plasma membranes

how is the fluid mosaic model fluid

phospholipids form a bilayer in which the phospholipid molecules are constantly moving

how is the fluid mosaic model mosaic

there are proteins of different shapes and sizes embedded in the phospholipid bilayer, so it looks like a mosaic pattern.

list the key components of the cell membrane

1. phospholipid bilayer

2. cholestrol

3. proteins (intrinsic and extrinsic)

4. glycoproteins

5. glycolipids

how does the arrangement of the phospholipids affect the bilayer.

hydrophilic heads face out and hydrophobic tails face inwards. This arrangement creates a hydrophobic centre in the bilayer, meaning water-soluble substances cannot pass through.

what substances can dissolve the bilayer

lipid-soluble substances can dissolve the bilayer and pass directly through the cell membrane.

what is the function of cholestrol molecules in the bilayer

they provide stability

where do the hydrophobic regions of cholestrol molecules bind in the bilayer and what is the affect

hydrophobic regions bind to the phospholipid fatty acid tails, causing them to pack more closely together. This reduces the fluidity of the cell membrane

what are intrinsic proteins and give examples

intrinsic proteins are proteins embedded through both sides of the phospholipid bilayer. They include channel and carrier proteins.

what do channel and carrier proteins transport across the membrane

large molecules and ions

what are extrinsic proteins and give their function

proteins present on only one side of the phospholipid bilayer. They provide support to the membrane and may be involved in cell signalling

what is the difference in the structure of glycoproteins and glycolipids

glycoproteins consist of intrinsic proteins attached to carbohydrates, whereas glycolipids consist of lipids attached to carbohydrates.

what are the 3 things that glycoproteins and glycolipids are involved in

1. cell adhesion

2. cell recognition

3. cell signalling

define cell adhesion

the attachment of cells to one another

define cell recognition

allows the cells to recognise one another

define cell signalling

the communication between cells

what are the two main types of cell membrane

1. cell-surface membranes

2. membranes around organelles

they are both partially permeable

what are cell surface membranes

they surround cells to act as barriers between the cell and its environment, controlling which substances enter and leave the cell

what are membranes around organelles

they surround organelles (e.g. mitochondria) to act as a barrier between the organelle and the cytoplasm, dividing the cell into different compartments.

what is compartmentalisation

when membranes surround organelles and act as a barrier between the organelle and the cytoplasm, dividing the cell into different compartments.

what are the two factors that affect the permeability of membranes

1. temperature

2. solvents

how does an increase in temperature affect the permeability of the membrane

1. as temp increases, phospholipids have more kinetic energy so they move faster and aren’t packed as closely together, which increases the permeability of the membrane.

2. When temperatures exceed 40*C, the phospholipid bilayer breaks down and channel and carrier proteins denature, meaning they canotn control what enters or leaves the cell. Permeanility increases.

how do solvents affect the permeability of the membrane

when cells are placed in solvents such as ethanol, the phospholipids dissolve, causing the membrane to become more fluid. This disrupts the structure of the cell membrane and makes it more permeable.

define diffusion

the net movement of particles from an area of high concentration to an area of low concentration

why is diffusion described as a passive process

because it does not require energy

define simple diffusion

the diffusion of molecules directly across cell membranes.

why do substances need to be small and non-polar for simple diffusion

1. small- they can pass through the spaces between phospholipids

2. non-polar- they can dissolve in the hydrophobic core of the cell membrane

what types of molecules and proteins are involved in facilitated diffusion

1. large or polar molecules

2. carrier and channel proteins

what type of molecules do carrier proteins mainly transport

large molecules

explain the stages of facilitated diffusion via carrier proteins

1. a large molecule attaches to a carrier protein

2. this causes the carrier protein to change shape

3. the carrier protein releases the molecule on the opposite side of the membrane

what molecule do channel proteins mainly transport

ions

how to channel proteins allow substances to pass through the membrane via facilitated diffusion

the channel proteins form pores in the cell membrane, which ions can travel through

what are the 5 factors affecting the rate of diffusion

1. temperature

2. concentration gradient

3. thickness of membrane

4. surface area

5. number of carrier or channel proteins

define active transport

the movement of particles from an area of lower concentration to an area of higher concentration. This process requires energy from respiration in the form of ATP

what type of process is active transport

an active process because particles move up (or against) a concentration gradient, so energy is needed

what protein does active transport across the membrane require

carrier protein

explain the stages in which carrier proteins assist in active transport across the membrane

1. The molecule or ion binds to the carrier protein

2. ATP binds to the carrier protein

3. Hydrolysis of ATP to ADP and phosphate (Pi) causes the carrier protein to change shape. This releases the molecule or ion on the opposite side of the membrane to which it entered.

4. The phosphate (Pi) is released from the carrier protein, causing the carrier protein to return to its original shape, ready to be used again.

what factors affect the rate of active transport

1. temperature

2. thickness of membrane

3. number of carrier proteins

4. rate of respiration

explain how temperature affects the rate of active transport

At higher temperatures, particles have more kinetic energy and travel faster. Respiration also increases with temperature. Very high temperatures denature carrier proteins, decreasing the rate of active transport.

what type of molecules are transported in bulk transport

large molecules such as enzymes or hormones, since they cannot fit through carrier and channel proteins.

what is the difference between the two types of bulk transport

1. endocytosis- transports materials into cells

2. exocytosis- transports materials out of cells

what happens during endocytosis

the cell-surface membrane engulfs the materials to form a vesicle which is then moved into the cytoplasm where the materials can be processed.

what happens during exocytosis

vesicles (mostly formed by the Golgi apparatus) move towards and fuse with the cell-surface membrane where materials are released outside the cell.

what are the two forms of endocytosis

1. phagocytosis- the uptake of solid materials

2. pinocytosis- the uptake of liquid materials

define solution

a mixture made up of a solute dissolved in a solvent

define water potential

the pressure exerted by water molecules on the membrane (or container) surrounding a solution. It is measured in kPa.

what is the highest water potential to exist

0 kPa

define osmosis

the diffusion of water molecules across a partially permeable membrane from an area of higher water potential to an area of lower water potential.

can water molecules diffuse directly through the cell membrane

yes because they are small molecules

give features of a hypotonic solution

a hypotonic solution has a higher water potential than the cell

water molecules move into the cell

the cell swells and bursts

give features of an isotonic solution

an isotonic solution has the same water potential as the cell

there is no net movement of water in or out of the cell

the cell stays the same size

give features of hypertonic solutions

a hypertonic solution has a lower water potential than the cell

water molecules move out of the cell

the cell shrinks

in what solution does a plant cell become turgid

hypotonic

in what solution does a plant cell become plasmolysed

hypertonic

what factors affect the rate of osmosis

1. temperature

2. water potential gradient

3. thickness of membrane

4. surface area