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Flashcards in Mod 2 Chap 5: Plasma Membranes Deck (27)
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Describe the roles of membranes within cells.

- may house pigments essential for chemical reactions e.g. Chlorophyll on thylakoid membranes of chloroplasts
- can form vesicles to transport substances e.g proteins from RER to Golgi
- compartmentalisation (all components needed for a specific function held in one place)
- folded membranes increases surface area
- contain enzymes for a specific chemical reaction


Describe the roles of membranes on the surface of cells.

- contain antigens so the organism's immune system recognises the cell as 'self', so does not attack it
- may release chemicals to signal to other cells
- separate cell's components from extracellular environment
- regulates transport of materials in and out of cells
- contains receptors for chemical signals, released by other cells
- may contain enzymes involved in metabolic pathways


What are cell membranes PARTIALLY permeable to?

- v small molecules that can diffuse easily across by moving between structural components of membrane e.g. O2
- lipid-soluble molecules can dissolve in lipid bi-layer and pass through
- large polar molecules pass proteins in membrane e.g. Glucose, water


Name the components of the Fluid-Mosaic Model of the plasma membrane and describe their roles.

Intrinsic / Integral proteins: are transmembrane (embedded in both layers of membrane), have amino acids w/ hydrophobic R-groups on their external surfaces, which interact w/ hydrophobic core of membrane, holding them in place. Examples:

- Channel proteins: provide hydrophilic channel allowing passive movement of polar molecules + ions down conc gradient through membranes.

- Carrier proteins: have role in both passive transport (down a conc gradient) + active transport (against a conc gradient) into cells, this often involves shape of protein changing

- Glycoproteins: embedded in surface membrane w/ attached carb (sugar) chains, have role in cell adhesion + as receptors for chemical signals (= process of cell communication / cell signalling).

Extrinsic / Peripheral proteins: present in one side of bilayer (can be either, some move between two), normally have hydrophilic R-groups on outer surfaces + interact w/ polar heads of phospholipids / w/ intrinsic proteins.

Cholesterol: a lipid w/ a hydrophilic end + a hydrophobic end, like a phospholipid, regulates fluidity of membranes, positioned between phospholipids in a membrane bilayer so hydrophobic-hydrophilic interactions pull them together, makes cholesterol add stability to membrane without being too rigid, stop phospholipid molecules grouping + crystallising, which stops membrane being too solid.

Glycolipids: similar to glycoproteins, lipids w/ attached carb (sugar) chains, called cell markers / antigens, can be recognised by immune system as self (part of organism) or non self (belonging to another organism).


Describe the process of cell communication / cell signalling, carried out by glycoproteins.

- when chemical binds to receptor, a response is elicited from cell
- causes a direct response / sets off cascade of events in cell
- (some drugs act by binding to cell receptors)

Example of this process:
- receptors for peptide hormones such as Insulin + glucagon, which affect uptake + storage of glucose by cells


Describe diffusion as a method of movement for molecules.

The net movement of molecules or ions from a region of high concentration to a region of low concentration until they are spread out evenly.

Simple Diffusion:
- a passive process, no energy required
- particles able to diffuse across plasma membrane: small non polar molecules that are small enough to fit between phospholipids e.g. O2 + CO2, or water small enough to fit through too, despite being polar


Describe the factors affecting the rate of diffusion.

- concentration gradient: steeper the conc gradient = faster rate of diffusion
- size of particles: larger particles = slower rate
- thickness of membrane: thicker membrane = slower rate
- surface area: larger SA:V ratio = faster rate
- temperature: higher temp = more KE molecules have = faster their movement = faster rate


Describe the difference between simple diffusion and facilitated diffusion.

Simple: diffusion in the absence of a barrier or membrane.

Facilitated: diffusion across a membrane through protein channels.


Describe facilitated diffusion as a method of movement for molecules across membranes.

- large polar molecules can only enter cell by facilitated diffusion, down their conc gradient w/ aid of proteins
- uses proteins to facilitate (allow) diffusion

The two types of proteins used:
- channel proteins
- carrier proteins

Membranes with channel proteins are selectively permeable.


Describe how channel proteins work in facilitated diffusion.

- charged ions / polar molecules like water cannot diffuse through non polar centre of phospholipid bilayer
- channel proteins form pores to allow them to pass through
- each is specific to one type of ion (so membrane is selectively permeable here)
- some can open and close pores, acting like gates


Describe how carrier proteins work in facilitated diffusion.

- allow diffusion of larger polar molecules e.g. Sugar and amino acids
- molecule attaches to specific binding site on carrier protein
- this causes carrier protein to change shape to 'deliver' molecule through membrane


Describe a practical investigation into the factors affecting diffusion rates in model cells.

Investigating factors of: temp and concentration

- dialysis tubing used as a membrane
- one end of tubing tied, fill with a solution, then tie other end = model cell
- place into another solution/s (of various concentrations / sizes of solute molecules)
- measure changes over time in conc of solute molecules, inside and out of model cell
- calculate rate of diffusion across tubing (changes in conc divided by time)


Describe active transport as a method of movement for molecules.

The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration.

- a process requiring energy and carrier proteins
- energy needed as particles are being moved up a conc gradient (opposite direction to diffusion)
- metabolic energy supplied by ATP
- carrier proteins span membranes + act as 'pumps'
- a selective process, specific substances transported by specific carrier proteins


Describe HOW the process of diffusion occurs for movement of molecules.

- particles in a gas / liquid have KE, so are moving randomly
- so unequal distribution of particles eventually becomes even, equilibrium is reached so movement is equal in both directions
- particles are moving at high speeds so are constantly colliding, slowing down their overall movement
- means that over short distance, diffusion is fast, but as diffusion distance increases, rate of diffusion slows as more collisions have taken place


Describe HOW the process of active transport occurs for movement of molecules into or out of a cell.

- molecule / ion to be transported binds to receptors in channel of carrier protein
- ATP binds to carrier protein + is hydrated into ADP and phosphate
- binding of phosphate to carrier protein causes protein to change shape, opening up to other side of cell
- molecule or ion is released to other side of cell
- phosphate molecule released from carrier protein + recombined w/ ADP to form ATP
- carrier protein returns to original shape


Describe Bulk Transport as another form of active transport.

- another form of active transport
- where large molecules e.g. Enzymes / hormones / whole cells are too large to move through channel / carrier proteins, so move into / out of cell via bulk transport


Describe Endocytosis as a form of Bulk Transport.

- the bulk transport of material INTO cells
- two types: Phagocytosis (for solids), + Pinocytosis (for liquids), but process = same for both

- cell surface membrane first invaginates (bends inwards) when in contact w/ material to be transported
- membrane enfolds material until membrane fuses, forming a vesicle
- vesicle pinches off + moves into cytoplasm to transfer material for further processing inside cell
ATP energy required for this


Describe Exocytosis as a form of Bulk Transport.

- reverse of Endocytosis (transport OUT OF cells)

- vesicles, usually formed by Golgi apparatus, move towards + fuse w/ cell surface membrane
- contents of vesicles then release outside of cell
ATP energy required for this


Describe osmosis as a method of movement for water molecules.

The net movement of water molecules from an area of high water potential to an area of low water potential across a partially permeable membrane.


Describe the relationships between the water potential of a solution and its concentration.

Pure water: has a high no. of free water molecules and no solute molecules, SO has Water Potential (Psi) = 0 kPa

Concentrated solution: has a low no. of free water molecules, as most are bound to solute molecules, SO has a negative Water Potential (Psi)

The more concentrated the solution, the more negative the Water Potential (Psi)


Describe how osmosis is caused between solutions of different concentrations / water potentials.

- when solutions of diff concentrations (so diff water potentials), are separated by partially permeable membrane, water molecules can move been the solutions but solutes usually can't
- so will be a net movement of water from solution w/ higher water potential (less concentrated) to solution w/ lower water potential (more concentrated)
- this net movement continues until water potential = equal on both sides of membrane (equilibrium is reached)


Describe the effects of osmosis on plant cells.

- water moves into plant cell, down water potential gradient, by osmosis
- cell swells and contents press outwards onto cell wall
- cell wall exerts pressure back against hydrostatic pressure
- no more net entry of water, so cell is TURGID

- water potential outside cell equal to water potential inside cell
- so no net movement of water, there is an equilibrium

- water potential inside cell > water potential outside cell
- water moves out of plant cell, down water potential gradient, by osmosis
- cell membrane starts to shrink away from cell wall
- cell is FLACID
- if water continues to leave cell, plasma membrane pulls away from cell wall


Describe the effects of osmosis on animal cells.

- water potential outside cell greater than water potential inside cell
- net movement of water into cell, increases hydrostatic pressure
- cell surface membrane cannot stretch much and cell bursts, (CYTOLYSIS)

- water potential outside cell = water potential inside cell
- no met movement of water, there is an equilibrium

- water potential outside cell is lower than water potential inside cell
- net movement of water out of cell
- cell shrivels and puckers, (CRENATION)


Describe a practical investigation into the effects of solutions of different water potentials on plant and animal cells.

- pieces of potato / onion can be placed into sugar / salt solutions w/ diff concentrations (so diff water potentials)
- water will move into / out of cells depending on water potential of solution relative to water potential of plant tissue
- as plant tissue gains / loses water it will increase / decrease in mass and size


What are the factors that affect membrane structure and permeability?

- temperature
- presence of solvents


Describe temperature as a factor affect membrane structure and permeability.

- phospholipids in a cell membrane are constantly moving
- when temp increases, phospholipids have more KE + so move more
- makes membrane more fluid, so begins to lose its structure, will eventually real down completely
- loss of structure increases permeability of membrane, so easier for particles to cross it
- carrier + channel proteins in membrane denatured at high temps, increasing membrane permeability too


Describe solvents as a factor that affect membrane structure and permeability.

- water (polar solvent) = essential in formation of phospholipid bilayer
- many organic solvents are less polar than water, or are non polar
- these will dissolve membranes, disrupting cells
- pure or v strong alcohols are toxic as they destroy cells in body, less concentrated solutions of alcohols will not dissolve membranes but can still cause damage
- non polar alcohol molecules can enter cell membrane between phospholipids, which disrupts membrane
- makes membrane more fluid + more permeable, in neuronal membranes for e.g., this means nerve impulses are no longer transmitted as normal
- also happens to neurones in brain, explain behavioural changes after alcohol consumption