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Cell Structure and Main Functions

Grade 8 Science Worksheets

The Basic Unit of Life – The Cell

All living things are made up of cells. Some organisms are simply one single cell (unicellular), others are made of millions and millions of cells (multicellular). Cells are “alive” – they take in food, they use it to grow and reproduce, they release waste, and they die. Each cell is made up of some basic parts, known as organelles, or the organs of a cell, which help carry out specific functions for the cell.


The cell’s central control unit is the Nucleus. It holds the genetic material – the Chromosomes – which are made up of DNA (deoxyribonucleic acid), the hereditary material of the cell. The DNA drives the growth and metabolism of the cell. Within the nucleus is a smaller structure, the Nucleolus, which keeps the RNA (ribonucleic acid). DNA directs the production of RNA. The RNA carries out the instructions of the DNA, mainly the production of special proteins.

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Surrounding the nucleus is a thick jellylike fluid called the Cytoplasm. This contains many tiny yet important structures.

Present in the cytoplasm is Mitochondria, which is the main site for cellular respiration. Glucose along with oxygen is converted into energy molecules known as ATP (adenosine triphosphate). Cells requiring more energy have more mitochondria, such as liver and muscle cells.

A membrane system with a series of flattened sacs called the Endoplasmic reticulum is also present within the cytoplasm, close to the nuclear membrane. This has a rough and a smooth side. The rough side contains Ribosomes, which build proteins based on instructions from the RNA. These proteins are essential for survival of organisms. For example, the cells of the digestive system make enzymes for digestion. The smooth side produces vital lipids.

Yet another folded membranelike structure is the Golgi apparatus or Golgi complex which store the proteins and eventually release them to other parts of the cell.

Many more structures such as Lysosomes and Centrioles are also found in the cytoplasm, each with their own specialized function.

Cell Membrane

The cytoplasm and all its tiny organelles are held in place around the nucleus by the Cell membrane. This is made of a lipid bilayer, embedded with proteins. The structure and composition of the cell membrane determines what materials can enter the cell.

Plant Cells

Chloroplasts are special organelles found inside plant cells. They contain a substance called chlorophyll which is essential in the process of Photosynthesis to produce vital sugars. Plant cells have an additional cell wall around their cell membranes. Cell walls are made up of Cellulose, which provides stiffness to the stems of plants.


Cyanobacteria are special unicellular bacteria that live in water and moist places and produce their own food through the same photosynthesis process as plants. The chloroplast of plant cells is also made up of these bacteria. For the important function that they play, they are the most important group of bacteria on our planet.

Cellular Respiration

All living cells release energy from food molecules in order to provide for the necessary functioning of life. This is the process of Cellular Respiration. It consists of the following steps –

  1. Glycolysis: Food in the form of glucose enters through the cell membrane and undergoes a process of Glycolysis to create ATP (adenosine triphosphate), which is the primary unit of energy for the cell.
  2. TCA Cycle: A subsequent process takes place within the mitochondria in the presence of oxygen to produce even more ATP. Since this involves oxygen, it is referred to as Aerobic Respiration.
  3. Fermentation: When there is lack of oxygen, a fermentation process takes place to produce lactate. Lactate further aids the glycolysis process to create more ATP. This is termed Anaerobic Respiration.
  4. Electron Transport Chain: The primary unit of cellular energy, ATP, is released in the form of an electron transport chain.

Aerobic Respiration

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Cell Division

The growth and development of cellular organisms take place through a process called Cell Division. Cells divide in two ways –

  1. Mitosis: A cell divides into two new exactly identical cells, which grow and divide again to form four cells, and so on.
  2. Meiosis: This involves special sex cells (male and female) which undergo two separate divisions so as to create four sex cells, each with only half the number of chromosomes as the parent. The male and female sex cells then combine to produce a single fertilized egg with the proper set of chromosomes to create a new child.

Cell Differentiation

Cell differentiation is a process that takes place in a growing embryo wherein embryonic cells become specialized to perform specific functions in an organism.  Special signaling molecules bring messages to each cell to help the cell know which activities or processes it needs to perform. Once this differentiation process is triggered in a group of cells, those cells continue down that path to create specialized tissues and organs. This is how some cells form bones, or muscles, or blood, or organs, and other such specialized parts of the organism.

Check Point

  1. The basic parts or organs of a cell are known as ______.
  2. DNA and RNA are inside the ______ of a cell.
  3. In a cell the primary process of respiration takes place in the –
    1. Nucleus
    2. Ribosomes
    3. Mitochondria
    4. Cellulose
  4. Cells divide either through ______ or ______.
  5. A generic embryonic cell develops into many specialized cells through the process of ______.

Answer Key

  1. Organelles
  2. Nucleus
  3. Mitochondria
  4. Mitosis or Meiosis
  5. Cell Differentiation

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