Chapter 2- 2.5- The ultrastructure of plant cells Flashcards
Explain the cellulose cell wall ?
Plant cells, unlike animal cells, are rigid structures.
They have a cell wall surrounding the cell-surface membrane.
Plant cell walls are made of cellulose, a complex carbohydrate. They are freely permeable so substances can pass into and out of the cell through the cellulose wall.
The cell walls of a plant cell give it shape.
The contents of the cell press against the cell wall making it rigid.
This supports both the individual cell and the plant as a whole.
The cell wall also acts as a defence mechanism, protecting the contents of the cell against invading pathogens. All plant cells have cellulose cell walls.
What are the structures unique to plants?
Vacuole
Chloroplasts
Explain the vacuole.
Membrane lined sacs in the cytoplasm containing cell sap.
Many plant cells have large permanent vacuoles which are very important in the maintenance of tugor, so that the contents of the cell push against the cell wall and maintain a rigid framework for the cell.
The membrane of a vacuole in a plant cell is called the tonoplast.
It is selectively permeable, which means that some small molecules can pass through it but others cannot.
If vacuoles appear in animal cells, they are small and transient (not permanent).
Explain the chloroplast.
Chloroplasts are the organelles responsible for photosynthesis in plant cells. They are found in the cells in the green parts of plants such as the leaves and the stems but not in the roots.
They have a double membrane structure, similar to mitochondria. The fluid enclosed in the chloroplast is called the stroma.
They also have an internal network of membranes, which form flattened sacs called thylakoids.
Several thylakoids stacked together from a granum (plural grana).
The grana are joined by membranes called lamellae. The grana contain chlorophyll pigments, where light-dependent reactions occur during photosynthesis.
Starch produced by photosynthesis is present as starch grains. Like mitochondria, chloroplasts also contain DNA and ribosomes.
Chloroplasts are therefore able to make their own proteins.
The internal membranes provide the large surface area needed for the enzymes, proteins and pigment molecules necessary in the process of photosynthesis.
