Let's get to know more about cells!

ALL cells have the following structural features in common:

• PROTOPLASM
• Protoplasm is the mass of living matter. 
• It is a complex jelly like substance of which 70-90% is water while the rest consists of mineral salts, organic compounds, etc. 
• Chemical reactions take place here.
• Protoplasm is a collective term for cell membrane, cytoplasm and nucleus.

• CELL MEMBRANE
• It is present in all plants and animal cells.
• It completely surrounds the call and holds in the constituents of the cell.
• It is a thin living structure that encloses the entire cell.
• It provides a high surface area for the absorption of nutrients and the disposal of wastes.
• It is a partially permeable membrane.
• It controls substances entering and leaving the cell.
• It stops cell contents from escaping. Small substances such as oxygen, water, glucose and other simple food substances are allowed to enter. Waste products are allowed to leave the cell and harmful substances are kept out. Large molecules like proteins are not able to leave the cell.
• It is a two-layered structure. The outer and inner layers are made of proteins and the region in between them contains a double layer of lipids (fats). This gives the cell its characteristic shape.
• Acids and heat can denature it.

• CYTOPLASM
• A jelly like substance where chemical or metabolic reactions of the cell take place
• It is the basic living material of all cells
• It is a complex mixture of chemicals of which 70-90% is water. 
• the main contents of the cytoplasm are proteins, fats, carbohydrates, mineral salts and digested food substances such as glucose and amino acids.

• NUCLEUS
• It is a spherical or oval body lying in the cytoplasm.
• It is present in all plant and animal cells.
• It contains a hereditary material known as chromosomes that are made of the chemical DNA (deoxyribonucleic acid) which is organised into genes and controls all cellular activities of the cell.

• RIBOSOMES
• They are very tiny round organelles present in the cytoplasm
• They are responsible for the synthesis of proteins from amino acids

• MITOCHONDRIA
• Mitochondria (singular : mitochondrion) are tiny, spherical or sausage or rod-shaped organelles present in plant and animal cells. 
• They are often called the power houses of the cell.
• They are most numerous in regions of rapid chemical activity (eg muscles).
• They are responsible for producing energy by oxidising food substances in a process known as aerobic respiration. the energy released can be used by the cell to perform vital activities (eg cell division, growth or respiration)

PLANT cells have the following additional structures:

• A LARGE CENTRAL VACUOLE
• Younger plant cells have very tiny vacuoles.
• Mature plant cells have a large fluid filled space called a vacuole. Animal cells have much smaller vacuoles which may contain food and water.
• A vacuole is a space in a cell surrounded by a membrane known as the tonoplast
• A vacuole contains cell sap, a solution made up of mostly sugars, mineral salts and amino acids and other substances including waste products.
• The vacuole is sometimes called the "sap vacuole".
• Vacuoles play an important role in osmosis.

• THE CELLULOSE CELL WALL
• The plant cell is completely enclosed by a non-living cell wall made of cellulose (a polysaccharide)
• Animal cells do not have cell walls
• Cellulose form fibres that criss-cross over each other to form a very strong covering of the cell.
• This helps to protect and support the cell. 
• The spaces between the fibres allow very large molecules to pass through the cellulose cell wall. It is therefore said to be fully permeable.
• It allows water and dissolved food substances to pass through.

IF the cell is involved in the process of photosynthesis, then it also contains:

• CHLOROPLASTS
• Chloroplasts are never found in animal cells.
• They are only present in plant ccells that perform photosythesis.
• They can be located in the cytoplasm.
• They are green in colour because they contain thee green pigment, chlorophyll, which absorbs sunlight.
• The light energy from the sun, plus carbon dioxide and water is the used for making glucose by photosynthesis.