Chapter 5Cell membrane structure and function

Chapter 5Cell membrane structure and function

Copyright © 2008 Pearson Prentice Hall, Inc.

Teresa Audesirk • Gerald Audesirk • Bruce E. ByersTeresa Audesirk • Gerald Audesirk • Bruce E. Byers

A brown hermit spider

and a rattlesnake

ready to attack.

Plasma membrane

• Protein categories according to their function:

– Receptor proteins– Recognition proteins– Enzymes– Attachment proteins – Transport proteins

Proteins

• Receptor proteins– They trigger a sequence of chemical

reactions with specific molecules. Example: hormones.

• Recognition proteins– They select the different molecules of

substances and tag them.

Receptor protein activationreactions

(interior)

(exterior)

• Enzymes– They catalyze chemical reactions that

synthesize or break up biological molecules.

• Attachment proteins – They bond the plasma membrane with the

cytoskeleton filaments.

• Transport proteins– There are 2 kinds of transport proteins:

channel proteins and carrier proteins. – They regulate the movement of hydrophilic

molecules.

How do substances move across membranes?

– Molecules in fluids move in response to gradients.

– The transport through membranes could be active or passive.

– Passive tranport do not requiere energy, and it includes: simple difusion, facilitated difusion, and osmosis.

– Active transport requires energy to move molecules against concentration gradient. It includes: endocytosis and exocytosis.

moléculade agua

Una gota de colorante se coloca en agua

Las moléculas de colorante se difunden en el agua; las moléculas de

agua se difunden en el colorante

Las moléculas de agua y de colorante

están dispersas de manera uniforme

gota de colorante

Dye in water diffusion

Passive transport:

Diffusion of substances through cell membranes occur down concentration gradients.

The phospholipids and the proteic channels of the plasma membrane control which ions or molecules can cross, but do not control the movement direction.

Active transport:

Substances travel through the cell membrane against the concentration gradient.

• Simple diffusion includes lipid-soluble molecules(vitamins A y E), dissolved gasses and water.

Kinds of passive transport

Gases like CO2 and Oxygen can pass through the phospholipidic bilayer.

• Facilitated diffusion includes water, ions, and water soluble molecules (like sugars or AA) via channel or carrier proteins.

Kinds of passive transport

This protein pores let some water soluble molecules, and mainly ions pass through the membrane.

AA and sugars getting inside the cell by facilitated diffusion.

c) Difusión facilitada a través de un portador

aminoácidosazúcares

proteínas pequeñas

(fluido extracelular)

Proteína portadora con sitio de unión para la molécula

(Citosol)

proteína portadora La molécula entra

en el sitio de unión. La proteína portadora cambia de forma, transportando la molécula al otro lado de la membrana.

La proteína portadora recupera su forma original.

• Osmosis includes water from regions of higher concentration to regions of lower concentration.

Kinds of passive transport

Isotonic solution

There is no water flow.

• Isotonic solutions have the same [] .

• Hypertonic solutions have more [] .

Water flows inside hypertonic solutions.

• Hypotonic solutions have less [] .

[ ] comparison

Hypotonic solution outside, hypertonic solution inside the bag. The bag swells and bursts.

Osmosis effects: Red blood cells in salty water.

Equal movment of water into and out of cells.

a) Isotonic solution

b) Hypertonic solution Water moves out of the cells.

c) Hypotonic solutionWater moves inside

the cells.

• Osmosis explains why sweet water protists have contractile vacuoles.

• Water filters continuously because the cytosol is hypertonic in relation to the sweet water they live in.

• The salts are pumped to the vacuoles and that makes them hypertonic in relation to the cytosol.

• Water, by osmosis fills the vacuole and then its expelled by contractions.

Kinds of passive transport

a) Water gets inside the protist by osmosis. Inside the cell, water is trapped by colector conducts and drained to the central vacuole.

b) When the deposit si full, it contracts and water is expelled by a pore in the plasma membrane.

• In active transport, proteins from the plasma membrane cross using ATP.

– Proteins that help in active transport (transport proteins) usually have 2 active spots, one is attached to the molecule and the otherone is attached to an ATP molecule. They are called pumps.

Active transport

Active transport works against concentration gradient. The transport protein (blue) has an ATP binding site and a recognition site for the molecules that need to be transported. Example: (Ca2+). When ATP donates its energy, it looses its third phosphate group and becomes ADP + P.

Endocytosis

• Cells can get fluids or particles from its environment by endocytosis.

– Kinds of endocytosis:• Pinocytosis• Receptor-mediated endocytosis• Phagocytosis

Pinocytosis

Pinocytosis

1 There are receptor proteins for specific molecules located in coated pits.

2 2 In the cytosol there is a vesicle released (“recovered vesicle”) that contains attached molecules.

3 The coated pit region encloses the molecules that are attached to the receptors.

4 The receptors are attached to the molecules in the membrane and they get inside the cell.

Phagocytosis

An Amoeba (a sweet water protist) eats a Parameciumby phagocytosis.

Phagocytosis

A white blood cell ´eats´ bacteria by phagocytosis.

Exocytosis is the opposite process to endocytosis. In exocytosis the vesicle expells the material out of the cell.

Cell size and shape

• The exchange of fluids affects the size and shape of the cell. – When spherical cells grow, their internal regions

get farther from the plasma membrane. – A very big cell has a relatively smaller area to

exchange nutrients than a small cell. Example: skin of a teenage boy and the skin of an obese man.

Volume and surface area relations.

How do specialized junctions allow cells to connect and communicate?

– Desmosomes are tight unions that join certain animal cells.

– The tight unions make cell attachments leakprof. (leaky skin or leaky bladder)

– Gap-junctions are cell to cell channels that connect their interior. They accept hormone traffic.

– Plasmodesmata are openings in the walls of adjacent plant cells, lined with plasma membrane and filled with cytosol.

Desmosome Thin intestine

Cells that cover the small intestine

microvilli

.

microvilli

Small intestine cells

Plasma membranedesmosome

Proteic fibers the connect cells

Proteic filaments in the cytosol

Urinary bladder

Urinary bladder cells

Células que revisten la vejiga

membranas plasmáticas

(corte)

Las uniones estrechas, formadas por fibras proteicas, sellan las membranas

de las células

FIGURA 5-18b (parte 2) Estructuras de unión de las células

Caribous´ technique not to freeze

• Caribou legs have cells with a plasma membrane adapted to very cold conditions. – Their plasma membranes are more fluid to

avoid freezing.They also have unsaturated fats.

Alaskan caribous

Vicious venoms

• Spider and snake venoms have certain enzymes that beakdown phospholipids and destroy cell membranes.

• When cell membranes are destroyed, cells are destroyed.

• When cells die, the tissue around the bite is destroyed.

Hermit spider bite.

Rattlesnake bite.