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Cell – Structure and Function
Notes
Diversity and Evolution of Life
CELL – STRUCTURE AND FUNCTION
INTRODUCTION
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 whereas the body of higher 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 biological cells. This lesson deals with the structure common to all types of the cells. You will also learn about the kinds of cell division and the processes involved therein in this lesson.
OBJECTIVES
After completing this lesson, you will be able to :
z justify that cell is the basic structural and functional unit of all organisms;
z list the components of the cell and state cell theory;
z differentiate between prokaryotic and eukaryotic cells;
z differentiate between plant and animal cells;
z illustrate the structure of plant and animal cells by drawing labelled diagrams;
z describe the structure and functions of plasma membrane, cell wall, endoplasmic reticulum (ER), cilia, flagella, nucleus, ribosomes, mitochondria, chloroplasts, golgi body, peroxisome, glyoxysome and lysosome;
z describe the general importance of the cell molecules-water, mineral ions, carbohydrates, lipids, amino acids, proteins, nucleotides, nucleic acids, enzymes, vitamins, hormones, steroids and alkaloids;
z justify the need for cell division;
z describe various phases of cell cycle;
z explain the term karyotype and mention the karyotype analysis and its significance.
Cell – Structure and Function Diversity and Evolution of Life
Notes
4.1 THE CELL AND CELL THEORY
4.1.1 Landmarks in the study of a cell Soon after Anton Van Leeuwenhoek invented the microscope, Robert Hooke in 1665 observed a piece of cork under the microscope and found it to be made of small compartments which he called “cells” (Latin cell = small room). In 1672, Leeuwenhoek observed bacteria, sperms and red blood corpuscles, all of which were cells. Much later, in 1831, Robert Brown, an Englishman observed that all cells had a centrally positioned body which he termed the nucleus.
4.1.2 The cell theory In 1838 M.J. Schleiden and Theodore Schwann formulated the “cell theory.” Which maintains that:
z all organisms are composed of cells.
z cell is the structural and functional unit of life, and
z cells arise from pre-existing cells.
The cells vary considerably, in shapes and sizes (Fig.4.1). Nerve cells of animals have long extensions. They can be several centimeter in length. Muscle cells are elongated in shape. Egg of the ostrich is the largest cell (75 mm). Some plant cells have thick walls. There is also wide variation in the number of cells in different organisms.
4.1.3 The Cell A cell may be defined as a unit of protoplasm bound 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, and pigments. The bacteria have neither defined cell organelles nor a well formed nucleus. But every cell has three major components: z plasma membrane z cytoplasm z DNA (naked in bacteria) and enclosed by a nuclear membrane in all other organisms
Two basic types of cells Cytologists recognize two basic types of cells (Fig. 4.1). 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.
Cell – Structure and Function Diversity and Evolution of Life
Notes
Table: 4.2 Differences between plant cell and animal cell Plant cell
- Cellulose cell wall present external to cell membrane.
- Vacuoles are usually large.
- Plastids present.
- Golgi body present in the form of units known as dictyosomes.
- Centriole absent.
Fig. 4.2a Generalised plant cell Fig. 4.2b Generalised animal cell
INTEXT QUESTIONS 4.
- From where do new cells arise? ............................................................................................................................
- Name the scientists who proposed the ‘cell theory’. ............................................................................................................................
- Name an organelle which a plant cell has but an animal cell does not. ............................................................................................................................
- Give two points of difference between a prokaryotic cell and a eukaryotic cell ............................................................................................................................
4.2 COMPONENTS OF THE CELL The major components of the cell are (1) cell membrane, (2) cytoplasm, and (3) nucleus.
4.2.1 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 internal 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 )
Animal cell
- No cell wall, outermost structure is cell membrane or plasma membrane
- Generally vacuoles are absent and if present, are usually small..
- Plastids absent.
- Golgi body well developed having 2 cisternae
- Centriole present.
Cell – Structure and Function
Notes
Diversity and Evolution The plasma membrane is made of proteins and lipids and several models were of Life proposed regarding the arrangement of proteins and lipids. The fluid mosaic model proposed by Singer and Nicholson (1972) is widely accepted. It is represented in Fig 4.3.
According to the fluid mosaic model, (i) The plasma membrane is composed of a lipid bilayer of phospholipid molecules into which a variety of globular proteins are embedded. (ii) 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 (iii) 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 the lipid bilayer partially or wholly.
Fig. 4.3 The fluid mosaic model of cell membrane.
Functions (i) The plasma membrane encloses the cell contents. (ii) It provides cell shape (in animal cells) e.g. the characteristic shape of red blood cells, nerve cells, and bone cells. (iii) It allows transport of certain substances into and out of the cell but not all substances so much 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: (i) Diffusion : molecules of substances move from their region of higher concentration to the regions of lower concentration. This does not require energy. Example : absorption of glucose in a cell. (ii) Osmosis: movement of water molecules from the region of their higher concentration to the region of their lower concentration through a semipermeable
Non-polar tail Polar head Protein molecule on one side of the membrane only
Lipid molecule
Plasma membrane in cross-section
Glycoprotein Glycolipid
Lipid bilayer
Cholesterol
Protein molecule that traverses the membrane and is exposed at both surfaces
Cell – Structure and Function
Notes
Diversity and Evolution (a) Structure of Life
- Outermost non-living layer present in all plant cells.
- Secreted by the cell itself.
- In most plants, it is chiefly made up of cellulose but may also contain other chemical substances such as pectin and lignin.
- The substance constituting the cell wall is not simply homogeneous 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.
(b) Functions
- The cell wall protects the delicate inner parts of the cell.
- Being rigid, it gives shape to the cell.
- As it is 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 pectinate.
INTEXT QUESTIONS 4.
- Define diffusion and osmosis.
............................................................................................................................
- What does active transport mean?
............................................................................................................................
- Give one point of difference between phagocytosis and pinocytosis.
............................................................................................................................
- Match the following :
(i) hydrophilic end (a) cell wall (ii) microfibrils (b) inner ends of lipids (iii) fluid-mosaic model (c) fluid droplets (iv) hydrophobic end (d) outer ends of lipids (v) pinocytosis (e) Nicholson and Singer
- Give two functions of the plant cell wall. (i) ................................................... (ii) .......................................................
Cell – Structure and Function Diversity and Evolution of Life
Notes
4.3 THE CYTOPLASM AND THE CELL ORGANELLES
The cytoplasm contains many cell organelles of which we shall learn about :
- those that trap and release energy e.g. mitochondria and chloroplasts;
- those that are secretory or involved in synthesis and transport e.g. Golgi, ribosomes and endoplasmic reticulum
- the organelles for motilily - cilia and flagella
- the suicidal bags i.e. lysosomes
- the nucleus which controls all activities of the cell, and carries the hereditary material 4.3.1 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 structures under light microscope. Approximately 0.5 - 1.00 μm (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 mitochondrion observed by means of electron microscope is shown in Fig. 4.5. Note the following parts.
Fig. 4.5 Structure of a mitochondrion
- Wall made up of double membrane
- The inner membrane is folded inside to form projections called ‘ cristae ’ which project into the inner compartment called the ‘matrix’. Function : Oxidises pyruvic acid (breakdown product of glucose) to release energy which gets stored in the from of ATP for ready use. This process is also called cellular respiration. That is why mitochondria are called the ‘power house’ of a cell.
Ribsome Ring of DNA
Inner membrane
Cristae
Outer membrane
Strand of DNA
Matrix
Site of enzymes that remove NH group from some amino acids
2
Cell – Structure and Function Diversity and Evolution of Life
Notes
z Inside of the chloroplast is filled with a fluid medium called stroma. z Function: chloroplasts are the site of photosynthesis (production of sugar, from carbon dioxide and water in the presence of sunlight).
Chloroplast versus mitochondria Can you now visualize how these two organelles are opposite to each other, one traps the solar energy locking it in a complex molecule (by photosynthesis), the other releases the energy by breaking the complex molecule (by respiration). Similarities between mitochondria and chloroplasts : both contain their own DNA (the genetic material) as well as their own RNA (for protein synthesis). Thus, they can self-duplicate to produce more of their own kind without the help of nucleus. Thought the chloroplasts and mitochondria contain their own DNA the hereditary molecule and also their own ribosomes, they are termed as semi-autonomous only because they are incapable of independent existence outside the cytoplasm for a long time. Since most of their proteins are synthesised witht he help of the nuclear DNA.
INTEXT QUESTIONS 4.
- What is a cell organelle? ............................................................................................................................
- Name the chemical which provides energy trapped in its bonds to the cell. ............................................................................................................................
- Which part of the chloroplasts is the site of light reaction? ............................................................................................................................
- Name the sac like–structure which form the grana? ............................................................................................................................
- Why is mitochondrion called the “power house” of the cell? ............................................................................................................................
- Which organelle contains enzymes for cellular respiration? ............................................................................................................................
- State two similarities between mitochondria and chloroplasts. ............................................................................................................................
- Which plastid imparts colour to flower petals? ............................................................................................................................
- Which plastid is green in colour? ............................................................................................................................
- Why are mitochondria and chloroplast called semi-autonomous? ............................................................................................................................
Cell – Structure and Function
Notes
Diversity and Evolution 4.3.3 Endoplasmic reticulum (ER), golgi body and ribosomes of Life The 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 proteins in the cell, Golgi bodies in secreting and the ER in transporting and storing the products. These three organelles operate together.
Fig. 4.7 and Fig. 4.8 show the diagram of ER and Golgi body as seen under an electron microscope. Note the ribosomes present in the ER.
Fig. 4.7 Golgi body Fig. 4.8 Endoplasmic reticulum
Endoplasmic reticulum (ER) Structure 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. Distributed hroughout the cytoplasm and is in contact with the cell membrane as well as the nuclear membrane.
Function Provides internal framework, compartment and reaction surfaces, transports enzymes and other materials through out the cell. RER is the site for protein synthesis and SER for steroid synthesis, stores carbohydrates.
Golgi body
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 in number of 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.
Ribosomes
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 (S- svedberg unit of measuring ribosomes). Site for protein synthesis.
Cisternae Nucleus Nuclear pore Rough endoplasmicreticulum
Ribosome
Smooth Endoplasmic reticulum
Cell – Structure and Function
Notes
Diversity and Evolution The main features of lysosomes are as follows : of Life
(i) Membranous sacs budded off from Golgi body.
(ii) May be in hundreds in a single cell.
(iii) Contain several enzymes (about 40 in number)
(iv) Materials to be acted upon by enzymes enter the lysosomes.
(v) 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
(i) help in nutrition of the cell by digesting food, as they are rich in various hydrolysing enzymes which enable them to digest almost all major chemical constituents of the living cell.
(ii) Help in defence by digesting germs, as in white blood cells.
(iii) Help in cleaning up the cell by digesting damaged material of the cell.
(iv) Provide energy during cell starvation by digestion of the own parts of the cells (autophagic, auto : self; phagos: eat up).
(v) Help sperm cells in entering the egg by breaking through (digesting) the egg membrane.
(vi) In plant cells, mature xylem cells lose all cellular contents by lysosome activity.
(vii) 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.
2. Peroxisomes 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.
z They often contain a central core of crystalline material called nucleoid composed of urate oxidase crystals.
z These bodies are mostly spherical or ovoid and about the size of mitochondria and lysosomes.
z They are usually closely associated with ER.
z They are involved in photorespiration in plant cells.
z They bring about fat metabolism in cells.
3. Glyoxysomes z The microbodies present in plant cells and morphologically similar to peroxisomes.
z Found in the cell of yeast and certain fungi and oil rich seeds in plants.
z Functionally they contain enzymes of fatty acid metabolism involved in the conversion of lipids to carbohydrates during germination.
Cell – Structure and Function Diversity and Evolution of Life
Notes
INTEXT QUESTIONS 4.
- Why are lysosomes called suicidal bags? ............................................................................................................................
- List the usefulness of intracellular digestion by lysosomes ............................................................................................................................
- What is the function of peroxisomes in plant cells ............................................................................................................................
4.3.5 Cilia and flagella (the organelles for motility) (i) Some unicellular organisms like Paramecium and Euglena swim in water with the help of cilia and flagella respectively. (ii) In multicellular organisms 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. (iii) Cilia beat like tiny oars or pedals (as in a boat) and flagella bring about whiplash like movement. (iv) Both are made up of contractile protein tubulin in the form of microtubules. (v) The arrangement of the microtubules is termed as 9 + 2, that is, two central microtubules and nine duplet sets surrounding them. Cilia shorter (5 to 10 μm) several 100 per cell structure : protoplasmic projection and membrane bound consist of 9 sets of peripheral duplet microtubules and 1 set of two singlet tubules in the centre
Centriole It is present in all the animal cells (but not in Amoeba ), located just outside the nucleus. It is cylindrical, 0.5 μm in length and without a membrane. It has 9 sets of peripheral triplet tubules but none in the centre (9 + 0). Each set has three tubules arranged at definite angles (Fig. 4.10). 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
Fig. 4.10 Centriole (showing 9 + 0 structure)
Flagella longer (15 μm) usually 1 or 2 in most cells
same as in cilia
Peripheral tubules
Cell – Structure and Function Diversity and Evolution of Life
Notes
z The number of chromosomes is fixed in an organism. During mitotic cell division chromosomes divide in a manner that the daughter cells receive identical amounts of hereditary matter.
4.4.3 Nucleolus z Membraneless, spheroidal bodies present in all eukaryotic cells except in sperms and in some algae.
z Their number varies from one to few, they stain uniformly and deeply.
z It has DNA, RNA and proteins.
z Store house for RNA and proteins; it disappears during early phase of cell cycle and reappears after telophase in the newly formed daughter nuclei.
z Regulates the synthetic activity of the nucleus. z Thus nucleus and cytoplasm are interdependent, and this process is equal to nucleo–cytopalsmic interaction.
INTEXT QUESTIONS 4.
- Why cannot the cell survive without the nucleus? ............................................................................................................................
- Explain the following terms: (a) chromatin network...... ............................................................................. (b) chromosomes ............................................................................................
- What is the function of the nucleolus in the cell? ............................................................................................................................
4.5 MOLECULES OF THE CELL The cell and its organelles are made of organic chemicals such as proteins, carbohydrates, nucleic acid and fats. These are aptly termed biomolecules. Inorganic molecules such as water and minerals are also present in a cell.
A. Water z Water with unique physical and chemical properties has made life possible on earth. z It is a major constituent of protoplasm. z It is a medium in which all the metabolic reactions occur. z It is a universal solvent in which most substances remain dissolved sparingly or completely. z It is responsible for turgidity of cells.
Cell – Structure and Function
Notes
Diversity and Evolution B. Elements necessary for life of Life
Elements Functions Hydrogen, Carbon, Oxygen, Nitrogen, 1. Required for organic compounds of Calcium, Potassium, Sodium, Magnesium, the cell and present as major Phosphorous, Sulphur, Chlorine, Iron, constituents. (Ca in plant cell wall, C, Boron, Silicon, Manganese, Copper, H, O, N as organic compounds) Zinc, Cobalt, Molybdenum, Iodine
- Act as major cations (Na, K) and anions (Cl) in most physiological processes.
- As cofactor of enzymes participate in most of the biochemical reactions of a cell (Fe, Cu, Mo, Zn, B)
- Involved in energy transfer reactions (P in ATP).
- Green pigment chlorophyll in plants have magnesium in the centre of tetrapyrrole ring.
C. Biomolecules
(i) Carbohydrate
Structure Functions
- Composed of C, H and O 1. Most abundant organic substance present in nature which occurs in the form of cellulose in plant cell wall.
- Simple six carbon sugar (glucose) 2. In both plants and animals it is used as a is called a monosaccharide. source of energy (sugar).
- Two molecules or units join 3. An important storage form in plants is together to form disaccharide starch and in animals it is glycogen. (sucrose).
- More than ten units of 4. Present in nucleic acids as five carbon monosaccharides join in a chain to sugar (Ribose in RNA, and deoxyribose in form a polysaccharide e.g. starch DNA). and cellulose.
(ii) Amino acid
- Basic amino acid structure shows 1. Plants have the ability to utilize inorganic that the central carbon atom is nitrogen and synthesize amino acid. attached with an amino group (–NH 2 ), a carboxylic acid group (–COOH), one hydrogen and one side group (R).
- There are 20 different side groups 2. In an animal, principal source of amino acids which give 20 different amino acids. is provided by the plants or animals that it consumes in its diet (pulses are rich in protein). (iii) Proteins
- Composed of C, H, O and N. 1. Structurally proteins form integral part of the membranes
Cell – Structure and Function
Notes
Diversity and Evolution
- Plants have the ability to synthesize 3. Vitamin A present in the carotene pigment of of Life vitamins from CO 2 , NH 3 and H 2 S. carrot. Vitamin D can be produced by man with the help of sunlight. Vitamin K is produced by bacteria in the human intestine.
(vii) Hormones
- Hormones are specific organic 1. In animals hormones are produced in ductless substances effective in low concen- glands called endocrine glands which control trations, synthesized by cells in one all the biochemical activities of the organism part of the organism and then 2. In animals hormones may be proteins, transported to another part of the peptides or steroids. organism, where it produces 3. In plants hormones (growth regulators) characteristic physiological responses. are generally produced in metabolically active cells and control the vegetitive and reproductive growth of the entire plant. Proteinaceous hormones are not found in plants.
(viii) Alkaloids
- Alkaloids are complex organic 1. The active principles of drugs from compounds made of C, H, O medicinal plants are generally and N. alkaloids e.g. Quinine from the Cinchona. Ephedrine from and Morphine from Papaver species
- Alkaloids in plants are produced from amino acids.
(ix) Steroids
- These are fat soluble lipid compounds synthesized from cholesterol.
- They are produced by the Most of the steroids act as life-saving drugs, reproductive organs like ovaries, and others act as hormones which are effective testes and placenta and also by in performing specific functions in specific adrenal glands. organs of animal body.
- They include testosterone, estrogen, and cortisol
INTEXT QUESTIONS 4.
- What is the importance of water in a living cell.
............................................................................................................................
Cell – Structure and Function Diversity and Evolution of Life
Notes
- Which is the basic molecule in starch? ............................................................................................................................
- What is a peptide bond and where will you find it? ............................................................................................................................
- Which is the most energy rich biomolecule in living organisms? ............................................................................................................................
- What are nucleotides? ............................................................................................................................
4.7 CELL DIVISION
A single cell divides many times and forms a multicelled organism. Unicellular bacteria and protozoa divide and increase in number. The injured tissues are replaced by new cells through cell division. Thus cell division is one of the most important activities in all organisms. In this lesson you will study about the two kinds of cell division and the processes involved in them. Majority of cells in a multicellular organism grow and then can divide. However, the cells like the nerve and muscle cells of animals and guard cells of plants do not divide. The process of cell division is almost same in all organisms. A cell passes through phases of growth after which are able to duplicate their chromosomes before they divide. These phases in the life of a cell constitute the cell cycle.
4.7.1 The cell cycle You can use the term mother or parent cell for the cell that undergoes division and the daughter cells for the ones that are the result of this division. Before each daughter cell undergoes division, it must grow to the same size as its mother cell. We can distinguish two main phases in the life of a cell. (i) Interphase - Non-dividing period (Growth phase) (ii) Dividing phase - Also called M-phase (M for mitosis or meiosis) (i) Interphase - ( Inter = in between ) The interval between two successive cell divisions is termed interphase (phase at which the cell is not dividing). It is the longest period in the cell cycle (Fig.4.11). The interphase is subdivided into three main periods - G 1 , S and G 2. G 1 (Gap-1 ) Phase i.e. First phase of growth – This is the longest phase. Lot of protein and RNA are synthesised during this phase. S or synthetic Phase - It comes next. Lot of DNA is (synthesised). A chromosome contains a single double helical strand of DNA molecule. After S-phase each chromosome becomes longitudinally double except at centromere,
