Organisation of a multicellular organism

  1. The cell is the most basic unit of a living organism that can be classified as living.
  2. A group of cells of the same type that are found near each other and carry out the same function comprises a tissue.
  3. An organ is made up of different tissues working together to perform a specific function or a group of functions within an organism. An organ has a distinct shape which allows it to carry out its function well.
  4. A group of functionally-related organs form an organ system.
  5. Many organ systems working together make up a multicellular organism.

Cell structure and organisation

All living things are made of cells.

All (typical) cells have: (i.e some for example the red blood cell do not have all these things, no nucleus)

Cell Membrane: a membrane that controls the entry and exit of dissolved substances and separates the cell’s contents from its surroundings.

Cytoplasm: contains water and dissolved substances such as sugars and salts

Nucleus: contains the genetic material (DNA). This carries the coded instructions for controlling the activities and characteristic of the cell.

Mitochondria: organelle where aerobic respiration happens.

A typical animal cell (e.g the liver cell) has all the above things.

Only plant cells have:

Chloroplast: Small organelle which contains chlorophyll (dye used for light absorption) and enzymes necessary for the production of glucose by photosynthesis.

(Large permanent) Vacuole: contains water necessary to provide turgor pressure and may store ions and molecules.

Cellulose cell wall: provides structural support, permeable for dissolved substances and water and prevents damage when the cell is in a hypotonic solution i.e. cell can’t explode.

A typical plant cell (e.g. the palisade cell) has all the above things.

Cytoplasm

Under the ordinary microscope (light microscope), cytoplasm looks like a thick liquid with particles in it. In plant cells it may be sen to be flowing about. The particles may be food reserves such as oil droplets or granules of starch. Other particles are structures known as organelles, which have particular functions in the cytoplasm. In the cytoplasm, a great many chemical reactions are taking place which keep the cell alive by providing energy and making substances that the cell needs.

The liquid part of cytoplasm is about 90% water with molecules of salts and sugars dissolved in it. Suspended in this solution there are larger molecules of fats (lipids) and proteins. Lipids and proteins may be used to build up the cell structures, such as the membranes. Some of the proteins are enzymes. Enzymes control the rate and type of chemical reactions which take place in the cells. Some enzymes are attached to the membrane systems of the cell, whereas others float freely in the liquid part of the cytoplasm.

Cell Membrane

This is a thin layer of cytoplasm around the outside of the cell. It stops the cell contents from escaping and also controls the substances which are allowed to enter and leave the cell. In general, oxygen, food and water are allowed to enter; waste products are allowed to leave and harmful substances are kept out. In this way the cell membrane maintains the structure and chemical reactions of the cytoplasm.

Nucleus (plural:nuclei)

Most cells contain one nucleus, which is usually seen as a rounded structure enclosed in a membrane and embedded in the cytoplasm. In drawings of cells, the nucleus may be shown darker than the cytoplasm because, in prepared sections, it takes up certain stains more strongly than the cytoplasm. The function of the nucleus is to control the type and quantity of enzymes produced by the cytoplasm. In this way it regulates the chemical changes which take place in the cell. As a result, the nucleus determines what the cell will be, for example, a blood cell, a liver cell, a muscle cell or a nerve cell.

The nucleus also controls cell division, as shown in Figure 1 (a) to (d) . A cell without a nucleus cannot reproduce. Inside the nucleus are thread like structures called chromosomes, which can be seen most easily at the time when the cell is dividing.

Figure 1 – (a)-(d)

Plant Cells

Plant cells differ from animal cells in several ways.

  1. Outside the cell membrane they all have a cell wall which contains cellulose and other compounds. It is non-living and allows water and dissolved substances to pass through. The cell wall is not selective like the cell membrane. (Note that plant cells do have a cell membrane but it is not easy to see or draw because it is pressed against the inside of the cell wall see Figure 1 (e) (f) Under the microscope, plant cells are quite distinct and easy to see because of their cell walls. It is only the cell walls (and in some cases the nuclei) which can be seen. Each plant has its own cell wall but the boundary between two cells side by side does not usually show up clearly. Cells next to each other therefore appear to be sharing the same cell wall.
  2. Most mature plant cells have a large, fluid-filled space called a vacuole. The vacuole contains cell sap, a watery solution of sugars, salts and sometimes pigments. This large, central vacuole pushes the cytoplasm aside so that it forms just a thin lining inside the cell wall. It is the outward pressure of the vacuole on the cytoplasm and cell wall which makes plant cells and their tissues firm. Animal cells may sometimes have small vacuoles in their cytoplasm but they are usually produced to do a particular job and are not permanent.
  3. In the cytoplasm of plant cells are many organelles called plastids. These are not present in animal cells. If they contain the green substance chlorophyll, the organelles are called chloroplasts. Colourless plastids usually contain starch, which is used as a food store. ( Note: the term plastid is not a syllabus requirement.)
Figure 1 – (e) (f) Structure of a palisade mesophyll cell. It is important to remember that, although cells look flat in sections or in thin strips of tissue, they are in fact three-dimensional and may seem to have different shapes according to the direction in which the section is cut. If the cell is cut across it will look like (f); if cut longitudinally it will look like (e).

TableSummary: the parts of a cell

Name of partDescriptionWhere foundFunction
(supplement only)
Animal plant cellscytoplasm jelly-like, with particles and organelles inenclosed by the cell membranecontains the cell organelles, e.g. mitochondria, nucleus site of chemical reactions
cell membranea partially permeable layer that forms a boundary around the cytoplasm around the cytoplasmprevents cell contents from escaping
controls what substances enter and leave the cell
nucleus a circular or oval structure containing DNA in the form of chromosomesinside the cytoplasmcontrols cell division
controls cell development
controls cell activities
plant cells onlycell walla tough, non-living layer made of cellulose surrounding the cell membranearound the outside of plant cellsprevents plant cells from bursting
allows water and salts to pass through (freely permeable)
vacuolea fluid-filled space surrounded by a membraneinside the cytoplasm of plant cellscontains salts and sugars helps to keep plant cell firm
chloroplast an organelle containing chlorophyllinside the cytoplasm of some plant cellstraps light energy for photosynthesis