Free Lecture notes cell and subcellular organelles and membrane transport pdf

1

Introduction

Prokaryotes & Eukaryotes

Components of cell

Cell membrane

Cytoplasm with its

organelles

Nucleus

Cell membrane

Components

Structure

Fluid Mosaic Model

Cytoplasm & its organelles

Endoplasmic Reticulum

Golgi apparatus

Mitochondria

Lysosomes

Peroxisomes

Nucleus

Structure & Functions

Membrane transport

Cell is the universal functional unit of all forms of life.

On the basis of differences in cell structure, all life

forms are divided into two major classes. They are

prokaryotes and eukaryotes.

INTRODUCTION

PROKARYOTES & EUKARYOTES

LECTURE NOTES ON CELL & MEMBRANE TRANSPORT

Dr Vijay Marakala, MBBS. M.D. Assistant professor,

Department of Biochemistry, SIMS & RC, MUKKA - SURATHKAL, MANGALORE.

[email protected]

2

A COMPARISON OF PROKARYOTES AND EUKARYOTES ORGANELLE PROKARYOTES EUKARYOTES

Nucleus No definite nucleus; DNA present but not separate from rest of cell

Present

Cell membrane Present Present

Mitochondria None; enzymes for oxidation reactions located on plasma membrane

Present

Endoplasmic reticulum

None Present

Ribosomes Present Present

COMPONENTS OF THE CELL

The outermost structure of the cell that decides its contour is the cell

membrane.

It is a lipid bi-layer. It also consist of proteins and small amounts of

carbohydrates

Membranes are asymmetric. The outer and inner surfaces have different

components and different enzymatic activities.

Membranes are fluid structures. The unsaturated fatty acids bound to

phospholipids contribute to the fluid state of the membrane. At body

temperature lipids are in a fluid state and this fluidity of the membrane is

essential for the normal functioning to occur.

Cell Membrane (Plasma Membrane)

Cell membrane

Cytoplasm with its organelles

Nucleus

3

FLUID MOSAIC MODEL OF CELL MEMBRANE

In 1972 Singer and Nicolson postulated a theory of membrane structure

called the fluid mosaic model.

A mosaic is a structure made up of many different parts. Likewise, the

plasma membrane is composed of different kind of macromolecules like

phospholipid, integral proteins, peripheral proteins, glycoproteins,

glycolipids and cholesterol.

According to this model the matrix or continuous part of membrane

structure, is a polar lipid bilayer. Phospholipids are arranged in bilayers

with polar head groups oriented towards the extracellular side and

cytoplasmic side with hydrophobic (nonpolar) tails face each other at the

core of the bilayer.

The lipid bilayer is fluid because of more number of unsaturated fatty

acids. The cholesterol content of the membrane alters the fluidity (More

cholesterol less fluid).

Proteins are interspersed in the lipid bilayer, of the plasma membrane,

producing a mosaic effect.

4

FUNCTIONS OF CELL MEMBRANE

1. It is fluid and dynamic.

2. It is semipermeable; only selected compounds are allowed to pass

through from out-side. The selective permeability is responsible for the

maintenance of internal environment of the cell and for creating

potential difference across the membrane.

3. The modification of the cell membrane results in formation of

specialized structures like axon of nerves, microvilli of intestinal

epithelium and tail of spermatids.

MEMBRANE LIPIDS

Three major classes – phospholipids, glycolipids and cholesterol

They are all amphipathic molecules, that is, they have both

hydrophobic and hydrophilic ends.

PROTEINS OF THE CELL MEMBRANE

Two major categories

1. Integral or intrinsic or transmembrane proteins: are either partially or

totally (transmembrane proteins) immersed in lipid bilayer. They serve

as channels (pores) and carrier proteins.

2. Peripheral or extrinsic proteins: function almost entirely as enzymes

and receptors.

Most of membrane proteins are glycoproteins. They have short chains of

carbohydrates on the exterior side of the membrane.

MEMBRANE CARBOHYDRATES (THE CELL GLYCOCALAYX)

Membrane carbohydrate is not free. It occurs as glycoproteins or

glycolipids.

Many of the carbohydrates act as receptor substances for binding

hormones such as Insulin

5

CYTOPLASM AND ITS ORGANELLES

The extra nuclear cell content that possess both organelles and other

material constitutes cytoplasm. Material other than subcellular

components in the cytoplasm makes up the cytosol. Cytosol contains

mainly dissolved proteins, electrolytes and glucose.

Five important organelles that are suspended in the cytoplasm are:

Endoplasmic reticulum

Golgi apparatus

Mitochondria

Lysosomes and

Peroxisomes.

MITOCHONDRIA (Power house of the cell)

Mitochondria consist of

outer and inner

membranes.

The outer membrane is

composed of equal

amount of protein and

lipids.

The outer membrane is

freely permeable to many

compounds.

The inner membrane consists of 75% protein and remainder is lipid.

The inner membrane is folded to form number of invaginations known as

cristae extending to matrix. Cristae give large surface area and are the

site of oxidative phosphorylation.

Mitochondrion is the power house of the cell. It is responsible for the

production of energy in the form of ATP.

Mitochondrial matrix contains enzymes of the citric acid cycle and β –

oxidation.

Mitochondria contains some DNA known as mitochondrial DNA

6

The endoplasmic reticulum (ER) is part of a continuous single-membrane

system throughout the cell; the membrane doubles back on itself to give

the appearance of a double membrane in electron micrographs. The

endoplasmic reticulum is attached to the cell membrane and to the nuclear

membrane. It occurs in two forms, rough and smooth. The rough

endoplasmic reticulum is studded with ribosomes bound to the membrane.

Ribosomes, which can also be found free in the cytosol, are the sites of

protein synthesis in all organisms. The smooth endoplasmic reticulum (SER)

does not have ribosomes bound to it

ENDOPLASMIC RETICULUM

FUNCTIONS OF ENDOPLSAMIC RETICULUM

Ribosomes and rough endoplasmic reticulum are involved in protein

synthesis.

SER of intestinal cells is involved in formation of triglycerides.

In the adrenal cortex, SER is the site of steroid formation.

Cytochrome P450 dependent monooxygenases are present in liver

cell SER.

7

GOLGI APPARATUS

Golgi apparatus is separate from the endoplasmic reticulum but is frequently

found close to the smooth endoplasmic reticulum. It is a series of

membranous sacs.

Functions

1. The Golgi apparatus is involved in secretion of proteins from the cell.

Material produced in the cell for export is processed by Golgi body and

is packaged as vesicle and is pinched off. The vesicles fuse with plasma

membrane and their content is released to exterior by the process

known as exocytosis. The digestive enzymes of pancreas and insulin are

produced and released in this way.

2. Golgi apparatus helps in the formation of other subcellular organelles

like lysosomes and peroxisomes.

3. Golgi apparatus is involved in protein targeting.

LYSOSOMES

They are small vesicles present in cytoplasm. They are surrounded by a

membrane enclosed sacs containing hydrolytic enzymes that could cause

considerable damage to the cell if they were not physically separated from

the lipids, proteins, or nucleic acids that they are able to attack. Lysosomes

are called as ‘Suicidal bags’ of the cell.

Functions

1. Lysosomes are rich in hydrolytic enzymes, which are active at acidic

pH. The lysosomal enzymes digest the molecules brought into the

cell by phagocytosis.

2. Macrophages are rich in lysosomes.

3. Lack of one or more of lysosomal enzymes cause accumulation of

materials in the cell resulting in lysosomal diseases.

8

PEROXISOMES

Peroxisomes are similar to lysosomes; their principal characteristic is that

they contain enzymes involved in the metabolism of hydrogen peroxide

(H2O2), which is toxic to the cell. The enzyme catalase, which occurs in

peroxisomes, catalyzes the conversion of H2O2to H2O and O2

MEMBRANE TRANSPORT

Biological membranes are semipermeable membranes. Permeability is

conferred by membrane proteins; these are called channel proteins and

carrier proteins.

There are two types of transport mechanisms

1. Passive transport or passive diffusion and

2. Active transport

PASSIVE TRANSPORT OR PASSIVE DIFFUSION

Transport of solute molecules from high concentration to low

concentration across membrane is called passive transport. It is a

spontaneous process because it is thermodynamically favourable. It is a

downhill transport and requires no energy.

Two types of passive transport,

Simple diffusion and

Facilitated diffusion.

Simple diffusion

Transport of solute molecules from high concentration to low

concentration across membrane is known as simple diffusion. Lipid

soluble (lipophilic) molecules can pass through the cell membrane,

without any interaction with carrier proteins.

9

Simple diffusion can occur through the cell membrane by ways.

Through the interstices of the lipid bilayer if the diffusing

substance is lipid soluble and

Through the channel proteins.

Example: Transport of O2, CO2, N2, ethanol and urea

Facilitated Diffusion (Carrier mediated diffusion)

Carrier molecules present in membranes mediate transport of many solute

molecules across membrane. Hence, mediated transport involves carrier

molecules. They are known as permeases, translocases, transporters or

pumps. Most of them are proteins.

10

Facilitated diffusion is faster than simple diffusion. It requires no energy.

Facilitated diffusion by carrier molecule involves conformational change of

carrier molecule. The carrier molecule exists in two conformations. It has

binding site to solute molecule. In the native conformation the carrier is

exposed to high concentration of solute. Then the solute molecules bind to

the sites on carrier molecule. A conformational change in carrier molecule

occurs. It exposes solute molecule to low concentration and solute

molecules are released into the cell. The empty carrier returns to the

native state to transport solute molecules once again.

Example: Transport of glucose and most of amino acids.

Facilitated diffusion displays

saturation behavior while the

rate of transport of a solute in

simple diffusion across a

membrane is directly

proportional to its concentration.

11

Active transport

It requires energy in addition to carrier molecules. It moves solute

molecules from low concentration to high concentration or against

concentration gradient.

Substances that are actively transported through cell membranes

include, Na+, K+, Ca++, Fe++, H+, Cl-, I-, several different sugars and

most of amino acids.

Active transport is classified into two types according to the source

of energy used as follows:

1. Primary active transport and

2. Secondary active transport.

Primary active transport

Energy is derived directly from hydrolysis of ATP.

Example: , Na+, K+ ,Ca++, H+ and Cl- transport across the membrane.

Primary active transport of Na+ and K+ / sodium-potassium pump

Active transport by Na+, K+-

ATPase. Three sodium ions bind

to the transporter protein on

the cytoplasmic side of the

membrane. When ATP is

hydrolyzed to ADP, the carrier

protein is phosphorylated and

undergoes a change in

conformation that causes the

sodium ions to be released into

the extracellular fluid.

12

Two potassium ions then bind on the extracellular side. Dephosphorylation

of the carrier protein produces another conformational change, and the

potassium ions are released on the inside of the cell membrane. The

transporter protein then resumes its original conformation, ready to bind

more sodium ions.

Importance of Na+-K+ pump

The Na+-K+ gradient created by this pump in the cells, controls cell

volume.

Renders neurons and muscles electrically excitable and

Drives the active transport of sugars and amino acids.

Secondary active transport

Are those active transport systems in which transport of molecules is

indirectly linked to hydrolysis of ATP. The Na+ gradient, which is maintained

by primary active transport, is used to power the transport of glucose,

amino acids, and many other compounds into the cell through secondary

active transport. An example is provided by the transport of glucose into

cells of the intestinal epithelium in conjunction with Na+ ions

13

Secondary active

transport of glucose

by the Na+-glucose

cotransporter.

Sodium ion binds to the carrier protein in the luminal membrane, stimulating

the binding of glucose. After a conformational change, the protein releases

Na+ and glucose into the cell and returns to its original conformation. Na+-K+

ATPase in the basolateral membrane pumps Na+ against its concentration

gradient into the extracellular fluid. Thus, the Na+ concentration in the cell is

low, and Na+ moves from the lumen down its concentration gradient into the

cell and is pumped against its gradient into the extracellular fluid.

Glucose, consequently, moves

against its concentration

gradient from the lumen into

the cell by traveling on the

same carrier as Na+. Glucose

then passes down its

concentration gradient into

the extracellular fluid on a

passive transporter protein.

14

UNIPORT, SYMPORT AND ANTIPORT

UNIPORT: Carries single solute

across the membrane. E.g. glucose

transporter

CO-TRANSPORT: If the transport

of one molecule depends on

simultaneous or sequential

transfer of another molecule, it is

called co-transport. The co-

transport system may either be a

symport or an antiport

SYMPORT: The transporter system

carries two solutes in the same

direction across the membrane.

E.g. Sodium dependent glucose

transporter

ANTIPORT: Carries two solutes or

ions in opposite direction. E.g.

Sodium pump or chloride-

bicarbonate exchange in RBC.