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)