Biology 3A – membranes

Cells exchange matter and energy

In: Out:Processes:

Cells exchange matter and energy

In:

Oxygen

Nutrients eg glucose

(carbon dioxide and light energy in plants)

Water

Ions

Out:

Carbon dioxide

Wastes eg urea

(oxygen in plants)

Water

Ions

Heat energy

Processes:

Respiration

Synthesis

Growth

Division

Photosynthesis (in plants)

Exchange of material

Occurs at the cell membrane

Involves the intake of essential materials eg nutrients and oxygen and the removal of unwanted materials eg wastes

The processes involved include diffusion, osmosis and active transport

Cell membranes

Sketch a typical cell membrane according to the Fluid Mosaic model

Describe the roles of:• Phospholipids

• Proteins

• Carbohydrates

CellsSketch a typical cell membrane according to the Fluid Mosaic model .

Describe the roles of:Phospholipids = A – waterproof, flexible layer, that lets in small particles

Proteins = B – provides channels for larger particles (facilitated diffusions), acts as pumps (active transport), acts as receptor sites

Carbohydrates = C - acts as receptor sites

A

B

C

Structure of the cell membrane

Fluid mosaic model

Describe the roles of:• PhospholipidsWaterproof barrier, acts as a bubble, main

constituent

• Proteins Act as carriers, receptor sites, antigens,

channels

• CarbohydratesAct as receptor sites, antigens

Evidence for membrane structure

• Biochemical analysis

• Electron microscopy

• Fluorescent and radioactive markersFluorescent markers added to cell membrane

Cells merged so markers on 1 side

Markers have moved around the entire membrane

Markers found on othermembranes within cell

Exchange of material

Small particles eg water, ions can pass through the gaps between the phospholipids

Larger molecules pass through channels in the proteins or are carried by special proteins

TerminologySolution

Solute

Solvent

Permeable

Differentially permeable

Hypotonic

Hypertonic

Isotonic

Mixture of solvent and solute

Thing added to solvent

Liquid to which solute is added

Allows substances to move through

Allows some substances to move through, but not others

Less concentrated (dilute)

More concentrated

Concentrations are equal

Movement through membranes

Diffusion

Draw a diagram to show how materials are exchanged by diffusion

Movement of particles - diffusion

Occurs without energy

Particles will move from higher concentrations to lower concentrations

Diffusion through membranesMembranes are differentially permeablePermeable substances (eg small particles) Impermeable substances (eg large particles)Movement is Energy

Diffusion through membranesMembranes are differentially permeablePermeable substances (eg small particles) Impermeable substances (eg large particles)Movement is Energy

Move through

Can’t move throughFrom areas of high concentration to

areas of low concentration

Is not needed

Facilitated diffusion

Allows larger or less permeable substances to cross

Involves channel proteins

Energy is not needed

Osmosis

Draw a diagram to show how materials are exchanged by osmosis

OsmosisMovement of waterThis occurs from areas of high water concentration (low

concentration of solute) to areas of low water concentration (high concentration of solute)

Concentration gradient

This refers to the difference in concentration across membranes

The steeper the gradient,

the greater the movement

Osmotic pressure refers to the pull of water due to concentration gradients

Active transport

Draw a diagram to show how materials are exchanged by active transport

Active transport

Movement against a concentration gradient.Energy is needed

Ion pumps

Involves special proteins that act as pumps

Allows cells to maintain ion concentrations that are very different from the external environment

Energy is used

EndocytosisMovement due to folding of membrane and bringing in

the trapped substancesEnergy is needed Phagocytosis – solid substances trappedPinocytosis – liquids are trapped

Exocytosis

Movement due to vacuoles moving to membrane and attaching to release contents

Energy needed

Exchange of material

Show the movements of water and ions that would occur in both these cases

Distilled water Sea water

Exchange of material

Show the movements of water and ions that would occur in both these cases

Distilled water Sea water

Water moves in

Water moves out Ions move inIons move out

Determining the components of the cell membrane

Beetroot lab

acid base

water Distilled water

detergent

Salt water

Beetroot experiment

• What hypothesis is being tested?

• What is the control?

• What is the independent variable?

• What is the dependent variable

• What variables are controlled?

• How will this tell you about the components of the cell membrane?

Size and surface area

As size increases what happens to surface area to volume ratio?

Why is this important?

Size and surface area

As size increases what happens to surface area to volume ratio? SA:Vol decreases

Why is this important? Exchange occurs at surfaces – the larger the SA:Vol, the more exchange will occur

Shape and surface areaWhat happens if cells change shape?

2x 2x2

1x1x8

4x2x1

Shape and surface areaWhat happens if cells change shape? – alters surface area

2x 2x2

1x1x8

4x2x1

How cells increase their surface area

• Size • Shape• Folding • Cell extensions

pseudopods

microvilli