Cell Membranes

Structure of the Plasma Membrane
1. Phospholipid bilayer
  1. Hydophobic Tails
    1. allow very small neutrally charged molecules like oxygen and carbon dioxide and water pass freely through the membrane
  2. Hydrophilic Heads
  3. Cholesterol
    1. provides stability to the membrane and cell (increases strength)
2. Glycolipid
  1. Glycoproteins and glycolipids may be involved in cell signalling. Glycoproteins can also bind cells together in tissues.
    1. Some hormone receptors are glycoproteins and some are glycolipids
  2. a lipid with a carbohydrate attached
    1. Responsible to cell recognition
3. Glycoprotein
  1. a protein with a carbohydrate attached
    1. helps stabalise the membrane, also they act as receptor molecules by binding with particular substances
4. Alpha Helix Protein
The Fluid Mosaic Model
1. Is used to describe the molecular arrangements in membranes. The lipid molecules give fluidity and proteins in the membrane gives it a mosaic (patchwork) appearance
Transport
1. Types of Transport
  1. Facilitated Diffusion
    1. The passive movement of molecules across the membrane, down their concentration gradient, this is aided by transport proteins and no metabolic energy is required. Facilitated Diffusion uses both channel and carrier proteins, the channel proteins carries small water soluble molecules or ions and carrier proteins carry specific molecules in either direction, depending on the diffusion gradient
  2. Diffusion
    1. The net movement of molecules and ions in a gas or liquid from an area of high concentration to an area of low concentration. The component used in diffusion is the phospholipid bilayer
  3. Active Transport
    1. The movement of substances across membranes against their concentration gradient, requiring the use of metabolic energy in the form of ATP. Active transport, transports substances including ions, glucose and amino acids across the membrane via carrier proteins.
  4. Exocytosis
    1. The process of removing from substances the cells by fusing vesicles containing the substance with the plasma membrane. For example, Hormones use exocytosis.
  5. Endocytosis
    1. The process of substances entering the cell by surrounding them with part of the plasma membrane, which then pinches off to form a vesicle inside the cell. For example, Hormones use endocytosis.
  6. Osmosis
    1. The net movement of water particles from an area of high water potential to an area of low water potential. This is a result of the random movement of particles through a partially permeable membrane
2. Ways across the membrane
  1. The Phospholipid bilayer
    1. Can fit, small non-polar substances through by the process of diffusion. Water-soluble substances can pass through.
  2. Carrier Proteins
    1. These carry large substances across the membrane by either active transport of facilitated diffusion. They have receptors so they can recognise the specific molecules
  3. Channel Proteins
    1. These carry polar substances (ions and polar atoms) across the membrane by diffusion
Roles of membranes
1. At the surface
  1. Separate cells from their external environment. Control entry and exit of molecules. To allow cell recognition due to the cell surface antigens. To allow cell to cell attachment in tissue formation. To allow cells signalling due to receptors specific to trigger molecules. Some enzymes attach to the cell surface membranes to allow reactions to occur.
2. Within cells
  1. Surrounds organelles to allow compartmentalisation. Divisions of labour, so that each reaction can occur independently. Some reactions take place on membranes within cells. Some membranes form organelles e.g. Golgi/ER etc.
Cell Signalling
1. When cells communicate with each other, so that cells can recognise or identify each other, in order to work together and enable coordinators between the action of different cells, to trigger a reaction.
Animal Cells
1. If an animal cell is placed in a solution with a higher water potential, the water will move into the cell and the cell will burst and die.
  1. If an animal cell is placed in a solution with a lower water potential outside of the cell then the cell will loose water by osmosis and the cell will shrink.
Plant Cells
1. If the cell is placed in a solution where the water potential is higher outside of the cell them the water will move in, pressure will increase against the walls, and the cell reaches equilibrium and becomes turgid.
  1. If the cell is placed in a solution which has a lower water potential outside of the cell then water will move out and eventually become plasmolysed

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Cell Membranes

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	Created by Hannah Clatworthy about 9 years ago