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Plasma Membrane & Cellular Transport

What does the cell membrane do?

  • Distinguish the cell from its external environment
  • Controls the movement of substances into and out of the cell
  • enables cells to recognize each other and also identify substances, for example hormones
  • Also called a plasma membrane

Semi Permeable

The cell membrane is semi-permeable - it allows certain particles to pass through it, but not all particles

Passage through the membrane depends on 3 things:

1. Size of the molecule

2. Charge of the molecule

3. Whether they are soluble or not in lipids

What is the membrane consisted of?

  • A double layer of lipids that have a phosphate group attached
  • Proteins (channel proteins and carrier proteins)

What is the membrane consisted of?

Fluid Mosaic Model

  • The cell membrane can be referred to as a fluid mosaic model.
  • A mosaic is a collection of different substances held together by a common material

  • The cell looks like a mosaic of tiles (proteins) held together by a fluid (the lipid bilayer)

Particle Model of Matter

  • All matter is made of particles, but they may be different in size and composition
  • All particles of matter are consistently moving or vibrating (move least in solids)
  • Particles of matter are attracted to one another or are bonded together
  • Particles have spaces between them (smallest in solids)

Diffusion

  • Diffusion is the natural movement of particles from an area of high concentration to an area of low concentration
  • Rate of diffusion can be increased by heating or stirring
  • Does NOT require energy (passive transport)

Diffusion

Diffusion in Cells

  • Diffusion can occur across a cell membrane if there is a difference in concentration on either side of the membrane (called the concentration gradient)
  • This gradient determines which way the water or solute will move

Diffusion in Cells

Osmosis

  • Osmosis is the diffusion of water across the cell membrane (high --> low)
  • Requires NO energy (passive transport)

Osmosis

Predicting direction of water movement

hypertonic: solution has a higher concentration of solutes than in the cell ("hyper" = lots) --> water flow OUT of the cell

isotonic: solution has the same concentration of solutes as the cell ("iso" = equal)

hypotonic: a solution has a lower concentration of solutes as the cell ("hypo" = less) --> water flows INTO the cell

Facilitated Diffusion

  • Substances that are soluble in water but not soluble in lipids need help crossing the membrane
  • A protein can help facilitate the movement across the membrane (this process is called facilitated diffusion)
  • NO energy is required (passive transport) because the substances are still following in response to their concentration gradient (high --> low)

Types of Proteins

  • Channel Protein - creates pores or channels through which small water-soluble particles are able to move
  • Carrier Protein - attaches to larger molecules and then the protein changes shap and physically moves the molecule across the membrane. Then returns to its original shape

Active Transport

  • Sometimes it is necessary to move molecules against the concentration gradient (low --> high). This is called active transport
  • A carrier protein acts like a pump to move molecules or ions across the membrane.
  • It requires energy (is produced by the mitochondria through cellular respiration)
  • Specific kind of energy called ATP (adenosine triphosphate)

Active vs. Passive Transport

Endocytosis and Exocytosis

  • In some cases, molecules are too large to pass across the membrane, even with carrier proteins
  • They must instead use vesicles (sacs that surround the large particles and contain it)
  • Requires energy!!

Endocytosis and Exocytosis

2 ways to move large molecules:

Endocytosis: A vesicle forms around the particle and the cell membrane pinches off around it so that the vesicle is inside the cell (brings molecules IN)

Exocytosis: A vesicle surrounds the particle, then moves to the membrane and fuses with it. The vesicle ruptures and releases contents to surroundings (moves molecules OUT)

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