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Q1L1 Cell physiology Dr. Hassan Yashar Hassan Rough endoplasmic reticulum (with ribosomes) Nuclear envelope Nucleus Chromatin Nucleolus Cytoplasm Figure 3.1 The simple cell. Plasma membrane The structure and functions of membranes are fundamental to cell survival, as they control passage of subs...

Q1L1 Cell physiology Dr. Hassan Yashar Hassan Rough endoplasmic reticulum (with ribosomes) Nuclear envelope Nucleus Chromatin Nucleolus Cytoplasm Figure 3.1 The simple cell. Plasma membrane The structure and functions of membranes are fundamental to cell survival, as they control passage of substances into and out of it, regulating the intracellular environment. Structure Mitochondria 1 The plasma membrane (Fig. 3.2) consists of two layers of phospholipids (p. 33) with proteins and sugars embedded in Centrasome them. In addition to phospholipids, the lipid cholesterol is also present. The phospholipid molecules have a head, which is electrically charged and hydrophilic (meaning 'water-loving'), Centrioles and a tail, which has no charge and is hydrophobic (meaning Lysosomes 'water-hating', Fig. 3.2A). The phospholipid bilayer is arranged like a sandwich with the hydrophilic heads aligned Smooth on the outer surfaces of the membrane and the hydrophobic endoplasmic reticulum tails forming a central water-repelling layer. These differences influence the transfer of substances across the membrane. Membrane proteins Golgi apparatus Those proteins that extend all the way through the membrane provide channels that allow the passage of, for example, Plasma membrane electrolytes and non-lipid soluble substances. Protein molecules on the surface of the plasma membrane are shown in Fig. 3.2B. The membrane proteins perform several functions: Some membrane protein molecules have branched carbohydrate molecules attached to the outside of the cell, giving the cell its immunological identity - 'self' markers . They can act as receptors (specific recognition sites) for hormones and other chemical messengers. Some are enzymes 1 Q1L1 Cell physiology Dr. Hassan Yashar Hassan Each cell is enclosed by its plasma membrane, which provides a selective barrier to substances entering or leaving. This property, called selective permeability, allows the cell (plasma) membrane to control the entry or exit of many substances, thereby regulating the composition of its internal environment. Particle size is important, as many small molecules, e.g. water, can pass freely across the membrane by simple diffusion, while large molecules cannot and may therefore be confined to either the interstitial fluid or the intracellular fluid. Pores or specific channels in the plasma membrane admit certain substances but not others. The membrane is also studded with specialised pumps or carriers that import or export specific substances. In Fig. 3.3C the membrane has pumps that actively import the pink particles and other pumps that actively export the blue particles; pink particles therefore concentrate within the cell and blue particles concentrate outside the cell. Selective permeability ensures that the chemical composition of the fluid inside cells is different from the interstitial fluid that bathes them. Transport mechanisms are explained in the next section. Carrier protein molecule Figure 3.4 Specialised membrane protein carrier molecules involved in facilitated diffusion and active transport. Passive transport This occurs when substances can cross the semipermeable plasma and organelle membranes, and move down maximum. the concentration gradient (downhill) without using energy. Q3.1 Diffusion Small molecules diffuse down their concentration gradient: 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. Transmembrane proteins form channels that are filled Extracellular with water and allow very small, water-soluble ions to fluid Plasma cross the membrane. membrane Some are involved in pumps that transport substances Intracellular across the membrane. fluid Transport of substances across cell membranes 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 (Fig. 3.4). The carrier sites are specific and can be used by only one substance. 2 Q1L1 Cell physiology Dr. Hassan Yashar Hassan Osmosis Osmosis is passive movement of water down its concentration gradient towards equilibrium across a semipermeable membrane. Active transport [J3.2 This is the transport of substances up their concentration gradient (uphill), i.e. from a lower to a higher concentration. Chemical energy in the form of adenosine triphosphate (ATP, p. 37) drives specialised protein carrier molecules that transport substances across the membrane in either direction (Fig. 3.4). 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 possess this pump, which indirectly supports other transport mechanisms such as glucose uptake, and is essential in maintaining the electrical gradient needed to generate action potentials in nerve and muscle cells. This active transport mechanism maintains the unequal concentrations of sodium (Na+) and potassium (K+) ions on either side of the plasma membrane. It may use up to 30% of cellular ATP (energy) 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+ into the cell. In order to maintain their concentration gradients, excess 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 ('cell-drinking') or phagocytosis ('cell-eating'). These particles are engulfed by extensions of the cytoplasm (Fig. 3.5; see also Fig. 15.1), which enclose them, forming a membrane-bound vacuole. Pinocytosis allows the cell to bring in fluid. In phagocytosis, larger particles (e.g. cell fragments, foreign materials, microbes) are taken into the cell. Lysosomes (see later) then adhere to the vacuole membrane, releasing enzymes that digest the contents. Extrusion of waste material by the reverse process through the plasma membrane is called exocytosis. Vesicles formed by the Golgi apparatus (see later) usually leave the cell in this way, as do any indigestible residues of phagocytosis. Organelles __________________ Organelles (see Fig. 3.1), literally meaning 'small organs', have individual and highly specialised functions, and are often enclosed by their own membrane within the cytosol. They include the nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes and cytoskeleton. Nucleus 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, p. 476); 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 (see Fig. 17.1). 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 there is a roughly spherical structure called the nucleolus, which is involved in synthesis (manufacture) and assembly of the components of ribosomes. Mitochondria Mitochondria are membranous, sausage-shaped structures in the cytoplasm, sometimes described as the 'power house' of the cell (Fig. 3.6). 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 (see Fig. 2.10).. The most active cell types have Nucleus Lysosomes Particle engulfed Formation Fusion of lysosomes Digestion of the particle Exocytosis by plasma membrane of a vacuole with vacuole by lysosomal enzymes Figure 3.5 Bulk transport across plasma membranes. (A-E) Phagocytosis. (F) Exocytosis. 3 Q1L1 Cell physiology Dr. Hassan Yashar Hassan permission.) Figure 3.6 Mitochondrion and rough endoplasmic reticulum. False colour transmission electron micrograph showing mitochondrion (orange) and rough endoplasmic reticulum (turquoise) studded with ribosomes (dots). (Bill Longcore/Science Photo Library, reproduced with permission.) the greatest number of mitochondria, e.g. liver, muscle and spermatozoa. Ribosomes These are tiny granules composed of RNA and protein. They synthesise proteins from amino acids, using RNA as the template (see Fig. 17.5). 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 (see Fig. 3.6 and next section), where they manufacture proteins for export from the cell. Endoplasmic reticulum Endoplasmic reticulum (ER) is an extensive series of interconnecting membranous canals in the cytoplasm (Fig. 3.6). There are two types: smooth and rough. Smooth ER synthesises lipids and steroid hormones, and is also associated with the detoxification of some drugs. 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 (Fig. 3.5F) to be used by cells elsewhere. Golgi apparatus The Golgi apparatus consists of stacks of closely folded flattened membranous sacs (Fig. 3.7). It is present in all cells but is larger in those that synthesise and export proteins. The proteins move from the ER 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, expelling the contents from the cell. This process is called exocytosis (Fig. 3.5F). 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 (e.g. RNA, DNA, carbohydrates, proteins) inside the cell into smaller particles that are either recycled, or exported from the cell as waste material. Lysosomes in white blood cells contain enzymes that digest foreign material such as microbes. 4

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