The Human Organization: The Cell (I) PDF

# The Human Organization: The Cell (I) ## Introduction - All living things are made of one or more cells - Cells are the smallest units of living things - The body contains many kinds of cells - All cells share certain characteristics such as that they are surrounded by a cell membrane, they breat...

# The Human Organization: The Cell (I) ## Introduction - All living things are made of one or more cells - Cells are the smallest units of living things - The body contains many kinds of cells - All cells share certain characteristics such as that they are surrounded by a cell membrane, they breath, reproduce and grow - Cells also differ in function, shape and size. - For example, nervous system cells differ from kidney cells ## Cell Shapes and Sizes - Some cells are rounded, oval, flat, cubical, columnar, spindle or fusiform - **Lymphocyte** is one of the smallest cells (6µm) - **Fat** and **ova** are the largest cells (160 µm) ## Cell Types - Cells are of two types: **prokaryotic** and **eukaryotic cells** ### 1. Prokaryotic Cells: - They lack a nuclear membrane or any other membrane covered organelles in their cytoplasm. - Surrounded by a cell wall - Considered to be "Primitive Cells" - Example: Bacteria & Blue-Green Algae ### 2. Eukaryotic Cell: - They possess a nuclear membrane as well as membrane bound organelles in their cytoplasm - Surrounded by cell membrane called (plasma membrane) - Example: Plant & Animal Cells ## Eukaryotic Cell Possess and Components - **Cell (plasma) membrane:** - **Protoplasm:** Is a fluid-like material - The protoplasm surrounding the cell nucleus is known as the cytoplasm or cytosol and that inside the nucleus as the nucleoplasm - It contains: - **Organelles:** Living structures, essential for cell life - **Inclusions:** non-living accumulations of the metabolites or cell products - **Other Substances:** such as water (75%), Proteins, Carbohydrates, Lipids, nucleic acid, inorganic substance and minerals - **Nucleus:** ## Cell Structure - It consists of: - **Cell membrane:** A very thin membrane (only 7.5 to 10 nanometers thick) that surrounds the cell and separates the intercellular from the extracellular environments. - Invisible by light microscope. Visible only with Electron Microscope. - Composed of: - Bilayer of phospholipids (lipid bilayer) - Proteins - Cholesterol - Short chain of sugars (glycoprotein and glycolipids) - Most protein and phospholipid molecules can move laterally forming a mosaic model - That means; cell membrane is flexible - **Protoplasm:** - Consists of **cytoplasm** and **nucleoplasm** - Surrounded by **plasma membrane** - **Plasma membrane** is made of: - **Phospholipid bilayer:** The phospholipids of the membrane are arranged in two layers forming a lipid bilayer. Each phospholipid has a hydrophilic head and hydrophobic tails. - **Cholesterol**: Cholesterol molecules in the membrane dissolve in the middle of the bilayer of the membrane. - **Proteins**: They exist as globular masses floating in the phospholipids bilayer. There are two types of proteins: - **Integral Proteins:** They can be transmembrane proteins (span the entire membrane, e.g. **ion channels**) or monotopic proteins (are only embedded into one side of the membrane and act as **enzymes**). - **Peripheral Proteins:** They do not penetrate the cell membrane but are loosely attached. ## Functions of Proteins of the Membrane: - **Structure support** - **Transport of molecules:** Through channels or as carriers. - **Cell-cell recognition:** Ability of a cell to distinguish one cell from another.. as basis for rejection of foreign cells by immune system. - **Receptors for hormones and antigens:** Identify the blood and tissue types - **Enzymatic control of chemical reactions:** At the cell surface ## The Carbohydrates Of Cell Membrane - **Carbohydrate components** of the cell membrane are attached to the **trans-membrane proteins (glycoprotein)** and **phospholipids molecules (glycolipid)**. They form the **cell coat or glycocalyx.** - Their functions are: - **Cell-adhesion molecules:** Bind one cell to another. - **Cell-cell recognition:** A glycocalyx acting as a cell identity. A marker distinguishing the body's own cells from foreign cells as during organ transplantation. ## Transport Across The Cell Membrane - The movement of materials across the cell membrane may occur either passively or actively. - The movement of large particles into a cell is by **Bulk Transport** ### Passive Transport: - **Does not use energy** - The movement of material occurs with the concentration gradient (high concentration => low concentration) **There are three main types of Passive Transport:** - **Simple diffusion:** The movement of small molecules across the lipid bilayer or the protein channels of the cell membrane (e.g. O2, CO2, etc.). The solutes are moved from their high concentration to low concentration across the membrane. - **Osmosis:** Is the diffusion of water molecules across a selectively permeable membrane from the area of water greater concentration to the area of lesser concentration, until they are equally distributed. - Since cytoplasm contains proteins and salts and is surrounded by a selectively permeable membrane, water will move in or out. - **Isotonic solution:** The concentration of solutes inside and outside the cell is the same. Therefore, there is no net movement of water. Our body cells are not affected in an isotonic solution. - **Hypotonic solution:** Has a greater concentration of water and a lesser concentration of solutes than the cell. This causes the cell to swell or even burst as water moves in. - **Hypertonic solution:** Has a greater concentration of solutes and a lesser concentration of water than the cell. This causes the cell to shrink as water moves out. - **Facilitated diffusion:** The movement of large or charged molecules through membrane proteins (e.g. ions, glucose, amino acids, etc.) - This process occurs by the means of protein carriers within the membrane. - The transport occurs only through protein channels. ### Active Transport: - A molecules are transported across a membrane through protein channels using carriers and use cellular energy (ATP). - The molecules move against their concentration gradient from the side of lower concentration to the side of higher concentration. ### Bulk Transport: - An active transport that includes the process of **endocytosis** and **exocytosis** - **Endocytosis:** Moves large, complex particles into a cell. - This includes: - **Phagocytosis:** The cell membrane extends pseudopodia around the particle, forming a vesicle which is then engulfed by the cell. - **Pinocytosis:** The cell membrane invaginates to produce a deep, narrow whole, forming a vesicle that is filled with extracellular fluid and then pinched off and enters the cell. - **Receptor-mediated endocytosis:** The interaction of specific molecules in the extracellular environment with specific membrane receptor proteins causes the membrane to invaginate, fuse, and pinch off to form a vesicle. - **Exocytosis:** Proteins and other molecules produced within the cell are packaged within vesicles and then exported. These vesicles fuse with the cell membrane and release their content into the extracellular environment. - Both **endocytosis** and **exocytosis** require energy.