2.3 cell requirements

• Cells in the human body need to be in a stable environment that continually supplies the materials they need and continually removes and processes the materials they produce.
• Extracellular fluid is the immediate environment of a cell.
• All cells have a thin layer of fluid between them which allows a continual exchange of materials into and out of cells.
• Homeostasis is when body systems work together to ensure that the cellular environment is kept constant.
• Cells need to take in certain substances from the tissue fluid as they process these substances, they produce materials that must then be removed from the cell.
• Different cells have different requirements, however all cells undergo cellular respiration, where glucose and oxygen are used to produce carbon dioxide, water, and energy.
• Many cells also produce substances that will be used elsewhere, such as hormones and enzymes as well as other wastes, which are all released into the tissue fluid.
• The cell membrane separates the internal and external environment and determines which substances will enter and exit the cell.
• The current accepted model for a cell membrane structure is the fluid mosaic model.
• The membrane is 'fluid' as the molecules it is composed of are constantly moving and changing position.
• The main structure of the cell membrane is composed of phospholipid molecules, lipid molecules containing a phosphate group.
• Phospholipid molecules are arranged in two layers (bilayer) and each has a head that is hydrophilic (water loving) and a tail that is hydrophobic (water hating).
• The heads of the phospholipid molecules are on the outside and the tails are on the inside.
• Phospholipid molecules move by moving their heads and tails, keeping the membrane fluid.
• Embedded in the phospholipid bilayer of the cell membrane are cholesterol and protein molecules.
• Cholesterol molecules are wedged in between the phospholipids and are essential for the function integrity and stability of the membrane.
• There are also a variety of protein molecules, including receptor proteins, channel proteins, carrier proteins, and cell-identity markers, embedded in the cell membrane.
• Vesicular transport is an active process, as energy from the cell is needed to form the vesicles.
• Facilitated diffusion is a passive process, as it does not require the input of energy.
• Active transport is similar to that of facilitated diffusion via carrier proteins, but it does not depend on a concentration gradient.
• Endocytosis is taking liquid or solids into the cell by vesicular transport.
• Active transport requires energy in the form of ATP because substances are transported across the membrane against the concentration gradient, from a lower to higher concentration.
• Liquid endocytosis is known as pinocytosis.
• The contents of the vesicle are then pushed out into the extracellular fluid.
• Exocytosis is when the contents of a vesicle inside the cell are passed to the outside.
• There are two main types of carrier-mediated transport: facilitated diffusion and active transport.
• Vesicular transport is the movement of substances across the cell membrane in membranous sacs called vesicles.
• A vesicle that is formed inside the cell migrates to the cell membrane and fuses with the membrane.
• Only 2% of the molecules in the cell membrane are proteins, however they make up about 55% the mass of the membrane as they are very large molecules.
• During facilitated diffusion, the molecule to be transported, such as glucose, attaches to a binding site on the specific carrier protein.
• Facilitated diffusion occurs when substances are transported through a protein along the concentration gradient, from a higher concentration on one side of the membrane to a lower concentration on the other.
• In endocytosis, the cell membrane folds around a droplet of liquid or solid particle until it is completely enclosed.
• The protein then changes shape and the molecule is released on the other side of the membrane.
• Solid endocytosis is known as phagocytosis.
• Carrier proteins are important in coordinating the activities of carrier proteins.
• The vesicle formed then pinches off and is suspended in the cell's cytoplasm.
• Using active transport, a cell can take in or pass out substances regardless of their concentrations inside or outside the cell.
• Net diffusion is the movement of liquid or gas molecules from places of higher concentration to places of lower concentration along a concentration gradient.
• Materials can pass through a cell membrane in different ways; some transfer mechanisms are passive processes (do not use energy) or active (uses the cell's energy in the form of atp).
• The cell membrane acts as a physical barrier and separates the cytoplasm from the extracellular fluid around the cell.