Osmosis

Osmosis is the net movement of freely moving water molecules down a concentration gradient through a partially permeable membrane (permeable to water and to certain solutes).

The pressure exerted by freely moving water molecules in a system is called the water potential, measured in kilopascals (kPa). By convention, the water potential of pure water is 0 kPa. The more solute the solution has, the less water potential. The contribution of solutes to the water potential of a system is called the solute potential. Since it always lowers water potential, it is always negative.

In a situation in which a solution with a higher water potential is separated by a partially permeable membrane from a solution with a lower water potential, more water molecules move from the first solution to the second than in the other direction. The movement continues until both sides of the membrane are equal. Water molecules keep moving, but with no net movement.


Osmosis in animal and plant cells

-Isotonic solution: the water potential is equal in both sides of the cell membrane.

• Animal and plant cells: no net water flow in or out of the cell.

-Hypotonic solution: the water potential is higher in the outside of the cell.

• Animal cells: water tends to enter the cell, increasing its volume. If the cell doesn't eliminate the excess, it will burst. However, if it is the natural environment of the cell, it will have a mechanism to eliminate it.
• Plant cells: water enters the cell, fills the vacuole, which pushes the cell surface membrane against the cell wall making the cell turgid. The pressure restricts the inflow of water.

-Hypertonic solution: the water potential is higher in the inside of the cell.

• Animal cells: the cell may lose water, causing it to shrink and shrivel.
• Plant cells: the water moves out, the cell vacuole shrinks and the cell surface membrane pulls away from the cell wall. The cell becomes flaccid because the contents are no longer pushing against the cell wall: Plasmolysed cell.

Pressure potential

The contribution made by the cell wall is the pressure potential. Turgid (positive value) or flaccid (0 kPa).

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Active Transport

It is used to import and export substances into and out of the cell. Active transport is:

• movement of substances


• usually against a concentration gradient


• across a cell membrane
  
• uses energy (usually from ATP provided by respiration in the mitochondria)

Different carrier proteins carry specific molecules or ions through the cell membrane. There are three types, since they move:

1. Uniport carriers: a single substance in a single direction.
2. Symport carriers: two substances in the same direction.
3. Antiport carriers: two substances in opposite directions.

The precise mechanism of active transport is unclear. There are two different hypothesis:

Hypothesis 1: Cotransport

The plumping of one substance indirectly drives the transport of one or more other substances against a concentration gradient.

Hypothesis 2

Protein molecules change shape to transport solutes form one side of the membrane to the other. To do this, ATP is hydrolysed to ADP. The phosphate group attaches directly onto the protein, causing it to change shape.

Cytosis

Active transport which involves infolding and outfolding of sections of the cell membrane.

• Exocytosis (transport out of a cell). Vesicles and vacuoles move to the cell surface membrane, fuse with it and release their cargo to the outside world.


• Endocytosis (transport into a cell). There are three types:

*Phagocytosis (cellular eating). Solid substances brought inside the cell by invagination. A vacuole is formed.

*Pinocytosis (cellular drinking). Smaller infoldings.

*Receptor-mediated endocytosis: receptor molecules on the cell surface membrane bind with a specific substance from the extracellular environment. As receptor sites are filled, the surface folds inwards until a coated vesicle finally separates from the cell surface membrane. (Following Fig)

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Diffusion

Diffusion is the net movement of molecule ions down a ocncentration gradient. When a uniform concentration is reached, equilibrium is established. Molecules still move, but there is not net movement in any particular direction. (Fig. 1)

Rate of diffusion (Flick's law)

(Surface area x Difference in concentration) / Lenght of diffusion path

The rate is also affected by the nature od the membrane (eg. permeability), size and type of molecule.
This law applies to situations in which there is no barrier to the movement of substances. The diffusion is a way of passive transport, that's to say, it does not require the expenditure of energy.

Adaptations of tissues to increase the rate

• Maintaining a steep concentration gradient


• Having a high surface area to volume ratio


• Being thin

Cell membranes are partially permeable: many substances can pass through them but others can't.

Facilitated diffusion

It is the passive movement of molecules down a concentration gradient, but involves special carrier proteins in the cell membrane (Fig. 2). It is important because glucose, nucleic acids, amino acids and proteins are not soluble in lipids and don't pass the bilayer of phospholipids. Therefore, this process takes place. Facilitated diffusion of relatively large molecules such as glucose is specific and involves a protein called permease. It changes shape when glucose binds to it, and can transport it in either way although it is always down a concentration gradient. Like enzymes, it can be inhibited.

Surface area to volume relationship

As the size of an organism increases, its surface area to volume ratio decreases. This relationship has important consequences on organisms. Some processes such as the rate of diffusion depende on the surface area, while others such as metabolic rate, depend on volume. Small organisms can satisfy their oxygen needs by simple diffusion. Large organisms, instead, have evolved special respiratory exchange surfaces suchs as lungs, gills and tracheae to compensate for their relatively small body surface areas.

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Cell Membranes

Electron micrograph studies revealed that cell membranes are 7-8 nm. wide and have three layers. Cell surface membranes are 45% lipid, 45% protein and 10% carbohydrate. According to the fluid-mosaic model of cell membrane structure, individual protein molecules shift and move on a fluid bilayer of phospholipid. They vary in function and structure, but they all contribute to the mechanical strenght of membranes. Proteins can be:

• Intrinsic (widht of membrane): some may act as carrier molecules, and other as enzymes.


• Extrinsic (confined to outer or inner surface): many of them combine with carbohydrate groups to form glycoproteins that act as chemical receptors, together with glycolipids. Some proteins attach to the cytoskeleton to anchor the membrane.

Functions of cell surface membrane

• Control of the passage of materials in and out of the cell
  
• Recognition of other cells


• Receptor sites for hormones and neurotransmitters


• Transmission of nerve impusles


• Insulation of nerves

Functions of membranes inside cell

• Acting as a reaction surface
  
• Acting as an intracellular transport system
  
• Providing separate intracellular ccompartments, isolating different chemical reactions.

Revealing the Ultrastructure of Cells

Even though the electron microscope was invented, biologists still couldn’t see many of the structures. They believed that many features observed in electron micrographs were artefacts, features not present in life.
Freeze-fracturing
This is a different technique for examining existent structures in cells:
• Living material is plunged into liquid nitrogen at -196º and pushed against a sharp blade in a precise way.
• The frozen tissue splits along lines of weakness, often in the middle of a membrane.
• The fractured surfaces are ‘etched’ with heavy metal so that the specimen can be examined by the Transmission Electron Microscope.
Freeze fracturing has confirmed the existence of structures in cells and has also revealed new features.
Cell fractionation
This is a technique used to prepare samples of the various cell organelles so that their functions can be studied. Organelles are separated into fractions according to their size (using differential centrifugation) or density (using density gradient centrifugation). For either method, the tissue is first cut into small pieces and then placed in a chilled, isotonic and buffered solution.
• The Tº of the solution is kept low to slow down metabolism and minimize self-digestion of the organelles.
• The salt concentration of the solution is made isotonic so that organelles do not change volume.
• The solution buffered to minimize changes in PH during the process; this prevents enzymes in the organelles from becoming denatured.

The Ultrastructure of Plant Cells

• Cell wall: made of rigid cellulose. This makes the plant a more uniform and gives a regular shape. The ones of neighboring cells are cemented together by a thin layer of pectic substances which form the middle lamella.
• Plasmodesmata: fine membrane-lined pores through which pass cytoplasmic threads. Each one has a central tubular core. It enables substances to be transported easily.
• Chloroplast: large organelle where photosynthesis takes place. It has an envelope of two membranes and contains stroma (gel-like; contains ribosomes, DNA and photosynthetic enzymes; stores starch and lipids) and a system of membranes. The membranes contain chlorophyll which harvests light energy for photosynthesis.
• Large central vacuole: sac bounded by a membrane (tonoplast). It contains sap (watery solution of various substances). When full of sap, the vacuole causes the cell surface membrane to press against the cell wall (pressure helps to support plant). Sometimes it functions as a lysosome.

Eukaryotic cells
These types of cells are double membraned, with a well defined nucleus containing genetic material. For example: plants, animals, fungi, protoctists.
Prokaryotes cells
These types of cells lack the double membrane organelles and do not have a membrane-bound nucleus.
Bacteria cell
• Cell surface membrane: encloses the cytoplasm. Contains enzymes, ribosomes and food granules.
• Cell wall: surrounds the cell surface membrane.
• Capsule: encloses the cell wall.
• Simple flagella
• Nucleoid: Single strand of DNA coiled into the centre of the cell.
• Plasmids: additional circular pieces of genetic material.

The Ultrastructure of Animal Cells

Cells are highly complex structures that contain organelles. Organelles are structures which carry out specific functions within the cell.

• Cell surface membrane: selectively permeable. Controls exchange of substances between the cell and the environment. Mostly protein and lipid.


• Cytoplasm: all the living parts of the cell without counting the nucleus. They consist of membrane-bound organelles and the cytosol (fluid part). The cytosol contains small molecules in solution and large molescules in suspention.


• Cytoskeleton: consists of miscrotubules, filaments and fibres that provide the physical support. This organelle is also involved in cell movement.


• Nucleus: enclosed by nuclear envelope (two membranes perforated by nuclear pors which control the exchange of material between the nucleus and the cytoplasm). The nucleus also contain one or more nucleoli that manufacture ribosomes.


• Chromatin: is the form of chromosomes when they are not dividing in DNA. There are two types: euchromatin (active DNA; lighter), and heterochromatin (coiled DNA, inactive; darker).


• The endoplasmic reticulum: a system of flattened membrane- bound sacs (cisternae, full of fluid) forming tubes and sheets. There are two types: the Rough Endoplasmic Reticulum (Some regions are coiled with bead-like structures, ribosomes); and the Smooth Endoplasmic Reticulum (has no ribosomes).

* The RER transports proteins made by the ribosomes.

*The SER synthesises secretes and storescarbohydrates, lipids and other non protein products.

* The SER of liver cells also contains enzymes.

• Ribosomes: tiny organelles with two subunits (one smaller) which are made of roughly equal parts of protein and RNA and they are the sites of protein synthesis.


• Mitochondria: cigar-shaped organelles. Involved in generating ATP. Each one of them is bounded by an envelope which consists of two membranes. The inner is deeply folded into shelves (cisternae). The watery matrix contains DNA, ribosomes, enzymes, calcium phosphate and granules.


• Golgi apparatus: a stack of flattened membrane-bound sacs (cisternae). A new membrane is constantly added to one end and buds off as vesicles at the other end. Processing and packing structure, enables cell materials (enzymes)to be secreted of the cell by vesicles and is involved in the formation of lysosomes. Lysosomes eliminate worn-out mitochondria and other redundant organelles. They are also involved in autolysis (breakdown of tissue).


• Centrioles: paired cylinders composed of a complex arangement of microtubules (involved in cell division).


• Flagela and celia: slender mobile extensions (involved in cell locomotion).