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A Level Bio Processes > 2.1.5 Biological Membranes (Foundations in Biology) > Flashcards

2.1.5 Biological Membranes (Foundations in Biology) Flashcards

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1
Q

What are membranes at the surface of the cells called?

A
  • plasma membranes
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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)
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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
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4
Q

What are membranes composed of?

A
  • lipids (mainly phospholipids)
  • proteins
  • carbohydrates
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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
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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
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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
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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
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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
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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

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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
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12
Q

What are membrane-bound receptors?

A
  • proteins in the cell membrane that act as receptors for messenger molecules
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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
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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

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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
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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

A Level Bio Processes (14 decks)

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