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Cell Structure And Function

Cell structure and function (ESG4S)

Cell theory (ESG4T)

The cell theory developed in 1839 by microbiologists Schleiden and Schwann describes the properties of cells. It is an explanation of the relationship between cells and living things. The theory states that:

  • all living things are made of cells and their products.
  • new cells are created by old cells dividing into two.
  • cells are the basic building blocks of life.

The cell theory applies to all living things, however big or small. The modern understanding of cell theory extends the concepts of the original cell theory to include the following:

  • The activity of an organism depends on the total activity of independent cells.
  • Energy flow occurs in cells through the breakdown of carbohydrates by respiration.
  • Cells contain the information necessary for the creation of new cells. This information is known as ‘hereditary information’ and is contained within DNA.
  • The contents of cells from similar species are basically the same.

DNA (the hereditary information of cells) is passed from ‘parent’ cells to ‘daughter’ cells during cell division. You will learn more about this in the following chapter: Cell division.

Cells are the smallest form of life; the functional and structural units of all living things. Your body contains several billion cells, organised into over 200 major types, with hundreds of cell-specific functions.

Some functions performed by cells are so vital to the existence of life that all cells perform them (e.g. cellular respiration). Others are highly specialised (e.g. photosynthesis).

Figure 2.9 shows a two-dimensional drawing of an animal cell. The diagram shows the structures visible within a cell at high magnification. The structures form the ultrastructure of the cell.

Figure 2.9: Diagram of the cell ultrastructure of an animal cell.

We will now look at some of the basic cell structures and organelles in animal and plant cells.

Cell wall (ESG4V)

The cell wall is a rigid non-living layer that is found outside the cell membrane and surrounds the cell. Plants, bacteria and fungi all have cell walls. In plants, the wall is comprised of cellulose. It consists of three layers that help support the plant. These layers include the middle lamella, the primary cell wall and the secondary cell wall.

Middle lamella: Separates one cell from another. It is a thin membranous layer on the outside of the cell and is made of a sticky substance called pectin.

Primary cell wall: Is on the inside of the middle lamella and is mainly composed of cellulose.

Secondary cell wall: Lies alongside the cell membrane. It is is made up of a thick and tough layer of cellulose which is held together by a hard, waterproof substance called lignin. It is only found in cells which provide mechanical support in plants.