fundamentals of cell biology and cell=cell interactions

Question 1

Five Principles of Cell Biology

Answer 1

All organisms are composed of one or more cells.

2.Cells are the main structural and functional units of life.

3.All important functions of an organism operate inside cells.

4.All cells originate from cells that have existed before.

5.Cells have hereditary information that regulates cell functions and is passed from one generation to the other.

Question 2

Cell Wall

Answer 2

The cell wall is an extracellular structure of plant cells that differentiates plant cells from animal cells. A cell wall is found in:

*Plant cells

*Fungi

*Many protists

Question 3

Function of the Cell Wall:

Answer 3

*It protects the plant cell.

*It maintains the plant’s shape.

*It adds strength to the plant.

*It prevents excessive uptake of water.

*It provides the skeletal support that keeps the plant upright on land. Thus, it prevents the plant from falling due to the force of gravity.

Question 4

Structure of the Cell Wall:

Answer 4

Plant cell walls are thicker than the cell membrane.

*The precise chemical composition of the cell wall differs from species to species and from one type of cell to another within the same species.

Question 5

*However, the fundamental design of the cell wall follows a similar pattern:

Answer 5

A plant cell that is growing secretes a very thin and flexible wall known as the primary cell wall.
Between the primary cell walls of adjacent cells, is the middle lamella, a slim layer comprising sticky polysaccharides known as pectin (pectin is used in the food industry as a thickening agent in jams and jellies).
The middle lamella joins the adjacent cells together.
The moment the cell matures and stops growing, it strengthens its wall by either secreting hardening substances into the primary cell wall or adding a secondary cell wall between the plasma membrane and the primary cell wall.

Question 6

The secondary cell wall:

Answer 6

Is usually deposited in a number of layers.
Has a strong and durable matrix that ensures both protection and support of the cell.
Is the main component of wood.

Question 7

Cell Membrane

Answer 7

he cell membrane, also often referred to as the plasma membrane, is composed of phospholipids and proteins. In animal cells, the steroid cholesterol is wedged into the bilayer.

Question 8

The cell membrane is commonly described as a fluid mosaic:

Answer 8

Mosaic” – Denotes a surface made of small pieces. A membrane is a mosaic in the sense that it has diverse molecules which include proteins and phospholipids.

*“Fluid” – The membrane is fluid in that most of the membrane molecules can drift about in the membrane

Question 9

Functions of the Cell Membrane:

Answer 9

*Keeps the internal components of a cell in a state of balance, i.e. homeostasis.

*It acts as a physical partition separating the internal contents of the cell from the external environment.

*It automatically allows nutrients into the cell and allows waste material to go out of the cell. This is because it is a semi-permeable membrane.

Question 10

Structure of the Cell Membrane:

Answer 10

*The cell membrane comprises mainly phospholipid molecules.

*These molecules have phosphorous and a mixture of other atoms at one end = this forms the phosphate end.

*The phosphate head is polar, which means that it carries electrical charges, making it readily mix with water = hydrophilic.

*There is an attachment to one side of the phosphate head called the fatty acid tails. These tails are made up of lipids or fat molecules and, thus, they are water-repelling in nature = hydrophobic.

*Thus, the cell membrane is a phospholipid bi-layer with other components such as cholesterol and various proteins surrounding it.

*The cell membrane is arranged in such a way that the hydrophilic phosphate heads come into contact with water and the fatty acid tails are tucked into one another due to their hydrophobic nature.

*This entire assembly is referred to as the Fluid Mosaic Model.

*The functions of the cholesterol wedged into the bilayer of animal cells include the following:

Helps to stabilise the membrane at warm temperatures.

Helps to keep the membrane fluid at low temperatures.

Acts as labels which help cells recognise other cells, e.g. when the sperm cell is trying to find the ovum during the process of fertilisation.

Question 11

unction of Cell Membrane Proteins

Answer 11

Various proteins determine much of the membrane’s specific functions.

Some proteins are embedded in the hydrophobic region. These are called integrins or transmembrane proteins if they span the membrane.
Peripheral proteins are not embedded in the interior, but are loosely bound to the surface.
Plasma proteins form intercellular junctions or passageways between cells, which allow molecules to pass through the membrane.
Six major functions of membrane proteins:
Enzymatic activity
Signal transduction
Cell-cell recognition which usually involves glycoproteins
Transport of specific solutes into or out of cells
Intercellular joining of adjacent cells with gaps or tight junctions
Attachment to the cytoskeleton and extracellular matrix, maintaining cell shape and stabilising the location of certain membrane proteins
It is important to remember that some cell membranes have more proteins than others, e.g. RBCs have more than 50 kinds of proteins embedded in their cell membranes.
It is also important to note that membrane structure results in selective permeability.

Question 12

Intercellular Junctions

Answer 12

*Intercellular junctions are structures which offer connection and communication between cells.

*Plant cell walls are perforated by channels between adjacent cells called plasmodesmata.

*In animal cells, there are three main types of intercellular junctions:
1.Tight junctions
2.Desmosomes
3.Gap junctions

Question 13

Plasmodesmata

Answer 13

Plasmodesmata
(Plural= Plasmodesmata, Singular = Plasmodesma)
*These are microscopic pores that pass through the cell walls of plant cells. These channels:
Enable transport and communication between the cells.
Connect the chemical environments of adjacent cells.
Allow the passage of water, small solutes, certain proteins and RNA molecules.
Allows tissues to function in a coordinated way.
*Due to these connections, the entire plant is united and able to function harmoniously.

Question 14

Tight Junctions/Occluding Junctions

Answer 14

*At tight junctions, the plasma membranes of adjacent cells are very tightly pressed against each other.
*They are bound together by a variety of specific proteins such as claudins.
*They form continuous seals around the cells, thus preventing leakage of fluid across a layer of epithelial, endothelial and myelinated cells.
*For example, tight junctions between skin cells make humans watertight, thus preventing the absorption of water when bathing.

Question 15

Anchoring Junctions/Desmosomes/Adherens

Answer 15

*They facilitate the adhesion of cells to other cells or the extracellular matrix.
*Intermediate filaments made of sturdy keratin proteins anchor desmosomes into the cytoplasm.
*Desmosomes are common in tissues subject to stretching or mechanical stress such as muscle cells. Desmosomes attach muscle cells to each other.
*“Muscle tears” involve the rupture of desmosomes.

Question 17

Gap Junctions/Communicating Junctions

Answer 17

*They provide cytoplasmic channels between neighbouring cells.
*In this way, they are similar in their function to plasmodesmata in plants.
*Gap junctions consist of membrane proteins that surround a pore allowing various molecules, ions and electrical impulses to directly pass through a regulated gate between cells.
*Gap junctions are vital for communicating between cells in many types of tissue, e.g. heart muscle, animal embryos, etc.

Question 17

Movement Processes

Answer 17

A steady traffic of all types of substances moves across the plasma membrane in both directions. These include:
*Atoms and molecules which move into and out of the cell constantly.
*Cell products arranged together in the Golgi apparatus needs to be transported outside the cell to designated areas of the body.
*Nutrients and building blocks are taken in from the outside of the cell.
*Waste products have to leave the cell.
*Water.

Question 18

There are two types of movement processes that are responsible for movements needed:

Answer 18

1.Passive movements – here particles move from an area of high concentration to low concentration. Thus, these processes do not require the use of energy. These processes involve:
Brownian motion
Concentration gradients and equilibrium
Diffusion

2.Active movements – here particles move from an area of low concentration to high concentration. Thus, these processes require energy. ATP supplies the energy for the most active movement. These processes involve:
Transport proteins
Exocytosis
Endocytosis – phagocytosis, receptor-mediated endocytosis, pinocytosis

Question 19

Passive Movements
Brownian motion

Answer 19

*Involves the random movement of atoms and molecules suspended in a fluid.
*The movement of atoms and molecules is very fast, thus this motion involves multiple collisions of atoms and molecules within a system.

Question 20

Concentration gradients and equilibrium

Answer 20

*A concentration gradient occurs when there is a difference in the concentration of atoms and molecules between two places.
*Atoms and molecules will then move from an area of high concentration to low concentration until an equilibrium is reached.

Question 21

Diffusion

Answer 21

*Diffusion works on the principles of concentration gradients. It is a passive process i.e. energy is not used.
*There are three types of diffusion:
1.Simple diffusion
2.Facilitated diffusion
3.Osmosis

Question 22

Simple diffusion

Answer 22

is when molecules move from an area of high concentration to an area of low concentration

Question 23

Facilitated diffusion

Answer 23

is the spontaneous transport of substances across a biological membrane from an area of higher concentration to an area of lower concentration by means of transmembrane integral proteins.

Question 24

Osmosis

Answer 24

is the diffusion of water through a selectively permeable membrane or cell membrane from an area of low solute concentration to an area of high solute concentration.

Question 25

Active Movements
Proteins

Answer 25

*Some transport proteins can move solutes across membranes against their concentration gradient, e.g. carrier proteins.

*ATP is usually involved in inducing a conformational change in carrier proteins, translocating the bound solute across the membrane.
*The sodium-potassium pump works this way by exchanging sodium ions for potassium ions. This pump is considered an electrogenic pump.
*In plants, bacteria and fungi, a proton pump is the major electrogenic pump.
*Because most transport proteins are involved in the flow of ions, a combined gradient of concentration and electrical charge called an electrochemical gradient is formed.
*In cotransport, a membrane protein couples the transport of two solutes.

Question 26

Bulk Transport
*Exocytosis

Answer 26

Exocytosis is when substances such as enzymes and hormones are expelled from the cell/or leave the cell.
Waste products also need to be expelled from the cell.
During exocytosis, the vesicle carrying products to be expelled from the cell, making their way to the edge of the cell. Its membrane then joins with the cell membrane. As the vesicle pushes against the cell membrane, its own membrane ruptures and it becomes part of the cell membrane until its content is released.

Question 27

Endocytosis

Answer 27

Endocytosis occurs when substances enter the cell.
There are three types of endocytosis:
Phagocytosis is where solid materials are absorbed by the cell either to act as food or to be destroyed (e.g. white blood cells absorbing bacteria to destroy it).
Receptor-mediated endocytosis is similar to phagocytosis in that solid materials will be engulfed and taken into the cell. This process requires a receptor to begin the process of engulfing the molecule.
Pinocytosis is cellular drinking. The cell membrane forms a vesicle around some extracellular fluid which will later be released into the cellular contents.