Q#1: Define the following |
1. Cell (Latin word Cella = chamber)
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The structural and
functional units of all living organisms is called a cell. |
2. Unicellular organisms
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Those organisms that are made up of only one cell are called unicellular organisms. |
For example, Amoeba, Euglena Paramecium, etc. are unicellular organisms.
3. Multicellular organisms
Those organisms that are made up of more than one cell are called multicellular organisms. |
For example, humans, horses, frogs, mustard plants, etc. are multicellular organisms.
Q#2: Define microscope. Also, discuss the term microscopy. |
Microscope (micro=small and scopy=to see)
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An instrument by which small and invisible objects/things can be made large and visible is called a microscope. |
Microscopy
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The use of a microscope for the purpose of observation of small things is termed microscopy. |
First microscope
The first microscope was developed by Zacharias Jansen in Holland in 1595. It was simply a tube with lenses. Its magnification ranged from 3x to 9x.
Leeuwenhoek’s microscope
A.V Leeuwenhoek a Dutch scientist made a much better microscope with a magnification of more than 250x. He is considered to be the first microscopist.
Q#3: Define and explain the following terms (as used in microscopy).(I) Magnification (ii) Resolution |
1. Magnification
The ability of a microscope to enlarge the apparent size of an object is called magnification. |
Unit of magnification
Magnification is expressed in the unit of ‘diameter’ abbreviated as ‘X’ e.g. 3x, 9x, 250x, etc.
2. Resolution
The ability of a microscope to distinguish between close objects. |
(OR)
The ability of a microscope to show two objects separately is called resolution of the microscope. |
For example, the human eye can differentiate between two parts at least 1 nm. So the resolution of the human eye is 0.1 mm.
Q#4: Discuss briefly the light microscope and electron microscope. |
1. Light microscope
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The type of microscope which uses light to make the image of an object is called a light microscope. |
Light passage
In a light microscope, light passes through the sample and then through two lenses.
Image formation
Lens produces an enlarged image of the sample and the second lens magnifies the image more. After passing through the object and lenses, the light is projected into the viewer’s eye where an enlarged and clear image is formed.
Magnification
The magnification of a light microscope is 1500 X.Resolution
The resolution of a light microscope is 2 um (micrometer).2. Electron microscope
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The type of microscope which uses a beam of electrons to make the image of an object is called an electron microscope. |
Magnetic lenses
Magnetic lenses focus the electron beam on a screen and make much-enlarged image.
Resolution
The resolution of the electron microscope is 0.2 nm where1nm=1/1000,000 mm.
Types of electron microscope
The electron microscope is of the following two types.1. Transmission electron microscope (TEM)
It is used to view the internal cell structures. Its magnification is 250,000X.2. Scanning electron microscope (SEM)
It is used to study the detailed structure of surfaces of cells or any other objects. The specimen (an object under observation is called a specimen) is coated with metals. Electrons are reflected from metal, which produces the image of an object. Its magnification is 10,000X to 100,000X.
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Comparison between light and electron microscope
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Distinctive features |
Light microscope |
Electron microscope |
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Radiation type |
Light |
Beams of electrons |
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Lenses |
Optical |
Magnetic |
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Magnification |
10,000 times greater than the naked eye |
100 times greater than a light microscope |
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Resolution |
500 times of the naked eye |
400 times of the light microscope |
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Images |
2D images |
TEM show 2D while SEM shows 3D images |
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History of the emergence of the cell theory
- Robert Hook (an English scientist), 1665, was the first person to observe cork cells with a small microscope.
- Anton van Leeuwenhoek (a Dutchman) around the same time made a much more powerful microscope. He magnified pond water 300 times (300X) and saw tiny one-celled animals (now we call protists).
- Dolland, in 1827, further improved the quality of lenses.
- Robert Brown, in 1833, described the nucleus as a spherical body in a plant cell.
- Mathias Schleiden (a German botanist), 1838, claimed that all plants were made up of cells.
- Theodor Schwann (a German Zoologist), 1839, claimed that animals were also made up of cells.
- Jan Evangelista Purkyne, 1840, gave the name protoplasm to the cell contents. Later on, the term cytoplasm was introduced.
- Rodalf Virchow (a German pathologist), 1855, gave the statement that “every cell comes from a cell”. The word cellula (cell) was used.
- Louis Pasteur, in 1862, gave experimental proof for Virchow’s hypothesis. He said that bacteria could be formed only from existing bacteria.
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Cell theory
This theory was presented by two German scientists, Mathias Schleiden (Botanist) and Theodor Schwann (Zoologist). Following are the main points of cell theory.
- All living organisms are made up of one cell (unicellular) or more than one cell (multicellular).
- The cell is a structural and functional unit of all living organisms.
- New cells arise from pre-existing cells by cell division.
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Cell
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The structural and functional unit of all living organisms is called a cell. |
Types of cell
Following are the two types of cells on the basis of the presence or absence of membrane-bound nucleus and membrane-bound organelles
1. Prokaryotic cell
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The cell that lacks a membrane-bounded nucleus and membrane-bounded organelles is called a prokaryotic cell. |
The organisms that are made up of prokaryotic cells are called prokaryotes. Such as Bacteria and cyanobacteria (blue-green algae) are prokaryotes.
2. Eukaryotic cell
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The cell which a true nucleus and membrane-bounded organelles is called a eukaryotic cell |
The organisms that are made up of eukaryotic cells are called eukaryotes. Such as amoeba, euglena, brassica, rose, frog, etc are eukaryotes.
General components of eukaryotic cell
Following are the main parts of the eukaryotic cell
- Cell wall (absent in an animal cell).
- Cell membrane
- Nucleus and its components.
- Cytoplasm and its organelles.
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Cell wall
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The outer-most boundary of the plant cell is known as the cell wall. |
- It is semi-rigid and non-living.
- The cell wall of plants cell is composed of cellulose while that of fungi is made of chitin.
- The thickness of the cell wall varies in different cells.
Structure of cell wall
Cell walls have the following layers
1. Primary wall: In this first layer cellulose molecules are arranged in a criss-cross manner.
2. Secondary wall: This inner layer is relatively thick and more rigid than the primary wall.
3. Middle lamella: It is an intercellular layer that binds the cell to form tissues.
Functions of cell wall
The cell wall provides a definite shape, rigidity, support, and protection to the cell.
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Cell membrane/ Plasma membrane
A Cell membrane is the outermost part of an animal cell while in a plant cell it lies next to the cell wall. The cell membrane is thin, delicate, and selectively permeable.
Chemical composition of cell membrane
Cell membrane is chemically composed of lipids and proteins.
Structure of cell membrane
Two scientists Singer and Nicolson, 1972, presented a fluid mosaic model of the structure of cell membrane. Due to the continuously changing position of the sheet-like structure, it was named the fluid mosaic model. According to this model
“cell membrane is a lipid bilayer membrane in which
some protein molecules are completely embedded
while some floats.”
The function of cell membrane
- Glucose, fatty acids, water gases, and ions can slowly diffuse through it. This property makes the cell membrane selectively permeable.
- It also acts as a receptor site for recognizing hormones, neurotransmitters, and chemicals.
- It protects the inner parts of the cell.
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Nucleus
It is considered the brain of the cell because it controls all the activities of the cell.
Location of the nucleus in the cell
In an animal cell nucleus lies in the center of the cell. In plant cell it is pushed to a side, due to the presence of a large vacuole in the middle.
Structure and composition of the nucleus
It is usually spherical in shape and bounded by a double membrane called a nuclear membrane. The protoplasm of the nucleus is called nucleoplasm. Nucleoplasm contains one or two nucleoli and chromosomes.
Chromosomes were discovered by Waldyer in 1876. They become visible in dividing cells, while in non-living cells (which don’t have the ability to divide) they appear as a network of fine threads.
Chromosomes comprise DNA (the hereditary material) and protein (which provide structural support). The number of chromosomes remain the same in a specie e.g. in a human cell it is 46, in an onion 16, in radish 18, in chicken 78, in horse 64, in tobacco 48, in pea 14, etc.
Function of nucleus
- The nucleus contains DNA, which is responsible for the transmission of hereditary characters.
- The nucleolus helps in the formation of RNA.
- Ribosomes are synthesized in nucleus.
- It controls all the activities of the cell.
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Cytoplasm
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That part of the cell which lies between the cell membrane and nuclear membrane is known as cytoplasm. |
Characteristic
It is an aqueous, living, and viscous substance.
Portions of cytoplasm
The cytoplasm is divided into the following two portions
a. Ectoplasm: The outer transparent portion is called ectoplasm.
b. Endoplasm: The inner granular portion is called endoplasm.
Chemical Composition of cytoplasm
Cytoplasm is composed of organic compounds such as proteins, carbohydrates, lipids, enzymes and some inorganic compounds e.g. water and salt. It contains about 90% water.
Functions performed by cytoplasm
- It is the storehouse of vital chemicals.
- It provides chemicals, sites, and conditions for various biochemical activities such as glycolysis, etc.
- It contains various cell organelles such as mitochondria, endoplasmic reticulum, ribosomes, Golgi bodies, etc.
Q#13: Discuss the structure and function of Mitochondria. |
Mitochondria (Latin word: Mito=thread, Chondria=granules)
Mitochondria are oval, rod-shaped, or filamentous bodies. They are typically 5 micrometers in length and 0.2 micrometers across. Their number varies, depending upon the activity of the cell e.g. liver cells have more than 1000 mitochondria while ear lobes have a very small number of it.
Structure of mitochondria
Mitochondria is a double membrane structure.- The smooth outer membrane controls the entry and exit of chemicals.
- The inner membrane forms many folds called cristae. The inner space is filled with a fluid called cell matrix. The matrix contains DNA and a variety of enzymes.
Function of mitochondria
- Mitochondria is known as the powerhouse of the cell due to storage of ATP (Adenosine-tri-phosphate) during cellular respiration.
- Most of the enzymatic activities of the cell are carried out by mitochondria.
Q#14: Discuss the structure and function of Golgi Bodies. |
Golgi bodies
Golgi bodies were first discovered by Italian scientist Camillo Golgi in 1898. Golgi bodies are in the form of granules, rods, threads, or canals.
Structure of Golgi bodies
Golgi bodies consist of cisternae, Golgi vesicles (lysosomes), and Golgian vacuoles. Cisternae are flattened sac-like structures stacked one above the other. Golgi vesicles are small droplet-like structures that develop from cisternae. The Golgi vacuoles are large spherical sacs formed by the expansion of the flattened sacs of cisternae.
Functions of Golgi bodies/ Golgi apparatus/ Golgi complex
They store the selections, convert them into finished products, and packed them into Golgian vesicles, which transport the secretion outside the cell.
Q#15: Discuss the structure and function of the Endoplasmic Reticulum. |
Endoplasmic reticulum (ER)
They are the membrane-bounded organelles that are extended from nuclear membrane to plasma membrane, forming a cytoskeleton.
Structure of Endoplasmic reticulum
The endoplasmic reticulum consists of
a. Cisternae: They are long flattened structures that occur in parallel rows.
b. Vesicles: They are spherical.
c. Tubules: They are found in the form of irregular branches.
Types of endoplasmic reticulum
The endoplasmic reticulum is of two types i.e.
1. Smooth endoplasmic reticulum (SER):
SER has no ribosomes attached to its surface. It is non-granular and plays a key role in lipid formation.
2. Rough endoplasmic reticulum (RER)
RER has ribosomes attached to its surface. It is granular and involved in protein synthesis because of the presence of ribosomes on its surface.
Functions of endoplasmic reticulum
- It helps in the transport of material within a cell.
- It provides mechanical support to the cell.
- SER is important for lipid formation while RER is important for protein synthesis.
- SER in liver cells detoxifies poison and drugs.
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Ribosomes
These tiny granular structures were first discovered by Palade in 1955. It is the only organelle found in prokaryotic cells. They are formed in the nucleolus. They are either freely dispersed in the cytoplasm or attached to RER.
Composition and Structure of ribosomes
They are composed of equal amounts of RNA and proteins hence they are called ribonucleic protein particles. Each eukaryotic granule has two subunits i.e. larger subunits and smaller subunits. Eukaryotic ribosome is 80S while prokaryotic ribosome is 70S (S means Svedberg, which shows sedimentation rate).
Function of ribosome
Ribosomes are involved in protein synthesis.
Q#17: Shortly discuss the structure and functions of Plastids. |
Plastids
They are present in plant cells only. They have one or more pigments. Plastids are of the following three types
1. Chloroplasts
Chloroplasts are larger, green, rounded, or oval organelles. They contain a green pigment called chlorophyll. They are present in green parts of the plant, particularly in leaves.
Structure of chloroplast
Following are the main structural components of the chloroplast
a. Stroma: Inside the chloroplast, a semi-fluid and colorless matrix is known as the stroma.
b. Thylakoid: Inner membrane is folded to form fluid-filled flattened sacs called thylakoids.
c. Granum: Thylakoids constitute a structure called granum (plural: grana).
d. Lamella/Intergrana: The grana are connected by membranous extensions called lamella or intergrana. Lamella and grana are all embedded in the stroma.
Function of chloroplast
The grana of chloroplast contain chlorophyll which is the main site for photosynthesis.
2. Chromoplasts:
Chromoplasts are colored plastids, but other than green. They are present in the petals of flowers, the skin of ripened fruits, etc.
3. Leucoplasts:
Leucoplasts are colorless, triangular, tubular, or any other shape. They are particularly common in storage parts such as roots, stems, and seeds.
Functions of plastids
- Chloroplasts are concerned with photosynthesis.
- Chromoplasts present in the petals of flowers attract insects for pollination.
- Leucoplasts store food material in plants.
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Centriole
They are found only in animal cells. They are hollow and cylindrical structures about 0.2 microns in diameter. They arise in a distinct region of the cytoplasm called the centrosome. They are in pair form i.e. two in number. Each centriole contains nine triplets of microtubules i.e. 27 microtubules.
Function of centriole
Their function is to help spindle formation and cell division. They are composed of protein fibers which on contraction separate the duplicated chromosome during the cell division. They are absent in plant cells but they do form spindles.
Q#19: Shortly describe the structure and function of a typical vacuole. |
Vacuole
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Membrane-enclosed fluid-filled spaces are called vacuoles. |
A mature plant cell usually has one large central vacuole. Animal cells have many small vacuoles. The membrane of the vacuole is called the tonoplast. A plant vacuole contains a solution of mineral salts, sugar, amino acids, wastes, and colorful sap with blue, red or purple pigments called anthocyanins.
In unicellular organisms, food is digested in a food vacuole while water and waste materials are excreted through contractile vacuoles.
Function of vacuoles
Vacuoles help the cells to remain turgid by absorbing more water from the surrounding.
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1. Cytoskeleton
It is a network of filaments or tubules scattered in the cytoplasm of eukaryotic cells. Cytoskeleton consists
a. Microfilaments:
These are small filament-like structures. They are made up of actin proteins. These are used by cells to change their shapes.
b. Microtubules:
These are small tubules. These are made of tubulin proteins. Cells use these tubules to hold their shape. They are also involved in the formation of cilia and flagella.
c. Intermediate filaments:
These filaments are involved in maintaining the cell shape.
2. Lysosomes (Lyso=splitting, soma=body)
These are single membrane-bounded organelles present in eukaryotic cells. They contain strong digestive enzymes. They break down food and waste within the cell.
During its function, a lysosome fuses with the vacuole that contains the materials and its enzymes break down the material.
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1. Prokaryotic cell (pro=before, karyon=nucleus )
The cell that lacks a membrane-bounded nucleus and membrane-bounded organelles is called a prokaryotic cell.
The organisms that are made up of prokaryotic cells are called prokaryotes. Such as Bacteria and cyanobacteria (blue-green algae) are prokaryotes.
2. Eukaryotic cell (Eu=true, karyon=nucleus)
The cell which a true nucleus and membrane-bounded organelles is called a eukaryotic cell.
The organisms that are made up of eukaryotic cells are called eukaryotes. Such as amoeba, euglena, brassica, rose, frog, etc are eukaryotes.
Differences between prokaryotic and eukaryotic cells
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Prokaryotic cell |
Eukaryotic cell |
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1 |
It lacks true nuclei and membrane-bounded organelles. |
It has a true nucleus and membrane-bounded organelles. |
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2 |
Chromosomes are scattered in the cytoplasm. |
Chromosomes are present in membrane-bounded nuclei. |
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3 |
Ribosomes are of small size (the 70S). |
Ribosomes are of large size (the 80S). |
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4 |
Ribosomes are freely dispersed in the cytoplasm. |
Ribosomes are freely present as well as attached to RER. |
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5 |
The cell wall is made of murine. |
The cell wall of plant cells is made of cellulose and that fungi is made of chitin. |
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6 |
These cells are simple, comparatively smaller in size (0.5 nm in diameter) |
These cells are complex and of larger size (10-100 nm in diameter). |
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7 |
Examples are bacteria and cyanobacteria. |
Examples are plants, animals, fungi, etc. |
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Cell specificity
The cell is the basic unit of structure and function of all living organisms. cell specificity means that cells, in living organisms, are specific for their functions.
A. Cell specificity in plants
The following examples illustrate cell specificity in plants.
- Xylem transports water and dissolved substances.
- Phloem transport prepared food from the leaves to other parts.
- Parenchyma cells transport food.
- Collenchyma cells and parenchyma cells provide support to plants.
B. Cell specificity in animals
The following examples illustrate cell specificity in animals.
- Nerve cells conduct messages through nerve impulses.
- Muscular cells contract and relax to produce movements.
- Connective tissues connect the body parts and provide support.
- RBCs (Red Blood Cells) carry oxygen.
- WBCs (White Blood Cells) defend the body from microorganisms.
- Platelets help in blood clotting.
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A cell works as an open system i.e. it takes in substances needed for its activities through its cell membrane, then it performs the metabolic processes. Products and by-products are formed in metabolism. Cell either utilizes the products or transports them to other cells. The by-products are either stored or excreted out of the cells.
Q#24: Discuss surface area to volume ratio.
Surface area to volume ratio
As a rule, large cells have less surface area in relation to their volume. While small cells have more surface area.
In figure shows large cell and 27 small cells. In both cases, the total volume is the same i.e.
Volume=30 μm x 30 μm x 30 μm=27,000 μm^3
In contrast to the total volume, the total surface areas are very different. Because the cell has 6 sides, its surface area is 6 times the area of 1 side. The surface areas of the cells are as follow
Surface area of 1 large cell=6 x (30 μm x 30 μm)=5400 μm^2
Surface area of 1 small cell=6 x (10 μm x 10 μm)=600 μm^2
Surface area of 27 small cells=27 x 600 μm^2=16,200 μm^2
The need for nutrients and the rate of waste production is directly proportional to cell volume. The cell takes up nutrients and excretes wastes through its surface cell membrane. So a large volume cell demands a large surface area. But, a large cell has a much smaller surface area relative to its volume than smaller cells. So, the cell membrane of small cells can serve their small volumes more easily than the membrane of the large cell.
Each unit of volume requires a specific amount of surface area to supply its metabolism with raw materials. The amount of surface area available to each unit of volume varies with the size of a cell. As a cell grow its SA/V (surface area to volume ratio) decreases. At some point in its growth, its SA/V becomes so small that its surface area is too small to supply its raw materials to its volume.
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Transport
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The movement of substances from one part to another part within the living organisms is called transport. |
As plasma membrane is semi-permeable, so it regulates the outflow and inflow of materials. Following are the two types of transport
1. Active transport
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The type of transport across a cell membrane against input is called active transport. |
Cell membrane uses energy in the form of ATP for active transport.
Example:
- Absorption of minerals by root hairs from soil.
- Exchange of ions across cell membrane.
2. Passive transport
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The movement of substances across the cell membrane without the expenditure of metallic energy is called passive transport. |
It always occurs from a higher concentration region to a lower concentration region.
Examples: Diffusion, Osmosis
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Diffusion
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The random movement of molecules or ions from the region of higer concentration to the region of lower concentration is called diffusion. |
Water, Oxygen, Carbon-di-Oxide, and some other simple molecules can diffuse across the cell membrane by the process of diffusion. Diffusion is a slow process, yet it is efficient to fulfill all the gaseous requirements of plants.
The efficiency of diffusion depends on
- Distance to be covered
- Concentration gradient difference.
- The kind of molecules i.e. solid, liquid, gas.
Examples:
- After food digestion, glucose is present in a higher concentrations in the small intestine, so glucose is transported to the villi from the small intestine through diffusion.
- When we spray a perfume in one corner of the room, its fragrance will be spread by diffusion in all parts of the room.
Facilitated diffusion
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The type of diffusion aided by carrier proteins is called facilitated diffusion. |
Some large-sized molecules cannot pass through cell membrane so they are brought about by certain proteins known as carrier proteins.
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Osmosis
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The movement of solvent (water) molecules from a region of higher concentration to a region of lower concentration through the partially permeable membrane is called osmosis. |
How does osmosis occur?
- When a cell is placed in a hypotonic solution (a solution that has a lower solute concentration than the cell), more water is moved inward. In such conditions, animal cells become turgid (due to their hard cell wall).
- When a cell is placed in an isotonic solution (solution in which the concentration of solute is equal to that of the cell), the animal cells remain normal while plant cells become flaccid (loose) because the net uptake of water is not enough.
- When a cell is placed in a hypertonic solution (solution with a higher solute concentration than the cell), water moves out by osmosis. Animal cells shrink while plant cells become plasmolyzed.
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Turgor (Latin word: Turgere=to swell)
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A state of turgidity resulting in rigidity of cells is called turgor. |
Turgidity
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The swelling of plant cells due to osmosis is called turgidity. |
Mechanism of turgidity
As the water enters the vacuole, it increases in size and pushes the cell contents against the wall. At this stage, the cell becomes hard and tough and is said to be a turgid cell and the condition is known as turgidity.
Importance of turgor in plants
- Due to turgor the young stem, branches and leaves remain upright.
- The Turgidity of guard cells helps in opening and closing stomata.
- It also helps in maintaining the shape of the plants.
- The uptake of salts, minerals, and water by the uphill movement of the plant takes place due to turgor.
- Some flowers open during day hours and close at night due to turgidity.
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Plasmolysis
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The shrinkage of protoplasm from the cell wall of a plant cell due to outward movement (exosmosis) from the cell due to hypertonic solution outside is called plasmolysis. |
When a cell is placed in a hypertonic solution (solution with a higher solute concentration than the cell), water moves out by osmosis. Animal cells shrink while plant cells become plasmolyzed.
Deplasmolysis
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Recovery of a plasmolyzed cell to its normal condition is known as deplasmolysis. |
If a plasmolysed cell is placed in a hypotonic solution the water moves to the cell and gains its turgidity, the process is called deplasmolysis and cell is said to be deplasmolysed.
Q#24: Define cytosis. What are its types. (OR) Differentiate between endocytosis and exocytosis.
Cytosis
When the materials are transported through the formation of vesicles or vacuoles inside or outside the cell, then such transport is called cytosis.
Types of cytosis
There are two types of cytosis
Endocytosis
The inward movement of the materials through the vesicles or vacuole formation is known as endocytosis.
It is a general term for group of processes that bring the large particles, small molecules and even small cells into the eukaryotic cell.
Mechanism
Plasma membrane fold inward around the materials forming a small pocket.
The pocket deepens and forms a vesicle.
The vesicle separates from the cell membrane and move to the cell.
Forms of endocytosis
There are two forms of endocytosis
Phagocytosis (cellular eating) in which cell takes in solid materials.
Pinocytosis (cellular drinking) in which cell takes in liquids.
Exocytosis
The outward movement of the materials from the cell through vesicle fromation is called exocytosis.
Mechanism
First the vesicle, containing material, moves towards the cell membrane.
These material are then released to outside environment.
The vesicle membrane is incorporated into the plasma membrane.
Q#25: Define the term filtration.
Filtration
The process in which small molecules are pushed across selectively permeable membrane with the help of hydrostatic pressure (blood pressure) is called filtration.
In humans, kidney filter out the waste materials through the process of filtration.
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