cell membranes

CM made up of 1

phospholipids, they form the basis of membrane structure, important component

reason 1

they can form bilayers with one sheet of PL molecules opposite another

reason 2

the inner layer of PL has its hydrophilic heads pointing in towards the cell and interacts with water in the cytoplasm

reason 3 

outer layer of PL has its hydrophilic heads pointing outwards interacting with water surrounding the cell

reason 4

the hydrophilic tails of the two PL layers point towards each other to the centre of the membrane

reason 5

the PL component of a membrane allows lipid soluble molecules across but not water soluble molecules

the CM is made up of 2

proteins - scattered throughout PL bilayer of the membrane

first way in which proteins are embedded

extrinsic proteins - on either surface of the bilayer

function of extrinsic proteins

provide structural support and form recognition sites by identifying cells and receptor sites for hormone attachment

the second way in which proteins are embedded

intrinsic proteins- extended across both layers of the PL bilayer

function of intrinsic proteins

some of them are carriers transporting water soluble substances across

others allow AT of ions across by forming channels

the official name of the way in which PL and proteins are arranged in a membrane

arrangement/ model of membrane structure name is fluid mosaic model

reason 1 for the name of model of membrane structure

the individual PL molecules can move within a layer relative to one another (fluid)

reason 2 

proteins are embedded in the bilayer vary in shape, size and pattern(mosaic) 

occuring in the membranes of animal cells

cholesterol between the PL molecules making the membrane more rigid and stable

extra feature of cell membranes 1

glycoproteins and glycolipids are found in the outer layer of the membrane

extra feature 2

the carbohydrate layer around the membrane is glycocalyx, some molecules of glycocalyx have roles as hormone receptors or in cell-to-cell recognition

lipid soluble substances 

dissolve in the PL and diffuse across the membrane 

this is because

the PL layer is hydrophobic so lipid soluble molecules move through the cell membrane more easily than water soluble molecules

examples include

vitamin A and small molecules such as oxygen, CO2

water soluble molecules

cannot readily diffuse through the PLs and must pass through intrinsic protein molecules which form water filled channels across the membrane

as a result

the cell surface membrane is selectively permeable to water and some solutes

definition of diffusion

passive transport

movement of molecules or ions from a region where they are in high concent to a region of lower concent down a concent gradient until equally distributed

ions and molecules

are always in a state of of random movement but if they are highly concent is one area there will be net movement away from the area until there is a uniform distribution

factor 1 affecting the rate of diffusion

the concent gradient → the greater the diff in the concent of molecules in two areas, the more molecules diffuse in a given time (the greater the RoD) 

different way of phrasing

as the concent gradient between the two areas increases the RoD increases

factor 2 affecting the RoD

the thickness of the exchange surface/ distance of travel over which diffusion takes place → the thinner the membrane/ the shorter the distance the more molecules diffuse in a given time (greater the RoD)

factor 3 affecting the RoD

the SA of membrane- the larger the A, the more molecules have room to diffuse across in a given time 

how can these 3 factors be expressed

in the equation: RoD =( SA x difference in concentration ) // length of diffusion path

extra factor 1 affecting the RoD

the size of the diffusing molecule → small molecules diffuse faster than larger molecules

extra factor 2 affecting the RoD

the nature of diffusing molecules → fat soluble molecules diffuse faster than water soluble molecules

non polar molecules diffuse faster than polar ones

extra factor 3

temp → increased temp increases rate as the molecules or ions have more KE

facilitated diffusion definition

passive transfer of molecules or ions down a CG across a membrane by protein carriers in the membrane

this is because

ions and molecules such as glucose cannot pass through the cell membrane because they are relatively insoluble in the PL bilayer , FD is speical form of D allowing movement of these Ms across a membrane 

how does it happen

occurs at specific sites on the plasma membrane where there are transport protein molecules, number and availability limit the rate of FD

transport proteins type 1

channel proteins: molecules with pores lined with polar groups

description

as the channels are hydrophilic ions being water soluble can pass through

channels open and close according to the needs of the cell e.g. when open ions are let through, when closed pores are too narrow for ions to pass through 

transport proteins type 2

carrier proteins allow diffusion of larger polar molecules across the membrane such as sugars and amino acids

explain process

a molecule attaches to its binding site on the carrier protein then the CP changes shape and releases the Ms on the other side of the membrane

example of a graph of carrier proteins

as the rate of uptake by FD increases the AC concent also increases but reaches a plateau as there is no increase in the rate of uptake because all the carriers are always occupied and their number has become limiting

AT

movement of molecules acorss a membrane against a CG using energy from the hydrolosis of ATP made by the cell in respiration 

exchange of substances between cells and surroundings in a way that involves metabolic energy 

processes involving AT

muscle contraction

nerve impulse transmission

reabsorption of glucose in the kidney

mineral uptake into plant root haris

describing AT

ions or Ms are moved from a L T A HC against the CG , requires energy from ATP therefore anything affecting respir will also affect AT

the rate of AT

limited by the number and availiability of carrier proteins because the process occurs through intrinsic carrier proteins spanning the membrane 

explain the steps of the active uptake of a single M or I

M or I combine with specific carrier protein on the outside of the membrane

step 2

ATP transfers a phosphate group to the carrier protein on the inside of the membrane 

step 3

the carrier protein changes shape, carries M or I across membrane to the inside of the cell releasing into the cytoplasm

step 4

P ion is released from carrier molecule back to the cytoplasm recombining with ADP to form ATP

finally

the carrier protein returns to its original shape 

co transport

type of FD that brings Ms and I into cells together on the same transport molecule

an example of this

sodium and glucose co transport- signifcant in absorbing glucose and sodium ions across CMs and into the blood in the ileum and the kidney nephron

step 1 of the process

a glucose molecule and two sodium ions outside the cell attach to a carrier protein in the CM 

step 2

carrier protein changes shaoe and deposits the glucose molecule and sodium ions inside cell

step 3

the glucose molecule and sodium ions separately diffuse through the cell to the opposite membrane

overall

the glucose passes into the blood by FDD and sodium ions are carried by AT 

another extra fact about CM that is in the textbook but I missed it out

Euk cells contain membrane bound organelles which are enclosed areas in the cytoplasm- adv is that potentially harmful chemicals such as enzymes are isolated and molecules with particular functions such as chlorophyll can be concentrated in one area

What is another advantage of membranes

They provide a large surface area for the attachment of enzymes involved in metabolic processes and they provide a transport system inside the cell

Two ways in which the memorable transports individual molecules or ions

Decide which does individual and which doesn’t do individual

A cell can also

Transport materials in bulk in by endocytosis or out by exocytosis

endocytosis

When materials are engulfed by extensions of the plasma membrane and the cytoplasm surrounding it making a vesicle

Type 1 of endocytosis

Phagocytosis: uptake of solid material that is too large to be taken in by diffusion of active transport

Example of phagocytosis

When granulocytes engulf bacteria a lysosome fuses with the vesicle formed and enzymes digest the cells and the products are absorbed into the cytoplasm

type 2 of endocytosis

Pinocytosis : uptake of liquid by the same mechanism but the vesicles produced are smaller

Exocystosis

Process by which susbtances may leave the cell having been transported through the cytoplasm in a vesicle which fuses with the cell membrane

For example

Digestive enzymes are often secreted in this way

Look at diagrams of both processes in the textbook

When ENDOC or EXOCY occurs

The cell membrane has to change shape which requires energy so these processes are active using ATP generated by the cell’s respiration

What is essential for these processes to occur

The property of fluidity of the cell membrane

Why

The cell membrane flows with endocy decreasing the overall area of the membrane and flows with exocyto increasing the overall area of the membrane

osmosis

special case of D which involves movement of water molecules only, the D of water from a region of high water potential to a region of low water potential through a selectively permeable membrane

CM and osmosis

most cell membranes are permeable to water and to certain solutes

water potential

measure of the free energy of water molecules, tendency for water to move measured in kPa 

pure water

there is no tendency for water molecules to move into pure water so pure water has a wp of 0

however this can change because

the addition of a solute to pure water tends to bring water molecules in, as the force pulls inwards it has a negative sign so the addition of a solute lowers the WP of PW giving it a negative value

therefore

the higher the concent the more strongly water molecules are pulled in the lower the WP

explaining this trend

where there is a high concent of water molecules in a dilute solution the water molcules are free to move so they have a high a high potential energy

whereas

in a solution water molecules are weakly bound to the solute so fewer are free to move and the system has a lower potential energy

however

external water molecules with higher potential energy will move down an energy gradient to the lower potential energy  

This is

The pulling force they experience which is the osmotic pull inwards i.e. the water potential

A more concentration solution has

Even fewer free water molecules so the pull on water molecules is greater so the WP is more negative, lower

Solute potential

A measure of the osmotic strength of a solution, reduction in WP due to the presence of solute molecules, how easily water molecules move out of a solution

When the WP

Is related only to the concentration of the solution this is called the solute potential

the trend of solute potential

The more solute present, the more tightly water molecules are held the lower the tendency of water to move out

Therefore a high concentration

Has a Lower more negative solute potential

In plant cells

The presence of a cell wall introduces an extra factor concerning water movement in and out of the cells

Explain

Water entering a plant cell expands the vacuole and pushes the cytoplasm against the cell wall

However

The cell wall can only expand a little so pressure outwards builds up resisting the entry of more water making the cell turgid

Turgid

A PC that holds as much water as possible, further entry of water is prevented as the cell wall cannot expand further

What is this called

This pressure is the potential pressure- a push outwards so it has a positive sign

Pressure potential

Pressure exerted by cell contents on the cell wall

In conclusion

Plant cells are under the influence of two opposing forces : solute potential and the pressure potential

Why

SP- due to the solutes in the vacuole and the cytoplasm pulling water in, the higher these concentrations the less likely the water is to move out

And the pressure potential- a force which increases tendency of water to move out

Equation

The balance of these two determines the WP of the cell and whether water moves in or out

WP of cell= pressure potential + solute potential

Where will water move

Always from higher WP to lower WP

Hypotonic

If an external solution has a higher WP than the solution inside the cell it is HYPOT to the cell and water flows into the cell

hypertonic

If the external solution has a lower WP than the solution inside the cell it is hydroponic to the cell, water flows out of the cell

Isotonic

If the cell has the same WP as the surrounding solution the external solution and cell are ISOT and there will be no net water movement

Plasmolysis

Plant cells in a hypertonic solution lose water by osmosis, the vacuole shrinks and the cytoplasm draws away from the cell wall , when complete the cell is flaccid