HOMEOSTASIS AND THE

PLASMA MEMBRANE

Biology

Maintaining a Balance

• (Cells must maintain a stable environment

regardless of its external environment)

• (Concentrations of dissolved substances,

food, and waste materials change constantly

both inside and outside of a cell)

• Certain substances pass through cell

membranes more easily than others

• Difficult

• Starch

• proteins

• Easy

• Lipid molecules

• Alcohol

• Ether

• Chloroform

• (Selective Permeability – the property of a

membrane that allows some materials to

pass through while rejecting others)

Structure of the Plasma

Membrane

• Two-layered

• Each layer is made up of a sheet of lipid

molecules, w/ proteins embedded in the

lipid layers

• Lipids have 2 fatty acids attached to a

glycerol

PHOSPHOLIPIDS

• Lipids also have a

phosphate group and

is called a

phospholipid

Phospholipid

• Phospholipid – a group of lipid compounds

occurring in plant and animal tissues with

stored fat

• Cells have a watery environment inside and

out

• Water attracts the phosphate ends of

phospholipid

Phospholipid

• They form double layers with the water

soluble phosphate ends toward the outside

of each layer

• (The nonpolar tails lie on the inside of the

bilayer)

Phospholipid

• The bilayer can be considered to be a fluid

material that flows

• Fluid mosaic – the ability of the

phospholipid and protein molecules of a

membrane to flow among one another

• The presence of cholesterol molecules

makes the fluid mosaic more stable

Plasma Membrane

• Diffusion – the net movement of particles

from an area of higher concentration to an

area of lower concentration

• Dynamic equilibrium – the condition in

which there is continuous movement but no

overall change

• Maintains homeostasis

Dynamic Equilibrium

• http://www.emc.maricopa.edu/faculty/farab

ee/BIOBK/BioBooktransp.html

Plasma Membrane

• Diffusion cannot occur unless a substance is

in higher concentration in one region than it

is in another

• Concentration gradient – the difference in

concentration of a substance across space

Osmosis

• Osmosis – the diffusion of WATER

molecules through a membrane from an

area of higher water concentration to an

area of lower water concentration

Osmosis

• Isotonic solution – a solution in which the

concentration of dissolved substances is the

same as the concentration inside the cell

• Concentration of water is the same as in the

cell

• Cell is in dynamic equilibrium

Osmosis

• Hypotonic solution – a solution in which the concentration of dissolved substances is lower than the concentration inside the cell

• The concentration of water is higher than inside the cell

• As water diffuses into the cell, the cell swells and its internal pressure increases (plant cells)

• Hypotonic solution – water goes into the cell

• Turgor pressure – the pressure that builds in a plant cell as a result of osmosis

• Causes pressure against the cell wall, making the cell rigid

• An animal cell may burst under the pressure (contractile vacuoles)

Turgor Pressure

• http://www.emc.maricopa.edu/faculty/farab

ee/BIOBK/BioBooktransp.html

• Hypertonic solution – the concentration of

dissolved substances is higher than the

concentration inside the cell

• Concentration of water is lower than inside

the cell

• Animal cells will shrivel up due to the

decreased pressure in the cells

• Plant cells will lose water from its central

vacuole, causing the plant to wilt

• Plasmolysis – the loss of water resulting in

a drop in turgor pressure

• Hypertonic solution - water goes out of

the cell

Passive Transport

• Passive transport -the net movement of substances across plasma membranes without additional energy

• Includes osmosis

• Transport proteins – proteins embedded in the lipid bilayer of the plasma membrane that allow needed substances or waste materials to move through the membrane

Passive Transport

• Facilitated diffusion – the diffusion of materials across a plasma membrane by transport proteins

• 1. Channel Proteins – tube-shaped proteins that allow molecules to pass through the membrane

• Provides convenient openings for them to pass through

Passive Transport

• 2. Carrier proteins – proteins that pick up ions and

molecules and move them through the membrane

• Have specific shapes and transport only 1 or 2

kinds of ions or molecules

• Change shape to fit the specific molecule it allows

through

• http://www.emc.maricopa.edu/faculty/farabee/BI

OBK/BioBooktransp.html

Active Transport

• Active transport - process in which the

movement of materials across a membrane

requires the expenditure of cellular energy

• Ex: kidney cells sort needed materials from

waste products

Active Transport

• Endocytosis – process in which a cell

surrounds and takes in material from its

environment

• Material is enclosed and engulfed by a

portion of the membrane and cytoplasm

• Material is then enclosed within a vacuole

Active Transport

• 2 types of endocytosis

• Phagocytosis – moves large particles of

solid food or whole cells into the cell

• Ex: white blood cells engulf & digest

bacteria

• Pinocytosis – transports liquid droplets or

small particles

Active Transport

• Exocytosis – the process by which waste

materials are expelled from the cell

• Also used to secrete sacs of molecules such

as hormones

Exocytosis

• http://www.emc.maricopa.edu/faculty/farab

ee/BIOBK/BioBooktransp.html