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Chapter 5 – The Plasma Membrane and Transport
State Standard
• Standard 1.a. – Cells are enclosed within semi-permeable membranes that regulate their interaction with their surroundings
Membrane Function
1. Forms a boundary between living cells and their surroundings.
2. Controls the movement of molecules into and out of the cell
- Selective permeability – allows some substances to cross more easily than others and blocks passage of some substances altogether
Membrane Function
3. Takes up substances the cell needs and disposes of the cell’s wastes.4. Membranes can contain enzymes that function in various chemical reactions.
Membrane Structure
• The plasma membrane is 8 nm thick.
• It is mainly made up of phospholipids and proteins.
Membrane Structure
Phospholipid bilayer
protein
cholesterolc
carbohydrate
Membrane phospholipids
• Phospholipids have a hydrophilic head and two hydrophilic tails.
Membrane phospholipids
• The phospholipids form a bilayer- The hydrophilic heads face outward and the hydrophobic tails face inward
Hydrophobictails
Membrane Proteins
• Membrane proteins are found within the phospholipid bilayer
• They can extend all the way through the bilayer – integral proteins
• They can be found embedded on either surface – peripheral proteins
Integral Protein
Phospholipid bilayer
Peripheral protein
Functions of Membrane Proteins
• Some proteins transport substances across the membrane by forming channels or by physically moving them across the membrane.
Membrane Proteins
Functions of membrane proteins include:• Transport of substances across the membrane• Join cells to one another• Act as enzymes• Bind to chemical messengers from other cells• Act as identification tags so cells can recognize
one another• Attach to cytoskeleton and the extracellular
matrix
Membrane cholesterol and carbohydratesCholesterol• Cholesterol is found within
the phospholipid bilayer• It helps stabilize the
phospholipids
Carbohydrates• Located on outer layer of
membrane only• Bound to proteins
(glycoproteins) or phopholipids (glycolipids)
• Act as identification tags for cell cell recognition
Fluid Mosaic Model
Fluid Mosaic Model
• The membrane is a fluid mosaic• Fluid – the phospholipids and the proteins
drift within the membrane• Mosaic – the diversity of proteins found within
the membrane and the arrangement
of these proteins in the membrane
Transport Across the Membrane
Diffusion• The tendency of particles of any kind to
spread from regions of high concentration to regions of low concentration
• Requires no energy, results from the random motion of atoms and molecules.
Transport Across the Membrane
Passive Transport• Substances diffuse through
the membrane without work by the cell
- They spread from areas of high concentration to areas of lower concentration
- At equilibrium the molecules continue to move but there is no net change in the concentration on either side of the membrane
Molecule of dye
MembraneEQUILIBRIUM
Transport Across the Membrane
• Osmosis is the passive transport of water
• Water travels from an area of lower solute concentration to an area of higher solute concentration
Transport Across the Membrane
• Hypertonic- the solution with the higher solute concentration
• Hypotonic- the solution with the lower solute concentration
• Water moves from the hypotonic solution to the hypertonic solution.
Transport Across the Membrane
• Water continues to cross the membrane until the solute concentrations are equal on both sides of the membrane
• Isotonic- solutions with equal solute concentrations
• Water molecules continue to move across the membrane but at the same rate in both directions
Osmosis Problems – Draw a diagram illustrating the problem, identify the solutions as hyper, hypo or isotonic, and show the direction osmosis will occur
1. A cell containing 2% solute is placed in a solution containing 10% solute.
2. A candy containing 15% solute is placed in a solution containing 12% solute.
3. A cell containing 7% solute is placed in a solution containing 9% solute.
4. A candy containing 10% water is placed in a solution containing 90% water.
5. A fish contains 95% water lives in freshwater that contains 68% water.
How Does Osmosis Affect Cells?Distilled Water 3x Saline
Induces turgor in cellsInduces turgor in animal cellsMakes plant cell pull away from cell wall
Makes plant cell lyse
Makes animal cell lyse
Makes animal cell shrivel
How Does Osmosis Affect Cells?
1. Explain the different outcomes of adding distilled water to the plant and animal cells.
2. Which conditions best model the effects of drought on plant cells? Explain your answer.
3. Hypothesize why plants grow better when the water surrounding their cells is slightly hypotonic to the cell.
Water balance between cells and their surroundings is crucial to organisms
• Osmosis causes cells to shrink in a hypertonic solution and swell in a hypotonic solution.
Transport Across the Membrane
• Small non-polar molecules diffuse freely through the phospholipid bilayer
• Many other kinds of molecules diffuse through pores created by membrane proteins
• This type of transport is called facilitated diffusion
Solute molecule
Transportprotein
Transport Across the MembraneActive Transport
Transport Across the Membrane
• Transport proteins can move solutes across the membrane against a concentration gradient.
- This is called active transport• Active transport requires ATP – cells use energy
to move the molecule across the membrane.• The solute is moved from low concentration to
high concentration.
Transport Across the Membrane
• To move large molecules or particles across the membrane a vesicle may fuse with the membrane and expel its contents – exocytosis
Transport Across the Membrane
• Or the membrane may fold inward, trapping material from the outside, and form a vesicle that carries the material into the cell – endocytosis
Transport Across the Membrane
• There are 3 kinds of endocytosis
Pseudopod of the amoeba
Food being ingested
Plasma membrane Material bound to receptor proteins
Transport Across the Membrane
There are 3 kinds of endocytosis:1. Phagocytosis – a cell brings in
macromolecules, other cells, or particles2. Pinocytosis – the cell takes in fluid and
dissolved solutes3. Receptor-mediated endocytosis – specific
molecules bind to receptors on the membrane that signal to the cell to take the molecule in
• Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors