The cellular components present inside cells are called cell organelles, it is a subcellular (present within the cell) structure that performs one or more specific jobs in the cell.
Cell organelles on the basis of membrane
The cell organelles includes membranous as well as non- membranous bound organelles. There are different types of organelles present inside the cell, these may be classified into three categories on the basis of the presence or absence of a membrane.
Cell organelles without membrane: These are cell walls, ribosomes, and cytoplasm. These organelles are present in both prokaryotic and eukaryotic cells.
Cell organelles with single membrane-bound: These include lysosome, vacuole, Golgi bodies, and Endoplasmic reticulum. These organelles are present only in eukaryotic cells.
Double membranous cell organelles: These include mitochondria, nucleus, and chloroplast. present only in the eukaryotic cells.
The plasma membrane is made up of proteins and lipids. It is a living membrane, the outermost covering in animal cells but inner to the cell wall in the plant cell.
Several models of Plasma membranes were proposed regarding the arrangement of proteins and lipids by many biologists but The fluid mosaic model is widely accepted. The fluid mosaic model proposed by Singer and Nicholson (1972)
According to Fluid Mosaic Model
(i) The plasma membrane is composed of a lipid bilayer of phospholipid molecules into which various globular proteins are embedded.
(ii) Each phospholipid molecule has two ends, an outer head hydrophilic means water-attracting, and an inner tail pointing centrally hydrophobic, i.e. having a nature of water-repelling
(iii) The protein molecules are shown in two different ways as-
(a) Peripheral proteins or extrinsic proteins: these are proteins present on the outer and inner surfaces of the lipid bilayer.
(b) Integral proteins or intrinsic proteins: These are proteins that penetrate the lipid bilayer partially or completely.
The function of plasma membrane:
As we know it is selectively permeable and helps in the movements of certain substances, it also helps in providing the shape of some cells such as RBCs, nerve cells, etc.
Diffusion: molecules of substances move from the region of higher concentration to the lower concentration. For this energy is not required, Ex: absorption of glucose in a cell.
Osmosis: During osmosis, water molecules move from the region of their higher concentration to the region’s lower concentration through a semipermeable membrane, and energy is not required for this.
Active Transport– in the active transport direction of movement of a certain molecule is reverse to diffusion i.e. from the region of their lower concentration to the region of their higher concentration, it requires energy. In this case, energy is provided by adenosine triphosphate i.e ATP.
As we are introduced to the cytoplasm in the first part of the blog, so from here we start from its constituents. The cytoplasm contains many cell organelles some help in the release of energy and some in synthesis and movement. eg. mitochondria, Golgi bodies, etc
Mitochondria– (Singular = mitochondrion)
Mitochondria are found in both animal and plant cells. Mitochondria contain their own DNA and RNA so they can self-duplicate to produce their own kind without the help of the nucleus. Some important facts about mitochondria are:
- They release energy
- In size approximately 0.5 to 1.00 μm (micrometer) having a double membranous wall
- The inner membrane is folded inside and forms projections termed ‘cristae’ which project into the inner side compartment known as the ‘matrix’.
- They Oxidise pyruvic acid which breakdown the product of glucose and takes place to release energy stored in the form of ATP for ready use.
- This process of ATP formation in mitochondria is called cellular respiration. Mitochondria are also called the ‘powerhouse’ of a cell as it releases energy.
The nucleus is the largest organelle. It is double membranous and found in all eukaryotic cells.
- The nucleus is the control center of cellular activities and it is functioning as the storehouse of the DNA of the cell. It is structurally dark and round, surrounded by a nuclear membrane.
- The nucleolus is found inside the nucleus. It carries chromosomes.
The chloroplast is found in the cytoplasm of green plant cells. It may be disc-shaped or laminate as in most plants cell. In some ribbon-shaped for eg. in an alga Spirogyra or cup-shaped as in alga Chlamydomonas.
Some important facts about chloroplast:
- wall of the chloroplast is a double membrane
- A fluid medium called stroma is filled in the inner side of the chloroplast.
- Chloroplast contains their own DNA and RNA so they can self-duplicate to produce their own kind without the help of the nucleus.
- Chloroplasts are the site of photosynthesis as plant synthesize their food by the process (with the help of carbon dioxide and water in the presence of sunlight sugar is synthesized by plants).
Plastids are found in plant cells only and may be colored or colorless. Plastids are of three types as-
- Leucoplast -These plastids are of white colour or colorless
- Chromoplast –The chromoplasts may be of colour blue, red, yellow, etc.
- Chloroplast – The chloroplasts are green-coloured organelles.
The Endoplasmic reticulum (ER) is a single membrane-bound structure having a thickness between 50 – 60A°. These are of two types–
- Rough endoplasmic reticulum (RER) i.e. when ribosomes are attached to ER, and Smooth endoplasmic reticulum (SER) when no ribosomes are attached to ER
- it Provides internal framework and compartment and reaction surfaces,
- it transports enzymes and other materials throughout the cell. RER is involved in protein synthesis and folding, modification, and transport of proteins.
- SER is a site for steroid synthesis and stores carbohydrates
The Golgi bodies are single membrane-bound structures. They vary in shape and size. Camillo Golgi (1898) first observed dense stained reticular structures near the nucleus, those are named after him i.e. Golgi bodies.
Some important facts about Golgi bodies:
- In animal cells, Golgi bodies are present around the nucleus and are 3 to 7 in number.
- In-plant cells, are many in number and present scattered throughout the cell termed dictyosomes.
- Golgi bodies consist of many flat, disc-shaped sacs called cisternae which play a significant role as it modifies and packs proteins and polysaccharides
- As Golgi bodies package proteins into membrane-bound vesicles inside the cell then the vesicles are transported to their recognized sites.
- The Golgi apparatus is the most important site for the formation of glycoproteins and glycolipids
The Ribosomes are spherical in shape and do not have membranes. Ribosomes are involved in the synthesis of proteins in the cell.
Some important Facts about ribosomes:
- The ribosomes are spherical in shape and about 150 – 250 Å in diameter,
- Ribosomes are made up of large molecules of RNA and proteins called ribonucleoproteins
- Ribosomes are present as free particles in the cytoplasm or attached to the ER.
- Also found stored in the nucleolus inside the nucleus. 80S types are found in eukaryotes
- The 70S is found in prokaryotes
- The ribosomes are the site for protein synthesis
Lysosome-(lysis = breaking down)
Lysosomes are found in almost all animal cells and some non-green plant cells. They consist of many enzymes capable to digest carbohydrates, proteins, lipids, and nucleic acids, by nature these are phagocytic and they digest their own damaged cell so these are also called “suicidal bags”.
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