Chap 4: Cell Membranes + Transport Flashcards
Cell membranes + diffusion + osmosis + AT
Describe 3 functions of PHOSPHOLIPIDS in the CELL MEMBRANE
- allow lipid-soluble substances to enter and leave the cell
- prevent water-soluble substances entering and leaving the cell
- make the membrane flexible and self-sealing.
Describe the phospholipid bilayer
- The hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides.
- The hydrophobic tails of both phospholipid layers point into the centre of the cell membrane, repelled by the water on both sides.
What are CHANNEL proteins, and what do they do?
-Protein channels form water-filled tubes
- to allow water-soluble ions to diffuse across the membrane.
(NOTE: channel and carrier proteins completely span the phospholipid bilayer from one side to the other)
What are CARRIER proteins, and what do they do?
Carrier proteins bind to ions or molecules like glucose and amino acids,
then change shape in order to move these molecules across the membrane.
Some proteins occur in the surface of the bilayer and never extend completely across it.
What is their purpose?
They act either to give mechanical support to the membrane or, in conjunction with glycolipids, as cell receptors for molecules such as hormones.
What are the functions of the proteins in the membrane? (5)
- provide structural support
- act as channels transporting water-soluble substances across the membrane
- allow active transport across the membrane through carrier proteins
- help cells adhere together
- act as receptors, for example for hormones.
- form cell-surface receptors for identifying cells
Describe cholesterol
- occur within the phospholipid bilayer of the cell-surface membrane.
-add strength to the membranes.
- very hydrophobic and therefore prevent loss of water and dissolved ions from the cell.
-pull together the fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules but without making the membrane as a whole too rigid.
What are the functions of cholesterol?
- reduce lateral movement of other molecules including phospholipids
- make the membrane less fluid at high temperatures
- prevent leakage of water and dissolved ions from the cell.
What are the functions of glycolipids in the membrane?
- act as recognition sites
- help maintain the stability of the membrane
- help cells to attach to one another and so form tissues.
What are the functions of glycoproteins in the membrane?
- act as recognition sites
- help cells to attach to one another and so form tissues
- allows cells to recognise one another, for example lymphocytes can recognise an organism’s own cells.
Why aren’t most molecules able to diffuse through freely in the cell membrane?
- not soluble in lipids and therefore cannot pass through the phospholipid layer
- too large to pass through the channels in the membrane
- of the same charge as the charge on the protein channels and so, even if small enough to pass through, they are repelled
- polar molecules (electrically charged) have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer.
Why is the fluid mosaic described as ‘fluid’ and ‘mosaic’?
Fluid: bc individual phospholipid molecules can move relative to one another. This gives a flexible structure that is constantly changing in shape.
Mosaic: bc the proteins embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as a mosaic.
What are the functions of membranes WITHIN cells?
- control the entry and exit of materials in e.g. mitochondria and chloroplasts
- separate organelles from cytoplasm so that specific metabolic reactions can take place within them
- provide an internal transport system, e.g., ER
- isolate enzymes that might damage the cell, e.g., lysosomes
- provide surfaces on which reactions can occur, e.g., protein synthesis using ribosomes on RER
What type of transport is diffusion?
(Active or passive?)
Passive transport
What type of molecules can diffuse through the cell membrane?
- small
- lipid soluble
- non polar (e.g. CO2, O2)
molecules
How do non polar (e.g. CO2, O2) molecules diffuse through the cell membrane?
(what process?)
simple diffusion —> through the phospholipid bilayer
Why are charged ions and non polar molecules not able to diffuse as easily?
Bc of the hydrophobic nature of the fatty acid tails of the phospholipids in the membrane.
So how are charged ions and non polar molecules able to diffuse through?
- Facilitated diffusion
- By using transmembrane channels and carriers that span the membrane
Define DIFFUSION
The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed.
How would you describe the movement of particles in general?
- all particles are constantly in motion due to KE they possess
- this motion is random, no set pattern to the way the particles move around
- particles are constantly bouncing off one another as well as off other objects
What is facilitated diffusion?
- passive
-relies only on the KE of the diffusing molecules.
- no external input of ATP from respiration.
- occurs down a concentration gradient, but it differs from simple diffusion in that it occurs at specific points on the plasma membrane where there are special protein molecules.
- Two types of protein are involved - protein channels and carrier proteins.
What are channel proteins?
- Formed from water-filled hydrophilic channels across the membrane.
- Allow specific water-soluble ions to pass through.
- Very selective, each opening in the presence of a specific ion.
- If particular ion is not present, the channel remains closed = there is control over the entry and exit of ions.
- The ions bind with the protein causing it to change shape in a way that closes it to one side of the membrane and opens it to the other side.
What are CARRIER proteins?
When a molecule (e.g. glucose) that is specific to the protein is present, it binds with the protein.
= change in shape in such a way that the molecule is released to the inside of the membrane
- No external energy is needed for this.
- molecules move from a region where they are highly concentrated to one of lower concentration, using only the KE of the molecules themselves.
What is diffusion proportional too??
Diffusion is proportional to the DIFFERENCE in concentration between two regions (the concentration gradient).
Where does diffusion takes place?
ONLY fluids
Describe characteristics of Faciliated diffusion?
- limited by no. of proteins
- more specific: assures transport proteins–> 1. carrier charge specific 2. channel charge specific
- ions / water soluble / larger molecules / non polar molecules able to diffuse through (EG glucose, amino acids)
Define osmosis
The passage of water from a region where it has a higher water potential
to a region where it has a lower water potential
through a selectively permeable membrane.
What does the addition of a solute to pure water mean for the water potential?
lower its water potential
Which direction will water move by osmosis according to water potential?
water will move by osmosis from a region of higher (less negative) water potential (e.g., -20 kPa)
to one of lower (more negative) water potential (e.g., -30 kPa).
How can we find out the water potential of cells or tissues?
- place cells/tissues in a series of solutions of different water potentials.
Where there is no net gain or loss of water from the cells / tissues, the water potential inside the cells / tissues must be = external solution.
When is a dynamic equilibrium established in osmosis?
When water potentials on either side of the plasma membrane are =,
a dynamic equilibrium is established and there is no net movement of water.
What is the highest value of water potential? (Pure water)
The highest value of water potential, that of pure water, is zero, and so all other values are negative.
The more negative the value, the lower the water potential.
Define Active Transport
The movement of molecules / ions into or out of a cell from a region of lower concentration
to a region of higher concentration
using ATP and carrier proteins.
Describe Na+-K+ pump
sodium ions are actively removed from the cell/organelle while potassium ions are actively taken in from the surroundings.
How is ATP used in Active Transport?
- directly move molecules
- individually move molecules using a concentration gradient which has already been set up by (direct) active transport. (AKA COTRANSPORT)
Describe CO-Transport
- Metabolic energy in the form of ATP is needed.
- Substances are moved against a concentration gradient, that is from a lower to a higher concentration.
- Carrier protein molecules which act as ‘pumps’ are involved.
- The process is very selective, with specific substances being transported.
