3.2.3 Transport across cell membranes Flashcards
Plasma membranes
All membranes around and within all cells
Phospholipid bilayer
- Hydrophilic phosphate heads: point to outside of the cell membrane on both sides
- Hydrophobic hydrocarbon tails: point into the centre of the cell membrane
Functions of phospholipids in the plasma membrane
- Allow lipid-soluble substances to enter + leave the cell
- Prevent water-soluble substances to enter + leave the cell
- Make the membrane flexible and self-sealing
Proteins in the plasma membrane
- Some occur on the surface of the bilayer = give mechanical support, or along with glycolipids act as cell receptors
- Some span the bilayer: protein channels, carrier proteins
Protein channels
Form water-filled tubes to allow water soluble ions to diffuse across membrane
Carrier proteins
Bind to ions or molecules (e.g. glucose, amino acids) then change shape in order to move these molecules across the membrane
* active transport
Functions of proteins in the plasma membrane
- structural support
- act as channels to transport water-soluble substances across the membrane
- carrier proteins allow active transport across the membrane
- form cell-surface receptors for identifying cells
- help cells adhere together
- act as receptors (e.g. for hormones)
Cholesterol
- very hydrophobic so prevent loss of water + dissolved ions from the cell
- pull together the fatty acid tails of the phospholipid molecules to limit their movement
Functions of cholesterol in the plasma membrane
- reduce lateral movement of other molecules (including phospholipids)
- make the membrane less fluid at high temperatures
- prevent leakage of water + dissolved ions from the cell
Glycolipids
Carbohydrate covalently bonded with a lipid
Glycolipids in the plasma membrane
Carbohydrate portion extends from the phospholipid bilayer into the water environment outside the cell where it acts as a cell-surface receptor for specific chemicals
Functions of glycolipids in the plasma membrane
- Act as recognition sites
- Help maintain the stability of the membrane
- Help cells to attach to one another and so form tissues
Glycoproteins
Act as cell-surface receptors, specifically for hormones + neurotransmitters
Role of the plasma membrane
To control the movement of substances into + out of the cell
Why do most molecules not freely diffuse across the plasma membrane?
The molecules are:
* insoluble in lipids: cannot pass through the phospholipid bilayer
* too large to pass through the protein channels
* of the same charge as the charge on the protein channels - so even if they are small enough, they are repelled
* polar so cannot pass through the non-polar hydrophobic tails in the phospholipid bilayer
Fluid-mosaic model of the cell surface membrane
The way in which all the various molecules are combines into the structure of the plasma membrane
Why is the fluid mosaic model called the fluid mosaic model?
- Fluid: individual phospholipid molecules can move relative to one another -> makes the membrane flexible (it is constantly changing shape
- Mosaic: the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern
Passive processes: explanation for simple diffusion
The energy comes from the natural motion of particles (not from an external source)
- All particles are constantly in random motion due to the K.E. they possess
- particles are constantly bouncing off each other and other objects (e.g. the side of the container)
Diffusion definition
The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower, until they are evenly distributed
How is facilitated diffusion different to simple diffusion?
The movement of polar molecules across the plasma membrane
Only occurs at specific points on the plasma membrane where there are special protein molecules (protein channels and carrier proteins)
Mechanism of protein channels for facilitated diffusion
- Hydrophilic channels which allow specific water-soluble ions to pass through
- Selective (only open in the presence of a specific ion)
- The ions bind with the protein causing it to change shape in a way that closes it on one side and opens it on the other
Mechanism of carrier proteins
When a molecule that is specific to the protein is present, it binds with the protein which causes it to change shape and release it to the inside of the membrane
- The molecules move from high -> low conv
Osmosis definition
The passage of water from a region where it has high water potential to a region where it has lower water potential through a selectively permeable membrane
Solute
Any substance that is dissolved in a solvent
Water potential of water under standard conditions
0
Value of water potential
- Adding pure water increases its water potential
- The value is always less than 0
How to find the water potential of cells or tissues
- To place them in a series of solutions of different water potentials
- Where there is no net gain or loss of water from the cells or tissues, the water potential inside the cells or tissues must be the same as that of the external solution
Why is the environment around a cell in an animal the same water potential as that of the cytoplasm inside the cell?
To avoid bursting of the cell which would release its contents
- in humans, the blood is the same water potential as that of the cell
Conditions of cell
- Turgid
- Incipient plasmolysis
- Plasmolysed
Turgid
Cell is in higher water potential solution
Incipient plasmolysis
equal
Plasmolysed
Cell is in lower water potential solution - protoplast is completely pulled away from the cell wall
Protoplast
Outer cell-surface membrane
Active Transport
The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins
What is ATP used for in active transport?
- Directly move molecules
- Co-transport
How does active transport (active process) differ from passive forms of transport?
- Metabolic energy in the form of ATP is needed
- Substances are moved against a concentration gradient
- Carrier proteins are involved, which act as ‘pumps’
- The process is selective
Process of direct active transport
- Carrier proteins span the plasma membrane and bind to the molecule or ion to be transported on one side of it
- The molecule or ion binds to receptor sits on the carrier protein
- On the inside of the cell/organelle, ATP binds to the protein, causing it to split into ADP and P1. This causes the protein molecule to change shape and open to the opposite side of the membrane
- The molecule or ion is released to the other side of the membrane
- The P1 is released from the protein -> the protein reverts to its original shape (ready for the process to be repeated)
- The P1 recombines with ADP to form ATP during respiration
Difference between facilitated diffusion and active transport
- Facilitated occurs down a concentration gradient, so doesn’t require metabolic energy
- Active transport occurs against a concentration gradient, so requires metabolic energy
Sodium-potassium pump
Sodium ions are actively removed from the cell/organelle while potassium ions are actively taken in from the surroundings
Role of diffusion in absorption
During digestion, there is a higher concentration of glucose + amino acids in the ileum compared to the blood
So glucose moves into the blood by facilitated diffusion
Role of active transport in absorption
With diffusion, glucose + amino acids can only be absorbed until the concentrations are the same on either side of the ileum = not all of the glucose + amino acids
So instead glucose + amino acidsa re drawn into the cells along with sodium ions that have been actively transported by the sodium-potassium pump
Process of co-transport
- Sodium ions are actively transported out of the epithelial cells by the sodium-potassium pump, into the blood. This takes place in 1 type of protein-carrier molecule found in the plasma membrane of the epithelial cells
- This maintains a higher conc. of sodium ions in the lumen compared to the epithelial cells
- Sodium ions diffuse into the epithelial cells down a concentration gradient through a different type of protein carrier in the plasma membrane, and as they do this they can either carry amino acids or glucose into the cell within them
- The glucose/amino acids pass into the lumen by facilitated diffusion using another type of carrier
Purpose of co-transport
Indirect form of active transport
