mitochondria & chloroplasts
have their own circular DNA, like bacteria
mitochondria & chloroplasts
move freely within the cell
mitochondria & chloroplasts
from the cell
Structures and function
All organisms are composed of structural and functional units of life called ‘cells’. The body of some organisms like bacteria, protozoans and some algae is made up of a single cell while the body of fungi, plants and animals are composed of many cells. Human body is built of about one trillion cells.
Cells vary in size and structure as they are specialized to perform different functions. But the basic components of the cell are common to all cells. This lecture deals with the structure common to all types of the cell.
The cell theory
In 1838 M.J. Schleiden and Theodore Schwann formulated the “cell theory.” The cell theory maintains that
_ all organisms are composed of cells.
_ cell is the structural and functional unit of life, and
_ cells arise from pre-existing cells.
A cell may be defined as a unit of protoplasm bounded by a plasma or cell membrane and possessing a nucleus. Protoplasm is the life giving substance and includes the cytoplasm and the nucleus. The cytoplasm has in it organelles such as ribosomes, mitochondria, golgi bodies plastids, lysosomes and endoplasmic reticulum. Plant cells have in their cytoplasm large vacuoles containing non-living inclusions like crystals, pigments etc.
The bacteria have neither organelles nor a well-formed nucleus. But every cell has three major components
_ plasma membrane
_ DNA (naked in bacteria and covered by a membrane in all other organisms.
Two basic types of cells
Cytologists recognize two basic types of cells. Their differences have been tabulated below in table 4.1. Organisms which do not possess a well formed nucleus are prokaryotes such as the bacteria. All others possess a well-defined nucleus, covered by a nuclear membrane. They are eukaryotes.
When the cell is fractionated or broken down into its components by
rotating in an ultracentrifuge at different speeds the ribosomes of eukaryotic and prokaryotic sediment (settle down) at different speeds. The coefficient of sedimentation is represented in Svedberg unit and depicted as S.
The plant cell and the animal cell also differ in several respects as follws:
COMPONENTS OF THE CELL
The major components of the cell are
(1) cell membrane,
(2) cytoplasm, and
Cell membrane (Plasma membrane)
Each cell has a limiting boundary, the cell membrane, plasma membrane or plasmalemma. It is a living membrane, outermost in animal cells but next to cell wall in plant cells.
It is flexible and can fold in (as in food vacuoles of Amoeba) or fold out (as in the formation of pseudopodia of Amoeba)
The plasma membrane is made of proteins and lipids and several models were proposed regarding the arrangement of proteins and lipids. The fluid mosaic model proposed by Singer and Nicholson (1972) is widely accepted.
According to the fluid mosaic model,
The plasma membrane is composed of a lipid bilayer of phospholipid molecules into which a variety of globular proteins are embedded.
Each phospholipid molecule has two ends, an outer head hydrophilic i.e. water attracting, and the inner tail pointing centrally hydrophobic, i.e. water repelling
The protein molecules are arranged in two different ways:
(a) Peripheral proteins or extrinsic proteins: these proteins are present on the outer and inner surfaces of lipid bilayer.
(b) Integral proteins or intrinsic proteins: These proteins penetrate lipid
bilayer partially or wholly.
The plasma membrane encloses the cell contents.
It provides cell shape (in animal cells) e.g. the characteristic shape of red blood cells, nerve cells, bone cells, etc
It allows transport of certain substances into and out of the cell but not all substance, so it is termed selectively permeable.
Transport of small molecules (such as glucose, amino acids, water, mineral ions etc).
Small molecules can be transported across the plasma membrane by any one of the following three methods:
Diffusion : molecules of substances move from their region of higher concentration to their region of lower concentration. This does not require energy. Example : absorption of glucose in a cell.
Osmosis : movement of water molecules from the region of their higher concentration to the region of their lower concentration through a semipermeable membrane. There is no expenditure of energy in osmosis. This kind of movement is along concentration gradient.
Active Transport : When the direction of movement of a certain molecules is opposite that of diffusion i.e. from region of their lower concentration towards the region of their higher concentration, it would require an “active effort” by the cell for which energy is needed. This energy is provided by ATP (adenosine triphosphate). The active transport may also be through a carrier molecule.
Transport of large molecules (bulk transport)
During bulk transport the membrane changes its form and shape. It occurs in two ways:
endocytosis (taking the substance in)
exocytosis (passing the substance out)
Endocytosis is of two types :
Phagocytosis and Pinocytosis Cell membrane regulates movement of substance into and out of the cell. If the cell membrane fails to function normally the cell dies.
In bacteria and plant cells the outermost cell cover, present outside the plasma membrane is the cell wall about which we shall study now.
Bacterial cell wall is made of peptidoglycan. Given below is the structure and function of the plant cell wall.
– Outermost non-living, layer present in all plant cells.
– Secreted by the cell itself.
– In plant, made of cellulose but may also contain other chemical substance such as pectin and lignin.
– The substance constituting the cell is not simply homogenous but it consists of fine threads or fibres called microfibrils.
– It may be thin (1 micron) and transparent as in the cells of onion peel. In some cases it is very thick as in the cells of wood.
– The cell wall protects the delicate inner parts of the cell.
– Being rigid, it gives shape to the cell.
– Being rigid, it does not allow distension of the cell, thus leading to turgidity
of the cell that is useful in many ways
– It freely allows the passage of water and other chemicals into and out of the cells
– There are breaks in the primary wall of the adjacent cells through which
cytoplasm of one cell remains connected with the other. These cytoplasmic strands which connect one cell to the other one are known as plasmodesmata.
– Walls of two adjacent cells are firmly joined by a cementing material called middle lamella made of calcium pectate.
THE CYTOPLASM AND THE CELL ORGANELLES
The cytoplasm contains many cell organelles of which we shall learn about :
1. those that trap and release energy e.g. mitochondria and chloroplasts;
2. those that are secretory or involved in synthesis and transport e.g. Golgi, ribosomes and endoplasmic reticulum
3. the organelles for motilily - cilia and flagella
4. the suicidal bags i.e. lysosomes
5. the nucleus which controls all activities of the cell, and carries the hereditary material
Mitochondria and chloroplast - the energy transformers
Mitochondria (found in plant and animal cells) are the energy releasers and the chloroplasts (found only in green plant cells) are the energy trappers.
Mitochondria (Singular = mitochondrion)
Appear as tiny thread like structure under light microscope. Approximately 0.5 - 1.00 m (micrometer)
Number usually a few hundred to a few thousand per cell (smallest number is just one as in an alga (Micromonas)).
Structure: The general plan of the internal structure of a mitochondria observed by means of electron microscope is shown in the following figure.
Wall made of double membrane
The inner membrane is folded inside to form projections called cristae which project into the inner compartment called matrix.
Function :Oxidizes pyruvic acid (breakdown product of glucose) to release energy which gets stored in the form of ATP for ready use. This process is also called cellular respiration.
Plastids are found only in a plant cell. They may be colourless or with colour. Based
on this fact, there are three types of plastids.
Leucoplast-white or colourless
Chromoplast – blue, red, yellow etc.
Chloroplast – green
_ Found in all green plant cells in the cytoplasm.
_ Number 1 to 1008
_ Shape: Usually disc-shaped or spherical as in most plants around you. In some ribbon - shaped as in an alga spirogyra or cup - shaped as in another alga Chlamydomonas.
_ Wall made of double membrane i.e. outer membrane and inner membrane numerous stack-like (piles) groups or grana (singular = granum) are interconnected by lamellae.
_ Sac like structures called thylakoids Placed one above the other constitute granum.
_ Inside of the chloroplast is filled with a fluid medium calleds stoma.
_ Function: chloroplasts are the seat of photosynthesis (production of sugar, from carbondioxide and water in the presence of sunlight).
Endoplasmic reticulum (ER), golgi body and ribosomes
Endoplasmic reticulum (ER) and Golgi body are single membrane bound structures.
The membrane has the same structure (lipid-protein) as the plasma membrane but ribosomes do not have membranes. Ribosomes are involved in synthesis of substances in the cell, Golgi bodies in secreting and the ER in transporting and storing the products. These three organelles operate together.
Endoplasmic reticulum (ER)
A network of membranes with thickness between 50 - 60A°. It is of two types– rough endoplasmic reticulum (RER) i.e. when ribosomes are attached to it and Smooth-endo-plasmic reticulum (SER) when no ribosomes are present.
Throughout the cytoplasm and is in with the cell membrane as well as the nuclear membrane.
Provides internal framework, compartment and reaction surfaces, transports enzymes and other materials throughout the cell. RER is the site for protein synthesis and SER for steroid synthesis, stores carbohydrates.
Is a stack of membranous sacs of the same thickness as ER. Exhibit great diversity in size and shape. In animal cells present around the nucleus, 3 to 7 in number. In plant cells, many and present scattered throughout the cell called dictyosomes.
Synthesis and secretion as enzymes, participates in transformation of membranes to give rise to other membrane structure such as lysosome, acrosome, and dictyosomes, synthesize wall element like pectin, mucilage.
Spherical about 150 – 250 Å in diameter, made up of large molecules of RNA and proteins (ribonucleo proteins)
Present either as free particles in cytoplasm or attached to ER. Also found
stored in nucleolus inside the nucleus. 80S types found in eukaryotes and
70S in prokaryotes (Ssvedberg unit of measuring ribosomes).
Site for protein synthesis.
The microbodies (tiny but important)
These are small sac-like structures bounded by their membranes. These are of different kinds of which we will take up three like lysosomes, peroxisomes and glyoxysomes.
1. Lysosomes (lysis = breaking down; soma = body)
Lysosomes are present in almost all animal cells and some non - green plant cells They perform intracellular digestion.
Some main features of lysosomes are as follows :
Membranous sacs budded off from golgi body.
May be in hundreds in single cell.
Contain several enzymes (about 40 in number)
Materials to be acted upon by enzymes enter the lysosomes.
Lysosomes are called “suicidal bags” as enzymes contained in them can digest the cell’s own material when damaged or dead.
Importance of intracellular digestion by the lysosomes
Help in nutrition of the cell by digesting food, as they are rich in various enzymes which enable them to digest almost all major chemical constituents of the living cell.
Help in defence by digesting germs, as in white blood cells.
Help in cleaning up the cell by digesting damaged material of the cell.
Provide energy during cell starvation by digestion of the cells own parts (autophagic, auto : self; phagos: eat up).
Help sperm cells in entering the egg by breaking through (digesting) the egg membrane.
In plant cells, mature xylem cells lose all cellular contents by lysosome activity.
When cells are old, diseased or injured, lysosomes attack their cell organelles and digest them. In other words lysosomes are autophagic, i.e. self-devouring
Found both in plant and animal cells. Found in the green leaves of higher plants.
They participate in oxidation of substrates resulting in the formation of hydrogen peroxide.
_ They often contain a central core of crystalline material called nucleoid
composed of urate oxidase crystals.
_ These bodies are mostly spherical or ovoid and about the size of mitochondria and lysosomes.
_ They are usually closely associated with E.R.
_ They are involved in with photorespiration in plant cells.
_ They bring about fat metabolism in cells.
_ Found in the cell of yeast and certain fungi and oil rich seeds in plants.
_ Functionally they contain enzyme of fatty acid metabolism involved in the conversion of lipids to carbohydrates during germination.
Cilia and flagella (the organelles for mobility)
Some unicellular organisms like Paramecium and Euglena swim in water with the help of cilia and flagella respectively.
In multicellular organism some living tissues (epithelial tissues) have cilia.
They beat and create a current in the fluid in order to move in a given direction e.g. in the wind pipe (trachea) to push out the mucus and dust particles.
Cilia beat like tiny oars or pedals (as in a boat) and flagella bring about whip – like lashing movement.
Both are made up of contractile protein tubulin in the form of microtubules.
The arrangement of the microtubules in termed 9 + 2, that is, two central microtubules and nine set surrounding them.
shorter (5 to 10 µm) several 100 per cell structure : protoplasmicprojection and membrane bound consist of 9 sets of peripheral microtubules and 1 set of tubules in the centre
longer (15 µm) usually 1 or 2 in most cells
It is present in all animal cells (but not in Amoeba), located just outside the nucleus.
It is cylindrical, 0.5 m in length and without a membrane. It has 9 sets of peripheral tubules but none in the centre. Each set has three tubules arranged at definite angles. It has its own DNA and RNA and therefore it is self-duplicating.
Function : Centrioles are involved in cell division. They give orientation to the ‘mitotic spindle’ which forms during cell division.
NUCLEUS (THE HEREDITARY ORGANELLE)
General structure of nucleus :
It is the largest organelle seen clearly when the cell is not dividing.
It stains deeply, is mostly spherical, WBC have lobed nuclei.
It is mostly one in each cell (uninucleate, some cells have many nuclei; (multinucleate).
Double layered nuclear membrane enclosing nucleoplasm which contains chromatin network and a nucleolus.
_ Maintains the cell in a working order.
_ Co-ordinates the activities of organelles.
_ Takes care of repair work.
_ Participates directly in cell division to produce genetically identical daughter cells, this division is called mitosis.
_ Participates in production of gametes through another type of cell division called meiosis.
The part of a nucleus are given here :
_ Double layered membrane is interrupted by large number of pores.
_ Membrane is made up of lipids and proteins (like plasma membrane) and has ribosomes attached on the outer membrane which make the outer membrane rough.
_ The pores allow the transport of large molecules in and out of nucleus, and the membranes keep the hereditary material in contact with the rest of the cell. Chromatin
_ Within the nuclear membrane there is jelly like substance (karyolymph or nucleoplasm) rich in proteins.
_ In the karyolymph, fibrillar structures form a network called chromatin fibrils,
which gets condensed to form distinct bodies called chromosomes during cell division. On staining the chromosomes, two regions can be identified in the chromatin material heterochromatin dark and autromaticn (light).
Heterochromatin has less DNA and genetically less active than euchromatin which has more DNA and genetically more active.
Number of chromosomes is fixed in an organism. During cell division chromosomes divide in a manner that the daughter cells receive identical amounts of hereditary matter.
_ Membraneless, spheroidal bodies present in all eukaryotic cells except in sperms and in some algae.