Lecture 11

Membrane transport and

Photosynthesis I

LIF1019-2-2014Sunday Dr. Jonaki Sen

Movement of water across membranes

Osmosis:

Diffusion of water molecules along a water concentration gradient

Effects of tonicityNormally the fluid inside the cells and the tissue fluid bathing them are isotonic.

If the extracellular fluid becomes very hypotonic the cells would take up so much water that they would burst. If the extracellular fluid becomes too hypertonic they lose water and shrink.Most cells have built in mechanism that adjusts to shifts in tonicity except red blood cells. That is the reason solutions isotonic to blood are given intravenously to dehydrated people.Hypotonic Hypertonic Isotonic

Effects of fluid pressureAnimal cells avoid bursting by engaging in ongoing selective transport of solutes across the plasma membraneCells of plants, protists and many fungi also resist bursting by exerting pressure against their cells walls

Compartment 1

Hypotonic solution

Pressure differences as well as solute concentrations influence the osmotic movement of water.

Water continues to diffuse from a hypotonic to a hypertonic solution until its concentration becomes the same on both sides of the membrane separating them.

Any volume of fluid exerts hydrostatic pressure , a force directed against the wall membrane or any structure enclosing it.Cells will continue to expand in volume if placed in a hypotonic solution until a point is reached where the hydrostatic pressure counters the inward diffusion of water. Osmotic pressure is the force that prevents any further increase in the volume of the solution.

Plant cells are also vulnerable to water loss which can happen when the soil dries up or is too salty .

Water stops diffusing in, cells lose water and internal fluid pressure drops.

Such osmotically induced shrinkage of cytoplasm is called plasmolysis.

Plasmolysis

Exocytosis and EndocytosisTransport proteins can move only small molecules in and out of the cell. For moving large molecules and proteins cells use exocytosis and endocytosis.

Exocytosis: A vesicle moves to and fuses with the cell membrane. The contents of the vesicle are released outside the cells where as the proteins embedded in the lipid bilayer of the vesicle wall now become proteins embedded in the plasma membrane.

Endocytosis: There are three different pathways of endocytosis. However in all three cases a small indentation forms at the plasma membrane, balloons inwards and pinches off. The resulting endocytic vesicle transports or stores its contents in the cytoplasm.

Three types of endocytosis

1) Receptor-mediated endocytosis

Receptors on the membrane recognize and specifically bind to ligands such as growth factors, lipoproteins, peptides etc. The receptors become concentrated in tiny indentations in the plasma membrane. Each pit looks like a woven basket from the cytoplasmic side. The basket is made of a protein called clathrin. The basket closes on itself and becomes the structural framework of the vesicle.

2) Bulk-phase endocytosisThis is a less selective pathway. An endocytic vesicle forms around a small volume of extracellular fluid irrespective of its content. This form of endocytosis pulls in patches of the membrane into the cell to compensate for the extra membrane being added to the plasma membrane when exocytic vesicles fuse with it.

3) PhagocytosisThis is an active form of endocytosis by which a cell engulfs microorganisms, large edible particles and cellular debris. Amoeba’s and some other protistans obtain their food in this way. In multicelled organisms macrophages and some other white blood cells engage in phagocytosis when they defend the body against microorgnisms.

A phagocytic cell has receptors on its surface that bind the target and send a signal inside. Actin microfilaments form a ATP-requiring network below the membrane. Pseudopods are formed around the target and fuse to engulf the target in a phagosome which fuses with the lysosome. The lysosome helps digests the target to fragments and produce smaller reusable molecules.

Photosynthesis IPlants are autotrophs or self-nourishing organisms i.e. they can produce their own food.Plants, some bacteria and many protistans are photoautotrophs, which means they capture sunlight energy to drive a metabolic process called photosynthesis.Many other organisms are heterotrophs i.e. they must feed on autotrophs, one another or organic wastes. That is how most bacteria, many protistans and all fungi as well as animals stay alive.

T. Englemann’s experiment

To figure out which part of the visible spectrum is most favored by plants to synthesize food.

By 1882 it was known that plants use sunlight, water and something in the air to make food.

Photosynthesis : the main pathway by which carbon and energy enter the web of life

The link between photosynthesis (the main energy-requiring process) and and aerobic respiration (the main energy releasing process) in living systems.

The main energy releasing process

ATP is avialble to drive cellular tasks

Carbon dioxide and water are required

Carbon dioxide

and water are

released

Oxygen is required

Oxygen is released

The main energy acquiring process

Plants capture energy from the sun which drives

photosynthesis

The main energy releasing process

ATP is available to drive cellular tasks

Where does photosynthesis take place?

The leaf

The upper and lower surface of the leaf has photosynthetic cells

The chloroplast

The thylakoid membrane

Stroma

The thylakoid membrane is organized into grana(stacks of coins) surrounded by fluid (stroma).

Light dependent reactions: On the thylakoid membrane

Light independent reactions: In the stroma

In light-dependent reactions the energy from sunlight is used to synthesize ATP from ADP + inorganic (Pi). Water molecules are split and the co-enzyme NADP + picks up their electrons and hydrogen atoms to produce NADPH.

In light-independent reactions ATP donates the energy to sites where carbon, hydrogen and oxygen are combined to produce glucose (C6H12O6 ). NADPH provides hydrogen and electrons and carbon dioxide provides the carbon and oxygen atoms.

Although glucose is shown as the end product to keep book-keeping simple, very few plant cells have glucose. The glucose is phosphorylated and primed to be incorporated into starch, sucrose or cellulose.

Sunlight as energy sourcePhotoautotrophs intercept only about 1% of the total energy that reaches the surface of the earth. They can absorb wavelengths between 380 and 750 nm (visible range) . Wavelengths shorter than this including the UV range have energies high enough to break covalent bonds.

The ozone layer protects most of the earth’s surface from the harmful effects of UV radiation. Before the ozone layer was there most photoautotrophs existed under the sea.

Engleman used prisms to identify the wavelengths that are preferred by green algae but he did not know the molecular basis of that.

Pigment molecules absorb different wavelengths and either reflect or transmit other wavelengths. Chlorophyll is the pigment present in photosynthetic organisms. Chrorophyll a is the main photosynthetic pigment absorbing all wavelengths except in the green-yellow range. Cholorophyll b acts as accessory pigment absorbing some wavelengths missed by chlorophyll a.

Other accessory pigments are carotenoids and phycobilins.

Why aren’t all pigments black?Photosynthetic life evolved in water.

Red and green light (500 to 600 nm range) can penetrate much deeper than blue light and ultraviolet light.

Thus green algae evolved in shallow waters and red algae which almost looks black evolved in deeper waters.

Light-dependent reactionsWhat happens to the absorbed energy?

The chloroplast thylakoid membrane has photosystems.Each photosystem comprises of a cluster of proteins and 200-300 pigment (chlorophyll) molecules.

Most of these pigments only harvest photon energy and instead of releasing the energy in the form of fluorescence they pass this energy onto a neighboring pigment which passes it on to another one and so on. Some energy is lost at each of these transfer steps as heat until only energy corresponding to a wavelength that can be trapped by the reaction center containing chlorophyll a remains. Chlorophyll a absorbs this energy and does not pass it onto another pigment, instead the suitably activated reaction center donates electrons to a nearby acceptor molecule poised next to the electron transport system.

Cyclic electron flow

Photosynthesis started out in ancient bacteria to produce ATP and not to synthesize organic compounds. They converted sunlight energy to the chemical bond energy of ATP by cycling it from a photosystem to a electron transport system and back to the photosystem.

This pathway still operates in photoautotrophs and has a reaction center designated p700. This electron transport chain does not have enough energy to drive synthesis of NADPH.

Noncyclic electron flow

More than 2 billion years ago another photosystem (II) was coupled with photosystem I which generated enough energy for electron transfer to generate NADPH.

P

Photosystem II absorbs photons and a reaction center p680 containing chlorophyll a gives up electrons. The energy input also triggers photolysis or splitting of water into oxygen, hydrogen ions and electrons. When excited p680 releases electrons, new electrons from split water replace them. The excited electrons move on to a transport system and then to p700 of photosystem I.These electrons have some energy and they also get more from photons bombarding photosystem I. Thus they get enough boost of energy to enter a electron transport system where NADP accepts two electrons and an Hydrogen ion to become NADPH.

The legacy-a new atmosphere

AA by product of the noncyclic pathway molecular oxygen can be seen as bubbles on the leaves of aquatic plants.

When this pathway first evolved a great amount of oxygen was found dissolved in the water.

About 1.5 billion years ago this large quantity of dissolved oxygen was escaping into the atmosphere which became rich in oxygen. This made aerobic respiration the most efficient way of releasing energy from stored food.