Cell Structure and Function

Define a cell organelle

Cell organelles are microscopic structures found in the cytoplasm which perform specific functions essential to normal cell function - they are the machinery of the cell.

Describe diffusion, osmosis and filtration and explain their functions

Diffusion is the movement of a substance from a region of high concentration to region of low concentration without the use of energy. Osmosis is the diffusion of water molecules from a region of high concentration of water molecules to region of low concentration of water molecules across a selectively permeable membrane. Filtration is a passive process in which hydrostatic pressure (blood pressure) forces water and small molecules across the plasma membrane.

Compare and contrast endocytosis and exocytosis

Endocytosis means taking substances into the cell. It involves encircling of the material to be taken into the cell with a plasma membrane sac that is then pinched off and moved into the interior of the cell as an endocytic vesicle. The process of exocytosis is the opposite of endoyctosis, moving substances out of the cell. Secretory vesicles produced within the cell move to the plasma membrane and fuse with it, releasing their contents outside the cell (e.g., hormones, mucous).

Explain the concept of facilitated diffusion and how it differs from simple diffusion. Give examples of a substance transported

Facilitated diffusion is when a carrier molecule is used to transport another molecule across the plasma membrane along its concentration gradient without the use of energy. This maybe because they are simply too large to pass through the 'pores' of the membrane, or because they are lipid insoluble in difference to simple diffusion, where the molecules are smaller and are lipid soluble. An example is the glucose molecule, which is too large to pass through the membrane 'pores' and uses a carrier protein to enter the cell.

Name and give a brief description of the organelles in a typical cell and describe their functions

Nucleus - The nucleus is a very important organelle in the cell. It contains DNA (deoxyribonucleic acid), which is a set of instructions necessary for the normal functioning of the cell - thus it is the 'control centre' of the cell. Endoplasmic reticulum (ER) - The endoplasmic reticulum is network of fluid filled channels (tubules) in the cytoplasm of the cell. It serves as a 'minicirculatory system' for the cell as it provides a network of channels for carrying substances from one part of the cell to another. Golgi apparatus - The Golgi apparatus is a characteristic series of disc-shaped membrane-lined channels stacked one above the other, all slightly curved so as to give a concave and convex surface to the stack resembling a stack of dinner plates. The Golgi can be thought of as the sorting and despatchment area of the cell. Mitochondria - Mitochondria have two membranes, the outer one encasing the inner one which is thrown into folds called cristae. Most of the energy needed by the cell is produced in mitochondria - therefore they are referred to as the 'powerhouses of the cell'. Lysosomes - Lysosomes are small membranous sacs containing digestive enzymes. Their function is to destroy debris from damaged cells, worn out cell components and foreign substances (pathogens) that enter the cell. Ribosomes - Ribosomes are spherical bodies about 15-20 nm in diameter. Ribosomes are found either free in the cytoplasm or attached to the rough endoplasmic reticulum. Ribosomes are the site of protein synthesis. Cytoskeleton - It consists of tubules and filaments that act as the cell's skeleton. Centrioles - The centrioles are a pair of rod shaped structures that lie close to the nucleus. The centrioles direct the formation of the mitotic spindle during cell division. Microvilli - Microvilli are small finger like projections of the plasma membrane on the outside surface. These structures increase the effective surface area between the cell and its external environment. Cilia and flagella - Cilia are hair like structures involved in aiding the movement of substances along the surface of the cell. Flagella are much longer extensions of the cell and are involved in the propulsion of the cell itself—e.g., sperm.

Compare and contrast passive and active transport mechanisms

Passive transport processes include: diffusion, osmosis and facilitated diffusion all of which are along their concentration gradients - from an area of high concentration to an area of low concentration. Active transport requires the use of carrier proteins to transport substances across the plasma membrane. Many carrier proteins only transport one substance at a time but some can move two substances simultaneously. If the two substances are being moved in the same direction, it is referred to as co-transport. If, however, one substance is being transported in and the other out, it is called counter-transport. Vesicular transport is another example of active transport, in which substances are transported into and out of the cell through a vesicle. The two main mechanisms for the vesicular transport are endocytosis (taking in) and exocytocis (taking out).

Describe the structure and function of the plasma membrane.

Structurally, the plasma membrane is a flexible barrier consisting of a lipid bilayer; which consists of phospholipids and small amounts of other lipids called glycolipids and cholesterol, proteins; transmembrane proteins which are embedded in the phospholipid bilayer and peripheral proteins which are usually bound to the exposed parts of transmembrane proteins on the internal and external surfaces of the plasma membrane and carbohydrates; which attach to proteins and phospholipids on the outer surface of the membrane. The plasma membrane is selectively permeable which means that it allows some substances to move across it but restricts the entry of others. Functions of the plasma membrane include physical isolation in which the plasma membrane is a physical barrier that separates the inside of the cell from the surrounding extracellular fluid. Secondly, regulation of exchange with the environment - the plasma membrane regulates the entry and exit of materials. Thirdly, sensing the environment - the plasma membrane is the first part of the cell to come in contact with the extracellular environment. Fourth, structural support - specialised connections between plasma membranes or between membranes and extracellular materials give tissues a stable structure.

Using the sodium/potassium pump as an example, explain active transport

The concentration of potassium inside the cell is generally 10 to 20 times higher than the outside and the reverse is true of sodium. Because the potassium and the sodium tend to leak slowly through the cell membrane along their concentration gradients - the potassium out of the cell and the sodium into the cell, the sodium/potassium pump operates to pump sodium out of the cell against its concentration gradient and pumps potassium back into the cell. The sodium/potassium pump is as an example of countertransport mechanism - the carrier protein moves three sodium ions out of the cytoplasm and moves two potassium ions into the cytoplasm.

Define the concept of tonicity and explain the difference between an isotonic, hypertonic and a hypotonic solution and their effect on cells

Tonicity it the ability of a solution to change the shape or tone of cells by altering their internal water volume. Isotonic solutions contain the same concentration of solutes as the cell, therefore a cell will retain its shape when immersed in an isotonic solution. Hypertonic solutions have a higher concentration of solutes than the cell, therefore cells immersed in hypertonic solutions lose water by osmosis, which causes them to shrink - this is called crenation. Hypotonic solutions contain a lower concentration of solutes than the cell therefore when cells are immersed in hypotonic solutions, they increase in size (swell) as water rushes in and may eventually burst (haemolyse).

Define passive and active transport

• Passive transport processes move substances across the plasma membrane without the use of energy. The substances move along their concentration gradient - that is, from high concentration to low concentration. • Active transport processes require energy, usually in the form of ATP (adenosine triphosphate). Active transport moves substances against their concentration gradient - that is, from an area of low concentration to an area of high concentration and hence the need for energy.

Name and define the three main components of a cell

• plasma membrane (cell membrane) - the flexible outer boundary that keeps all the contents of the cell together and separates the cell's internal environment from it's external environment. • nucleus - the control centre of the cell - it controls activity of the cell • cytoplasm - consists of all the contents of the cell other than the nucleus (the cytosol and organelles).