Biochemical Nature of The Cell Membrane

 

Biochemical Nature Of The Cell Membrane

Cellular Level of Organization

  • Compartmentalization of chemical reactions within the specialized structures
  • Regulate the materials in and out
  • Genetic material is used to direct cell activity

Cytology is the study of a cellular structure

Cell Physiology is the study of cellular function

Structure

Fluid Mosaic:

  • Is made up of a lipid bilayer (proteins, carbohydrates and cholesterol)
  • (Cholesterol)Unsaturated Fatty Acids make the cell membrane fluid rather than rigid
  • (Protein and Carbohydrates)The outer surface communicates with hormones and neurotransmitters.

Plasma Membrane:

  • Flexible barrier that surrounds the cytoplasm of the cell. 50% lipid and 50% protein held together by hydrogen  bonds.
  • 50 Lipid molecule to every 1 protein.
  • Lipids are the entry and exit barrier of polar substances and proteins are gate keepers.

Phospholipids:

  • Make up 75% of lipids
  • Have a bilayer (2 parallel layers of molecules)
  • Each molecule has both polar (hydrophilic heads) and non-polar (hydrophobic tails)areas (amphipathic). The hydrophilic heads are on the outside and the hydrophobic tails like up next to each other inside. Does my picture makes sense ?

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Glycolipids within the Cell Membrane:

  • Make up 5% of lipids of the cell membrane
  • Carbohydrate groups form polar (hydrophilic) heads only on the side facing the extracellular fluid.
  • Glycocalyx* is a carbohydrate covering the surface of a red blood cell. It’s role is to protect the cell’s surface and to give cell recognition and blood type (BT A has A antigen, BT B has B antigen, BT AB has A and B antigen, BT O has neither antigen)

Cholesterol:

  • Makes up the other 20% of the lipids. (75% phospholipid, 5% glycolipid)
  • Stuck between the other lipids in bilayer
  • Steroid rings and hydrocarbons are nonpolar and so like to live in the middle of the cell membrane.

Proteins:

-Intergral:

  • Extend across cell (if it reaches it is a transmembrane protien)
  • All are amphiathic with hydrophobic tails hiding with phospholipid tails.
  • Glycoproteins have sugar portion facing extracellular fluid (forms a Glycocalyx*)
  • Protects cell from digestion, makes the stickiness to hold to a fluid layer (making a slippery surface)

-Peripheral:

  • Attached to inner or outer surface of cell membrane
  • Easily removable

Function of Membrane Protein

  • Makes a channel = allows specific substances to pass through
  • Transports proteins = binds a substance and changes its shape so it can move across membrane
  • Receptor proteins = cellular recognition so binding can occur
  • Cell Identity Marker = can be recognised by other cells
  • Linker = attaches proteins to other cells in cell membrane
  • Cell shape and structure allow cell moment
  • Act as an Enzyme allows reactions to be sped up.

Membrane Fluidity

  • Self sealing (will reseal when penctured)
  • Membrane molecules can rotate and move freely – but it is hard for hydrophilic molecules to pass the hydrophobic core of the lipid bilayer
  • The fluidity is affected by the cholesterol and the temperature, it stiffens in higher temps.

Permeable Membrane

  • Semipermeable  (“it allows nutrients to enter the cell and waste products to leave”)

The Lipid Bilayer is Permeable to:

  • nonpolar molecules
  • uncharged molecules
  • oxygen
  • CO2
  • Steroids
  • Water – flows through gaps that form in the hydrophobic membrane as phospholipids move around

Transmembrane Proteins Acts as a Permeable Channel to:

  • Small and medium polar molecules
  • Charged particles

Macromolecules that are unable to pass through the membrane use vesicular transport.

Transport Across Membrane

Concentration Gradient :

  • Inside and outside the Membrane have a different concentration of substances (concentration gradient).
  • The membrane can maintain the difference in charged ions inside and outside of the membrane (membrane potential). Things move down their concentration gradient towards the opposite charged area – Trying to even it out it makes them move. This is another way for transport across the plasma membrane.

Ways to cross:

  • Transporter proteins assistance (mediated)
  • Doesn’t use a transporter protein (non-mediated)
  • ATP is used to drive the substance against their gradient (a way which doesn’t occur without energy)
  • Passive Transport – where they go with the flow (follow the gradient with kinetic energy)
  • Vesicular transport – moves substance across membrane in small vesicles (usinf endocytosis or exocytosis)

Diffusion:

  • Random mixture of particles  as a result of particle potential energy: movement from higher concentration to lover concentration gradient
  • The further it is from the current concentration gradient or surface area the faster diffusion happens
  • The higher the temperature the faster it diffuses
  • The further the distance, the slower diffusion happens.
  • Only when it is even will equilibrium be reached.
  • (Facilitated diffusion us when a substance binds to a transporter protein and only occurs during down concentration gradient, without a gradient to travel down there is no movement. In the facilitated diffusion of glucose, once glucose had binded to a protein – the transport protein changes shape then moves across the down concentration gradient)

Osmosis:

  • Movement through water through a selectively permeable membrane from a level of high water concentration to a low water concentration (like diffusion)
  • Diffusion through lipid bilayer (important for absorption of nutrients and getting rid of waste)
  • Only happens if the membrane is permeable to water but not o specific solids.
  • Channels which the water can pass through (aquaporins a transmembrane protein) can be open all the time or gated (closed at random or by order).

ATP – Active Transport

  • Movment of polar or charged substances move against their gradient, the energy used comes from the hydrolysis of ATP
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