section 4_cell membranes

8/3/2019 Section 4_Cell Membranes

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CellMembranes


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Plasma/Cell membrane

structure They are made oflipids- phospholipids to be

exact, a lipid with a phosphate group attached to

it.

Proteins,

Carbohydrates, which are usually attached to

the previous two.

In 1972 -fluid mosaic model used to describethe arrangement of molecules in the membrane.


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Cell Membrane structure

-Phospholipid molecules form a continuous

double layer (bilayer).

-The molecules are constantly moving, making

the bilayer fluid. -Protien molecule are scattered in the layer, like

tiles in a mosaic, because the phospholipid

bilayer is fluid the proteins can move around in

it.

Some proteins have a carbohydrate

(polysaccharide) chain attached they are called

glycoproteins.


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Cell Membrane structure

-There is also cholesterol in the

membrane, this is found in between the

phospholipids forming bonds with them,making the membrane more rigid.

-The membrane is partially permeable as

some molecules can fit in the small gaps

between phospholipids, others are too

large and must go in through carrier

proteins and channel proteins. These are

special membrane proteins.


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Scientific evidence on the fluid

mosaic model-In the past people believed that cell membranes

were made out of a phospholipids layer in-between

two protein layers because electron microscope

images made it look like there were 3 layers. This iswrong.

Newer electron microscope techniques show a

bilayer of phospholipids with proteins randomlyscattered within it.

Experiment to prove bilayer is fluid:

A mouse and human cell were fused- mouse and


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Permeability of the cell

membrane There are two ways to increase the

permeability of a cell membrane:

-Increase the temperature-Increase the alcohol concentration


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Temperature

Below 0 degrees

-Phospholipids do not have much energy, so

they cannot move around much.-They are packed closely together- very rigid.

channel/carrier proteins deform, increasing

permeability.-Ice crystals may form and pierce the

membrane making it highly permeable

when it melts.


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Temperature

Temperatures between 0 and 45

degrees

-Phospholipids can move around andarent packed as tightly together

The membrane is partially permeable.

As temp increases phospholipids movemore as they have more energy.

This increases permeability of the

membrane.


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Temperature

Temperatures between above 45

degrees

-P

hospholipids begin to melt, making themembrane more permeable.

-The water begins to expand, putting pressure

on the membrane.

-Channel and carrier proteins deform -Permeability increases as they are unable to

control what leaves or enters the cell.


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alcohol concentration

As you increase the concentration, the

permeability increases.

This is because alcohol dissolves lipids, sothe membrane loses its structure.


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How permability can be

investigated in a labYou can test how temp and alc. Conc. Affect

permeability by doing a beetroot

experiment.

Beetroot contain coloured pigment that

leaks out.

The higher the permeability the more

pigment leaks out of the cell.


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To investigate how alcohol conc.

affects permeability 1. Cut 5 equal pieces of pieces of beetroot.

2. 5 different concentrations of alcohol in 5 beakers.

3.Place the on piece beetroot in each beaker for thesame amount of time.

4.Remove the beetroot, leaving just the coloredliquid.

5. Use a colorimeter

This machine passed light through a liquid andmeasures how much is absorbed.

-The higher the absorbance number

- the more pigment released

-the higher the permeability


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Transport across the cell

membraneDiffusion

- passive movement of particles down the

concentration gradient. (no energy is required)

-Molecules move both ways but net movement is

to the area with low conc, until particles are

evenly distributed.

Particle can diffuse through cell membranes if theyare small enough

CO2 and O2


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Gaseous exchange

-O2 and CO2 diffuse across a surface called the gas

exchange surface.

these surfaces have two things in common.

They are thin, only one cell thick- this gives it a shortdiffusion pathway across the gas exchange surface

The have a large surface area.

The organism also maintains a steep conc. gradientof gases across the exchange surface.

These features above increase the rate of diffusion.


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How lungs are adapted for efficient

gaseous exchange. O2 diffuses out of alveoli, across the alveoli epithelium,and capillary endothelium, into the blood.

Co2 diffuses out of blood into alveoli and is breathed out.

Human Lungs have these features which increasethe rate of gaseous exchange.

-Alveoli provide a large surface area for diffusion tohappen across.

-The alveoli epithelium and capillary endothelium is onecell thick- giving it a short diffusion pathway.

-Breathing in and out refreshes air in alveoli keepingconcentration gradients high.

-Alveoli have a good supply of blood from capillaries

The capillaries take o2 and bring co2 constantly ,maintaining the concentration gradient.


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Osmosis

Diffusion of water molecules across

partially permeable membrane.

H

igh-> low Diffusion happens both ways net

movement is to lower conc. of water.


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Transport across cell membrane

Facilitated diffusion- passive.

Larger molecules/charged atoms cannot diffuse

directly through bilayer.

They diffuse through carrier proteins or channel

proteins.

Called facilitated diffusion.

High -. Low / down conc. grad.


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Channel proteins

Form pores in membrane for charged

molecules to diffuse through.

Different channel proteins for different

charged particles.


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Active Transport

Uses energy to move molecules and ions across

cell membranes.

Against conc. Grad./ low-> high

Molecule attaches to carrier protein

Protein changes shape.

Moves molecule across the membrane,

releasing it on the other side. Only diff, from facilitated diff. AT uses energy

from ATP, to move solute against conc. Grad./

Low->high


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Endocytosis

A way cells can take in substances.

Some molecules are too large even for carrier proteins'.

Lipids, proteins,

carbohydrates.

A cell can surround these substance with a section of its

membrane.

This creates a vesicle.

This enters the cell pushing the membrane inwards,

whilst some of the membrane and cytoplasm bulges

around it.

Some cells use it to take in larger objects.

white blood cells (phagocytes) also use endocytosis to

take in dead cells and microorganism to destroy them.


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Exocytosis

How Some substances produced by cells are

released. Eg. Digestive enzymes, hormones and

lipids.

Vesicles containing these substances pinch offfrom sacs of the Golgi apparatus and move

towards the cell membrane.

Vesicles fuse with the cell membrane and

release contents outside cell.

Some substances arent released outside call,

but are inserted into cell membrane.

section 4_cell membranes

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