Structure and functions of Cell
The cell: structure and functions
The human body develops from a single cell called the zygote, which results from the fusion of the ovum (female egg cell) and the spermatozoon (male sex cell). Cell division follows and, as the fetus grows, cells with different structural and functional specialisations 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 a 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 cytosol.
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.
The plasma membrane consists of two layers of phospholipids with proteins and sugars embedded in them. In addition to phospholipids, lipid cholesterol is also present. The phospholipid molecules have a head, which is electrically charged and hydrophilic (meaning ‘water loving’), and a tail which has no charge and is hydrophobic (meaning ‘water hating’). 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 influence the transfer of substances across the membrane.
Those proteins that extend 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. The membrane proteins perform several functions:
• Branched carbohydrate molecules attached to the outside of some membrane protein molecules give the cell its immunological identity
• They can act as receptors (specific recognition sites) for hormones and other chemical messengers
• Some are enzymes
• Transmembrane proteins 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.
All body cells have a nucleus, with the exception of mature erythrocytes (red blood cells). Skeletal muscle fibres and some other cells contain several nuclei.
The nucleus 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 (DNA); 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 structures called chromosomes. A related substance, ribonucleic acid (RNA) is also found in the nucleus.
There are different types of RNA, not all 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 the synthesis (manufacture) and assembly of the components of ribosomes.
Mitochondria are membranous, sausage-shaped structures in the cytoplasm, sometimes described as the ‘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 stages of aerobic respiration, a process that requires oxygen. The most active cell types have the greatest number of mitochondria, e.g. liver, muscle and spermatozoa.
These are tiny granules composed of RNA and protein. They synthesise proteins 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 (ER)
The endoplasmic reticulum is an extensive series of interconnecting membranous canals in the cytoplasm. There are two types: smooth and rough. Smooth ER synthesizes 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 exocytosis to be used by cells elsewhere.
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 (secreted) from the cell. This process is called exocytosis.
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 (e.g. RNA, DNA, carbohydrates, proteins) inside the cell into smaller particles that are either recycled or extruded from the cell as waste material. Lysosomes in white blood cells contain enzymes that digest foreign material such as microbes.
This consists of an extensive network of tiny protein fibres.
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.
These are larger contractile protein fibres that are involved in the movement of:
• organelles within the cell
• chromosomes during cell division
• cell extensions (see below).
This directs the organisation of microtubules within the cell. It consists of a pair of centrioles (small clusters of microtubules) and plays an important role in cell division.
These project from the plasma membrane in some types of cell and their main components are microtubules, which allow movement.
• Microvilli – tiny projections that contain microfilaments. They cover the exposed surface of certain types of cells, e.g. absorptive cells that line the small intestine. By greatly increasing the 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 projections 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
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