Cells - Structure of Eukaryotic Cells

Question 1

What is the Cell Theory?

Answer 1

1. All living organisms are composed of one or more cells.
2. The cell is the basic organisational unit of life.
3. All cells arise from pre-existing cells.

Question 2

What are the biological levels (largest to smallest) of a living thing?

Answer 2

organism -> system -> organ -> tissue -> cell -> organelle

Question 3

What is the plasma membrane?

Answer 3

Membranes consisting of a phospholipid bilayer found around and within all cells. The cell-surface membrane is the plasma membrane that surrounds cells.

Question 4

What is a phospholipid?

Answer 4

Triglyceride in which one of the three fatty acid molecules is replaced by a phosphate molecule. Phospholipids are important in the structure and functioning of plasma membranes.

Question 5

What is a bilayer?

Answer 5

A membrane consisting of two layers of phospholipids.

Question 6

What is permeability?

Answer 6

How permeable a substance is depends on the size, polarity and charge of the molecule. If it is small, non-polar and fat soluble it is very permeable and can pass through the cell membrane.

Question 7

What is the fluid mosaic model?

Answer 7

The arrangement of the various molecules of the cell-surface membrane. Fluid because the individual phospholipid molecules can move relative to one another and mosaic because the proteins vary in shape, size and pattern.

Question 8

What is the nucleus?

Answer 8

The nucleus is the largest, most prominent organelle and is surrounded by its own membrane called the nuclear envelope (double membrane) which contains many pores. It contains the organism’s hereditary material and controls the cell’s activities.

Question 9

What does the nucleus contain?

Answer 9

• the hereditary material (DNA) of the cell which codes for the synthesis of proteins in the cytoplasm
• chromosomes (which are made from protein-bound linear DNA)
• one or more nucleoli.

Question 10

What is the nucleolus?

Answer 10

The nucleus contains a small dark structure called the nucleolus. It is a small spherical region within the nucleoplasm. It manufactures ribosomal RNA and assembles the ribosomes. There may be more than one nucleolus in a nucleus.

Question 11

What is chromatin?

Answer 11

The DNA + histone = chromatin. The DNA double helix in the cell nucleus is packaged by special proteins termed histones. The formed protein/DNA complex is called chromatin.

Chromosomes consist of a linear molecule of DNA tightly wrapped around proteins called histones. The cell’s DNA is in this form only during cell division. When the cell isn’t dividing, the DNA is more loosely associated with the histones. DNA in this form is enclosed in the nucleus and is called chromatin.

Question 12

What is the mitochondria?

Answer 12

Mitochondria are the sites of aerobic respiration (the Krebs cycle and the oxidative phosphorylation pathway). They provide all of the energy a cell requires - so more active cells (muscles) will have greater numbers of mitochondria. They too have a double membrane - the inner one is folded to form cristae - where the energy carrier molecule ATP is produced, from respiratory substrates such as glucose. Inside is the matrix which contains enzymes involved in respiration.

Question 13

What is the nuclear envelope?

Answer 13

The nuclear envelope is a double-layered structure consisting of two adjacent membranes that surrounds the nucleus. The envelope is perforated by pores. The pores enable communication to occur between the nucleus and the cytoplasm. Its outer membrane is continuous with the endoplasmic reticulum of the cell and often has ribosomes on its surface. It controls the entry and exit of materials in and out of the nucleus and contains the reactions taking place within it.

Question 14

What is the golgi body/apparatus?

Answer 14

• The Golgi apparatus occurs in almost all eukaryotic cells and is similar to the SER in structure except that it is more compact.
• It consists of a stack of membranes that make up flattened sacs (cisternae) with small rounded hollow structures called vesicles.
• The proteins and lipids produced by the ER are passed through the Golgi apparatus in strict sequence.
• The Golgi modifies these proteins often adding non-protein (prosthetic) components, such as carbohydrates, to them.
• It also ‘labels’ them, allowing them to be accurately sorted and sent to their correct destinations.
• Once sorted, the modified proteins and lipids are transported in Golgi vesicles which are regularly pinched off from the ends of the Golgi cisternae.
• These vesicles may move to the cell surface, where they fuse with the membrane and release their contents to the outside.

Question 15

What is the function of the golgi apparatus?

Answer 15

• Modifying proteins produced in the endoplasmic reticulum by adding carbohydrates to them (e.g. to form glycoproteins).
• Labelling of proteins allows directing to their final destination in the cell. Transport to their destination occurs via Golgi vesicles.
• Production of secretory enzymes, such as those secreted by the pancreas.
• Production of lysosomes and lysosomal enzymes.
• Secretion of carbohydrates, such as those needed to make up the cell wall.
• Transportation, modification and storage of lipids.
• Formation of lysosomes.

Question 16

What is the rough endoplasmic reticulum?

Answer 16

Rough endoplasmic reticulum is a series of flattened sacs enclosed by a membrane with ribosomes on the surface. It provides a large surface area for the synthesis of proteins and glycoproteins. It also provides a pathway for the transport of materials, especially proteins, throughout the cell.

Question 17

What is the smooth endoplasmic reticulum?

Answer 17

Smooth endoplasmic reticulum is a system of membrane bound sacs. It is similar to the RER but has no ribosomes and is often more tubular in appearance. It’s not connected to the nucleus membrane directly. It is where lipids/steroids and carbohydrates are synthesised, stored and transported.

Question 18

What are ribosomes?

Answer 18

Ribosomes are small cytoplasmic granules found in all cells. Eukaryotic ribosomes may be either free, meaning that they are floating around in the cytoplasm, or bound, meaning that they are attached to the endoplasmic reticulum or the outside of the nuclear envelope.

Question 19

What are lysosomes?

Answer 19

Lysosomes are a type of Golgi vesicle that releases lysozymes.

• Formed when the vesicles produced by the Golgi apparatus contain enzymes such as proteases and lipases.
• Also contain lysozymes (enzymes that hydrolyse the cell walls of certain bacteria).
• Lysosomes isolate these enzymes from the rest of the cell before releasing them, either to the outside or into a phagocytic vesicle within the cell.

Question 20

What is the nuclear pore?

Answer 20

Nuclear pores allow the passage of large molecules, such as messenger RNA, out of the nucleus.

Question 21

What is the nucleoplasm?

Answer 21

The granular, jelly-like material that makes up the bulk of the nucleus.

Question 22

What is the cell wall?

Answer 22

A rigid layer of polysaccharides lying outside the plasma membrane of the cells of plants, algae, fungi, and bacteria. It provides mechanical strength to prevent the cell bursting under the pressure created by the osmotic entry of water, to give mechanical strength.

• cellulose: plants and algae
• chitin: fungi
• peptidoglycan: bacteria

Question 23

What are centrioles?

Answer 23

Protein structures found in animal cells. Centrioles are involved in producing spindle fibres for cell division.

Question 24

What are chloroplasts?

Answer 24

Chloroplasts are the organelles that carry out photosynthesis in plant cells. They vary in shape and size.

Chloroplasts are oval-shaped and have two membranes: an outer membrane and an inner membrane. Between the outer and inner membrane is the intermembrane. The space within the inner membrane is the stroma, the dense fluid within the chloroplast.

Question 25

What is the cytoplasm?

Answer 25

Liquid portion of a cell that contains organelles and enzymes. Many cell reactions take place here.

Question 26

What is a eukaryotic cell?

Answer 26

A cell that has a membrane-bound nucleus and chromosomes. The cell also possesses a variety of other membranous organelles, such as mitochondria and endoplasmic reticulum.

Question 27

What is an organelle?

Answer 27

Sub-cellular structures surrounded by a membrane that has a specialised function.

Question 28

What is the vacuole?

Answer 28

A fluid-filled sac bounded by a single membrane may be termed a vacuole. Within mature plant cells there is usually one large central vacuole. The single membrane around it is called the tonoplast. A plant vacuole contains a solution of mineral salts, sugars, amino acids, wastes and sometimes pigments such as anthocyanins.

Question 29

What is the middle lamella?

Answer 29

The middle lamella is a layer which cements the cell walls of two adjoining plant cells together. It is the first formed layer which is deposited at the time of cytokinesis. The cell plate that is formed during cell division itself develops into middle lamella or lamellum.

Question 30

What is the cell wall made from?

Answer 30

Characteristic of all plant cells, the cell wall consists of microfibrils of cellulose, embedded in a matrix. Cellulose microfibrils have considerable strength and so contribute to the overall strength of the cell wall.

Question 31

What do chromosomes consist of?

Answer 31

Protein-bound, linear DNA.

Question 32

What does the nucleus contain?

Answer 32

• nuclear envelope
• nuclear pores
• nucleoplasm
• chromosomes
• nucleolus

Question 33

What are the functions of the nucleus?

Answer 33

• act as the control centre of the cell through the production of mRNA and tRNA and hence protein synthesis
• retain the genetic material of the cell in the form of DNA and chromosomes
• manufacture ribosomal RNA and ribosomes

Question 34

What does the double membrane around the mitochondrion do?

Answer 34

Around the organelle is a double membrane that controls the entry and exit of material. The inner of the two membranes is folded to form extensions known as cristae.

Question 35

What are cristae?

Answer 35

Cristae are extensions of the inner membrane, which in some species extend across the whole width of the mitochondrion. These provide a large surface area for the attachment of enzymes and other proteins involved in respiration. It is where the energy carrier molecule ATP is produced by respiratory substrates such as glucose.

Question 36

What is the matrix?

Answer 36

The matrix makes up the remainder of the mitochondrion. It contains protein, lipids, ribosomes and DNA that allows the mitochondria to control the production of some of their own proteins. Many enzymes involved in respiration are found in the matrix.

Question 37

Why is there a larger amount of mitochondria in cells that have a high level of metabolic activity?

Answer 37

Mitochondria are responsible for the production of ATP as they are the sites of the aerobic stages of respiration. Because of this, the number and size of the mitochondria, and the number and density of their cristae, are high in cells that have a high level of metabolic activity and therefore require a plentiful supply of ATP. Examples of metabolically active cells include muscle and epithelial cells. Epithelial cells in the intestines require a lot of ATP in the process of absorbing substances from the intestines by active transport.

Question 38

What is the chloroplast envelope?

Answer 38

A double plasma membrane that surrounds the organelle. It is highly selective in what it allows to enter and leave the chloroplast.

Question 39

What is the grana?

Answer 39

The grana are stacks of up to 100 fluid filled disc-like structures called thylakoids. Grana are interconnected by lamellae. Within the thylakoids is the photosynthetic pigment (e.g. chlorophyll) that absorbs sunlight. Some thylakoids have tubular extensions that join up with thylakoids in adjacent grana. The grana are where the first stage of photosynthesis (light absorption) takes place.

Question 40

What is the stroma?

Answer 40

The stroma is a fluid-filled matrix where the second stage of photosynthesis (synthesis of sugars) takes place. Within the stroma are a number of other structures, such as starch grains. They also contain circular DNA and ribosomes.

Question 41

How are chloroplasts adapted to their function of harvesting sunlight and carrying out photosynthesis?

Answer 41

• The granal membrane provide a large surface area for the attachment of chlorophyll, electron carriers and enzymes that carry out the first stage of photosynthesis. These chemicals are attached to the membrane in a highly ordered fashion.
• The fluid of the stroma possesses all the enzymes needed to make sugars in the second stage of photosynthesis.
• Chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture some of the proteins needed for photosynthesis.

Question 42

Why do some plant cells not need chloroplasts?

Answer 42

E.g. root hair cells. These are below the soil surface where light rarely penetrates and so no photosynthesis is possible.

Question 43

What is the endoplasmic reticulum?

Answer 43

The endoplasmic reticulum is an elaborate, three-dimensional system of sheet-like membranes, spreading through the cytoplasm of the cells. It is continuous with the outer nuclear membrane. The membranes enclose a network of tubules and flattened sacs called cisternae.

Question 44

Which cells have an extensive endoplasmic reticulum?

Answer 44

Cells that manufacture and store large quantities of carbohydrates, proteins and lipids have a very extensive ER. Such cells include liver and secretory cells, for example the epithelial cells that line the intestines.

Question 45

Where is the Golgi apparatus well developed?

Answer 45

The Golgi apparatus is especially well developed in secretory cells, such as the epithelial cells that line the intestines.

Question 46

What are the functions of lysosomes?

Answer 46

• Hydrolyse material ingested by phagocytic cells, such as white blood cells and bacteria.
• Release enzymes to the outside of the cell (exocytosis) in order to destroy material around the cell.
• Digest worn out organelles so that the useful chemicals they are made of can be re-used.
• Completely break down cells after they have died (autolysis).

Question 47

Where are lysosomes especially abundant?

Answer 47

Given the roles that lysosomes perform, it is not surprising that they are especially abundant in secretory cells, such as epithelial cells, and in phagocytic cells.

Question 48

What are the two types of ribosomes?

Answer 48

80S - found in eukaryotic cells

70S - found in prokaryotic cells, mitochondria and chloroplasts, is slightly smaller

Question 49

What do the subunits of ribosomes contain?

Answer 49

Ribosomes have subunits - one large and one small - each of which contains ribosomal RNA and protein. Despite their small size, they occur in such vast numbers. Ribosomes are the site of polypeptide synthesis.

Question 50

What features do cell walls have?

Answer 50

• They consist of a number of polysaccharides, such as cellulose.
• There is a thin layer, called the middle lamella, which marks the boundary between adjacent cell walls and cements adjacent cells together.

Question 51

What are the functions of the cellulose cell wall?

Answer 51

• To provide mechanical strength in order to prevent the cell bursting under the pressure created by the osmotic entry of water.
• To give mechanical strength to the plant as a whole.
• To allow water to pass along it and so contribute to the movement of water through the plant.

Question 52

What are cell walls of algae made up of?

Answer 52

Either cellulose or glycoproteins, or a mixture of both.

Question 53

What are the cell walls of fungi made up of?

Answer 53

The cell walls of fungi comprise a mixture of a nitrogen-containing polysaccharide called chitin, a polysaccharide called glycan and glycoproteins.

Question 54

What functions do plant vacuoles serve?

Answer 54

• they support herbaceous plants, and herbaceous parts of woody plants, by making cells turgid
• the sugars and amino acids may act as a temporary food store
• the pigments may colour petals to attract pollinating insects

Question 55

How would you relate cell ultrastructure to function?

Answer 55

As each organelle has its own functions, it is possible to deduce, with reasonable accuracy, the role of a cell by looking at the number and size of the organelles it contains.

For example, as mitochondria produce ATP that is used as a temporary energy store, it follows that cells with many mitochondria are likely to require a lot of ATP and therefore have a high rate of metabolism. Even within each mitochondrion, the more dense and numerous the cristae, the greater the metabolic rate of the cell possessing these mitochondria.

Question 56

How do cells provide indirect evidence for evolution?

Answer 56

All cells have basic features in common. Differences between cells are due to the addition of extra features.

Question 57

What is cell signalling?

Answer 57

communication between cells