General Biology/Cells/Cell Structure

Cell structure

What is a cell? The word cell comes from the Latin word "cella", meaning "small room", and it was first coined by a microscopist observing the structure of cork. The cell is the basic unit of all living things, and all organisms are composed of one or more cells. Cells are so basic and critical to the study of life, in fact, that they are often referred to as "the building blocks of life". Organisms - bacteria, amoebae and yeasts, for example - may consist of as few as one cell, while a typical human body contains about a trillion cells.

According to Cell Theory, first proposed by Schleiden and Schwann in 1839, all life consists of cells. The theory also states that all cells come from previously living cells, all vital functions (chemical reactions) of organisms are carried out inside of cells, and that cells contain necessary hereditary information to carry out necessary functions and replicate themselves. It is the basis of human life.

All cells contain:

Lipid bilayer boundary (plasma membrane)
Cytoplasm
DNA (hereditary information)
Ribosomes for protein synthesis

Eukaryotic cells also contain:

At least one nucleus
Mitochondria for cell respiration and energy

Cells may also contain:

Lysosomes
Peroxisomes
Vacuoles
Cell walls

Concepts

Plasma Membrane: Phospholipid bilayer, which contains great amount of proteins, the most important functions are the following:

It selectively isolates the content of the cell of the external atmosphere.
It regulates the interchange of substances between the cytoplasm and the environment.
Communicates with other cells.

Model of the fluid mosaic: Describes the structure of the plasma membrane, this model was developed in 1972 by cellular biologists J. Singer and L. Nicholson.
Phospholipid bilayer: Is in the plasma membrane and produces the fluid part of membranes.
Proteins: Long chains of amino acids.
Glucose proteins: Proteins together with carbohydrates in the plasma membrane, mostly in the outer parts of the cell.
Functions of proteins: Transport oxygen, they are components of hair and nails, and allow the cell interact with its environment.
Transport Proteins: Regulate the movement of soluble water molecules, through the plasma membrane. Some transport proteins called channel proteins form pores or channels in the membrane so that water soluble molecules pass.
Carrying proteins: Have union sites that can hold specific molecules.
Reception proteins: They activate cellular responses when specific molecules join.
Proteins of recognition: They work as identifiers and as place of union to the cellular surface.
Fluid: It is any substance that can move or change of form.
Concentration: Number of molecules in a determined unit of volume.
Gradient: Physical difference between two regions of space, in such a way that the molecules tend to move in response to the gradients.
Diffusion: Movement of the molecules in a fluid, from the regions of high concentration to those of low concentration.
Passive transport: Movement of substances in a membrane that doesn’t need to use energy.
Simple diffusion: Diffusion of water, gases or molecules across the membrane.
Facilitated diffusion: Diffusion of molecules across the membranes with the participation of proteins.
Osmosis: Diffusion of the water across a membrane with differential permeability.
Transport that needs energy: Movement of substances across a membrane generally in opposition to a gradient of concentration with the requirement of energy.
Active transport: Movement of small molecules using energy (ATP).
Endocytosis: Movement of big particles towards the interior of the cell using energy. The cells enclose particles or liquids.
Pinocytosis: (Literally cell drinking) Form in which the cell introduces liquids.
Phagocytosis: Way of eating of the cells. It feeds in this case of big particles or entire microorganisms.
Pseudopodia: False feet (the amoeba).
Exocytosis: Movement of materials out of the cell with the use of energy. It throws waste material.
Isotonic: The cytoplasm fluid of the interior of the cells is the same that the outer.
Hypertonic solution: The solutions that have a higher concentration of dissolved particles than the cellular cytoplasm and that therefore water of the cells goes out with osmosis.
Hypotonic: The solutions with a concentration of dissolved particles lower than the cytoplasm of a cell and that therefore do that water enters the cell with osmosis.
Swelling: Pressure of the water inside the vacuole.
Endoplasmic Reticulum: It is the place of the synthesis of the cellular membrane.

Structure and function of the cell

Rudolf Virchow: Zoologist, who proposed the postulates of the cellular theory, observes that the living cells could grow and be in two places at the same time, he proposed that all the cells come from other equal cells and proposed 3 postulates:

Every living organism is formed from one or more cells
The smallest organisms are unicellular and these in turn are the functional units of the multicellular organisms.
All the cells come from preexisting cells.

Common characteristics of all the cells

Molecular components: Proteins, amino acids, lipids, sweeten, DNA, RNA.
Structural components: Plasmatic membrane, cytoplasm, ribosomes.
Robert Hooke: He postuled for the first time the term cell
Prokaryotes: Their genetic material is not enclosed in a membrane ex. Bacterias
Eukaryotes: Their genetic material is contained inside a nucleus closed by a membrane

History of cell knowledge

The optical microscope was first invented in 17th century. Shortly thereafter scientists began to examine living and dead biological tissues in order to better understand the science of life. Some of the most relevant discovery milestones of the time period include:

The invention of the microscope, which allowed scientists for the first time to see biological cells
Robert Hooke in 1665 looked at cork under a microscope and described what he called cork "cells"
Anton van Leeuwenhoek called the single-celled organisms that he saw under the microscope "animalcules"
Matthias Jakob Schleiden, a botanist, in 1838 determined that all plants consist of cells
Theodor Schwann, a zoologist, in 1839 determined that all animals consist of cells
Rudolf Virchow proposed the theory that all cells arise from previously existing cells

In 1838, the botanist Matthias Jakob Schleiden and the physiologist Theodor Schwann discovered that both plant cells and animal cells had nuclei. Based on their observations, the two scientists conceived of the hypothesis that all living things were composed of cells. In 1839, Schwann published 'Microscopic Investigations on the Accordance in the Structure and Growth of Plants and Animals', which contained the first statement of their joint cell theory.

Cell Theory

Schleiden and Schwann proposed spontaneous generation as the method for cell origination, but spontaneous generation was later disproven. Rudolf Virchow famously stated "Omnis cellula e cellula"... "All cells only arise from pre-existing cells." The parts of the theory that did not have to do with the origin of cells, however, held up to scientific scrutiny and are widely agreed upon by the scientific community today.

The generally accepted portions of the modern Cell Theory are as follows: (1) The cell is the fundamental unit of structure and function in living things. (2) All organisms are made up of one or more cells. (3) Cells arise from other cells through cellular division. (4) Cells carry genetic material passed to daughter cells during cellular division. (5) All cells are essentially the same in chemical composition. (6) Energy flow (metabolism and biochemistry) occurs within cells.

Microscopes

Allow greater resolution, can see finer detail
Eye: resolution of ~ 100 μm
Light microscope: resolution of ~ 200 nm
Limited to cells are larger organelles within cells
Confocal microscopy: 2 dimension view
Electron microscope: resolution of ~0.2 nm
Laser tweezers: move cell contents

Cell size

One may wonder why all cells are so small. If being able to store nutrients is beneficial to the cell, how come there are no animals existing in nature with huge cells? Physical limitations prevent this from occurring. A cell must be able to diffuse gases and nutrients in and out of the cell. A cell's surface area does not increase as quickly as its volume, and as a result a large cell may require more input of a substance or output of a substance than it is reasonably able to perform. Worse, the distance between two points within the cell can be large enough that regions of the cell would have trouble communicating, and it takes a relatively long time for substances to travel across the cell.

That is not to say large cells don't exist. They are, once again, less efficient at exchanging materials within themselves and with their environment, but they are still functional. These cells typically have more than one copy of their genetic information, so they can manufacture proteins locally within different parts of the cell.

Key concepts: Cell size:

Is limited by need for regions of cell to communicate
Diffuse oxygen and other gases
Transport of mRNA and proteins
- Surface area to volume ratio limited
Larger cells typically:
- Have extra copies of genetic information
- Have slower communication between parts of cell

Structure of Eukaryotic cells

Eukaryotic cells feature membrane delimited nucleii containing two or more linear chromosomes; numerous membrane-bound cytoplasmic organelles: mitochondria, RER, SER, lysosomes, vacuoles, chloroplasts; ribosomes and a cytoskeleton. Also, plants, fungi, and some protists have a cell wall.

Structure of the nucleus

The nucleus is the round object in the cell that holds the genetic information (DNA) of the cell. It is surrounded by a nuclear envelope and has a nucleolus inside.

Nuclear envelope

The nuclear envelope is a double-layered plasma membrane like the cell membrane, although without membrane proteins. To allow some chemicals to enter the nucleus, the nuclear envelope has structures called Nuclear pores. The nuclear envelope is continuous with the endoplasmic reticulum.

Nucleolus

The nucleolus appears in a microscope as a small dark area within the nucleus. The nucleolus is the area where there is a high amount of DNA transcription taking place.

Chromatin

Chromosomes consist of chromatin. This is made up of strings of DNA, which typically measure centimeters in length if stretched out. This DNA is wound around a histone core and organized into nucleosomes.

The chromatin must be uncoiled for gene expression and replication. Chromosome micrograph

Endoplasmic reticulum

The endoplasmic reticulum is a cellular organelle made up of a series of extended folded intracellular membranes. It is continuous with the nuclear membrane.

There are two main types of endoplasmic reticulum:

RER: rough endoplasmic reticulum (site of protein synthesis) associated with ribosomes
SER: smooth endoplasmic reticulum (site of lipid synthesis)

Rough Endoplasmic Reticulum

Proteins are directed to the RER by a signal sequence of a growing polypeptides on the ribosome. This is recognised by a signal recognition particle which brings the ribosome/polypeptide complex to a channel on the RER called a translocon. At the translocon, the signal sequence and ribosome/polypeptide complex interact with the translocon to open it. The signal sequence becomes attached to the translocon. The ribosome can continue to translate the polypeptide into the lumen of the RER. As synthesis continues, 2 processes can happen.

If the protein is destined to become a membrane bound protein then the protein synthesis will continue until termination. The ribosome can then dissociate, allowing protein folding within the RER lumen to occur and continuation to the golgi apparatus for processing of the polypeptide.
If the protein is destined for storage for later secretion after stimulation or for continuous secretion then a protease -an enzyme which cuts proteins at the peptide bond- can cut the signal sequence from the growing polypeptide. Continuation to the golgi etc. can then occur.

When produced, proteins are then exported to one of several locations. The proteins are either modified for extracellular membrane insertion or secretion. Note, this is in contrast with ribosomes which do not associate with the RER and produce proteins which will become cytosolic enzymes for example.

Smooth Endoplasmic Reticulum

Smooth endoplasmic reticulum produces enzymes for lipid and carbohydrate biosynthesis and detoxification RER

Sarcoplasmic Reticulum

This is a specialised form of endoplasmic reticulum found in some muscle cell types-particularly striated, skeletal muscle. Its main function is different from the other 2 types in that is mainly acts as a storage of calcium. This reticulum has voltage gated channels which respond to signals from 'motor neurones' to open and release calcium into the cytoplasm. This can then bring about the next part in muscle contraction.

Figure 1 : Image of nucleus, endoplasmic reticulum and Golgi apparatus.

Nucleus.
Nuclear pore.
Rough endoplasmic reticulum (RER).
Smooth endoplasmic reticulum (SER).
Ribosome on the rough ER.
Proteins that are transported.
Transport vesicle.
Golgi apparatus.
Cis face of the Golgi apparatus.
Trans face of the Golgi apparatus.
Cisternae of the Golgi apparatus.

The Golgi apparatus

The golgi apparatus is made up of multiple stacks of bilipid membranes.

Proteins made on the RER are modified and then sorted
Formation of secretory vesicles
Formation of lysosomes (intracellular digestion)

Other membrane-bound cytoplasmic organelles include:

Microbodies (generic term)
Glyoxysome (transforms fat into carbohydrate in plants)
Peroxisome (uses oxidative metabolism to form hydrogen peroxide and is destroyed by catalase)

Ribosomes

Ribosomes are the site of protein synthesis. Ribosomes themselves are synthesized in the cell nucleoli and are structured as two subunits, the large and the small. These parts are composed of RNA and protein.

Prokaryotic and eukaryotic ribosomes are different, the eukaryotic ones being larger and more complicated.

DNA-containing organelles

Mitochondria

Double membrane
Aerobic metabolism, internal membrane
DNA, ribosomes
Give rise to new mitochondria

Chloroplast

Double membrane
Photosynthesis, internal membrane
DNA, ribosomes
Give rise to new chloroplasts

Centrioles

Microtubule organizing centers
- Animal cells and many protists
- Pair constitutes the centrosome
- Give rise to flagellum during spermatogenesis
Consist of 9 triplet microtubules
Mitosis, meiosis

Cytoskeleton

Cytoskeleton is a collective term for different filaments of proteins that can give physical shape within the cell and are responsible for the 'roads' which organelles can be carried along.

Gives the cell shape
Anchors other organelles
Vital to intracellular transport of large molecules

The cytoskeleton is composed of 3 main types of filaments:

Actin filaments (7 nm)
Microtubules: (25 nm) polymer of tubulin; 13/ring.
Intermediate Filaments

Both actin and microtubules can have associated motor proteins.

Intermediate Filaments

These are rope like filaments, 8-10 nm in diameter and tend to give the structural stability to cells. Examples include Vimentin, neurofilaments and keratin. It is keratin which principly makes up hair, nails and horns.

Actin Filaments

Growth

These filaments are 2-stranded and composed of dimeric subunits called G-Actin. They contain a GTP molecule in order to bind (polymerise). As GTP is hydrolysed then the structure becomes unstable and depolymerisation occurs. The growth of actin filaments is concentration dependant-that is, the higher the concentration of free G-actin, the greater the polymerisation. The are also polar, having a + and a - end (not related to charge) and polymerisation tends to happen faster at the + end.

Cilia and flagella are threads of microtubules that extend from the exterior of cells and used to move single celled organisms as well as move substances away from the surface of the cell. motor proteins-move, wave motion