2.1.5 - BIOLOGICAL MEMBRANES

Question 1

What are the functions of membranes at the surface of cells?

Answer 1

• They are a barrier between the cell and its environment, controlling which substances enter and leave the cell - partially permeable (letting some molecules in but not others) | Can osmose, diffuse or enter by active transport
• They allow recognition by other cells (e.g. immune system cells)
• They allow cell communication (cell signalling)

Question 2

What are the function of membranes within cells?

Answer 2

• The membranes around organelles divide the cell into different compartments - they act as a barrier between the organelle and the cytoplasm | makes different functions more efficient (e.g. the substances needed for respiration are kept together inside mitochondria)
• They can form vesicles to transport substances between different areas of the cell
• They control which substances enter and leave the organelle (e.g. RNA leaves the nucleus via the nuclear membrane)
• Can get membranes within organelles - acting as barriers between the membrane contents and the rest of the organelle (e.g. thylakoid membranes in chloroplasts)
• Membranes within cells can be the site of chemical reactions (e.g. the inner membrane of a mitochondrion contains enzymes needed for respiration)

Question 3

Explain why the plasma membrane can be described as having a fluid mosaic structure.

Answer 3

• The phospholipid bilayer is ‘fluid’ because the phospholipids are constantly moving
• Protein molecules are scattered through the bilayer, like tiles in a mosaic

Question 4

Describe the structure of a cell membrane

Answer 4

• Phospholipid molecules form a continuous, double layer (bilayer)
• Cholesterol molecules are present within the bilayer
• Protein molecules are scattered through the bilayer
• Some proteins have a polysaccharide chain attached (glycoproteins)
• Some lipids also have a polysaccharide chain attached (glycolipids)
• Phospholipid bilayer about 7 nm thick

Question 5

Describe the structure of the phospholipid bilayer

Answer 5

• Phospholipid molecules have a hydrophilic head + hydrophobic tail
• The molecules automatically arrange themselves into a bilayer - heads face out towards the water on either side of the membrane
• The centre of the bilayer is hydrophobic so the membrane doesn’t allow water soluble substances (like ions) through it - acts as a barrier to these dissolved substances (but fat-soluble substances, e.g. fat-soluble vitamins, can dissolve in the bilayer and pass directly through the membrane

Question 6

Describe the function of cholesterol in the phospholipid bilayer

Answer 6

• Cholesterol is a type of lipid
• Present in all cell membranes (except bacterial cell membranes)
• Cholesterol molecules fit between the phospholipids. They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together
  ^— makes membrane less fluid + more rigid
• At lower temperatures, cholesterol prevents phospholipids from packing too close together and so increases membrane fluidity

Question 7

Describe the functions of proteins in cell membranes

Answer 7

• Some proteins form channels in the membrane - allow small or charged particles through
• Other proteins (carrier proteins) transport molecules + ions across the membrane via Clive transport and facilitated diffusion
• Proteins also act as receptors for molecules (e.g. hormones) in cell signalling | When a molecule binds to the proteins, a chemical reaction is triggered inside the cell

Question 8

Describe the functions of glycolipids and glycoproteins in cell membranes

Answer 8

• Glycolipids and glycoproteins stabilise the membrane by forming hydrogen bonds with surrounding water molecules
• Sites where drugs hormones and antibodies bond
• Receptors for cell signalling
• Antigens - cell surface molecules involved in the immune response

Question 9

Describe, using an example, how cells communicate with one another

Answer 9

• One cells releases a messenger molecule (e.g. a hormone)
• This molecules travels (e.g. through the blood) to another cell
• This messenger molecule is detected by the cell because it binds to a receptor on its cell membrane

Question 10

Describe how the permeability of a cell membrane changes as the temperature changes

Answer 10

• Temp below 0 - phospholipids don’t have much energy, so can’t move move. Packed closely together + membrane is rigid | channel and carrier proteins in membrane deform, increasing permeability. Ice crystals may form, piercing the membrane making it highly permeable when thawed
• Temp between 0 and 45 - Phospholipids can move around and aren’t packed as tightly together. membrane is partially permeable. As temp increases, phospholipids move more due to more energy (increases permeability)
• Temp above 45 - Phospholipid bilayer starts to melt (break down) + membrane becomes more permeable. Water inside cell expands, putting pressure on the membrane | Channel + carrier proteins deform so they can’t control what enters or leaves cell. Increases permeability

Question 11

Describe how the permeability of a cell membrane changes with the type of solvent

Answer 11

• Surrounding cells in a solvent (e.g. ethanol) increases permeability of cell membranes
• Because solvents dissolve the lipids in a cell membrane, so membrane loses structure
• Some solvents increase cell permeability more than other (e.g. ethanol more than methanol)
• Can investigate effects off different solvents with beetroot practical
• Increasing conc. of solvent will increase membrane permeability

Question 12

What is diffusion?

Answer 12

The net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration

Until equilibrium is reached

Question 13

What is a passive transport process?

Answer 13

A process of transport that requires no energy for it to happen

Question 14

What type of molecules can diffuse through a cell membrane?

Answer 14

• Small, non-polar molecules (e.g. oxygen and carbon dioxide) can diffuse through spaces between phospholipids
• Water is small enough to fit through phospholipids, so can diffuse across plasma membranes even though it’s polar (osmosis)

Question 15

What factors affect the rate of diffusion?

Answer 15

• Concentration gradient (higher it is, the faster the rate of diffusion)
• Thickness of the exchange surface (the thinner the exchange surface, the faster the rate of diffusion)
• The surface area (the larger the surface area, the faster the rate of diffusion)
• Temperature (the warmer it is, the faster the rate of diffusion - due to kinetic energy)

Question 16

What is facilitated diffusion?

Answer 16

Larger molecules, ions and polar molecules can’t directly diffuse through the cell membrane, so they diffuse through carrier or channel proteins

Question 17

Describe the role of carrier proteins in facilitated diffusion.

Answer 17

• Carries proteins move large molecules into or out of the cell, down their concentration gradient
• A large molecule attaches to a carrier protein in the membrane
• The proteins changes shape
• This releases the molecules on the opposite side of the membrane

Question 18

Describe the role of channel proteins in facilitated diffusion.

Answer 18

• Channel proteins form pores in the membrane for charged particles to diffuse through (down their concentration gradient)
• Different channel proteins facilitate the diffusion of different charged proteins

IMAGINE A TUNNEL

Question 19

What is active transport?

Answer 19

The transport of molecules from an area of low concentration to an area of high concentration against a concentration gradient. Requires energy
Involves carrier proteins

Question 20

Describe the process of active transport using carrier proteins

Answer 20

• A molecule attaches to the carrier proteins
• The protein changes shape and this moves the molecule across the membrane, releasing it on the other side
• Energy is used to move the solute against its concentration gradient

Question 21

Which molecule provides the energy for active transport?

Answer 21

ATP (Adenosine Triphosphate)

Question 22

Describe endocytosis

Answer 22

• Some molecules are way too large to be taken in to a cell be carrier proteins (e.g. proteins, lipids + some carbohydrates)
• Instead, a cell can surround a substance with a section of its plasma membrane
• The membrane pinches of to form a vesicle inside the cell containing the ingested substance

Some cells take much larger objects (e.g. some white blood cells - phagocytes - use endocytosis to take in microorganisms and dead cells to destroy them
USES ATP FOR ENERGY

Question 23

Describe exocytosis

Answer 23

• Some substances produced by the cell (e.g. digestive enzymes, hormones, lipids) 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
• The 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 inserted straight into plasma membrane

USES ATP FOR ENERGY

Question 24

What is osmosis?

Answer 24

The diffusion of water molecules across a partially permeable membrane down a water potential gradient

Question 25

What is water potential?

Answer 25

The potential (likelihood) of water molecules to diffuse out of or into a solution

Pure water has the highest water potential (no molecules dissolved/present inside it)

Question 26

What is a hypotonic solution?

Answer 26

Solution with a higher water potential than the cell

Question 27

What is an isotonic solution?

Answer 27

Solution with the same water potential as the cell

Question 28

What is a hypertonic solution?

Answer 28

A solution with a lower water potential than the cell

Question 29

What happens to an animal cell if it is placed in a hypotonic solution?

Answer 29

Net movement of water molecules is into the cell
Cell bursts

Question 30

What happens to an animal cell if it is placed in an isotonic solution?

Answer 30

Water molecules pass into and out of the cell in equal amounts
Cell stays the same

Question 31

What happens to an animal cell if it is placed in a hypertonic solution?

Answer 31

Net movement of water molecules is out of the cell
Cell shrinks

Question 32

What happens to a plant cell if it is placed in a hypotonic solution?

Answer 32

Net movement of water is into cell
Vacuole swells
The vacuole and cytoplasm push against the cell wall
The cell becomes turgid (swells)

Question 33

What happens to a plant cell if it is placed in an isotonic solution?

Answer 33

Water molecules move into and out of the cell in equal amounts
Cell stays the same

Question 34

What happens to a plant cell if it is placed in a hypertonic solution?

Answer 34

Net movement of water is out of the cell
The cell becomes flaccid (limp)
The cytoplasm and the membrane pull away from the cell wall
This is called plasmolysis