Topic 3: Cells
All plants and animals are made up of units called cells. Some organisms, like protozoans, diatoms and bacteria consist of single cell. Others are made up of many millions; for example, there are about thirty million million cells in an adult human body. In many-celled (multicellular) organisms there are different types of cells, with different functions, shapes and sizes.
The smallest cells are bacteria, which can only last be seen with a microscope, and the largest cells are birds' eggs (each egg is a single cell). Most cells are between 0-25mm and 0-025mm in diameter.
FIGURE 1.5: The two types of cells eukaryotic left and prokaryotic right.
The cell structure of living things was first seen by Hooke in 1667, when he examined fine slices of cork and other plant material with a primitive microscope. He believed that .the cells were empty and the cell wall was the most important part. We now realise that the most important part of living cells is their contents.
The internal structure of a plant cell
The cells of the 'flesh' of the tomato are thin-walled sacs. When mounted for microscope examination, the cells become flabby and their walls wrinkled. The wall of the cell is made of cellulose, and is permeable to water. Inside the cell wall is the cytoplasm, which is a thick jelly
Cell from tomato fruit. A External view ( x 400) B Section (diagrammatic) jelly, like uncooked white of egg. It consists of a complicated mixture of chemical compounds and contains a high proportion of water.
Also inside the cell wall is the plasmic membrane. This is a thin film on the surface of the cytoplasm which is important because it controls the passage of substances into and out of the cytoplasm.
Inside the cytoplasm is a ball-shaped or oval body called the nucleus, which cannot usually be seen unless the cell has been stained with certain dyes. As well as the nucleus, the cytoplasm contains many reddish-orange granules which give the tomato cell its colour.
Colourless granules are also present. Many plant cells contain small structures called chloroplasts, which contain the green pigment chlorophyll. They can be seen in the cells of green, unripe tomatoes.
The living material of the cell consists of the nucleus and cytoplasm, and is called the protoplasm. The nucleus is extremely important as it controls the living processes of the cell. The cytoplasm and nucleus cannot live without each other. If a cell is divided into two parts, one with the nucleus and one without it, only the part with the nucleus will live.
The granular cytoplasm of the tomato cell appears to have spaces in it. These 'spaces' are really the cell vacuoles which contain the cell sap. This is a watery solution containing sugars and other substances. Each vacuole is surrounded by a film of cytoplasm similar to the plasmic membrane.
In young plants the whole cell is filled with cytoplasm. As the cell grows older, its wall gets bigger, but the amount of cytoplasm only increases slightly, and vacuoles appear. In some plant cells (like the tomato), the cell wall is made of cellulose throughout its life, but in others it becomes thickened by impregnation with other substances. This gives many plant cells a constant and distinctive shape.
The internal structure of animal cells
Fig. 1.8 Two human cheek cells ( x 300),
In general structure animal cells are similar to plant cells, but they differ in the following ways.
Their protoplasm is denser and contains many small vacuoles.
They do not have a thick cellulose wall, but are enclosed by a thin limiting membrane.
Chloroplasts are never present.
Cells were discovered by Hooke in 1667 AD. Under his rough microscope, he viewed cork and saw the cells as compartments. Hooke thought that the walls of the cells were the only important things and that the inside of the cell was only empty space. He was wrong at this thought.
All plants and animals are made of cells. Most cells are microscopic but some can be over a metre long for example nerve cells, and others are as large as a small orange for example an ostrich egg.
Cells contain organised structures suspended in the cytoplasm called organelles. Examples of organelles include chloroplasts for photosynthesis, mitochondria for respiration, etc.
Prokaryotic cells like those of bacteria do not contain a nuclear membrane bounding the nuclei. Eukaryotic cells contain a nuclei membrane bounding the nucleus.
The cell membrane
This is a thin membrane made of protein and fat molecules. It is for retaining the content of the cell and for controlling substances leaving and entering the cell. So it is called a selectively permeable membrane or semi-permeable membrane i.e. it allows certain substances to pass through but not others.
The cell wall
In plants and plants only there is a wall outside the membrane. This cell wall is made of cellulose.
This wall gives shape to the cell and maintains rigidity. It is a freely permeable membrane i.e. it allows substances to pass through in either directions. When plant cells divide, a layer forms between them and is called the middle lamella.
Protoplasm
This is the living matter found inside cells and there are two types of protoplasm: Cytoplasm and nucleoplasm. Cytoplasm is a jelly-like substance which contains organelles. Examples of such organelles include chloroplasts for photosynthesis; mitochondria for respiration. The cytoplasm forms a semi-permeable membrane which allows some substances to enter or leave the cell but not others. Nucleoplasm is the fluid found in the nucleus.
The nucleus
This is the control centre of the chemical activities of the cell and is usually bounded by a nuclear membrane.
The nucleus is lighter in colour than the cytoplasm but in diagrams the nucleus is always drawn darker because most microscopic preparations are stained with dyes to show the nucleus up clearly.
The nucleus is less easily visible in unstained cells.
Functions of the nucleus
1. It determines the size and function of the cell.
2. It also controls all the processes within the cell. For instance, if a cell is cut into two parts: one part containing the nucleus and the other without the nucleus; the part with the nucleus will live and the one without will die.
3. The nucleus also controls the process of cell division.
4. The nucleus has got threads in it called chromosomes. On these chromosomes are long molecules of an acid called De-oxyribonucleic acid (DNA). The chromosomes are important in inheritance and protein synthesis in the body.
5. Some cells lack a nucleus for example in fungi.
The vacuole
These are spaces in cells. In animal cells they are small, scattered and temporary.
In plant cells there is usually one vacuole which is large, permanent and central. It is also filled with a fluid called cell sap containing salts, sugars and pigments all dissolved in water.
Detailed structure of a cell:
In the electron microscope the animal cell looks like this:
The golgi apparatus is for secretion; the mitochondria are the ‘power house' for respiration; the endoplasmic reticulum is for storage; the ribosomes are for protein synthesis; the lysosomes for digestion; and the nucleolus is where a chemical called ribonucleic acid (RNA) is made. It is this RNA which passes out of the nucleus to the cytoplasm to carry information for control of the cell processes.
Shape of a cell
Cells are of different shapes because they perform different functions. Some cells are spherical for example the spongy cell in a leaf; others have no definite shape like the white blood cell.
The shape of a cell drawn on paper depends on the direction in which a section is cut. For instance a cell may be cubed or cylindrical.
Cell division:
Many cells divide to produce new cells. Cells divide in order to cause growth by a process called mitosis.
Growing parts therefore usually have cells dividing by mitosis for example plant meristems of the terminal bud and root tips, cambium of roots and stems.
A cell about to divide has a large nucleus, a thick soft cell wall and no vacuole. The nucleus divides first and then the cells are separated by a middle lamella.
Mitosis is a process of cell duplication, or reproduction, during which one cell gives rise to two genetically identical daughter cells. Strictly applied, the term mitosis is used to describe the duplication and distribution of chromosomes, the structures that carry the genetic information.
Prior to the onset of mitosis, the chromosomes have replicated and the proteins that will form the mitotic spindle have been synthesized. Mitosis begins at prophase with the thickening and coiling of the chromosomes. The nucleolus, a rounded structure, shrinks and disappears. The end of prophase is marked by the beginning of the organization of a group of fibres to form a spindle and the disintegration of the nuclear membrane.
The chromosomes, each of which is a double structure consisting of duplicate chromatids, line up along the midline of the cell at metaphase. In anaphase each chromatid pair separates into two identical chromosomes that are pulled to opposite ends of the cell by the spindle fibres. During telophase, the chromosomes begin to decondense, the spindle breaks down, and the nuclear membranes and nucleoli re-form. The cytoplasm of the mother cell divides to form two daughter cells, each containing the same number and kind of chromosomes as the mother cell. The stage, or phase, after the completion of mitosis is called interphase.
Mitosis is absolutely essential to life because it provides new cells for growth and for replacement of worn-out cells. Mitosis may take minutes or hours, depending upon the kind of cells and species of organisms. It is influenced by time of day, temperature, and chemicals.
Some cells that produce gametes carry out a type of cell division called meiosis where gametes are formed. It is also called Reduction Division.
This is actually a division of a germ cell involving two fissions of the nucleus and giving rise to four gametes, or sex cells, each possessing half the number of chromosomes of the original cell.
The process of meiosis is characteristic of organisms that reproduce sexually. Such species have in the nucleus of each cell a diploid (double) set of chromosomes, consisting of two haploid sets (one inherited from each parent). These haploid sets are homologous-i.e., they contain the same kinds of genes, but not necessarily in the same form. In humans, for example, each set of homologous chromosomes contains a gene for blood type, but one set may have the gene for blood type A and the other set the gene for blood type B.
Prior to meiosis, each of the chromosomes in the diploid germ cell has replicated and thus consists of a joined pair of duplicate chromatids. Meiosis begins with the contraction of the chromosomes in the nucleus of the diploid cell. Homologous paternal and maternal chromosomes pair up along the midline of the cell. Each pair of chromosomes-called a tetrad, or a bivalent-consists of four chromatids. At this point, the homologous chromosomes exchange genetic material by the process of crossing over. The homologous pairs then separate, each pair being pulled to opposite ends of the cell, which then pinches in half to form two daughter cells. Each daughter cell of this first meiotic division contains a haploid set of chromosomes. The chromosomes at this point still consist of duplicate chromatids.
FIGURE 3.7
Eukaryotic Cell. Compare and contrast the eukaryotic cell shown here with the prokaryotic cell. What similarities and differences do you see?
In the second meiotic division, each haploid daughter cell divides. There is no further reduction in chromosome number during this division, as it involves the separation of each chromatid pair into two chromosomes, which are pulled to the opposite ends of the daughter cells. Each daughter cell then divides in half, thereby producing a total of four different haploid gametes. When two gametes unite during fertilization, each contributes its haploid set of chromosomes to the new individual, restoring the diploid number.
Differences between plant and animal cells:
- Plant cell are comparatively larger and easier to see than animal cells.
- In addition to cell membranes, plant cells have cell walls made of cellulose where as animal cells have only cell membranes.
- Plant cells have a thin lining of cytoplasm with a large central vacuole where as animal cells have cytoplasm with small and temporary vacuoles for excretion and secretion.
- Plant cells have chloroplasts for trapping light whereas animal cells do not have chloroplasts.
- Plant cells have the nucleus on the sides whereas animal cells have the nucleus in the centre.
- Plant cells have starch grains whereas animal cells have glycogen granules.
Cheek cells:
If a finger is run the inside of a cheek, some cells will be scraped off.
ORGANISATION OF CELLS:
Tissues:
In an organism, each cell carries out some life process but it can not exist on its own.
For instance, a muscle cell for locomotion cannot get its food or oxygen. It is other special cells that collect food or carry oxygen.
So individual cells must be grouped together in large numbers in order to work together by coordinating. Such a group of cells each with more or less identical function is called a tissue.
Thus each tissue has a specific function. A tissue may have one to three different types of cells.
Examples of tissues include blood tissue which transports substances; nerve tissue which transmits impulses and xylem tissue which carries water to the leaves of a plant.
The simplest tissue in man is the epithelial tissue which is for protection and for allowing diffusion of substances. There is also the connective tissue that has tough fibres for binding tissues together.
In plants there is the epidermal tissue also for protection.
Organs:
Tissues cannot exist on their own. Instead, several tissues must be grouped together for a particular function. Such a collection of tissues forms an organ.
In man, the organ called the leg for movement consists of blood tissue, nerve tissue, muscle tissue, etc.
In plants the organ called the stem consists of phloem tissue, xylem tissue, cork tissue, etc.
In man the most diversified organ is the liver with many functions and the least diversified organ is the heart, which has the sole function of pumping blood.
Systems:
Organs cannot exist on their own. Instead a series of organs with a coordinated function must be grouped together for effective action. Such a series of organs is called a system.
Examples include: The urinary system which consists of organs like the kidneys, ureters, a urethra and bladder; the nervous system which consists of organs like the brain, spinal cord and the nerves.
Systems cannot exist on their own. Instead organs, tissues and cells must coordinate to form an individual capable of separate existence. This individual is called an organism.
Specialisation of cells:
Each cell is adapted to a particular function in an organ. Here are some examples:
Ciliated cells: These are usually found in the air passages like the trachea, bronchi, bronchioles and nose. They have tiny hairs which keep flicking to and fro thus sweeping mucus with trapped germs and dust away from the lungs.
White blood cells: These are also called leucocytes. They have no definite shape. They are found in the blood and are for engulfing harmful germs. They also destroy germs by producing antibodies. They can move out of blood through the walls of capillaries to go and fight bacteria.
Dead bacteria and white blood cells form what we know as pus.
Phloem cells: These are long living cells joined end to end and where they meet, perforations occur in the wall. Dissolved food passes through these perforations during transport from the leaves to storage organs.
The companion cell beside the phloem cell contains a nucleus and mitochondria for release of energy to transport food.
Nerves cells: These are also called neurons. The nerve cell conducts messages called impulses along its fibres which are usually very long. A nerve fibre may start from the foot up to the spinal cord.
Nerve cells which carry impulses from the sense organs towards the brain are called sensory nerve cells or sensory neurons.
Nerve cells that carry impulses from the brain towards the muscles are called motor nerve cells or motor neurons like the one drawn below.
Lesson Summary
- Discoveries about cells using the microscope led to the development of the cell theory. This theory states that all organisms are made of one or more cells, all the life functions of organisms occur within cells, and all cells come from already existing cells.
- All cells are very small because they need to pass substances across their surface. Their small size gives them a relatively large ratio of surface area to volume, facilitating the transfer of substances. The shapes of cells may vary, and a cell's shape generally suits its function.
- Cells are diverse, but all cells contain a plasma membrane, cytoplasm, ribosomes, and DNA.
- Prokaryotic cells are cells without a nucleus. They are found in single-celled organisms. Eukaryotic cells are cells with a nucleus and other organelles. They are found mainly in multicellular organisms.
Review Questions
Recall
1. What did Hooke and Leeuwenhoek discover about cells by using a microscope?
2. What does the cell theory state? Name the three scientists mainly responsible for developing the cell theory.
3. List the four parts that are found in all living cells.
Think Critically
5. Why are all cells very small? Explain what limits the size of cells.
6. Compare and contrast prokaryotic cells and eukaryotic cells.
7. Explain why viruses are not considered to be living.
Points to Consider
Cells have many different structures that carry out the processes of life.
- Beside the cell parts described in this lesson, what other structures do you think cells might have? What life processes might these other structures carry out?