Fluid Mosaic Membranes

 

Fluid Mosaic Model of the plasma membrane

• The fluid-mosaic model describes the plasma membrane that surrounds animal cell.
• The membrane has two layers of phospholipids (fats with phosphorous attached), which at body temperature are like vegetable oil (fluid).

 

Fluid Mosaic Model

• mosaic part is due to the scattered arrangement of proteins through the membrane
• fluid partis due to the phospholipids which allows the proteins to float and rotate on their axis

 

 

The role of the components of cell membrane

 

component	roles
Phospholipids	Have a hydrophobic head and a fatty acid tail to form a bilayer separating the cell from the outside. They are fluid ie components can move around freely. Permeable to small and/or non-polar molecules Impermeable to large molecules and ions ie it prevents these substances from passing thorough.
cholesterol	Maintains the fluidity of the membrane Increases the stability of the membrane and prevents molecules like water and ions from passing through the membrane. Without cholesterol the membrane would easily split apart.
Proteins and glycoproteins	Channel proteins allow the movement of some substances, such as the large molecule sugar, into and out of the cell as they can‘t travel directly through the cell surface membrane Carrier proteins actively move substances across the cell surface membrane, using energy from ATP Cell surface receptors are glycoproteins responsible for the binding of an extracellular signaling molecule (hormones and cell surface antigens) and cell signaling molecules.
Glycolipids	Cell signaling Cell surface antigens Cell adhesion

 

Phospholipid bilayer

•  This provides the basic structure of membranes; it is selectively permeable and acts as a barrier to most water-soluble substances.
  • The more unsaturated the tails, the more fluid the membrane as unsaturated fatty acid tails are bent and therefore fit together more loosely.
  • The longer the tail, the less fluid the membrane.

 

• Cholesterol: regulates the fluidity of a membranes. Its hydrophobic region prevents polar molecules from passing through the membrane e.g. in myelin sheath
  • At low temperatures: cholesterol increases the fluidity of the membrane, preventing it from becoming too rigid.
  • At higher temperatures: cholesterol helps stabilize cells when the membrane could otherwise become too fluid.
  • Helps with mechanical stability

 

Glycolipids and glycoproteins

• Carbohydrate chains that are attached to membrane protein (glycoprotein) and phospholipids (glycolipid) project out into the watery fluids surrounding the cell where they form hydrogen bonds to stabilize the membrane structure.
• Carbohydrate chains act as receptors, mainly:
  • • Signalling receptors: The receptors recognise messenger molecules like hormones and neurotransmitters. When the messenger molecule binds to the receptor, a series of chemical reactions is triggered inside the cell.
    • Endocytosis: These group of receptors bind to molecules that are to be engulfed by the cell surface membrane.
    • Cell adhesion: binding cells to other cells in tissues and organs. Some glycolipids and glycoproteins act as antigens, allowing cell–cell recognition.

 

Proteins

• Transport proteins provide hydrophilic channels for ions and polar molecules. Enzymes catalyse the hydrolysis of molecules. Cytoskeleton made of protein filaments help maintain the shape of the cell.
  • Intrinsic/integral proteins: Proteins that are found embedded within the membrane. They may be found in the inner layer, the outer layer or, most commonly, spanning the whole membrane, known as transmembrane proteins.
  • Extrinsic/peripheral proteins found on the inner or outer surface of the membrane. Many are bound to intrinsic proteins or to phospholipids.
  • Channel proteins: water-filled pores that allow charged substances, usually ions, to diffuse through the membrane. They have a fixed shape and can be gated to control ion exchange. This does not use ATP and is in facilitated diffusion.

• • Carrier proteins: can flip between two shapes and is mainly in active transport where it uses ATP to change shape and carry ions/molecules up the concentration gradient. It is also involved in passive transport (facilitated diffusion) down the concentration gradient without the use of energy.

 

 

Cell surface receptors

•  These are present in membranes and bind with particular substances, e.g.: hormones which are chemical messengers which circulate in the blood but only bind to specific target cells.

 

Cell surface antigen

•  These act as cell identity markers. Each type of cell has its own antigen. This enables cells to recognise other cells and behave in an organized way.

 

Cell signaling

• • Cells communicate by sending and receiving signals.
  • A signal arrives at a specific protein receptor in a cell surface membrane that recognizes the signal.
  • The signal brings about a conformational change in the shape of the receptor, spanning the membrane, and the message is passed to the inside of the cell (signal transduction).
  • Changing the shape of the receptor allows it to interact with the G protein, which brings about the release of a ‘second messenger’ (a small molecule which diffuses through the cell relaying the message).
  • The second messenger activates a cascade of enzyme catalyzed reactions which brings about the required change.
  • This is an active process involving ATP use.

 

Roles of cell surface membranes

• Structural, keeping the cell contents together.
• Separate cell components from the outside environment
• Allows cells to communicate with each other by cell signaling.
• Allows recognition of other external substances.
• Allows mobility in some organisms, e.g. amoeba.
• Selectively permeable barrier.
• Regulating the transport of materials into or out of cells
• The site of various chemical reactions.

 

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Practice question

• Describe how membrane structure is related to the transport of materials across a membrane.

 

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Movement of Substances into & out of Cells

 

Passive and active transport across cell membranes

Substances can enter or leave a cell in two ways:

1. Passive
2. • Simple Diffusion
   • Facilitated Diffusion
   • Osmosis (water only)
3. Active transport

• • Molecules
  • Particles

 

Diffusion

•  the net movement of molecules or ions from a region of high concentration to a region of low concentration. It is a passive process (molecules have natural kinetic energy). As a result of diffusion, molecules reach equilibrium.
  • Steeper concentration gradient, higher temperature and increased surface areas all increase rate of diffusion.
  • Non-polar molecules can pass directly through the membrane e.g. steroid hormones
  • Gases can diffuse through the membrane directly
  • Water can diffuse through directly as it is a small molecule despite being polar.

 

Facilitated diffusion

• is the movement of specific molecules down a concentration gradient with the aid of special channel or carrier protein
• Used to transport molecules such as Glucose, Fructose, non-fat-soluble vitamins, urea and many ions across the membrane.
• There are four key steps involved in facilitated diffusion:

• • Glucose binds with transport protein molecules on the cell surface. Different cells have different types of glucose transporter.
  • The transport protein changes shape.
  • The glucose is transported through the membrane to the inside of the cell
  • The glucose detaches from the transporter protein and the protein reverts to its original shape.
• Each carrier protein has its own shape and only allows one molecule (or one group of closely related molecules) to pass through.
• The rate of facilitated diffusion is proportional to the concentration gradient and to the number of channels or transporter proteins that are available.

 

Factors which determine the ability of passage of substance through the cell membrane

• polarity
• size
• solubility

 

Osmosis

• the diffusion of water molecules from a region of higher water potential (ψ) (less negative) to a region of lower ψ (more negative) through a selectively permeable membrane.
• ψ is the tendency of water to move out of a solution; pressure potential (ψp) on liquid increases ψ
• Pure water has 0 ψ
• Negative ψ means that solution has more solute than solvent, therefore solute potential (ψs) reduces ψ.

 

In red blood cells

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In plant cells: ψ = ψs + ψp

• Protoplast: the living part of the cell inside the cell wall
• In pure water: water enters the cell by osmosis, and the cell wall pushes back against the expanding protoplast, building up pressure rapidly, becoming turgid. This is the ψp, and it increases the ψ of the cell until equilibrium is reached.
• In concentrated solution: water will leave the cell by osmosis. The protoplast gradually shrinks until it is exerting no pressure on the cell wall. The ψp = 0, so ψ = ψs. The protoplast continues to shrink and pulls away from the cell wall, so the cell is plasmolysed. The point at which ψp has just reached 0 and plasmolysis is about to occur is referred to as incipient plasmolysis.

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How plasmolysis occurs

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Active transport

• the movements of ions from a region of their lower concentration to a region of their higher concentration up a concentration gradient using ATP energy
• The active transport is done using carrier (transporter) proteins in the cell membrane.
• These use energy from the breakdown of ATP to move the ions into the cell.
• The carrier proteins are ATPases.
• Each carrier protein is specific to just one type of ion or molecule.
• Cells contain many different carrier proteins in their membranes

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Endocytosis and exocytosis (bulk transport)

• Macromolecules are too large to move with membrane proteins and must be transported across membranes in vesicles.

Exocytosis

• the transport of macromolecules out of a cell in a vesicle
• the object is surrounded by a membrane inside the cell to form a vesicle
• the vesicle is moved to the cell membrane.
• the membrane of the vesicle fuses with the cell membrane, expelling its contents outside the cell.

Endocytosis

• the transport of macromolecules into a cell in a vesicle
• the cell puts out extensions around the object to be engulfed
• the membrane fuses together around the object, forming a vesicle
• there are two types of endocytosis: phagocytosis (cell eating) and pinocytosis (cell drinking)