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Cell Membranes

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sadie skiles

on 8 November 2016

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Transcript of Cell Membranes

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Plasma membrane
semi-permeable, lipid bilayer, in all cells,

controls flow in and out of the cell.

the cell

– allows some things to pass through, while blocking others

- keeps stable internal conditions
The Plasma Membrane
Phospholipid bilayer
: two layers of phospholipids per membrane, with proteins and some cholesterol

molecule, looks like a head and two tails
face out, are
(like water) &
face in, are

(repel water) &
Transport across the membrane
Movement of particles from an area of
high concentration
to an area of

low concentration

Happens naturally
(caused by constant vibration of atoms based on energy)

Continues until an
is reached
*particles are equally spread out*
No energy is needed to move particles
Simple diffusion:
Passive, spontaneous, no energy use
Pass through with out help
Only small

CO2, O2, ethanol
Facilitated diffusion:
Passive, spontaneous, no energy use
Cross with help of
transport proteins
Channel proteins:
tunnels particles “fall” through
Carrier proteins:
bind specific molecules, then deposit across the membrane
large or
polar molecules
Active transport proteins:
Carrier proteins
: change shape to open and close passages, for
specific bigger molecules
only- ex: glucose.
Protein pumps:
move ions
against gradient
to build up steeper gradient (more on this later!),
proton (H+) pumps
in electron transport chain
Active transport
Energy is needed
to move particles
opposite direction of diffusion or osmosis,
against concentration gradient
Passive transport
Membrane proteins
Lipid bilayer
Includes diffusion and osmosis
All cells acquire the molecules and ions they need from extracellular fluid
unceasing traffic goes in and out of the cell through membrane
Membrane proteins:
permanently embedded (stuck)
3-D shape & folding extremely important to function
main functions:
transporters- carriers, channels/pores, pumps
If a cell must move molecules/ions
against a concentration gradient
it always

Movement "upstream", costs energy
Bulk flow
Factors Affecting Diffusion Rate?
Difference in how 'crowded' on either side
Steeper concentration gradient, faster diffusion
Molecule size
Smaller molecules, faster diffusion
Higher temperature, faster diffusion
Vesicles: used to transport large particles across the membrane, requires energy

Exocytosis: vesicle from inside merges with the membrane and releases its contents out

Endocytosis: membrane sinks inward, pinches off and forms a vesicle
Made mainly of

double layer of
phospholipid molecules
; tails in, heads out

embeded throughout

Fluid mosaic model:

membranes are fluid layers; both lipids and proteins are constantly in motion in their horizontal layer.
Transport proteins: regulate the transport of polar molecules and ions through the membrane

Channel proteins: are pores or channels lined with certain amino acids that allow certain molecules and ions to pass through

Carrier proteins: have binding sites that grab certain molecules then changes shape and moves the molecule across the membrane

Receptor proteins: trigger cellular responces when hormones or nutrients in the extracellular fluid bind to them, trigger cellular changes like increasing metabolism, cell division, movement, secretion of hormone

Recognition proteins: serve as identification tags, are often glycoproteins, have a branched carbohydrate attached to the protein (glyco = carbohydrate) Allow cells to recognize other cells, protect your cells from your bodies immune system
Passive (no energy input)
diffusion of WATER
across membrane
goes from
high concentration OTHER STUFF -->--> low concentration OTHER STUFF
(other stuff) dissolved in
more solute = more concentrated
*keeps going until water to OTHER STUFF ratio is equal both sides
To compare solutions (like inside & outside a cell):
Isotonic solutions
– the same amount dissolved solute
HYPOtonic solution
– less dissolved solute
HYPERtonic solution
– more dissolved solute
lets draw an example....
Osmosis & solutions
Mini Quiz! open note but work alone!
1. What are the differences between osmosis and diffusion?
---what molecule type?
---are proteins involved?

2. What do osmosis and diffusion have in common?
---what is the driving factor?
---when do they stop?
---Do they need energy?

Group Quiz!
1. Diagram the Structure of a cell membrane
Work together and on one piece of paper
write full names of who is doing each group role (leader, data, equipment)

Draw and label a diagram of a section of cell membrane with the following:
each gr0up member add 1/3 of the double layer of phospholipids, 5 long (whole thing 15 long on top and 15 on bottom)
label hydrophobic and hydrophilic parts of double layer (leader)
2 Transport proteins, one with a non-polar molecule going through into cell (data)
label area that is the inside of the cell and outside (equipment)
circle one phospholipid, label hydrophobic tails and hydrophilic head (equipment)
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