CELL BIOLOGY 1

Cell Biology (Cytology) is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level. Cell biology research encompasses both the great diversity of single-celled organisms like bacteria and protozoa, as well as the many specialized cells in multicellular organisms such as humans. 2

3 Knowing the components of cells and how cells work is fundamental to all Biological Sciences. Appreciating the similarities and differences between cell types is particularly important to the fields of cell and molecular biology as well as to biomedical fields such as cancer research and developmental biology. Research in cell biology is closely related to genetics, biochemistry, molecular biology, immunology, and developmental biology.

4 Cellular Anatomy There are several hundred distinct human cell types. There are between 50 and 75 trillion cells in the human body. [1] The human body contains two classes of cells: Germ cells or reproductive cells (a) sperm of a male. (b) ovum (oocyte) of a female. Somatic cells include all the other cells of the body. 1- Molecular Biology of the Cell, Fourth Edition, Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter

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Plasma membrane Definition Plasma membrane The plasma membrane in plants and prokaryotes is usually covered by a cell wall. Plasma membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of lipids (hydrophobic fat-like molecules and hydrophilic phosphorus molecules. Hence, the layer is called a phospholipid bilayer.

Plasma membrane Definition It also be called a fluid mosaic membrane. Embedded within this membrane is a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is said to be selectively permeable, in that it can either let a substance (molecule or ion ) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as hormones.

Plasma membrane Components

10 The general function of the cell membrane include: Physical Isolation: It acts as a physical barrier that separates inside the cell from the surrounding extra-cellular fluid. Regulation of exchange with the environment: It controls the entry of ions and nutrients such as glucose, the elimination of wastes and release of secretions. Plasma membrane Function

11 Sensitivity: It contains a variety of receptors that allow the cell to recognize and respond to specific molecules in the environment. Structural support : Specialized connections between cell membranes or between membrane and Extra-Cellular Materials ECM. Plasma membrane Function

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18 Membrane structure The membrane thickness ranges from 6 to 10nm. The cell membrane contains : *Lipids: In the form of phospholipids & cholesterol. *Proteins. *Carbohydrates in the form of glycolipids. Plasma membrane Structure

19 Membrane lipid : The cell membrane is called a phospholipid bilayer because the phospholipids form two distinct layers. In each layer, the phospholipid molecules lie with their hydrophilic heads at the membrane surface and the hydrophobia tails on the inside. In addition, cholesterol molecules are also present and also with their heads at the membrane surface and their tails on the inside. Plasma membrane Structure

20 Membrane proteins : Proteins account for 55% of the weight of the cell membrane Two types of proteins are present: (1) lntegral proteins: which are part of the membrane structure and most of them span the membrane width and therefore called trans membrane proteins. (2) Peripheral proteins: which are bounded to the outer or inner surface of the membrane. Plasma membrane Structure

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22 Membrane protein function: 1.Anchoring proteins.which may attach the membrane to other structures and stabilize its position. 2.Recognition proteins (identifiers),the cells of the immune system recognize the cells of the body as self on the bases of the presence or absence of recognition proteins that are characteristic for each individual. 3.Enzymes, which catalyze reactions in the extracellular fluid or inside the cytoplasm. Plasma membrane Structure

23 4.Receptor proteins, which bind specifically to molecules called ligands. When a receptor protein binds to its specific ligand, it triggers changes in the activity of the cell. 5.Carrier proteins, bind solutes and transport them across the cell membrane. The transport process involves a change in the shape of the carrier. The carrier returns to its original form when the solute is released. Carrier proteins may or may not require energy from ATP. Plasma membrane Structure

24 5.Channels, some integral proteins act as channels that form a passage way that permits the movement of water and small solutes across the membrane. Channels are of two types: (1) Leak channels, which permits water and ions movement at all times. (2) Gated channels, which can open or close to regulate ion passage according to the cell demands. Plasma membrane Structure

25 N.B. proteins are embedded in the phospholipid bilayer and can drift from place to place across the surface of the membrane like the ice cubes in a bowl of punch.

26 Functions of glyco calyx: The glycoproteins and glycolipides form a viscous layer that lubricates and protects the cell membranes. Because it is sticky, glycocalyx can help anchor of the cell in place and also participates in the locomotion of specialized cells. Glycoproteins are recognized as normal (self) or abnormal (foreign) by cells involved with the immune response. The characteristics of the glycocalyx are genetically determined. For example the blood group (A,B, AB & O ) of any individual is determined by the presence or absence of certain glycoproteins on the membrane of red blood cells. Plasma membrane Structure

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