8th Class Science Cell - Structure and Functions Cell

Cell

Synopsis

• The structural and functional unit of living organisms is a cell. Organisms that consist of only one cell are called unicellular. A tissue is made of cells. Tissues organise to form organs which in turn form organ systems and the organism.

• Cells without well organised nucleus, i.e., lacking nuclear membrane, are called prokaryotic cells and cells with definite nucleus are called eukaryotic cells.

• The cell is enclosed by a cell membrane enclosing the cytoplasm, nucleus and cell organelles.

• The cell membrane is selectively permeable, i.e., it lets the passage of certain substances only.

• The cytoplasm contains organelles, water and many dissolved substances such as proteins, carbohydrates, inorganic substances, fats, etc.

• The nucleus is bound by the nuclear membrane. Inside it, the nucleoplasm and a dense network of chromatin are present. Chromatin condenses during cell division and separates into chromosomes which bear genes.

• Mitochondria are the power houses of the cell. They use oxygen to oxidise food and release energy

• Endoplasmic reticulum helps in the transport of substances within the cell. It is a network of tube like structures.

• Golgi body is a stack of tubes and vesicles. It helps in the synthesis and storage of many substances. Some vesicles form lysosomes that digest and destroy cells. Hence they are called suicidal bags.

• Centrioles are present in animal cells. These help in the formation of astral fibres during cell division.

• Plastids are exclusively present in plants. They are of three types. Chloroplasts are photosynthetic in function. Leucoplasts are for storage and chromoplastsare coloured.

• Vacuoles are present in plant cells. It is covered by a tonoplast and has a vacuolar sap which stores food, wastes and water.

• Plant cells have a cell wall. It is thick, rigid and is made of cellulose. It gives support, shape and protection to the cell.

• The outermost region of the stem of a plant is called the epidermis. Ground tissue occupies most of the inner region. It consists of different types of tissues. The central core is called the pith, while the region inside the epidermis and extending up to the vascular bundles is called the cortex.

• The vascular bundles are distinct groups of cells arranged in a ring inside the stem. They consist of xylem and phloem tissues. Xylem cells transport water and minerals, while phloem cells transport food.

• There are four types of animal tissues epithelial, muscular, connective and nervous. Epithelial tissue is protective. Connective tissue has cells embedded in a matrix. Muscular tissue helps in movement. Nervous tissue consists of nerve cells. It makes up the brain and spinal cord.