Cell PDF

The Cell Dr. Ahmed Esmat Lecturer of physiology, Zoology & Entomology department, Faculty of Science, Helwan University [email protected] Cell Membrane (plasma membrane or plasmalemma) The cell membrane forms a selectively permeable barrier between the cyt...

The Cell Dr. Ahmed Esmat Lecturer of physiology, Zoology & Entomology department, Faculty of Science, Helwan University [email protected] Cell Membrane (plasma membrane or plasmalemma) The cell membrane forms a selectively permeable barrier between the cytoplasm and the external milieu (environment). Plays a dynamic role in cellular activity Formed from trilaminar stucture (bi-lipid layers with intermadiate clear thin layer). Double Layered (bi-layer) of lipids with imbedded proteins. Bi-layer consists of phospholipids, cholesterol, and glyco- lipids. Glycolipids are lipids with bound carbohydrate Phospholipids have hydrophobic and hydrophilic bipoles  phospholipid molecule of the lipid bilayer is composed of a polar head, located at the surface of the membrane, and two long nonpolar tails projecting into the center of the plasmalemma. Because the phospholipid molecule is composed of a hydrophilic head and a hydrophobic tail, the molecule is said to be amphipathic. Philic = Love Phobic = Hate  Transmembrane proteins form channel proteins and carrier proteins. Channel proteins participate in the formation of hydrophilic pores, called ion channels. The hydrophobic amino acids are positioned peripherally. Most channels are gated channels; only a few are ungated. Gated channels are classified according to the control mechanism required to open the gate.  Voltage-Gated Channels: The most common example is during the transmission of nerve impulses, such as Na+ channels. Ligand-Gated Channels: Channels that require the binding of a ligand (signaling molecule) to the channel protein to open their gate. G-Protein-Gated Ion Channels: Channels require the interaction between a receptor molecule and a G-protein complex with the resultant the activated G protein interacts with the channel protein to open or close it.  Ungated Channels: Like potassium (K+) channel and these type of channels not under the cell's control. Carrier Proteins: Primary Active Transport “NA+- K+ Pump” and Secondary Active Transport “NA+- Glucose”. Carrier proteins are multipass membrane transport proteins, when a solute binds to the binding site, the carrier protein undergoes reversible conformational changes; as the molecule is released on the other side of the membrane, the carrier protein returns to its previous conformation. Passive and active transport across the membrane Facilitated diffusion  Carrier mediated transport: can be passive or active Uniport Cotransport – Symport – Antiport Active transport  Cell/Material movement SIMPLE DIFFUSION FACILITATED DIFFUSION ACTIVE TRANSPORT OSMOSIS  Cells/Material movement LARGE OBJECTS Large substances –ENDOCYTOSIS PHAGOCYTOSIS Exocytosis Endocytosis PINOCYTOSIS Eject out substance Take in substance –EXOCYTOSIS ( secretion – wastes) By vesicles Cells/Material movement Cells/Material movement Body water The 70 kg 'standard male' contains 42 liters of water - 60% of his body weight. The hypothetical adult female contains 55% of her body weight as water: this lower percent being due to a higher fat content. Variations in Water Content 1. Variation due to Age (Neonates contain more water then adults: 75-80% water) 2. Variation between Tissues (Plasma: 93% water, Fat: 10-15% water and Bone: 20% water) 3. Variation between Individuals Compartments The water in the body is distributed in 2 compartments. The major division is into Intracellular Fluid (ICF: about 23 liters; 55%) and Extracellular Fluid (ECF: about 19 liters; 45%) based on which side of the cell membrane the fluid lies. 1. Intracellular fluid (ICF): it is the fluid (water) present inside the cell. 2. Extracellular fluid (ECF): it is the fluid present outside the cell. It is divided into: a- Interstitial fluid (ISF): it is present between cells. b- Intravascular fluid (plasma): it is present inside the blood vessels.  Homeostasis Definition: it is keeping the internal environment of the body (ECF) constant [Volume (water content) / Ions concentration / Temperature / PH] to make it optimum for cellular function. Homeostasis is regulated by rapid control system (nervous system) and slow control system (endocrine system) as homeostatic control systems.  Feedback control in homeostasis 1- Negative feedback: in which the resultant effect of a function inhibits that function (Negative feedback mechanisms consist of reducing the output or activity of any organ or system back to its normal range of functioning). 2- Positive feedback: in which the resultant effect of a function stimulates that function. One example is the onset of contractions in childbirth. Another examples are the process of blood clotting and lactation.