1.2: cell membranes Flashcards
What is the structure/function of cell surface membranes?
- separates cell contents from the outside environment
- holds components of some metabolic pathways on place
- regulates transport of materials in and out of the cell
- cell recognition and signalling
What permeability are cell surface membranes?
partially permeable, as they allow water and some solutes through
What are the components of the plasma membrane?
- phospholipids
- proteins
- cholestrol
- glycolipids
- glycoproteins
what components make up a phospholipid?
Hydrophilic head
hydrophobic tails
Phospholipids - hydrophilic head structure
attracted to water, called polar
Phospholipids - hydrophobic tail structure
not attracted to water, called non-polar
What do the phospholipids form?
Bilayers (2 layers of phospholipids with hydrophobic tails protected inside hydrophilic heads)
What is the phospholipid bilayer?
basic structure of membranes
How thick is the phospholipid bilayer?
7nm
What is the model for the structure of the membrane called?
fluid mosaic model
Why is the structure of the membrane called the ‘fluid’?
because individual phospholipids and proteins can move around freely within the layer, by diffusion, like it’s a liquid
Why is the structure of the membrane called the ‘mosaic’?
because of the pattern produced by the scattered protein molecules when the membrane is viewed from above
What is the correlation between membrane and temperature?
- A higher temp gives molecules in the membranes more kinetic energy, so they move faster
- this makes the membrane leaky
- increases permeability
What is the structure of protein in cell membranes?
- most protein molecules float like icebergs in layers, some are fixed ro structures inside the cell
- some proteins are embedded in just the outer or inner layer
- some span the two layers
What is the name of the proteins embedded in just the outer or inner layer?
extrinsic proteins
What is the name of the proteins that span the two layers?
intrinsic proteins
Where do the hydrophobic and hydrophilic parts of the protein sit?
They sit next to the hydrophobic and hydrophilic portions of the membrane. Ensuring proteins stay in the membrane
What are the phospholipids with carbohydrate chains attached to called?
glycolipids
What are the proteins with carbohydrate chains attached to called?
Glycoproteins
What do the membranes also contain molecules of?
Cholesterol
what is the role of phospholipids?
- forms a bilayer
- allow lipid soluble substances in and out of the cell
- prevent water soluble substances moving in/out of the cell
What is the role of cholesterol?
- reduces side to side movement of phospholipids in the bilayer
- regulates fluidity
- prevents leakage of ions and water
- maintains the mechanical stability of the cell
What is the role of proteins?
- may act as channel proteins for water soluble substances
- can work as enzymes
- form receptor sites for molecules
- cell adhesion
- transport proteins - actively move substances using ATP
What is the role of glycolipids?
- forms binding sites for molecules
- helps to maintain stability of membrane
- role in cell attachment
What is the role of glycoproteins?
- receptor site for hormones and neurotransmitters
- role in cell adhesion
- cell signalling
- act as antigens so cells can recognise each other
- intercellular lubrications
What are passive transport methods through cell membranes?
- osmosis
- simple diffusion
- facilitative diffusion
What are active methods through cell membranes?
- active transport
- bulk transport
what is the difference between active and passive methods?
active requires ATP
what is simple diffusion?
diffusion is the passive net movements of molecules from a region of higher concentration to a region of lower concentration
Simple diffusion - movement
- Moves down a concentration gradient
- molecules have kinetic energy, which makes them move about randomly. It is passive and does not require ATP
What determines the rate of diffusion?
- concentration gradient
- temperature
- area over which diffusion takes place
- size and nature of the molecule
Factors rate of diffusion - concentration gradient
The steeper the conc gradient the quicker the rate of diffusion
Factors rate of diffusion - Temperature
Higher temp gives molecules or ions more kinetic energy. Molecules move around faster, so diffusion is faster
Factors rate of diffusion - Area over which diffusion takes place
The greater the surface area the faster the rate of diffusion
Factors rate of diffusion - Size and nature of the molecule
- large molecules need more energy to move so they diffuse more slowly
- Non-polar molecules diffuse more easily than polar molecules because they are soluble in the non-polar phospholipid tails
Distance traveled will also effect rate
What is facilitated diffusion?
Large polar molecules (E.g. glucose and amino acids, and ions E.g. Na + or Cl- can’t diffuse across the phospholipid bilayer)
- diffuse through channel or carrier proteins often shaped to allow only one type of molecule through
- passive
What is the definition of osmosis?
The diffusion go water molecules down a water potential gradient, from an area of higher water potential to an area of lower water potential across a partially permeable membrane
What is water potential?
The tendency of water molecules to move from one place to another
- Measured in Kpa
What is a solute?
Dissolves in water
When is equilibrium reached in osmosis?
When the water potential is equal on either side of the cell
What is the definition of Active Transport?
- Against a concentration gradient
- from a lower to a higher concentration
- uses energy supplied by ATP
- Carrier proteins are involved - complementary in shape to the molecule it transports
Is diffusion or active transport faster rate?
Diffusion
What is an hypotonic solution (ANIMALS)
high water potential outside the cell, low water potential inside cell.
Causes the cell to burst (LEFT PICTURE)
`what is an isotonic solution?
Equal water potential inside and outside the cell.
EQUILIBRIUM REACHED
(MIDDLE PICTURE)
What is an hypertonic solution?
Higher water potential inside the cell, lower water potential outside the cell.
Cell shrinks (RIGHT PICTURE)
What molecules can diffuse through cell membranes?
- Oxygen
- Carbon Dioxide
- Water
How quickly do each of these molecules diffuse through cell membrane and why:
- oxygen
- carbon dioxide
- water
- oxygen: Non-polar so diffuses very quickly
- Carbon dioxide: Polar but very small so diffuses quickly
- Water: Polar but also very small so diffuses quickly
What are turgid plant cells in relation to osmosis?
When water moves into a cell because the water potential outside the cell is higher than inside the cell.
Why do the turgid cells not burst when water enters the cell via osmosis?
Because the cellulose cell wall is strong enough to stop the cell bursting. It pushes back against the protoplast
What are flaccid plant cells in relation to osmosis?
When water leaves the cell because the water potential is higher in the cell compared to lower water potential outside the cell.
What happens to flaccid cells in terms of the shape?
The protoplast shrivels and plasmolysis occurs
What is plasmolysis?
When cell membrane pulls away from the cell wall
What is incipient plasmolysis?
When a cell is placed in an equal water potential so there is no net movement of water.
What is the equation for water potential?
water potential of cell = solute potential of the cell + pressure potential of the cell
What does the solute potential do to the water potential?
decreases it, makes it more negative in Kpa
What does the pressure potential do to the water potential?
increases it, makes it less negative in Kpa
In incipient plasmolysis what is the water potential in Kpa?
0 Kpa
What is the process of exocytosis?
- vesicle moves towards plasma membrane
- Vesicle joins with plasma membrane
- Vesicle contents released, the vesicle membrane is now part of the plasma membrane
What is the process of endocytosis?
- the cell spreads around an object or area of the solution outside the cell
- the area enclosed becomes a vesicle
- The contents of the vesicle are absorbed into the cytoplasm and the vesicle membrane is recycled
What is cell signalling?
When cells communicate with signals which can be molecules such as hormones, neurotransmitters or cytokines.
What signals must cells be able to detect which coordinates processes?
- growth
- development
- movement
- excretion
How does a cell detect a signal?
The cell has receptors (usually glycoproteins) on the cell surface membrane
- communication is often by hormones (chemical messengers)
- a cell with a receptor for the hormone molecule is a target cell
- when the signal molecule binds to the receptor, a response then occurs
What is an example of a medicinal drug using cell signalling?
insulin
What is the mechanism of cell signalling ‘receptor acts as an ion channel’
When a chemical signal attaches to the receptor, it makes the ion channel open, letting ions into the cell causing a response.
What is the mechanism of cell signalling ‘receptor activates a G-protein’
When this happens, the G-protein then activates an enzyme which brings about a response
What is the mechanism of cell signalling ‘receptor acts as an enzyme’
The receptor is made up of 2 parts. The signal molecule attaches to both parts, connecting them together and forming an active enzyme, which brings about reactions in the cell.
How does the structure of the water channel protein relate to its function?
The phospholipid heads are hydrophilic so position themselves on the outside.
The fatty acid tails are hydrophobic so position themselves in the inside of the cell.
This means that the water can pass through the whole cell membrane.
Draw and label a simple diagram of facilitated diffusion
What reasons are exchange between the cell and the environment essential
- allows them to obtain nutrients and energy
- allows them to eliminate waste products
- in order for communication
What two types of molecules can diffuse through the bilayer?
- ions
- polar molecules (charged)
What characteristic of a protein channel allows it to maintain its position within the phospholipid bilayer?
They are made up of amino acids which are non polar side chains. This allows them to contact the hydrophobic environment of the lipid bilayer
What characteristics of a protein channel allows molecules such as glucose to pass through them
They have open pores in their membranes
What is bulk transport?
Moving large quantities of materials in or out of the cell. This requires ATP
What are the two types of bulk transport?
- Endocytosis
- Exocytosis
What is endocytosis?
Bulk transport of a material into a cell
What is exocytosis?
Bulk transport of a material out of a cell
What is bulk uptake of solids called? Give an example
Endophagocytosis, White blood cells
What is bulk uptake of liquids called?
Endopinocytosis
Draw and label a diagram of bulk transport
What do the receptors have in cell signalling?
A specific and complementary shape to the shape of the signalling molecule
What are the 3 types of cell signalling?
- Autocrine
- endocrine
- paracrine
How does Autocrine signalling work?
- cell stimulates a response within itself releasing chemical signals for its own cells.
E.g. Immune cells
How does Endocrine signalling work?
- Occurs over large distances
- signalling molecule is usually a hormone transported in the blood to the target cell
How does Paracrine signalling work?
Between cells close together.
E.g. Neurotransmitters released between neurones
Examples of hydrophobic molecules passing across cell membrane?
O₂ , CO₂ , N₂ , Steroids
Examples of small uncharged polar molecules passing across cell membrane?
H₂o, Urea, ethanol
Examples of large uncharged polar molecules passing across cell membrane?
Glucose, Sucrose, Amino acids
Examples of ions passing across cell membrane?
Na +, K +, cl -
