Transport across the cell membrane

Question 1

Passive transport

Answer 1

• no energy required
• goes along the concentration gradient (movement from high concentration to low concentration)
• e.g. simple diffusion: process resulting from the random movement of ions and molecules to requiring proteins e.g. H2O, O2, CO2
• e.g facilitated diffusions: some particles are too large to move across the lipid bilayer so they use a channel or carrier protein with a specific shape

Question 2

Active transport

Answer 2

• energy required in the from of ATP (adenosine triphosphate) ATP → ADP + Pi + energy
• substances are transported against the concentration gradient
• e.g. vesicular transport: an active process in which materials are moved in membrane-bound sacs
• e.g. some facilitated transport: ones involving carrier proteins that can go against the concentration gradient

Question 3

Simple diffusion

Answer 3

• passive transport mechanism
• movement of particles from an area of high concentration to low concentration along/down the concentration gradient (difference in concentration that brings about diffusion) until equilibrium is reached
• equilibrium is when the concentration of substances in a system is same throughout
• the greater the difference in concentration the steeper the concentration gradient and the faster the rate of diffusion
• the primary direction of most of the movement is called the net movement (direction of diffusion)
• diffusion constantly occurs until there is no net movement
• particles spread out randomly, collide randomly and constantly move
• when equilibrium is reached, particles continue moving randomly across the membrane but the same amount of particles move from each side

Question 4

Particles that can’t move through diffusion

Answer 4

• water-soluble substances
• molecules that are too large (e.g. glucose)
• charged particles

Question 5

Facilitated transport

Answer 5

• when porteins in the cell membrane allow molecules to be transported cross the membrane
• these proteins are channel proteins (which form protein channels) and carrier proteins
• protein transport is specific

Question 6

Protein Channels

Answer 6

• channels provide a pathway for hydrophilic particles to travel through to cross the cell membrane without coming in contact with the hydrophobic inner portion, thus allowing facilitated diffusion
• still, passive transport as it doesn’t require energy as it moves with/down the concentration gradient

Question 7

Carrier-mediated transport

Answer 7

• only 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 where it is released
• carrier proteins can be saturated as once all the carriers are full, any increase in the concentration of molecules to be transported cannot increase the rate of movement
• carrier protein activity is regulated by substances such as hormones
• two main types of carrier-mediated transport:
  
  • facilitated diffusion: when substances are passed
    through carrier proteins along the diffusion gradient
    (passive)
  • active: when substances require ATP as they pass through carrier proteins against the concentration gradient (i.e low to high concentration)

Question 8

Vesicular transport

Answer 8

• movement of substances across the cell membrane in membranous sacs called vesicles
• active process because energy is required to form the vesicles

Endocytosis
- taking in of materials by vesicular transport
- cell membrane folds around the material until the
material is completely enclosed
- the vesicle formed then pinches off and is
suspended in the cell’s cytoplasm
- pinocytosis: taking of liquids into the cell (drinking)
- phagocytosis: when vesicle contains solid (eating)

Exocytosis
- when contents of a vesicle are passed to the
outside
- a vesicle that is formed inside the cell membrane is
fused with the membrane
- the contents of the vesicle are then pushed out into
the extracellular fluid
- e.g. secretion of proteins such as enzymes,
hormones, antibodies from different cells

Question 9

Endocytosis

Answer 9

• taking in of materials by vesicular transport
• cell membrane folds around the material until the material is completely enclosed
• the vesicle formed then pinches off and is suspended in the cell’s cytoplasm
• pinocytosis: taking of liquids into the cell (drinking)
• phagocytosis: when vesicle contains solid (eating)

Question 10

Exocytosis

Answer 10

• when contents of a vesicle are passed to the
  outside
• a vesicle that is formed inside the cell membrane is
  fused with the membrane
• the contents of the vesicle are then pushed out into
  the extracellular fluid
• e.g. secretion of proteins such as enzymes, hormones, antibodies from different cells