Cell Biology: Topic 1.1 Cell Introduction

Question 1

The cell theory states that

Answer 1

1. All living things are composed of cells (or cell products)
2. The cell is the smallest and functional unit of life
3. Cells only arise from pre-existing cells

Question 2

What are 4 common features cells share?

Answer 2

1. Evert living cell consists of a cell membrane which separates the cell contents from everything else outside
2. Cells contain genetic material which stores the instructions needed for cellular activities
3. Cells contain enzymes which catalyses chemical reactions that are the activities
4. Cells have their own energy release system that powers all of the cell’s activities

Question 3

Why are striated muscles an exception to cell theory?

Answer 3

They are formed from division of pre-existing cells, have their own genetic material and energy release system
Muscle cells fuse to form fibres that may be very long (>300mm)
Consequently, they have multiple nuclei despite being surrounded by a single, continuous plasma membrane
Challenges the idea that cells always function as autonomous units

Question 4

Why are aseptate fungi an exception to cell theory?

Answer 4

Fungi may have filamentous structures called hyphae, which are separated into cells by internal walls called septa

• Have cell membrane and cell wall
• Some fungi are not partitioned by septa and hence have a continuous cytoplasm along the length of the hyphae with many nuclei spread along it
• Challenges the idea that living structures are composed of discrete cells

Question 5

Why is giant algae an exception to cell theory?

Answer 5

Certain species of unicellular algae may grow to very large sizes (e.g. Acetabularia may exceed 7 cm in length)

* Have genetic material and system of nutrition and energy release (photosynthesis)
* Challenges the idea that larger organisms are always made of many microscopic cells

Question 6

List the functions of life (MR HGREN)

Answer 6

Metabolism – The web of all essential chemical reactions catalysed by enzymes in living things
Reproduction – Living things produce offspring, either sexually or asexually
Homeostasis – Living things maintain a stable internal environment
Growth – Irreversible increase in cell size and shape or cell mass or both
Response – Living things are responsive to internal and external stimuli
Excretion – Living things exhibit the removal of metabolic waste products
Nutrition – Living things exchange materials and gases with the environment needed for growth

Question 7

Investigate the functions of life in Paramecium (reproduction)

Answer 7

Often the reproduction is asexual with the parent cell dividing to form two daughter cells (fission) although horizontal gene transfer can occur via conjugation

Question 8

Investigate the functions of life in Paramecium (nutrition and growth)

Answer 8

Food vacuoles contain smaller organisms that the Paramecium has consumed. These are gradually digested and the nutrients are absorbed into the cytoplasm where they provide energy and materials needed for growth.

Question 9

Investigate the functions of life in Paramecium (excretion)

Answer 9

The cell membrane controls what chemicals enter and leave. It allows the entry of oxygen for respiration. Excretion happens simply by waste products diffusing out through the membrane.

Question 10

Investigate the functions of life in Paramecium (homeostasis)

Answer 10

The contractile vacuoles at each end of the cell fill up with water and then expel it through the plasma membrane of the cell, to keep the cell water content within tolerable limits.

Question 11

Investigate the functions of life in Paramecium (metabolism)

Answer 11

Metabolic reactions take place in the cytoplasm, including the reactions that release energy by respiration. Enzymes in the cytoplasm are the catalysts that cause these reactions to happen.

Question 12

Investigate the functions of life in Paramecium (response)

Answer 12

Beating of the cilia moves the Paramecium through the water and this can be controlled by the cell so that it moves in a particular direction in response to changes in the environment.

Question 13

Investigate the functions of life in Chlamydomonas (reproduction)

Answer 13

The nucleus of the cell can divide to produce genetically identical nuclei for asexual reproduction. Nuclei can
also fuse and divide to carry out a sexual form of reproduction.

Question 14

Investigate the functions of life in Chlamydomonas (metabolism)

Answer 14

Metabolic reactions take place in the cytoplasm, with enzymes present to speed them up.

Question 15

Investigate the functions of life in Chlamydomonas (nutrition)

Answer 15

Produces its own food by photosynthesis in chloroplasts that occupies much of the cell in the cytoplasm.

Question 16

Investigate the functions of life in Chlamydomonas (growth)

Answer 16

Increases in size and dry mass as carbon compounds produced by photosynthesis are used for growth, and in the dark carbon compounds are absorbed from other sources through the plasma membrane if available

Question 17

Investigate the functions of life in Chlamydomonas (homeostasis)

Answer 17

The contractile vacuoles at the base of the flagella fill up with water and then expel it through the plasma membrane of the cell, to keep the cell water content within tolerable limits.

Question 18

Investigate the functions of life in Chlamydomonas (excretion)

Answer 18

The cell wall is freely permeable and it is the membrane inside it that controls what chemicals enter and leave. Oxygen is a waste product of photosynthesis and is excreted by diffusing out through the membrane.

Question 19

Investigate the functions of life in Chlamydomonas (response)

Answer 19

Beating of the two flagella moves the Chlamydomonas through the water. A light sensitive eyespot allows the cell to sense where the brightest light is and respond by swimming towards it.

Question 20

Explain the importance of surface area to volume ratio as a factor limiting cell size

Answer 20

As a cell grows, volume (units3) increases faster than surface area (units2), leading to a decreased SA:Vol ratio

• If metabolic rate exceeds the rate of exchange of vital materials and wastes (low SA:Vol ratio), the cell will eventually die.

If the ratio is too small then substances will not enter the cell as quickly as they are required and waste products will accumulate because they are produced more rapidly than they can be excreted. Surface area to volume ratio is also important in relation to heat production and loss. If the ratio is too small then cells may overheat because the metabolism produces heat faster than it is lost over the cell’s surface.

Question 21

Which cells importantly need to optimise surface area to volume ratio in relation to its function? Give examples

Answer 21

Cells and tissues that are specialised for gas or material exchanges will increase their surface area to optimise material transfer such as:

• Intestinal tissue of the digestive tract may form a ruffled structure (villi) to increase the surface area of the inner lining
• Alveoli within the lungs have membranous extensions called microvilli, which function to increase the total membrane surface
• Cells may adopt an elongated or flattened shape to increase SA:Vol ratio (e.g. bacillus)
• Cells may possess cellular extensions (e.g. root hairs)
• Cells may reduce effective cell volume by including large central vacuoles (e.g. plant cells)

Question 22

What are emergent properties

Answer 22

Emergent properties arise when the interaction of individual component of a complex structure produce new functions or distinctive overall properties of an organism

Question 23

How do multicellular organisms function based on the principle of emergent properties?

Answer 23

Multicellular organisms are capable of completing functions that unicellular organisms could not undertake – this is due to the collective actions of individual cells combining to create new synergistic effects

In multicellular organisms:

Cells may be grouped together to form tissues
Organs are then formed from the functional grouping of multiple tissues
Organs that interact may form organ systems capable of carrying out specific body functions
Organ systems collectively carry out the life functions of the complete organism

With each step in this hierarchy of biological order, new properties emerge that were not present at simpler levels of organization.

Question 24

What is cell differentiation?

Answer 24

Differentiation is the process during development whereby newly formed cells become more specialised and distinct from one another as they mature to carry out a specific function

Question 25

Advantage of cell differentiation

Answer 25

As cells become specialised, the role can be carried out more efficiently by developing the ideal structure with the enzymes needed to carry out all of the reactions associated with the function (cell’s shape and metabolism changes as cell differentiates and becomes specialised)

Question 26

How does differentiation occur in cells?

Answer 26

All cells of an organism share an identical genome – each cell contains the entire set of genetic instructions for that organism

The activation of different instructions (the expression of certain genes by switching on some base sequences but not others) within a given cell by chemical signals will cause it to differentiate

Question 27

What is chromatin?

Answer 27

Within the nucleus of a eukaryotic cell, DNA is packaged with proteins to form chromatin

Question 28

How are active genes packaged?

Answer 28

Active genes are usually packaged in an expanded form called euchromatin that is accessible to transcriptional machinery

Question 29

How are inactive genes packaged?

Answer 29

Inactive genes are typically packaged in a more condensed form called heterochromatin (saves space, not transcribed)

Question 30

Outline the appearance of gene packaging in differentiated cell

Answer 30

Differentiated cells will have different regions of DNA packaged as euchromatin and heterochromatin according to their specific function

Question 31

Stem cells are unspecialised cells that have two key qualities:

Answer 31

1. Self Renewal – They can continuously divide and replicate
2. Potency – They have the capacity to differentiate into specialised cell types

Question 32

There are four main types of stem cells present at various stages of human development:

Answer 32

Totipotent – Can form any cell type, as well as extra-embryonic (placental) tissue (e.g. zygote)

Pluripotent – Can form many different cell types (e.g. embryonic stem cells)

Multipotent – Can differentiate into a number of closely related cell types (e.g. Bone marrow stem cells, epithelial stem cells, bone stem cells (osteoblasts). (mostly adult stem cells)

Unipotent – Can not differentiate, but are capable of self renewal (e.g. progenitor cells, muscle stem cells)

Question 33

Describe the importance of stem cells in differentiation

Answer 33

“Stem cells are necessary for embryonic development as they are an undifferentiated cell source from which all other cell types may be derived
stem cells can differentiate in many ways which involves expressing some genes but not others
Cell types that are not capable of self-renewal (e.g. amitotic nerve tissues) are considered to be non-stem cells
As these tissues cannot be regenerated or replaced, stem cells have become a viable therapeutic option to repair/replace tissues/heal wounds;”

Question 34

Describe the use of stem cells to treat Stargardt’s disease

Answer 34

”* An inherited form of juvenile macular dystrophy that causes progressive vision loss to the point of blindness

• Caused by a gene mutation that causes a membrane protein for active transport in retinal photoreceptor cells to malfunction, causing them to degenerate
• Treated by replacing dead cells in the retina with functioning ones derived from stem cells by injecting it into the retina, where the retinal cells attach to the retina and become functional
• As more retinal cells become functional, vision significantly improves with no side effects. “