What is Cell
The human body develops from a single cell called the zygote, which results from the fusion of the ovum and the spermatozoon. Cell division follows and, as the foetus grows, cells with different structural and functional specialisation develop, all with the same genetic make-up as the zygote. Individual cells are too small to be seen with the naked eye. However, they can be seen when thin slices of tissue are stained in the laboratory and magnified using microscope.
A cell consists of a plasma membrane enclosing a number of organelles suspended in a watery fluid called cytosol. Organelles, literally 'small organs', have individual and highly specialised functions, and are often enclosed in their own membrane within the cytosine. They include : the nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, Lysosomes and the cytoskeleton. The cell contents, excluding the nucleus, is the cytoplasm, I.e. the cytosol and other organelles.
plasma membrane
The plasma membrane consists of two layers of phospholipids with proteins and sugars embedded in them. In addition to phospholipids, the lipid cholesterol is also present. The phospholipids molecules have a head, which is electrically charged and hydrophilic and a tail which has no charge and is hydrophobic. The phospholipid bilayer is arranged like a sandwich with the hydrophilic heads aligned on the outer surfaces of the membrane and the hydrophobic tails forming a central water-repelling layer. These differences influences the transfer of substances across the membrane.
Membrane proteins
Those proteins that extends all the way through the membrane provide channels that allow the passage of, for example, electrolytes and non-lipid soluble substances. Protein molecules on the surface of the plasma membrane .
Membrane protein function
The membrane proteins performs several functions:
- Branched carbohydrate molecules attached to the outside of some membrane protein molecules give the cell its immunological identity.
- They can acts as receptors for hormones and other chemical messengers
- Some are enzymes
- Transmembrane protein form channels that are filled with water and allow very small, water-soluble ions to cross the membrane
- Some are involved in pumps that transport substances across the membrane
ORGANELLES
Nucleus
All body cells have a nucleus, with the exception of mature erythrocytes . Skeletal muscles fibres and some other cells contain several nuclei. The nuclei is the largest organelle and is contained within the nuclear envelope, a membrane similar to the plasma membrane but with tiny pores through which some substances can pass between it and the cytoplasm
The nucleus contains the body's genetic material, in the form of deoxyribonucleic acid ; this directs all its metabolic activities. In a non-dividing cell DNA is present as a fine network of threads called chromatin, but when the cell prepares to divide the chromatin forms distinct structure called chromosomes. A related substance, ribonucleic acid(RNA) is also found in the nucleus, but which are in general involved in protein synthesis.
Within the nucleus is a roughly spherical structure called the nucleolus, which is involved in synthesis and assembly of the components of ribosomes.
What is power house of Cell
Mitochondria are membranous, sausage-shaped structures in the cytoplasm, sometimes described as 'power house' of the cell. They are central to aerobic respiration, the processes by which chemical energy is made available in the cell. This is in the form of ATP, which releases energy when the cell breaks it down. Synthesis of ATP is most efficient in the final stage of aerobic respiration, a process which requires oxygen . The most active cell types have the greatest number of mitochondria, e.g. liver, muscle and spermatozoa.
Ribosomes
These are tiny granules composed of RNA and protein. They synthesis protein from amino acids using RNA as the template. When present in free units or in small clusters in the cytoplasm, the ribosomes make proteins for use within the cell. These include the enzymes required for metabolism. Metabolic pathways consist of a series of steps, each driven by a specific enzyme. Ribosomes are also found on the outer surface of the nuclear envelope and rough endoplasmic reticulum. Where they manufacture proteins for export from the cell.
Endoplasmic reticulum
Endoplasmic reticulum is an extensive series of interconnecting membranous canals in the cytoplasm. ER synthesis lipids and steroid hormones, and is also associated with the detoxification of some drugs. Some of the lipids are used to replace and repair the plasma membrane and membranes of organelles. Rough ER is studded with ribosomes. These are the site of synthesis of proteins, some of which are 'exported' from cells, I.e. enzymes and hormones that leave the parent cell by exostosis to be used by cells elsewhere.
Gold apparatus
The Golgi apparatus consists of stacks of closely folded flattened membranous sacs. It is present in all cells but is larger in those that synthesise and export proteins. The proteins move from the endoplasmic reticulum to the Golgi apparatus where they are 'packaged' into membrane-bound vesicles. The vesicles are stored and when needed, they move to the plasma membrane and fuse with it. The contents are expelled from the cell. This process is called exocytosis.
Lysosomes
Lysosomes are small membranous vesicles pinched off from the Golgi apparatus. They contain a variety of enzymes involved in breaking down fragments of organelles and large molecules inside cell into smaller particles that are either recycled, or extruded from the cell as waste material. Lysosomes is white blood cells contains enzymes that digest foreign material such as microbes.
Cytoskeleton
This consists of an extensive network of tiny protein fibres.
Microfilaments.
These are the smallest fibres. They provide structural support, maintain the characteristic shape of the cell and permit contraction, e.g. actin in muscle cells.
Microtubules
These are larger contractile protein fibres that are involved in movement of:
- Organelles within the cell
- Chromosomes during cell division
- Cell extension
Centrosome
This directs organisation of microtubules within in cell. It consists of a pair of centrioles and plays an important role in cell division.
Cell extensions
These projects from the plasma membrane in some types of cell and their main components are microtubules, which allow movement. They include :
Microvilli-Tiny projections that contain microfilaments. They cover the exposed surface of certain types of cell,e.g. absorptive cells that line the small intestineBy greatly increasing the surface area, microvilli make the structure of these surface area, microvilli make the structure of these cells ideal for their function- maximising absorption of nutrients from the small intestine.
Cilia-microscopic hair-like projection containing microtubules that lie along the free borders of some cells. They beat in unison, moving substances along the surface, e.g. mucus upwards in the respiratory tract.
Flagella-single, long whip-like projections, containing microtubules, which form the 'tails' of spermatozoa that propel them through the female reproductive tract.
THE CELL CYCLE
Many damaged, dead, and worn out cells can be replaced by growth and division of other similar cells. The frequency with which cell division occurs varies with different types of tissue. This is normally carefully regulated to allow effective maintenance and repair of body tissue. At the end of their natural lifespan, ageing cells are programmed 'self destruct' and their components are removed by phagocytosis, a process known as apoptosis.
Cells with nuclei have 46 chromosomes and divide by mitosis, a process that results in two new genetically identical daughter cells. The only exception to this is the formation of gametes I.e. ova and spermatozoa, which takes place meiosis.
The period between two cell divisions is known as the cell cycle, which has phases that can be seen on light microscopy: mitosis and interphase.
CELL DIVISION
Interphase
This is the longer phase and three separates stages are recognised: First gap phase- the cell grows in size and volume. This is usually the longest phase and most variable in length. Sometimes cells do not continue round the cell cycle but enter a resting phase; e.g. secretion, absorption.
Synthesis of DNA - the chromosomes replicate forming two identical copies of DNA. Therefore following the S phase, the cell now has 92 chromosomes, I.e. enough DNA for two cells and is nearly ready to divide by mitosis.
Second gap phase- there is further growth and preparation for cell division.
Mitosis
This is a continuous process involving four distinct stages visible by light microscopy.
Prophase
During this stage the replicated chromatin becomes tightly coiled and easier to see under the microscope. Each of the original 46 chromosomes is paired with its copy in a double chromosomes unit. The two chromatid are joined to each other at the centromere. The mitotic apparatus appears; this consists of two centrioles separated by the mitotic spindle, which is formed from microtubules. The centrioles migrate, one to each end of the cell, and the nuclear envelops disappear.
Metaphase
The chromatids align on the centre of the spindle, attached by their centromeres
Anaphase
The centromeres separate, and one of each pair of sister chromatids migrates to each end of the spindles as the microtubules that form the mitotic spindle contract.
Telophase
The mitotic spindle disappears, the chromosomes uncoil and the nuclear envelope reforms. Following telophase, cytokinesis occurs: the cytosol, intracellular organelles and plasma membrane split forming two identical daughter cells.
Transport of substances across cell membrane
The structure of the plasma membrane provides it with the property of selective permeability, meaning that not all substances can cross it. Those that can , do so in different ways depending on their size and characteristics.
Passive transport
This occurs when substance can cross the semipermeable plasma and organelle membranes and move down concentration gradient without using energy.
Diffusion
Small molecules diffuse down their concentration gradients:
Small molecules diffuse down their concentration gradients:
- Lipid-soluble materials, e.g. oxygen, carbon dioxide , fatty acids and steroids, cross the membrane by dissolving in the lipid part of the membrane.
- Water soluble materials, e.g. sodium, potassium and calcium , cross the membrane by passing through water-filled channels.
Facilitated diffusion
This passive process is used by some substances that are unable to diffuse through the semipermeable membrane unaided , e.g. glucose, amino acids. Specialised protein carrier molecules in the membrane have specific sites that attract and bind substances to be transferred, like a lock and key mechanism. The carrier then changes its shape and deposits the substance on the other side of the membrane . The carrier sites are specific and can be used by only one substance. As there are a finite number of carriers, there is a limit to the amount of a substance which can be transported at any time. This is known as the transport maximum.
Osmosis
Osmosis is a passive movement of water down its concentration gradients towards equilibrium across a semipermeable membrane .
Active transport
This is the transport of substances up their concentration gradient, i.e. from a lower to a higher concentration. Chemical energy in the form of ATP drives specialised protein carrier molecules that transport substances across the membrane in either direction. The carrier sites are specific and can be used by only one substance; therefore the rate at which a substance is transferred depends on the number of sites available.
The sodium-potassium pump
All cells posses this pump, which indirectly supports other transport mechanism such as glucose uptake, and is essential in maintaining the electrical gradient needed to generate action potential in nerve and muscle cells.
This active transport mechanism maintains the unequal concentration of sodium and potassium ions on either side of the plasma membrane. It may use up to 30% of cellular requirements. Potassium levels are much higher inside the cell than outside - it is the principal intracellular cation. Sodium levels are much higher outside the cell than inside- it is the principal extracellular cation. These ions tend to diffuse down their concentration gradients, K+ outwards and Na+ is constantly pumped out across the cell membrane in exchange for K+.
Bulk transport
Transfer of particles too large to cross cell membranes occurs by pinocytosis or phagocytosis . These these particles are engulfed by extensions of the cytoplasm which enclose them, forming a membrane-bound vacuole. Pinocytosis allow the cell to bring in fluid. In phagocytosis larger particles are taken into the cell. Lysosomes then adhere to the vacuole membrane, releasing enzymes which digest the content.
Extrusion of waste material by the reverse process through the plasma membrane is called exocytosis. Vesicles formed by the Golgi apparatus usually leave the cell in this way, as do any indigestible residues of phagocytosis.
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