2.1.5 Biological Membranes (Foundations in Biology) Flashcards
1
Q
What are membranes at the surface of the cells called?
A
- plasma membranes
2
Q
What are the functions of plasma membranes?
A
- they are a barrier between the cell and its environment
- controls which substances enter and leave the cell
- they are partially permeable, so only allow some molecules through
- substances can move across the plasma membrane by diffusion, osmosis or active transport
- they allow recognition by other cells
- they allow cell communication (cell signalling)
3
Q
What are the functions of membranes within cells?
A
- membranes around organelles divide the cell into different compartments: barrier between organelle and cytoplasm. This makes different functions more efficient
- they can form vesicles to transport substances between different areas of the cell
- they control which substances enter and leave the organelle
- they are partially permeable
- membranes within organelles act as barriers between membrane contents and rest of the organelle
- membranes within cells can be the site of chemical reactions
4
Q
What are membranes composed of?
A
- lipids (mainly phospholipids)
- proteins
- carbohydrates
5
Q
Describe the fluid mosaic model
A
- phospholipid molecules form a continuous, bilayer
- it is ‘fluid’ because phospholipids are constantly moving
- cholesterol molecules are present within the bilayer
- protein molecules are scattered through the bilayer
- some proteins have a polysaccharide chain attached, which are called glycoproteins
- some lipids also have glycolipids attached
6
Q
What are the roles of phospholipids?
A
- they form a barrier to dissolved substances
- they have a ‘head’ and ‘tail’
- the head is hydrophilic (attracts water)
- the tail is hydrophobic (repels water)
- molecules automatically arrange themselves into a bilayer: the heads face out towards the water on either side of the membrane
- centre of the bilayer is hydrophobic, so the membrane doesn’t allow water-soluble substances through it
- fat-soluble substances can pass through
7
Q
What is the role of cholesterol in cell membranes?
A
- gives the membrane stability
- cholesterol is a type of lipid
- it is present in all cell membranes (except bacterial cell membranes)
- the molecules fit between the phospholipids
- they bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together
- this makes the membrane less fluid and more rigid
8
Q
What are the roles of proteins in cell membranes?
A
- controls what enters and leaves the cell
- some proteins form channel in the membrane: these all small or charged particles through
- carrier proteins transport molecules or ions across the membrane by active transport and facilitated diffusion
- proteins act as receptors for molecules in cell signalling: when a molecule binds to the protein, a chemical reaction is triggered inside the cell
9
Q
What are the roles of glycolipids and glycoproteins in cell membranes?
A
- receptors for messenger molecules
- glycolipids and glycoproteins stabilise the membrane by forming hydrogen bonds with surrounding water molecules
- they’re the sites where drugs, hormones and antibodies bind
- they act as receptors for cell signalling
- they’re also antigens
10
Q
What is cell signalling?
A
- how cells communicate with each other
- they need to communicate to control processes inside the body and to respond to changes in the environment
11
Q
How do cells communicate?
A
- they communicate using messenger molecules
- one cell releases a messenger molecule (e.g. a hormone)
- this molecule travels (e.g. in the blood) to another cell
- the messenger molecules is detected by the cell because it binds to a receptor on its cell membrane
12
Q
What are membrane-bound receptors?
A
- proteins in the cell membrane that act as receptors for messenger molecules
13
Q
Why are membrane-bound receptors important in cell signalling?
A
- receptors porteins have specific shapes
- only messengers with a complementary shape can bind to them
- different cells have different types of receptors: they respond to different messenger molecules
- a cell that responds to a particular messenger molecule is a target cell
14
Q
How do drugs work?
A
- many drugs work by binding to receptors in cell membranes
- they either trigger a response in the cell, or block the receptor and prevent it from working
15
Q
How does increasing the temperature increase membrane permeability?
A
below 0 degrees:
- phospholipids don’t have much energy and are packed closely together
- membrane is rigid
- however, channel proteins and carrier proteins in the membrane deform, increasing permeability of the membrane
- ice crystals may form and pierce the membrane, making it highly permeable when it thaws
temp between 0 and 45 degrees:
- phospholipids can move around and the membrane is partially permeable
- as temp increases, phospholipids move more because the have more energy and this increases permeability of membrane
above 45:
- phospholipid bilayer starts to melt and membrane becomes more permeable
- water inside the cell expands, putting pressure on the membrane
- channel proteins and carrier proteins deform, so they can’t control what enters or leaves the cell, increasing permeability
16
Q
How do different solvents affect membrane permeability?
A
- surrounding cells in a solvent increase the permeability of their cell membranes
- this is because solvents dissolve the lipids in a cell membranes, so the membrane loses its structure
- some solvents increases cell permeability more than others e.g. ethanol
- increasing concentration also increase membrane permeability
17
Q
What is diffusion?
A
- diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration
18
Q
What is the concentration gradient?
A
- the path from an area of higher concentration to an area of lower concentration
- particles diffuse down a concentration gradient
19
Q
Why is diffusion a passive process
A
- no energy is needed for it to happen
20
Q
What molecules diffuse through cell membranes?
A
- small, non-polar molecules such as oxygen and carbon dioxide are able to diffuse easily through spaces between phospholipids
- water is also small enough to fit between phospholipids, so its able to diffuse across plasma membranes, even though its polar. this is osmosis
21
Q
What factors change the rate of diffusion?
A
- concentration gradient: higher it is, faster rate of diffusion
- thickness of exchange surface: thinner, the faster rate of diffusion
- surface area
- temperature
22
Q
What is facilitated diffusion?
A
- some larger molecules, ions and polar molecules dont diffuse directly through the phospholipid bilayer
- instead they diffuse through carrier proteins or channel proteins in the cell membrane
- they move down a concentration gradient
23
Q
How do carrier proteins work?
A
- they move large molecules into or out of the cell, down their concentration gradient
- different carrier proteins facilitate the diffusion of different molecules
- a large molecules attaches to a carrier protein in the membrane
- then protein changes shape
- this releases the molecule on the opposite side of the membrane
24
Q
How do channel proteins work?
A
- they form pores in the membrane for charged particles to diffuse through
- different channel proteins facilitate the diffusion of different charged particles
25
Q
What is active transport?
A
- active transport uses energy to move molecules and ions across plasma membranes against a concentration gradient
- this process involves carrier proteins
26
Q
How does active transport work?
A
- a molecule attaches to the carrier protein
- the protein changes shape and this moves molecule across the membrane, releasing it to the other side
- energy is used from ATP to move solute against its concentration gradient
27
Q
What is endocytosis?
A
- some molecules are way too large to be taken into a cell by carrier proteins e.g. proteins, lipids
- a cell can surround a substance with a section of its plasma membrane
- the membrane then pinches off to form a vesicle inside the cell containing the ingested substance
- this process also uses ATP for energy
28
Q
What is exocytosis?
A
- some substances produced by the cell (e.g. digestive enzymes, hormones) need to be released from the cell
- vesicles contains these substances pinch of from the sacs of the Golgi apparatus and move towards the plasma membrane
- vesicles fuse with the plasma membrane and release their contents outside the cell
- some substances (e.g. membrane proteins) aren’t released outside the cell, instead they are inserted straight into the plasma membrane
- exocytosis uses ATP as an energy source
29
Q
What is osmosis?
A
- osmosis is the diffusion of water molecules across a partially permeable membrane down a water potential gradient
- water molecules move from an area of higher water potential to an area of lower water potential
30
Q
What is water potential?
A
- the potential of water molecules to diffuse out of or into a solution
31
Q
What happens to an animal cell when there is a solution with a higher water potential than the cell?
A
- this is a hypotonic solution
- net movement of water molecules into the cell
- cell bursts
32
Q
What happens to an animal cell when the solution has the same water potential as the cell?
A
- this is an isotonic solution
- water molecules pass into and out of the cell in equal amounts
- cells stays the same
33
Q
What happens to an animal cell when the solution has a lower water potential than the cell
A
- this is a hyper tonic solution
- net movement of water molecules is out of the cell
- cell shrink
34
Q
What happens when there is a hypotonic solution in plant cells?
A
- net movement of water is into the cell
- vacuole swells
- the vacuole and cytoplasm push against the cell wall
- cell becomes turgid
35
Q
What happens when there is an isotonic solution in plant cells?
A
- water molecules move into and out of the cell in equal amounts
- cell stays the same
36
Q
What happens when there is an hypertonic solution in a plant cell?
A
- net movement of water is out of the cell
- cell becomes flaccid
- the cytoplasm and membrane pull away from the cell wall
- this is plasmolysis
