INTRODUCTION A Cell is the basic unit of life which can function independently. A human body contains about 50 million cells. Cell arise from other cells by cell division. FINDINGS SECTION ONE: CELL AS A UNIT OF LIFE Organisms are made up of cells. Cell is termed a unit of life because it possesses the characteristics of a living organism. Cells must be able to move, respond to stimuli and adapt to their environment. They must be able to acquire food for growth and manufacture energy for metabolism. Like most organisms, cells must be able to permanently increase their size or number of their cells and must be able to reproduce (Cell division) and remove excess/toxic substances. Apart from viruses, all organisms are either prokaryotic or eukaryotic. Prokaryotic cells (e.g. Bacteria) are the earliest form of cell; they have no separate nucleus and can tolerate harsh environmental conditions. Eukaryotic cells (e.g. Fungi) have a true nucleus, they are larger and more complex. Fig 1.1 cell types. (Wikipedia,2013) Differences between Prokaryotic and Eukaryotic cells Basis for comparison Prokaryotic cells Eukaryotic Cells Organisms Bacteria Plants, animals, fungi Cell walls Present Present in plant and fungi but not in animals Internal structure Few organelles Many organelles Number of cells Unicellular Multicellular Motility Uses flagella Uses cilia Site of genetic material DNA in cytoplasm DNA inside distinct nucleus Organisation of genetic material DNA is circular and does not condense at cell division DNA is linear and condenses into visible chromosomes before cell division Diameter of cells Smaller (0.1-10 pm) Larger (10-100 pm) Mode of reproduction Uses binary fission Uses Mitosis and Meiosis. (Boyle and Senior 2002). Both prokaryotic and eukaryotic cells have plasma membrane, ribosomes, cytoplasm, and DNA. Prokaryotes carry out the process of respiration anaerobically while eukaryotes do theirs aerobically through their mitochondrion to generate energy needed for their metabolic demands. Viruses are infectious agents which cannot grow or reproduce outside of another living host cells. They are of different shapes and sizes. Fig 1.2 Structure of Viruses. (PMG biology, 2015) Viruses infect animal, plant or bacteria depending on the specific virus and the nature of the host cell. The Bacteriophage from fig 1.2 affects bacteria while Influenza virus affects mammals and birds. Viruses are different from cells (both prokaryotic and Eukaryotic) because they have no cell, they are unable to self-reproduce like the cells, and they do not respond to stimuli or exhibit metabolism. Viruses cannot grow or reproduce without a cell. All species of animals and plants are made of eukaryotic cells. Eukaryotic cells have different organelles which play specific role to aid its survival. These organelles include; Nucleus which is the largest organelle in the cell. It contains the cell`s DNA, which allows the cell to carry out its cellular processes, Cytoplasm; a solution within the cell membrane which contains enzymes and properties required for the cell to function. Nucleolus is the dark region of chromatin which is involved in manufacture of ribosomes. Mitochondrion; a “sausage shaped “organelle often referred to as the power house of a cell. Its function is to make ATP via the process of aerobic respiration. ATP is a molecule that diffuses around the cell and provides instant chemical energy to the processes that require it. Fig 1.3 Mitochondria. (Biochemist01, 2013). Chloroplast are one of a group of plant cells organelles known as plastids. They are surrounded by a double membrane and contain an elaborate internal membrane system that houses the chemicals of photosynthesis (Boyle and Senior 2002). Ribosomes are the most numerous and smallest organelles. They can be found on the surface of the Rough ER or free in the cytoplasm. Protein synthesis occurs in the ribosome. Smooth Endoplasmic Reticulum (SER) is the organelle responsible for the metabolism of fats and carbohydrates. It transports lipids materials to area where they are needed in the cell. Rough Endoplasmic Reticulum (RER) is similar to SER but has attached ribosomes. It modifies proteins made by the ribosome and transports them from the cell via the Golgi body. Golgi body is a group of flattened cavities or vesicles formed from the rough ER. They transport proteins from the rough ER to the cell membrane. Vacuoles are fluid-filled membrane bound sac which acts as a storage for food substances. The plant cells have a large central vacuole that becomes turgid when filled with water. They also store the pigment that gives plant its colours. Cytoskeleton is a network of protein fibres which extends throughout the cytoplasm and gives the cell its shape. It supports and moves the whole cell, and also forms the spindle during cell division. Cilia and flagella are like flexible tails used for movement. Flagella are larger and can move a whole cell. Cilia are used to move substances over or around the cell. Microvilli are often found in the lining cells such as epithelial cells. They increase the surface area of cell for absorption of materials. Lysosome function as the digestive system of the cell. It contains lytic enzymes which destroys worn out cells and organelles. Lysosomes are formed from the rough ER. Cell membrane is a phospholipid bilayer studded with proteins. It separates the internal contents of the cell from its environment and controls the materials that enters or leaves the cell. Cell wall is found in plants and fungi. It gives protection, support and shape to the cell. Centrioles are short bundles of microtubes found near the nucleus in animal cells. They form the spindle which guides the chromosome during cell division. Peroxisomes are small, round organelles enclosed by single membranes; they carry out oxidation reactions that break down fatty acids and amino acids. Peroxisomes also detoxify many poisons that may enter the body. There are over 200 different cells in the human body. Some are stem cells while others are differentiated cells. Stem cells are undifferentiated cells which have the potential to become any type of human cell once they are well developed. They can self-renew or multiply without losing the potential to develop into other kind of cells but cannot perform specific functions in the body. For example, they can become blood, heart or skin cells. The three main types of stem cells are the embryonic, foetal and adult stem cells. Fig 1.4 stem cell differentiation into various tissues (prleap.com, 2017). Differentiated cells on the other hand, are specialised cells which perform specific function in the body and will only become one type of cell when they mature and develop. E.g. are epithelial cells, liver cells and smooth muscle cells. SECTION TWO: Understanding Cellular Metabolism. The cell controls what enters and leaves it through the cell membrane. The cell membrane is a selectively permeable structure which is made up of a phospholipid bilayer which has a water loving (hydrophilic)head and tail with a middle layer that doesn`t like water (hydrophobic). It also contains proteins, glycoproteins and cholesterol which work in a fluid state to absorb incoming nutrients, extract waste and give the cell a stable structure (SAC 2017). Fig 2.1 Structure of a cell membrane (Socratic.org., 2016) The nature of a substance will depend on how it crosses the cell membrane. There are different possible methods by which substances and nutrients can cross the cell membrane. It could be through Lipid diffusion, where lipid molecules (e.g. steroids, water, Oxygen) pass through the lipid layer of the cell membrane by moving down their concentration gradients (diffusion) into the cell or through Osmosis, where water molecules move by diffusion across the membrane only. Water moves from a solution where it is in abundance to area where there is lower water potential. The simplest forms of transport across a membrane is passive. Passive transport does not require the cell to expend any energy and involves a substance diffusing down its concentration gradient across a membrane. However, during Active transport, dissolved molecules move across a cell membrane from a lower to a higher concentration. Here, protein will bind a molecule to be transported on one side of the membrane and release it on the other side. In active transport, unlike passive transport, the cell expends energy (for example, in the form of ATP) to move a substance against its concentration gradient (Khan Academy, 2017). Fig 2.2 Active Transport. (Byjus, 2017) Vesicles are also used for transporting larger molecules like polysaccharides and nucleotides. When such substances are transported into a cell, it is termed endocytosis whilst out of a cell is exocytosis (SAC,2017). ANIMAL CELL All living animal cell require an energy source for growth, movement and cell division. Cell get its energy through cellular respiration which is a process that converts biochemical energy from food molecules into adenosine triphosphate (ATP), and then release waste products. Fig 2.3 Cellular Respiration (Room 114, 2017). Respiration Occurs in two stages; glycolysis and the Kreb`s Cycle. Glycolysis occurs in the cytoplasm, where glucose is broken down to form 2 molecules of pyruvic acid. These are then transported to the mitochondria where it combines with co enzyme A to form acetyl-CoA. Energy for the rest of the cell`s function is produced through the creation of ATP, NADH and FADH2 by the chemical reactions occurring in the Kreb`s Cycle. When a Phosphate is released to a site which needs energy, the ATP becomes ADP. ADP is recharged in the Mitochondria to become ATP again. To aid movement, chemical energy is changed into kinetic energy and heat energy. The process of respiration also manufactures heat energy needed for movement. The movement within the cell can involve endocytosis, exocytosis and active transport. To grow, animal cell uses anabolism which involves breaking down simple molecules from food to form complex molecules. This requires stored energy known as ATP to be hydrolysed into ADP and Pi. ADP and Pi releases energy to wherever it is needed. During cell division, key structures will move in the nucleus with the aid of energy. The centrioles move to form spindle required for the pulling apart of chromosomes during Mitosis. Protein also play a major role in growth and repair in animal cells. There are 20 amino acids found in the body. One protein differs from another by the sequence in which the amino acids align in a polypeptide chain of the protein. The DNA will determine the exact order proteins align in a polypeptide chain. Protein is synthesised by producing proteins using information coded by DNA, located in the nucleus of a cell. Fig 2.4 Protein synthesis. (Creative Biostructure, 2018). Protein synthesis occurs in 3 stages; Transcription occurs in the nucleus where the genetic code in the DNA is transferred to a messenger RNA (mRNA) which carries the code for building a specific protein to where it is needed. The RNAs exit the nucleus into the cytoplasm where Activation commences by binding amino acid to a site on the transfer RNA (tRNA) molecule with the aid of an enzyme. Each tRNA molecule is distinct to one of the 20 different amino acids and will connect at the base (anticodon). The last stage is the Translation stage which occurs in the ribosome. The tRNA picks up specific amino acids from the cytoplasm and brings them into position on the surface of a ribosome where they can be joined together at the codon in specific order to make a specific protein. Nuclei acids has to be in play for protein to be synthesised. Nuclei acids are macromolecules which keeps hereditary information in a cell so that the cell can perform its function effectively. They are made up from chains of nucleotides, and are required for protein synthesis. There are two types of Nuclei acids; One is Deoxyribonucleic acid (DNA) which controls all activities of the cell, stores genetic information, supplies information for protein synthesis and controls synthesis of RNA in the cell. Fig 2.5 DNA and RNA Structure. (Lieff, 2014) The other is the Ribonucleic acid (RNA) which carries instructions from the DNA in the nucleus to the ribosome in the cytoplasm where it controls the synthesis of proteins from amino acids. There are 3 forms of RNA; Ribosome RNA (rRNA) which is manufactured by the DNA in the nucleus but located in the cytoplasm. It speeds up the formation of peptide bonds between amino acids. Transfer RNA (tRNA) is where an amino acid can attach to during the activation stage in protein synthesis. There are 20 different tRNA, each specific to each form of the 20 amino acids. The Messenger RNA (mRNA) which is formed into a helix shape by a thousand nucleotides is made in the nucleus and occurs during the transcription stage of protein synthesis. The table list out the differences between DNA and RNA. Comparison DNA RNA Shape Double stranded (Helix) Single stranded Length Long Molecule Shorter than DNA Nitrogenous Bases Contains thiamine Contains Uracil Types Several rRNA, mRNA and tRNA. Found in Nucleus Nucleus and cytoplasm Sugar Deoxyribose Ribose Function Genetic role Protein synthesis Replication Self-replicating Synthesised from DNA SECTION THREE: HOW CELL GROW AND DIVIDE The series of events a cell goes through when it grows and divides is called a cell cycle. The Cell cycle consists of two Key aspects; the Interphase and Mitosis. INTERPHASE The interphase is the place where a cell spends most of its life. Duplication of DNA and cell organelles occurs during interphase. Fig 3.2 Stages in Interphase (Slideplayer.com, 2016). Interphase consists of 3 phases. At the G1 Phase, majority of the cell organelles are copied. Cell grows bigger, protein synthesis occurs and the volume of cytoplasm increases. At S Phase, the DNA molecule and centrosome are replicated or copied. Cells only enter this stage if they are going to divide. Centrosome helps in the separation of DNA during Mitosis. The final is the G2 Phase where the cell grows rapidly, synthesises proteins, and begin to reorganize its contents in preparation for mitosis. G2 phase ends when mitosis begins. Some cells remain in the interphase for months, while some like the brain and nerve cells which rarely divides will always be in interphase. MITOSIS This is a type of cell division that leads to the formation of two daughter cells from an existing parental cell, with each cell having the same number and kind of chromosomes as the parental cell. Mitosis is used in growth and repair of animal cells. It is the step after the Interphase. Fig 3.4 Stages of Mitosis ( Bigstock, n.d.) There are four stages in Mitosis. The first stage is the Prophase where the duplicated chromosomes from the previous phase condenses and become visible. In the Metaplase, the mitotic spindle is formed and chromosomes become lined up at the middle of the spindle. The pairs of chromatids are attached to the spindle at the centromere. The next stage is the Anaphase, where the pairs of chromatids are pulled apart by movements of the spindle fibres. The centromeres split and sister chromatids move in opposite directions towards the poles of the cell. Telophase is where the nuclear envelope which had previously broken down to allow the microtubules to access and recruit the chromosomes to the middle of the dividing cell, reforms as two new nuclear envelopes around the separated sister chromatids. The cell cytoplasm is then split into two by constriction from the edges of the cell. This results in two identical daughter cells being produced. This process is called cytokinesis. The two cells are genetically identical daughter cells. Cell division is a complex process that requires a lot of energy to pull off. Numerous proteins are required to move molecules, membranes, and DNA in appropriate ways that do not result in damage. Factors that affect cell division could be physical or Chemical (SAC, 2017). Physical factors would include Stress which could lead to cell damage and may result in cancer. If cells are crowded or lack nutrients, they could stop growing. Chemical factors would include some toxic pesticides can alter the genetic code for proteins to be synthesized. Also, a defect in any of the below growth factors could affect cell division. CDK-Cyclin which controls the movement of the cell through the stages in the cell cycle. Maturation Promoting Factor (MPF) which controls entry into mitosis from G2 stage. Since cells mature during the cell cycle, and cancerous cells divide before they reach maturity. MPF screens what goes into mitosis. P53- which is a regulatory protein in the cell cycle that can cause cell death (apoptosis). Conserving P53 helps maintain stability of the cell and prevent cancer from happening. P27- which is a regulatory protein that blocks the entry into the S phase of the cell cycles. It does this by binding with CDK and cyclin. Severe breast cancer patients have been found to have low levels of P27. Cancerous cells are different both physically and characteristically to normal cells. Fig 3.3 Normal cell vs Cancer cell (Agegard, 2016). Cancer cells are damaged versions of normal cells. They can continue growing until they damage the healthy cells of the body. They grow sporadically and possess the ability to spread over large areas. They lack the immunity that normal cells possess. Cancerous cells are usually down to a combination of abnormalities rather than a single mutation or protein abnormality (SAC, 2017). Differences between Cancer cell and Normal cell Characteristics or Organelle Cancer cell Normal cell Cell Reproduction Reproduce in an uncontrollable way Reproduce in a normal way Cell communication Does not communicate with other cells Communicates with other cells Cell specialisation Cannot differentiate into different cells but can replicate themselves Can differentiate into specialised cells Cell Death Cannot self-destruct Ability to self-destruct when damaged or diseased. Energy Source Energy obtained in the absence of oxygen Energy obtained from the molecule ATP and a small amount through glycolysis. Growth (reproducing) Will continue to grow when not required to cells will stop reproducing when a repair work is done Evading the immune system Evades the immune system. Immune system identifies and removes damaged cells Function (ability to complete a task) May function abnormally Function Normally Cell can also develop from an embryonic stem cell. Embryonic stem cells are undifferentiated cells which can differentiate into almost all other cell types (They are pluripotent). They are derived from 4-5 days old embryos of a human source. Embryos develop naturally from a fertilised egg called a zygote. The zygote undergoes mitosis and after 4-6 days becomes known as the blastocyst. To generate specialised tissues from ESCs, the stem cells are isolated and placed in a nutrient rich culture dish where they divide and replicate themselves without undergoing differentiation or losing their pluripotency. The next step is the collection of healthy, dividing and undifferentiated cells which is known as a stem cell line. Fig 3.1 Embryonic (pluripotent) stem cell ( Derobio, 2010) An embryonic stem cell line is created from cells taken from the inner cell mass of a blastocyst (embryoblast). Depending on the usage, they are treated to develop into required cell types. Stem cells provides a range of treatment for conditions that may not be treated by conventional drugs. These includes making new brain cells to treat people with Parkinson’s disease, bone marrow transplant to treat Leukaemia, making replacement heart valves and giving people their sight back. CONCLUSION Cell is the building block of life, therefore there cannot be life without cell structures.