ABN1 Plasma Membrane PDF Notes - 2024

Cells: The plasma membrane and movement of substances N1239: Applied Biosciences for Nursing Practice 1 Natasha Price September 2024...

Cells: The plasma membrane and movement of substances N1239: Applied Biosciences for Nursing Practice 1 Natasha Price September 2024 1  Recap: The plasma membrane is a phospholipid bilayer. Hydrophilic heads and hydrophobic tails. Proteins are embedded in the membrane which act as carrier molecules and receptors. Some substances can move into the cell freely through the membrane, others cannot. Phospholipid layer. The lipids are arranged so their fatty acid chains (hydrophobic or water hating) are pointing inwards and their hydrophilic (water loving) polar heads pointing out to the aqueous phase. It is very stable, and proteins are either across the lipid layer or anchored it in in various ways. The structure is important in influencing how substances transfer across the membrane. Membrane proteins attach to carbohydrate molecules to give the cell immunological identity; the protein molecules also act as receptors for specific hormones and chemical messengers; some proteins are enzymes, and some transmembrane proteins form channels that are filled with water and allow small water-soluble ions to cross; and some are involved in pumps to transport substances across the membrane, for example the sodium/potassium pump. 2  Movement across the cell membrane: diffusion and active transport Passive transport (diffusion and osmosis): oxygen, carbon dioxide, sodium, calcium Facilitated diffusion: glucose, amino acids Active transport: e.g., Na/K pump Passive transport: diffusion and osmosis Substances such as molecules and ions cross the semipermeable membrane and move down the concentration gradient without using energy- lipid soluble materials such as oxygen, C02, fatty acids and steroids; water soluble materials such as sodium, potassium, calcium pass through water-filled channels (aquaporins*). Facilitated diffusion means there is a helper molecule. Examples are glucose transport proteins, and aquaporins which facilitate water movement. Water also moves passively through the SPM of the membrane Osmosis is diffusion of water (solvent) from an area with more water to an areas with less water and the water compartments are separated by a semipermeable membrane. Water moves to the more concentrated solution and appears to be pulled- water diffuses into a more concentrated saline solution. *For more on aquaporins if interested: Aquaporins (colostate.edu) Separate slide for active transport. 3 4 Simple diffusion Diffusion This is the term used for movement of molecules from an area of high concentration to an area of low concentration, down a concentration gradient. It is passive and does not require energy. It occurs mainly in gases, liquids and solutions. Diffusion can occur across a semipermeable membrane if the molecules are small enough (and are not charged particles). Because the phospholipid bilayer has hydrophobic heads and hydrophilic tails, water soluble molecules cannot pass through without a channel protein (a channel in the membrane). 5 An example of facilitated diffusion is glucose entering the cell. In this case, insulin is the “stimulus molecule” (green in picture), which works like a key to open the protein channel. Once the channel is open, glucose can move down its concentration gradient into the cell. 6 Water movement into the cell Aquaporins are a relatively recent scientific discovery. These are literally “water pores”, amino acid molecules in the plasma membrane which allow single water molecules to bind and enter the cell freely. Water can move down its concentration gradient. Water also moves across the cell membrane in other ways (e.g. some moves via osmosis through the bilayer)- this mechanism is still not fully understood. 7  Electrolytes have to be kept in a narrow range and cells can adjust their fluid levels by changing the concentration of electrolytes inside and out. Sodium (Na+) is the main electrolyte outside the cells (extracellular) Potassium (K+) is the main electrolyte inside the cell (intracellular) The cell achieves this via the Na/K pump, which is an active transport mechanism. Because this is working against the concentration gradient, it requires energy in the form of ATP. Another example is the proton pump in the stomach. 8  Movement across the cell membrane: Simple summary Simple summary 9 Movement across the cell membrane: more detail (for the enthusiastic) 10 Receptors The cell membrane plays an important role in signalling through the action of receptors. Receptors are proteins embedded in the plasma membrane surrounding the cell which bind to chemicals from outside called ligands. A ligand can be a hormone, neurotransmitter, small protein, a drug molecule, or part of an infectious agent Receptors detect the specific ligands to which they are sensitive and modify the cell (or target cell)’s activity. 11 12  Receptors modify the activity of target cells in various ways, for example: By opening/closing an ion channel By activating a receptor that activates another protein in the membrane. By activating a receptor which initiates enzyme activity. By activating an intracellular receptor to adjust the transcription of specific genes. Do not worry about the names of the different types of receptor at this stage- the idea is to learn that the plasma membrane has highly selective functions and numerous different receptors for substances within the body (e.g. hormones, neurotransmitters) and these can be targeted by extrinsically administered substances therapeutically. We will look at drug receptors specifically in a different lecture. 13  Further reading: Ross and Wilson’s Anatomy and Physiology in Health and Illness, Chapter 2 and 3. 14