O LEVEL BIOLOGY COMPLETE NOTES

Cells

Cells are the smallest structural and functional unit of life. A single cell can carry out all the characteristics of a living thing.

An animal cell is different from a plant cell. Almost all cells contain nuclei. Exceptions include red blood cells as they do not have a nucleus in order to efficiently carry out their functions which will be discussed in an upcoming chapter.

A cell consists of protoplasm which includes cell membrane, cytoplasm and nucleus along with double membrane organelles. It's where all the biochemical reactions take place in the cell.

AN OVERVIEW OF ALL CELL STRUCTURES

The NUCLEUS is a a tiny spherical mass of denser protoplasm called nucleoplasm- consisting of a network of long thread-like structures known as chromatin that contain hereditary material such as DNA. Chromatins can condense to form highly coiled rod-shape structures called chromosomes. Along with it is the largest part of the nucleus-a smaller spherical structure called nucleolus (plural: nucleoli). All this is surrounded by the nuclear envelope.

FUNCTIONS OF NUCLEUS:
The nucleus is commonly referred to as the 'headquarter' of the cell's as it controls all cell activities such as repair and reproduction of the cell. As it contains genetic material, it is responsible for inheritance. All proteins are made by the chromosomes.
*NOTE- Ribosomes are also sites of protein synthesis.

The CYTOPLASM is a gel-like substance that usually occupies most of the cell.

FUNCTIONS OF CYTOPLASM:
Most of the chemical reactions occur here. As it is a suspension, it holds the cell structures together and prevents them from mixing up. 90% of the cytoplasm is water.

The CELL SURFACE MEMBRANE is a thin and flexible membrane that surrounds the inner structures i.e cytoplasm.

FUNCTIONS OF CELL MEMBRANE
The cell membrane is partially permeable. Meaning, it controls the entry and exit of substances of the cell- allowing only required substances to move in or out. It also holds and protects inner structures and provides identification to the cell.

Double-membrane organelles such as MITOCHONDRIA or CHLOROPLASTS may also be present.

FUNCTIONS OF MITOCHONDRIA (singular: mitochondrion)
It is commonly referred to as the powerhouse of the cell because it carries out aerobic respiration to produce large amounts of energy which may temporarily be stored in small adenosine triphosphate (ATP) molecules.

FUNCTIONS OF CHLOROPLASTS
They contain a green pigment called chlorophyll which traps the energy from sunlight for photosynthesis.

Sometimes for some organisms an external CELL WALL is also present. The substance from which it is made of can vary. E.g plant cell walls are made of cellulose (carbohydrate) while a fungus cell wall is made of chitin (protein).