Bio- Chapter 3

LI:

To learn about the structure and function of the

plasma membrane.

SC: I can

- Describe the structure of the plasma membrane

- Explain the fluid mosaic model

- Outline the function of the plasma membrane

and its components

The Plasma Membrane

All cells have a plasma (cell) membrane separating the intracellular

and extracellular environments. Plasma membranes have 3 functions:

1. Control what enters/exits the cell (semi-permeability)

Permeable to lipo-soluble or very small molecules

Impermeable to ions or big molecules

2. Provide shape and structure

3. Maintain homeostasis (stable internal environment)

The Fluid Mosaic Model is accepted as the plasma membrane structure.

The membrane is fluid as components can move laterally, and mosaic as

a range of molecules are embedded to provide specific functions.

Peripheral/

/INTEGRAL

Phospholipid Bilayer

Phospholipids are the main component of the plasma

membrane, and are composed of:

A phosphate head

- Made of glycerol and a phosphate group

- Negatively charged, hydrophilic (water-loving) and polar

Two fatty acid tails

- Uncharged, hydrophobic (water-fearing) and non-polar

- Flexible, not solid at room temperature

As phospholipids are amphipathic, they organise into two

layers (a bilayer) with the hydrophilic portions in contact

with the watery exterior and interior of the cell to form a

stable plasma membrane.

Proteins

Proteins are amino acid sequences folded into specific shapes specific to their function. Membrane

proteins are categorised into 2 groups depending on how they attach to the plasma membrane:

● Permanently

embedded in the

membrane.

● Called transmembrane

if they span the bilayer.

● May act as pumps or

channels (in facilitated

diffusion)

● Temporarily attached to

the plasma membrane

surface, and connected

often to the cytoskeleton.

● Assist with transport

and communication

Carbohydrates

Carbohydrates are used for cell recognition, so our bodies can tell if the cells belong to

us (self cells) or are foreign cells that may cause harm. Glycoproteins are also used for

intracellular joinings (connections between cells) and anchorage.

Proteins & lipids on the extracellular side of the plasma membrane surface often

have short chains of carbohydrates (sugars) attached to them, forming:

Cholesterol

● A lipid steroid embedded between fatty acid tails in animal cells

● Regulates the fluidity (fluid movement) and permeability

(movement of molecules) of the plasma membrane.

- High Temperatures: keeps phospholipids packed together

- Low Temperatures: disrupts membrane from being solid

3B

Passive Transport

LI:

To learn how molecules are transported across

the plasma membrane without energy.

SC: I can

- Contrast active and passive transport

- Explain the processes of diffusion, facilitated

diffusion and osmosis

Passive Transport

does not require energy.

Active Transport

requires energy.

Permeability

Remember, plasma membranes are

semi-permeable, so it allows some

substances through but not others.

The rate of diffusion depends on:

● Molecule size (smaller = faster)

● Concentration gradient (steeper = faster)

● Temperature (hotter = faster)

Permeable to... Impermeable to...

- Non-polar

- Hydrophobic

- Small

Eg water, oxygen

or carbon dioxide

- Polar

- Hydrophilic

- Large

Eg sugar, ions or

amino acids

Concentration Gradients

When molecules are concentrated on one side we say it has

a high concentration. When molecules are not concentrated

on the other side we say it has a low concentration.

The difference between the high and low concentration is

called the concentration gradient.

When molecules move from an area of high concentration to

low concentration, we call this going down the concentration

gradient.

Simple Diffusion

Diffusion is the passive movement of molecules (solutes) across the phospholipid bilayer

from an area of higher concentration to an area of lower concentration. This occurs until

equilibrium is achieved, where concentration is equal inside and outside of the cell.

Facilitated Diffusion

Protein channels are narrow, water-filled pores in the

membrane that open and create a hydrophilic passageway

to let a specific substance through only after binding with

the relevant molecule (can be gated).

Carrier proteins bind to the substance being transported

and undergo a conformational change (change shape) to

push the substance through to the other side of the

membrane, before returning to their original shape.

Substances impermeable to the plasma membrane are ‘helped’ into or out of cells by facilitated

diffusion, which is the passive movement of molecules from an area of higher concentration to an

area of lower concentration through a membrane-bound protein.

Osmosis

Osmosis is the passive movement of water from

low solute concentration (salt or sugar) to high

solute concentration.

Water molecules can move through the phospholipid

bilayer despite being hydrophilic due to how small

they are. Water movement can be increased through

water-specific transmembrane proteins called

aquaporins.

For solutes that struggle to cross the semipermeable

plasma membrane, osmosis is important as diluting

(adding water to) concentrated molecules will help to

equalise the concentration gradient inside and outside

of the cell.

Tonicity

In osmosis, we are always comparing the solute

concentration between two solutions. There are 3 types

of tonicity (comparisons) that characterise a solution:

Hypertonic solutions have higher solute concentrations

than the cell, so water moves towards the area of high

concentration outside the cell.

Hypotonic solutions have lower solute concentrations

than the cell, so water moves towards the area of high

concentration inside the cell. .

Isotonic solutions have equal concentrations of solute

to the cell, so there is no net movement of water. This

means water flows equally in and out of the cell.

Effects of Tonicity

The tonicity of solutions can impact cell size.

*Note: plant cells have a rigid cell wall that animal cells do not and so

their reaction will vary.

Plasmolysis

In hypertonic environments, plant and animal cells lose water. Animal

cells shrivel and die, while plant cells (protected by cell walls) lose the

water in its large vacuole and all cell contents shrink to the centre. This

makes a plant look wilted (plasmolysed).

Turgor Pressure

In hypotonic environments, plant and animal cells gain water. Plant cells

expand slightly and becomes rigid as the vacuole pushes against the cell

wall (turgor pressure), which prevents it from bursting. This causes a

plant to stand upright. As animal cells lack cell walls, they also lack

turgor pressure, so the cell will swell and most likely die.

Example: Red Blood Cells

If red blood cells are placed in freshwater, the cells absorb so much

water that they swell and can eventually burst. If red blood cells are

placed in a solution more concentrated than their cytosol, water leaves

the red blood cells and causes them to shrink.

3C

Active Transport

LI:

To understand active transport.

SC: I can

- Explain the process of active transport

- Understand the importance of active

transport in living things

Passive Transport

does not require energy.

Active Transport

requires energy.

Active Transport &

Concentration Gradients

Passive Transport:

From high → low concentration (down conc. gradient)

Active Transport:

From low → high concentration (against conc. gradient)

Active transport is the movement of substances across

the cell membrane against the concentration gradient,

from an area of low concentration to an area of high

concentration. This process requires energy (often

adenosine triphosphate, ATP), and protein pumps or

carrier proteins.

There are 2 types of active transport:

1. Protein-Mediated Active Transport

2. Bulk Transport

Active Transport

If there is a big difference in the concentration of substances

inside the cell compared to outside the cell, the cell must use

energy and protein pumps to move impermeable substances (ie

ions) against their concentration gradient into the cytoplasm.

Specialised transport proteins are used for active transport,

with each substance requiring a specific pump to force specific

molecules to move against their concentrations gradient.

Some pumps have enzymes attached which are used for an

energy-releasing reaction (ATP → ADP + Pi + Energy)

Active transport occurs in 3 steps:

1. Binding: the target molecule binds to its specific protein pump

2. Conformational change: the protein pump changes shape

using energy from the energy-releasing reaction.

3. Release: the target molecule is pushed through the protein

and released to the other side of the membrane.

The Sodium-Potassium Pump

Bulk Transport

1. Exocytosis (export materials OUT OF the cell)

This is an important process as cells must release

products like hormones, neurotransmitters and

antibodies, or molecules too large for even protein

channels, in large amounts.

Three Steps of Exocytosis:

- Vesicular Transport: a vesicle containing secretory

products from the Golgi Apparatus is transported to

the cell membrane.

- Fusion: vesicle fuses with plasma membrane

- Release: secretory products are released from the

vesicle outside the cell.

Bulk Transport is a type of active transport that involves the movement of large molecules (ie amino acids, proteins,

signalling molecules or pathogens) or groups of molecules across the plasma membrane using vesicles. There are two

forms of bulk transport:

Bulk Transport

2. Endocytosis (importing materials INTO the cell)

This process is essential as many substances too large for protein

channels must enter the cell to be broken down for metabolic process or

used as a structural element of the cell. There are two types of

endocytosis:

- Phagocytosis (‘cell eating’) is when vesicles containing large bacterial

cells or food particles fuse with lysosomes to be digested (ie white blood

cells defence).

- Pinocytosis (‘cell drinking’) is when engulfed molecules are dissolved in

extracellular fluid.

Three Steps of Endocytosis:

- Fold: cell membrane forms a cavity fills with fluid and target molecules.

- Trap: cell membrane continues folds back on itself until the two ends of

the membrane meet and fuse, trapping target molecules in the vesicle.

- Bud: vesicle pinches off from the membrane, to be transported to the

appropriate cellular location or fused with a lysosome for digestion.