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