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PinkMonkey Online Study Guide-Biology

Mitochondria : is another organelle of the cell. It is called the "power house of the cell" because it stores and releases the energy of the cell. The energy released is used to form ATP (adenosine triphosphate).

Figure 3.3 A cross-section of a Mitochondrion

Nucleus : Prokaryotic cells donít have a nucleus but eukaryotic cells have a nucleus situated in the cytoplasm. The nucleus mainly contains DNA ( i.e., Deoxyribonucleic acid). DNA is organized into linear units called chromatin. Genes are the functional segments within the chromosome. Each chromosome consists of approximately 1,000,000 genes. The chromatin is coiled around nuclear protiens called histones. When chromatin is coiled, it forms chromasomes. Genes contain the coding for all the protiens in a cell of an animal or plant. The nucleus of the cell is surrounded by an outer membrane called the nuclear envelope. The nuclear membrane resembles the plasma membrane in its function. It is also a double layer membrane consisting of two lipid layers similar to those in the plasma membrane. Pores in the membrane allow the internal nuclear part to communicate with the cytoplasm of the cell.

Prokaryotic cells donít have a nucleus but they do possess DNA which exists freely in the cytoplasm. In bacteria a single looped chromosome consists of 4,000 genes.In plant cells, organelles called chloroplasts exist. Due to chloroplasts plants look green in color. The chloroplasts function in the process of photosynthesis.

During this process, chloroplasts convert the energy in the sunlight into energy of carbohydrate molecules. Energy from the sun comes in the form of photons i.e. a package of energy which gets converted into carbohydrate energy. Chloroplasts consist of a green pigment called chlorophyll. Because chlorophyll molecules absorb most wavelenghths of light except green, they reflect green light and appear green to our eyes. chlorophyll is normally present in that area of a plant where sunlight can reach easily. For example, the leaves and stem of a plant are green. On the contrary, the roots donít have chloroplast so they are not green.

Cytoskeleton : is an interconnecting system of fibers and threads and interwoven molecules that give structure to the cell. The main component of the cytoskeleton are microtubules, microfilaments and intermediate filaments. They are all made up of proteins.

Centriole : is another organelle present in the cell. It is cylindrical in shape and always occurs in pairs. Centrioles are involved in cell division. Vacuole :Another organelle seen in the plant cell is the vacuole. The vacuole forms about 75% of the plant cell. In the vacuole, the plant stores nutrients as well as toxic wastes. If pressure increases within the vacuole it can increase the size of the cell. In this case the cell will become swollen. If the pressure increases further the cell will get destroyed.

Many cells have structures attached to them called Flagella or Cilia. Flagella are seen in the single-celled plant and protozoans, and cilia are commonly seen in animal cells. Flagella are long hairlike extensions that extend from the cell and help in locomotion. Animal sperm has flagella which permit locomotion. Cilia are shorter and more numerous than flagella. Rows of cilia move in waves to move the cell (prokaryotes like paramecia), or to move fluids around the cell (e.g., respiratory epithelial cells).These cells help in the removal of particles from the tract.

Cell Wall : Plant cells possess a cell wall. It is a structure which is present outside the cell membrane. It is not very thick. In bacteria the cell wall is very thick and rigid: this gives shape to the bacteria. In a eukaryote cell, the cell wall is not identical in different animals. In fungi, the cell wall is made up of chitin which is a polysaccharide. In plant cell there is no chitin. Cell walls are composed of another polysaccharide called cellulose.

The cell wall provides support to the structure to the cell. It also saves the cell from mechanical pressure.; it is not a selective (semipermeable) membrane like the plasma membrane. When bacteria enter the human body the cell wall this is recognized as a foreign substance in the body; this is how our immune system recognizes and destroys bacteria.

Table of Contents

3.0 Introduction
3.1 Modern cell theory
3.2 Structure of cell
3.3 Movement through the plasma membrane

Chapter 4


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