tranport in cell-membranes

Cell membrane definition

Controls what substances enter and leave the cell

Characteristics of cell membrane

• allow cell to change shape

• Enables communication with external environment

• Partially permeable

Partially permeable definition.

Allows small but not large molecules to pass through through

What are glycoproteins

Proteins with carbohydrate attached

What are glycolipids

Lipids with carbohydrate attached

What are intrinsic proteins

Channel/carrier proteins that allow large, polar molecules or ions to pass through the membranes

Channel proteins

• they act like a open and. Close channel/gate

• To allow specific ions and water moleules to pass through via facilitated diffusion

Carrier proteins

• they must change shape for each molecule transported

• Slower transport rate

• Facilitate the movement of ions and small molecules by binding to them

Extrinsic/peripheral proteins

Act as receptors allowing the cell to detect chemicals released from other cell s

Function of phospholipid bilayer

• forms a barrier to dissolved substances

• Doesn’t allow water soluble substances

• Does allow small polar molecules to diffuse through

Structure of phospholipid bilayer

• polar, hydrophilic phosphate heads - soluble in water

• Two non-polar, hydrophobic fatty acid tails - insoluble in water

• Tails face inwards and heads face outwards forming a permeable barrier

Structure of cell membrane

• phospholipid bilayer

• Intrinsic proteins and peripheral proteins on the surface

• Cholesterol - between the phospholipids

• Carbohydrates - exterior surface attached to proteins (glycoproteins) or lipids (glycolipids)

Why is the cell membrane structure described as the fluid-mosaic model?

• fluid - phospholipids are constantly moving

• Mosaic - proteins are scattered through bilayer like tiles in a mosaic

Function of cholesterol

• gives stability by binding phospholipid tails packing them more closely together

• Restricts movement of other molecules

In general how does temperature affect cell membranes

As temp increases the phospholipids have more kinetic energy so they move faster making the bilayer more fluid like

How does the temp being below 0 affect cell membranes

• phospholipids packed closely together

• Channel and carrier proteins can denature which increases the permeability

• Ice crystals may form and pierce the membrane making it more permeable when it thaws

How does the temp between 0-45 degrees affect the cell membrane

• partially permeable membrane

• Temp has increased so, kinetic energy has increased so it becomes more permeable

• Phospholipids move freely

How does the temp above 45 degrees affect the cell membrane

• phospholipids melt

• Water expands increasing pressure

• Proteins denature so membrane is more permeable

How do solvents affect cell membranes

• increased phospholipid movement

• Solvents can denature proteins by disrupting bonds

How do ethanol molecules affect cell membrane

• they can form hydrogen bonds with a phospholipid molecule near an ester bond

• This disrupts the membrane structure, increasing the permeability

Simple diffusion definition

The net movement of molecules from an area of high concentration to low concentration

Characteristics of simple diffusion

• passive -(so requires no energy)

• Moves molecules down a concentration gradient

• Happens across a partially permeable membrane

Overall net movement of simple diffusion

Towards the lower concentration

When does simple difffusion occur

When molecules are not too large

How does surface area affect simple diffusion - explain

• larger surface area = faster rate of diffusion

• Larger SA means more particles can be exchanged in the same amount of time - increasing the rate

How does concentration gradient affect the rate of simple diffusion

• greater the concentration gradient = faster rate of simple diffusion

• A good blood/air supply helps maintain concentration gradients by transporting substances to and from the membrane

How does membrane thickness affect the rate of simple diffusion

• thinner membrane = faster rate of diffusion

• Thin membranes create a shorter pathway for the particles to travel - increasing rate

How does temperature affect the rate of simple diffusion

• higher temp = faster rate of diffusion

• Increasing temp means particles have more kinetic energy so they are able to move faster

Facilitated diffusion definition

• movement of molecules from areas of high concentration to low concentration requiring transmembrane protein

Characteristics of facilitated diffusion

• moves particles down concentration gradient

• Passive - no energy required

• Faster than simple diffusion

When is facilitated diffusion needed to be used

• for larger or charged particles

• They need to diffuse through specialise carrier or channel proteins

Carrier proteins for facilitated diffusion

• move large molecules across membrane

• Different proteins facilitate the diffusion of different large molecules

How does the carrier proteins work - facilitated diffusion

• large proteins attach

• Protein changes shape releasing molecule on the other side of membrane

Channel protein for facilitated diffusion

• form pores for charged particles to diffuse through

• Different protein channels facilitate the diffusion of different charged particles

How do channel proteins work - facilitated diffusion

• create a “tunnel” For charged particles to cross the membrane

• No changing shape

How does concentration gradient affect the rate of facilitated diffusion

• greater concentration difference = faster the diffusion

• Particles still moving down the concentration gradient until reached equilibrium

How does the number of channel/carrier proteins affect the rate of facilitated diffusion

• Greater number = faster facilitated diffusion

• Once all the proteins are in use the diffusion can’t happen any faster so it becomes a limiting factor

Osmosis definition

Net movement of water molecules from areas of high water potential to low water potential across a partially permeable membrane

Water potential definition

Likelihood of water molecules to diffuse out of or into a solution

What is the water potential of pure water

Zero

What does more negative water potential mean

• the more concentrated a solution is

What happens when you keep adding more solvent to a solution

• lowers the water potential

Hypotonic solution meaning

• solution has higher water potential than inside the cell so water moves in by osmosis

Characteristics of hypotonic solution

• higher water potential

• Pure water

What happens to animal cell in a hypotonic solution.

• cell swells up

• Membrane cannot withstand the pressure

• Cell eventually bursts

What happens to a plant cell in hypotonic solution

• Cell swells up

• vacuole expands and cell becomes turgid

• Cell wall protects the cell from bursting

Hypertonic solution meaning

Solution has lower water potential than inside the cell so water moves out by osmosis

Characteristics of hypertonic solution

• lower water potential

• Concentrated solution

What happens to animal cell in a hypertonic solution

• cell loses water in cytoplasm

• Cell shrivels

What happens to plant cell in a hypertonic solution

• vacuole shrinks but cell doesn’t shrink

• Membrane pulls away from cell wall called PLASMOLYSIS

Isotonic solution meaning

Two solutions with the same water potential

Characteristics of isotonic solution

• no net movement of water

What happens to animal cell in isotonic solution

• stays normal

What happens to a plant cell in isotonic solution

• becomes flaccid

How does membrane thickness affect the rate of osmosis

• thinner membrane = faster rate of osmosis

• Thinner membranes reduce the distance that molecules have to travel

How does water potential gradient affect the rate of osmosis

• higher gradient = faster rate of osmosis

• During osmosis the difference in water potential on either side of the membrane decreases

How does surface area affect the rate of osmosis

• larger SA = faster rate of osmosis

• More molecules move accross in the same amount of time - increasing the rate

Active transport definition

Movement of molecules from area of low concentration to high concentration using energy

What is significant about active transport

• requires ATP that is produced by respiration

• Also used for when carrier proteins require energy to change shape

How does the umber of carrier proteins affect the rate of active transport

• greater number = faster rate of AT

• Some cells are specially adapted with high numbers to carry out lots of A.T -(eg root hair cell)

How does the speed of proteins affect the rate of active transport

• faster = increases rate of A.T

• Decreasing concentration gradient has no effect. But proteins moving slowly do

What is co-transport

The coupled movement of two substances accross a cell membrane via a carrier protein

When is co transport often used

Often seen In the ileum for glucose/amino acid absorbtion

Describe co transport in the absorbtion of glucose/amino acids

• sodium ions diffuse down their gradient providing energy from glucose/amino acids to pull against their gradient

• Requires active transport to maintain the sodium gradient