Cell Boundaries

Biology chapter 7.3

Mr. Hines

Where does the cell begin and end

Animal cells – the cell membrane separates regions between cells.

All animal cells will have a thin, flexible layer which holds all of the materials inside. (made of a lipid bilayer)

Embedded within the cell membrane are various protein and carbohydrate molecules.

Outsideof cell

Insideof cell(cytoplasm)

Cellmembrane

Proteins

Proteinchannel Lipid bilayer

Carbohydratechains

Section 7-3

Figure 7-12 The Structure of the Cell Membrane

Plant cellsAll plant cells will have a cell membrane

AND a second layer called the “cell wall.”The cell wall adds a layer of protection and

adds support (made of carbohydrates). The term plant cell can be misleading since

other forms of life will also contain a cell wall.

Fungi, algae, some protists, and some prokaryotes will have a cell wall too.

Diffusion through cell walls

The most important function of the cell membrane is to select what materials can enter the cell and exit the cell.

In other words, the cell membrane acts as a complicated security gate.

Measuring concentration

Important terms

Solute

solvent

solution.

The solute is measured in grams and the Solvent in measured in Liters.

Therefore a concentration measurement will include these units.

For example –

Solution A has 3grams/Liter of Salt (g/L)Solution B has 6 grams/Liter of salt. (g/L)Which is more concetrated?Which is more dilute?

Diffusion

Because all of the molecules within a solution are moving around randomly, they will eventually come to equilibrium.

Equilibrium is when a solution is evenly mixed so that the concentration is the same all over. (equal)

This process is called diffusion.

• Click the image to play the video segment.

Video 1

Diffusion

Diffusion can happen through a membrane if the membrane is

permeable to a substance.Diffusion is important because a cell can get

materials in and out of a cell without using energy.

If a membrane allows some substances through, but not others, it is said to be “selectively permeable.”

If a membrane allows a substance through, the membrane is said to be “permeable” to the substance.

If a membrane does NOT allow a substance through, the membrane is said to be “impermeable” to the substance.

Selectively permeable

diffusionFor example: A cell membrane will allow salt

molecules through it, but not Carbohydrate molecules.

Therefore the membrane is permeable to salt molecules and impermeable to carbohydrate molecules.

In this case, the salt molecules would move across the membrane until both concentrations are equal (equilibrium)

Diffusion

What does diffusion have to do with the cell membrane?

Remember that the cell membrane allows materials into the cell.

Diffusion can cause molecules to move through a membrane so that the concentration outside of the membrane is equal to the inside.

This can happen in both directions (inside/ouside)

Osmosis

Osmosis is the diffusion of water through a selectively permeable membrane.

• Click the image to play the video segment.

Video 2

Osmosis

How osmosis works

Important terms

Isotonic – condition when concentration are the same on both sides of the membrane.

Hypertonic – name given to the solution on one side of the membrane that is above strength.

Hypotonic – name given to the solution on one side of the membrane that is below strength.

When there is a hypertonic/hypotonic condition, water will move across the membrane until an Isotonic condition is reached.

This is called “osmotic pressure.”

Question

Too much of any substance can “poison” a living thing.

If you drink too much water, you can die.

Think of how this might happen using the concepts of osmosis.

Section 7-3

Figure 7-15 Osmosis

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Facilitated diffusion

How can molecules such as carbohydrates get across the membrane if they are too large for diffusion?

There are protein molecules embedded within the cell membrane which acts as a doorway (has enlarged entrance)

These proteins are referred to as “protein channels.”

The protein helps the carbohydrate across the membrane – facilitated diffusion.

Facilitated diffusion is still a form of diffusion – even though it is assisted.

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Active Transport

If substances can not get across the membrane by diffusion, there must be another other methods.

If a cell needs to move materials across the membrane against the concentration difference – it is called active transport.

Active transport requires energy

Active transport is carried out by special proteins on the cell membrane.

These proteins are there for the purpose of “pumping” certain chemicals in and out.

The shape of a protein can play an important role in active transport.

Energy can change the shape of a protein which causes release of various chemicals inside/outside a cell.

• Click the image to play the video segment.

Video 4

Active Transport

Molecule tobe carried

Moleculebeing carried

Energy

Section 7-3

Figure 7-19 Active Transport

EndocytosisEndocytosis – method for transporting large

amounts of materials into the cell by means of infolding of the cell membrane.

There are 2 kinds of Endocytosisa. Phagocytosis – This is when a cell partially

surrounds material with its cell membrane and engulfs it (cell eating).

b. Pinocytosis – pockets of the cell membrane fill with liquid and then pinch of inside of the cell. (cell drinking)

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Exocytosis

Exocytosis is the process by which cells move materials from the inside of the membrane to the outside (exit).

• Click the image to play the video segment.

Video 5

Endocytosis and Exocytosis

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