What is a cell?
- Cell is the basic unit of life.
- Life requires a structural compartment separate from the external environment in which macro molecules can perform unique functions in a relatively constant internal environment.
- Most living things are made of cells. Cell structure varies according to its function.
Plant and animal cells differ in size, shape and structure (plants cells are usually larger than animal cells).
- Cells range in size from a few micrometers to several centimeters
- Most cells are small because larger cells do not function efficiently because small cells can more easily transport materials into and out.
- A large surface-to-volume ratio is beneficial, as cell structure increases, the volume grows more rapidly than surface area.
- The larger the structure of a cell, the faster a cell can take in substances and remove waste products.
- Whereas large internal volume relative to surface area makes it more difficult to transport materials into and out of the cell.
Features of a plant cell:
- Presence of cellulose cell wall which is firm and rigid so gives the shape to the cell.
- Cell wall is fully permeable.
- Plant cell is also surrounded by cell membrane which is partially permeable.
- Presence of chloroplast contain green pigment called chlorophyll, green in colour and also contain different enzymes.
- A large permanent central vacuole that take up a large part of the interior of plant cell contain cell sap helps to maintain the turgidity in the cell.
- Cytoplasm is a thick jelly like substance contain more dissolved substances and organelles( for example Mitochondria, Endoplasmic Reticulum, Golgi apparatus etc)
- Presence of starch which is a stored food present in form of grains in cytoplasm.
- Immediate food that is made in the result of photosynthesis is, glucose while moving food is called sucrose and stored food is called starch.
Features of an animal cell:
- Irregular in shape because cell wall is absent but it contains cell membrane which holds the cell together and controls the substances entering and leaving the cell.
- Vacuoles are many but small and temporary.
- Cytoplasm fill the space in the cell and a site for many chemical reactions.
- Chloroplast is absent.
- Stored food is in form of glycogen.
Difference between cell wall and cell membrane:
- Cell walls are permeable to salts and water, which can pass by diffusion. Cell wall in plants is made up of cellulose fiber, which is tough as well as flexible to some extent. Cellulose is the major component of cotton fiber and wood, and it is used in paper production. It is fully permeable to all substances.Bacterial and fungi cells also have a cell wall, but it is not made of cellulose. When water enters and fills the plant cell, turgor produced but cell wall prevent the cell bursting. Thus the cell wall has many important functions in a cell including protection, structure, and support. The cell wall sends signals for the cell to enter the cell cycle in order to divide and grow.
- Cell membrane The cell membrane is a biological membrane that separates the interior of all cells from the outside environment which protects the cell from its environment. The cell membrane consists of a lipid bi layer, including cholesterol’s that sit between phospholipids to maintain their fluidity at various temperatures. Cell membrane is partially permeable membrane to different substances. Water can pass through osmosis, from high concentration to low concentration, down the osmotic gradient. Permeability to water causes turgor in vacuole of plant cell. Salts and mineral ions can pass selectively through the process of active transport where energy is required.
All organisms begin from a single cell. This single cell carries the DNA coding for all the proteins the adult organism will use. However, if this cell expressed all of these proteins at once it would not be functional. This cell must divide repeatedly, and the cells must begin the process of cell differentiation as they divide. The cell lines begin to emerge, and the cells get more and more specific. Eventually, an entire organism is formed with hundreds of different cell types from this process of cell differentiation.
Specialized cells, tissues, organs, and systems:
· A specialized cell is designed to do a particular job.
· Nerve cells have long fibres to carry massages.
· Muscle cells can contract and relax.
· White blood cells attack bacteria.
· Platelets help clotting
Detail of some specialized cells are as follows:
Root hair cell:
- Long and narrow shape to increase surface area to volume ratio for efficient absorption of water and mineral salts from the soil.
Red blood cell:
- Contain a red pigment called hemoglobin, enabling the cell to transport oxygen from the lungs to all parts of the body
- Circular biconcave shape to increase surface area to volume ratio so oxygen can diffuse into and out of the cell at a faster rate
- Lacks nucleus, provide more space for hemoglobin.
- Transport water and mineral salts from the roots to the stem and leaves
- No cross walls and protoplasm that enables water to move easily through the lumen
- Lignin strengthens the walls and prevents collapse of the vessel
- When bundled together, xylem vessels provide mechanical support to the plant
· Large numbers of specialized cells make up tissue.
· Muscles, blood and nerves are all tissues.
· Blood tissue contains red cells for carrying oxygen, white cells for destroying harmful bacteria, and platelets to cause clotting in cuts.
· Various tissues together make up an organ.
· Each organ has its own specific job.
· The heart, the stomach and the brain are all organs.
· The Heart has to pump blood around the body. It is made up
of muscle tissue, blood vessels and nerves.
· Arteries and veins are usually thought of as organ as they consist of
several tissue layers.
Various organs together make up an organ system. E.g. the circulatory
system carries blood to all parts of the body. It is made up of heart,
arteries, veins, capillaries and blood.