Genetics Basic Concepts
The cell is the smallest unit of life.
In all living beings, cells share common abilities: nutrition, adaptation, response and reproduction. Nutrition allows a cell to support its vital functions by converting matter into energy. A cell interacts with its environment, but it can also adapt to it by modifying its physiology or behaviour when changes occur. It also relates to its environment by responding to the various stimuli it encounters. Lastly, as is the case with all living beings, a cell can reproduce and, through cellular division, generate other similar cells.
Animal and plant components
Under a microscope, a cell looks like a tiny sac filled with a gelatinous liquid called cytoplasm. Cells contain structures that perform specific jobs, in the same way the organs in our body all carry out tasks. These structures are called organelles (see interactive image). For example, vacuoles are small organelles used as storage. In animals, they store waste products before they get eliminated or substances the cell may need later (nutrients, fat, etc.). In plant cells, vacuoles are used to store water. Plant and animal cells are different in other ways as well. For instance, plants are capable of converting carbon dioxide into sugars using energy from the sun. This process is known as photosynthesis. To be able to carry out photosynthesis, plant cells contain chloroplasts, light-sensitive organelles that contain chlorophyll. Plant cells also have an extra protective barrier: the cell wall. The cell wall withstands the physical and chemical stresses in its environment, acting as a kind of exoskeleton.
The cell nucleus is the largest organelle. The nucleus holds most of the DNA (deoxyribonucleic acid) and the machinery needed for its decoding and replication. Cells that have a nucleus are called eukaryotic cells and those without a nucleus are called prokaryotic cells. They are more primitive. In eukaryotic cells, the genetic material is separated from the rest of the cytoplasm by a membrane surrounding the nucleus, known as the nuclear membrane. Bacteria and archaebacteria are prokaryotic cells. Their genetic material is found in an irregular-shaped region within the cell referred to as a nucleoid. Unlike the nucleus of plant and animal cells, this region is not clearly defined.
The content of the cell is enveloped by a cellular membrane. This membrane is more than just a protective barrier; it also plays a role in communication and interaction between the inside and outside of the cell. Embedded in the membrane are proteins, which are capable of transmitting signals that trigger reactions within the cell, for example, when a hormone attaches to it. Some proteins also allow substances to pass through the membrane, which is why it is said to be semi-permeable. Solute molecules (e.g., minerals) can move from the highest concentrated environment to the lowest concentrated environment by diffusion. Solvent molecules (e.g., water) move from the lowest concentrated environment to the highest concentrated environment by osmosis. Inputs (e.g., nutrients) travel toward the inside of the cells, while outputs (e.g., waste) leave the cell.
Unicellular or multicellular
Some organisms are unicellular, meaning that they are composed of a single cell; bacteria are one example. Other organisms, however, are made of many billions of cells that work together; these are multicellular organisms. While cells all contain the same DNA, they do not necessarily all look alike because they specialize in performing specific jobs. Some cells make up the skin, muscles, skeleton, brain or so on.