cell membranes Flashcards
Cell surface membranes
Surround cells to form a barrier between cell and environment, controlling what enters/leaves the cells.
Partially permeable- only let through some molecules.
Membranes within cells
Membranes around organelles divide cells into compartments.
Act as a barrier between organelle and cytoplasm.
Also partially permeable.
Membrane structure
Lipids, proteins and carbohydrates.
Fluid mosaic model- phospholipids form a continuous bilayer which is constantly moving.
Membrane components
Channel and carrier proteins- allow large molecules/ions to pass.
Receptor proteins (on surface)- allow cell to detect chemicals released from other cells.
Glycoproteins- protein with carbohydrate attached.
Glycolipid- lipid with carbohydrate attached.
Cholesterol
Membrane components
phospholipids
Form a barrier to water soluble substances. Hydrophilic head and hydrophobic tail. Molecules arrange into a bilayer with a hydrophobic centre stopping diffusion of water soluble substances. Small non polar molecules can diffuse.
Membrane components
cholesterol
Lipid that gives the membrane stability. Binds to the hydrophobic phospholipid tails causing them to pack more closely together. Movement of the bilayer reduces making the membrane less fluid and more rigid.
Also helps maintain the shape of animal cells, especially those without other supporting cells.
Also has hydrophobic regions so creates a further barrier to polar substances.
Temperature and membranes
Affects how much phospholipids in the bilayer can move.
Below 0 degrees- phospholipids have little energy so don’t move much . Channel/carrier proteins denature increasing membrane permeability. Ice crystals may form and pierce the membrane making it permeable when thawed.
0-45 degrees- Phospholipids move more and aren’t as tightly packed. Membrane is partially permeable, increasing temp mean the phospholipids can move more (more energy) so membrane permeability increases.
Above 45 degrees- bilayer breaks down and becomes more permeable. Water in the cell expands, putting pressure on the membrane. Channel/carrier proteins denature so don’t control what enters/leaves the cell.
Diffusion
The net movement of particles (molecules/ions) from an area of higher to lower concentration.
Net movement towards the area of lower conc. This continues until particles are evenly distributed.
Passive- no energy needed, particles move freely through membrane.
Factors affecting rate of diffusion
Conc gradient
Thickness of exchange surface
Surface area
Facilitated diffusion
Large molecules diffuse slowly due to size. Charged particles diffuse slowly because they’re water soluble. To speed it up channel/carrier proteins are used. Particles still move down the concentration gradient and it is a passive process.
Facilitated diffusion
Carrier proteins
A large molecule attaches to the carrier protein in the membrane. The protein changes shape which releases the molecule to the other side of the membrane.
Facilitated diffusion
Channel proteins
Form pores in the membrane for charged particles to diffuse through. Different channel proteins facilitate the diffusion for different charged particles.
Factors affecting rate of facilitated diffusion
Conc gradient
Number of channel/carrier proteins
Osmosis
Diffusion of particles from an area of higher to lower water potential across a partially permeable membrane. Pure water has a potential of 0. Adding solutes lowers it, so the water potential of a solution is always negative. More negative water potential=stronger conc of solutes.
Isotonic solutions
Same water potential. Cells in the solution won’t lose or gain any water. There is no net movement because there is no difference in water potential.
Solutions with higher water potential compared to the inside of the cell are hypotonic.
Solutions with lower water potential compared to the inside of the cell are hypertonic.
Factors affecting rate of osmosis
Water potential gradient
Thickness of exchange surface
Surface area
Active transport
Uses energy to move molecules and ions across a plasma membrane against a concentration gradient. Carrier proteins and co-transporters are used.
Active transport
carrier proteins
Molecule attaches to carrier protein, protein changes shape and this moves the molecule across the membrane. It is moved against the concentration gradient using energy produced from the breakdown of ATP.
Active transport
co-transporters
A type of carrier protein. They bind 2 molecules at the same time. The concentration gradient of one molecule is used to move the other molecule against its own concentration gradient.
