Cell Membrane and Transport Maintaining homeostasis and providing nutrients to cells.

Cell Membrane and Transport

Maintaining homeostasis and providing nutrients to cells

Cell Membrane Structure

The cell membrane is composed of lipids and proteins.

The lipids are arranged in a bilayer.The bilayer is a “barrier” that is

impermeable to most molecules.The proteins are embedded in the

bilayer.Specific molecules can be helped across

the membrane by these proteins.

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What are membranes?

keeping all cellular components inside the cell

allowing selected molecules to move in and out of the cell

allowing a cell to change shape.

isolating organelles from the rest of the cytoplasm, allowing cellular processes to occur separately.

Membranes cover the surface of every cell, and also surround most organelles within cells. They have a number offunctions, such as:

a site for biochemical reactions


Membranes: timeline of discovery


It was discovered (using images from a TEM) that the phospholipid “heads” face the exterior/interior of a cell and the phospholipid “tails” are in the middle.

2nd cell membrane

1st cell membrane

intracellular space (blue)

1 light layer = phospholipid tails

2 dark layers: phospholipid heads

Evidence for the Fluid Mosaic Model

This picture shows the cell membranes of two adjacent cells.


Evidence from freeze-fracturing

E-face: looking up at outer layer of membrane

This revealed a smooth surface with small bumps sticking out. These were later identified as proteins.

In 1966, biologist Daniel Branton used freeze-fracturing to split cell membranes between the two lipid layers, revealing a 3D view of the surface texture.

P-face: looking down on inner layer of membrane


The fluid mosaic model

This model suggested that proteins are found embedded within, not outside, the phospholipid bilayer.

The freeze-fracture images of cell membranes were further evidence against the Davson–Danielli model.

They led to the development of the fluid mosaic model, proposed by Jonathan Singer and Garth Nicholson in 1972.

E-face

P-face protein


Exploring the fluid mosaic model

The chemical properties of lipids determines the bilayer nature of the cell membrane.

Cell membranes contain special lipids known as phospholipids

Phospholipids: Composed of two fatty acid chains attached to a glycerol

molecule and phosphate group.

Fatty Acid Chains

Glycerol

Phosphate Group

Phospholipids, continued:

Have a hydrophilic “head” that “loves” water (both are polar)

Hydrophilic heads form outside of layer so they can touch water inside and outside cell

Have a hydrophobic “tail” (hydrocarbon chain) that “fears” water (is non-polar)

Hydrophobic tails face interior of bilayer so they can avoid water

Phospholipid Bilayer

Role of the Cell Membrane

The cell membrane is described as “selectively permeable”How does this feature relate to the

job/function of the cell membrane?Cell membrane acts as a “guard”

Allows nutrients into cellAllows for removal of wastes and release of

substances made by the cell that are needed by other cells.

Factors that affect Passive Transport:

Whether a molecule can move through the membrane depends on:

1) the size of the molecule

2) the type of molecule (polar or nonpolar, charged, etc.)

Molecules move by one of the following methods: diffusion, facilitated diffusion, or osmosis.

The direction of movement (in/out) depends on the concetration gradient.

Concentration Gradient

Concentration Gradient = a difference in concentration of a substance in one area compared to another.

Direction of movement:

Out of the blood, into the lungs

All 3 types are passive transport

Passive Transport: movement of substances without any energy input by a cell.

1. Diffusion: molecules move straight through the membrane.

2. Facilitated diffusion: molecules or ions move through protein channels embedded in the membrane.

3. Osmosis: water molecules move through the membrane (mostly through protein channels).

Methods of Transport

Direction of transport

In passive transport, the net movement is always “down the concentration gradient”.Molecules move from an area of higher

concentration to an area of lower concentration.

It’s “passive” because it doesn’t require cellular energy.

Net Movement

Think of dye or sugar molecules in water. (Water particles NOT shown.) Dye or sugar molecules will DIFFUSE through the water.

What happens in the alveoli?

Example: Transport of Oxygen

Concentration gradient for O

2

HIGH

LOW

Example: Osmosis

If the membrane is permeable to both water and solutes, both will diffuse to reach equilibrium.

Often, the membrane is NOT permeable to the solute(s). In this case osmosis occurs; water diffuses (high to low) to balance the concentration on both sides. (egg lab)

LO

W

HIG

H

Net movement

Predicting osmosis

Osmosis in action

Example: Transport of Glucose

HIGH

LOW

Net m

ovement

Facilitated Diffusion:

Integral Proteins can “facilitate” or assist in transporting a substance by:1. acting as a channel or tunnel2. acting as a carrier or transporter

Diffusion through a carrier protein

Active Transport

Cells move molecules from an area of LOW concentration to an area of HIGH concentration

Molecules move AGAINST the concentration gradient

Requires the cell to use energy in the form of ATP

Animation


What is active transport?

Substances can move passively in and out of cells by diffusion until the concentration on both sides of the cell membrane reaches an equilibrium.

Substances can continue to move in and out of a cell using a process called active transport.

As the name suggests, active transport requires energy from the cell, which is made available by respiration (ATP).

During active transport, protein carriers in the cell membrane ‘pick up’ particles and move them against the concentration gradient.


What is active transport?


Active transport in plants

Plants need to absorb mineral elements such as nitrogen, phosphorus and potassium from the soil for healthy growth.

When the concentration of minerals in soil is lower than inside the plant, active transport is used to absorb the minerals against the concentration gradient.

What would happen if the plant relied on diffusion to absorb minerals?

The cells would become drained of minerals because they would travel down the concentration gradient.

minerals

Example: Sodium-Potassium Pump

Three Na+ ions in the cytoplasm bind to carrier protein Shape of protein is changed, allowing the three Na+ out

of cell Two K+ ions outside of cell bind to protein Shape of the protein is changed The two K+ are allowed into the cytoplasm Similar to facilitated diffusion:

Uses a carrier protein, Different from facilitated diffusion:

requires energy. Overall: Na+ (sodium) becomes concentrated on the

outside of a cell. Important in the proper functioning of neurons and the kidneys.

Animation 1

Animation 2

Other types of active transport

Not all active transport moves molecules from a low concentration to a high concentration

Active transport used in two other situations:Moving very large molecules through

membraneMoving large quantities of smaller

molecules through membrane

Other examples of active transport:

Endocytosis: Process in which cells ingest fluids, macromolecules, and large particles that are outside the cell

Animation 1

Animation 2

Other examples of active transport:

Exocytosis: how cells release large molecules (proteins) or get rid of large amounts of wastes

Cell Membrane and Transport Maintaining homeostasis and providing nutrients to cells.

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