3. Cells

Question 1

Cell Membrane

Structure & Function

Answer 1

Structure:

• Partially permeable
• Made up of 2 layers of phospholipids - phospholipid bilayer
• Also contains various proteins, carbohydrate chains, and cholesterol attached to or embedded within the phospholipid bilayer
• The fluid mosaic model is used to describe the structure of the cell membrane

Function:

• Separates and protects the cell from its surrounding environment
• Controls the movement of substances in and out of the cell

Question 2

What does the fluid mosaic model suggest about the cell membrane?

Answer 2

The fluid mosiac model suggests that:

• The cell membrane is fluid in that the phospholipids and protiens are not stationary but can move rapidly across the surface of the membrane
• The cell membrane is a mosaic in that many different protein molecules are randomly scattered and embedded throughout the phospholipid bilayer

Question 3

What substances can and cannot pass through the cell membrane without transport proteins? Why?

Answer 3

Due to the nature of phospholipids,

• Only small hydrophobic molecules can pass through the phospholipid bilayer
• Hydrophilic or water-soluble molecules are unable to pass through the phospholipid bilayer

Question 4

Cellulose Cell wall

Strucutre & Function

Answer 4

Structure:

• Consists of cellulose fibres embedded in a polysaccharide matrix
• Fully permeable

Function:

• To provide mechanical support for plant cell and to the plant
• To resist expansion when water enters by osmosis, ensuring integrity of plant cell and to provide turgidity

Question 5

Cytoplasm

Structure

Answer 5

Cytoplasm contains:

• Cytosol - aqueous solution of essential ions, soluble proteins (e.g. enzymes), and soluble organic compounds (e.g. sugars & amino acids)
• Cytoskeleton - network of protein fibres that give support, motility, and regulation to the cell
• Orgnalles - structures with specialised functions, suspended in the cytosol

Question 6

Cytoplasmic Streaming

What it is + Where it occurs + Function

Answer 6

Cytoplasmic streaming is the movement of the cytoplasm within a plant or animal cell.

Cytoplasmic streaming takes place in the cytoplasm of cells.

Cytoplasmic streaming aids in the transport of materials and organelles around the cell.

Example: Cytoplasmic streaming of choroplasts within a plant cell to regions with more light.

Question 7

Nucleus

Structure & Function

Answer 7

Structure:

• Spherical
• Consists of nuclear envelope (double membrane) which separates the nucleus from the cytoplasm
• Nuclear pores
• Contains nucleolus which synthesises ribosomes
• Contains DNA

Function:

• Contains hereditary material (DNA)
• Controls the activities of the cell
• Plays an important role in DNA replication and protein synthesis (transcription)

Question 8

Rough Endoplasmic Reticulum (RER)

Structure & Function

Answer 8

Structure:

• A system of flattened membrane-bound sacs
• Appears “rough” under an electron miscrope because of the presence of ribosomes on its surface
• Extends from and is contiguous with the nuclear envelope

Function:

• Plays a role in protein synthesis
• The polypeptide chain synthesised by ribosomes is folded in the RER

Question 9

Smooth Endoplasmic Reticulum (SER)

Structure & Function

Answer 9

Structure:

• Consists of fine tubules

Function:

• Plays a role in lipid synthesis
• Detoxification of drugs and poisons
• Storage and release of calcium ions

Question 10

Golgi Apparatus

Structure & Function

Answer 10

Structure:

• Consists of stacks of flattened membrane-bound sacs

Function:

• Chemically modifies products received from the ER and packages them, before sending them to destinations inside or out of the cell using vesicles

Question 11

Lysosomes

Structure & Function

Answer 11

Structure:

• Small, spherical vesicles surrounded by a single membrane
• Contains hydrolytic enzymes (digestive enzymes) such as amylases, lipases, proteases, and nucleases

Function:

• Digests materials made in the cell or taken in from outside by phagocytosis
• Digests worn-out organelles in the cell, a process known as autophagy

Question 12

Animal Vacuoles

Structure + How it is formed

Answer 12

Strucutre:

• Fluid-filled sac bound by a single membrane
• Relatively smaller and exist temporarily
• Formed either by the pinching-off part of the cell membrane, or by enlargement of a vesicle from the Golgi apparatus

Food vacuoles are formed by phagocytosis in the case of intracellular digestion by macrophages

Question 13

Plant Vacuoles

Structure & Function

Answer 13

Structure:

• Singular, large, central vacuole is permanant and occupies over 80% of the cell volume
• Enclosed by a single, partially permeable membrane called the tonoplast
• Filled with cell sap, a solution of dissolved substances such as sugars, ions, waste products, and pigments

Function:

• Storage of nutrients (e.g. protein storage in seeds) and inorganic ions (e.g. K+ and Cl-)
• Disposal sites for metabolic by-products that would endanger the cell if they accumulated in the cytosol
• May contain pigments that colour the cells (e.g. red and blue pigments of petals that attract pollinators)

Question 14

Mitochondria (plural) / Mitochondrion (singular)

Structure & Function

Answer 14

Structure:

• Rod shaped or cylindrical
• Bound by a double membrane

Function:

• Involved in aerobic respiration to produce ATP

Mitochondria are abundant in metabolically active cells e.g. muslce and liver cells

Question 15

Chloroplast

Structure & Function

Answer 15

Structure:

• Bound by a double membrane
• Contain chlorophyll
• Contain thylakoids and stroma

Function:

• Site of photosynthesis

Question 16

Ribosomes

Structure & Function

Answer 16

Structure:

• Small, round structures
• Made up of ribosomal RNA and proteins
• Synthesised by the nucleolus
• May occur as free ribosomes suspended in the cytosol, or bound ribosomes bound to rough ER

Function:

• Site of polypeptide synthesis
• Free ribosomes generally make proteins that will function within the cytosol
• Bound ribosomes generally make proteins that are destined for insertion into membranes, for packaging within certain organelles, or for export from the cell

Question 17

Centrioles

Structure & Function

Answer 17

Structure:

• Small, hollow cylinders that occur in pairs
• Found near the nucleus

Function:

• Play a role in cell division in animal cells

Centrioles are only found in animal cells and are absent in plant cells

Question 18

Describe the process of protein synthesis and secretion in a cell

6 Steps

Answer 18

1. Polypeptide chains are synthesised at the ribosomes through translation and are folded at the rough endoplasmic reticulum.
2. Transport vesicles containing the proteins made by the endoplasmic reticulum pinch off from the endoplasmic reticulum.
3. These transport vesicles fuse with the Golgi apparatus and release the proteins into the Golgi apparatus.
4. The proteins are chemically modified and packaged inside the Golgi apparatus.
5. Secretory vesicles containing these modified proteins pinch off from the Golgi apparatus.
6. The secretory vesicles move towards and fuse with the cell membrane, releasing the proteins outside the cell by exocytosis.

Question 19

Describe the process of the formation of lysosomes

2 Steps + Why enzymes have to be kept in lysosomes

Answer 19

1. Enzyme contents are synthesised in the rough endoplasmic reticulum and transported by transport vesicles to the Golgi apparatus for further processing
2. A vesicle containing the processed enzymes buds off from the Golgi apparatus, forming a lysosome

The enzymes have to be kept in lysosomes apart from the rest of the cell or else they would destroy the cell

Question 20

Compare the strucutre of typical animal and plant cells

Similarities & Differences (Note those seen under electron microscope)

Answer 20

Similarities:

Both cells contain

• Cell membrane
• Mitochondria (only electron microscope)
• Nucleus
• Cytoplasm

Differences:

• Cell wall is present in plant cell while absent in animal cell
• Chloroplasts are present in large numbers in plant cells while absent in animal cells
• Vacuole is present in plant cell as a single, large, central vacuole while it is present in animal cells as numerous, small vacuoles
• Centrioles are absent in plant cells while present in animal cells (only electron microscope)

Question 21

What is cell specialisation / cell differentiation?

Answer 21

It is a process where the structure of a cell is differently adapted to perfom a specific function.

Question 22

How is a root hair cell specially adapted to perform its function?

Answer 22

1. Function: To absorb water by osmosis

• The cells have a long and narrow extension to increase its surface area to volume ratio, thus increasing the rate of absorption of water by osmosis and mineral salts by active transport.
• The cell sap has a lower water potential than the soil solution, thus water will continuously enter the root hair cell by osmosis.

2. Function: To absorb mineral salts by active transport

• The root hair cell contains mitochondria to provide energy for the active transport of mineral salts into the cell.

Question 23

How is a red blood cell specially adpated to perform its function?

Answer 23

Function:

• Transports oxygen to body cells and carbon dioxide away from body cells.

Structural adaptations:

1. Presence of haemoglobin: which combines with oxygen reversibly to form oxyhaemoglobin, to enable red blood cells to transport oxygen from the lungs to all cells in the body.
2. Circular, flattened biconcave disc: increases surface area to volume ratio to increase rate of diffusion of oxygen and carbon dioxide.
3. Absence of nucleus: allows more haemoglobin to be packed in the cell.
4. Elastic, flexible and can turn bell-shaped: to be able to squeeze through blood vessels smaller than itself in diameter without breaking.

Question 24

How is a muscle cell specially adapted to perform its function?

Answer 24

1. Contains many nuclei and mitochondria: To provide the large amount of energy needed for the contractions of the muscle cell

Question 25

State the level of complexity of biological molecules in increasing order

Answer 25

Macromolecule
Organelle
Cell
Tissue
Organ
Organ System