2.1 Cell structure: subcellular components:

Ribosomes:

• Ribosomes are made of primarily ribosomal RNA

• They are the site of translation and are responsible for making all of the proteins for the cell

• There are 2 kinds found in different locations

• Free ribosomes are in the cytosol and make proteins that stay in the cell for various functions.

• Bound ribosomes are attached to the rough endoplasmic reticulum and mainly make proteins for export.

• Ribosomes synthesize proteins according to mRNA sequences that they receive during the process of translation.

Endoplasmic Reticulum:

• The endoplasmic reticulum (ER), made up of two parts, serves to make other products that the cell needs.

• The smooth ER has many functions. It performs synthesis of lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and stores calcium ions.

• The rough ER, however, secretes proteins made by bound ribosomes. Proteins then are moved to the transitional ER, where they are wrapped in a transport vesicle to head to the Golgi apparatus.

Golgi Apparatus:

• modifies, stores, and sends proteins that come from the rough ER.

• Things like glycoproteins are modified in the Golgi

• There are 2 sides on the Golgi apparatus, cis and trans face

• Vesicles enter the Golgi apparatus via the cis face and depart via the trans face.

Mitochondria:

• have a double membrane, which is a phospholipid bilayer.

• The outer membrane is smooth, while the inner has many folds, called cristae.

• These folds help to increase the surface area available for the Electron Transport Chain.

• The inside of the inner membrane is called the mitochondrial matrix, which is the site of the Krebs Cycle.

• The Mitochondria creates ATP for the cell to use via cellular respiration. Mitochondria also have their own circular DNA.

Lysosomes:

• hydrolyze most foods, amino acids, and other molecules.

• The inside of the lysosome is extremely acidic.

• Lysosomes can digest foods by using phagocytosis or engulfing nutrients to digest them.

• The hydrolytic enzymes inside of the lysosome work to break down anything that comes into contact with it.

• The lysosome is also used to recycle and digest old or damaged parts of the cell.

Vacuoles:

• are large vesicles which store many different things, such as food or water.

• Many unicellular eukaryotes have contractile vacuoles to pump water out of the cell.

• plants generally have a large central water vacuole which stores water and ions.

Chloroplast

• is the site of photosynthesis

• These organelles have a double membrane and have green pigments called chlorophyll that allow for the absorption of photons

• The chloroplast is made up of the stroma, or liquid filling of the chloroplast, and the thylakoids, flat sacs of membranes that allow for the absorption of light.

2.2 cell structure and function:

Mitochondria:

• The inner foldings of the mitochondria membrane, referred to as cristae, are essential to the increased surface area of the inner membrane

• This increased surface area allows for much more space for the Electron Transport Chain (ETC), an essential part of cellular respiration.

• The matrix, used as the site of the Krebs cycle, is right next door and can therefore transfer its products easily to the ETC.

Chloroplast:

• The stacked thylakoid membranes of the chloroplast allow for an increase in surface area necessary for the ETC of the light-dependent reactions of photosynthesis.

• membranes are also lined with photosystems and chlorophyll, increasing the amount of energy that the plant can get from light.

• the proximity of the stroma to the thylakoids allows for the sharing of products between the Calvin Cycle (which happens in the stroma) and the light-dependent reactions

Plasma membrane:

• designed to be semi-permeable

• Some molecules are able to travel through without any problem, while others need specific proteins in order to help them across.

• The plasma membrane allows for the creation of a concentration gradient, an essential part of many biological processes.

Lysosome:

• The plasma membrane that contains the lysosome is incredibly important.

• If the hydrolytic enzymes inside of the lysosome were to burst, all of the cell's organelles may burst as well, and the cell would die.

• In order to prevent this, the membrane is specially designed to keep these enzymes in.

• will bind with other vesicles that contain contents necessary for digestion when needed.

2.3 cell size:

Comparative cell size:

• Cells, as you all know, are incredibly tiny

• The cell needs to maintain a small size in order to have the maximum amount of space for the transfer of nutrients and waste (surface area) with the smallest volume of cell.

• As the volume increases, the cell will need more and more nutrients to enter, and the higher the surface area, the more nutrients can enter.

• The greater the surface area to volume (SA/V) ratio is, the more efficient the cell becomes

• the cell reaches a point where the surface area doesn’t allow enough nutrients to pass that the cell needs it divides

• the cell reaches a point where the surface area doesn’t allow enough nutrients to pass that the cell needs it divides

• Tissues and membranes that have folds have them to increase the surface area.

2.4 Plasma membrane:

• The plasma membrane is made up of a phospholipid bilayer

• These have 2 parts, a hydrophobic (water-hating) part, and a hydrophilic (water-loving) part.

• there are proteins embedded into the plasma membrane

• proteins can be hydrophobic, hydrophilic, charged, uncharged, polar, or non-polar depending on the configuration of the amino acids in the protein

• The cell membrane also has glycoproteins and glycolipids attached to it along with steroids

• These groups help with cell signaling and the attachment of the cell to other structures

• Models of the plasma membrane are given the name: fluid mosaic model

• This represents the fact that the membrane is fluid and somewhat moveable

• The proteins embedded in the membrane, which serve a variety of functions, create the mosaic portion of the name

2.5 Membrane permeability:

Membrane permeability:

• the structure of the membrane, with the hydrophobic tails and hydrophilic heads, the cellular membrane has selective permeability

• This allows some substances to cross easily, while others may not be able to cross or may require a special transport protein to do so.

• The membrane acts like a barrier separating the inside of the cell from the external environment of the cell

• Small, non-polar molecules are able to freely cross the cell membrane, while polar or charged molecules require transport proteins to cross

• If a molecule is small, polar, and uncharged (like water!) it may be able to pass through the membrane in small quantities but requires a transport protein to move across in any larger quantities

• The hydrophobic fatty acid tails are what controls the movement of substances described above

• hey repel charged and polar molecules and make it very challenging for them to come across

2.6 Membrane transport

• The membrane allows for various types of transport

Different modes of membrane transport:

Simple diffusion:

• is a passive movement of a substance from an area of higher to lower concentration

• With no energy needed, a substance will diffuse down its concentration gradient, until equilibrium occurs

• This is called passive transport

Passive transport:

• plays a large role in getting rid of wastes, and importing needed materials

• Carbon dioxide and oxygen gas frequently rely on simple diffusion in order to enter and exit the cell membrane

Facilitated diffusion:

• is also a passive movement of a substance from an area of higher to lower concentration

• No energy is required for this movement, but a transport protein is needed in order for the substance to get across the membrane

• This is used by small polar molecules who cannot get across the membrane without a transport protein. When water utilizes facilitated diffusion, it is called osmosis

Active transport:

• uses energy, in the form of ATP, to transport molecules against their concentration gradient.

• Because of the selective permeability of the membrane, concentration gradients can form.

• Active transport may also be used for large or bulky molecules or to transport large quantities of small molecules.

• These processes are referred to as exocytosis and endocytosis

• When a large amount of molecules are entering a cell it is called endocytosis

In this there are 3 kinds of endocytosis

• In phagocytosis, the cell engulfs a large molecule, and brings it into the cell. These are generally food vacuoles

• pinocytosis. The cell “gulps” surrounding solutes into small vesicles that are covered in a layer of protein.

• receptor-mediated endocytosis  In this, a receptor binds to a cell. When solute binds to the receptor, the plasmid pulls away creating a vesicle with the solutes.

• When molecules are secreted from the cell, it is called exocytosis

• A transport vesicle from the Golgi apparatus moves along the microtubules in the cell until it reaches the plasma membrane

• Then the vesicle fuses with the Plasma membrane, releasing the contents out of the cell

2.7 Facilitated Diffusion;

When and why does Facilitated Diffusion occur:

• When molecules cannot move easily enough through the plasma membrane, facilitated diffusion occurs

• Molecules cannot pass through the phospholipid bilayer of the plasma membrane easily when particles are either charged or polar

• Facilitated dIffusion is a form of passive transport which does NOT require energy

• Passive transport occurs when molecules go down the concentration gradient

• A concentration gradient is when particles or solutes move from a highly concentrated area of particles to a less concentrated area of particles

• This process is aided by proteins located on the plasma membrane (membrane proteins) such as transport proteins: Channel proteins and carrier proteins.

Channel proteins:

• Channel proteins are laid throughout the membrane to provide a Hydrophilic passage through for the molecules to avoid the Hydrophobic core

• An example of a channel protein is Aquaporins which allow water (polar H20) to diffuse through the membrane

• Aquaporins are essential for plant cells, red blood cells

• Nerve and muscle cells have gated ion channel proteins to enable the flow of charged ions such as sodium and potassium present in the sodium potassium pump of action potentials

• If a signal such as an electrical signal is activated these channels open their gate to transmit these signals through cells.

Carrier Proteins:

• Carrier proteins alter their shape to allow the flow of molecules through the concentration gradient of the membrane similarly to an enzyme substrate complex

• Their rate of transport is slower than that of channel proteins

• Carrier proteins provide an easy way for hydrophilic molecules to pass through the concentration gradient

2.8 Tonicity and osmoregulation:

Tonicity & Osmoregulation:

• The movement of water inside and out of the cell is essential to its survival

• ater, just like all other substances, travels from a higher concentration of itself to a lower concentration of itself

• This movement can have large impacts on the cell

• Depending on the amount of material outside of a cell compared to inside, the environment outside of a cell can be hypotonic, hypertonic or isotonic to the internal environment of a cell

Hypotonic

• A hypotonic solution is one that has LESS solute than the inside of the cell

• In this case, water will move to where there is MORE solute (and therefore less water!)

• Water will move into the cell, causing the cell to expand

Hypertonic

• A hypertonic solution is one that has MORE solute in it than there is inside of the cell.

• In this case, water will still move to where there is MORE solute.

• Water will, therefore, move out of the cell causing the cell to shrink

Isotonic

• An isotonic solution is one that has EQUAL solute in it to that of the cell

• In this case, water moves equally in and out of the cell, with no net movement.

• The cell does not change shape

• Water will attempt to move from an area of high concentration to an area of low concentration until there are equal amounts of water on both sides of the membrane

Osmosis

• Osmosis allows organisms to control their internal solute composition and water potential.

Animal cell                                                            Plant cell 

2.10 Cell Compartmentalization:

Cell compartmentalization:

• One of the major differences between eukaryotes and prokaryotes is that eukaryotes compartmentalize their internal processes in membrane-bound organelles

Eukaryotic cells:

• in eukaryotic cells, after RNA is made from DNA in the process of transcription, it moves to the ribosome to go through the process of translation

• The RNA has to move out of the nucleus to either a free-standing ribosome or to the rough endoplasmic reticulum

Prokaryotic cells:

• In prokaryotes, RNA is converted to proteins right after being made from DNA, as they do not have a nucleus or endoplasmic reticulum

• This will lead to more differences in transcription and translation

• By reducing the amount of competing space and surface area, and also reducing the amount of competing reactions, eukaryotic cells are able to be more efficient than prokaryotic cells

2.10 Origins of cell compartmentalization:

Endosymbiotic Theory:

• The endosymbiotic theory is the current theory of how eukaryotic membrane-bound organelles existed in eukaryotic cells

• The endosymbiotic theory states that an early ancestor of eukaryotic cells engulfed a prokaryotic cell, and the prokaryotic became an endosymbiont, a cell living in another cell

• These smaller cells were capable of producing extra energy for the other cell, leading to a selective advantage

• Overtime, cells with an extra cell inside were able to become more able to survive and reproduce quickly

• The eukaryotic cell was born!

• While prokaryotes generally lack internal membrane bound organelles, they still have internal regions with specialized structures and functions

• These regions are just not defined by outer membranes