Transport across cell membranes

Question 1

What are plasma membranes?

Answer 1

All membranes around and within all cells have the same basic structure

• boundary between cytoplasm and environment
• it allows different conditions to be established

Question 2

Phospholipids

Answer 2

Important components of cell surface membranes for the following reasons:

• hydrophillic heads of both layers point outside of the cell surface membrane attracted by water on both sides
• hydrophobic tails of both phospholipid layers point into the centre of the cell membrane, repelled by the water

Question 3

Function of phospholipids

Answer 3

• allow lipid-soluble substances to enter and leave the cell
• prevent water-soluble substances entering and leaving the cell
• make the membrane flexible and self sealing

Question 4

Proteins

Answer 4

They’re embedded in the bilayer in two ways:

• proteins act as mechanical support to the membrane or with glycolipids as cell receptors for molecules such as hormones
• some are protein channels which form water filled tubes to allow water-soluble ions to diffuse across
• others are carrier proteins that bind to ions or molecules like glucose and amino acids then change the shape to move these molecules across the membrane

Question 5

Function of proteins

Answer 5

• provide structural support
• act as channels transporting water-soluble substances across the membrane
• allow active transport across the membrane through carrier proteins
• form cell surface receptors with glycolipids
• act as receptors for homones

Question 6

Chloesterol

Answer 6

• they’re hydrophobic and play an important role in preventing loss of water and dissolved ions from the cell
• they pull fatty acid tails of the phospholipid molecules limiting their movement without making it too rigid

Question 7

Functions of cholesterol

Answer 7

• add strength to membranes
• reduce movement
• make the membrane less fluid at high temps
• prevent leakage of water and dissolved ions from cell

Question 8

Glycolipids

Answer 8

made up of carbohydrate and lipid
Functions:
- act as recognition sites
- help maintain the stability of the membrane
- help attach to one another and so form tissues

Question 9

Glycoproteins

Answer 9

• they’re attached to extrinsic proteins on outer surface
• they also act as cell- surface receptor for hormones and neurotransmitters
  Functions:
• act as recognition sites
• help attach to cells and form tissues
• allow cells to recognise one another e.g lymphocytes can recognise an organism’s own celss

Question 10

Permeability of cell surface membrane

Answer 10

Cell surface membrane controls the movement of substances in and out of cell
Most molecules don’t freely diffuse across because:
- not soluble in lipids and can’t pass through bilayer
- too large to pass through
- of the same charge as the protein channel
- electrically charged and have difficulty passing through non polar hydrophobic tales

Question 11

Fluid mosaic model of cell surface membrane

Answer 11

• fluid: because the individual phospholipid molecules can move relative to one another giving the membrane a flexible structure so its constantly changing shape
• Mosaic: proteins that are embedded in the phospholid vary in shape, size and pattern

Question 12

Diffusion

Answer 12

definition: the net movement of molecules or ions from a region where they’re more highly concentrated to one where their concentration is lower

Question 13

Facilitated Diffusion

Answer 13

• small non polar molecules like oxygen can diffuse easily
• charged ions and polar molecules don’t diffuse easily
• FD is a passive process that relies on the kinetic energy of the molecules
• Like diffusion, it occurs down a concentration gradient
• two proteins are involved: protein channels and carrier proteins

Question 14

Protein Channels

Answer 14

• they form water filled channels across membrane
• allow water- soluble ions to pass through
• ions bind with protein causing it change shape so it can move across

Question 15

Carrier proteins

Answer 15

• When a molecule like glucose that is specific to the protein is present, it binds with the protein. This causes it to change shape resulting the molecule to be released into the membrane
• no energy is needed. molecules from a region of high concentration to a low concentration only using its kinetic energy
• they have a specific tertiary structure and will only transport particular substances

Question 16

Osmosis

Answer 16

Definition: passage of water from a region of high water potential to a low water potential through a selectively permeable membrane

Question 17

Solutions and Water potential

Answer 17

• Solute: any substance that is dissolved in a solvent e.g water
• Solution: solute and solvent together form a solvent
• water potential is represented by Ψ and is measured in units of pressure, kiloPascals (kPa)
• pure water is said to have a WP of zero

Question 18

Water potential

Answer 18

• the addition of a solute to pure water will lower its WP
• the WP of a solution will always be less than zero- neg value
• The solute that is added the lower it’s WP
• water will move by osmosis from a region of higher (less negative) WP to one of lower (more negative)
• one way of finding the WP of cells is to place them in a series of solutions of different WP
• where there’s no gain or loss of water the WP is the same inside and in the external solution

Question 19

Understanding WP

Answer 19

The more negative the value, the lower the WP

Question 20

High ( less negative ) WP - animal cell

Answer 20

Water enters cell and cell swells and bursts so haemoglobin is released

Question 21

Equal WP - animal cell

Answer 21

Water doesn’t leave or enter and there’s no change to the cell

Question 22

Lower (more negative) WP - animal cell

Answer 22

Water leaves cell and cell shrinks so haemoglobin is more concentrated and gives cell a darker colour

Question 23

High (less negative) WP - plant cell

Answer 23

water enters cell and cell swells and becomes turgid

Question 24

Equal WP - plant cell

Answer 24

water doesn’t leave or enter the cell so there’s no change and the condition is incipient plasmolysis

Question 25

Lower (more negative)

Answer 25

Water leaves the cell and it shrinks leaving it plasmolysed

Question 26

Plant cell can be divided into 3 parts:

Answer 26

• central vacuole which contains a solution of salts, sugars and organic acids
• the protoplast consisting of the outer- cell surface membrane, nucleus and the inner vacuole membrane
• the cellulose cell wall, a tough, inelastic covering that is permeable to even large molecules

Question 27

Active transport definition

Answer 27

movement of molecules or ions in and out of a cell from a region of lower concentration to a higher concentration using ATP and CARRIER PROTEINS

Question 28

In Active Transport ATP is used to:

Answer 28

• directly move molecules

- individually move molecules using a concentration gradient which has been set up by active transport

Question 29

Its different to the other forms of transport

Answer 29

• ATP is needed
• Substances are moved against a concentration gradient
• carrier protein act as pumps

Question 30

Direct AT stages

Answer 30

• carrier proteins bind to molecule or ion to be transported
• molecule or ion binds to receptor sites on carrier protein
• ATP binds to protein causing it to split into ADP and Pi. As a result the protein molecule changes shape
• molecule or ion is released to other side
• phosphate molecule is released from the protein which causes the protein to revert to its original shape ready for the process to be repeated. The phosphate then recombines to ADP to form ATP in respriration

Question 31

Sodium potassium pump

Answer 31

sometimes more than one molecule or ion may be moved in the same direction at the same time by AT. the molecule or ion is moved into cell at the same time as a different one is being removed from it

Question 32

sodium potassium pump cont

Answer 32

sodium ions are actively removed from the cell while potassium ions are actively taken in from the surroundings. This process is essential to a number of important processes

Question 33

Increasing the rate of movement across membranes:

Answer 33

• epithelial cells lining the ileum possess microvilli which are on the cell surface membrane. They provide more surface area for insertion of carrier proteins through which diffusion, facilitated diffusion and AT can take place.
• another way to increase transport is to increase the number of protein channels and carrier proteins

Question 34

The role of diffusion in absorption

Answer 34

As carbohydrates and proteins are being digested there is a greater concentration of glucose and amino acids within the ileum than in the blood. There’s a concentration gradient so glucose moves by facilitated diffusion from inside the ileum into the blood. The glucose is absorbed into blood and is being removed by cells in respiration. this helps to maintain concentration gradient between inside ileum and blood.
The rate of movement by facilitated diffusion across epithelial cell surface membranes is increase

Question 35

Role of Active Transport in absorption

Answer 35

Glucose and amino acid are also being absorbed by AT
- They’re absorbed from the small intestine by co-transport. Glucose or amino acids are drawn into the cells along with sodium ions that have been actively transported out by the sodium potassium pump

Question 36

Co transport of a glucose molecule

Answer 36

1) Sodium ions are actively transported out of epithelial cells by the sodium potassium pump into the blood. This takes place in the protein carrier found in the surface of epithelial cells
2) This maintains a much higher concentration of sodium ions in the lumen of the intestine that inside of the epithelial cells
3) Sodium ions diffuse into epithelial cells down this concentration gradient through a different protein carrier. As sodium ions diffuse through the second protein carrier, they either carry amino acid molecules or glucose molecules into the cell
4) glucose/ amino acid pass into the blood plasma by facilitated diffusion using another type of carrier

Question 37

Sodium ions or glucose

Answer 37

sodium ions move down their concentration gradient, the glucose molecules move against the concentration gradient. it is the concentration gradient rather than ATP that powers the movement of glucose and amino acids into the cells. This makes it an indirect form of AT

Question 38

Co transport

Answer 38

substances are transported across a membrane by one protein, or protein complex which does not have ATPase activity.

Question 39

Damage caused by diarrhoea

Answer 39

• damage to the epithelial cells lining the intestine
• loss of microvilli due to toxins
• excessive secretion of water due to toxins e.g cholera toxin

Question 40

Drinking water is ineffective to treat diarrhoeal reasons:

Answer 40

• Water isn’t being absorbed from the intestine, in cholera, water is lost from cells
• Drinking water doesn’t replace electrolytes

Question 41

A rehydration solution needs to contain

Answer 41

• water to rehydrate tissues
• sodium ions to replace the ones lost from the epithelium of the intesting
• glucose to stimulate the uptake of sodium ions from the intestine and to provide energy
• potassium ions to replace the lost ones and to stimulate appetite
• other electrolytes such as chloride ions and citrate ions to help prevent electrolyte imbalance