2.1.5: Biological membranes Flashcards
Diffusion
Net movement of particles from an area of higher concentration to an area of lower concentration.
Diffusion occurs _____ the concentration gradient.
Diffusion occurs down the concentration gradient.
Diffusion stops when:
The concentration on both sides is equal (equilibrium has been reached).
Factors affecting diffusion:
- Temperature
- Concentration gradient
- Surface area
- Distance/thickness
- Size of molecule (smaller = faster)
Simple diffusion
Molecules diffuse across a membrane by passing between the phospholipids.
Molecules for simple diffusion must be:
- Small
* Non-polar (polar diffuse through very slowly)
Facilitated diffusion
Molecule diffuses across a membrane with the aid of a channel or carrier protein. PASSIVE.
Channel protein:
hydrophilic channel allowing polar molecules and ions to pass through; can be ‘gated’.
Carrier protein:
Protein that changes shape to allow molecule to pass across a membrane.
How cells control diffusion:
- Number of channel proteins
- Number/presence of carrier proteins
- Gated channels
Active transport
Movement of particles from an area of low concentration to high concentration (against the concentration gradient)
Active transport requires:
• Energy in the form of ATP
Involves carrier proteins which are specific to particular ions/molecules
Endo and exocytosis
Movement of fluids/solids across a plasma membrane in vesicles.
Endocytosis
Uptake into cells
Exocytosis
Export from the cell
Endocytosis steps
1) Small region folds in from the rest of the plasma membrane
2) Region pinches off forming a vesicle containing the substance
⟶ Proteins carry out this process using energy from ATP
3) Ends of the membrane reattach
Exocytosis steps (for protein)
1) Ribosomes on the rER synthesise proteins, which enter the cisternae
2) Vesicle buds off cisternae (ATP required) and carries protein to Golgi apparatus
3) Vesicle fuses with Golgi
4) Protein is modified as it moves through the Golgi cisternae
5) Protein released in more vesicles
6) Vesicle moves to plasma membrane and fuses with it, releasing proteins from cell
Why endocytosis and exocytosis are active processes:
∵ ATP is needed for:
• Formation of vesicles
• Movement of vesicles along cytoskeleton
• Changing shape of cells to engulf (pinching of plasma membrane)
• Fusion of vesicles and membrane
Osmosis
the net movement of water from a solution with a high water potential to a solution with a low water potential, through a partially permeable membrane.
Water potential
- A measure of how many ‘free’ water molecules there are
- Always 0 or -ve; pure water = 0
- Measured in kPa –> pressure exerted by water molecules as they hit membrane/container
- ψ
Solution all have ______ water potential
negative
Animals cells placed in distilled water:
Cells swell and burst - “cytolysis”
If red blood cells then “haemolysis”
Animal cells placed in concentrated salt solution:
Cells shrink and shrivel - “crenation”
Plant cells placed in distilled water
- Increased turgor pressure –> cell is turgid
* Cell stiffens but generally retains shape
Plant cells placed in concentrated salt solution:
• Hydrostatic pressure drops, cell becomes flaccid
• Plasma membrane pulls away from cell wall
Cell is “plasmolysed”
Function of biological membranes
- Compartmentalisation
- Cell signalling
- Control of substances entering/exiting cell
- Site of chemical reaction
Explain compartmentalisation (function of membrane)
• Keep cell contents separate from surroundings
• Keep organelles separate from cytoplasm
⟶ Metabolism includes many different and often incompatible reactions
⟶ Separation of parts of cell enables specific conditions (e.g. chemical gradients) to be maintained
Explain role of membranes in controlling what enters/exits cell
- Small and uncharged molecules can enter cell easily (pass through phospholipid bilayer)
- Large and charged articles must enter thriugh specific channels (can be gated to control how much enters)
Explain the role of membranes in cell signalling
• e.g. binding of neurotransmitters to receptors on membranes allows impulses to be transmitted
Explain the role of membranes as providing sites for chemical reactions
• e.g. highly folded membranes in mitochondria (cristae) provide surface for aerobic respiration reactions
Name for biological membrane molecular structure
Fluid mosaic model
Amphipathic
Molecules with both a hydrophobic and hydrophilic end
Explain how the structure of phospholipids helps maintain the structure of cell membranes
• Phospholipid molecules: partly hydrophobic (head), partly hydrophilic (tail)
• Arrange themselves as a bilayer:
⟶ hydrophilic (phosphate-glycerol) head in contact with the water,
⟶ hydrophobic (fatty acid) tails facing each other on inside
• Creates a strong, stable barrier
Types of membrane proteins
- Intrinsic (integral)
* Extrinsic (peripheral)
Intrinsic protein
Embedded through both layers of a membrane
Extrinsic protein
Present on one side of the membrane
Examples of intrinsic proteins
- Channel proteins
- Carrier proteins
- Glycoproteins
Channel proteins
• Hydrophilic core
• Allows passive movement of polar molecules/ions
⟶ Needed because phospholipid tails repel charged molecules
Carrier proteins
- Involved in passive and active transport
* Shape of protein may change to allow a substance through
Glycoproteins
• Proteins with carbohydrate chains attached
• Role in cell signalling:
⟶ For neurotransmitters at synapses
⟶ For peptide hormones like insulin and glucagon (not lipid hormones as these can pass between phospholipids)
• Role in cell adhesion
⟶ e.g. sperm cell binding to egg mediated by glycoproteins
Glycolipids
- Lipids with carbohydrate chains attached
* Cell markers (self/non-self)
Cholesterol
- Regulates fluidity of membranes
- Adds stability without rigidity
- Have hydrophilic and hydrophobic end
- Stops phospholipids from grouping too closely and crystallising
Factors affecting membrane permeability
- Temperature
* Solvents
How an increase in temperature affects membrane permeability
• Increase in kinetic energy
⟶ Phospholipids (esp. tails) become more fluid
⟶ Gaps between phospholipids form
⟶ Increases membrane permeability
Also:
⟶ Excess kinetic energy breaks bonds maintaining protein’s tertiary structure
⟶ Possibility of denaturing intrinsic/extrinsic/channel proteins
⟶ Increases permeability
Effect of solvents upon membrane permeability
• Organic solvents (e.g. alcohols) generally less polar than water so disrupt or dissolve membranes
⟶ Form temporary bonds to phospholipid heads, causing gaps in membrane
⟶ Affect bonding in proteins, can cause denaturation at higher conc.
How antiseptic works; dissolves membrane
Active transport steps
1) Molecules/ions need to be transported into cell against concentration gradient –> enters carrier protein
2) Carrier protein activated by reaction with ATP
3) Change in shape of carrier protein (let molecule through)
4) ADP and P released from carrier protein, which reverts to receptive shape
