O Level Biology (5090) - Cells

Cells: The Building Blocks of Life

1.1 Cell Structure and Organization

A cell is a a unit of life. It consists of a mass of living matter called protoplasm.

Protoplasm

• Living matter of a cell.
• A complex jelly like substance.
• 70%-90% of water.
• Rest are dissolved organic compounds (carbohydrates,Proteins,etc)
• Composition varies from cell to cell and exact composition is unknown.
• It exists in Gell state (semi-solid) and Sol state (liquid)

Protoplasm

↙               ↘

Cytoplasm                  Nucleoplasm

Organelle

A specialize part of a cell the is membrane bounded and that performs a specific function. For example: Mitochondria, Chloroplasts, Cell membrane, Ribosomes, etc.

Structure of Cells

1. Cytoplasm:

• A part of protoplasm in between the cell membrane and nucleus envelop is called cytoplasm.
• It is the basic medium of the cell where all the cellular activities take place.
• All the cellular organelles are embedded here.
• Mostly it forms the biggest part of the cell.

2. Cell Membrane:

• Also called plasma membrane/cell surface membrane.
• It is the outermost layer in animal cell where as it is found next to the cell wall in plant cell.
• It is a partially permeable membrane (It allows only certain substances to pass through, those which can pass through its pores.)
• It controls the traffic of substances in and out of the cell.
• It surrounds the cytoplasm.
• It is made up of carbohydrate, proteins and fats.

3. Nucleus:

• It is the brain or the control center of the cell.

1. Prokaryotes:- Organisms that do not have a true nucleus and other membrane bounded organelles. For example; Bacteria.
2. Eukaryotes:- Organisms the have a true nucleus and other membrane bounded organelles. For example; Plants, Animals, etc.

      True nucleus has:-

1. Nuclear Envelop
2. Nucleolus (Nucleoli): It is a darkly stained structure within the nucleus. It is associated with protein synthesis.
3. Nucleoplasm: Protoplasm within the nucleus. Medium where all nuclear activities take place.
4. Chromatin network: Network of threads. At the time of cell division chromatin threads condense and become chromosomes that help in cell division.

4. Cell Wall:

• It is absent in animal cell.
• It is present in plant cells, bacteria cells And fungi cells.
• It is the outermost layer.
• It provide regular shape to a cell.
• It is completely permeable.
• In plant cells it provides turgidity.
• It protects the cell from any mechanical injury.

5. Centrioles:

• A pair of centrioles is found in animal cells.
• It is absent in plant cells.
• It helps in cell division.

6. Mitochondria:

• It is the powerhouse or the battery of the cell.
• It is the site of respiration.
• It is associated with ATP synthesis.
• ATP (Adenosine Triphosphates) is the energy currency of a cell.

7. Vacuoles:

• Found in both plant and animal cells.
• Plant cells have large permanent vacuoles with Tonoplast (membrane of the plant vacuole)
• Inside the plant vacuole there is a mixture of water, nutrients and salts called Cell Sap.
• Animal cells have small, many and temporary vacuoles.
• Animal vacuole can be Food vacuole (stores food) or Contractile vacuole (store waste products)

8. Chloroplasts:

• Found only in plant cells.
• Have green photosynthetic pigment chlorophyll.
• It imparts green color to the plant.
• They convert light energy into chemical energy.

1.2 Differentiation

Cells specialize on the basis of division of tasks. During process of growth and development, cells grow and take different forms. This change is called differentiation.

For Example:

• Animal Cells:

          RBC - Transport oxygen

          Neuron - Transmits nerve impulses

          Goblet cells - secrete mucus

• Plant Cells:

          Mesophyll cells - carry out photosynthesis

          Xylem cells - transports water and mineral ions.

          Phloem cells - transports synthesized food.