Cells-Structures and Functions

The Cell: The Basic Unit of Life

• All living organisms perform basic functions such as digestion, respiration, and excretion.

• Different organs work together to perform these functions. For example, digestive organs form the digestive system.

• Each organ is composed of smaller parts called

  tissues, which are made up of similar cells.

• The cell is the basic structural unit of an organism, comparable to how bricks are the basic units of a building.

Discovery of the Cell

• In

  1665, Robert Hooke observed thin slices of cork under a simple magnifying device.

• He noticed partitioned boxes that looked like a honeycomb and coined the term "

  cell" for each box.

• The cells Hooke observed were actually dead cells.

• The detailed structure of living cells could only be studied after the invention of more powerful microscopes.

Variety in Cell Number, Shape, and Size

Cell Number

• Organisms can be single-celled or multicellular.

• Unicellular organisms are single-celled (e.g., Amoeba, Paramecium). A single cell performs all necessary functions like capturing food, respiration, excretion, growth, and reproduction.

• Multicellular organisms are made of more than one cell (e.g., humans, elephants, trees). The human body has trillions of cells.

• An organism with billions of cells begins its life as a

  single fertilized egg cell, which then multiplies.

Cell Shape and Size

• Cells have different shapes and sizes related to their specific functions.

• Some cells, like the

  Amoeba, have an irregular shape that constantly changes due to the formation of projections called pseudopodia ("false feet").

• Common cell shapes include round, spherical, or elongated. Some cells are long and pointed at both ends, giving them a

  spindle shape , while others, like

  nerve cells (neurons), are long and branched to transfer messages.

• Cell size has no relation to the body size of the organism. For example, a nerve cell in an elephant is similar in size and function to a nerve cell in a rat.

• Most cells are microscopic and are not visible to the naked eye, but some are large enough to be seen without a microscope, such as a hen's egg.

• The smallest cell is 0.1 to 0.5 micrometres in bacteria, and the largest is the egg of an ostrich, measuring 170 mm x 130 mm.

Cell Structure and Parts

The basic components of a cell are the

cell membrane, cytoplasm, and nucleus.

Cell Membrane

• The cell membrane, also known as the

  plasma membrane, encloses the cytoplasm and nucleus.

• It separates cells from one another and also separates the cell from its surrounding environment.

• The membrane is porous and allows substances to move both inward and outward.

• This membrane provides shape to the cells of plants and animals.

Cell Wall

• This is an

  additional, thick outer layer found in plant cells and bacterial cells, located outside the cell membrane.

• The cell wall provides shape and rigidity to the plant cell and protects it from variations in temperature, wind, and atmospheric moisture.

Cytoplasm

• The cytoplasm is a jelly-like substance that fills the space between the cell membrane and the nucleus.

• Various cell organelles like mitochondria, Golgi bodies, ribosomes, etc., are present in the cytoplasm.

Nucleus

• The nucleus is a spherical, centrally located organelle in a cell. It contains genetic material and controls the cell's activities.