The cell was first discovered in 1665 by an English scientist named Robert Hooke. While looking through a microscope, he observed tiny box-like objects in a slice of cork (bark from an oak tree) and named these boxes cells. Cells are the basic units of life, which make up all living things. This idea forms the basis of the Cell Theory.
The three main parts of the cell theory are:
- All living things are made of cells.
- The cell is the basic unit of structure and function in all living things.
- Cells only come from other pre-existing cells by cell division.
While some organisms are single-celled, others are made up of many cells. These organisms are called multicellular (having many cells). Cells differ in their size and complexity.
Eukaryotes are organisms which are made up of large and complex cells, whereas prokaryotes are organisms which are made up of small and simple cells. Animals and plants are examples of eukaryotes (have eukaryotic cells) while bacteria are examples of prokaryotes (have prokaryotic cells).
Plant Cell Structure and Function
In spite of the differences in size and complexity, all cells are mostly composed of the same substances and they all carry out similar life functions. These include growth and metabolism and reproduction by cell division.
Cells are made up of subcellular structures that are responsible for different and specific functions. These structures are known as organelles. A number of these organelles are common to both animal and plant cells. This section will focus on those parts which plants have.
Cell Structures (Cell Organelles)
- Cell Wall: This is the rigid outermost layer of a plant cell. It makes the cell stiff -providing the cell with mechanical support - and giving it protection. Animal cells do not have cell walls.
- Cell Membrane: This is a protective layer that surrounds every cell and separates it from its external environment. It is found just inside the cell wall and is made up of complex lipids (fats) and proteins.
- Cytoplasm: The cytoplasm is a thick, aqueous (water-based) solution in which the organelles are found. Substances such as salts, nutrients, minerals and enzymes (molecules involved in metabolism) are dissolved in the cytoplasm.
- Nucleus: The nucleus is the ‘control center’ of the cell. It contains Deoxyribonucleic acid (DNA), the genetic material that directs all the activities of the cell. Only eukaryotic cells have nuclei (plural for nucleus), prokaryotic cells do not. The nucleus is separated from the cytoplasm by a specialized membrane called the
- nuclear membrane.
- Ribosomes: These are little round structures that produce proteins. They are found in the cytoplasm or attached to the endoplasmic reticulum.Endoplasmic Reticulum (ER): The ER is a membrane system of folded sacs and tunnels. The ER helps move proteins within the cell as well as export them outside of the cell. There are two types of endoplasmic reticulum –
- smooth endoplasmic reticulum and
- rough endoplasmic reticulum. The rough endoplasmic reticulum is covered with ribosomes.
- Golgi body: The Golgi body is a stack of membrane-covered sacs that prepares proteins for export from the cell.
- Mitochondrion (plural mitochondria): This is the ‘powerhouse’ of the cell. It converts the energy stored in food (sugar and fat) into energy-rich molecules that the cell can use (Adenosine triphosphate – ATP for short).
- Lysosome: The lysosome is the digestive center of a cell that produces many different types of enzymes which are able to break down food particles and recycle worn out components of the cell.
- Vacuoles: These are large membrane-enclosed compartments that store toxic wastes as well as useful products such as water. These are mainly found in plants.
- Chloroplast: Chloroplasts contain a green pigment that traps sunlight and converts it into sugars by a process called photosynthesis. The sugars are a source of energy for the plants and the animals that eat them.
What Makes Plant Cells Unique
- Plant cells have a cell wall.
Plant cells are different from animal cells in a number of ways. Perhaps the most obvious difference is the presence of a cell wall. The cell wall provides strength and support to the plant, much like the exoskeleton of an insect or spider (our skeleton is on the inside of our body, rather than on the outside like insects or spiders).
The plant cell wall is mainly made up of the carbohydrates molecules cellulose and lignin. Cellulose is used extensively by humans for making paper. Cellulose can also be converted into cellulosic ethanol, a type of biofuel. Some animals, such as cows, sheep and goats, can digest cellulose with the help of bacteria in their stomachs. Humans cannot digest cellulose, which passes through our bodies and is better known as dietary fiber, something that we should eat to keep our waste moving as it should! Lignin fills in the spaces between cellulose and other molecules in the cell wall. Lignin also helps water molecules move from one side of the cell wall to the other – an important function in plants.
- Plant cells contain vacuoles.
Most adult plant cells have one large vacuole that takes up more than 30% of the cell's volume. At certain times and conditions the vacuole takes up as much as 80% of the cell’s volume! In addition to storing wastes and water, the vacuole also helps to support the cell because the liquid inside the vacuole exerts an outward pressure on the cell, much like the water inside of a water balloon. This is called turgor pressure and keeps the cells from collapsing inward.
- Plant cells contain chloroplasts.
Unlike animal cells, plant cells can harness the energy of the Sun, store it in the chemical bonds of sugar and later use this energy. The organelle which is responsible for this is the chloroplast. Chloroplasts contain chlorophyll, the green pigment that gives leaves their colour and absorbs light energy. Cyanobacteria, a type of prokaryote capable of photosynthesis, are considered to be the ancestors of chloroplasts!
Did you know?
Red algae (multicellular marine algae) have chloroplasts that contain the pigment phycobilin rather than chlorophyll, which gives them a reddish, rather than green, colour.
Plant and animal cells also have many common organelles, including the nucleus, cell membrane (called the plasma membrane in animals) endoplasmic reticulum, mitochondria and cytoplasm, as well as several others.