Section 2 - Transport across cell membranes Flashcards
What is the name given to membranes around and within all cells?
Plasma membranes
What does the cell-surface membrane surround?
This surrounds the cell and forms a boundary between the cell cytoplasm and the environment
What is the function of the cell-surface membrane?
It allows different conditions to be established inside and outside a cell. It also controls the movement of substances in and out of the cell.
Why is the phospholipid bilayer important in the cell-surface membrane?
- 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 is the function of phospholipids in the membrane?
- Allows 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
What are the main ways that proteins are embedded in the phospholipid bilayer?
- some proteins occur in the surface of the bilayer and never extend completely across it. They act either to give mechanical support to the membrane or, in conjunction with glycolipids, as cell receptors for molecules such as hormones - Other proteins completely span the phospholipid bilayer from one side to the other. Some are protein channels, which form water-filled tubes to allow water-soluble ions to diffuse across the membrane. Others are carrier proteins that bind to ions or molecules like glucose and amino acids, then change shape in order to move these molecules across the membrane.
What are carrier proteins?
They bind to ions or molecules like glucose and amino acids, then change shape in order to move these molecules across the membrane.
What are the functions of proteins in the membrane?
- provide structural support - act as channels transporting water-soluble substances across the membrane - Allow active transport across the membrane through carrier proteins - form cell-surface receptors for identifying cells - help cells adhere together - act as receptors, for example for hormones
Where is cholesterol found?
Cholesterol molecules occur within the phospholipid bilayer of the cell-surface membrane
What is the structure of cholesterol?
The molecules are very hydrophobic and so plays an important role in preventing loss of water and dissolved ions from the cell.
What is the function of cholesterol in the membrane?
- Reduce lateral movement of other molecules including phospholipids - Make the membrane less fluid at high temperature - Prevent leakage of water and dissolved ions from the cell
Why does cholesterol Pull together the fatty acid tails of the phospholipid molecules ?
limiting their movement and that of other molecule but without making the membrane as a whole too rigid
What are glycolipids made of?
a carbohydrate covalently bonded with a lipid
What does the carbohydrate do in the glycolipids?
It extends from the phospholipid bilayer into the watery environment outside the cell where it acts as a cell-structure receptor for specific chemicals eg. the human ABO blood system
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 carbohydrate chains attached to?
Many extrinsic proteins on the outer surface of the cell membrane. They also act as cell-surface receptors and more specific for hormones and neurotransmitters
What is the function 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, eg. Lymphocytes
What do lymphocytes do?
Can recognise an organism’s own cells
Why can molecules not freely diffuse across the 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 they are small enough to pass through, they are repelled - Electrically charged (in other words polar) and therefore have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer
Define - polar
An electrically charged molecule
Give an example of a non-polar molecule
hydrophobic tails in the phospholipid bilayer
Why is the arrangement of molecules called the fluid-mosaic model?
- Fluid because the individual phospholipid molecules can move relative to one another. This gives the membrane a flexible structure that is constantly changing in shape. - Mosaic because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as the stones or tiles of a mosaic
What are the functions of membranes within cells?
- Control the entry and exit of materials in discrete organelles such as mitochondria and chloroplast - Separate organelles from cytoplasm so that specific metabolic reactions can take place within them - Provide an internal transport system - Isolate enzymes that might damage the cell - Provide surfaces on which reactions can occur
Give an example of a system that provides an internal transport system
Endoplasmic reticulum
Give an example of a system that provides isolation for enzymes that might damage the cells.
Lysosomes
Give an example of a molecule that provides a surface on which reactions can occur
Protein synthesis using ribosomes on rough endoplasmic reticulum
Give an example of a passive transport
- Diffusion - Osmosis
Give an example of an exchange between cells and the environment that occurs in ways of requiring metabolic energy
active transport
Define - Active Transport
The exchange of substances between cells and the environment occurring in ways that require metabolic energy.
What does passive mean in turns of transport?
The energy comes from a natural, inbuilt motion of particles, rather than from some external source such as ATP.
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
Give two examples of non-polar molecules that can diffuse across membranes
- Oxygen - Carbon Dioxide
Why don’t polar molecules diffuse easily?
The hydrophobic nature of the fatty acid tails of the phospholipids in the membrane.
How is movement in diffusion made easier?
Transmembrane channels and carriers that span the membrane. This is called facilitated diffusion.
What is facilitated diffusion?
- Transmembrane channels and carriers that span the membrane. - A passive process - Relies only on the in bulit motion (Kinetic energy) of the diffusing molecules. - Occurring down the concentration gradient - Either using protein channels or carrier proteins.
State the two types of proteins used in facilitated diffusion
- Protein channels - Carrier Proteins
What do protein channels produce?
- Water-filled hydrophilic channel across the membrane
What do protein channels allow through?
Specific water soluble ions to pass through
Explain the structure of protein channels
- They are selective and each opening in the presence of a specific ion - If this specific ion is not present the channel will remain closed - This gives them control over the entry and exit of ions to the cell - 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
How do protein channels open and close?
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?
- A form of facilitated diffusion - Spanning the plasma membrane
Explain how a carrier protein may react to the presence of glucose
If it is specific to the protein it binds with the protein causing it to change shape in such a way that the molecule is released to the inside of the membrane. No external energy is used for this using only the kinetic energy of the molecules themselves.
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 is meant by selectively permeable?
They are permeable to water molecules and a few other small molecules but not to larger molecules.
What is a solute?
Any substance that i dissolved in a solvent eg. water.
What makes up a solution?
A solute and the solvent.
How can you represent water potential?
Greek Letter Psi
What is water potential measured in?
Units of pressure KiloPascals (kPa)
What is water potential?
The pressure created by water molecule.
Under what conditions does pure water have a water potential of zero
Under standard conditions of temperature and pressure 25*C and 100kPa
What can lower pure water’s water potential?
Adding a solute
What does adding a solute do to pure water’s water potential?
Lower’s the water potential
What is the rule of water potential?
The water potential of a solution must always be less than zero. A negative value
Explain osmosis in terms of water potential
Water will move by osmosis from a region of higher (less negative) water potential (eg. -20kPa) to one of lower (more negative) water potential (eg. -30kPa)
How can you find the water potential of cells or tissue?
To place them in 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.
