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Transcript
Cell Boundaries
The roles of the cell membrane and
the cell wall….it’s all about transport
(active and passive)
Structures that enclose the cell
• Cell membrane is the outer boundary of
every cell
• Role is to separate and protect the cell from
its surroundings
• Cell membrane is selectively permeable
• Phospholipids and the lipid bilayer
• Role of proteins and carbohydrates
In plants: the cell wall
• Cell wall located outside the cell membrane
and serves to support and protect the cell
• Found in PLANTS ONLY
• Cell walls are very porous; allow water,
gases to pass through easily
• Primarily cellulose (carbohydrate) and some
protein
It’s about Transport
• Cell membranes help the cell regulate its
internal chemistry
• Cells maintain a constant internal
environment by compensating for
environmental changes.
• This is called homeostasis
• Use a combination of Passive Transport and
Active transport
Passive Transport
• Substances cross the cell membrane without
the cell expending energy
• Process is diffusion: this is how substances
spread through a liquid or a gas
• Substances move from regions of high
concentration to regions of lower
concentration
Passive transport (diffusion)
• Small molecules like alcohol, water and
small lipids diffuse directly across the cell
membrane.
• Cell membranes have hundreds of different
kinds of protein channels to allow diffusion
of specific molecules (like glucose)
• This use of channels is called facilitated
diffusion
Special case: diffusion of water
• Diffusion of water across the cell membrane
is called OSMOSIS
• Like all other substances water diffuses
form regions of high concentration to
regions of low concentration
• Pure water has a higher concentration of
water than a solution does
• This has important consequences for a cell
The consequence: Osmotic
pressure
• Movement of water causes powerful
pressure; high enough to burst the cell
• Cells deal with this:
– By having cell walls (physical strength)
– Pumping out water (special structures)
– By having blood to bathe the cells (blood and
cells have same concentrations of dissolved
substances)
Active Transport
• Movement of substance against a
concentration gradient
• ALWAYS requires energy; can be compared
to a pump
• Used to transport large molecules into the
cell, pareticles of food and sometimes
whole other cells
Active Transport
• Endocytosis: the cells engulfs large
particles and takes the particle into the cell
• Endocytosis is also called phagocytosis
– Protozoans
– White blood cells
• Exocytosis: cells use energy to expel
particles from the cell