CP BIO: Ch. 7 The Cell Membrane

and transport across the membrane

The Plasma membraneBoundary between the cell and its environment

FLUID MOSAIC MODEL - Describes structure of cell membranes

• “mosaic” – sea of lipids with scattered proteins • “fluid” – molecules float and move around within

the layer

PhospholipidPhospholipid = lipid molecule with phosphate on one end

• glycerol + two fatty acids + phosphate group

• phosphate group = polar end

(hydrophilic “head”)

• Fatty acids = nonpolar end

(hydrophobic “tails”)

Phospholipids form a double layer

Nonpolar tails are on the inside (away from water)Polar heads are on the outside ( touch water)

Features of the Cell Membrane

• Semipermeable = some substances can pass through - some cannot- depends on: molecule size, charge, polar or nonpolar

• Cholesterol – molecules scattered among the phospholipids

- in animal cells

- keep membrane flexible in changing temperatures • Carbohydrates – glucose chains on outside of cell - Identification “tags”

- Receptor sites for messenger molecules

Fluid Mosaic Model

Membrane Proteinshave many functions

Transport Enzyme Receptor Allows a specific Catalyzes a Site for molecule to reaction inside messenger pass through the cell molecule to the membrane attach

Membrane proteins (2)

Identification Junctions Structure“SELF” – cell belongs Cells join to Keeps internal in this organism, form tissues, parts organized immunity communicate

How do membranes keep homeostasis?

Cell membranes are selectively permeable• The lipid layer blocks most

substances• Some molecules can cross the

membrane– By passive or active transport

• Some are too big to cross at all

PASSIVE TRANSPORTUSES NO CELL ENERGY

Molecules move randomly – spread out until evenly distributed• From an area of higher concentration to an area of

lower concentration

DIFFUSION Diffusion: movement of particles from an area

of high concentration to an area of lower concentration

• Two areas with different concentrations = concentration gradient

• Particles move in all directions (random)• NET movement is from high concentration to low• “Down the concentration gradient”

Diffusion

• Particles spread out until evenly distributed equilibrium (homogeneous)• AT equilibrium, particles continue moving, but

in all directions equally NO further change in concentration

Cell Membranes allow some particles to cross

Particles can diffuse across the lipid bilayer if they are:• Small • Nonpolar (lipid-soluble) Examples: CO2 , O2, fatty acids can diffuse easily

FACILITATED DIFFUSIONTransport proteins “help” particles move across

the membrane – DOWN their gradient • PASSIVE transport – no cell energy used• Proteins are SPECIFIC

– each allows only a certain substance to pass

Particles can cross by facilitated diffusion if they are:• Small • POLAR (water soluble) or CHARGED (ions)Examples: H2O, glucose, Ca+2, Cl-

Transport Proteins• Pores and Channels – open path through membrane• Carrier proteins – take particle on one side of

membrane and release on the other

OSMOSISDiffusion of water across a membrane Important process in cell homeostasis

Water crosses the cell membrane easily- Small enough to pass between lipid molecules- Also pass through special proteins, aquaporins

Why osmosis matters• Water crosses membrane easily, faster than

many solutes• Will try to reach equilibrium • If NET water moves into or out of cell changes

homeostasis• Unequal water on one side of cell membrane =

osmotic pressure

TONICITY = osmotic pressure in cells

ISOTONIC Equal concentrations of solutes inside and outside cell– Equal concentrations of water– Water goes in and out of cell at equal rates

Isotonic pressure in cells

• No NET movement of water into or out of cell

• Normal water pressure in animal cells

• Wilted (“flaccid”) water pressure in plant cells

When solute concentration is different on two sides of a membrane

lower solute concentration = hypotonicHigher solute concentration = hypertonic

Solutes will move down their gradient IF THEY CAN CROSS THE MEMBRANE

Water concentration is OPPOSITE of solutes

Low solutes HIGH WATER concentration High solutes LOW WATER concentration

Water WILL diffuse down its gradient and crosses the cell membrane easily

Goes TO whichever side has more solutes

Cells in hypotonic solutions

Cytoplasm swells - animal cells: swell, may burst (“lyse”) - plant cells: high osmotic pressure “turgor” - won’t burst - cell wall - “Turgid” – stiff and firm, upright stem

Solutes are lower outside cell, water is higherWater enters cell by osmosis

Cells in hypertonic solutionsSolutes are higher outside cell, water is lower

Water leaves cell by osmosis

Cytoplasm shrinks - “plasmolysis” - animal cells: shrivel - plant cells: low turgor pressure - cytoplasm pulls away from cell wall - but cell wall does not shrink

Osmosis in Animal Cells Animal cells like ISOTONIC conditions best

Osmosis in Plant Cells Plant cells like HYPOTONIC conditions best

Contractile Vacuoles

Fresh-water protists (like Paramecia or Amoeba) must constantly remove water that comes into the

cell by osmosis

ACTIVE TRANSPORT – uses cell energy

Two kinds:1. Molecule transport against the gradient

a. from low concentration to highb. pushed across by membrane proteins (pumps)

c. uses energy ATP d. small molecules and ions

Why would cells use active transport?1) To concentrate substances: Examples: kidneys concentrate wastes in urine, - intestine concentrates nutrients in blood2) To maintain an ion concentration

- sodium-potassium pump – allows nerve impulses

2) Bulk Transport – uses energy

To bring larger particles into or out of cellEndocytosis = brings material into cell - fold cell membrane around it form a vacuole a. Phagocytosis = “cell eating” - large particles or whole cells - examples: amoeba, white blood cells b. Pinocytosis = “cell drinking” - small folds of membrane take in liquids

Pinocytosis – “cell drinking”

Example: small intestine absorbs some water this way

Exocytosis – send OUT of cellVacuole containing substance fuses with cell membrane opens to outside of cell - for cell secretions - Examples: hormones from endocrine glands digestive juices from pancreas or intestine