Onion and Cheek Cells

Objective

Our aim is to prepare stained temporary mounts of onion peel and human cheek cells and to record the observations.

The Theory

Onion Cell

An onion is a multicellular plant organism. As in all plant cells, the cell of an onion peel consists of a cell wall, cell membrane, cytoplasm, nucleus and a large vacuole. The nucleus is present at the periphery of the cytoplasm. The vacuole is prominent and present at the centre of the cell. It is surrounded by cytoplasm. The presence of a cell wall and a large vacuole are indicators that help identify plant cells, such as seen in the onion peel.

Human Cheek Cell

As in all animal cells, the cells of the human cheek do not possess a cell wall. A cell membrane that is semi-permeable surrounds the cytoplasm. Unlike plant cells, the cytoplasm in an animal cell is denser, granular and occupies a larger space. The vacuole in an an animal cell is smaller in size, or absent. The nucleus is present at the centre of the cytoplasm. The absence of a cell wall and a prominent vacuole are indicators that help identify animal cells, such as cells seen in the human cheek.

 

Cell Organelles

Have you ever wondered what the inside of a cell looks like? If you think about the rooms in our homes, the inside of any animal or plant cell has many similar room-like structures called organelles. Here are some names and descriptions of organelles and other parts commonly found in cells.

Cell Wall: Protective Coat in Plant Cells

The cell wall is what provides the most significant difference between plant and animal cells, as it is present only in plant cells and covers the cell membrane. The cell wall is rigid and is composed of cellulose fibre, polysaccharides, and proteins. Despite the rigidity of the cell wall, chemical signals and cellular excretions are allowed to pass between cells.

Cell Membrane: Protective Coat in Animal Cells

The Cell Membrane is found in both plants and animals, and it is the outer most layer in the animal cell, that separates the contents of the cell from the outside world. It consists of both lipids and proteins and is selectively permeable, which means it permits only some molecules to pass through it.

Cytoplasm: Cell’s Inner Space

It is a jelly-like material that is eighty percent water and is usually clear in colour. It is also called cytosol. Cytoplasm contains all the organelles inside the cell membrane. The cytosol contains dissolved nutrients, helps break down waste products, and moves material around the cell through a process called cytoplasmic streaming.

Nucleus: The Control Centre

The nucleus is known as the control centre of the cell. It is the regulatory machinery responsible for all the activities inside the cell. It is the largest organelle in the cell and it contains the DNA of the cell. DNA contains all the information that helps cells live, perform their functions and reproduce. The nucleus has a double layered covering called nuclear membrane.

Vacuoles: Cell’s Storage Space

A vacuole is a membrane-bound organelle that stores solid and liquid contents. Vacuoles are found in both animal and plant cells, but are much larger in plant cells. Vacuoles are formed by the fusion of multiple membrane vesicles and are effectively just larger forms of these. The organelle has no basic shape or size; its structure varies according to the needs of the cell.

Similarities and Differences between Plant and Animal Cells

Animal and plant cells have a number of key similarities, but also some noted differences. Here are some of the common similarities and differences between plant and animal cells.

Features Animal Cell Plant Cell
Cell Shape Round (irregular shape) Rectangular (fixed shape)
Cell Wall Absent Present and is formed of cellulose
Cell Membrane Present Present and is covered by the cell wall
Nucleus Present Present
Vacuole One or more small vacuoles A large central vacuole taking up 90% of the cell volume
Plastids Present Present
Chloroplast Absent  Present and make their own food 
Endoplasmic Reticulum Present Present
Ribosomes Present Present
Mitocondria Present Present

Learning outcomes:

  • Students understand the different organelles inside the cell and their functions.
  • Students understand difference between animal cells and plant cells.
  • Students acquire skills to do the temporary mounts of cells by observing animation and simulation