves.