2.1.5 Biological Membranes Flashcards
what is a membrane
a membrane is part of a cell and creates an enclosed space separating the cell from the external environment
what is the roles of membranes within cells
- membranes separate organelles from cytoplasm
- it provides internal transport system
- membranes around organelles can act selectively permeable to regulate passage of molecules into/out of organelles.
- provides a site of chemical reactions
what are the roles of membranes around a cell
- barrier to external environment
- has antigens which is involved in cell recognition/cell signalling
-membranes is selectively permeable to regulate transport of substances
define intracellular membranes
membranes which are inside of cells that cover organelles inside of cells
define hydrophilic
a molecule which is attracted to water
define hydrophobic
a molecule which repels water
what are the components in the fluid mosaic model of membrane structure
- phospholipids
- cholesterol
- glycolipids
- glycoproteins
- transport proteins ( carrier and channel proteins)
what is the role of the phospholipids in the membrane
O< - A phospholipid
- phospholipids form the basic structure of the membrane ( PHOSPHOLIPID BILAYER)
- the head is hydrophilic and the tail is Hydrophobic
- the phospholipid bilayer is composed of two layers of phospholipids : there hydrophobic tails pointing inward the hydrophilic head facing outward
- the phospholipid bi layer acts as a barrier to most water soluble substances
( the non-polar, hydrophobic fatty acids tails prevent polar molecules from entering - this ensures water soluble molecules such as sugar, amino acids and proteins cant escape
- and some water soluble substances cant enter the membrane
- phospholipid molecules can be chemically modified to act as signalling molecules by moving within the bilayer to activate other molecules
what is the role of cholesterol in the membrane
- cholesterol maintains the fluidity of the phospholipid bilayer
- it can increase and decrease the fluidity under different conditions.
- when there is low temps, cholesterol increases the fluidity of the cell, stopping it from being too rigid, by stopping the phospholipid tails from packing too closely together.
- cholesterol also stabilises the cell membrane at higher temperatures, by cholesterol binding to the hydrophobic tails and stabilising them to pack more closely together.
- cholesterol increases the mechanical strength and stability of membranes
( membranes would break down without cholesterol
what is the role of glycolipids and glycoproteins in a membrane
- carbohydrate attached to a lipid forms a hydrophilic glycocalyx
- carbohydrates attached to intrinsic proteins are glycoproteins
- these are used for cell adhesion ( attachment on cells to one another)
- used for cell recognition, as glycolipids and glycoproteins can act as antigens, allowing for cell to cell recognition
- use for cell signalling, glycolipids and glycoproteins can be used as signalling receptors for hormones and neurotransmitters
what is the roles of transport/integral proteins ( channel and carrier proteins)
- these proteins are both intrinsic proteins
- transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane
- channel proteins allow target molecules to pass through via diffusion
- carrier proteins will change shape in order to transport a substance across the membrane
- transport proteins allow the cell to control which substances enter or leave
what is the role of membrane - bound receptor on the membranes
- they are sites were hormones and drugs can bind
explain how temperature is able to affect membrane permeability and structure
- as temperature increases, the lipid bilayer becomes more fluid, so reduces the effectiveness of the cell membrane as a barrier to polar molecule. ( polar molecules are able to pass through)
- as temperature increases channel proteins are effected as diffusion now happen faster due to more kinetic energy.
- often around 40 degrees proteins will begin to denature meaning the membrane structure is no longer an effective barrier
explain how solvent concentration is able to affect membrane permeability and structure
- organic solvents damage membranes as they dissolve lipids.
- this increase the cell membranes permeability and causes the membrane to lose its structure
describe the practical investigation into factors affecting membrane structure and permeability
- cut beetroot into 5 pieces that have equal surface areas and volumes
- rinse and pat dry the beetroot
- add the betroot to the test tube and add the same vlume of water to each test tube
- leave in different temperatures, 10, 20, 30, 40, 50, 60 for same length of time
- use a colorimeter to see how much light is absorbed as it passes through the 5 liquids
- the higher the absorbance, the more pigment released due to greater membrane permeability
- as temp increase membrane permeability also increase
explain the result of the practical
- as temperature increases, membrane permeability also increases
- the increased movements causes phospholipids to be less tightly packed together.
- a high temps the phospholipid bilayer may start to breakdown causing further permeability of the membrane
- so the permeability increases so the pigment is release.
define diffusion
the net movement of a substance from a region of its higher concentration to a region of its lower concentration
(down a conc gradient)
(high to low)
explain how diffusion works in the movement of molecules across membranes
- molecules or ions move down a concentration gradient
- molecules go from a high conc outside of the membrane into a low conc which is inside the cell
what are some of the factors that affect diffusion
- temperature
- diffusion distance
- surface area
- size of substance
- concentration gradient
explain how the factor of steepness of concentration gradient effects the rate of diffusion
- the steeper the gradient the higher the rate of diffusion.
- a greater difference ion concentration means a greater difference in the number of molecules passing in the two directions and therefore a faster rate of diffusion
explain how the factor temperature affects diffusion
- if high, molecules have more energy so they move faster resulting in a high rate of diffusion
explain how the factor of diffusion distance affects diffusion
- the thicker the membrane the slower the diffusion due to longer diffusion distance
explain how the factor surface area of the membrane affects the rate of diffusion
- the greater the surface area the greater the number of molecules that can cross at any one moment
- so the diffusion happen faster
explain how the factor of size of molecules affects the rate of diffusion
- large molecules diffuse more slowly due to needing more energy to move
explain how facilitated diffusion works
- some molecules cant diffuse through diffusion so use channel and carrier proteins to get inside the cell instead
- channel proteins are water - filled pores which allows certain charged substances to diffuse through the cell membrane
- the channel protein is gated meaning in the inside surface of the of the membrane can move in order to close or open the pores
- carrier proteins can switch between two shapes, the binding site of the carrier proteins open to enter the molecule or ion, then the carrier protein opens on the other side of the membrane and the molecule /ion exits.
when would facilitated diffusion occur
- it would occur to take large polar molecules such as glucose and ions such a sodium.
- these molecules cannot diffuse through the phospholipid bilayer
what is active transport
the movement of molecules and ion through a cell membrane from region of lower concentration to a region of higher concentration using energy from respiration
(low to high)
(up the concentration gradient)
explain how active transport work across membranes
- active transport uses energy and carrier proteins.
- the energy is provided by ATP produces during respiration, ATP is hydrolysed to release energy.
- the energy makes the carrier protein change shape allowing for it to transfer the molecules or ions across the cell membrane
what is active transport important in
- the reabsorption of useful molecules and ions into the blood after filtration in the kidney
- loading of sugar from cells of leaves into the phloem tissue for transport around the plant
- the loading of inorganic ions from the soil into root hair
what three process require ATP/energy to move molecules across a membrane
- active transport, endocytosis and exocytosis all use energy/atp for movement of molecules across a membrane
what is endocytosis
bulk transport of larger quantities into cells
what is exocytosis
bulk transport of lager quantities out of cells
what are the bulk transport of larger quantities taken by endocytosis and exocytosis
- large molecules such as proteins or polysaccharides
- parts of cells
- whole cells eg bacteria
describe the process of endocytosis
- the process by which cell surface membrane engulfs material and forms a small sac around it
what are the two forms of endocytosis
phagocyosis and pinocytosis
describe what occurs in phagocytosis
- the bulk intake of solid material by a cell
- the cells that specialise in this are phagocytes
- the phagocytes form a phagocytic vacuole around it and engulf the material
describe what occurs in pinocytosis
- this is the bulk intake of liquids
- part of the plasma membrane surrounds and encloses the substance, and takes it in the cell in a vesicle
describe the process of exocytosis
- it is the process where materials are transported out of cells
- the Golgi body forms secretory vesicles and the substances go into the secretory vesicles
- these substance are enzymes, hormones or cell wall building materials
- the vesicles go to the cell membrane and diffuse and release there contents outside of the cell
what is osmosis
the diffusion of water molecules across a partially permeable membrane from a region of high water potential to a region of low water potential
what is water potential
measure of the tendency of water molecules to diffuse
why are effects for the loss and gain of water for animal cells more severe than for plant cells
animal cells do not have a supporting cell wall, unlike plant cells
when do animal cells loose water
- when animal cells are placed in a solution with a lower water potential than the cell. ( there is more water in the cell)
what happens when animal cells loose water
- crenation occurs
- water leaves the cell through its partially permeable surface membrane
- the cell will then shrink and shrivel up ( become crenated)
- this is fatal to the cell
when does crenation occur
- when the cell is in a hypertonic environment and looses too much water
( the solution outside the cell has a high solute concentration)
When do animal cells gain water
- when animal cells are placed in pure/distilled water, where the water has a higher water potential than the cell. ( more water outside the cell)
what happens when animal cells gain water
- the cell will gain water and the cell membrane will stretch out too far and burst called cytosis.
- this is fatal to the cell
when does cytosis occur
- when the animal cell is in a hypotonic environment ( the solution outside the cell has a lower solute concentration than inside the cell) and the cell gains too much water
define hypertonic environements
the solution outside of the cell has a higher solute concentration than the inside of the cell
define hypotonic environments
the solution outside of the cell has a lower solute concentration
define isotonic environment
the solution outside of the cell has the same solute concentration as the inside of the cell
what happens to animal cells in isotonic environments
the movement of water molecules into and out of the cell membrane occur at the same rate so no change occurs to the cell
when do plant cells lose water
plant cells gain water when they are in a solution with a lower water potential than the plant cell.
what occurs when plant cells lose water
- water leaves the vacuole and out via osmosis out through the partially permeable membrane
- the volume of the plant cell will decrease
- the protoplast will shrink and pull away from the cell wall.
what is the protoplast of a plant cell
all the contents/living parts of the plant cell except the cell wall
when do plant cells gain water
when plant cells are placed in pure/distilled water.
- when the plant cell is in a solution with a higher water potential than the plant cell.
what occurs when plant cells gain water
- water enters the cell through the partially permeable membrane via osmosis
- the volume of the cell increases
- the protoplast expands which increases the pressure
- the cell wall is able to withstand the pressure so the cell becomes rigid and firm (turgid)
- the cell is fully inflated with water so no more water can enter
