2.5 - Plasma Membranes Flashcards
Define hydrophobic
Repels water
Define hydrophilic
Soluble in water
Outline the basic structure of a phospholipid
- Phosphate head
- Glycerol backbone
- Two fatty acid chains
How wide is the phospholipid bilayer?
7-10nm
What is the function of the cell surface membrane?
- Controls movement in and out of cell
- Recognition site for body’s immune system
- Receptor for hormones and neurotransmitters
- Helps cell attachment and tissue formation
What substances can pass through the phospholipid bilayer?
- Non-polar molecules
- Very small molecules e.g. O₂
- Impermeable to ions and larger polar molecules.
Name the two types of embedded proteins
Intrinsic and extrinsic proteins
Define intrinsic protein
Transmembrane proteins
Define extrinsic
Through one layer (outside)
Describe a carrier protein
- Intrinsic protein
- Plays an important role in both passive and active transport
Describe a channel protein
- Intrinsic protein
- Provide a hydrophilic channel which allows passive movement of polar molecules and ions down a CG through a membrane
- Held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of proteins.
What is a glycoprotein?
- Intrinsic protein with attached carbohydrate chains (cell markers/antigens)
- Play a role in cell adhesion
- Act as receptors for chemical signals
What is a glycolipid?
- Lipid with attached carbohydrate chains (cell markers/antigens)
- Can be recognised by the immune system as self/non-self
What is cholesterol?
- Stabilises/maintains phospholipid bilayer
- Regulates fluidity of phospholipid bilayer
What is the fluid mosaic model?
A model representing the features of the phospholipid bilayer.
What is the function of plasma membranes within cells? (i.e. membrane-bound organelles)
- Separates cell components from cytoplasm
- Isolates enzymes which might damage the cell e.g. lysosomes
- Controlling movement of molecules into organelles
Name three lipid-soluble substances
can’t travel across
- Steroid hormone
- O₂
- CO₂
Name three lipid-insoluble substances
can travel across
- Glucose
- Protein
- Lipids
Name two factors affecting membrane permeability
- Temperature
- Solvents
How does temperature affect membrane permeability?
- Higher temps = higher KE
- Membrane becomes more fluid and more permeable
- If temp becomes too high proteins will denature and membrane won’t hold its shape
- Too low temps means less movement in the fatty acid tails.
- Membrane becomes rigid and less permeable
- Membrane may break.
How does solvent concentration affect membrane permeability?
- Non-polar substances will disrupt organisation of fatty acids in membrane
- Like dissolves like (i.e. fatty acid tails won’t be dissolved by alcohols/acetones)
- The less polar the molecules, the more disrupted the membrane will be.
Define cell signalling
Processes that lead to communication and coordination between cells.
Why must cells be able to detect internal and external signals?
To:
- Grow
- Develop
- Respond
What types of signals do cells detect?
- Hormones
- Drugs
- Growth factors
- Extracellular matrix
- Adhesion factors on other cells
- Antigens
Describe the primary messenger system of cell signalling
- Chemicals produced by specific cells released into organism
- Recognised by target cell (cell with a receptor for a specific hormone on membrane)
- Binds to a receptor due to a complementary shape
- Causes target cell to respond
- Initiates a second messenger system
Describe the secondary messenger system of cell signalling
- The hormone is the ‘first messenger’ which binds to the receptor
- The receptor transduces this signal
- The first messenger never crosses the membrane
- So the response in the cell comes about due to ‘second messenger’ molecules, many of these are enzymes
Give two examples of drugs which block cell signalling
- Beta blockers
- Botox (toxin from bacteria)
Define facilitated diffusion
Diffusion across a membrane through a channel protein
Describe the features of facilitated diffusion
- Selectively permeable process
- Involves carrier proteins which change shape when a molecule binds.
List the factors which affect facilitated diffusion
- Temperature
- Concentration gradient
- Membrane surface area and thickness
- Number of channel proteins present
Define active transport
The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of high concentration, using energy.
List the requirements for active transport
- Energy
- Carrier proteins
Describe the process of active transport into a cell
- Molecule/ion binds to receptors in the channel of the carrier protein on the outside of the cell
- On the inside, ATP binds to the carrier protein and is hydrolysed into ADP and phosphate
- Binding of the phosphate molecule to the carrier protein to change shape - opening up the inside of the cell
- Molecule/ion is released into the inside of the cell
- The phosphate molecule is released from the carrier protein and recombines with ADP to form ATP
- The carrier protein returns to its original shape.
This process is selective.
What is bulk transport?
- Another form of active transport
- Large molecules such as enzymes/hormones or even entire cells like bacteria are too large to fit though the channel or carrier proteins, so they are moved into and out of the cell via bulk transport.
Define endocytosis
The bulk movement of material into cells.
It requires energy in the form ATP.
What are the two types of endocytosis?
- Phagocytosis (for solids)
- Pinocytosis (for liquids)
Outline the process of endocytosis
- Cell surface membrane invaginates when in comes into contact with the material to be transported.
- The membrane enfolds the material until eventually the membrane fuses forming a vesicle.
- The vesicle pinches off and moves into the cytoplasm to transfer the material for further processing within the cell. e.g. vesicles containing bacteria will me moved towards lysosomes for digestion.
What is exocytosis?
The bulk movement of material out of cells.
It requires energy in the form ATP.
Define osmosis
The diffusion of water across a partially permeable membrane.
What is water potential?
The pressure exerted by water molecules as they collide with a membrane or container. Measured in Pa or kPa.
State the water potential of pure water
0 kPa
As a solution becomes more concentrated, what happens to the water potential?
Water potential decreases (becomes more negative).
Define cytolysis
The process of an animal cell surface membrane breaking causing the cell to burst when placed in a solution of higher water potential than itself.
Define crenation
The process of water leaving an animal cell when placed in a solution of a lower water potential than itself.
Define isotonic
A solution with an equal water potential to the material it is placed in.
What is tugor pressure?
The hydrostatic pressure exerted on a plant cell wall. Caused by water entering the cell via osmosis, down the WP gradient.
What is plasmolysis?
The reduction of the volume of the cytoplasm when the cytoplasm pulls away from the plant cell wall. Caused by water diffusing out from the cell via osmosis, down the WP gradient.
What happens when a plant cell is placed in a hypotonic solution?
Water diffuses in via osmosis causing the cell to be turgid.
What happens when an animal cell is placed in a hypertonic solution?
Water diffuses out via osmosis causing the cell to crenate.
