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Tilia stem cross-section (claudio9divizia, iStockphoto)
Tilia stem cross-section (claudio9divizia, iStockphoto)
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. Cell TheoryThe three main parts of the cell theory are:
Cells seen in a plant stem cross-section (Source: RolfDieterMueller [CC BY 3.0] via Wikimedia Commons). 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 FunctionIn 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)
What Makes Plant Cells Unique
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 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 carbohydratescarbohydrates 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.
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 on the cell, much like the water inside of a water balloon. This is called turgor pressure and keeps the cells from collapsing inward.
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 , the green pigment that gives leaves their colour and absorbs light energy. CyanobacteriaCyanobacteria , a type of prokaryote capable of photosynthesis, are considered to be the ancestors of chloroplasts! Chloroplasts (Source: Kristian Peters -Fabelfroh [CC BY-SA 3.0] via Wikimedia Commons).
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. |