ch2.3 cell requirements

• cell membrane: external boundary of the cell and determines what materials enter and leave
• homeostasis: the maintenance of a constant internal environment despite changes in the external environment
• the cell membrane model is based on the fluid mosaic model
• phospholipids form phospholipid bilayer of the cell membrane

  they have a hydrophilic head and a hydrophobic tail
• embedded in phospholipid bilayer are cholesterol and protein
• cholesterol is important for function integrity and stability
• proteins embedded in cell membrane include channel proteins, carrier proteins, and cell-identity markers
• main functions of the cell membrane

  • acts as physical barrier
  • regulates passage of materials
  • sensitive to changes
  • supports the cell shape and structure
• cell membrane is semi-permeable
• two kinds of transport processes:

  • passive - requires no energy as material moves along concentration gradient
  • active - requires energy in the form of adenosine triphosphate (ATP) as material moves against concentration gradient
• 3 basic processes: simple diffusion, facilitated transport, vesicular transport
• cell membrane structure

  A) cell-identity marker

  B) cholesterol

  C) hydrophilic head

  D) hydrophobic tail

  E) carrier protein

  F) channel protein

  G) phospholipid

  
• simple diffusion is the random movement of particles from areas of high concentration to low concentration along the concentration gradient
• alcohol, steroids and other fat soluble substances can easily diffuse between the phospholipid bilayer
• simple diffusion is a passive process as no energy is require because materials move along the concentration gradient
• osmosis refers to the diffusion of water
• osmotic pressure is the pressure due to differences in concentration on either side of a semipermeable membrane
• isotonic solution has the same solute concentration compared to the intracellular solute concentration

  therefore there is no movement of water
• hypotonic solution has lower solute concentration compared to the intracellular solute concentration

  therefore water moves into the cell
• hypertonic solution has higher solute concentration compared to the intracellular solute concentration

  therefore water moves out of the cell
• tonicity

  A) isotonic solution

  B) hypertonic solution

  C) hypotonic solution

  
• water soluble molecules pass through the membrane via protein channels

  this is so the hydrophilic particles can cross the membrane without coming into contact with the hydrophobic inner portion
• facilitated diffusion is a passive process as the molecules move along the concentration gradient
• carrier proteins open on one side of the membrane at a time. When the specific substance binds to the binding site within the protein, the protein changes shape and opens to the other side of the membrane so the substance can be released
• 3 characteristics of carrier mediated transport:

  • carrier proteins are specific - only bind to a particular molecule
  • carriers can become saturated - once all are occupied any increase cannot increase rate of movement
  • carrier activity is regulated by substances such as hormones
• vesicular transport is movement of substances across the cell membrane in membranous sacs called vesicles
• vesicular transport is an active process as it requires energy to make the vesicles
• endocytosis is the process where a cell takes in materials through vesicles
• pinocytosis (a type of endocytosis) is the process where cells enfold and take in liquid
• phagocytosis (a type of endocytosis) is the process where cells surround and take in solid particles
• exocytosis is the process where a cell pushes out materials through vesicles