Transport Across Cell Membranes:
- Created by: hotzmc
- Created on: 09-04-18 14:15
Plama Membrane Basics
- All membranes around all cells have the same basic structure
- The cell surface-membrane is given specifically to membrane that surrounds cells and froms a boundary between the cell cytoplasm and the environemt
- It allows different conditions to be established inside and outside a cell
- It controls the movement of substances into and out of the cell
Plama Membranes are made up of:
- Phospholipids
- Proteins
- Cholesterol
- Glycolipids
- Glycoproteins
Phospholipids:
Phospholipids form a bilayer. They're important because:
- The hydrophilic heads point to the outside of the cell-surface membrane, attracted by water on both sides
- The hydrophobic tails point into the centre of the cell membrane, repelled by water on the outside
Lipid soluble material moves through the membrane via the phospholipid portion. Functions of phospholipids are to:
- Allow lipid-soluble substances to enter & leave the membrane
- Prevent water soluble substances entering & leaving the membrane
- Make the membrane flexible
Proteins:
Proteins are interspread and embeded throughout the cell-surface membrane in 2 main ways:
- Occur on the surface of the bilayer, and never expand completely across it. Act to either give mechanical support or in conjuction with glycolipids as a cell receptor for molecules like hormones
- Span the bilayer from one side to the other. Some are protein channels, forming water filled tubes to allow water soluble ions to diffuse across the membrane. Others are carrier proteins, they bind to ions or molecules like glucose and amino acids, then change shape to move these molecules acorss the membrane
Functions of Proteins in the membrane are to:
- Provide structural support
- Act as channel transporting water soluble substances across the membrane
- Allow active transport across the membrane through carrier proteins
- Form and act as cell-surface receptors for identifying cells
- Helps cells adhere together
Cholesterol:
Cholesterol molecules occur within the phospholipid bilayer of the cell-surface membrane. They add strength and are very hydrophobic. They play an important role in preventing loss of water and dissolved ions from the cell. They pull together the fatty acid tails of the phospholipid, limiting their movement and that of other molecules, but without making the membrane too rigid.
Fuctions of cholesterol in the membrane are:
- Reduce lateral movement of other molecules including phospholipids
- Make the membrane less fluid
- Prevent leakage of water and dissolved ions from the cell
Glycolipids:
Are made up of a carbohydrate covalently bonded with a lipid. The carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell, it acts as a cell-surface receptor for specific chemicals
The functions of glycolipids are to:
- Act as recognition sites
- Help maintain stability of the membrane
- Help cells to attach to one another and therefore form tissues
Glycoproteins:
Carbohydrate chains are attached to many extrinsic proteins on the outer surface of the cell membrane. They act as cell-surface receptors for hormones and neurotransmitters.
The functions of glycoproteins in the membrane are to:
- Act as recognition sites
- Help cells to attach to one another, forming tissues
- Allow cells to recognise one another, e.g. lymphocytes can recognise an organisms own cells
Permeability of the cell-surface membrane:
the cell-surface membrane controls the movement of substances into and out of the cell. Most molecules do not freely diffuse across it becuase many are:
- 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, so they're repelled
- Electrically charged (are polar), therefore they have dificulty passing through the non-polar hydrophobic tails in the phospholipid bilayer
Fluid-Mosaic Model of the Cell-Surface Membrane:
The way in which all various molecules are combined into the structure of the cell-surface membrane is known as the fluid-mosaic model for the following reasons:
Fluid: The individual phospholipid molecules can move relative to one another. Giving the membrane a flexible structure that is constantly changing shape
Mosaic: The proteins that are embeded in the phospholipid bilayer vary in shape, size and pattern
Explanation of Simple Diffusion:
The energy comes from the natural, inbuilt motion of particles, rather than an external source like ATP.
- All particles are constantly in motion due to the kinetic energy they possess
- Their 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. E.g. the sides of the vessel
Particles that are concentrated together in part of a closed vessel will distribute themselves evenly thoughout the vessel as a result of diffusion
Defined as:
The netmovement of molecules from a region of high concentration to a region of low concentration until evenly distributed
Facilitated Diffusion:
Plasma membranes are not readily permeable to molecules. Only small, non-polar molecules can diffuse across them easily. Charged ions and polar molecules do notdiffuse easily becuase of the hydrophobic nature of the fatty acid tails in the membrane. Movement of these molecules is facilitated by transmembrane channels and carriers spanning the membrane.
Facilitated diffusion is a passive process, It relies on inbuilt motion of the diffusing molecules. There's no external input from ATP, but it occurs at specific points in the membrane; where there are protein channels or carrier proteins
Protein Channels:
- Form water filled hydrophilic channels across the membrane.
- Allow specific water-soluble ions to pass through.
- The channels are selective, each opening in the presence of a specific ion. If the particular ion is not present, the channel remains closed, therefore there is control over the entry and exit of ions
- 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
Carrier Proteins:
- They span the plasma membrane. When a molecule that is specific to the protein is present, it binds with the protein, causing it to change shapein such a way that the molecule is released to the inside of the membrane
- No external energy is needed
- Molecules move from a region of high concentration to a region of low concentration, using only kinetic energy of the molecules themselves
Osmosis:
Osmosis is defined as:
The passge of water from a region of high water potential to a region of low water potential, through a selectively permeable membrane
- Cell-surface membranes and other plasma membranes are selectively permeable
- They're permeable to water and some other small molecules, but not larger molecules
Osmosis and Animal Cells:
- Animal cells, e.g. red blood cells, contain a variety of solutes dissolved in their cytoplam. If a red blood cell is placed into pure water, it will absorb water by osmosis because it has a lower water potential.
- Cell surface membranes are very thin, and cannot stretch much. Therefore the cell will take in too much water and the cell surface membrane will break, bursting the cell and releasing its contents.
- To prevent this happening, animal cells normally live in a liquid which has the same water potential as the cells, in this case it is blood plasma
- This and the red blood cells have the same water potential
- If a red blood cell is placed into a solution with a lower water potential than its own, water leaves by osmosis
- This causes the cell to shrink and become shrivelled
Solutions and Water Potential:
A solute is any substance that is dissolved in a solvent, e.g. water. The solute and solvent together form a solution.
Water postential is measured in units of pressure, usually kiloPascals (kPa). Water potential is the pressure created by water molecules. Pure water is said to have the potential of zero.
- The addition of a solute to pure water will lower its water potential
- The water potential of a solution must always be a negative value
- The more solute added, the more negative the water potential
- Water will move by osmosis from a region of higher water potential to a region of lower water potential
Where there is no net gain or loss of water from cells or tissues, the water potential inside the cells or tissues must be the same as the external solution
What is Active Transport:
The movement of molecules into or out of a cell from an area of low concentration to an area of high concentration (against the concentration gradient) using ATP and carrier proteins.
Use of ATP in Active Transport:
- Directly move molecules
- Individually move molecules using a concentration gradient which has already been set up by active transport (co-transport)
Active Transport Differs from Passive Transport:
- ATP energy is needed
- Substances are moved against the concentration gradient, from low to high
- Carrier proteins are involved, as pumps
- It's a selective process, only specific substances are transported
Direct Active Transport of a single Molecule/Ion:
- Carrier proteins span the membrane, and bind to the molecule or ion being transported on one side of it
- Molecule or ion binds to the receptor site on carrier protein
- On the inside of the membrane, ATP binds to the protein, causing it to split into ADP and a phosphate molecule
- This causes the protein to change shape and opens to the opposite side of the membrane
- The molecule or ion is then release to the other side of the membrane
- The phosphae molecule is released from the protein, causing the protein to revert to its original shape
- The phosphate molecule will recombine with ADP to make ATP during respiration
Sometimes more than one molecule can be moved in the same direction at the same time by active transport. Occasionally a molecule is moved in while another is moved out, for example in a sodium-potassium pump
In a sodium-potassium pump, sodium ions are acitvely removed from the cell, and potassium ions are taken in. This is very important for the creation of nerve impulses
Increasing the Rate of Movement Across Membranes:
- Epithelial cells lining the ileum posess microvilli
- They're finger like projections of the cell-surface membrane, about 0.6um in length
- Collectively termed a 'brush-boarder' as under a microscope they look like bristles on a brush
- The microvilli provide more surface area for carrier proteins and so more diffusion, facilitated diffusion and active transport can take place
- Another way to increase the rate of transport, is to increase the number of protein channels and carrier proteins in any area of the membrane
The Role of Diffusion in Absorption:
Diffusion is 'the net movement of molecules/ions from an area of high concentration to an area of low concentration'
- Carbohydrates and proteins are continuously being digested, there's a greater concentration of glucose and amino acids in the ileum that the blood
- Therefore there's a concentration gradient down, which glucose moves by facilitated diffusion, from the inside of the ileum to the blood
- As blood is constantly being circulated by the heart, glucose absorbed into the blood and is constantly being pumed round and being removed by cells as they use it for respiration
- This helps maintain the concentration gradient between the inside of the ileum and the blood
- Meaning the rate of movement by facillitated diffusion across epithelial cell-surface membranes is increased
The Role of Active Transport in Absorption:
Diffusion only results in the concentrations ethier side of intestial epithelium becoming equal. Meaning, not all the available glucose and amino acids can be absorbed in this way and some may pass out of the body. However, this doesn't happen becuase glucose and amino acids are also being apsorbed by active transport. Meaning, that all glucose and amino acids should be absorbed into the blood.
The actual mechanism that they're absorbed from in the small intestine is an example of co-transport. This term is used as when either glucose or amino acids are drawn into cells along with sodium ions that have been acitively transported out by the sodium potassium pump.
Co-Transport - Sodium-Potassium Pump:
1) Sodium ions are actively transported out the epithelial cells by the sodium-potassium pump, into the blood. Takes place in protein-carrier molecules found in the cell-surface membrane of the epithelial cells
2) This maintains a higher concentration of sodium ions in the lumen of the itestine, than inside the the epithelial cells
3) Sodium ions diffuse into the epithelial cells down this concentration gradient through a co-transport protein in the cell-surface membrane. As sodium ions diffuse in through this protein they carry either a molecule of glucose or an amino acid into the cell with them
4) The glucose/amino acid pass into the blood plasma by facilitated diffusion using another type of carrier
Both sodium ions and glucose/amino acids molecules move into the cell. Sodium moves down its concentration gradient, but glucose/amino acids move against its concentration gradiet. The sodium ion concentration gradient powers the movement of glucose/amino acid into the cells, making in an indirect form of active transport.
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