3. Cells Flashcards
Cell Membrane
Structure & Function
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
What does the fluid mosaic model suggest about the cell membrane?
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
What substances can and cannot pass through the cell membrane without transport proteins? Why?
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
Cellulose Cell wall
Strucutre & Function
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
Cytoplasm
Structure
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
Cytoplasmic Streaming
What it is + Where it occurs + Function
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.
Nucleus
Structure & Function
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)
Rough Endoplasmic Reticulum (RER)
Structure & Function
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
Smooth Endoplasmic Reticulum (SER)
Structure & Function
Structure:
- Consists of fine tubules
Function:
- Plays a role in lipid synthesis
- Detoxification of drugs and poisons
- Storage and release of calcium ions
Golgi Apparatus
Structure & Function
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
Lysosomes
Structure & Function
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
Animal Vacuoles
Structure + How it is formed
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
Plant Vacuoles
Structure & Function
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)
Mitochondria (plural) / Mitochondrion (singular)
Structure & Function
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
Chloroplast
Structure & Function
Structure:
- Bound by a double membrane
- Contain chlorophyll
- Contain thylakoids and stroma
Function:
- Site of photosynthesis
Ribosomes
Structure & Function
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
Centrioles
Structure & Function
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
Describe the process of protein synthesis and secretion in a cell
6 Steps
- Polypeptide chains are synthesised at the ribosomes through translation and are folded at the rough endoplasmic reticulum.
- Transport vesicles containing the proteins made by the endoplasmic reticulum pinch off from the endoplasmic reticulum.
- These transport vesicles fuse with the Golgi apparatus and release the proteins into the Golgi apparatus.
- The proteins are chemically modified and packaged inside the Golgi apparatus.
- Secretory vesicles containing these modified proteins pinch off from the Golgi apparatus.
- The secretory vesicles move towards and fuse with the cell membrane, releasing the proteins outside the cell by exocytosis.
Describe the process of the formation of lysosomes
2 Steps + Why enzymes have to be kept in lysosomes
- Enzyme contents are synthesised in the rough endoplasmic reticulum and transported by transport vesicles to the Golgi apparatus for further processing
- 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
Compare the strucutre of typical animal and plant cells
Similarities & Differences (Note those seen under electron microscope)
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)
What is cell specialisation / cell differentiation?
It is a process where the structure of a cell is differently adapted to perfom a specific function.
How is a root hair cell specially adapted to perform its function?
- 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.
- 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.
How is a red blood cell specially adpated to perform its function?
Function:
- Transports oxygen to body cells and carbon dioxide away from body cells.
Structural adaptations:
- 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.
- Circular, flattened biconcave disc: increases surface area to volume ratio to increase rate of diffusion of oxygen and carbon dioxide.
- Absence of nucleus: allows more haemoglobin to be packed in the cell.
- Elastic, flexible and can turn bell-shaped: to be able to squeeze through blood vessels smaller than itself in diameter without breaking.
How is a muscle cell specially adapted to perform its function?
- Contains many nuclei and mitochondria: To provide the large amount of energy needed for the contractions of the muscle cell
State the level of complexity of biological molecules in increasing order
Macromolecule
Organelle
Cell
Tissue
Organ
Organ System
