Plant cell structure and plant polymers (4.7, 4.8, 4.9, 4.10)
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Organelles unique to animal cells are: centrioles and lysosomes (although plants have their own version).
Organelles unique to plant cells are: cell wall, chloroplasts, amyloplasts, tonoplast, plasmodesmata, pits, middle lamella and permanent vacuoles.
Chloroplasts are double membrane–bound organelles found in plant and algal cells that are the site of photosynthesis.
Chloroplasts contain flattened membrane sacs called thylakoids, which contain chlorophyll and are stacked into grana connected by lamellae. These membranes are the site of the light-dependent reactions. The fluid-filled stroma contains enzymes involved in the light-independent reactions.
Chloroplasts also contain circular DNA, ribosomes and starch grains for temporary carbohydrate storage.
A cell wall is a freely permeable, rigid outer layer surrounding the cell–surface membrane in plants, algae and fungi. Plant and algal cell walls are mainly made of cellulose, whereas fungal cell walls are made of chitin.
The cell wall provides mechanical strength and support, helping maintain cell shape and preventing the cell from bursting when water enters by osmosis. The resistance of the cell wall to expansion allows turgor pressure to develop, making plant cells rigid.
Amyloplasts are double membrane-bound organelles found in plant cells that store starch as starch granules. They act as energy storage sites, and the stored starch can be broken down into sugars for respiration when required.
Plasmodesmata are narrow, fluid-filled channels that pass through plant cell walls, connecting the cytoplasm of adjacent cells. They allow the movement and exchange of substances between cells (e.g., ions and sugars) and enable cell-to-cell communication.
In plant cells, the vacuole is a large, membrane-bound sac containing cell sap, a solution of water, sugars, amino acids, ions, pigments, and wastes.
The vacuole is surrounded by a selectively permeable membrane called the tonoplast, which regulates the movement of substances into and out of the vacuole. This helps maintain osmotic balance and ) within the cytoplasm and can isolate harmful substances.
The uptake of water into the vacuole maintains turgor pressure, keeping plant cells rigid and helping support the plant.
Pits are thin regions in plant cell walls where there is no secondary wall thickening. They are often found in pairs between adjacent cells and often contain plasmodesmata, allowing movement of substances between neighbouring cells.
Pits are thin regions in plant cell walls where there is no secondary wall thickening. They are often found in pairs between adjacent cells and often contain plasmodesmata, allowing movement of substances between neighbouring cells.
Cellulose is a structural polysaccharide in plants. It is composed of long, linear chains of -glucose molecules joined by 1,4 glycosidic bonds.
Every other -glucose molecule in cellulose is inverted, producing a linear structure with extensive hydrogen bonding within and between polysaccharide chains.
Cellulose’s high tensile strength is a combined result of several key structural features:
- The strong 1,4 glycosidic bonds within a polysaccharide chain.
- There hydrogen bonding between polysaccharide chains within a microfibril.
- The alignment of microfibrils into macrofibrils.
- The alignment of macrofibrils into fibres.
Individual cellulose chains are held together by hydrogen bonds to form microfibrils.
Microfibrils are organised into larger macrofibrils, further stabilised by additional hydrogen bonds.
The alignment of macrofibrils can vary depending on their role within a plant.
Cellulose is the main structural component of plant cell walls. The cell wall matrix includes other molecules such as lignin, to further increase cell wall strength.
Its structure gives it the following properties:
- Its high tensile strength enables cell walls to withstand turgor pressure and maximise leaf surface area for light absorption.
- Spaces between macrofibrils make cellulose fibres permeable to water and solutes, facilitating diffusion into and out of plant cells.
- Cellulose is a source of dietary fibre (roughage). Few organisms have the enzyme cellulase, which hydrolyses cellulose, meaning that it is not absorbed during digestion.
Cellulose is a polysaccharide formed from -glucose units bonded together with 1,4 glycosidic bonds. Cellulose molecules are straight.
Hydrogen bonds form between parallel cellulose chains, forming fibrous bundles called microfibrils that are laid down at different angles in layers in the plant cell wall. This structure is further reinforced by hemicelluloses and pectins, making cell walls both strong and flexible.
The arrangement of cellulose microfibrils and secondary thickening in plant cell walls gives xylem vessels and sclerenchyma fibres their strength.
Fibre strength depends on length and degree of lignification; the cells in stems of taller plants are reinforced with more lignin.
These plant fibres are used in clothing, rope, flower coverings, and paper because they are long, thin, flexible, and strong.