The Cell Membrane

Cell Membrane

Also known as the plasma membrane

In both plant and animal cells

Main function: to protect the cell

Made of a lipid (fat) double layer with special embedded proteins

Cell Transport

The membrane also controls what goes in or out of the cell (cell transport)

It is semi-permeable Some molecules, like

oxygen & water, are permeable go directly through the membrane

Other molecules, like glucose & salt, are impermeable and only pass through a protein

Cell Transport

There are lots of ways substances enter or exit a cell

It depends on the size of the substance:

Small Particles move through the membrane by: Passive or Active Transport

Large Particles move through the membrane by: Endocytosis or Exocytosis

Passive Transport

Does not require energy (thus passive…)

Two Types1. Diffusion2. Facilitated Diffusion

1) Diffusion

Diffusion is when molecules spread from an area of high concentration to an area of low concentration

This is called a concentration gradient

1) Diffusion

Think about spraying air freshener into a room. It gradually spreads out. Same thing if someone farts…

Or, if we pushed everyone here into a corner, but let you move where you wanted, you would tend to spread out around the room instead of staying crammed into the corner. You’re moving from an area of high concentration (the corner) to an area of low concentration (the rest of the room).

1) Diffusion

1) Diffusion

Diffusion occurs in cells through the membrane.

Oxygen needed for cellular respiration enters cells by diffusion

CO2 produced by cellular respiration leaves cells by

diffusion.

Osmosis

Osmosis is when water diffuses through a membrane.

Think about how important it is for water to get into and out of cells.

2) Facilitated Diffusion

Sometimes, molecules can’t get through the membrane without a little help (facilitation…)

In facilitated diffusion, a special protein embedded in the membrane helps molecules move across

No energy is required, this is still passive transport

Molecules such as glucose, sodium & chloride ions get into the cell this way

Active Transport

Requires energy (ATP)

Sometimes, cells have to move a substance against the concentration gradient or from an area of low to high concentration

Active Transport

Active transport also occurs through a special protein or pump, like with facilitated diffusion

This is one of the main reasons why cells require energy – they need the ability to remove substances.

For example, kidney cells filter & remove salt from your blood through active transport.

Passive versus Active Transport

Large Particles

Sometimes particles are too big to simply cross the membrane.

Instead, they must get pulled into or pushed out of the membrane in one of two ways1. Endocytosis

2. Exocytosis

1) EndoCytosis

Occurs when large particle(s) are captured by a pocket in the membrane The particle is too big to fit

through a protein channel

The pocket breaks off & forms a package that moves into the cell

Requires energy

Essential nutrients, like iron, are absorbed into cells this way

1) EndoCytosis

2) ExoCytosis

Occurs when a large bit of material needs to be removed from a cell

A package of the materials travels through the cell to the membrane, joins it, and is expelled

Opposite of endocytosis

Requires energy

Cells use this to flush out waste such as hormones

2) ExoCytosis

Big Ideas

1. What are two functions of the cell plasma membrane?

2. What types of substances need to be transported across the membrane?

3. How does a particle’s size affect how its transported through a cell?

4. List 3 differences between passive & active transport.