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The Fundamental Unit of Life: The
Cell
Structure and Function of the Cell
The Cell as the Structural and Functional Unit of Life
The structural and functional unit of all living organisms is the cell. To study general features of cells, the light microscope would be the most appropriate tool. To study the detailed anatomy of internal cell parts, the transmission electron microscope (TEM) would be the best choice.
The Plasma Membrane
The plasma membrane regulates the movement of materials into and out of the cell. The environment outside the plasma membrane is most appropriately referred to as the extracellular environment. The fluid-mosaic model suggests that the plasma membrane is neither rigid nor static in structure, but rather a dynamic structure where proteins are free to move about within a double layer of phospholipids. According to the most current model, the plasma membrane is a fluid structure with proteins that are free to move about within a double layer of phospholipids. One function of the plasma membrane is recognition of bacterial cells by the immune system. Plasma membrane phospholipids have a hydrophilic head and hydrophobic tails. Receptor molecules attach to ligand molecules, which is a function of the plasma membrane. Water-soluble molecules diffuse through protein channels , while lipid- soluble molecules diffuse through the bilipid layer. Cell membrane phospholipids form a bilayer , have nonpolar fatty acid tails , have polar phosphate heads , and create a selectively permeable barrier. Voltage-gated channels are the type of channels that close when a sperm cell comes into contact with an egg cell, changing the electrical charge across the plasma membrane. Communication between cells occurs when chemical messengers from one cell bind to receptor molecules on another cell. Channel proteins provide a 'door' through which extracellular molecules can enter the cell. Molecules that serve as chemical signals in cell-to-cell communication are called ligands.
Cells that respond to ligands possess receptor sites for specific ligands. Membrane-bound receptors have their receptor sites on the outer surface of the plasma membrane. The last event in the sequence is that Na+ ions diffuse into skeletal muscle cells after acetylcholine binds to membrane-bound receptors. G proteins are found associated with the plasma membrane. Cell communication involves the lipid bilayer of the plasma membrane and chemical signal molecules released by cells. The plasma membrane is selectively permeable , meaning that only certain substances can pass through it. Vitamin A, a fat-soluble vitamin, would move across the plasma membrane into the cell by dissolving in the lipid bilayer. Lipid-soluble substances pass through the membrane by dissolving in the lipid bilayer.
Diffusion and Osmosis
The distribution of the aroma of cookies baking throughout the house is an example of simple diffusion. In the process of diffusion, net movement of substances is always from a region of higher concentration to a region of lower concentration. Increasing the temperature will increase the rate of diffusion. After 24 hours, the percentage of salt in the top and bottom samples would be approximately equal. The movement of oxygen from the alveoli of the lungs into the bloodstream is an example of diffusion. Osmosis is the diffusion of water across a selectively permeable membrane. If the solutions are separated by a selectively permeable membrane, with solution B containing less sugar than solution A, most of the water molecules will move by osmosis from solution B to solution A. The greater the concentration of a solution, the greater the osmotic pressure of the solution. A red blood cell placed in a hypertonic solution loses water. A 0.5% saline solution is hypotonic to a cell that is isotonic to a 0.9% saline solution. As a result of fluid evaporation from burned areas, the cells will tend to shrink. A solution isosmotic with plasma has an osmolality of 300 mOsm. After drinking large volumes of water, the body cells of the runner will swell. The movement of a solution across a plasma membrane because of a pressure gradient is called filtration.
Membrane Transport Processes
Increasing the number of available carrier molecules would increase the rate of mediated transport.
Factors Affecting Facilitated Diffusion
Increasing the concentration gradient of the transported molecule can increase the maximum rate of facilitated diffusion. Decreasing the concentration gradient, increasing the concentration of competitive molecules, or increasing ATP synthesis do not affect the maximum rate of facilitated diffusion.
Effects of Inhibiting Cellular Processes
If a toxic drug inhibits mRNA (messenger RNA) synthesis, the most directly affected process would be protein synthesis.
Cytoplasm and Cytoskeleton
Cytoplasm
Cytoplasm is found outside the nucleus but inside the plasma membrane.
Cytoskeleton
The cytoskeleton consists of microtubules, actin filaments, and intermediate filaments. The absence of a cytoskeleton can affect cell shape. Microtubules provide structure and support to the cytoplasm.
Organelles
General Characteristics of Organelles
Organelles vary in number and type depending on cell function.
Specific Organelles and Their Functions
Lysosomes function in the destruction of nonfunctional organelles. Ribosomes are responsible for protein synthesis. The Golgi apparatus functions in the packaging and distribution of proteins and lipids. The endoplasmic reticulum (ER) with ribosomes attached is called rough ER. Cells that produce many proteins for secretion would have large numbers of rough ER and Golgi apparatus. The sequence of events in protein transport and packaging is: Protein made by ribosomes on rough ER. Protein moves through ER and then carried in vesicles to Golgi. Golgi modifies protein and then packages them into vesicles. Vesicle pinches off from Golgi and carries product to plasma membrane.
Lysosomes contain hydrolytic enzymes and are part of the intracellular digestive system. Peroxisomes are involved in detoxification activities. Damaged cells can destroy nearby healthy cells when they release lysosomal enzymes.
Mitochondria
Mitochondria are the cell's power plants, responsible for energy production. Increased energy demands can be met by an increase in the number of mitochondria.
Cilia and Flagella
Cilia and flagella are distinguished by their length and numbers.
Nucleus
The nucleus is the control center of the cell, coordinating and controlling cellular activities. The nucleolus produces ribosomal subunits. Nuclear pores allow molecules to move between the nucleus and cytoplasm.
Cellular Respiration
Glycolysis
Glycolysis converts glucose to pyruvic acid.
Aerobic and Anaerobic Respiration
Aerobic respiration occurs when oxygen is available. Anaerobic respiration occurs when oxygen supplies do not meet cell demands.
Genetic Information Transfer
Transcription and Translation
Transcription is the transfer of information from DNA to mRNA. Translation is the synthesis of proteins based on the information in mRNA. The correct sequence is: transcription → translation → protein synthesis.
Cell Transport and Metabolism
Transport Mechanisms : The different transport mechanisms include active transport (requires energy), facilitated diffusion (uses carrier molecules but no energy), and diffusion (movement from high to low concentration).
Cell Organelles : The cell organelles and their functions include the nucleus (contains chromosomes), ribosomes (sites of protein synthesis), lysosomes (contain hydrolytic enzymes), mitochondria (produce energy), and centrioles (source of spindle fibers).
Metabolic Pathways : The metabolic pathways include aerobic respiration (uses oxygen and produces ATP, CO2, and H2O) and anaerobic respiration (occurs without oxygen and produces lactic acid).
Cell Aging and Death : Theories about cell aging and death include DNA damage, mitochondrial damage, free radicals, and the presence of "death genes" that turn on late in life.
