Cell structure

all cells are surrounded by a membrane made of phospholipids and proteins. Biological membranes are so thin that their structure cannot be distinguished in the light microscope, and in the electron microscope, they appear as a single line.

Eukaryotic cells:

contains membrane-bound organelles, which are enclosed areas in the cytoplasm. The advantage is that potentially harmful chemicals, such as enzymes, are enzymes, are isolated and molecules with particular functions, such as chlorophyll, can be concentrated in one area. Membrane provides a large surface area for the attachment of enzymes involved in metabolic processes, and they provide a transport system inside the cell.

Organelles

Nucleus:

The most prominent feature in the cell. It is usually spherical and 10-20 micrometres in diameter. It contains DNA which, with protein, comprises the chromosomes. The chromosomes direct protein synthesis because they are the site of transcription.

The nucleus has a number of components:

• It is bounded by two membranes, called the nuclear envelope, with pores which allow the passage of large molecules, such as mRNA, and ribosomes. The outer membrane is continuous with the endoplasmic reticulum.

• The granular material in the nucleus is nucleoplasm. It contains chromatin, which is made of coils of NA bound to protein, During cell division, chromatin condenses into chromosomes.

• Within the nucleus are one or more small spherical bodies, each called a nucleolus. They are the sites of formation or rRNA, a constituent of ribosomes.

Mitochondria:

Mitochondria are often cylindrical and 1- 10 Micrometres in length. They comprise of:

• Two membranes, separated by a narrow, fluid-filled inter-membrane space. The inner membrane is folded inwards to form cristae.

• An organic matrix, which is a solution containing many compounds, including lipids and proteins.

• Small (70S) Ribosomes and a small circle of DNA which enable mitochondria to make some of their own proteins and self-replicate.

The functions of the Mitochondria is to produce ATP in aerobic respiration. Some of the reactions occur in the matrix and others on the inner membrane. The cristae provide a large surface area for the attachment of enzymes involved in respiration.

Metabolically active cells, such as muscle cells, need a plentiful supply of ATP. They contain many mitochondria, reflecting the high metabolic activity taking place

Being cylindrical, mitochondria have a large surface area than a sphere of the same volume, in other words, their surface area to volume ratio is bigger. Compared with a sphere, being a cylinder reduces the diffusion distance between the edge and the centre, making aerobic respiration more efficient.

Chloroplasts:

Chloroplasts occur in the cells of photosynthesising tissue. In many plants the highest concentration is in the palisade mesophyll cells, just below the upper surface of the leaf. Each chloroplast is surrounded by two membranes, comprising the chloroplast envelope.

• The stroma is fluid-filled and contains some of the products of photosynthesis, including lipid droplets and starch grains, which can take up a large part of the stroma.

• Like Mitochondria, they contain 70S ribosomes and circular DNA which enable them to make some of their own proteins and self-replicate.

• Within the stroma are many closed, flattened sacs called thylakoids. A stack of thylakoids is called a granum. Each granum comprises between two and a hundred parallel sacs. The photosynthetic pigments, such as chlorophyll, are found in the thylakoids. This arrangement produces a large surface area, efficient for trapping light energy.

Endoplasmic reticulum:

The endoplasmic reticulum is an elaborate system of parallel double membranes forming flattened sacs with interconnected, fluid-filled spaces between them, called cisternae. These are pink in the electron micrograph. The ER is connected with the nuclear envelope. This system allows the transport of materials through the cell. There are two types of ER:

• Rough ER (RER) has ribosomes on the outer surface and transports the proteins made there. RER is present in large amounts in cells that make a lot of proteins, such as making amylase in the salivary glands.

• Smooth ER comprises membranes that lack ribosomes. It is associated with the synthesis and transport of lipids.

Ribosomes:

Ribosomes are smaller in prokaryotic cells than eukaryotic cells. In prokaryotic cells they are 70S in size, whereas those in the cytoplasm of eukaryotic cells are 80S, where they occur singly or attached to membranes on the RER. Ribosomes have one large and one small subunit. They are assembled in the nucleolus from ribosomal RNA (rRNA) and protein. They are important in protein synthesis, as they are the site of translation, where mRNA is used to assemble the polypeptide chain.

Golgi body:

The structure of the Golgi body resembles the structure of ER, but it is more compact. Vesicles containing polypeptides pinch off from RER and fuse with the stack of membranes which constitute the Golgi body. Proteins are modified and packaged in the Golgi body. At the other end of the Golgi body, vesicles containing the modified proteins are pinched off. These may carry proteins elsewhere in the cell or move to and fuse with the cell membrane, secreting the modified proteins by exocytosis.

the functions of the Golgi body include:

• Producing secretory enzymes, packaged into secretory vesicles.

• Secreting carbohydrates, e.g. for the formation of plant cell walls.

• Producing glycoprotein.

• Transporting and storing lipids.

• Forming lysosomes, containing digestive enzymes.

Lysosome:

Lysosomes are small, temporary vacuoles surrounded by a single membrane, formed by being pinched off from the Golgi body. They contain and isolate potentially harmful digestive enzymes from the remainder of the cell. They release these enzymes in lysosomes can also digest material that has been taken into the cell, e.g. lysosomes fuse with the vesicle made when a white blood cell engulfs bacteria by phagocytosis and their enzymes digest the bacteria.

Centrioles:

Centrioles occur in all animal cells and most protoctistans but not in the cells of higher plants. They are located just outside the nucleus. Centrioles are two rings of microtubules, making hollow cylinders positioned at right angles to one another. Together, they are sometimes called centrosomes. During cell division, centrioles organise the microtubules that make the spindle.

Vacuole:

Most plants cells have a large permanent vacuole which consists of a fluid-filled sac bounded by a single membrane, the tonoplast. Vacuoles contain cell sap, a solution which stores chemicals such as glucose, amino acids and minerals, and may store vitamins and pigments, as in oranges. Vacuoles have a major role in supporting soft plant tissues.

Cell Wall:

The cell wall of a pant cell consists largely of cellulose. Cellulose molecules molecules are held together in microfibrils, which are aggregated into fibres, embedded in a polysaccharide matrix called pectin. The cell wall has the following functions:

• Transport - the gaps between the cellulose. Cellulose fibres make the cell was fully permeable to water and dissolved molecules and ions. This space outside the cells through which solution moves, is called the apoplast. The apoplast pathway is the main way that water crosses the plant root.

• Mechanical strength - the structure of cellulose microfibrils and their laminated arrangement make the cell wall very strong. When the vacuole is full of solution, the cell contents push against the cell wall, which resists expansion and the cell becomes turgid, supporting the plant.

• Communications between cells - cell walls have pores, called pits, through which strands of cytoplasm, called plasmodesmata, pass. The plasmodesmata occurs where there is no cellulose thickening between two cells. The strands of cytoplasm runs from one cell to the next. The network of cytoplasm in connected cells is called the symplast. The symplast pathway is important in water transport through a plant.

Prokaryotic cells:

An example of prokaryotic cells is a bacterium. The major distinguishing feature feature of prokaryotic cell is that they have no nucleus, or any internal membranes, so, unlike eukaryotic cells, they have no membrane-bound organelles. In some prokaryotes, infolding of the cell membrane in a mesosome or photosynthetic lamellae increases the membrane’s surface area. They are unicellular and rarely from multicellular structures.

Viruses:

Viruses are so small they cant be seen in the light microscope. They pass through filters that can trap bacteria and although experiments in the late nineteenth century suggested their existence, they could not be seen until the electron microscope was invented. They are not made out of cells and so they are described as acellular. There are no organelles, no chromosomes and no cytoplasm. Outside a living cell, a virus exists as an inert virion. However, when they invade a cell, they are able to take over the cell’s metabolism and multiply inside the host cell. Each virus particle is made up of a core nucleic acid. either DNA or RNA, surrounded by a protein coat, the capsid. In some viruses, a membrane derived from the host cell surrounds the capsid.

cells of all groups of organisms can be infected with viruses. The viruses that attack bacteria are called bacteriophages

All Prokaryotic cells have:

• DNA molecule loose in the cytoplasm

• Peptidoglycan cell wall

• 70S Ribosomes

• Cytoplasm

• Cell membrane

Some prokaryotic cells have:

• A slime coat

• Flagella (one, some or many)

• Photosynthetic lamellae holding photosynthetic pigments

• Mesosome - possible site of aerobic respiration

• Plasmids

Tissues

Cells near each other in the embryo often differentiate in the same way and group together as a tissue.

Mammalian tissue

Mammals have several tissue types including epithelial, muscular and connective tissue.

Epithelial tissue

Epithelial tissue forms a continuous layer, covering or lining the internal and external surfaces of the body. Epithelial have no blood vessels buy may have nerve endings. The cells sit on a basement membrane, made of collagen and protein and they vary in shape and complexity. They often have a protective or secretory function.

• The simplest form is simple cuboidal epithelium, in which cells have a cube shape, and the tissue is just one cell thick. It occurs in the proximal convoluted tubule of the kidney nephron and the ducts of salivary glands.

• Columnar epithelium has elongated cells. Those lining tubes that substances move through, such as the oviduct ( Fallopian tube) and trachea, have cilia.

• Squamous epithelium consists of flattened cells on a basement membrane. They form the walls of the alveoli and line the renal capsule of the nephron.

• Stratified epithelium - made from multiple layers, with the top ones dead to avoid damaging the ones underneath. Forms the skin.

• Ciliated epithelium - moves substances and particles using cilla, hairlike projections that move in a wavelike manner. For example in fallopian tubes.

Muscle tissue

Muscle tissue comes in three main types, with a slightly different structures and function:

• Skeletal muscle is attached to bones and generates locomotion in mammals. It has bands of long cells, or firbres, which give powerful contraction, but the muscle tires easily. You can choose whether or not to contraction these muscle, so they are called voluntary muscles. Because you can see stripes on them in the microscope, they are also called striped or striated muscle.

• Smooth muscle has individual spindle-shaped cells that can contract rhythmically, but they contract less powerfully than skeletal muscle. They occur in the skin, in the walls of blood vessels and in the digestive and respiratory tracts. You cannot control these muscles so they are involuntary muscles. They do not have stripes and so are also called unstriped or unstriated muscle.

• Cardiac muscle is only found in the heart. Its structure and properties are somewhat in between skeletal and smooth muscle. The cells have stripes, but lack the long fibres of skeletal muscle. They contract rhythmically, without any simulation from nerves or hormones, although these can modify their contraction. Cardiac muscle does not tire. They are involuntary muscles.

Connective tissue

connective tissue connects, support or separates tissues and organs. It contains elastic and collagen fibres in an extracellular fluid or matrix. Between the fibres are fat-storing cells (adipocytes) and cells of the immune system.

Organs

An organ comprises of several tissues working together, performing a specific function. In humans, for example, the eye contains nervous, connective, muscle and epithelial tissues and is the organ of sight. In plants, the leaf contains epidermal tissue, vascular tissue and packing or ground tissue between the vascular bundles, and is specialised for photosynthesis.

Organ systems

An organ system is a group of organs working together with a particular role. Some examples of mammalian organ systems are shown in the table on the left.

Organisms

All the systems of the body work together, making an organism, which is a discrete individual.