Definitions

Cells are the basic structural and functional unit of an organism. All living cells consist of living material called protoplasm : the cell surface membrane, cytoplasm, and the nucleus. Plant cells contain an additional cell wall.

Example of cells in animals include muscle cells, skin cells, nerve cells and red blood cells. Examples of cells in plants include root hair cells and mesophyll cells.

Nucleus

The nucleus contains chromatin,a mass of long threadlike structures. It is made of proteins and deoxyribonucleic acid (DNA), that stores hereditary material and instructions for the cell. Thus, it controls cell activity and plays a role in cell division.

During cell division, the chromatin condense and become chromosomes, which appear as thick rod-shaped structures under a microscope.

Cell surface membrane

The cell surface membrane is a selectively permeable membrane that encloses the contents of a cell. The partial permeability allows regulation of the movement of substances across the membrane.

Cytoplasm

The cytoplasm contains specialised structures called organelles. It is gel-like and composed of water mainly and dissolved proteins, nutrients and enzymes. Most cellular reactions occur here.

Chloroplasts

Chloroplasts are oval structures found in plants ONLY. They are enclosed by a double membrane, have its own DNA and chlorophyll. Chlorophyll absorbs light energy essential for photosynthesis.

Vacuoles

A vacuole is a fluid-filled bubble enclosed by a partially permeable membrane that stores substances within the cell. 

Animal cells have  many small vacuoles containing water and nutrients. Plant cells have a large central vacuole which contains a liquid called cell sap, that consists of dissolved substances such as sugars, mineral salts, and amino acids. Vacuoles in plant cells contribute to turgidity.

Cell wall

The cell wall is the outermost part of the cell. It is composed of cellulose and is fully permeable to small molecules. It provides structural support to the plant and protects the plant from injury.

Functions

Now that you have a basic idea of what the organelles in a cell look like, its time to get to know what they do. This table summarises the main functions of each cell component.

Differences between a plant cell and animal cell

Plant cells and animal cells share many common organelles. However, there are some very notable differences.

Differentiation

Differentiation is the development of specific features that set cells apart. 

Multicellular organisms are made of different types of cells which perform different specific functions. Cells produced by cell division differentiate by losing or gaining certain structures. These new cells replace the worn-out or damaged ones and take over their functions.

Specialised Cells

The table below shows us the unique structure of different types of cells and their functions.

Division of Labour

In multicellular organisms, division of labour refers to how each type of cell is specialised in performing one particular function. Such division of labour is only possible due to specialisation. But how is it important?

Relationship between cells, tissue, organ, organ system and organism

Cells --> Tissue --> Organ --> Organ system --> Organism

Tissues

A tissue is a group of cells with similar structures which work together to perform a specific function.

Examples in animals include muscular tissues, epithelial tissues and blood (red blood cells, white blood cell, platelets, etc.).

Examples in plants include mesophyll tissues and phloem (mainly sieve tubes and companion cells).

Organs

An organ contains more than one type of tissue, all working together for a specific function. 

For example, the stomach is an organ. It contains gland tissue, muscular tissue and nervous tissue. They work together for the digestion of food.

Another example is the leaf, which contains mesophyll tissue, xylem tissue and phloem tissue, that work together for plant nutrition and transport of food.

Organisms

Various systems work together to support a whole living organism.