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C hapter 5 THE FUNDAMENTAL UNIT OF LIFE While examining a thin slice of cork, Robert avoid air bubbles while putting the...

C hapter 5 THE FUNDAMENTAL UNIT OF LIFE While examining a thin slice of cork, Robert avoid air bubbles while putting the d Hooke saw that the cork resembled the cover slip with the help of a mounting needle. Ask your teacher for help. We structure of a honeycomb consisting of many have prepared a temporary mount of he little compartments. Cork is a substance onion peel. We can observe this slide which comes from the bark of a tree. This under low power followed by high was in the year 1665 when Hooke made this powers of a compound microscope. chance observation through a self-designed microscope. Robert Hooke called these boxes is Eyepiece cells. Cell is a Latin word for ‘a little room’. This may seem to be a very small and insignificant incident but it is very important bl in the history of science. This was the very Coarse adjustment Body tube first time that someone had observed that Fine adjustment living things appear to consist of separate pu Arm units. The use of the word ‘cell’ to describe Objective lens Clip these units is used till this day in biology. Microscope slide Let us find out about cells. Stage be T Swivel Condenser 5.1 What are Living Organisms re Mirror o R Made Up of? Base tt E Activity ______________ 5.1 Fig. 5.1: Compound microscope Let us take a small piece from an onion What do we observe as we look through C bulb. With the help of a pair of forceps, the lens? Can we draw the structures that we can peel off the skin (called epidermis) from the concave side (inner we are able to see through the microscope, on an observation sheet? Does it look like no N layer) of the onion. This layer can be put immediately in a watch-glass Fig. 5.2? containing water. This will prevent the peel from getting folded or getting dry. What do we do with this peel? © Nucleus Let us take a glass slide, put a drop of water on it and transfer a small piece of the peel from the watch glass to the Cells slide. Make sure that the peel is perfectly flat on the slide. A thin camel hair paintbrush might be necessary to help transfer the peel. Now we put a drop of safranin solution on this piece followed by a cover slip. Take care to Fig. 5.2: Cells of an onion peel  We can try preparing temporary mounts Chlamydomonas, Paramoecium and bacteria. of peels of onions of different sizes. What do These organisms are called unicellular we observe? Do we see similar structures or organisms (uni = single). On the other hand, different structures? many cells group together in a single body and assume different functions in it to form What are these structures? various body parts in multicellular organisms These structures look similar to each other. (multi = many) such as some fungi, plants Together they form a big structure like an and animals. Can we find out names of some onion bulb! We find from this activity that more unicellular organisms? onion bulbs of different sizes have similar Every multi-cellular organism has come small structures visible under a microscope. from a single cell. How? Cells divide to d The cells of the onion peel will all look the produce cells of their own kind. All cells thus same, regardless of the size of the onion they come from pre-existing cells. he came from. These small structures that we see are Activity ______________ 5.2 the basic building units of the onion bulb. We can try preparing temporary These structures are called cells. Not only mounts of leaf peels, tip of roots of onions, but all organisms that we observe is onion or even peels of onions of different around are made up of cells. However, there sizes. are also single cells that live on their own. After performing the above activity, let us see what the answers to the following bl Cells were first discovered by questions would be: Robert Hooke in 1665. He observed (a) Do all cells look alike in terms of the cells in a cork slice with the help shape and size? pu (b) Do all cells look alike in structure? of a primitive microscope. (c) Could we find differences among Leeuwenhoek (1674), with the cells from different parts of a plant improved microscope, discovered the be T body? free living cells in pond water for the (d) What similarities could we find? re first time. It was Robert Brown in o R 1831 who discovered the nucleus in Some organisms can also have cells of More to know the cell. Purkinje in 1839 coined the different kinds. Look at the following picture. term ‘protoplasm’ for the fluid It depicts some cells from the human body. tt E substance of the cell. The cell theory, that all the plants and animals are composed of cells and that the cell is C the basic unit of life, was presented by two biologists, Schleiden (1838) and Schwann (1839). The cell theory no N was further expanded by Virchow Blood (1855) by suggesting that all cells cells Nerve Cell Smooth arise from pre-existing cells. With the muscle discovery of the electron microscope © cell in 1940, it was possible to observe and understand the complex structure of the cell and its various organelles. Bone Fat cell cell The invention of magnifying lenses led to the discovery of the microscopic world. It is Ovum Sperm now known that a single cell may constitute a whole organism as in Amoeba, Fig. 5.3: Various cells from the human body 58 SCIENCE  The shape and size of cells are related to every cell; plasma membrane, nucleus and the specific function they perform. Some cells cytoplasm. All activities inside the cell and like Amoeba have changing shapes. In some interactions of the cell with its environment cases the cell shape could be more or less are possible due to these features. Let us see fixed and peculiar for a particular type of cell; how. for example, nerve cells have a typical shape. Each living cell has the capacity to 5.2.1 P LASMA MEMBRANE OR CELL perform certain basic functions that are MEMBRANE characteristic of all living forms. How does a living cell perform these basic functions? This is the outermost covering of the cell that We know that there is a division of labour in separates the contents of the cell from its d multicellular organisms such as human external environment. The plasma membrane beings. This means that different parts of allows or permits the entry and exit of some he the human body perform different functions. materials in and out of the cell. It also The human body has a heart to pump blood, prevents movement of some other materials. a stomach to digest food and so on. Similarly, The cell membrane, therefore, is called a division of labour is also seen within a single selectively permeable membrane. cell. In fact, each such cell has got certain is How does the movement of substances specific components within it known as cell take place into the cell? How do substances organelles. Each kind of cell organelle move out of the cell? performs a special function, such as making bl Some substances like carbon dioxide or new material in the cell, clearing up the oxygen can move across the cell membrane waste material from the cell and so on. A by a process called diffusion. We have studied cell is able to live and perform all its pu the process of diffusion in earlier chapters. functions because of these organelles. These We saw that there is spontaneous movement organelles together constitute the basic unit of a substance from a region of high called the cell. It is interesting that all cells be T concentration to a region where its are found to have the same organelles, no concentration is low. re matter what their function is or what Something similar to this happens in cells o R organism they are found in. Q when, for example, some substance like CO 2 (which is cellular waste and requires to be uestions excreted out by the cell) accumulates in high tt E concentrations inside the cell. In the cell’s 1. Who discovered cells, and how? 2. Why is the cell called the external environment, the concentration of C structural and functional unit of CO2 is low as compared to that inside the life? cell. As soon as there is a difference of concentration of CO2 inside and outside a cell, no N CO2 moves out of the cell, from a region of high concentration, to a region of low 5.2 What is a Cell Made Up of? concentration outside the cell by the process of diffusion. Similarly, O2 enters the cell by What is the Structural © the process of diffusion when the level or Organisation of a Cell? concentration of O2 inside the cell decreases. Thus, diffusion plays an important role in We saw above that the cell has special gaseous exchange between the cells as well components called organelles. How is a cell as the cell and its external environment. organised? Water also obeys the law of diffusion. The If we study a cell under a microscope, we movement of water molecules through such would come across three features in almost a selectively permeable membrane is called THE FUNDAMENTAL UNIT OF LIFE 59  osmosis. The movement of water across the Activity ______________ 5.3 plasma membrane is also affected by the amount of substance dissolved in water. Osmosis with an egg Thus, osmosis is the passage of water from a (a) Remove the shell of an egg by dissolving it in dilute hydrochloric acid. The shell region of high water concentration through a is mostly calcium carbonate. A thin semi-permeable membrane to a region of low outer skin now encloses the egg. Put water concentration. the egg in pure water and observe after What will happen if we put an animal cell 5 minutes. What do we observe? or a plant cell into a solution of sugar or salt The egg swells because water passes in water? into it by osmosis. One of the following three things could (b) Place a similar de-shelled egg in a d happen: concentrated salt solution and observe for 5 minutes. The egg shrinks. Why? 1. If the medium surrounding the cell has Water passes out of the egg solution he a higher water concentration than the into the salt solution because the salt cell, meaning that the outside solution solution is more concentrated. is very dilute, the cell will gain water We can also try a similar activity with dried by osmosis. Such a solution is known raisins or apricots. is as a hypotonic solution. Water molecules are free to pass Activity ______________ 5.4 across the cell membrane in both Put dried raisins or apricots in plain bl directions, but more water will come into the cell than will leave. The net water and leave them for some time. Then place them into a concentrated (overall) result is that water enters the solution of sugar or salt. You will cell. The cell is likely to swell up. pu observe the following: 2. If the medium has exactly the same (a) Each gains water and swells when water concentration as the cell, there placed in water. be T will be no net movement of water (b) However, when placed in the across the cell membrane. Such a concentrated solution it loses water, re solution is known as an isotonic and consequently shrinks. o R solution. Unicellular freshwater organisms and Water crosses the cell membrane most plant cells tend to gain water through in both directions, but the amount osmosis. Absorption of water by plant roots tt E going in is the same as the amount is also an example of osmosis. going out, so there is no overall Thus, diffusion is important in exhange of gases and water in the life of a cell. In C movement of water. The cell will stay the same size. additions to this, the cell also obtains 3. If the medium has a lower nutrition from its environment. Different molecules move in and out of the cell no N concentration of water than the cell, meaning that it is a very concentrated through a type of transport requiring use solution, the cell will lose water by of energy. osmosis. Such a solution is known as The plasma membrane is flexible and is made up of organic molecules called lipids © a hypertonic solution. and proteins. However, we can observe the Again, water crosses the cell structure of the plasma membrane only membrane in both directions, but this through an electron microscope. time more water leaves the cell than The flexibility of the cell membrane also enters it. Therefore the cell will shrink. enables the cell to engulf in food and other Thus, osmosis is a special case of diffusion material from its external environment. Such through a selectively permeable membrane. processes are known as endocytosis. Amoeba Now let us try out the following activity: acquires its food through such processes. 60 SCIENCE  Activity ______________ 5.5 What do we infer from this activity? It appears that only living cells, and not dead Find out about electron microscopes cells, are able to absorb water by osmosis. from resources in the school library or Cell walls permit the cells of plants, fungi through the internet. Discuss it with your teacher. and bacteria to withstand very dilute Q (hypotonic) external media without bursting. uestions In such media the cells tend to take up water by osmosis. The cell swells, building up 1. How do substances like CO2 and pressure against the cell wall. The wall exerts water move in and out of the cell? an equal pressure against the swollen cell. Discuss. d Because of their walls, such cells can 2. Why is the plasma membrane withstand much greater changes in the called a selectively permeable he surrounding medium than animal cells. membrane? 5.2.3 NUCLEUS 5.2.2 CELL WALL Remember the temporary mount of onion peel is Plant cells, in addition to the plasma we prepared? We had put iodine solution on membrane, have another rigid outer covering the peel. Why? What would we see if we tried called the cell wall. The cell wall lies outside observing the peel without putting the iodine bl the plasma membrane. The plant cell wall is solution? Try it and see what the difference mainly composed of cellulose. Cellulose is a is. Further, when we put iodine solution on complex substance and provides structural the peel, did each cell get evenly coloured? pu strength to plants. When a living plant cell loses water According to their chemical composition through osmosis there is shrinkage or different regions of cells get coloured be T contraction of the contents of the cell away differentially. Some regions appear darker from the cell wall. This phenomenon is known than other regions. Apart from iodine solution re as plasmolysis. We can observe this we could also use safranin solution or o R phenomenon by performing the following methylene blue solution to stain the cells. activity: We have observed cells from an onion; let us now observe cells from our own body. tt E Activity ______________ 5.6 Mount the peel of a Rheo leaf in water Activity ______________ 5.7 C on a slide and examine cells under the high power of a microscope. Note the Let us take a glass slide with a drop of small green granules, called water on it. Using an ice-cream spoon no N chloroplasts. They contain a green gently scrape the inside surface of the substance called chlorophyll. Put a cheek. Does any material get stuck on strong solution of sugar or salt on the the spoon? With the help of a needle mounted leaf on the slide. Wait for a we can transfer this material and minute and observe under a © spread it evenly on the glass slide kept microscope. What do we see? ready for this. To colour the material Now place some Rheo leaves in boiling we can put a drop of methylene blue water for a few minutes. This kills the solution on it. Now the material is ready cells. Then mount one leaf on a slide and observe it under a microscope. Put for observation under microscope. Do a strong solution of sugar or salt on not forget to put a cover-slip on it! the mounted leaf on the slide. Wait for What do we observe? What is the shape a minute and observe it again. What of the cells we see? Draw it on the do we find? Did plasmolysis occur now? observation sheet. THE FUNDAMENTAL UNIT OF LIFE 61  Was there a darkly coloured, spherical present in eukaryotic cells. Many of the or oval, dot-like structure near the functions of such organelles are also centre of each cell? This structure is performed by poorly organised parts of the called nucleus. Were there similar cytoplasm (see section 5.2.4). The chlorophyll structures in onion peel cells? in photosynthetic prokaryotic bacteria is The nucleus has a double layered covering associated with membranous vesicles (bag called nuclear membrane. The nuclear like structures) but not with plastids as in membrane has pores which allow the transfer eukaryotic cells (see section 5.2.5). of material from inside the nucleus to its outside, that is, to the cytoplasm (which we d will talk about in section 5.2.4). Plasma Ribosomes The nucleus contains chromosomes, membrane which are visible as rod-shaped structures he only when the cell is about to divide. Cell wall Chromosomes contain information for inheritance of features from parents to next Nucleoid generation in the form of DNA (Deoxyribo is Nucleic Acid) molecules. Chromosomes are composed of DNA and protein. DNA molecules contain the information necessary for bl constructing and organising cells. Functional Fig. 5.4: Prokaryotic cell segments of DNA are called genes. In a cell which is not dividing, this DNA is present as 5.2.4 CYTOPLASM pu part of chromatin material. Chromatin material is visible as entangled mass of thread like structures. Whenever the cell is about to When we look at the temporary mounts of be T divide, the chromatin material gets organised onion peel as well as human cheek cells, we can see a large region of each cell enclosed re into chromosomes. o R The nucleus plays a central role in cellular by the cell membrane. This region takes up reproduction, the process by which a single very little stain. It is called the cytoplasm. cell divides and forms two new cells. It also The cytoplasm is the fluid content inside the tt E plays a crucial part, along with the plasma membrane. It also contains many environment, in determining the way the cell specialised cell organelles. Each of these organelles performs a specific function for the C will develop and what form it will exhibit at maturity, by directing the chemical activities cell. of the cell. Cell organelles are enclosed by no N In some organisms like bacteria, the membranes. In prokaryotes, beside the nuclear region of the cell may be poorly absence of a defined nuclear region, the defined due to the absence of a nuclear membrane-bound cell organelles are also membrane. Such an undefined nuclear region absent. On the other hand, the eukaryotic © containing only nucleic acids is called a cells have nuclear membrane as well as nucleoid. Such organisms, whose cells lack membrane-enclosed organelles. a nuclear membrane, are called prokaryotes The significance of membranes can be (Pro = primitive or primary; karyote ≈ karyon illustrated with the example of viruses. = nucleus). Organisms with cells having a Viruses lack any membranes and hence do nuclear membrane are called eukaryotes. not show characteristics of life until they enter Prokaryotic cells (see Fig. 5.4) also lack a living body and use its cell machinery to most of the other cytoplasmic organelles multiply. 62 SCIENCE  Q uestion 5.2.5 ((ii ) ENDOPLASMIC RETICULUM (ER) 1. Fill in the gaps in the following The endoplasmic reticulum (ER) is a large table illustrating differences network of membrane-bound tubes and between prokaryotic and sheets. It looks like long tubules or round or eukaryotic cells. oblong bags (vesicles). The ER membrane is similar in structure to the plasma membrane. There are two types of ER– rough endoplasmic reticulum (RER) and smooth endoplasmic Prokaryotic Cell Eukaryotic Cell reticulum (SER). RER looks rough under a 1. Size : generally 1. Size: generally microscope because it has particles called d small ( 1-10 μm) large ( 5-100 μm) ribosomes attached to its surface. The 1 μm = 10–6 m ribosomes, which are present in all active he cells, are the sites of protein manufacture. 2. Nuclear region: 2. Nuclear region: The manufactured proteins are then sent to _______________ well defined and various places in the cell depending on need, _______________ surrounded by a using the ER. The SER helps in the and known as__ nuclear membrane manufacture of fat molecules, or lipids, is important for cell function. Some of these 3. Chromosome: 3. More than one proteins and lipids help in building the cell single chromosome membrane. This process is known as bl membrane biogenesis. Some other proteins 4. Membrane-bound 4. _______________ and lipids function as enzymes and cell organelles _______________ hormones. Although the ER varies greatly in pu absent _______________ appearance in different cells, it always forms a network system. be T 5.2.5 CELL ORGANELLES re o R Every cell has a membrane around it to keep its own contents separate from the external environment. Large and complex cells, tt E including cells from multicellular organisms, need a lot of chemical activities to support their complicated structure and function. To C keep these activities of different kinds separate from each other, these cells use membrane-bound little structures (or no N ‘organelles’) within themselves. This is one of the features of the eukaryotic cells that distinguish them from prokaryotic cells. Some of these organelles are visible only with an © electron microscope. Fig. 5.5: Animal cell We have talked about the nucleus in a previous section. Some important examples Thus, one function of the ER is to serve as of cell organelles which we will discuss now channels for the transport of materials are: endoplasmic reticulum, Golgi apparatus, (especially proteins) between various regions lysosomes, mitochondria, plastids and of the cytoplasm or between the cytoplasm vacuoles. They are important because they and the nucleus. The ER also functions as a carry out some very crucial functions in cells. cytoplasmic framework providing a surface THE FUNDAMENTAL UNIT OF LIFE 63  for some of the biochemical activities of the Camillo Golgi was born at cell. In the liver cells of the group of animals Corteno near Brescia in called vertebrates (see Chapter 7), SER plays a crucial role in detoxifying many poisons and 1843. He studied medicine at the drugs. University of Pavia. After graduating in 1865, he continued to work in Pavia at the Hospital of St. Matteo. At that time most of his investigations d were concerned with the nervous system, In 1872 he accepted the post of Chief Medical Officer at the Hospital for the he Chronically Sick at Abbiategrasso. He first started his investigations into the nervous system in a little kitchen of this hospital, which he had converted into a laboratory. is However, the work of greatest importance, which Golgi carried out was a revolutionary method of staining individual nerve and cell bl structures. This method is referred to as the ‘black reaction’. This method uses a weak solution of silver nitrate and is pu particularly valuable in tracing the processes and most delicate ramifications of cells. All through his life, he continued be T Fig. 5.6: Plant cell to work on these lines, modifying and improving this technique. Golgi received re o R the highest honours and awards in ii)) G OLGI APPARATUS 5.2.5 ((ii ii recognition of his work. He shared the Nobel prize in 1906 with Santiago Ramony The Golgi apparatus, first described by tt E Cajal for their work on the structure of the Camillo Golgi, consists of a system of nervous system. membrane-bound vesicles arranged C approximately parallel to each other in stacks called cisterns. These membranes often have 5.2.5 ((iii iii iii)) LYSOSOMES connections with the membranes of ER and no N therefore constitute another portion of a Lysosomes are a kind of waste disposal complex cellular membrane system. system of the cell. Lysosomes help to keep The material synthesised near the ER is the cell clean by digesting any foreign material packaged and dispatched to various targets as well as worn-out cell organelles. Foreign © inside and outside the cell through the Golgi materials entering the cell, such as bacteria apparatus. Its functions include the storage, or food, as well as old organelles end up in the lysosomes, which break them up into modification and packaging of products in vesicles. In some cases, complex sugars may small pieces. Lysosomes are able to do this because they contain powerful digestive be made from simple sugars in the Golgi enzymes capable of breaking down all organic apparatus. The Golgi apparatus is also material. During the disturbance in cellular involved in the formation of lysosomes [see metabolism, for example, when the cell gets 5.2.5 (iii)]. 64 SCIENCE  damaged, lysosomes may burst and the 5.2.5 ((vi vi vi)) VACUOLES enzymes digest their own cell. Therefore, lysosomes are also known as the ‘suicide Vacuoles are storage sacs for solid or liquid bags’ of a cell. Structurally, lysosomes are contents. Vacuoles are small sized in animal membrane-bound sacs filled with digestive cells while plant cells have very large vacuoles. enzymes. These enzymes are made by RER. The central vacuole of some plant cells may occupy 50-90% of the cell volume. 5.2.5 ((iv iv iv)) MITOCHONDRIA In plant cells vacuoles are full of cell sap Mitochondria are known as the powerhouses and provide turgidity and rigidity to the cell. of the cell. The energy required for various Many substances of importance in the life of d chemical activities needed for life is released the plant cell are stored in vacuoles. These by mitochondria in the form of ATP include amino acids, sugars, various organic (Adenosine triphopshate) molecules. ATP is he acids and some proteins. In single-celled known as the energy currency of the cell. The organisms like Amoeba, the food vacuole body uses energy stored in ATP for making contains the food items that the Amoeba has new chemical compounds and for mechanical work. Mitochondria have two membrane consumed. In some unicellular organisms, specialised vacuoles also play important roles is coverings instead of just one. The outer membrane is very porous while the inner in expelling excess water and some wastes from the cell. Q membrane is deeply folded. These folds create bl a large surface area for ATP-generating chemical reactions. uestions Mitochondria are strange organelles in the pu sense that they have their own DNA and 1. Can you name the two organelles ribosomes. Therefore, mitochondria are able we have studied that contain to make some of their own proteins. their own genetic material? be T 2. If the organisation of a cell is 5.2.5 (V) PLASTIDS destroyed due to some physical re o R or chemical influence, what will Plastids are present only in plant cells. There are two types of plastids – chromoplasts happen? (coloured plastids) and leucoplasts (white or 3. Why are lysosomes known as tt E colourless plastids). Plastids containing the suicide bags? pigment chlorophyll are known as 4. Where are proteins synthesised chloroplasts. Chloroplasts are important for inside the cell? C photosynthesis in plants. Chloroplasts also contain various yellow or orange pigments in Each cell thus acquires its structure and addition to chlorophyll. Leucoplasts are no N ability to function because of the organisation primarily organelles in which materials such of its membrane and organelles in specific as starch, oils and protein granules are ways. The cell thus has a basic structural stored. organisation. This helps the cells to perform The internal organisation of the plastids © functions like respiration, obtaining nutrition, consists of numerous membrane layers and clearing of waste material, or forming new embedded in a material called the stroma. Plastids are similar to mitochondria in proteins. external structure. Like the mitochondria, Thus, the cell is the fundamental plastids also have their own DNA and structural unit of living organisms. It is also ribosomes. the basic functional unit of life. THE FUNDAMENTAL UNIT OF LIFE 65  What you have learnt The fundamental organisational unit of life is the cell. Cells are enclosed by a plasma membrane composed of lipids and proteins. The cell membrane is an active part of the cell. It regulates the d movement of materials between the ordered interior of the cell and the outer environment. he In plant cells, a cell wall composed mainly of cellulose is located outside the cell membrane. The presence of the cell wall enables the cells of plants, fungi and bacteria to exist in hypotonic media without bursting. is The nucleus in eukaryotes is separated from the cytoplasm by double-layered membrane and it directs the life processes of the cell. bl The ER functions both as a passageway for intracellular transport and as a manufacturing surface. pu The Golgi apparatus consists of stacks of membrane-bound vesicles that function in the storage, modification and packaging of substances manufactured in the cell. be T Most plant cells have large membranous organelles called plastids, which are of two types – chromoplasts and leucoplasts. re o R Chromoplasts that contain chlorophyll are called chloroplasts and they perform photosynthesis. The primary function of leucoplasts is storage. tt E Most mature plant cells have a large central vacuole that helps to maintain the turgidity of the cell and stores important C substances including wastes. Prokaryotic cells have no membrane-bound organelles, their chromosomes are composed of only nucleic acid, and they have no N only very small ribosomes as organelles. Exercises © 1. Make a comparison and write down ways in which plant cells are different from animal cells. 2. How is a prokaryotic cell different from a eukaryotic cell? 3. What would happen if the plasma membrane ruptures or breaks down? 66 SCIENCE  4. What would happen to the life of a cell if there was no Golgi apparatus? 5. Which organelle is known as the powerhouse of the cell? Why? 6. Where do the lipids and proteins constituting the cell membrane get synthesised? 7. How does an Amoeba obtain its food? 8. What is osmosis? 9. Carry out the following osmosis experiment: Take four peeled potato halves and scoos each one out to make d potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. he Now, (a) Keep cup A empty (b) Put one teaspoon sugar in cup B (c) Put one teaspoon salt in cup C is (d) Put one teaspoon sugar in the boiled potato cup D. Keep these for two hours. Then observe the four potato cups bl and answer the following: (i) Explain why water gathers in the hollowed portion of B and C. pu (ii) Why is potato A necessary for this experiment? (iii) Explain why water does not gather in the hollowed out be T portions of A and D. re o R tt E C no N © THE FUNDAMENTAL UNIT OF LIFE 67