topic 2B: transport across cell membranes Flashcards by sehansa panditharatne

function of cell membranes

FUNCTION 1
-regulates movement of substances into and out of the cell, it is partially permeable to allow certain molecules to pass through but not others

FUNCTION 2
-surrounds the cell and to act as a barrier between cell and environment

FUNCTION 3
-site for metabolic reactions e.g. respiration and photosynthesis

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components in a membrane structure

-phospholipids
-cholesterol
-glycolipids
-glycoproteins
-proteins

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structure of phospholipid + arrangement

-phospholipids form a bilayer
-phospholipid molecules have a ‘head’ and ‘tail’

-HYDROPHILLIC (head): a polar, charged phosphate head, it attracts water so points to water
-HYDROPHOBIC (tail): a non polar, uncharged fatty acids tail, it repels water so point away from water

-the phospholipid molecules automatically arrange themselves into a bilayer - the heads face out towards the water (aqueous environment), shielding tails

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phospholipid bilayer structure aid to function

-the phospholipid bilayer acts as a barrier to prevent water-soluble substances (e.g. polar/ charged molecules/ions) passing through, as they repel the fatty acids tails

-hydrophobic/non polar molecules can pass through as they do not repel in fatty acids tails

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structure of glycolipids in membrane

-made of a carbohydrate covalently bonded to a lipid
-extends from the bilayer into the aqueous environment

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function of glycolipids

-helps cells attach together to form tissues
-acts as a recognition site
-helps maintains stability of membranes

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structure of glycoprotein

-a short chain, branched carbohydrate attached to proteins in the membrane
-extends from the bilayer into the aqueous environment

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function of glycoprotein

-acts as receptors for hormones and neurotransmitters or antigens
-recognition site
-helps cells to attach eachother to form tissues

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structure of cholesterol

-type of lipid, present in all cell membranes
-fit between the phospholipids
-bind to the hydrophobic tails of the phospholipids as theyre mainly non polar, causes the tails to pack more closely together becoming more rigid, less fluid

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function of cholesterol

-restricts the movement of phospholipids and other molecules making up the membrane so it decreases the fluidity and permeability of the membrane and increases rigidity

-prevents loss of water and dissolved ions from the cells, as tails are hydrophobic and closer together - this maintains the fluidity, stability and strength of the membranes

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types of membrane proteins

intrinsic
extrinsic

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intrinsic protein structure + function

(either channel proteins or carrier proteins)

-a protein that goes across the whole membrane

-carriers and channels to transport polar, water-soluble substances across the membrane (e.g. amino acids, ions, glucose)

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extrinsic proteins structure + function

-proteins that are on the surface layer of membrane

-acts as a receptor - binding to hormones or cell signalling
-enzymes
-detects chemicals

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why is the plasma membrane called a fluid mosaic?

fluid: flexible structure, all the different molecules can move around and changing shape

mosaic: different types of molecules varying in shape and size (e.g. proteins, carbohydrates - glycoproteins and glycolipids) all within the phospholipid layer floating, this creates a mosaic like pattern above

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simple diffusion definition

-diffusion is the net movement of particles (gas or liquid) from a region of higher concentration to a region of lower concentration, down a concentration gradient through a partially permeable membrane, it is a passive (no energy) process.

-simple diffusion means that molecules diffuse directly through the membrane

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equilibrium of simple diffusion

-until equilibrium is reached where particles have spread out evenly and equal on both sides, diffusion slows down overtime and stops

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What molecules can move through the phospholipid bilayer by simple diffusion?

Lipid-soluble (non-polar) or very small substances move from an area of higher concentration to an area of lower concentration, down a concentration gradient, across phospholipid bilayer. This is a passive process which doesn’t require energy from ATP but only kinetic energy of substances

-small molecules: fit between phospholipids
-lipid-soluble(capable of dissolving in lipids): dissolve in phospholipid bilayer to move through it
-nonn polar/uncharged: dont repel from the hydrophobic fatty acid tails

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limitations imposed by the phospholipid bilayer

-restricts movement of water soluble polar + larger substances - e.g. sodium ions and glucose, this is due to hydrophobic fatty acid tails in interior of bilayer

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factors affecting the rate of simple diffusion(5)

-surface area
-temperature
-size of particles
-distance/ thickness of surface
-concentration gradient

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surface area affecting the rate of diffusion

-surface area: the greater the surface area the greater the rate of diffusion as more area is exposed for particles to use and diffuse through

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concentration gradient affecting the rate of diffusion

-the greater the concentration gradient, the faster the rate of diffusion. as diffusion takes place, the concentration difference decreases until it reaches equilibrium so diffusion slows down overtime

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distance/thickness of surface affecting the rate of diffusion

-distance/thickness of surface: the thinner layer/ short diffusion pathway (short distance for substances to diffuse through) the faster the rate of diffusion

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temperature affecting the rate of diffusion

-temperature: the warmer the temperature is, the faster the rate of diffusion - more kinetic energy in particles so they can move faster and diffuse

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size of particles affecting the rate of diffusion

-size of particles: the smaller/ lighter the particles are, the faster the rate of diffusion as smaller particles can move faster

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examples of efficient simple diffusion

-microvilli are projections (Epithelial cells) which increase the surface area (x600), larger surface area means more particles can be exchanged in the same amount of time, increasing the rate of diffusion

-LUNGS - alveoli: thin exchange surface - 1 cell thick, thin layer means a short diffusion pathway, so faster rate of diffusion

facilitated diffusion definition

facilitated diffusion is the passive, net movement of water soluble, polar substances (or some larger molecules) (e.g. glucose, amino acids) across a cell membrane from a region of higher concentration to a region of lower concentration and moves down a concentration gradient through specific channel proteins or carrier proteins, which transport (large and charged) molecules in the membrane.

why are protein channels and carriers used in faciliated diffusion?

-larger molecules would diffuse extremely slowly through the phospholipid bilayer

-so to speed things up, protein channels and protein carriers are used in the membrane

protein carriers

-move large molecules (e.g. amino acids, glucose)across the membrane, down their concentration gradient

a specific large molecule attaches to a specific binding site on carrier protein in the membrane
then the protein changes shape due to it’s tertiary shape altering to transport substance
this release the molecule on the opposite side of the membrane

protein channels

-moves charged particles (e.g. ions + polar molecules) - they’re water-soluble as the centre of the bilayer is hydrophobic

-forms hydrophillic pores filled with water in the membrane for POLAR (CHARGED)/ HYDROPHILIC particles to diffuse through the membrane (down their concentration gradient)

-protein channels are SELECTIVE: different channel proteins facilitate the diffusion of different charged particles, so only some chemicals can pass through

what can pass through by facilitated diffusion + examples?

-larger molecules (e.g. glucose + amino acids)
-charged (polar)/ hydrophilic particles
-water-soluble particles

factors affecting the rate of facilitated diffusion

-concentration gradient
-number of protein channels/ carriers

concentration gradient affecting facilitated diffusion

-the greater the concentration gradient, the faster the rate of facilitated diffusion, up to a point, as EQUILIBRIUM is reached, the rate of reaction decreases and levels off

number of protein channels/carriers affecting facilitated diffusion

-the greater number of protein channels and carriers, the faster the rate of reaction
-once all the proteins in a membrane are in use, the rate of facilitated diffusion can’t happen any faster, even if other factors increase

osmosis definition

osmosis is the movement of water molecules across a partially permeable membrane, from an area of higher water potential (less negative) to an area of lower water potential (more negative), this is a passive process, but requires only kinetic energy of substances.

what is water potential(ψ)?

water potential (ψ) is the likelihood (potential) of water molecules to diffuse out of or into a solution

what water potential does pure water have?

-pure water is 0, which is the highest water potential - cant get a water potential higher than 0

-the water potential of a solution is always negative

-dilute solutions have a higher WP as there are more water molecules FREE TO MOVE
-a concentrated solution of a solute will decrease the WP as there are fewer water molecules free to move

water potential in hypotonic and hypertonic

-hypotonic - higher water potential
-hypertonic - lower water potential
water moves from hypotonic (high) to hypertonic (low)

types of solution (hypotonic, isotonic, hypertonic)

-hypotonic: WP of solution is higher (closer to 0) than WP of cell
-isotonic: WP same inside and outside cell
-hypertonic: WP of solution is lower (more negative) than WP of cell

animal cells in isotonic solution

-there is no net movement as the cell is in equilibrium, no change of shape or optimum conditions.

plant cells in isotonic solution

-no net movement of water (equilibrium), the cell becomes flaccid - rigid

animal cells in hypotonic solution

-higher water potential in the solution, water moves into the cell from high to low WP (more water will move into the cell by osmosis)
-animal cells have no cell wall so no support, the pressure will cause the cell to SWELL AND BURST, as membrane capacity is exceeded

plant cells in hypotonic solution

-higher water potential outside the cell, water moves into the cell by osmosis from high to low concentration down a concentration/ WP gradient.
-the cell swells up and becomes TURGID, doesn’t burst as the plant cell has a strong cell wall strengthened by cellulose

animal and plant cells in hypertonic cells

-higher WP inside cell, water moves out of cell by osmosis from high to low concentration of water, causing cells to shrink and become shrivelled - due to large volumes of water leaving the cell

factors affecting the rate of osmosis

-water potential gradient
-thickness of exchange surface
-surface area of exchange surface

water potential gradient affecting the rate of osmosis

-the higher the water potential gradient, the faster the rate of osmosis
-as osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off over time and stops

thickness of exchange surface affecting the rate of osmosis

-the thinner the exchange surface, the faster the rate of osmosis, as there is a shorter distance for the water molecules to travel

surface area of exchange surface affecting the rate of osmosis

-the larger the surface area, the faster the rate of osmosis, as there is more area for water molecules to travel through in a set amount of time

active transport definition + how is it selective

the movement of molecules or ions from a region of lower concentration to an area of higher concentration, against a concentration gradient, hydrolysis of ATP and specific carrier proteins

-active transport is SELECTIVE as only certain molecules can bind to the carrier protein and release the molecule on the opposite side

how is active transport similar to facilitated diffusion?

-carrier proteins involved in active transport

-similar process - a specific molecule attaches to the carrier protein, the protein changes shape and the molecule moves across the membrane, releasing it on the other side

differences in active transport and facilitated diffusion

-moves molecules from a lower to a higher concentration
-requires energy, facilitated diffusion does not

process of the hydrolysis of ATP active transport

a complementary substance BINDS to a specific carrier protein - each carrier protein is specific and will only transport one type of molecule/ion
ATP binds to the carrier protein from INSIDE of the cell
ATP HYDROLYSED into ADP and phosphate, the phosphate is attached to the protein
CONFORMATIONAL CHANGE is carried out: the protein carrier changes shape, altering the shape of the tertiary structure
the phosphate is RELEASED on the other side of the membrane
the carrier protein returns to its ORIGINAL SHAPE (when the inorganic phosphate molecule is released from the protein and RECOMBINES with ADP)

energy for active transport

-ATP (metabolic energt) is required - produced from mitochondria during respiration - mitochondria present in cells during active transport
-hydrolysis reaction Phosphate attaches to the protein causing change in tertiary structure, after molecules/ ions are transported, the inorganic phosphate is released and ADP and Phosphate recombine during respiration later

ATP –> ADP + Pi

co-transport definition + describe how movement across membranes occurs by co-transport

-co-transporters are a type of carrier protein,

Two different molecules bind and move simultaneously (Same time) by a co-transporter protein. The transport of two molecules, one of the molecules going down a concentration gradient and the other molecule against its own concentration gradient.

co-transport of sodium ions and glucose-transportation and where?

-sodium ions move into the cell down their concentration gradient
-glucose moves into the cell against its concentration gradient

factors affecting the rate of active transport

-speed of individual carrier proteins - the faster they work, the faster the rate of active transport
-the number of carrier proteins - the more proteins there are, the faster the rate of active transport
-the rate of respiration in the cell and the availability of ATP - if respiration is inhibited, active transport cant take place

what is an ileum?

the final part of the small intestine

absorption of glucose in ileum & low and high concentration of glucose - location

-glucose is absorbed by co-transport in the ileum, first glucose is absorbed into the bloodstream in the small intestine

-ileum –> epithelial cells —> capillary

-in the ileum, the concentration of glucose is too low (in epithelial cells lining the ileum) for glucose to diffuse out into the blood, so glucose is absorbed from the lumen (middle) of the ileum by co-transport

step 1: sodium ions transportation

-sodium ions are actively transported from the ileum epithelial cells into the bloodstream, by the SODIUM-POTASSIUM PUMP (carrier protein between bloodstream and epithelial cell)
-there is a higher concentration of sodium ions in the lumen of the ileum and there is a lower concentration of sodium ions in the epithelial cells of the ileum (inside the cell) - this creates a CONCENTRATION GRADIENT between the lumen of ileum and the epithelial cells

STEP 2: sodium glucose co-transporter (sodium ions + glucose diffuses/absorbed in)

-sodium ions from the lumen of the ileum into the epithelial cell by facilitated diffusion down their concentration gradient by a co-transporter protein, with glucose against it’s concentration gradient -as a result the concentration of glucose inside the epithelial cells increases

STEP 3: glucose diffuses out

-glucose diffuses out of the epithelial cell into the bloodstream, down a concentration gradient through a protein channel by facilitated diffusion

adapations of small intestine

-cell membranes folded e.g. villi is highly folded to form microvilli on the epithelial cell increase the surface area, which increases rate of movement
-more protein channels/carriers, for facilitated diffusion (or active transport for carrier proteins only)
-large number of mitochondria, this makes more ATP by aerobic respiration for active transport

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topic 2B: transport across cell membranes Flashcards

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