2:1:5 Biological Membranes Flashcards
What are the roles of membranes in cells
- Seperates internal and external cell environment
- Intracellular membranes form compartments (organelles)
- Membranes control exchange of materials passing through them as they’re partially permeable
- Act as an interface for communication and cell signalling
What is the fluid mosaic model of membranes
- Explains how biological molecules are arranged to form cell membranes, as well as cell to cell interactions and cell signalling
- It is fluid due to the ability of the phospholipids and proteins ability to move
- It is mosaic due to the scattered pattern of proteins in the bilayer
- Components include phospholipids, cholesterol, glycoproteins and glycolipids, transport proteins
Why are phospholipids present in membranes
- Form a bilayer which acts as a barrier to most water soluble substances due to the hydrophobic fatty acid tails
- Prevents water soluble molecules (e.g. amino acids, sugars, proteins) can’t leak out the cell
- Can be chemically modified to act as signalling molecules by moving within the bilayer to activate other molecules
Why is the amphipathic nature of phospholipids important
So the hydrophilic heads can form the outside of the bilayer and the hydrophobic tails can form the inside
Why is cholesterol included in the membrane
- Increases the fluidity of the membrane by stopping it from becoming rigid in low temperature
- Does this by stopping the phospholipids from being packed too tightly
- Phospholipid and cholesterol interactions stabilise the cell me,brand at high temperatures by stopping it from being too fluid (by binding to fatty acid tails)
- Affects impermeabukity of membrane to ions
- Increases the mechanical strength and stability of membranes
Why are glycolipids and glycoproteins included in the membrane
- Contain carbohydrate chains on the cell surface, so they can act as receptor molecules when binding to substances
- Act as signalling receptors for hormones and neurotransmitters, receptors for endocytosis, and receptors for cell adhesion and stabilisation
- Act as cell markers for antigens for cell to cell recognition
Difference between glycolipids and glycoproteins
- Glycolipids: carbohydrate chains attached to phospholipids (or any lipid)
- Glycoproteins: carbohydrate chain attached to protein
Why are transport proteins included in the membrane
- Create hydrophilic channels to allow ions and polar molecules through the membrane
- Specific to particular ions/molecules
- Control entering/leaving of substances
Label a cell membrane diagram
What are the factors affecting the permeability and structure of membranes
- Temperature
- Solvent concentration
What are the factors affecting membrane fluidity and why
- Temperature (high temperature = more fluid)
- Length of fatty acid tails (longer = less fluid due to more intermolecular bonds)
- Unsaturated/saturated (saturated means there are more kinks = less fluid)
How does temperature affect membrane structure and permeability
- Temperature increase causes lipids to become more fluid, and reduces the barrier effectiveness so becomes more permeable, and diffusion across the membrane speeds up (reversible)
- Temperature increase (as well as water volume increase) can denature proteins which disrupts the membrane structure and barrier effectiveness (irreversible)
- Temperature increase affects the conformation of proteins as amino acid intermolecular forces are broken affecting protein specificity
How does solvent concentration affect membrane structure and permeability
- Increase cell membrane permeability by dissolving the lipids in membrane
- Membrane loses structure
Describe the process to observe factors affecting membrane structure and permeability
- Cut equal sized beetroot cubes (equal surface areas and volumes) and rinse to remove excess pigment
- Add the pieces to separate test tubes with the same volume of water
- Place test tubes in water baths of varying temperature for the same length of time
- Use a colorimeter to measure how much light is absorbed in the solution
- The higher the absorbency, the more pigment has been released, and the greater the membrane permeability
Define diffusion
The net movement, as a result of the natural kinetic energy of molecules or ions, from a region of high concentration to low concentration, down a concentration gradient
What factors affect the rate at which a substance diffuses across a membrane
- Steepness of the concentration gradient
- Temperature
- Surface area
- Properties of molecules or ions
What are channel proteins
- Water filled pores in the cell membrane
- Allow charged substances to flow through
- Are gated, so area of protein on the inside of the membrane controls when it’s opened/closed
- Controls the exchange of ions into and out of cells
What are carrier proteins
- Can undergo conformational changes via ATP hydrolysis allowing the binding site of the protein to be open on one side of the membrane and not on the other
- Control the movement of molecules or ions out of/into the cell
Describe the process of measuring the rate of diffusion
- Coloured agar is cut into cubes of increasing degrees of volume (made with NaOH and universal indicator)
- They are placed in boiling tubes containing a diffusion solution (e.g. dilute HCl)
- Measure the time taken for the acid to change the colour of the block
What is active transport
- The movement of molecules and ions through a cell membrane from a region of low concentration to a region of high concentration via ATP, against the concentration gradient
- Requires energy for the carrier proteins to change shape and transfer the substance
- Energy provided by ATP from respiration
- E.g. The loading of inorganic ions from the soil into root hairs, or the reabsorption of useful ions into the blood after filtration in the kidneys
What is endocytosis
- Bulk active transport into cells, requiring energy (e.g. phagocytosis and pinocytosis)
- Cell surface membrane engulfs the substance forming an endocytic vacuole
What is exocytosis
- Bulk active transport out of cells, requiring energy
- Substances are packaged in secretory Golgi vesicles
- Vesicles fuse to the cell surface membrane and release the content outside the cell (e.g. secretion of digestion enzymes)
What is osmosis
The net movement of water molecules from areas of low water potential to areas of high water potential across a partially permeable membrane, down the water potential gradient
What happens when animal cells lose water
- If placed in a concentrated solution, water will leave the cell through its partially permeable cell membrane by osmosis
- The cell will shrivel and crenate which is fatal
- Crenation occurs in hypertonic solutions
What happens when animal cells gain water
- If a cell is placed in dilute solution, water will enter through its partially permeable membrane
- The cell membrane will burst (cytolysis) as there’s no cell wall to support it, which is fatal
- Lysis occurs in hypotonic environments
What happens to animal cells in isotonic environments
The movement of water into and out of the cell occurs at the same rate, so there is no change
What happens when plant cells lose water
- If cell is places in concentrated solution, water will leave via osmosis
- As water leaves the vacuole, the cell volume decreases
- The protoplast (everything but the cell wall) shrinks and stops exerting pressure on the cell wall
- It begins to pull away from the cell wall (plasmolysis) and becomes placid
