Maharashtra Board Class 8 Science Chapter 10 Cell and Cell Organelles PDF Download

10. Cell and Cell Organelles

Can You Recall?

1. How many types of cells are found in living organisms?

2. Which instrument had you used to observe cells?

Last year, we studied that cell is the structural and functional unit of living organisms. According to the functions, cells of different sizes and structures are found in different organs.

Structure Of The Cell

Observe the following figures, write the names and complete the chart.

We must study each and every part of the cell to understand the processes occurring within the cell.

Components Of A Cell

1. Cell Wall: It is found around cells of algae, fungi and plants. Animal cells lack cell wall. Cell wall is an elastic and strong coat around cell membrane. It is mainly composed of carbohydrates like cellulose and pectin. Afterwards, polymers like lignin, suberin, cutin are added in cell wall as per need. Functions of cell wall are to support and to protect the cell by preventing entry of excess water in the cell.

2. Plasma Membrane/Cell Membrane: It is a thin, fragile and elastic covering that separates the cell components from outer environment.

Teacher's Note

The cell wall is like the protective wall around your house. It keeps the cell safe and strong. Just like our house wall protects us from rain and wind, the cell wall protects the plant cell.

Exam Trick

Remember: Only plant cells have cell walls. Animal cells do not have them. Think of it like this - plants stay in one place, so they need strong walls. But animals move around, so they don't need walls.

Points To Remember

Cell wall is only in plant cells, algae and fungi cells.
Cell wall is made of cellulose and pectin.
Cell wall supports and protects the cell.
Plasma membrane is in all cells.
Plasma membrane separates the cell from outside.

Structure Of Plasma Membrane: Protein molecules are embedded in two layers of phospholipids. Plasma membrane is said to be a selectively permeable membrane as it allows some substances to enter the cell, while prevents other substances. Due to this property, useful molecules of water, salt and oxygen enter the cell and CO2 exits the cell. If any changes occur outside the cell, the cellular environment does not change due to plasma membrane. This condition is called homeostasis.

Teacher's Note

The plasma membrane is like a security guard at a door. It allows good things to enter (like water and oxygen) and lets bad things leave (like waste). This keeps the cell healthy and balanced.

Exam Trick

Remember: Selectively permeable means it chooses what goes in and out. Like a filter in your water bottle - it lets water pass but stops dirt. The plasma membrane does the same for the cell.

Points To Remember

Plasma membrane has two layers of phospholipids.
Proteins are embedded in these layers.
It is selectively permeable - it allows some things in and keeps others out.
Oxygen and water can enter through it.
Carbon dioxide can exit through it.

How Do Substances Travel In The Cell?

Activities That Consume Cellular Energy:

1. Endocytosis - To gulp or engulf food and other substances from outer environment.

2. Exocytosis - To excrete waste materials out of cell.

Processes That Don't Consume Cellular Energy:

1. Diffusion - Entry or exit of small molecules like O2 and CO2.

2. Osmosis - The travelling of water from a part with more water to a part with less water, through a selectively permeable membrane is osmosis. It is a physical process with 3 possibilities.

Teacher's Note

Osmosis is how raisins swell up when you put them in water. The water enters the raisin because it wants to balance the amount of water inside and outside. This is how dry food absorbs water.

Exam Trick

Remember: Osmosis = water movement. Diffusion = other substance movement. Think of it this way - osmosis is only for water, but diffusion is for other gases like oxygen. In the exam, if they mention water, think osmosis first.

Points To Remember

Osmosis is the movement of water through a selectively permeable membrane.
Diffusion is the movement of gases like oxygen and carbon dioxide.
Isotonic solution has the same amount of water inside and outside the cell.
Hypotonic solution has less water outside, so water enters the cell.
Hypertonic solution has more water outside, so water leaves the cell.

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