What are living things made up of?
All living things consist of small units called cells; cells are the smallest structural and functional unit of an organism.
In \(1665\), Robert Hooke used his self-designed microscope to observe a cork and discovered dead, honeycomb-like compartments, which he named cells.
Onion peel experiment:
Aim: To observe the cell structure of an onion peel under a compound microscope.
Procedure: A thin onion peel is placed on a slide with a drop of water, stained with iodine, and covered with a coverslip, avoiding air bubbles. It is then observed under a compound microscope.
Observation: Rectangular plant cells with a distinct cell wall, cytoplasm, nucleus, and stained cell structures are visible.
An onion cell under a microscope
Conclusion: Onion peel consists of regularly arranged plant cells showing prominent cell walls and nuclei.
The word 'microscope' comes from 'micro' (small) and 'scope' (to see), meaning an instrument to observe tiny objects. This topic addresses the compound and an electron microscope.
1. Compound microscope: A compound (light) microscope uses objective and eyepiece lenses to magnify tiny specimens up to \(1500\) times, helping study cells and microorganisms in biology and pathology labs.
2. Electron microscope: An electron microscope provides high-resolution images of biological and non-biological specimens, allowing detailed study of tissues, cells, organelles, and macromolecular structures in biomedical research.
Classification of cells:
1. Based on cell size.
2. Based on cell number.
3. Based on cell shape.
4. Based on nuclear membrane development.
Cell size:
Cell size varies widely. Cell sizes can range from nanometers to micrometres. A smaller cell has more volume and surface area than a larger cell.
The smallest cell found is a mycoplasma cell or PPLO (Pleuro Pneumonia Like Organisms, which is about 0.1micron in diameter.
Mycoplasma
The most longest are nerve cells, measuring about a meter in length.The longest part of a neuron is the axon.
Neuron
The largest cell is the egg of the ostrich, which is about \(15cm\) in diameter.
Human female ovum and the structure of sperm
Ovum is the largest cell in the human body, and sperm is the male gamete, which is the smallest cell in the human body.
Based on cell shape:
- Cell shape can vary widely, including spherical, oval, elliptical, spindle -shaped, polygonal, isodiametric, or flat plate-like.
- Each cell shape has a specific function.
- Plant cells and animal cells have almost permanent shapes.
- Different parts of the body perform individual functions. In carrying out their role, these cells have distinct shapes of their own.
Different types of human cells according to their shapes
Surface area to volume ratio:
As plant size increases, nutrient needs rise while root surface area decreases, limiting absorption. To overcome this, specialised structures such as microvilli in the human intestine and root hairs in plants increase the surface area for efficient nutrient uptake.
3. Cell number:
i. Unicellular organisms: Unicellular organisms consist of a single cell that performs all life processes such as respiration, digestion, and excretion. Examples include bacteria, amoeba, plankton, paramecium, euglena, and chlamydomonas.
Unicellular organisms
ii. Multicellular organisms: Multicellular organisms are made up of many cells that form tissues, organs, and organ systems to perform specialized functions. Examples include fungi, plants, birds, insects, humans, and animals.
4. Nuclear membrane development:
i. Prokaryotic cells do not have a nucleus surrounded by a complex nuclear membrane, where a single, circular chromosome is situated in a nucleoid. (Pro: primitive; Karyon: nucleus).
ii. Eukaryotic cells have a nuclear membrane that surrounds the nucleus, where the well-defined rod-shaped chromosomes are present. (Eu: True or well; Karyon: nucleus). Eukaryotic cells are further classified into plant cells and animal cells.
Types of cells:
i. Plant cells are the fundamental unit of life for plants. They are eukaryotic cells which have a well-defined nucleus with membrane-bound organelles that perform different functions. A plant cell has unique features compared to an animal cell. Viz. Cell wall, vacuole and chloroplasts.
ii. An animal cell is a type of cell that is seen specifically in animal tissues. It is characterized by the absence of a cell wall, with membrane-bound cell organelles enclosed within the cell membrane. Generally, it is more complicated than prokaryotic cells.
